From net energy to zero energy buildings: Defining life cycle zero ...

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as defined in ecological economics and in the renewable energy field. 3.1. Zero energy buildings. Some of the first documented attempts towards zero energy.
Energy and Buildings 42 (2010) 815–821

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Energy and Buildings journal homepage: www.elsevier.com/locate/enbuild

From net energy to zero energy buildings: Defining life cycle zero energy buildings (LC-ZEB) Patxi Hernandez a,*, Paul Kenny b a b

UCD Energy Research Group, School of Architecture, Landscape & Civil Engineering University College Dublin, Richview, Belfield, Dublin 4, Ireland UCD School of Architecture, Landscape & Civil Engineering University College Dublin, Belfield, Dublin 4, Ireland

A R T I C L E I N F O

A B S T R A C T

Article history: Received 18 September 2009 Received in revised form 4 December 2009 Accepted 10 December 2009

There are various definitions of ‘zero energy’ and ‘net-zero’ energy building. In most cases, the definitions refer only to the energy that is used in the operation of the building, ignoring the aspects of energy use related to the construction and delivery of the building and its components. On the other hand the concept of ‘net energy’ as used in the field of ecological economics, which does take into account the energy used during the production process of a commodity, is widely applied in fields such as renewable energy assessment. In this paper the concept of ‘net energy’ is introduced and applied within the built environment, based on a methodology accounting for the embodied energy of building components together with energy use in operation. A definition of life cycle zero energy buildings (LC-ZEB) is proposed, as well as the use of the net energy ratio (NER) as a factor to aid in building design with a life cycle perspective. ß 2009 Elsevier B.V. All rights reserved.

Keywords: Net energy Building life cycle Zero energy building Embodied energy Net-zero energy building Life cycle energy analysis

1. Introduction: the concept of ‘net energy’ Within the built environment the term ‘net energy’ is often used to describe a balance between energy used by the building and its occupants and systems and energy produced by its renewable energy systems. Employing this meaning, various definitions of ‘net-zero energy building’ are widely used and will be discussed later in this paper. However the original concept of ‘net energy’, as it is used in the field of ecological economics, has a very different meaning. It relates to whole life cycle energy accounting and has been evolving for more than a century, currently remaining a widely discussed topic, particularly in the fields of renewable energy and biofuels. The first notions of the concept of ‘net energy’ can be attributed to Podolinsky who tried to analyse aspects of society and the production of goods, mostly related to agriculture, in energy terms [1]. Influenced by the ideas of the Physiocrats from the 18th Century and integrating Carnot and Clausius’ descriptions of thermodynamics in the 19th Century, this Ukrainian socialist physician described the necessity for a ‘precise and conscientious system of accounting in which figures are neither hidden nor distorted’. He attempted to relate thermodynamic principles to economic production by way of considering the ‘‘accumulated solar energy’’ of human activity in what could be considered as the first ‘net energy’ study in history. A few decades later, in the 1920s,

* Corresponding author. Tel.: +353 87 1324058; fax: +353 1 2838908. E-mail address: [email protected] (P. Hernandez). 0378-7788/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.enbuild.2009.12.001

English chemist Frederick Soddy further analyzed contemporary social and economic systems from a thermodynamic perspective [2] suggesting that human development was highly dependent on biological and physical events, which are not accounted for in mainstream economics. Soddy suggested that detailed accounting for energy use could be a good alternative to the monetary system since energy is part of virtually any commodity. As such problems related to the generation of ‘virtual wealth’ and debts could be avoided as those concepts were ‘artificial’ not having any physical energy value. Such an argument could be considered timely given the current international economic crisis. Also in the 1920s a group of scientists and engineers in the United States, called the ‘Technical Alliance’, carried out a detailed analysis of various industries and processes in what were probably the first known set of detailed ‘net-energy’ analyses. From this group emerged the ‘Technocracy’ movement, which proposed a society where ‘energy units’ would replace the market system and which developed a methodology for energy accounting [3]. That movement gained much media attention during the 1930s, mainly in the United States, and has remained active ever since in United States, Canada, and Europe. However the momentum gained in the 1930s vanished during the following decades together with the global interest in the ‘net energy’ concept. It was not until the 1970s, and perhaps linked with the sharp decrease in the ‘net energy’ yield from fossil fuel reserves [4], that the concept of ‘net energy’ was again re-visited, properly formulated and gained general attention. Georgescu-Roegen is frequently acknowledged as the first economist that introduced

