Definition of nearly zero-energy building requirements based on a large building sample

Energy Policy ∎ (∎∎∎∎) ∎∎∎–∎∎∎

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Definition of nearly zero-energy building requirements based on a large building sample Zsuzsa Szalay a,n, András Zöld b a b

Department of Architectural Engineering, Budapest University of Technology and Economics, Műegyetem rkp. 3-9., Budapest 1111, Hungary Debrecen University, Department of Building Service Systems, Ótemető utca 2-4, Debrecen 4028, Hungary

H I G H L I G H T S







We We We We

analyse the European nearly zero-energy building definition. present a method for setting requirements based on a large building sample. demonstrate the method for residential buildings in Hungary. compare the results with the European targets.

art ic l e i nf o

a b s t r a c t

Article history: Received 8 April 2014 Received in revised form 17 June 2014 Accepted 4 July 2014

According to the recast of the Energy Performance Building Directive, Member States must give an exact definition for nearly zero-energy buildings to be introduced from 2018/2020. The requirement system stipulating the sustainable development of the building sector is usually based on the analysis of a few reference buildings, combining energy efficiency measures and HVAC systems. The risk of this method is that depending on the assumptions either the requirements do not provide sufficient incentives for energy saving measures and renewables or the requirements cannot be fulfilled with rational solutions in many cases. Our method is based on the artificial generation of a large building sample, where the buildings are defined by geometric and other parameters. Due to the large number of combinations, the effect of many variables appear in the results, with the deviations reflecting the sensitivity of the energy balance. The requirements are set based on some fundamental considerations and the statistical analysis of the sample. The method is demonstrated on the example of setting the requirements for residential buildings in Hungary. The proposed requirements are validated against the common European targets. The suggested method is suitable for developing building energy regulation threshold values, certification schemes or benchmarking values. & 2014 Elsevier Ltd. All rights reserved.

Keywords: Nearly zero-energy building Reference building Requirements Building population EPBD

1. Introduction The goal of the European Union is to drastically cut its domestic greenhouse gas emissions by 80% by 2050 compared to 1990 levels (BPIE, 2011a). Since the building sector has been identified as one of the key sectors for cost-efficient savings, at least 88–91% reduction is necessary in this field to reach these ambitious targets (BPIE, 2011a). This can only be achieved if the energy consumption

n

Corresponding author. E-mail addresses: [email protected] (Z. Szalay), [email protected] (A. Zöld).

of both the existing building stock and new buildings is reduced, and the share of renewable energy sources is increased in the energy supply. The 88–91% reduction cannot be guaranteed in the existing building stock, which means that new buildings still to be built by 2050 must compensate with even larger reductions. In line with these goals, the Energy Performance Building Directive has been revised in 2010 (EPBD recast, 2010). According to Article 9 of the EPBD recast, by 31 December 2020 all new buildings must be nearly zero-energy buildings (nZEB) in the Member States. Public authorities are supposed to take an exemplary role, and new buildings occupied and owned by public authorities must be nearly zero-energy buildings already after 31 December 2018 (EPBD recast, 2010). According to the definition of

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Please cite this article as: Szalay, Z., Zöld, A., Definition of nearly zero-energy building requirements based on a large building sample. Energy Policy (2014), http://dx.doi.org/10.1016/j.enpol.2014.07.001i

