Network Scan Data

Cool roofs and the energy performance of residential buildings: experimental optimization, numerical analysis and field tests

A Mamma e Papà, Giammi, Filippo e Leo, la mia famiglia.

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Abstract - Sintesi

ACKNOWLEDGMENTS “Wherever a man turns he can find someone who needs him.” Albert Schweitzer (1875-1965); Philosopher, Physician, Nobel Peace Prize Winner

Foremost, I would like to express my deepest gratitude to my advisor Professor Franco Cotana, for continuously supporting my PhD study and this research with his key cues and his lively enthusiasm as the best method for research. I am greatly indebted to him, also for the deep trust he gave to me about this one and other projects, since I arrived here at University of Perugia, for continuously encouraging me to overrun my research efforts and myself as well. I also wish to express my deep gratitude to H2CU, the Honors Center of Italian Universities, for giving me the deepest opportunity to grow I have ever had, from both scientific and personal point of view. For the warm hospitality at Columbia University, and for the exciting matches, thanks to: Professor John Eric Taylor, a great guide, Xiaouqi Xu, Dr. Jiayu Chen, Dr. Josh Iorio. For welcoming me in the United States, thanks to: Professor Michael Bobker, Professor Maurizio Porfiri, Dr. Maria Grillo, Professor Vittorio Canuto, and Professor Arthur Rosenfeld. Furthermore, I would like to express my gratitude to the whole group of Professors at University of Perugia, for keeping close to me during the last three years, thus special thanks to: Professor Cinzia Buratti, for the scientific rigors and the deep attention paid and time spent for every shared initiative; Professor Federico Rossi, for teaching me how to transform curiosity in research; Professor Francesco Asdrubali, for always sharing with us his passion and his fruitful effort. The present work would not be possible without the personal and scientific support of the team, thus thanks so much to: Dr. Catia Baldassarri, Dr. Michele Goretti, Dr. Elisa Moretti, Dr. Cleofe Merico, Dr. Emanuele Bonamente, Dr. Riccardo Paolini, Dr. Francesco Bianchi, Dr. Elisa Belloni, Dr. Mirko Filipponi, Dr. Sara Rinaldi, Dr. Samuele Schiavoni, Ms. Claudia Bastianini, Ms. Serena Gallicchio, and Mr. Roberto Barone. Words are not enough to express my gratitude to my parents and my brother: at least half of this PhD is theirs, for their patience in listening to me and my troubles. They all have become Cool Roof addicted… Finally, Thank you Filippo, for always being there when I need it, even when I do not need it. th Perugia, October 30 2012

Anna Laura

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SINTESI Un cool roof, in italiano letteralmente “tetto freddo”, è un sistema di copertura caratterizzato da un elevato valore di albedo, ossia da una spiccata capacità di riflettere la radiazione solare incidente sulla sua superficie, associato ad un’altrettanto spiccata emissività nell’infrarosso, che permette alla copertura di emettere in atmosfera la massima parte della irradiazione solare assorbita dal tetto, attraverso irraggiamento termico. Un sistema di copertura con queste caratteristiche permette di ottenere vari effetti positivi in termini energetico-ambientali, sia di tipo diretto sul bilancio energetico dell’edificio, che di tipo indiretto su scala urbana e di clima globale. In primis infatti, un cool roof permette di abbattere le temperature superficiali esterne della copertura, con il risultato di ridurre il carico termico di raffrescamento per le zone termiche adiacenti la copertura. La riduzione quindi anche delle emissioni causate da tali sistemi impiantistici, tipicamente in Italia molto energivori, provoca una riduzione a sua volta dello stress ambientale e del fenomeno di isola di calore urbana, anche grazie al ridotto accumulo termico da parte delle strutture di copertura che, essendo in grado di riflettere la radiazione solare incidente, non contribuiscono ad aggravare il bilancio energetico urbano a livello di “urban canopy”. Questo sistema tecnologico è già stato ampiamente investigato ed ottimizzato negli Stati Uniti, i cui governi degli stati membri hanno supportato importanti contributi in termini di ricerca e sviluppo sui cool roof, per poi incentivarne in maniera diffusa lo sviluppo e la diffusione, anche mediante sovvenzionamenti. In Italia ancora non esistono provvedimenti legislativi in merito ed anche la ricerca sul campo dei nuovi materiali da cool roof è piuttosto limitata. L’attività scientifica sui cool roof, tuttavia, ha riguardato fino ad oggi perlopiù sistemi di pitture per l’edilizia e membrane elastomeriche o poliuretaniche ad elevato albedo, sistemi che quindi ben si adattano a coperture di edifici in generale nuovi, ad elevato contenuto tecnologico, con coperture tipicamente piane, e soprattutto edifici adibiti ad uffici, o settore industriale e terziario in genere, dove anche i carichi interni sono spesso così gravosi, da implicare un consumo copioso di energia per il raffrescamento in tutto l’arco dell’anno, perfino alle alte latitudini. E’ invece ancora piuttosto ridotta la letteratura che riguarda i cool roof sugli edifici residenziali, ed ancor meno che ne valuta le vi

Abstract - Sintesi

prestazioni sia in regime estivo che invernale, se non per qualche studio di carattere puramente numerico. Ad ogni modo, il contributo effettivo nella riduzione dei fabbisogni di raffrescamento raggiunti dai cool roof in generale, è stato comprovato da varie attività sperimentali e di simulazione negli ultimi anni, ed ha incoraggiato anche ulteriori ricerche per lo studio e lo sviluppo di materiali innovativi e sistemi tecnici e tecnologici di copertura finalizzati ad innescare appunto il comportamento da “tetto freddo”. In questo quadro, la presente ricerca ha lo scopo di elaborare un innovativo elemento tegola in laterizio ad elevato albedo mediante analisi di riflettanza ed emissività in laboratorio, per poterne valutare gli effetti mediante monitoraggio in continuo indoor-outdoor sia in regime estivo che invernale, quando installata su una copertura di un edificio residenziale situato in un’area di clima temperato. Le tegole tradizionali presentano infatti una riflettanza piuttosto ridotta (0.30.4) che, associata ad una ridotta inerzia delle strutture di copertura, spesso ha reso impraticabili tutti gli ambienti mansarda e attico, sia nelle aree urbane che in quelle rurali in Italia. La presente ricerca risponde quindi a questa esigenza energeticoambientale mediante lo sviluppo di una tegola innovativa ad elevato albedo, con comprovati benefici in termini di ottimizzazione dell’efficienza energetica dell’edificio residenziale valutati mediante bilancio energetico annuale, tenendo quindi anche in considerazione le gravose forzanti climatiche della stagione invernale che caratterizzano il clima temperato del centro Italia, in cui si trova l’edificio caso di studio. Dopo quindi una prima fase di analisi delle proprietà suddette ed ottimizzazione delle stesse, è stato prodotto un innovativo elemento tegola in laterizio, le cui prestazioni in opera sono state valutate sia in termini di comportamento termico della copertura, che dell’ambiente indoor ad essa adiacente. L’attività di monitoraggio in continuo, sia della configurazione di tegole tradizionali che ottimizzate, ciascuna per la durata di un intero anno, ha poi permesso la elaborazione dei modelli numerici calibrati e validati dell’edificio caso di studio. Tale analisi numerica successiva ha quindi permesso di estendere i risultati sperimentali a tutte le zone climatiche Italiane significative, e per vari livelli di isolamento termico della partizione perimetrale di copertura. La ricerca dimostra come complessivamente la tecnologia proposta sia in grado di ridurre ampiamente lo stress termico del tetto, abbattendone il surriscaldamento estivo e quindi i carichi termici relativi gravanti sulla zona termica dell’attico. Durante la stagione invernale, altrettanto importante nella maggioranza del territorio Italiano, tale contributo risulta trascurabile in termini di temperatura operativa indoor. Dall’estensione di tali risultati sperimentali

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mediante analisi dinamica, emerge poi come la soluzione proposta risulti avere un’ottima efficacia in termini di riduzione del fabbisogno annuale di energia primaria in tutti i climi italiani, ad eccezione di quello alpino, con effetti largamente maggiori per configurazioni di copertura scarsamente o non isolate. Parole chiave: cool roof; albedo; efficienza energetica degli edifici; monitoraggio in continuo; simulazione dinamica; calibrazione del modello dell’edificio.

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Abstract - Sintesi

ABSTRACT A cool roof is a roof system characterized by high albedo properties, that make it able to reflect the solar radiation incident on its surface, combined to an as much high infrared emissivity, that allows the roof to emit the maximum quote of solar radiation previously absorbed, through thermal radiation. Such a roof system allows to achieve several energy-environmental benefits, both direct effects on the building energy balance, and indirect effects, at urban scale and in terms of global climate. First, a cool roof indeed allows to cut off the external surface temperature of the roof, with the main result to reduce the cooling thermal load for those thermal zones that are adjacent to the roof. Then, the corresponding reduction of the emissions produced by those cooling thermal plants, that in Italy are typically very low efficiency systems, produces also the cutback of the environmental stress and of the Urban Heat Island phenomenon, also thanks to the low cool roof thermal storage, that does not contribute to the exasperation of the urban energy balance calculated in proximity of the urban canopy. This technology has already been investigated and optimized in the United States, which Governments have supported important research and development activities about cool roofs, to finally stimulate their spreading diffusion also through incentives and specific energy policies. In Italy legislative guidelines still lack, and also the research around new materials is limited. The scientific investigation about cool roofs has however widely concerned high albedo coatings typologies, elastomeric and polyurethane membranes for buildings. All those systems usually fit much more for new buildings’ no-sloped roofs, with high technology level. These fabrics are usually office buildings, industrial or service sector buildings in general, where also the internal gains are as much onerous, to produce important cooling requirements all over the year, also at high latitudes locations. Scientific literature about cool roofs for residential buildings just consists of few contributions, and the contributions that evaluate both winter and summer performance are even less diffuse, except for some purely numerical studies. Anyway, the effective cool roof contribution in reducing cooling requirement is demonstrated with evidence of experimental and simulation activities during recent years. These results have encouraged further research for studying and developing new materials and technical roof systems aimed at triggering the cool roof behavior.

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In this panorama, the purpose of this research consists of the elaboration of an innovative clay tile with high albedo properties through lab reflectance and emissivity analyses, to finally evaluate its effect by continuous coupled indooroutdoor monitoring both in summer and winter conditions, when this prototyped tile is installed on the roof of a residential building located in temperate climate. Traditional clay tiles usually present low reflectance levels (0.3-0.4) that have often produced, together with low roof inertia characteristics, the unsuitability to live in attics, both in urban and rural areas in Italy. This research basically answers to this energy-environmental request through the development of a high albedo innovative tile, with proven benefits in terms of residential building energy efficiency optimization evaluated by yearround energy balance, thus considering also the severe climate forcing of winter season that characterizes the temperate climate of Central Italy, where the case study is located. After a first step focused on the analysis of the mentioned properties and their optimization, an innovative clay tile has been produced, whose in-field performance has been evaluated in terms of thermal behavior of the same roof and of the indoor adjacent zone. The continuous monitoring, both concerning the traditional tiles’ configuration and the optimized one, each one lasting for one entire year, has then allowed the elaboration of calibrated and validated models of the case study building. This further numerical analysis has then allowed to extend the experimental results to the main Italian climatological zones, and also to different roof insulation levels. The research demonstrates that the proposed technology is able to highly reduce roof thermal stress, reducing its summer overheating and thus also cutting the thermal loads burdening the attic. During winter, that is as much important in the majority of Italian territory, such contribution is actually negligible in terms of indoor operative temperature. From the extension of these experimental results through dynamic analysis, it emerged that the proposed solution has excellent efficacy in reducing the year-round requirement of primary energy in all Italian climate conditions, except for the Alps area, with very much larger effects for not or poorly insulated roofs. Keywords: cool roof; albedo; building energy efficiency; continuous monitoring; dynamic simulation; whole building model calibration.