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thermodynamic analysis and particularly the entropy law to economic theory [5] and has had a profound influence in the field of ecological economics [6,7]. Also in the 1970s, ecologist Howard T. Odum, working on energy flow analysis, stated that ‘the true value of energy to society is the net energy, which is that after the costs of getting and concentrating that energy are subtracted’ [8]. This definition and approach had a great impact at the time even influencing US policy with the publication of the Research and Development Act in 1974, still part of US code today. It states that ‘the potential for the production of net energy by the proposed technology at the stage of commercial application shall be analyzed and considered in evaluating proposals’ [9]. Odum also had a major influence in the field of ecological economics and amongst other achievements is known for developing the concept of ‘emergy’ in the 1990s, which is still being developed and used today in a wide range of applications including the building environment [10,11]. Since the revival of the ‘net energy’ concept in the 1970s, net energy analysis has been applied in many different fields, from the fossil fuel and nuclear industries to renewable technologies [4,12– 16], being recognized as a valuable tool to consider life cycle aspects of energy systems. The net energy analysis has been defined as a ‘technique for evaluating which seeks to compare the amount of energy delivered to society by a technology to the total energy required to find, extract, process, deliver, and otherwise upgrade that energy to a socially useful form’ [17]. There has been an emerging debate about the usefulness of the ‘net energy’ concept in decision-making particularly in the field of biofuels, with different experts highlighting both the usefulness of the concept and its potentially misleading application [18–20]. One of the reasons for the disagreements about the usefulness of the concept of ‘net energy’ is due to the difficulties of setting boundaries as happens with any life cycle analysis methodology. Different methods and degrees of accuracy have been used for ‘net energy’ analysis over the years and results have being expressed in different terms such as energy payback, energy return of investment, energy yield ratio, net energy ratio, life cycle energy analysis (LCEA). A recent paper by Mulder and Hagens [21] analyzed a range of different studies and proposed a consistent framework of analysis for energy return of investment (EROI) of energy production technologies. Richards and Watt [22] also attempted to review the different types of ‘net energy’ indicators that have been used over the years, focusing on studies carried out on photovoltaics, and proposes the energy yield ratio (EYR) as the most adequate indicator, having as its main advantage the consideration of the lifetime of the product. 2. Net energy and the built environment In the field of the built environment the ‘net energy’ concept and analysis has not been introduced into mainstream calculation and certification methods. Energy evaluation of buildings typically only considers the energy use in the form of electricity or fossil fuels for the operation of a building without considering the other energy inputs from building construction process as, for example, the manufacturing of materials. Some voluntary environmental assessment methods such as LEED [23] or BREEAM [24], do account for a wider perspective than annual energy in use and include issues such as material selection, transport, and usage. It can be argued that ‘net energy’ analysis is indirectly considered in these methods as, for example, reusing and recycling buildings and products (which effectively saves energy in the extraction, manufacturing, processes and transport industries) are rewarded by the rating method. In a more direct approach and over the past few decades studies have taken a more direct approach to the application of the concept of ‘net energy’ by considering the embodied energy in buildings and their constituent components. Accounting for embodied energy, which is referred to as energy