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the recast, a nearly zero-energy building is “a building that has a very high energy performance…. The nearly zero or very low amount of energy required should be covered to a very significant extent by energy from renewable sources, including energy from renewable sources produced on-site or nearby” (EPBD recast, 2010). However, the exact definition, reflecting their national, regional or local conditions is the responsibility of the Member States, including a numerical indicator of primary energy use expressed in kW h/m2 yr. Another important directive is the Renewable Energy Directive that orders Member States to require minimum levels of energy from renewable sources in new buildings and in existing buildings subject to major renovation in building regulations and codes by 31 December 2014 (Renewable Energy Directive, 2009). District heating and cooling with a large renewable share will be also accepted. A benchmarking mechanism for buildings is the principle of cost-optimality. The EPBD recast prescribes that Member States shall determine the cost-optimal levels of minimum energy performance requirements based on a comparative methodology framework established by the Commission (EU, 244/2012, 2012). If the gap between the calculated cost-optimal levels and the current requirements exceeds 15%, the Member States should plan steps to approach the cost-optimal levels. The cost-optimal methodology can also assist Member States in determining the nearly zeroenergy requirements. BPIE recommends setting the nZEB requirements in a corridor, where the upper limit is the cost-optimal and the lower limit is the best available technology (BPIE, 2011a). MS can decide their individual position within the corridor depending on the national circumstances. It is expected that currently there is a gap between cost-optimal and nZEB levels, but by 2021 this gap will narrow due to the increase in energy prices and decrease in technology costs. A core element of the cost-optimal methodology is the definition of reference buildings (EU, 244/2012, 2012). The effect of different energy saving measures is assessed for these buildings, and the associated primary energy demand and global costs are calculated. According to the Directive (EU, 244/2012, 2012) reference buildings shall be established for single-family buildings, apartment blocks and multifamily buildings, office buildings, and for other non-residential building categories for which specific energy performance requirements exist. Usually only a few reference buildings per building category are analysed, and these reference buildings are used for validating the pre-set requirements. However, this approach does not consider some special features of real buildings, such as the effect of geometry, orientation, shading etc. The nZEB requirements should be demanding, but at the same time they should be realistic. Defining the requirement system stipulating the sustainable development of the building sector based only on a few reference buildings is highly risky: if the requirements are too ‘soft’, it will not stimulate energy saving and the European targets will not be met, but if the requirements are too strict, the effectiveness of the regulation will be compromised if many buildings cannot comply using rational solutions. This paper shows a methodology for defining the requirements for nZEB with a bottom-up approach, based on the analysis of a large, artificially generated sample of buildings. As a consequence of the large number of combinations, the effect of many parameters can be studied on the energy balance. The use of a building sample instead of a few reference buildings may be very helpful, for example, for defining energetic requirements, benchmark values or labelling categories. The methodology was applied in Hungary when proposing the requirements. The structure of the paper is the following: first, approaches for setting the nZEB requirements in the European Union, and

methods for selecting reference buildings are summarised. In the second part, our fundamental considerations for setting the requirements are established. Then the methodology is presented in detail, and finally the usability of the method is demonstrated on the example of the proposed Hungarian requirements. In this paper only residential buildings are shown, but the method was also applied for offices and educational buildings. It has to be mentioned that the methodology presented here was applied in a background study for setting the requirements prepared for the Ministry, but the requirements have not been officially approved (Zöld et al., 2012, 2013).