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TABLE OF CONTENTS

Abstract

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Sintesi (in Italian)

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1. Introduction

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1.1 Research Topic

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1.2 Motivation

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1.3 Purpose of the work

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1.4 Outline of the work

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2. Cool roofs: state of the art and perspectives

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2.1 Overview

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2.2 Cool Roof as a building cooling strategy

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2.3 Cool Roofs to reduce Urban Heat Island effect

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2.4 The role of cool roofs in global warming mitigation

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2.5 Nomenclature

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2.6 References of the chapter

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3. Analytical formulations for thermal-energy performance assessment of buildings 3.1 Overview

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3.2 Analysis of results of building dynamic simulation and experimental monitoring

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3.3 Thermal Deviation Index

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3.2.1 Formulation

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3.2.2 Case study and correlations

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3.4 Overheating and overcooling assessment

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3.5 Enlargement of perspectives: inter-building context

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3.5.1 Formulation

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3.5.2 Case study

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3.6 Nomenclature

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4. Development and optimization of a new cool roof prototype

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4.1 Overview

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4.2 Mainframe of the experimental activity

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4.3 In-lab analysis and optimization

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4.3.1 Reflectance effective characterization and over time analysis

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4.4 Industrial prototype elaboration

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4.5. On-going research about “cool tiles”

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performance assessment

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4.6 Observations

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4.7 Nomenclature

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Table of contents

5. In-field application of the cool roof prototype

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5.1 Overview

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5.2 Choice of the building

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5.3 Architectural characterization of the case study

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5.4 Experimental thermal characterization of the

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building envelope 5.5 References of the chapter

6. Description of the continuous monitoring system

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6.1 Overview

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6.2 Experimental monitoring campaign setup

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6.3 Monitoring devices

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6.3.1 Outdoor weather station characterization

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6.3.2 Indoor microclimate station characterization

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6.4 Continuous monitoring calendar

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6.5 Main remarks

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7. Two-years continuous monitoring of the building thermal-energy performance

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7.1. Overview

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7.2. Analysis of the roof reflectance

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7.3. Analysis of the roof thermal behavior

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7.4. Analysis of the indoor thermal behavior

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7.5. Analysis of the ceiling thermal behavior

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7.6. Results interpretation

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7.7. Year-round assessment

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7.8. Concluding remarks

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7.9. Nomenclature

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8. Numerical dynamic simulation modeling

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8.1 Overview

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8.2 Building envelope modeling

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8.3 Building equipment characterization

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8.4 Operational schedules characterization

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8.5 Concluding remarks

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9. Calibration and validation of the model through real energy consumption assessment

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9.1 Overview

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9.2 Whole building calibrated simulation approach

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9.3 Methodology

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9.3.1 Whole building calibration and validation

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9.3.2 Cool roof validation procedure

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9.4 Elaboration of the year-round weather file of the location

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9.5 Occupants’ behavior survey and realistic activity schedules

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9.6 Whole-building model refining iterative process: statistical analysis and matching calculations

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9.7 Reliability of the simulation engine to predict cool roof impact 151

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9.8 Discussion

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9.9 Nomenclature

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Table of contents


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10. Cool roof year-round assessment in Italian climate 159 10.1 Overview

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10.2 Impact of cool roof on Primary Energy requirement

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10.2.1 Not insulated roofs

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10.2.2 Insulted roofs

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10.3 Impact of cool roof through TDI assessment in free running conditions

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10.4 Cost benefit analysis

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10.5 Discussion

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10.6 Nomenclature

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11. Conclusions

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11.1 Overview

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11.2 Analytical proposed methodology

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11.3 Prototype realization

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11.4 Continuous monitoring

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11.5 Dynamic calibrated simulation

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11.6 Directions for further research

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Chapter 1 – Introduction

Chapter 1

INTRODUCTION 1.1. Research Topic 1.2. Motivation 1.3. Purpose of the work 1.4. Outline of the work

1.1

Research topic

In this work, the effect of an innovative prototyped cool roof on the thermalenergy behavior of traditional Italian residential buildings is investigated, in order to demonstrate that this cool roof solution, and in general high albedo roofs, represent an effective strategy to optimize building energy efficiency and indoor thermal comfort during the whole course of the year in temperate climate.

1.2

Motivation

Residential buildings contribute for more than 30% to the demand upon the electrical system [1], while also the demand for electricity is registering a growing trend. Related economic and environmental issues are guiding research and technical challenges for the electrical generation, transmission, and distribution sectors [2], and also for final energy use reduction in buildings [3]. In many countries also characterized by temperate or continental heating dominated climates, the time schedules of peaking demands for electricity have moved to summer afternoons, when high outdoor temperature and high internal gains in buildings are able to produce increasing cooling demands for the whole construction sector, in both residential and commercial buildings [4]. Also, the rapid recent increase in comfort cooling requirements and thus in air conditioning sprawling usage have highlighted this phenomenon and the urgent research issue to respond to summer energy building requirement through sustainable solutions [5]. To exacerbate this effect, also the global and meso-climate conditions describing an increasing urban temperature produce an intensification of this

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The impact of cool roofs on the energy performance of residential buildings: experimental optimization, numerical analysis and field tests

issue in urban areas, affected by the heat island phenomenon and the undeniable climatic change [6]. In order to investigate these phenomena and to elaborate innovative effective solutions for buildings and environment, an intensive research effort is carried out about these topics. Thus both urban heat island effect and the related building energy efficiency in summer performance are much better documented [7-8]. In this panorama, roofs represent key element of the urban canopy for both city and building energy balance. Akbari and Rose in [9] estimate that roof area corresponds to about 20-25% of four American cities. Given the huge research development about meso-scale cost-effective solutions for mitigating these urban effects [10-12], the strategic role played by roofs is investigated for multipurpose aims. In fact roofs provide excellent space to apply mitigation techniques at limited costs, largely less than urban green areas [13-14], together with the potential important energy savings for buildings associated to roof thermal-energy performance optimization. The most beaten energy efficiency technologies associated to roofs basically consist of: cool roofs and green roofs. The former is aimed at increasing the surface albedo and emissivity [15-16], the latter is based on the benefits produced by live vegetation covering [17-18]. Both these solutions have radical effects in lowering roof surface temperatures and thus decreasing the energy requirement and emissions for cooling. Given the more suitable application to the Italian residential sector, an innovative cool roof solution represents the fulcrum of this research, following the recent developments on the field of new materials and technologies for cool roofs. In particular, the first generation of cool roof coverings concerns naturally high reflective materials with reflective capability rarely higher than 75% [19-20]. The second generation consists of the first experimentally developed coverings with high performance in reflecting solar radiation, often more than 85% [21-22]. The third generation is aimed at developing high performing roof coatings characterized by high infrared reflectance, thus optimizing the cool roof effect with respect to the conventional ones of the same color [23-24]. The forth recent generation specifically concerns those innovative materials, i.e. nanobased, thermochromic paints, or phase change materials [25-26]. With the same purpose, this work consists of the development of a traditional roof element characterized by high reflective surface properties, to quantify the year-round thermal-energy characterization of a single-family house in temperate climate of Central Italy. To evaluate cool roof thermal performance in reducing cooling requirement while increasing winter penalty, many studies have been performed [27-29]

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showing that cool roofs efficacy is mainly related to climate and building envelope features. Typically summer benefits are around 10-40%, while winter penalties are around 5-10% both estimated by dynamic simulation and fieldmonitoring of several case studies [30-31]. Nevertheless these already investigated applications typically concern commercial or tertiary buildings, which generally have cooling dominated requirement all year long, because the indoor heat gains represent prominent boundary conditions of the problem. On the contrary, the cool roof efficacy within residential buildings is still debated. A few contributions just concern simulation studies [32 -33] or specific envelope components, such as reflective solar protection systems [34]. Furthermore, the majority of these simulation studies are just operated by considering statistical weather files as boundary conditions, without effectively taking into account the specific weather of each case of study, that could largely differ with respect to reference weather files available in literature. These basically are collected by weather stations positioned close to airports, that often do not represent either urban climate, nor suburban one [35]. In residential buildings peculiar case, with varying climate conditions, building positioning (winds exposure, mutual shading among constructions, etc.), and occupants’ attitudes, the acknowledged summer benefits could be associated to not negligible winter penalties. Thus, the year-round balance could move to the negative side, which is especially true in middle latitudes area with temperate and continental climate, still to be investigated by experimental and numerical studies. This work bridges this research gap by integrating the simulation study with a huge experimental campaign consisting of in-lab measurements and twoyears long continuous monitoring. This latter activity in particular allows to elaborate the specific weather file for the case study for both cool roof and traditional scenarios. It indeed takes into account all the indoor-outdoor parameters for assessing cool roof effect both in severe winter and summer conditions, in addition to providing all the required data for the model calibration and validation, and the relative results extension. The unpredicted variability of the indoor occupants’ attitudes represents another important weak point of previous experimental researches [31-36]. The cool roof efficacy assessment in terms of energy requirement and indoor thermal comfort is indeed directly affected by occupants’ and equipments schedules, that often impact building energy performance more than this kind of passive solution is able to do. Important results shown by M. Kolokotroni et al. in [31] indicate that in the case of temperate climates, operation characteristics of the building should be considered carefully to determine potential benefits of the application for year-round assessment. This research also bridges this literature

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gap, for the case of residential buildings, by extending the continuous monitoring duration, and by choosing a specific building thermal zone that is not occupied for the entire duration of the monitoring. The long duration of the data collection allows to define a precise occupants’ schedule, also thanks to the survey compiled by the dwellers; the unoccupied assumption allows to consider the attic as free-running thermal zone, which thermal behavior is highly dependent to outdoor weather conditions, itself continuous monitored as well.

1.3

Purpose of the work

Coherently with the wide background described in previous section, this research proposes an innovative integrated methodology to study, optimize and produce a kind of new cool roof solution. The performance of such proposed solution is tested through continuous two-years long monitoring where the coupling between the indoor and the outdoor environment characterization allows to assess the actual efficacy of the system, both in hot summer and in severe winter. To this aim, the proposed solution is applied to a residential building located in Central Italy, where the high seasonal thermal variance allows also to describe cool roof impact in such heating dominated climate, and for traditional Italian residential buildings, not deeply investigated by other researches before through such combined experimental and numerical studies. Finally, the calibration and validation of the dynamic simulation model of the building is operated through the collected indoor-outdoor data, and the fitted building numerical model is elaborated. Therefore, the dynamic simulation tool allows the same study to be extended to take into account all the six Italian climate zones and the roof technical characterization in terms of insulation layer presence. Finally, this work consists of an analytical, numerical and experimental integrated study aimed at evaluating, through field measurement and predictive thermal-energy tools, how innovative cool roof solutions, such as the one proposed within this research, could be useful for optimizing the year-round energy performance of traditional residential buildings in Italy.

1.4

Outline of the work

Four main parts can be distinguished in this work. Chapters 1, 2, and 3 extensively review the state of the art about cool roof technology and expose

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useful analytical tools to evaluate building thermal-energy performance. Also the main motivations of this research, and further research application concerning high reflective surface and their impact on urban and global climate are carried out. The central part of the work (Chapters 4-5-6-7) deals with the whole experimental analysis about in-lab and in-field tests, from the prototyping to the continuous monitoring of the case study building. The simulation study, also integrated with the experimental analysis, will be described in Chapters 8 and 9, that specifically deal with the calibration and validation procedure, to be able to extend the main results to several climatological and building envelope configurations in Chapter 10. Detailed description of chapters themes is dealt with as follows. Chapter 2 reviews the research background and the main perspectives of cool roof application and albedo-control solutions for mitigating urban heat island and global warming phenomena. Chapter 3 describes the most significant methodologies applied within this thesis to analyze the thermal-energy behavior of buildings through both simulated and experimental data. Chapter 4 overviews the experimental activity carried out to elaborate and produce the roof clay tile innovative element and the work in progress about its characterization. Chapter 5 concerns the experimental analysis characterizing the case study building, starting from the motivations around the choice of the building, to its specific envelope characterization through field tests. Chapter 6 details the environmental monitoring setup consisting of the indoor microclimate station and the outdoor weather station. Both these instrumentations are designed to operate the continuous monitoring of the combined indoor-outdoor environment behavior. Chapter 7 describes the experimental results of the two-years long continuous monitoring. First the reflective tile potential is analyzed in terms of effective reflectance increase, second its effect is evaluated through assessing the roof thermal behavior, third the indoor thermal environment of the attic is investigated also applying the analytical methods dealt with in Chapter 3, and finally also the ceiling thermal performance is considered to understand the reflective tiles’ effect within the whole attic volume. At the end of these analyses, an year-round assessment of the attic thermal-energy performance is carried out, to globally quantify the proposed cool roof’s effectiveness in lowering minimum, average and maximum operative indoor temperatures of the attic.