necessary to deliver products and services, can effectively serve as a form of ‘net energy’ analysis when compared to the energy used by the building in operation over the life cycle. Probably the first studies using this approach were those by Hannon et al. [25] in which they used embodied energy values extracted from an inputoutput model of energy flows through the US economy and compared them with the typical energy use of family residences. Most of the more recent studies use existing detailed LCA tools such as SIMAPRO [26] or ATHENA[27], which offer the possibility of analyzing, in detail, a wide range of environmental aspects of materials including embodied energy, gathered in most cases through life cycle inventory analysis such as described in ISO 14040 and relative standards [28]. A review of 60 case studies where this type of analysis had been performed was carried out by Sartori and Hestnes [29], highlighting the increasing importance and relevance of this type of analysis as we move towards ‘lowenergy’ buildings. In buildings with a ‘zero energy’ balance in use (energy delivered to a grid is equal to energy in use) the life cycle energy is solely due to the process of delivering and maintaining the building and its components. At this stage it is important to clarify the definition of ‘zero energy building’, and ‘net-zero energy building’. 3. Definition of zero energy and net-zero energy buildings Historical definitions of zero energy are based mainly on annual energy use for the building’s operation (heating, cooling, ventilation, lighting, etc.). The term ‘net-zero energy’ is frequently used to present the annual energy balance of a grid connected building but it does not consider the energy inputs to deliver the building and its components. As such it is not directly associated with the use of the term ‘net energy’ as related to life cycle energy accounting and as defined in ecological economics and in the renewable energy field. 3.1. Zero energy buildings Some of the first documented attempts towards zero energy were in reality an attempt to achieve zero-heating in the form of solar houses. Early examples include the 1939 MIT Solar House I, which included a large solar thermal collection area and water storage [30], or the 1955 ‘Bliss House’ [31] using solar air collectors and rock mass storage. Examples from the 1970s include the Vagn Korsgaard Zero Energy Home in Denmark [32] or the Saskatchewan Conservation House [33]. These were also designed for zero or near zero heating incorporating higher insulated envelopes. The Saskatchewan Conservation House used some features that are now becoming mainstream in passive and low-energy construction such as good air tightness (1.3 air changes at 50 Pa) and air-toair heat exchangers. This approach allowed a reduction in the area of solar collection surface and solar storage when compared to previous zero-heating installations. These early examples have been influential in current approaches to building design and indeed contributed to the definition and upgrade of building standards and regulatory codes. Voluntary standards for low-energy buildings using the principles of high insulation, good air tightness and heat recovery ventilation systems are increasingly popular, such as the scheme R-2000 in Canada [34] or the Passivhaus in Germany [35], and are now extending to other parts of the world. While those standards are not zero energy nor zero-heating they do achieve reductions in heating energy demand using a practical and cost-efficient approach, which most experts would consider a good first step towards zero energy building (ZEB). These solutions also generally use lower quantities of material than more extreme zero energy solutions. If analyzed from a life cycle energy perspective such

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cases can be a better option than zero energy autonomous solutions [36] as autonomous houses need to use some energy storage system, typically batteries for electricity storage, that represent an additional input of energy when their production processes are considered.

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energy used in the building in operation plus the energy embodied within its constituent materials and systems, including energy generating ones, over the life of the building is equal to or less than the energy produced by its renewable energy systems within the building over their lifetime. An appropriate methodology is provided in the following sections.

