Dynamic simulation of buildings

Dynamic simulation of buildings: Problems and solutions Università degli Studi di Trento Paolo BAGGIO

The basic problem To design (and operate) energy efficient buildings, accurate modeling tools are needed. The “consolidated” approach was to perform a “static calculation” of the heating energy need based on “average values” . Things are changing: better performance required from building (heating, cooling, lighting); more complex interacting systems (boilers, heat pumps, solar-wind-biomass power sources, etc)

Dynamic simulation of buildings: problems and solutions

The basic problem In short, we want energy efficient buildings, but, the tools used to design such buildings may not be completely adequate. The approach needs an update


The possible solution To optimize the global behavior of a building the interaction between envelope, structure, systems and climate must be carefully evaluated. Since the energy balance of a building is continuously changing dynamic calculation tools are needed for design and operation. In addition building management , monitoring of actual energy use and flexible control, in a word Building Automation and Control Systems (BACS) are needed Efficiency should last in time !


Describing the behavior of High Performance Buildings Dynamically changing loads and gains (solar radiation, temperature) Factoring people and occupation schedules Building envelope and structures: heat storage and release Different systems interacting (heating, cooling, lighting, etc) with changing part load performance Buildings are definitely dynamic systems


Dynamically changing loads and gains Four major types of building loads: (1) Climate-driven envelope loads - Type most often associated with energy consumption, externally imposed on the building (temperature, solar radiation, humidity, wind) - change from hour to hour

(2) Internal loads/gains - Due to heat gain from: People, Lighting, Equipment and Appliances, Losses from HVAC systems

(3) Stored-released energy - Energy stored in thermal mass: Floors and walls, Other building materials, Furniture. Dependent on thermal capacity of interior materials. These effects are time, material-, and location-dependent.

(4) HVAC system loads Inefficiencies due to: Frictional, Leakage, or heat transfer losses within ducts, Inefficiencies in driving mechanisms (e.g. fans or pumps). Impose additional loads (= increased energy use)

Considering only average values ignores the influence of hourly load patterns - loads need to be analyzed on an hourly and daily basis Dynamic simulation of buildings: problems and solutions

Dynamically changing loads and gains EXTERNAL LOADS

• • •

Solar gains Conduction losses across envelope surfaces Infiltration

INTERNAL LOADS

•

Heat gain from lights, equipment, and people

(Source: ASHRAE Standard 90.1 User’s Manual, 2008) Dynamic simulation of buildings: problems and solutions

Factoring people and occupation schedules People and occupation schedules - contribute to internal gains from the four primary sources: Occupants (Heat and moisture gains from occupants, typically the smallest contribution to internal loads), Equipment/appliances, Processes, Lighting. -

Important to understand: when occupants are present Occupant behavior - e.g. air conditioning versus opening windows or lights on or off . - Important to understand: thermal and visual comfort levels they require Major source of uncertainties in building energy simulations.


Buildings are dynamic systems Optimal design (i.e. high performance) of building – need to consider that the building is a dynamic energy system • Components and sub-systems are coupled through energy mass flows driven by end-use requirements • BUT, until recently – building energy research focused on individual components rather than the whole building as a system • In addition, the interaction between the building itself and the HVAC systems must be aqccounted for


Buildings are dynamic systems

Figure: Building as dynamic energy system (Source: Busch, 1996)

*** Figure highlights the interdependence of loads and energy use Dynamic simulation of buildings: problems and solutions

Detailed Dynamic Simulation of a Building What is involved in a dynamic simulation: define boundary conditions (hourly values !) describe building behavior describe systems behavior


Boundary Conditions and Indoor Parameters


Climate data Hourly data, test reference years • • • • •

Temperature Solar radiation Relative humidity Wind velocity (and possibly wind direction) Possibly rainfall

Not always available for the exact building location Usually acceptable to use nearby weather stations (some judgment required by the operator)


Climate data Solar radiation • Intensity of solar radiation is a function of the angle of incidence (angle at which sun’s rays reach the earth’s surface), which varies in time • May need to consider obstructions or reflections in calculations


Climate data (Test reference year for Trento Italy)


Heat gains Internal heat gains are derived from: • • • • •

Occupants (people) Lights Equipment and appliances Infiltration Systems

Occupation schedules • Require accurate descriptions of building usage patterns • Occupant behavior – E.g. air conditioning versus opening windows • Heat and moisture gains from occupants (sensible and latent heat generated by people) • Important to understand: – Times at which occupants are present – Thermal comfort levels they require Dynamic simulation of buildings: problems and solutions