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Chapter 8 describes the numerical modeling procedures and the characterization of the building envelope and equipments to elaborate the input parameters for dynamic simulation. Chapter 9 specifically deals with the calibration and the validation procedure at whole-building and thermal-zone level. This phase concerns also the preliminary elaboration of the real weather files of the location through monitored data, and the understanding of the occupants’ behavior and energy requirements through occupants’ survey. Chapter 10 concerns the analysis of the results of dynamic simulation, where the case study building has been simulated varying Italian climate zones and roof insulation level. In this way a preliminary study of the proposed clay tile impact on the thermal-energy performance of buildings is carried out. Chapter 11 reports the main conclusions of each complementary research issue cured within this work, also outlining the recommendation about further interesting developments.

1.5 [1]

[2] [3]

[4]

[5]

[6]

[7]

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References of the chapter L.G. Swan, V.I. Ugursal: Modeling of end-use energy consumption in the residential sector: A review of modeling techniques, Renewable and Sustainable Energy Reviews, Vol. 13, n. 8, 2009, pp. 1819–1835. th IEA: Climate and electricity annual 2011 Technical report, 978-92-64-11154-7 International Energy Agency, Paris (2011). R. Saidur, H.H. Masjuki, M.Y. Jamaluddin: An application of energy and exergy analysis in residentialsector of Malaysia, Energy Policy, Vol. 35, n.2, 2007, pp. 1050–1063. A. Pietila, I. Beausoleil-Morrison, G. R. Newsham: Zero peak housing: Exploring the possibility of eliminating electrical draws from houses during periods of high demand on the electrical grid, Building and Environment, Vol. 58, 2012, pp. 103– 113. IEA International Energy Agency: Report for the Clean Energy Ministerial: Transforming global markets for clean energy products. Technical report, 2010, http://www.iea.org/papers/2010/global_market_transformation.pdf, Last access on 2012-09-27. M. Santamouris: Cooling the cities – A review of reflective and green roof mitigation technologies to fight heat island and improve comfort in urban environments, Solar Energy, Available online 30 July 2012, DOI: 10.1016/j.solener.2012.07.003. M. Santamouris: Heat island research in Europe – the State of the art, Journal Advances Building Energy Research, Vol. 1, 2007, pp. 123–150.


[8]

[9] [10]

[11]

[12]

[13]

[14]

[15] [16] [17] [18]

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Chapter 10 – Cool roof year-round assessment in Italian climate

Chapter 10

COOL ROOF YEAR-ROUND ASSESSMENT IN ITALIAN CLIMATE 10.1. Overview 10.2. Impact of cool roof on Primary Energy requirement 10.2.1 Not insulated roofs 10.2.2 Insulted roofs 10.3. Impact of cool roof through TDI assessment in free running conditions 10.4. Cost benefit analysis 10.5. Discussion 10.6. Nomenclature 10.7. References of the chapter

10.1

Overview

In previous analyses, the examination of experimental results and the elaboration of the numerical model through effective energy requirement data, have been carried out. In this part of the work, this numerical model is applied to assess cool roof benefits-penalties with more generality for Italian climate and several envelope configurations. To this aim, the validated model has been run for the six climatological Italian contexts described by different zones ([1] Figure 10.1), and also the roof analysis has been operated considering both the insulated and the not-insulated case of study. The purpose of this general analysis is to evaluate the year-round cool roof performance for all the climatological contexts, and for different construction features (i.e. presence or not of the insulation layer), given also the acknowledged importance of climate in building energy performance [2-3].

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Fig. 10.1: Climate zones in Italy [4].

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In particular, the insulation layer variability allows to consider the cool roof strategy both as intervention in new constructions (with insulated roof) and also as retrofit strategy for existing low performance building (without insulation). In this way, an evaluation of cool roof solution is also evaluated as isolated strategy or as part of a retrofit intervention, coupled with the rehab of the overall roof structure and thermal performance optimization. Also, the analysis in the six climatological Italian contexts from the very south hot climate to the very north cold climate allows to understand and to discover where cool roof solutions for residential buildings are able to provide effective thermal-energy performance optimization by year-round assessment. The thermal optimization is assessed through Thermal Deviation Index calculation, which method has been described in Chapter 3. As anticipated in previous sections of this work, this method allows to assess the effect of retrofit implementation in free-running conditions, considering the indoor operative temperature as control parameter for both adaptive comfort and potential energy requirement assessment. The energy performance optimization is provided to evaluate the primary energy requirement for cooling and heating. In this way, through yearround assessment, this overall analysis allows an exhaustive understanding of the phenomenon, where cool roof applications are convenient in Italy for residential applications and where the cold weather conditions make this strategy not convenient from an economic and energetic point of view. The considered climatological contexts are those reported in [1], characterized by different ranges of Degree Days as climatological reference parameter: − Zone A: less than 600 degree days, Lampedusa weather file [5]; − Zone B: 600 ÷ 900 degree days, Palermo Point Raisi weather file [5]; − Zone C: 900 ÷ 1400 degree days, Naples Capodichino weather file [5]; − Zone D: 1400 ÷ 2100 degree days, Rome Fiumicino weather file [5]; − Zone E: 2100 ÷ 3000 degree days, Bologna Casalecchio weather file [5]; − Zone F: more than 3000 Degree days, Dobbiaco weather file [5]; The choice of the cities has been determined through evaluating important places in Italy, also providing an overall homogeneous involvement of different latitudes levels, to basically compare cool roof benefits-penalties balance with Italian latitudes. This is the reason why basically Perugia (Zone E), the city where the case study is located, has not been evaluated again, being at close latitudes with respect to Rome. Thus Bologna has been chosen to describe the centralnorth position within the Italian peninsula territory. A final consideration concerns the description of the thermal zones’ schedules that is different from the case study analysis, where the specific occupants’ attitudes have been simulated, considering all the peculiarities. Thus the thermal

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zones’ characterization of internal gains and occupants behavior has been carried out following the acknowledged approach described in [6-8]. This approach is based on the evaluation of the occupancy times, equipment, lighting, and HVAC operation, heating and cooling temperature set-points coherently with the UK National Calculation Tool, elaborated following the Energy Performance Based Design Directive 2002/91/EC [9].

10.2

Impact of cool roof on Primary Energy requirement

This section specifically concerns the analysis of roof reflectance impact on primary energy requirement for cooling and heating of the case study building layout, positioned in the main Italian climatological contexts. The same analysis has been also aimed at characterizing both insulated and not insulated roof typologies, to quantify the possible cool roof penalty-benefit year-round balance for these main roof structures, basically characterizing new or old constructions. The insulated roof case has been assessed following the methodology just described, thus simulating the same case study building, within the six mentioned climatological conditions, but re-elaborating the roof technical composition by adding the insulation EPS 10cm layer over the structural element of the roof. In this way, it is possible to evaluate the potential effect of the cool roof strategy also for those structures that have been recently energy retrofitted coherently with Italian regulations, or for new structures. Also, the not-insulated roof could represent an overall traditional example, or a building realized at least 10 years ago in Italy, while the insulated roof technologies could describe the newer structures. The comparison between these two cases makes able to define when and where the roof reflectance increase could represent an effective building efficiency retrofit solution in year-round assessment. The reflectance levels considered in this analysis are the cool roof optimized tile (R=74%), the natural brick red tile (R=48%), and the dark brown tile originally installed on the case study (R=22%).

10.2.1 Not insulated roofs This numerical analysis consists of the assessment of the results of dynamic simulations of the case study building, when located in several Italian climates. In particular the considered results concern the global heating and global cooling requirement in year-round assessment, and a comparison of reflective roof is operated with respect to the original dark brown roof with 22% of reflectance, as

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measured through in lab spectrophotometer procedures (see Chapter 4 of this work). Figure 10.2 represents the primary energy saving-increase with respect to the R=22% base case, in Dobbiaco (coldest F zone), in Bologna (zone E), in Rome (zone D), in Naples (zone C), in Palermo (zone B), in Lampedusa (hottest A zone). These results show that all over Italy, summer benefits for reducing cooling system requirement are much more important than winter penalties, and also there is not a clear link between cool roof impact and degree days values. In fact also the coldest zones such as F and E zones are characterized by high benefits in summer, when winter penalties are actually almost negligible in winter, considering relative results with respect to the base case. Anyway these results are also affected by a possible misunderstanding deriving from the high climate variability in Italy. In fact higher summer benefits in cold climate conditions are also produced from very low values of cooling requirement in those zones, thus the relative percentage values seems higher than the real phenomenon. For this reason, the results in terms of effective kWh/m2 year are reported in Figure 10.3, where it is possible to quantify the actual primary energy requirement variation produced by roof reflectance increase. Figure 10.3 shows that the cool roof strategy is able to produce up to 18.9 kWh/m2 year of cooling reduction in Zone B (city of Palermo, Sicily), while in the same area the heating penalty is just 5.3 kWh/m2 year, registering a net positive year-round contribution of 13.6 kWh/m2. Actually for A,B,C,D climate zones the net year-round benefit is always higher than 4 kWh/m2 year, while for the two coldest zones there is a small net positive effect or small negative global effect, but for both these locations, the cool roof effect is in general negligible. Also the very much interesting effect is represented by the high reflective tiles. In fact the natural brick tiles, even producing bigger positive effect than winter penalties, are not able to reduce building energy consumption with significance in terms of yearround assessment.

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Fig. 10.2: Primary Energy requirement variation [%] with respect to R22 base case

Fig. 10.3: Primary Energy requirement variation [kWh/m2year] with respect to R22 base case

10.2.2 Insulated roofs The insulated roof case has been assessed following the same methodology adopted in the not-insulated case, thus simulating the same case study building, within the six mentioned climatological conditions, taking into account a 10cm EPS insulation layer within the roof layout.

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Figure 10.4 reports the results for both heating and cooling expressed in terms of percentage of primary energy reduction-increase with respect to the base case (R=22%). Coherently with the not-insulated case, the highest reflectance level produces about a double effect with respect to the intermediate case (R=48%). Also, the summer benefits are about three times higher than the winter penalties. In this particular case, the insulation contribution lowers the climatic variability of the cool roof effect, producing more similar results varying the weather conditions in Italy. Figure 10.5 represents the cool roof assessment in terms of kWh/m2 year of primary energy requirement, that shows a more objective evaluation of this strategy impact. The results show how the 10 cm insulation layer is able to produce a notable reduction of the cool roof effect. The choice of such big thickness of the EPS has been guided by Italian contemporary regulation that establishes a minimum value of transmittance value for roofs for each climate zone [1]. This assumption is justified by the choice to represent a new roof structure, and its relative cool roof impact with respect to traditional low performance building envelope. The reported values show how high reflective tiles are able to produce up to 2.9 kWh/m2 year of cooling reduction in zone C (Rome), while the heating penalty reaches a some kind of effect just within the coldest weather conditions, and just for the highest reflectance level. Thus it is possible to note that such high insulated roof typologies strongly reduces both the impact of the external layer reflectance and of the climate impact in terms of primary energy overall requirement.