3.2. Net-zero energy buildings 4.1. Selection of primary energy as an indicator In current practice, the most common approach to ZEB is to use the electricity grid both as a source and a sink of electricity, thus avoiding the on-site electric storage systems. The term ‘net’ is used in grid connected buildings to define the energy balance between energy used and energy sold, the term ‘net-zero energy’ being applied when the balance is zero. Vale and Vale [37] offer a thorough philosophical and practical examination of the implications of an ‘autonomous’ house versus a ‘‘net-zero energy’’ house and with a global market perspective. Their conclusion and approach is that connecting a domestic renewable system to the electricity grid and achieving a ‘‘net-zero energy’’ home can have better life cycle performance than an autonomous house as using electric storage systems is avoided and some flexibility in the use of appliances is gained. This argument being even more valid for non-domestic buildings, the ‘net-zero’ concept has been adopted widely while autonomous buildings are reserved for very specific site conditions, education, and research. Considering ‘net-zero energy’, there have been different definitions. The European Parliament recently approved a recast of the Energy Performance of Buildings Directive in which they define net-zero energy building as ‘‘a building where, as a result of the very high level of energy efficiency of the building, the overall annual primary energy consumption is equal to or less than the energy production from renewable energy sources on site’’, and aims to release a more detailed definition by 2011 [38]. Perhaps the two most frequently cited definitions in current literature are ‘‘net-zero site energy’’ and ‘‘net-zero source energy’’, as discussed by Torcellini et al. [39]. Net-zero site energy means that a site produces at least the same energy as it uses in a year, independent of the type of energy produced or used. In the ‘net-zero energy source’ definition, imported and exported energy are multiplied by a primary energy conversion factor, thus allowing for some flexibility in the use of heating fuels. For example, if electricity is being sold directly to the grid in a location where the electricity primary factor is high, the ‘net-zero energy source’ definition would allow the use of larger quantities of a heating fuel from a source with a smaller primary energy factor. A fuller list of measured ‘net-zero energy’ buildings in the US corresponding to the mentioned definitions can be found at the US Department of Energy website [40]. As it can be gathered from this brief review of ‘zero energy’ case studies and definitions, none of them directly takes into account energy input other than those of the building in operation and do not consider the concept of ‘net energy analysis’ and its life cycle perspective. 4. Definition of life cycle zero energy buildings (LC-ZEB) In this paper the definition of zero energy building is extended to include the embodied energy of the building and its components together with the annual energy use, which will serve to introduce a life cycle perspective and therefore bring the concept of ‘net energy’, as used in ecological economics, into the built environment within a consistent methodology. To denominate buildings with a zero life cycle energy balance, and not to confuse with the frequently used term ‘net-zero’ as described in the previous section, the term life cycle zero energy building (LC-ZEB) is proposed. A LC-ZEB is one where the primary

Primary energy has been selected as the indicator for annual energy use in operation and for embodied energy. Primary energy allows differentiation between electricity and fossil fuel use and includes an indication of the efficiency of delivering heating, hot water, lighting, etc. The authors acknowledge that some approaches to accounting for exergy [41] or emergy [10,11] could be appropriate for use in a definition of a LC-ZEB from a ‘net energy’ perspective as they relate more adequately to the actual thermodynamic principles of use of resources. It is also acknowledged that in some cases carbon dioxide emissions related to energy use might be considered a more appropriate indicator. Primary energy is suggested here as a more common indicator, reflective or actual energy use and currently applied in many building energy calculation methodologies, which at the moment do not generally support the calculation of exergy or emergy. The same applies for life cycle and embodied energy databases where primary energy is generally a more common indicator. In cases where carbon dioxide is considered appropriate, conversion factors from primary energy to carbon dioxide can easily be integrated within the proposed methodology and definition being available for most regions or countries. 4.2. Calculating annual energy use (AEU) Methodologies for calculating annual energy use of buildings are well developed and used throughout the world. Energy use calculations have evolved from steady state heat loss and semistatic monthly energy demand calculations to complex dynamic energy performance simulation tools which can model annual energy use over very short intervals (hours, minutes, even to a fraction of a second). Simulation programs have been compared in various papers [42,43] and detailed building energy simulation practice is extensive not only within the research community but also in the building industry. International standards such as EN ISO 13790:2008 ‘Energy performance of buildings—calculation of energy use for space heating and cooling’ [44], which include monthly calculations methodologies, are considered of sufficient accuracy for application in energy certification. Although ideally every energy use should be considered in a building energy calculation method, factors such as plug-in loads and equipment are generally excluded in some calculation methods, particularly in energy rating and certification methods. The methodology presented in this paper does not restrict the calculation of annual energy use to any particular methodology as it is suggested that any validated building energy calculation method could be expanded to include a ‘life cycle’ perspective as proposed. The only pre-requisite for the application of the proposed methodology is the conversion of the energy use results into ‘primary energy’ values. Some available software already provides results directly in the form of primary energy using national average factors for the different fuels used. Where the calculation tool does not directly offer this possibility conversion factors for the different fuels used need to be applied and implemented in accordance with national guidelines. For countries which have no defined national primary energy factors definitions can be found, for example, in EN 15603:2008 ‘Energy performance of buildings—overall energy use and definition of energy ratings’ [45]. However it must be noted