Heat gains Lighting schedules • Represent one of the largest loads (either external or internal) – e.g. lighting levels in office buildings can be 2 – 3W/ft2 of floor area • All heat from lighting = sensible (no latent heat) • All electrical energy supplied to light fixtures is converted to heat. Heat given off is either: – Radiant heat (visible light) – Heat convected to the surrounding air – Heat conducted to adjacent building materials • The rate at which electricity is converted to heat by light fixtures is a function of: – Lamp type (incandescent, fluorescent, etc) – Type of fixture (recessed, pendant mounted, etc) – Efficiency of the light fixture itself


Indoor air quality, ventilation rates No common standard index for indoor air quality exists • Indoor air quality is therefore expressed as the required level of ventilation or CO2 concentrations

Indoor air quality influenced by emission from people and their activities (e.g. bio effluent, smoking), from the building and furnishings, and HVAC system Required ventilation is based on health and comfort criteria • Health effects attributed to specific components of emission • Comfort related to the perceived air quality (odor, irritation)


Describing Building Dynamic Behavior


Behavior of building structures

Figure: The effect of thermal storage on cooling loads (Source: Mull, 1997) Dynamic simulation of buildings: problems and solutions

Non-stationary heat transfer (Fourier Equation) Transient heat transfer = heat transfer and temperature distribution under unsteadystate (varying with time) conditions • Time lag between application of heat to one surface and the time when the heat begins to appear on the other side is a result of thermal (or heat) storage of the material • Thermal mass of building directly influences time lag between peak heat gain and peak cooling load, and inversely affects intensity of peak cooling load – The greater the thermal mass, the greater the time lag, the lower the peak intensity of the cooling load


Solving transient heat transfer Different possible approaches: must balance between accuracy and complexity Non-stationary heat equation (Fourier), thermal diffusivity and thermal capacity Heat transfer matrix, time shift, decrement factor Response factor Conduction Transfer Functions Method Finite differences and finite elements direct building simulation Simplified EN ISO 13790 method


Ventilation, dry air mass transfer Ventilation = “intentional introduction of air from the outside into a building” (ASHRAE, 2005) • Natural or forced – Natural = air flow through open windows, doors, grilles, and other envelope openings – Forced = air flow via fans and intake and exhaust vents (also referred to as mechanical ventilation)

Infiltration = “uncontrolled flow of outdoor air into a building” (ASHRAE, 2005) • Via cracks, unintended openings, use of exterior doors • Also known as ‘air leakage’


Latent heat balance Latent heat balance of a building

. . m.ge + mv = m.gi

External Air . m.ge

.

m = air mass flow rate (kg/h) . mv = vaporisation rate (kg/h) g = absolute humidity (kgH2O/kgAIR) e = external i = internal

. . gi = ge + mv/m

. mv

Internal Air Exhaust . m.gi


Systems Simulation


Systems Simulation System and component simulation • Semi-dynamic approaches: nominal and part load efficiencies • Detailed simulation: operating curves energy and mass balances • True dynamic simulations: governing equations for systems components • Water (hydronic) loops • Air loops


Available software Various software programs available, including: • TRNSYS • ESP-r • DOE-2 and eQUEST • EnergyPlus Some effort should be probably devoted to develop an European (Open) Software platform (following the DOE example) Dynamic simulation of buildings: problems and solutions





Standards and Energy Performance


Dynamic simulation as the ultimate certification tool Three (3) types of methods (EN ISO 13790): (1) Simplified monthly or seasonal methods • Monthly based • Correct results on annual basis • Results for individual months subject to error, especially months close to the start and end of warm and cool seasons (2) Simple hourly dynamic calculation method (3) Detailed (e.g. hourly) dynamic simulation method • Procedures ensure compatibility and consistency between the application of different types of methods • Provides common rules for boundary conditions and physical input data Dynamic simulation of buildings: problems and solutions

Standards and Energy Performance While the European standards allow for detailed dynamic simulation of a a building (see EN 15265:2008 and EN 13790:2008 para 5.4.1) a coherent framework of standards, (hourly) data (climate, schedules), system component data (part load performance of boilers, heat pumps, etc.) is still missing. To simplify the use of dynamic simulation, updated standards (similar to ASHRAE 90.1) and an European software validation procedure are badly needed.