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Fig. 10.4: Primary Energy requirement variation [%] with respect to R22 base case.

Fig. 10.5: Primary Energy requirement variation [kWh/m2year] with respect to R22 base case

10.3 Impact of cool roof through TDI assessment in free running conditions This section specifically concerns the cool roof year-round performance assessment varying Italian climate conditions. This analysis has been carried out through applying the methodology proposed in Chapter 3 consisting of the Thermal Deviation Index calculation (Section 3.3). As already mentioned in Chapter 3, TDI calculation allows to determine indoor thermal behavior of specific thermal zones where energy plants for heating and cooling are turned off (free running conditions), for example to evaluate the impact of specific envelope solutions in optimizing energy efficiency and indoor thermal comfort. To this aim, the calibrated-validated simulation model has been fitted turning off the HVAC system of the house, to specifically assess the attic thermal performance through summer and winter TDI assessment [10-11]. Coherently with the formulation proposed in Section 3.3.1, the TDI for the attic thermal zone has been performed considering both winter and summer conditions and different integration domains as defined in (3.2) and (3.3). Following the same approach described above, and considering both the insulated and the not-insulated roof configuration, also the models of the same building equipped with the roof insulation layer have been run, in order to determine the cool roof efficacy also varying the technical envelope layout. The final aim of this analysis, in fact, is to evaluate if the proposed cool roof could represent an interesting solution for energy retrofit of existing buildings and for

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new constructions. The first case, i.e. existing buildings’ application, could be represented by the not-insulated roof, that usually characterizes the Italian residential constructions built before 1973. The second case, i.e. new constructions, could be represented by the insulated roof configuration, where it is possible to understand how much the reflecting tile could impact the year-round benefit-penalty balance. Figure 10.6 below represents summer results of both these configurations and Figure 10.7 concerns winter results. The first element to note is that severe climate conditions during winter, that characterizes almost the whole Italian territory, produce higher TDI values in winter (Figure 10.7) than in summer (Figure 10.6). Coherently with previous analysis, the not-insulated roof conditions present a more important cool roof effect in reducing TDI during summer and increasing TDI in winter, because of the higher sensitivity of the roof without any insulation panel. The comparison between summer and winter results is useful to discover that in general, Italian climate is much more comfortable in summer for residential buildings’ context than winter, and passive solutions such as reflective tiles or cool roof in general are able to produce determinant TDI reduction. In general, TDI values are lower than 1, that basically represent a thermal behavior of the attic more comfortable than the base case conditions, as calculated through equation (3.4). On the contrary winter climatological constraint could unlikely be reduced by passive solutions in residential typical buildings, without greenhouses or high solar gain available. Thus cool roof strategy or other solutions to reduce cooling requirement in summer could represent an interesting opportunity to lower, up to remove, cooling requirement for residential single family building such as the case study architecture. Considering now the specific climate zones in Italy in summer, all these locations take advantage by cool roof application except for the F coldest zone, both for insulated and not-insulated conditions. The same phenomenon happens in winter for zones A and C, where although the colder climate conditions, high irradiated and mild spring-fall period in these zones is able to produce important benefit with cool roof configuration in not-insulated roof. On the contrary, for the insulated case, this effect is not visible. The dark brown tile is the best performing, while the cool roof application basically produce very similar results with respect to traditional red clay tiles (R=48%).

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Fig. 10.6: TDI summer assessment varying climate conditions and roof insulation

Fig. 10.7: TDI winter assessment varying climate conditions and roof insulation

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10.4

Cost benefit analysis

A cost benefit analysis (CBA) has been carried out to calculate the return of the investment (ROI) for the prototyped cool roof retrofit solution. The extra-cost due to the cool roof tile respect to a traditional coated tile corresponds to 0.07 euro/each. The price of the high reflective tile is indeed around 0.61 euro/each, while the price of the already commercialized coated tile is around 0.54 euro/each. Considering the year-round primary energy balance for Italian typical climates reported in Figure 10.3, and excluding a few singularities represented by zones A and F, the cost benefit analysis has been conducted using the method reported in the Life-Cycle Cost Analysis published by the US Department of Commerce [10], designed for buildings that apply cool roof systems. The base-year costs are escalated from year to year at rates projected by Energy Information Administration (EIA), as reported in [11] to arrive at the total energy cost over a given period. The following CBA is basically carried out for a 100% cool roof replacement. Table 10.1 reports the input parameters and variables used in the CBA. By applying the EnergyPlus simulation model to evaluate the cool roof primary energy year-round balance, the Return On Investment (ROI) has been calculated based on the Net Present Value (NPV), taking into account the general inflation rate (GIR) and the energy Escalation Rate (EER), and the Depreciation Rate (DR). The effect of electricity rate inflation was considered to encompass future price rate inflation [12]. The NPV will exceed the additional cost of the cool roof retrofit in 4-5 years in zone B, and in 11-12 years in zone E. Both of these results are actually significantly less than the 15-20 years life expectancy of a conventional roof membrane, and very much less than the typical clay tile life, usually exceeding the overall house life.

10.4

Discussion

In this section a numerical analysis of cool roof effect for the case study residential building has been carried out. The main parameters to be evaluated in this analysis basically are the climate conditions and the presence of an important thickness of the insulation layer. Thus an exhaustive analysis of roof reflectance variation with respect to climate and roof technology has been operated in order to understand and to quantify if the cool roof strategy could be considered as an effective strategy in several Italian climates, for both high and low performance envelope residential buildings.

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Tab. 10.1: Cost Benefit analysis input parameters and variables.

Additional cost for cool roof retrofit Electricity cost for kWh Interest rate (IR) Discount rate (DR)2 Energy escalation rate (EER)2 General inflations rate (GIR)3 Annual mantainance Study period

1708 euro, for a 200 m2 roof covering 0.301 euro1 0% 3% 2.7% 3% 1% 20 years

The case study modeled and simulated within the climate zones in Italy is the same case study of the work, thus these results mainly represents a residential single family building in Italy. The choice of the insulation layer and its thickness are aimed at comparing more than 10 years old buildings with respect to new high performance constructions, or major renovation, coherently with the prescription introduced within the recent Italian energy efficiency regulation [13-14]. The results basically show, coherently with [15-16] that cool roof implementation is able to produce very notable effects for not insulated constructions, high cooling benefits, and low relative winter penalties. The absence of the insulation layer also produces a higher sensitivity with respect to climate. Thus it is possible to conclude that in climate zones A, B, C, D (from Southern up to Central-North Italy) the overall net effect in terms of primary energy reduction is always higher than 10 kWh/m2 year. For climate zones E (Northern Italy) the same year-round net effect is a benefit around 4 kWh/m2 year. In Alps very cold climate conditions, cool roof applications are not convenient in terms of year-round assessment. But also, these climate conditions have to be considered very extreme conditions for Italy, thus also the construction practice should be reviewed specifically considering the architecture and technological practice in high altitude zones, which is different from all the rest of peninsula typicality. Thus this last case (zone F) just represents a limit case for the numerical study, but further analysis should be carried out to evaluate optimization strategies specifically based on this architecture, more wooden based than the traditional Italian one. 1

or a single family house with more than 3 kWh electricity capability. Values deducted by the Regulatory Authority for Electricity and Gas (Aeeg), http://www.autorita.energia.it/it/inglese/index.htm, last access on 2012-10-02.

2

Discount rate and energy escalation rate were calculated based on the enrgy price indices and discount factors published by the U.S. Department od Commerce [10]. Nominal escalation rate was used in the study, which includes inflaction.

3

Genaral inflation rate was adepte by the latest consumer price index (CPI) data published by the Bureau of Labor Statistics [11].

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For highly insulated roofs, the cool roof application produces relatively large effect in summer, while almost negligible effects are registered in winter. But the absolute values of primary energy reduction are lower than the not-insulated case of about 9 times. Also, the insulation layer strongly contributes to reduce the climate effect on the building energy performance, through registering very similar values for all the climate zones in Italy. Figure 10.8 reports the overall results considering: the insulation layer presence, the climate variability, the roof reflectance levels. The final overall analysis shows hot cool roof implementation, i.e. roof brick tiles reflectance optimization, could represent a very effective, simple and also low-cost strategy to reduce the year round energy requirement in traditional buildings, where there is not insulation layer (or where it is thicker than the recent regulation prescriptions in Italy). Different results are observed just in Alps climate, where cool roof (R=74%) implementation produces small but not negligible penalties in year round assessment.

Fig. 10.8: Primary Energy requirement variation with respect toc limate and roof insulation

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10.5

Nomenclature

R22 = R48 = R74 = TDI = TDIsummer TDIwinter CBA = ROI = EIA = NPV = GIR = EER = DR =

10.6

configuration with dark brown tiles, which reflectance is 22%; configuration with natural red clay tiles, which reflectance is 48%; configuration with white reflecting tiles, which reflectance is 74%; Thermal Deviation Index of the building; = Thermal Deviation Index calculated in summer months; = Thermal Deviation Index calculated in winter months; cost benefit analysis; return of the investment; Energy Information Administration; net present value; general inflation rate; energy escalation rate; depreciation rate.

References of the chapter

[1] Dpr 26 agosto 1993 n.412, Regolamento recante norme per la progettazione, l'installazione e la manutenzione degli impianti termici degli edifici, ai fini del contenimento dei consumi di energia, in attuazione dell'art. 4, comma 4 della legge 9 gennaio 1991, n.10. GU n.242 del 14-10-1993 - Suppl. Ordinario n. 96. [2] M.F. Jentsch, , A.S. Bahaj, P.A.B. James: Climate change future proofing of buildings—Generation and assessment of building simulation weather file, Energy and Buildings, Vol. 40, n. 12, 2008, pp. 2148–2168. [3] L. Yang, J.C. Lam, J. Liu, C.L. Tsang: Building energy simulation using multiyears and typical meteorological years in different climates, Energy Conversion and Management, Vol. 49, n. 1, 2008, pp. 113–124. [4] http://www.edilportale.com/csmartnews/immagini/1851_01.gif, last access on 2012-09-22. [5] U.S. Department of Energy, Energy Efficiency and Renewable Energy, Building Technologies Program. http://apps1.eere.energy.gov/buildings/ energyplus/ cfm/weather_data3.cfm/region=6_europe_wmo_region_6 /country =ITA/cname=Italy. WMO Station Region 6 : Italy, (IGDG). [6] L. Tronchin, K. Fabbri: Energy performance building evaluation in Mediterranean countries: Comparison between software simulations and operating rating simulation, Energy and Buildings, Vol. 40, n. 7, 2008, pp. 1176-1187. [7] A.A. Chowdhury, M.G. Rasul, M.M.K. Khan: Thermal-comfort analysis and simulation for various low-energy cooling-technologies applied to an office building in a subtropical climate, Applied Energy, Vol. 85, n. 6, 2008, pp. 449-462.

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[8] A.L. Pisello, J.E. Taylor,X. Xu, F. Cotana: Inter-building effect: Simulating the impact of a network of buildings on the accuracy of building energy performance predictions, Building and Environment, 2012, Vol. 58, pp. 37-45. [9] European Parliament and Council: Directive 2002/91/EC of the European Parliament and of the Council of 16 December 2002 on the energy performance of buildings, Official Journal of the European Communities, 2003, pp. L.1/65-1/70. [10] US DOC: Energy price indices and discount factors for life-cycle cost analysis. April 2008. http://www1.eere.energy.gov/femp/pdfs/ashb08.pdf, last access on 2012-10-05. [11] J.H. Jo, J.D. Carlson, J.S. Golden, H. Bryan: An integrated empirical and modeling methodology for analyzing solar reflective roof technologies on commercial buildings, Building and Environment, 2010, Vol. 45, N. 2, pp. 453-460. [12] A. Black: Does it pay? Figuring the financial value of a solar or end energy system, Solar today, 2008, Vol. 28. [13] A.L. Pisello, M. Goretti, F. Cotana: A method for assessing buildings' energy efficiency by dynamic simulation and experimental activity, Applied Energy, 2012, Vol. 97, pp. 419-429. [14] A.L. Pisello, M. Goretti, F. Cotana: Building energy efficiency assessment by integrated strategies: dynamic simulation, sensitivity analysis and experimental activity. Proceedings of Third International Conference on Applied Energy, 2011, Perugia, Italy, pp. 1395-1412. [15] E. Bozonnet, M. Doya, F. Allard: Cool roofs impact on building thermal response: A French case study, Energy and Buildings, Vol. 43, n.11, 2011, pp. 3006-3012. [16] C. Romeo, M. Zinzi: Impact of a cool roof application on the energy and comfort performance in an existing non-residential building. A Sicilian case study, Energy and Buildings, 2011, Article in Press. DOI: 10.1016/j.enbuild.2011.07.023.