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Fig. 1. Flow diagram of proposed methodology.

that there are other issues related to calculating ‘primary energy’, in particular when renewable energies are considered, which are the subject of debate, as discussed by Segers [46]. 4.3. Calculating annualized embodied energy (AEE) A first step to including life cycle aspects of the energy use of constituent construction or system components within annual calculations is to estimate their service life. For HVAC or renewable energy systems, it may be as straightforward as estimating a service life based, for example, on manufacturer’s guarantees, although it is preferable to use real life expectancy if national or regional data available. For other components of a building such as envelope and structure, life expectancy could be much more difficult to establish as some buildings will last for hundreds of years with little refurbishment while others might experience various major refurbishments or even be demolished within 50 years or less. Certain types of buildings, such as speculative office developments, can often be refitted within much shorter periods. ISO 15686 ‘Buildings and constructed assets—service-life planning’ [47] provides some guidance on how to predict the service life of buildings and products. 50 years is considered a typical value for the service life of buildings before they undertake major renovations in most studies [29], so this value is suggested here where no other data is available. A second step is to consider the boundaries of the buildings life cycle, which is a key aspect in any analysis. Most common approaches are the ‘Cradle to Grave’, ‘Cradle to Site’ or ‘Cradle to Gate’, the latter including all energy inputs to a product, expressed in primary energy, from extraction to manufacturing, until the product leaves the factory gate. This approach is proposed as the basis for this methodology, as it is the most commonly used value referenced in embodied energy studies [48]. However this approach ignores some important aspects such as transport to building site, end of life disposal and maintenance, which could have a potentially high impact as discussed by various authors [49–51], and it is suggested that an extended life cycle should be used if reliable embodied energy data is available. The embodied energy data for each material, system, or product will be finally presented in kWh of primary energy per year of service life and denominated as ‘annualized embodied energy’ (AEE).

4.4. Annualized life cycle energy (ALCE) Annualized embodied energy (AEE) expressed in primary energy units per year of service life and annual energy use (AEU) expressed in primary energy units per year can be directly summed representing the impact of the building components and materials together with the energy use for running the building as in Eq. (1). ALCE ¼ AEU þ AEE

(1)

The sum of these two energy components gives a life cycle perspective of energy use and is defined as ‘annualized life cycle energy’ (ALCE), also expressed in primary energy units. A diagram outlining the proposed methodology is presented in Fig. 1. A LC-ZEB can now be redefined as one whose annualized life cycle energy is zero. AEU þ AEE ¼ 0

(2)

Fig. 2. XY graphic showing annualized life cycle energy of some generic buildings, as the distance to LC-ZEB line.