Designing High Performance Buildings


Dynamic simulation as the ultimate design tool Buildings account for approximately 40% of European energy consumption Demand for lighting, heating/cooling, and hot water in homes/workplaces > energy consumed by transport or industry

Need to decrease energy consumption to meet current European objectives (Directive 2002/91/EC) and future ones, (Directive recast toward “Near Zero Energy Building”) Require common methodology to measure energy efficiency of buildings

DYNAMIC SIMULATION Dynamic simulation of buildings: problems and solutions

Dynamic simulation as the ultimate design tool Building energy performance modeling = powerful tool Analyses how the form, size, orientation, and type of building systems/designs affect building energy consumption • Accurately quantifies interactions between different building systems Improved decision making capability for designers • Better understand implications of design choices Provide added value to building operators (e.g. reduce utility bills) • i.e. saves energy and money Enhances productivity Reduces risk


Optimizing energy performance Working along with civil engineers/architects to improve envelope design and with the mechanic to integrate HVAC systems Passive solar heating Inexpensive method of heating buildings


Optimizing envelope Optimal design (i.e. high performance) of building envelope can significantly reduce heating and cooling loads • Additional costs for high-performance envelope can be offset by savings made due to the installation of smaller HVAC equipment

“A tight, well-insulated building envelope allows designers to specify smaller, more energy efficient HVAC systems. Smaller units cost less, take up less space, and bring lower energy bills” (Voith, 2008)


Optimizing systems Diversification of zone control • Account for diverse occupant schedules = more efficient energy use

HVAC systems • Often run at full load for every occupied hour, therefore operate at high load factors • Modulating heating/cooling distribution via pumps/fans with the load = increase energy efficiency

Correct installation, maintenance and operation of control systems vitally important for managing energy use


Tuning building operation Optimizing operation schedules of energy systems within existing building to reduce energy usage (i.e. restore lost energy efficiency of buildings) • Plant operation – e.g. making heating and cooling load calculations for selecting or sizing HVAC equipment – ensures equipment is not oversized or undersized for the intended application – e.g. zone thermostatic controls capable of providing a temperature range of at least 2.50oC within which the supply of heating and cooling energy to the zone is shut off or reduced to a minimum – e.g. automatic shutdown controls, capable of starting and stopping the system under different time schedules – E.g. occupant sensors, capable of shutting off the system when no occupant is sensed for a period of up to 30 minutes

• Schedules – e.g. lighting schedules: over-lit areas, lack of occupancy sensors in common areas – e.g. occupancy schedules: building systems running during summer months when buildings unoccupied

• Working time Dynamic simulation of buildings: problems and solutions

BAC – Building Automation and Control •

To achieve high performance an integrated control system is required acting from the building level (e.g. main distribution network water/air temperature) down to the room level (individual room automatic control temperature, air flow rate, lighting).

•

In addition, both to keep the building energy efficient and to validate simulation results, Technical Building Management (TBM) functions are required to provide information about operation, maintenance, services and management of buildings especially for energy management – Measurement, recording trending, and alarming capabilities and diagnosis of unnecessary energy use.


BAC – Building Automation and Control BACS and TBM for energy performance, monitoring and data collection

Std. EN 15232:2007 considers 4 BAC efficiency classes: •

Class A high energy performance BACS and TBM.

•

Class B advanced BACS with some specific TBM functions.

•

Class C standard BACS.

•

Class D non energy efficient BACS. Building with such systems shall be retrofitted. (New buildings shall not be built with such systems).


Designing buildings with high energy performance is an iterative process Iterative process = calculating a desired outcome via a repeated cycle of operations = convergent process (i.e. each cycle brings the operator closer to the desired result) Dynamic simulation = iterative process • Best results achieved when incorporated early into design process • Start with simple design, continue to refine and update simulation model as design evolves


Conclusions Dynamic simulation will become more and more important as a design and optimization tool as we progress toward Near Zero Energy Buildings (NZEB). The (European and National) standards should be updated in order to give explicit and unambiguous indications about the use of detailed dynamic simulation. There are difficulties to be removed, we need: Ready to use time series data for climate, schedules, etc; User friendly software to simplify the process, especially regarding the input of the building geometry; Ready to use data about the HVAC components.


Thank you for your attention

Pampeago – Fiemme Valley (Italy)