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Chapter 11 – Conclusions

Chapter 11

CONCLUSIONS 11.1 Overview 11.2 Analytical proposed methodology 11.3 Prototype realization 11.4 Continuous monitoring 11.5 Dynamic calibrated simulation 11.6 Directions for further research

11.1

Overview

In this study, a fundamental question is posed to assess cool roof impact on the thermal-energy performance of Italian single family residential buildings. A wide multi-function analysis is carried out aimed at responding to this research issue by proposing innovative analytical methodologies, by performing field and lab experimental activity and numerical calibrated simulation procedures. The short answer to this research question is that the innovative cool roof prototyped tile is able to cool both the roof and the indoor thermal environment in temperate climate of Central Italy, and that the magnitude of this effect produces, first, a notable reduction of the energy requirement for cooling and, second, an optimization of the indoor thermal comfort condition of the attic. Also, the winter penalties do not produce the same magnitude of effects. Therefore, the continuous monitoring of the case study building basically shows that the innovative clay tile is able to produce notable year-round benefits for this kind of traditional Italian architecture. This final chapter discusses the merits of the numerical models and analytical formulations introduced herein, and it also sets the foundations for further research exploration.

11.2

Analytical proposed methodology

The analytical part of this work deals with the proposal of several multipurpose methodologies aimed at evaluating the results of both building dynamic simulations and continuous in-situ monitoring. In particular, the

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Thermal Deviation Index (TDI) allowed to relate the results in terms of operative indoor temperature to the evaluation of building thermal-energy performance. It has been applied to the final simulation study (Chapter 10) to describe the cool roof effect in free-running indoor thermal conditions of the attic. The study of passive solutions that improve building thermal performance are indeed unlikely investigated through simple and synthetic methodologies, thus it seems often difficult to assess and to compare several strategies in a concise and effective way. This method basically responds to this research gap. The second important proposed analytical approach specifically concerns the assessment of roof impact on indoor thermal comfort and related energy efficiency. It concerns the two series of overheating and overcooling indexes that allowed to quantify the cool roof impact in reducing daily peak temperatures, average daily temperatures and nightly minimum temperatures, by comparing both the analyzed scenarios. The third briefly mentioned approach, the Inter-Building Effect (IBE), basically responds to the research question aimed at considering, for both experimental and numerical studies, the outdoor building surrounding to assess building thermal-energy performance. Given the intrinsically inter-building characterization of roof and envelope reflectance in general, for both the mutual impact among close buildings and urban heat island phenomenon, this analysis basically suggests some important key issues for further exploration.

11.3

Prototype realization

The in-lab optimization and the industrial prototyping of the reflecting clay tile consisted of a huge research cooperation with industrial companies involved in the construction sector, in order to finalize an innovative roof product whose properties are documented within this work. In particular, also economic and technical evaluations have been considered during the optimization procedure. First, a group of clay prototypes has been selected for cool roof capabilities. Second one selected tile has been chosen to optimize also its properties during the production, especially for the severe conditions of the cooking process. Third, this same process has been assessed also from an economical point of view, by considering the market price and life cycle cost elements, in comparison to the other commercialized tiles in Italy. The return of investment for the proposed cool roof energy retrofit is 4-5 years in climate zone B, and in 11-12 years in climate zone E. The prototype elaboration, its performance optimization, and the attention paid to the cost-efficacy balance since the early stage of the research, show that

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the proposed cool roof solution represents an interesting innovative strategy for both energy and money saving, in Italian traditional houses.

11.4

Continuous monitoring

The experimental in-field campaign represents the fulcrum of the innovative contribution of this work, for both the campaign development and the relevant achieved results. Thanks to the combined indoor-outdoor continuous two-years long monitoring, that is still on-going, the cool roof performance in terms of roof reflectance capability, roof thermal behavior and indoor thermal-energy assessment are carried out. Both hot summer and severe winter conditions have been taken into account to demonstrate the efficacy of such technology in all weather constraints and operative conditions of the case study. Final results show that: − the effective roof reflectance basically triplicates during both summer and winter, and it registers important variations due to geometrical issues between sun and roof mutual variable position; − typical hysteretic cycles are observed when plotting measured global reflected radiation versus measured global radiation. Such peculiar hysteretic cycles, that are not expected in the case of large planar roofs, are related to the slope and orientation of the roof with respect to the sun position and to the surrounding environment, as well. This phenomenon demonstrates that, in the case of sloped roofs, the cool roof effect is strongly influenced by the orientation of the building roof which therefore has to be considered in cool roof design. − the most relevant cool roof contribution concerns the thermal maximum peak daily reduction, the capability of which also increases the tiles’ overall durability performance; − the daily peak temperature of the roof external surface decreases of about 10-18°C in summer and 1-3°C in winter; − the roof internal surface has lower daily peak temperatures of about 69°C in summer, and about 1°C in winter; − the summer indoor operative temperature of the attic presents daily average values about 3°C lower in cool roof scenario, while in winter the same average daily effect is basically negligible; − by applying the analytical proposed formulations, these same results are also synthesized through the proposed overheating and overcooling indexes, in order to highlight how the proposed cool roof solution is able to determine up to 24.3% overheating decrease in July in terms of daily peak (Smax values), and 12.6% in terms of Smean. While these same values

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for the coldest month (January) correspond to 8.8% (Smax values), and 2.6% (Smean values).

11.5

Dynamic calibrated simulation

The dynamic simulation modeling and calibration procedure presents an important innovative contribution by itself. It indeed consists of the proposal of an integrated whole building and thermal zone combined thermal-energy assessment. Both the quantitative energy year-round assessment and the qualitative indoor operative temperature comparison show an acceptable agreement for both the scenarios. The performance of the dynamic simulation engine is verified for both traditional roof reflectance configuration and the innovative extreme values of the cool roof tiles. In particular, important findings show that EnergyPlus tool is able to predict both summer benefits, as already investigated in previous researches, but it is also able to predict the winter penalties according to the same registered values in terms of monthly average indoor operative temperature of the attic. This high level correspondence between registered and predicted trends is also enforced by the elaboration of the real weather files of the examined location for the entire duration of the study, thanks to the experimental in-field monitoring of all the weather parameters reported in the boundary conditions file. Also a survey to investigate occupants’ attitudes is carried out, to simulate the effective dwellers’ behavior in terms of energy use and internal gains. The elaboration of the validated model of the case study also allows to extend the results of the analysis to the six Italian climate zones and to different levels of roof reflectance and insulation. The results achieved through this final parametric analysis show that the proposed cool roof solution, for not-insulated roofs, is able to save from 9.9 kWh/m2year of summer cooling in alpine climate, up to 18.9 kWh/m2year of summer cooling in Palermo, Sicily. The corresponding winter penalties are 12.6 kWh/m2year and 5.3 kWh/m2year for alpine climate and Palermo respectively. The same assessment for insulated roofs report that the cool roof effect is largely reduced for all the climate locations. The analysis of free-running conditions operated through TDI methodology reports coherent results. The overall evaluation of results shows that the proposed cool roof solution represents an effective strategy to improve energy efficiency and indoor thermal comfort conditions, in particular for Italian milder climates (Central and Southern areas), and for traditional not insulated envelope configurations, where the cool roof potential is able to optimize summer benefits while the winter penalties are relatively much smaller, given the less important impact of the solar radiation during winter period.

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11.6

Directions for further research

Several topics addressed in this work may warrant further research. Some notable development has already been mentioned within the body of the work, the research of which has just started up. In particular the main future contributions will concern: − The assessment of albedo measurements singularities due to geometric single-building and inter-building urban configuration, especially for sloped roofs. To this aim, an analytical and experimental research has been implemented to assess and optimize the directional albedo properties of some reflector coatings compared to the same color coatings with diffusive optic behavior. This specific idea arises from the observation of the monitored reflectance hysteretic progress during the day, and the measured reflectance variation during the course of the year. It has the final purpose to evaluate possible solutions for increasing the potentialities of reflecting roofs also in very dense urban areas, at both single-building and global Earth energy balance perspectives, for global warming mitigation. − The analysis of the thermal, energy and optic properties of the tiles during several operational conditions, such as urban aggressive and rural environmental boundaries. As already mentioned, this research takes part to a bigger project involving Politecnico di Milano, ENEA and University of California, Berkeley, where the experimental setup has already been implemented on April 18th, 2012. First results show that the in-time reflective capability of clay tiles is much higher compared to that of common cool roof membranes such as white synthetic and polymer-bitumen based elements. In fact, for both three-months and six-months measurements and three-years projections of these same results, the decay median is around 7.8% for membranes and 4.4% for clay tiles. − The study of cool roof effect in optimizing cooling system performance when the external units of the system are located over cool roofs. To this aim two more experimental campaigns have been designed and already set up in Perugia and Rome. These applications are aimed at discovering this coupled passive-active cool roof effect consisting of the reduction of the indoor energy requirement for cooling (passive contribution), and the contemporary increase of the system operative efficiency (active contribution). − The evaluation of cool envelope (roof plus walls) effects through continuous indoor-outdoor monitoring of more “controlled” buildings, such as a sort of testroom prototypes already realized in Perugia. Several traditional and innovative envelope configurations will be optimized and tested through inlab and in-field experiments, in order to elaborate a sensitivity profile for several building features impacting the thermal energy balance.

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− The integration of the computational fluid dynamics theory in modeling and simulating the attic behavior, for integrating also this third validation procedure to the calibration and validation method proposed in this work. − The development of further optimized cool roof tiles by increasing the infrared reflective properties, thus obtaining notable results also with notwhite clay elements. − The analysis of further cool roof innovative materials, also with nano-based formulation, for the year-round performance optimization of visible and infrared reflectance and emissivity. − The life cycle assessment of such cool materials and cool envelope applications, aimed at comparing the environmental impact of both the operational and production-construction phase, spanning the research limits beyond the just energy efficiency assessment. − The inter-building effect analysis of cool roof envelopes, to discover limits and potentialities of this kind of applications for several urban and climate contexts.

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10

Chapter 2 – Cool roofs: state of the art and perspectives

Chapter 2

COOL ROOFS: STATE OF THE ART AND PERSPECTIVES 2.1. Overview 2.2. Cool Roof as a building cooling strategy 2.3. Cool Roofs to reduce Urban Heat Island 2.4. The role of cool roofs in global warming mitigation 2.5. Nomenclature 2.6. References of the chapter

2.1

Overview

The previous chapter described the backgrounds, purposes and methodology of the research. The state of the art that represents the research basis of this analysis of cool roof systems and the consequent building energy assessment has though only briefly been dealt with. This chapter specifically concerns the research motivation and the previous notable contributions more in detail. In particular, given the necessarily articulated research issue, this chapter describes the threefold characterization of high albedo strategy through an inductive approach. First, high reflective solutions will be dealt with as a passive cooling strategy at a building scale of the problem, i.e. cool roofs. Second, high albedo surfaces will be analyzed in terms of potential way to mitigate Urban Heat Island effect, at urban scale. Third, new approaches concerning the role of albedo in global climate, especially for global warming mitigation, will be considered. The whole chapter will deal with the analysis of the most important tools used in this research to assess cool roof impact both in experimental and in theoretical analyses, focusing on building dynamic simulation and environmental monitoring as strategic tools to investigate the thermal-energy performance of buildings, starting from experimental and numerical integrated data.