P. Hernandez, P. Kenny / Energy and Buildings 42 (2010) 815–821

Instead of quantifying energy use numerically the annualized life cycle energy equation (Eq. (2)) can be represented by an XY graph where the horizontal axis is the annualized embodied energy (AEE) and the vertical axis the annual energy use (AEU). LCZEB is represented by a line at 458, and typically occupies the fourth quadrant representing buildings where AEU is negative and equal to the AEE. Buildings along the horizontal axis represent zero energy buildings (with zero annual energy use), but it must be noted that they could have different embodied energy, therefore appearing closer or further from being LC-ZEBs. Fig. 2 illustrates the representations of some generic building cases and showing how they relate to the concept of a LC-ZEB. As the annualized embodied energy (AEE) is likely to be always above zero, representing the energy used in the delivery process of the building and its components and systems, a LC-ZEB would generally have an annual energy use (AEU) below zero. To achieve an AEU less than zero requires that a building would need to produce more energy than is used for running it and so requiring the installation of some form of renewable energy systems. However the renewable energy systems must be considered as any other building component and so their additional embodied energy is also annualized and enters the equation as a part of the AEE. Therefore renewable energy installations with a high ‘net energy’ input, that is a large ratio of energy produced to their embodied energy, would be part of an optimum solution towards LC-ZEB. Over-sizing of building components or renewable energy systems with the sole intention of bringing the AEU to zero could result in a high increase of AEE, meaning that the total annualized life cycle energy (ALCE) might not be significantly reduced or could even increase. The ALCE indicator would provide a true value of the efforts to minimize energy use in the built environment and, as an analogy to Odum’s ‘net-energy’ definition, what ‘the true value of a design decision that decreases the annual energy use of a building is after the embodied energy of additional building components and systems are subtracted’. 5. Building optimization—towards LC-ZEB The building industry’s advance towards zero energy means the probable integration of additional energy saving and/or clean energy producing components and systems. The main advantage of the methodology presented here is that it allows building designers to carry out comparative analysis of the life cycle relevance of design decisions related to building envelope design, materials, HVAC and renewable energy systems. All such components can be included in the analysis through their annualized embodied energy and annual energy use. Historically, this approach has not been considered in very-low energy building design. There could arise situations where systems or building components are unintentionally over-specified, and it can be argued that some of the first and remarkable examples of ‘zero energy houses’ in history, which had very large solar collector and storage systems [30–32], would probably have had a lower life cycle energy use and would be closer to LC-ZEBs with smaller solar collecting surfaces. Care must be taken in current ‘zero energy’ building design to avoid over-specification of certain components, as discussed by Hernandez and Kenny [52,53], who argued how large areas of thermal solar collectors for water and space heating together with high levels of insulation, as is often promoted, might not be the most efficient way of reducing the life cycle energy for some building typologies. In the context of LCZEB, other design strategies might offer more appropriate solutions, particularly in less extreme climates such as in maritime Europe. As used in the renewable energy field, the ‘net energy ratio’ (NER), sometimes also called as energy return of investment (EROI)

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or energy yield ratio, can also be introduced into the building energy analysis to aid in decision-making and to optimize building design towards LC-ZEBs. The NER corresponding to a change between two building options 1 and 2 can be presented as: NER ¼

AEU1  AEU2 AEE2  AEE1

(3)