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2.2

Cool Roof as a building cooling strategy

A “cool roof” system is a kind of roof that is able to reflect solar radiation and emit heat, keeping the roof cooler than a traditional roof even under highly solar loads [1]. In practice, to the increase of solar reflectance corresponds a larger quote of solar radiation reflected by roof, with respect to the absorbed portion. Thus a smaller heat load goes through the roof and penetrates into the thermal zones of the building. This phenomenon has been documented in many studies concerning residential and non-residential buildings, through experimental and numerical analysis. Haberl and Cho in their literature review about cool roof impact on building cooling requirement [2] found that the application of cool roof technologies is able to save about 20% of cooling requirement in residential and commercial buildings. In fact their studies report a cooling energy saving from 2% to 44%, and a peak cooling reduction between 3% and 35%, depending on the ceiling insulation level, attic configuration, climate and building features in general. Synnefa et al. in [3] studied the effect of cool coatings on both cooling loads and indoor thermal comfort conditions within residential buildings, given that energy consumption for residential cooling shows an increasing trend in many developed countries, given the sprawling diffusion of low-efficiency cooling systems. Their findings are particularly interesting because they used energy simulation procedures by TRNSYS [4] in order to estimate the effect of cool colored materials for envelopes in 27 cities around the world, linking cool roof potentialities with climatological conditions of: (i) Mediterranean area, (ii) humid continental area, (iii) subtropical arid area, and (iv) desert conditions (Figure 2.1). Keeping constant the building layout of the modeled prototype and its technical-architectural features, they were able to quantify the cooling energy savings and potential wintertime penalties. A parametrical assessment has been carried out to consider different reflectance levels: 0.2 (low), 0.6 (moderate), 0.85 (extreme), with the same infrared emissivity value of 0.9. A further application of that research also concerned the indoor thermal comfort analysis through UNI EN ISO 7730 reference [5], thus simulating the models in freerunning conditions. They found that an increase in roof solar reflectance by 0.65 is able to reduce: cooling loads of 8-48 kWh/m2, discomfort hours by 9-100%, and the peak temperature by 1.2°C to 3.7°C. The main variables to consider when performing cool roof assessment are climate and roof insulation level. They found that winter potential penalties (0.2-17 kWh/m2) are in general lower

12


than summer benefits (9-48 kWh/m2), with higher effects for high-transmittance roofs (Figure 2.2) and almost linear dependence varying reflectance level (Figure 2.3). New important results has been achieved by coupling experimental and simulations results of calibrated models carried out by Bozonnet et al [6]. This contribution focused the analysis to a French social housing case study in temperate climate.

Fig. 2.1: Climate effect of cooling and heating load changes for a change in roof solar reflectance of 0.65 [3].

13


Fig. 2.2: The effect of U-value on the net energy savings resulting from changing the roof reflectance by 0.4 [3].

Fig. 2.3: The effect of roof solar reflectance changes on cooling load reduction for a roof U-

14


value equal to 0.84 [3].

The research [6] concerns the evaluation of a roof refurbishment made with cool coating, to optimize summer thermal comfort, roughly taken into account in French regulation, such as in Italian one. Bozonnet et al. found that cool roof system is able to lower the outside peak surface temperature of the roof of about 10°C, while minimum effect are found at lower temperatures during the day. The indoor operative temperature is found to be not so sensitive to roof reflectance in the high insulation level of the monitored roof, while in the notinsulated case study the same room registers a peak indoor temperature decrease of 9.3°C. They also found that the operational daily profile has an important role in determining the thermal-energy performance of the building area, given the high variability of dwellers’ attitudes. All the mentioned researches agree about the importance of calibrated modeling and real weather data to perform building simulation, especially because typical weather station positions are located close to airports, and typically far from the urban centre, characterized by peculiar climatological meso-scale conditions [7-8]. Cool roofs, and in general high albedo surfaces, together with the reduction of cooling loads in buildings (about 13-14 kWh/m2 of roof area [9]), produce notable effect also in terms of direct CO2 reduction associated with reduced energy use for cooling (about 11-12 kgCO2/m2, [9]). To assess this twofold issue, Xu et al. in [9] proposed an innovative field-based analytical method, supported by the yearlong concurrent monitoring of two commercial twin buildings in Hyderabad, India. They basically found the mentioned important reductions in terms of required energy and GHG emissions, and they consider possible further benefits produced by indirect benefits to the urban area, such as cooling outside air. Given these important benefits produced by cool roof applications, also the materials’ science has been involved within this research issue, and also the thermal performance of new and existing reflective envelope coatings has been dealt with. In [10] a comparative study aimed at investigating the effect of reflecting coatings on lowering surface temperature of buildings has been carried out. Also urban surface applications have been considered, basically to integrate building cool roof assessment with urban scale environment issue reported in the following section.

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2.3

Cool Roofs to reduce Urban Heat Island

Modern urban areas usually have darker surfaces and less vegetation then the surrounding areas. As analyzed in section 2.2, at the building scale, dark envelopes become hotter than lighter ones, thus the summer cooling requirement increases [11]. Given the relatively recent population growth in urban areas, and the consequent urban sprawling development, old forest and open spaces are being replaced by constructions. This phenomenon results in higher solar energy quote absorbed by concrete and paved surfaces, producing the urban surfaces temperature increase. The surface overheating consequently produces an overall ambient temperature increase that determines the called “Urban Heat Island” (UHI) effect. UHI phenomenon consists in the air temperature increase of about 2.5°C within cities, with respect to their surroundings [12], that causes an increase in energy demand for cooling, accelerating the smog production, human thermal discomfort and health problems [13]. In general UHI determines large differences between urban and rural climate, and the entity of such differences varies depending on weather conditions, urban thermophysical layout and geometrical features. Also urban pollutant can be up to ten times higher than rural “clean” areas, together with the mentioned air temperature increase [14]. UHI is a fundamental part of the same energy and environmental concern related to building energy efficiency and envelope reflectance, and the issue produces important consequences at several ranges of scale. The single-building and the urban canopy levels interact and mutually increase because the UHI and the cooling requirement increase are produced by three fundamental elements: the surfaces albedo decrease (i.e. roofs, envelopes, pavements, etc.), the reduction of evapotranspiration from vegetation, and the anthropogenic heating sources. The surface energy balance expressed in (2.1) simply describes the role of surfaces’ reflectance, that is the element we are focusing on in this research. Such equation for a unit surface area is represented by:

α S Q + α S q + α L L + Qf = εσTO4 + hC (TS − T A ) + k

Where:

16

dTS dz

+ λE Z =0

(2.1)


αS and αL = absorbance levels for short and long wave respectively; Q and q = direct and the diffuse short wave radiative fluxes; L = long wave radiative flux; Qf = anthropogenic heat flux; ε = emissivity of the surface; σ = Stefan-Boltzmann constant; TS and TA= surface and the air temperature respectively; hC = convective heat transfer coefficient; k = thermal conductivity of the ground; λ= latent heat of vaporization; E = evaporation rate.

radiation

Thus the (2.1) could be expressed as (2.2):

(1 − a )I + L * +Qf

= H + λE + G

(2.2)

Where: a = solar albedo of the surfaces; I = solar radiation; L* = net long wave radiation at the surface; H, λE, G = sensible, latent, and ground heat fluxes respectively. Given that our focus is surfaces’ (i.e. building envelopes) reflectance and their impact both on building energy performance and urban thermal behavior, the term “a” represents an important determiner for cool roof assessment. It is defined as the surface hemispherically and wavelength-integrated reflectivity [14]. Results from meteorological simulation operated by Taha in [14] suggests that cities could feasibly reduce their UHI, also reducing building cooling requirement, by increasing albedo of the buildings’ envelope and urban pavements, together with reforesting urban areas, where possible. Takebayashi and Moriyama in [15] following the Akbari et al. [16] approach proposed by the Lawrence Berkeley National Laboratory (Heat Island Group), studied the surface heat balance of cities considering both cool and green roofs for the mitigation of UHI. They found that with high albedo coatings, obviously the sensible heat flux is small because of the low net radiation, while on green roofs and surfaces in general, sensible flux is small too, thanks to the evaporation, although the net radiation is large. Similar analysis has been operated by Zinzi and Agnoli in [17] where they studied building energy performance of cool and green roofs in

17


Mediterranean region through building energy simulation. Their assessment effectively describes the issue bridging the gap between cooling demand reduction through cool roof and UHI mitigation, analyzing also their mutual “winwin” impact. They found that cool roofs represent the most effective solution for centre and southern Mediterranean areas, especially in not-insulated roofs, where they actually are able to avoid the installation of cooling systems. Also, green roofs performance is highly related to the water content; the dryer is the roof, the lower is the heating demand and viceversa for cooling in summer conditions. Santamouris et al. in [18] describe the impact of urban climate, i.e. UHI phenomenon, on building energy requirement through field measurement in Athens, Greece, that registers very important overheating conditions. They found that for a representative building, the cooling load almost doubles in the central Athens area, while peak electricity load may be tripled with so high outside air temperatures. At the same time, they open the discussion about heating benefits of UHI effect, calculating up to 30-50% of heating demand reduction in urban buildings with respect to the rural environment. Implementing the most important findings about UHI and building energy performance, Akbari et al. in [19] clarify the necessary mutual effect defining direct and indirect contributions. Direct effects are related to the building energy performance and they derives for example from cool roof implementation and planting trees. But these two strategies also produce indirect effect that cannot be taken into account by single-building energy use modeling tools, but that are able to produce city-wide changes in climate, referred to as indirect effects. Registering the necessity to consider and quantify these indirect effect and their impact on building energy performance, Taha in [20] proposes an innovative method integrating simulations at meso and meso-urban scales into a fine-resolution meso-urban meteorological model applied to evaluate the effect of urban albedo in UHI mitigation. Given the widely acknowledged interaction between urban and building environment issue in this field, Bretz et al. in [21] operate a sort of excursus of existing solar-reflective materials and potential surfaces for increasing urban albedo. About roof, they indicate three main membranes: (i) built-up membranes, realized by alternating layers of bitumen and felt; (ii) liquid applied membranes; (iii) single ply membranes-sheets, including shakes, shingles, tiles and other panels, mechanically attached to the roof substrate. Given the huge importance of the integrated research issue, and the typical bitumen or polymeric nature of cool roofs and cool paintings for buildings, this research finds its importance also in the proposal of an innovative clay tile to

18


realize a new cool roof using Mediterranean traditional materials and technologies such as brick elements.

2.4

The role of cool roofs in global warming mitigation

Cool roofs and cool pavements, especially in urban areas, can reduce summertime temperatures, improve urban air quality and saving energy in buildings. Other researchers are focusing on the global climate effect of high albedo surfaces exposed to solar radiation. The purpose of these researches is to quantify this effect in terms of negative radiative forcing. Akbari et al in [22] quantify the negative radiative forcing induced by world-wide albedo increase of urban roofs and pavements equal to the opposite effect of 44 Gt of CO2 emissions. They also considered a $25/tone of CO2 and calculated a worth of about $1,100 billion. The primary assumption of this theory is that changing albedo of urban surfaces and changing atmospheric CO2 concentrations both result in a change in radiative forcing. With respect to previous analyses in 2.2 and 2.3, this effect could be defined as another kind of indirect effect of cool roof and cool surfaces in general. In [23], Menon et al., coherently with the previous approach, analyze the global radiative forcing associated with land use and land cover change from pre-industrial times to present due to land albedo modifications. They find a value around -0.2±0.2 W/m2. This small environmental benefit has to be compared with the CO2 effect valued around 1.6 W/m2. They focus their research on how surface albedo over urban areas affect negative radiative forcing, calculating an average surface quote of roofs and pavements varying from 49% to 69% in typical US cities. Thus they conclude that the potential modification of surfaces’ albedo could have strong effects. Considering the equations proposed in [22], they calculated, through AK09, that the global change in radiative forcing for a 0.01 increase in urban albedo is 1.27 W/m2, with mean global cloud cover and insulation. The contribution of cool roofs and cool pavements, deriving from an albedo increase of 0.25 and 0.15 respectively, correspond to 57 Gt of CO2 emissions offset. An important contribution to this research has been reached by Akbari et al. in [24] where they analyze this same effect considering long-term effect of increasing urban albedo. Applying a global climate model, and suggesting to increase the albedo of all the land areas between ±20° and ±45° latitude, they find a long-term global cooling effect of 3x10-15 K for each square meter of surface with an albedo increase of 0.01. This square meter corresponds to 7 kg of equivalent CO2 emission offset.

19


Cotana and Rossi in [25] propose an innovative model to relate albedo effects and global warming applied to a specific case study in [26]. In other ongoing studies of the same authors, they also consider the economical opportunities deriving from albedo increase and its recognition within the Emission trading. Also, they compare the cost and the effectiveness of such a solution with respect to other sprawled renewable energies such as photovoltaic panels, solar thermal, wind generator, hydroelectric plant. The overall research about cool roofs-pavements and high albedo surfaces in general agree about the threefold aim of such a strategy, as specified in [27]. In fact cool roofs represent an effective geo-engineering solution to save building cooling requirements, combat global warming, and reduce CO2 emissions (see 2.2). Cool surfaces in urban areas reduce UHI effects, improving urban comfort and urban air quality (see 2.3). Given that these strategies basically have negligible costs with respect to the same commercialized solutions, cooling cities through high albedo urban envelopes could represent a strategic way to face the environmental constraint both in terms of efficacy and cheapness.

2.5 Q q L Qf ε σ TS TA hc k λ E a I L* H λE G

20

Nomenclature = = = = = = = = = = = = = = = = = =

direct short wave radiative flux; diffuse short wave radiative flux; long wave radiative flux; anthropogenic heat flux; emissivity of the surface; Stefan-Boltzmann constant; surface temperature; air temperature; convective heat transfer coefficient; thermal conductivity of the ground; latent heat of vaporization; evaporation rate. solar albedo of the surfaces; solar radiation; net long wave radiation at the surface; sensible heat flux; latent heat flux; ground heat flux.


2.6 [1]

[2]

[3]

[4] [5]

[6] [7]

[8]

[9]

[10]

[11]

[12]

References of the chapter H. Akbari, R. Levinson, L. Rainer: Monitoring the energy-use effects of cool roofs on California commercial buildings Energy and Buildings, Vol. 37, n.10, 2005, pp. 1007-1016. J.S. Haberl, S. Cho: Literature review of uncertainty of analysis methods (Cool Roofs) Report to the Texas Commission on Environmental Quality Energy systems laboratory Texas Engineering Experiment Station Texas A&M University System, 2004. A. Synnefa, M. Santamouris, H. Akbari: Estimating the effect of using cool coatings on energy loads and thermal comfort in residential buildings in various climatic conditions Energy and Buildings, Vol. 39, n. 11, 2007, pp. 1167-1174. TRNSYS (Version 15), A Transient System Simulator Program, Solar Energy Laboratory, University of Wisconsin, Madison, USA. ISO-DIS 7730 Ergonomics of the thermal environment – Analytical determination and interpretation of thermal comfort using calculation of the PMV and PPD indices and local thermal comfort International Organization for Standardization, Ginevra, 2003. E. Bozonnet, M. Doya, F. Allard: Cool roofs impact on building thermal response: A French case study, Energy and Buildings, Vol. 43, n.11, 2011, pp. 3006-3012. A.M. Papadopoulos: The influence of street canyons on the cooling loads of buildings and the performance of air conditioning systems, Energy and Buildings, Vol. 33, n. 6, 2001, pp. 601-607. A.L.S. Chan: Developing a typical meteorological year weather file for Hong Kong taking into account the urban heat island effect, Building and Environment, Vol. 46, n. 12, 2011, pp. 2434-2441. T. Xu, J. Sathaye, H. Akbari, V. Garg, S. Tetali: Quantifying the direct benefits of cool roofs in an urban setting: Reduced cooling energy use and lowered greenhouse gas emissions, Building and Environment, Vol. 48, n.1, 2012, pp. 1-6. A. Synnefa, M. Santamouris, I. Livada: A study of the thermal performance of reflective coatings for the urban environment, Solar Energy, Vol. 80, 2006, pp. 968–981. A.H. Rosenfeld, H. Akbari, S. Bretz, B.L. Fishman, D.M. Kurn, D. Sailor, H. Taha: Mitigation of urban heat island: materials, utility programs, updates, Energy and Buildings, Vol. 22, 1995, pp. 255-265. A. Synnefa , M. Santamouris, K. Apostolakis: On the development, optical properties and thermal performance of cool colored coatings for the urban environment, Solar Energy, Vol. 81, 2007, pp. 488–497.

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[13]

[14] [15]

[16] [17]

[18]

[19]

[20] [21]

[22] [23]

[24]

[25]

[26]

[27]

22

A. Synnefa, T. Karlessi, N. Gaitani, M. Santamouris, D.N. Assimakopoulos, C. Papakatsikas: Experimental testing of cool colored thin layer asphalt and estimation of its potential to improve the urban microclimate, Building and Environment, Vol. 46, 2011, pp. 38-44. H. Taha: Urban climates and heat islands: albedo, evapotranspiration, and anthropogenic heat, Energy and Buildings, Vol. 25, 1997, pp. 99-103. H. Takebayashi, M. Moriyama: Surface heat budget on green roof and high reflection roof for mitigation of urban heat island, Building and Environment, Vol. 42, 2007, pp. 2971-2979. H. Akbari, S. Konopacki: Calculating energy-saving potentials of heat-island reduction strategies, Energy Policy, Vol. 33, n. 6, 2005, pp. 721-756. M. Zinzi, S. Agnoli: Cool and green roofs. An energy and comfort comparison between passive cooling and mitigation urban heat island techniques for residential buildings in the Mediterranean region, Energy and Buildings, in press. DOI: 10.1016/j.enbuild.2011.09.024. M. Santamouris, N. Papanikolaou, I. Livada, I. Koronakis, C. Georgakis, A. Argiriou, D. N. Assimakopoulos: On the impact of urban climate on the energy consumption of buildings, Solar Energy, Vol. 70, n.. 3, 2001, pp. 201–216. H. Akbari, M. Pomerantz, H. Taha: Cool surfaces and shade trees to reduce energy use and improve air quality in urban areas, Solar Energy, Vol. 70, n. 3, 2001, pp. 295-310. H. Taha: Meso-urban meteorological and photochemical modeling of heat island mitigation, Atmospheric Environment, Vol. 42, n. 38, 2008, pp. 8795-8809. S. Bretz, H. Akbari, A. Rosenfeld: Practical issues for using solar-reflective materials to mitigate urban heat islands, Atmospheric Environment, Vol. 32, n. 1, 1998, pp. 95-101. H. Akbari, S. Menon, A. Rosenfeld: Global cooling: Increasing world-wide urban albedos to offset CO2, Climatic Change, Vol. 94, n. 3-4, 2009, pp. 275-286. S. Menon, H. Akbari, S. Mahanama, I. Sednev, R. Levinson: Radiative forcing and temperature response to changes in urban albedos and associated CO2 offsets, Environmental Research Letters, 2010, Vol. 5, n. 1, art. no. 014005. H. Akbari, D. Matthews, D. Seto: The long-term effect of increasing the albedo of urban areas, Environmental Research Letters, 2012, Vol. 7, n. 2, art. no. 024004. F. Cotana, F. Rossi: Solutions for global warming control and experimental laboratory validation, Proceedings of XXIV IUGG General Assembly, 2007, Perugia, Italy. A.L. Pisello, F. Rossi, F. Cotana: Increasing roof albedo: a retrofitting strategy for buildings and environment, 48° International Conference AiCARR, 2011, Baveno, Italy. H. Akbari, H., H.D. Matthews: Global cooling updates: Reflective roofs and pavements, Energy and Buildings, Article in Press, 2012, DOI: 10.1016/j.enbuild.2012.02.055.

Chapter 3

ANALYTICAL FORMULATIONS FOR THERMALENERGY PERFORMANCE ASSESSMENT OF BUILDINGS 3.1. Overview 3.2. Analysis of results of building dynamic simulation and experimental monitoring 3.3. Thermal Deviation Index 3.3.1 Formulation 3.3.2 Case study 3.4. Overheating and overcooling assessment 3.5. Enlargement of perspectives: inter-building context 3.5.1 Formulation 3.5.2 Case study 3.6. Nomenclature 3.7. References of the chapter

3.1

Overview

This chapter specifically concerns the proposed methodologies to assess building energy performance through analyzing several kinds and sources of data. First, in paragraph 3.3 the Thermal Deviation Index (TDI) is proposed as a synthetic but also exhaustive method for thermal dynamic analysis and experimental assessment of whole buildings. Second, paragraph 3.4 presents the formulation of two important indexes specifically concerning overheating and overcooling phenomena that should be evaluated in cool roof application. Third, paragraph 3.5 concerns Inter-building Effect. The analysis of this effect is operated by proposing a new method to assess building global energy performance in different urban layouts and climatological contexts. All these methodologies and formulations have been applied in evaluating the thermal-energy performance of the proposed cool roof system, representing the fulcrum of the thesis, through experimental measurements integrated with dynamic simulation results.

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3.2 Analysis of results of building dynamic simulation and experimental monitoring The idea of this section arises from this research nature, considering the necessity to integrate different methods and tools for analyzing the same complex building energy efficiency issue. Considering the final purpose of the whole research, the same focus is aimed at bridging the gap between building architectural design, dynamic simulation procedures and energy systems design [1]. Thus this section proposes a method to evaluate building thermal global performance after (i) dynamic calibrated and validated simulation, (ii) experimental continuous monitoring, (iii) field and lab tests about thermal-opticenergy properties of building components. These procedures, basically concerning the proposal of synthetic performance indexes, can be applied also for sensitivity analysis and optimization procedures [2]. First, the Thermal Deviation Index [3] concerns a whole-building approach expressing the performance level using an objective function numerical value characterized by an immediate meaning. It could be applied to whole building and thermal zone assessments, and the input data could be both experimental monitored data and predicted ones, in terms of operative temperature as the control parameter of indoor environment [4]. Focusing on the specific roof performance analysis, two further formulations have been proposed to assess the cool roof performance [5]. Also in this case they could be calculated through both experimental data and predicted ones. Given the high importance of urban, i.e. inter-building, phenomenon in determining the impact of envelope reflectance both in terms of environmental impact and building energy performance, a further investigation concerns the Inter-Building Effect that takes into account several possible links among groups of buildings: occupants’ relationship level [6], geometrical inter-building layout [7] and the climate context [8].

3.3

Thermal Deviation Index (TDI)

The main purpose of TDI assessment is the proposal of a simple and effective method to guide the evaluation of building thermal performance after dynamic simulations or experimental monitoring (or both of them), together with the expression of results using a concise not-dimension objective function [3]. The proposed index could be used to define the thermal global performance of both existing and new buildings, of the whole construction or each thermal

24

Chapter 3 – Analytical formulations for thermal-energy performance assessment of buildings

zones. It can be used also for comparing several optimization strategies or to represent the output parameter within sensitivity analyses [9]. The proposed methodology is based on the building analysis in real climatic conditions, through experimental continuous monitoring or dynamic hourly simulation. The procedure is composed of: − preliminary study and adaptation of the method considering the specific aim to pursue; − building continuous monitoring or dynamic calibrated and validated simulation; − assessment of the optimization solutions, i.e. cool roof implementation; − calculation of Thermal Deviation Index to assess the building performance of each prototype case study (TDIb); − calculation of Thermal Deviation Index specific of each prototype in relation to the site (TDIb-site); − final evaluation of the results and definition of the best solutions, with the relative performance assessment.