The NER, defined for the built environment as the ratio of the decrease on annual energy use to the increase in annualized embodied energy, can be used to compare design options that are intent on to improving AEU but increase the AEE. The higher the NER of a particular technology or the NER associated to a design decision that affects building energy performance, the more effective it will be in reducing the life cycle energy use and moving towards LC-ZEB. Options where the NER is greater than one will contribute to a reduction of the life cycle energy. These need not only include technologies but might also include decisions made at the early design stage such as those around building form, orientation, layout, etc. These options can decrease AEU without increasing AEE would have an NER value of infinity, meaning that they represent ideal options from a life cycle perspective and, in many cases, could mean prioritising architectural solutions over those of high AEE system based technologies. This introduction of the NER to the built environment allows building design and construction options to be compared with NER values of renewable energy systems, which are extensively published and discussed [21]. For example, the first layer of insulation in a house would normally have a very high NER and would save a large amount of energy with a small amount of material. Subsequent layers of insulation, while adding to the total embodied energy, would not deliver an equivalent energy saving and so would represent a diminishing NER. Technologies such as solar water or space heating systems would also represent a diminishing NER as the annual solar input rate per square meter of installation decreases as can occur with large installations oversized for the summer, as in those cases, increasing collector size and embodied energy does not proportionally increases the solar energy input. Technologies such as PV, however, will have a practically constant NER independent of their size as the production of electricity will be proportional to their quantity of materials used in their production and installation. To illustrate the method, Fig. 3 compares the NER of different insulation thicknesses of an energy intensive material (polystyrene) in its application in a highly efficient house in a maritime climate. The studied house is a passive design with no cooling demand and a very low heating demand of 15 kWh/m2 as the base case. Details of the house and calculations can be found in Hernandez and Kenny [53]. This house employs an average of 115 mm of insulation for walls, 85 mm for floors, and 235 mm for roofs. Four different polystyrene upgrades are tested, increasing insulation by 50 mm each time. It can be observed in Fig. 3 that the slope of the lines in between upgrade options, which is equal to the NER, diminishes as the insulation level increases. The NER drops below 1 in the last upgrade from UP3 to UP4, which corresponds to the increase of insulation from an average of 250 mm to an average of 300 mm. This NER 1) would represent a decrease in ALCE and a life cycle energy saving. A reduction of AEE that does not represent a increase in AEU would have a NER of infinity and would always be a preferred choice from a life cycle energy perspective. An example would be when choosing between different insulation materials with similar performance and durability characteristics but different embodied energy as discussed by different authors [56–58]. 6. Conclusion and application of the methodology Building regulations and standards are evolving towards ‘zero energy’. The European Parliament recently approved a recast of the Energy Performance of Buildings Directive proposing that all new buildings in the EU be at least ‘net-zero energy’ by 2019. Other countries, such as the UK, have already established comparable targets for all new housing which will see ‘net-zero’ achieved by 2016 [59]. However most regulations refer to ‘net-zero’, focusing on energy in use only and ignoring factors such as embodied energy. This paper proposes that a life cycle perspective should be added to the ‘equation’. Authors such as Cole and Kernan [60],

Adalberth [61,62], Harris [63] or [64] have developed methods to integrate embodied energy with annual energy use analysis. The integration of a life cycle perspective with regulation and certification has been discussed by Zold and Szalay [65] and Casals [66]. This paper has provided a model and definition of a simplified methodology to account for embodied energy together with energy use in operation and reclaims the original concept of ‘net energy’ to define a life cycle zero energy building (LC-ZEB). A LC-ZEB is defined here as a building whose primary energy use in operation plus the energy embedded in materials and systems over the life of the building is equal or less than the energy produced by renewable energy systems within the building. This paper has also introduced the concept of net energy ratio (NER) for its application to the built environment, to aid in the selection of different building options that inform can decisions to progress towards LCZEB. The authors acknowledge some limitations to this methodology as discussed in the selection of primary energy as an indicator, the energy use calculation methodology and the boundaries and simplifications of embodied energy calculations. It is understood by the authors that the concept of LC-ZEB does not take into account aspects of the decision and policy-making processes such as the integration of other environmental and socio-economic aspects of building construction. However, the accuracy and applicability of proposed methodology can only improve as more detailed data on life cycle of building products and systems becomes available. In the meantime the use of the LC-ZEB, even with the assumptions proposed in this paper can offer a simple life cycle perspective to building energy assessment not currently addressed in commonly used ‘net-zero energy building’ definitions. While it has the potential to inform future building energy certification and energy policy, the use of this proposed LC-ZEB concept and methodology, reinstating the original meaning of the ‘net energy’ concept, has the potential to make clearly visible the true environmental impact of design decisions over the full life cycle of a building, naturally promoting long-term best management of energy resources. Acknowledgement The authors would like to acknowledge the support of the Irish Research Council for Science, Engineering and Technology, funded by the National Development Plan of Ireland.

P. Hernandez, P. Kenny / Energy and Buildings 42 (2010) 815–821

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