3.3.1 Formulation The proposed index formulation concerns both seasonal and annual periods. Following each research purpose guideline, TDIb (TDIbuilding) defines the performance levels through synthesizing the results. Basically TDIb allows to quantify the distance from the thermal target condition of the indoor environment both in terms of frequency and intensity of the gap. The indoor parameter is represented by the Operative Indoor Temperature seasonal ranges typical of winter and summer, as explained in EN15251 [10]. Thus the formulation of TDIb is expressed as follows (3.1) for each period of assessment “s” [3]:

∫ [f (Tin ) − TO,M −s ]dτ + ∫

TDI b - s =

Ph

Pc

TDI BC −s

[Tm −s − f (Tin )]dτ ⋅

t s − t T −s ts

[-] (3.1)

Where: Tin= indoor operative temperature, calculated in the middle of each thermal zone.

25


TM-s, Tm-s= highest and lowest values of the indoor operative temperature target range. Maximum values considered are 26°C and 25°C and minimum values are 23°C and 20°C respectively for summer and winter conditions [10]; seasonal and annual periods of the analysis respectively; ts,ty= period during which TO-in is included within the thermal seasonal tT-s= target. The summer range is 23-26°C and the winter one is 20-25°C [10]. Ph,Pc= integration domains. In these periods the TO-in is out of the seasonal target. They are defined in (3.2) and (3.3):

{ [

] ( )

}

[h] (3.2)

{ [

] ( )

}

[h] (3.3)

Ph = τ ∈ 0, t s : f T in ≥ TM −s Pc = τ ∈ 0, t s : f T in ≤ T m −s

Basically the mathematical formulation of TDI (3.1) is a non-dimensional product of two terms. The first term is the ratio between the sum of the areas in which the TO-in is out of the thermal target, and a base case index TDIBC-s. TDIBCs is the arbitrary base case scenario described by a constant operative indoor temperature 3°C far from each seasonal target (Figure 3.1). TDIBC-s is calculated as follows in (3.4): TDIBC-s =

∫ [(TM−s + 3) −TM−s ]dτ = 3 ⋅ ts

[°C·h] (3.4)

ts

The right member of the product in (3.1) represents the frequency of the TOdistance from the indoor seasonal thermal target; it is a weighting factor always between 0 and 1. A null value of TDIb represents the final aim to pursue by design strategies. In fact it indicates that the thermal zone is characterized by TO-in value comprised between the thermal target extremes. When TDIb is higher than the unity, the thermal zone is far from the target more than the base case scenario is. Higher TDIb values represent further distance from the target. Thus the thermal zones that register frequent and intense deviation far from the target are characterized by high value of TDIb obtained both for an important deviation far from the thermal target (first member of the product) and for a frequent deviation (second member of the product close to unitary value). indoor

26


Fig. 3.1: Realistic *, reference ** and base case *** temperature trends and relative TDIb values [3].

Also local climate conditions are considered in this formulation, to evaluate building thermal-energy performance in relation to the climate of the location. This aspect is described through TDIsite index. Determining this index and considering the building effective performance allows to avoid important misunderstandings that often lead to judgments of a building’s performance being better than another building just because the climate condition are different. For instance, the same building in a less severe climate may perform more positively than a building in a more severe climate, even if they have the same features. Each weather file, and each location is characterized by its value of TDIsite index typical for seasonal or annual period. TDIsite in a seasonal period is calculated as follows in (3.5): ⎧ ⎫ t −t ⎪ ⎪ TDIsite,s = ⎨ ∫ f Ta-s − TM-s,site dτ + ∫ Tm-s,site − f Ta-s dτ ⎬ ⋅ s T - site,s ts ⎪⎩Ph ,site ⎪⎭ Pc ,site

[( )

]

[

( )]

[°C⋅h] (3.5)

Where: Ta-s = is the air-sun temperature of the location, calculated on a reference horizontal surface; are extreme values of the seasonal thermal target, TM-s,site, Tm-s,site = expressed by Ta-s,site . This thermal range is obtained as an extension of the

27


internal range on the hotter and colder temperature for 3°C. They are TM-S,site = 26°C+3°C = 29°C, Tm-S,site=26°C-3°C = 23°C and TM-W,site=25°C+3°C = 28°C and Tm-W,site=20°C-3°C = 17°C for summer and winter respectively. The overall evaluation of building performance with respect to site climate conditions is described in (3.6) and (3.7), for a season “s” and for the whole year respectively: ⎧ ⎫ t −t ⎪ ⎪ s T −s − + − f T T d τ T f T dτ ⎨∫ ⎬⋅ ∫ M-s in M-s in t ⎪⎩Ph ⎪⎭ Pc seas = ⎧ ⎫ t −t ⎪ ⎪ s T −s ,site − + − f T T d τ T f T dτ ⎬⋅ ⎨ ∫ ∫ m-s, site a-s,site M s, site a-s,site t ⎪⎩Ph −site ⎪⎭ Pc − site s [-] (3.6)

[( )

TDI b ,site −s

[(

]

]

)

TDI b - site, y =

[

( )]

[

(

∑ TDI b-site,s - i i

⋅

t s-i ty

)]

[-] (3.7)

Where Ph,site and Pc,site are the time periods during which the Ta-s, site is external to the thermal target range, typical of the location. So they are expressed as follows in (3.8) and (3.9):

{ [ ] (

)

}

[h] (3.8)

{ [ ] (

)

}

[h] (3.9)

Ph ,site = τ ∈ 0, ts : f T a −s,site ≥ TM −s,site

Pc ,site = τ ∈ 0, t s : f T a −s,site ≤ Tm −s,site

A null value of TDIb,site expresses the final aim in terms of TDIb. The further TDIb,site is from zero, the more the thermal zone registers thermal conditions far from the target. High TDIsite corresponds to severe climatological conditions, while low TDIsite corresponds to regular climatic contexts. An indoor condition far from the target corresponds to low design quality or to very severe climate conditions, which is necessary to consider, especially in this research where we need to evaluate the performance of cool roof that could produce at the same time large benefits or penalties just varying the weather forcing.

28


3.3.2 Case study and correlations In this section the results of some interesting TDI formulation application are reported. The first case concerns the application of this methodology in order to evaluate the role of architectural building layout with respect to climate while addressing the sensitivity of several envelope features, i.e. reflectance, insulation, inertia. Finally, the results expressed by TDI are compared to the adaptive comfort indicator expressed in terms of DH (Degree hour) index [10]. Figure 3.2 [3] describes the TDIb trends for summer and winter, for three different Italian climates. Buildings’ prototypes represent different architectural layouts: Sshape is optimized for summer performance, W for winter, and T represents the traditional Italian duplex single family house. Figure 3.3 describes the sensitivity analysis of the envelope properties concerning one building typology located in Perugia, both in summer and in winter conditions.

Fig. 3.2: a) TDIb-s varying architectural layout, climatic location, seasonal period of analysis. (b) TDIb-site,year varying architectural layout and climatic location [3].

29


Fig. 3.3: Sensitivity analysis results expressed in terms of TDIb for the city of Perugia and winter assessment [3].

In order to evaluate the correlation between TDI and other common thermalenergy performance indicators, here the TDIb results are compared with the results expressed in terms of an adaptive comfort indicator represented by Degree Hours (DH) [°C⋅h] described in [10]. To this aim, DH index is calculated with the same climatic conditions, assuming a “II building category” (“normal level of expectation and should be used for new buildings and renovations” as suggested in [10]). As shown in Figure 3.4 [3], the proposed TDIb-s could effectively represent also the adaptive thermal comfort conditions, both in winter and summer. In fact, the correlation coefficient values are 99.6% for winter and 96.1% for summer [3].

30


Fig. 3.4: Correlation between TDIb and DH values varying envelope IPs (transmittance, reflectance, inertia) for winter (a) and summer (b) analyses [3].

3.4

Overheating and overcooling assessment

The assessment of overheating and overcooling carried out for the proposed cool roof technology has been evaluated through simple indexes calculated through experimental monitored data. Cool roof assessment through continuous monitoring, that represents the focus of this research, has allowed to analyze the overall effect of this new cool brick tile in terms of: (i) tile superficial properties’ variation such as reflectance, (ii) thermal behavior of the roof, (iii) thermal-energy behavior of the indoor environment adjacent to the roof, i.e. the monitored attic of the house. The specific research occasion has guided toward the elaboration of two series of synthetic indexes to describe building thermal performance with varying the roof reflectance. The first series is formed by three indexes that describe the attic monthly overheating expressed by mean (Smean, 3.10), maximum (Smax, 3.11) and minimum (Smin, 3.12) values of operative indoor temperature. The sense of these indexes is the description of cool roof potentialities for reducing the daily peak, the average temperature and the lowest temperature, typically concerning the nocturnal behavior of the attic. Smean, Smax, and Smin are calculated as follows:

Smean − m =

n ⋅ Tin ,mean

S max −m =

∑ (Tin,mean − i − Tout ,mean − i ) n

1

i =1

1 n ⋅ Tin ,mean

∑ (Tin,max −i n

i =1

− Tout ,max −i

)

[-] (3.10)

[-] (3.11)

31


Smin − m =

1 n ⋅ Tin ,mean

∑ (Tin,min− i − Tout ,min − i ) n

i =1

[-] (3.12)

Where: - n is the number of days i in each considered month; - the subscript m represents the considered month of the calculation; - Tin,mean is the mean attic indoor temperature calculated in the monitored year; - Tin,min-i Tin,max-i, Tin,mean-i are the minimum, maximum, average values of operative temperature in the attic of each day i; - Tout,min-i Tout,max-i, Tout,mean-i are the minimum, maximum and mean values respectively of outdoor temperature of each day i during the month m. In the current research, these not-dimentional synthetic indexes are aimed at defining the role of innovative tiles on the thermal behavior of the indoor thermal zone but, in general, they could be applied also to define the efficacy of several efficiency optimization strategies in buildings. The second series of proposed indexes is aimed at defining the roof capability to cut down the overheating (3.13) and the same roof capability to emphasize the indoor overcooling (3.14). This last index in particular takes into account possible winter penalties to be balanced with summer benefits of cool roof implementation. Also in this analysis, the control indoor parameter is the monitored operative indoor temperature calculated with respect to the outdoor dry bulb temperature. Overheating decrease is expressed through the OHm index calculated in (3.13) and the overcooling increase is expressed through the OCm index in (3.14) as follows:

1 OH m = tm OCm =

1 tm

tm

∫ {[Tin (τ ) − Tout (τ )]⋅ HOH (Tin (τ ) − Tout (τ ))}

o

tm

∫ {[Tout (τ ) − Tin (τ )]⋅ HOC (Tout (τ ) − Tin (τ ))}

o

Where: - tm is the time period of each considered month m;

32

[°C] (3.13)

[°C] (3.14)


- Tin(τ) is the attic operative indoor temperature; - Tout(τ) is the outdoor dry bulb temperature; - H(f(τ)) are Heaviside functions described in (3.15) and (3.16):

⎧1, Tin (τ ) − Tout (τ ) ≥ 0 HOH Tin (τ ) − Tout (τ ) = ⎨ ⎩0, Tin (τ ) − Tout (τ ) < 0

(3.15)

⎧1, Tout (τ ) − Tin (τ ) ≥ 0 HOC Tout (τ ) − Tin (τ ) = ⎨ ⎩0, Tout (τ ) − Tin (τ ) < 0

(3.16)

(

(

)

)

As represented also in Figure 3.5, the physical meaning of OH and OC is the difference between the subtended areas of the operative indoor temperature and outdoor temperature respectively. The overheating index takes into account of this area just when the indoor thermal zone is hotter than the outdoor environment (Tin(τ)>Tout(τ)), through the Heaviside function. On the contrary, the overcooling index just considers those situations when Tin(τ)

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