2. Background 2.1. nZEB principles in the European Union In existing highly efficient building concepts, the high level of insulation, efficient windows, high level of airtightness and balanced mechanical ventilation are typically combined with passive and active solar measures and other renewable energy sources. The advantage of the qualitative definition for nearly zeroenergy buildings in the EPBD recast is that it leaves room for the Member States (MS) to adapt the definition to their specific conditions and climate. However, the level of ambition, the emission thresholds and renewable share remain vague and there is a risk that MS will defy common understanding. To assist the Member States in developing a uniform approach for implementing nZEB, BPIE has identified 10 main challenges (for example the consideration of both energy and CO2 targets, the time and local disparities of energy consumption and production, the inclusion of household electricity, the balance of energy efficiency and renewable energy, etc.) and has defined some common principles for sustainable and realistic nZEBs (BPIE, 2011a): – a threshold for the maximum allowable energy need should be defined; – a threshold for the minimum share of renewable energy demand should be defined; – a threshold for the overarching primary energy demand and CO2 emissions should be defined. BPIE has derived that the maximum CO2 emission of a new nZEB should not exceed 3 kg/m2 a to reach the long term EU targets, and suggested a corridor between 50% and 90% (or 100%) for the share of renewable energy in the total energy demand. This is the result of a top-down analysis (BPIE, 2011a). Simulations were carried out on two reference buildings for three European climates to validate the proposed principles (BPIE, 2011a). The thermal characteristics of the building envelope were chosen to be significantly better than actual standards but above the best available technology and close to the economic feasibility. Combinations of a number of HVAC options were evaluated. BPIE also proposed nZEB definitions for Bulgaria, Romania and Poland (BPIE, 2012a, 2012b, 2012c). For every country, many combinations of measures were analysed for three reference buildings from a technical and financial point of view. The conclusion was that it is possible to achieve the nZEB levels even without changing the most common building shapes. The proposed requirements are, however, less ambitious than the requirements proposed for Western Europe, because affordability was considered with a larger weight. In Hermelink et al. (2013) dynamic simulations were carried out for a single-family house and an office building for four representative European climates, and meteorological data based

Please cite this article as: Szalay, Z., Zöld, A., Definition of nearly zero-energy building requirements based on a large building sample. Energy Policy (2014), http://dx.doi.org/10.1016/j.enpol.2014.07.001i

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on the latest 15 years. Five technological packages of increasing performance were compiled in three areas (envelope, windows, passive cooling), and these were combined with various heating/ cooling elements plus photovoltaics and solar collectors. For every variant, the net primary energy demand and the global costs were calculated for 2010 and 2020, and the cost-optimal zone and the nearly zero-energy zone were identified. The benchmark area for the nearly zero-energy zone was indicated by choosing the variant with the minimum net primary energy, then increasing the energy by 15–25 kW h/m2 yr, and at this level choosing the variant with minimum global costs and increasing the global costs by 15–25%, so that this area contains about 200 variants. REHVA elaborated a calculation framework and system boundaries to assist MS to define nZEB in a uniform way (Kurnitski et al., 2011). The European neZEH project collected and analysed national nZEB definitions (Kurnitski et al., 2014), and revealed large differences in content (e.g. not all were based on primary energy) and ambitions level, for example nZEB primary energy values varied between 20 and 200 kW h/m2 yr in the ten analysed countries. This high variation was observable even for the same building type in countries with similar climate. 2.2. Reference buildings A reference building is either a hypothetical or a real building representing “the typical building geometry and systems, typical energy performance for both building envelope and systems, typical functionality and typical cost structure in the Member State and is representative of climatic conditions and geographic location” (EU, 244/2012, 2012). Reference buildings are used for example to evaluate the effect of energy saving measures on typical buildings or to develop benchmark energy consumption (Olofsson and Mahlia, 2012; Attia et al., 2012; Wan and Yik, 2004; Hernandez et al., 2008; Aste et al., 2013). Hrabovszky-Horváth et al. (2013) set up a generalised building typology for the Hungarian residential building stock to assess the climate change mitigation potential and vulnerability of buildings to climate change. Reference buildings are also required by the EPBD recast for determining the minimum energy requirements. The cost-optimal methodology requires reference buildings for at least three building categories: single-family buildings, apartment blocks/multifamily buildings, and office buildings (EU, 244/2012, 2012). Additionally, reference buildings for other non-residential building categories shall be created for which specific energy performance requirements exist (EU, 244/2012, 2012). For each building category at least three reference buildings shall be defined (one for new buildings and two for existing buildings), i.e. altogether at least 9 reference buildings are necessary (EU, 244/2012, 2012). The regulation allows a reference building to be used for more than one building category if that is justifiable, and the number of reference buildings can be reduced this way (EU, 244/2012, 2012). Reference buildings can be defined by dividing the building categories into subcategories (based on e.g. size, age, cost structure, construction material, use pattern or climatic zone) (EU, 244/ 2012, 2012). Reference buildings should represent the typical and average building stock as accurately as possible (EU, 244/2012, 2012). However, currently there is no standardised methodology for the creation of reference buildings (Corgnati et al., 2013). Corgnati et al. gives a comprehensive overview on the state-of-the-art in reference buildings. Two recent European projects, TABULA (Loga and Diefenbach, 2010) and Asiepi (Spiekman, 2010) have focused on developing a harmonised structure for building typologies and a set of reference buildings. TABULA distinguished three ways to classify reference buildings:

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a) example building: a fictional building based on experts’ judgement, regarded as typical; b) real building: the most typical real building in a certain category with average characteristics regarding geometry, materials, service systems and energy carriers. c) theoretical building: a statistical composite of the features found within a category of buildings in the stock (ECBS, 2004). The difficulty in defining reference buildings is usually the lack of information. According to Corgnati et al. and DOE, data collection is necessary in four main areas: form, envelope, system and operation (Deru et al., 2008, 2011). Data collection is usually based on official statistics and experts judgements. The problem is the limited availability of statistics. For many parameters, only aggregated statistics are available, while for other features there are no statistics available at all (e.g. use patterns). Additionally, even national statistics only cover a sample of the building stock. Corgnati et al. (2013) established a general methodology for the creation of reference buildings. In a comprehensive Italian research on office buildings two reference buildings were defined for the existing building stock, based on statistics and surveys. In the paper, the medium office building was modelled which can be regarded as representative for office buildings built after 1971. It has a rectangular plan with 2400 m2 floor area on five floors and is built with traditional construction techniques. The size of openings was assumed to be different depending on the climate. Altogether four buildings (one for existing buildings and three for new buildings) were examined with identical shape, area and systems, but different building envelope and plan layout, and standard values for the use of the building. The aim was to determine the benchmark energy consumption for a typical Italian office building. In ASIEPI pilot comparison studies were carried out on a typical single family house (Spiekman, 2010). European MS followed different approaches when determining, and later revising the maximum primary energy demand according to EPBD. One typical approach is to set a threshold and then verify it on some reference buildings. For example BPIE created two example buildings as reference buildings to check the nZEB principles for three European climates: a single-family house and a multi-storey office building to represent non-residential categories (BPIE, 2011a). According to an extensive evaluation of the European building stock (BPIE, 2011b), residential buildings represent 75% in terms of floor area, 64% of them are single-family houses and 36% are multi-family buildings. The single-family house was chosen as this category is regarded as the most critical due to the large envelope to volume ratio. The chosen office building can also represent a multi-family building with respect to size. In BPIE (2012a, 2012b, 2012c) a multi-family house was also selected, while in BPIE (2011a) the same model was used for offices and multi-family houses. In Hermelink et al. (2013) two reference buildings were considered. The residential building was a terraced 2-floor house located at the end of the row with a net floor area of 117 m2, which was chosen because of its large envelope to volume ratio and large share in new housing constructions in Europe. The other reference building was a medium size office building with 4 floors and a total area of 924 m2. As it can be seen, normally only a few reference buildings are selected in the literature based on a statistical analysis or experts judgements, and these are used to define a benchmark energy consumption or set requirements. The building stock is, however, very heterogeneous in every aspect. Modelling the building stock as accurately as possible would be an option for more reliable results, but such a model would be very complex and unrealistic (Corgnati et al., 2013). But using just one or two buildings per

Please cite this article as: Szalay, Z., Zöld, A., Definition of nearly zero-energy building requirements based on a large building sample. Energy Policy (2014), http://dx.doi.org/10.1016/j.enpol.2014.07.001i

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category is the other extreme. A building built with identical structures and systems may have a very different specific energy consumption than the reference building of the category, due to different size or height, but also due to orientation and shading. We should also consider that buildings deemed to be typical today may not be typical in the future. Building trends are changing, even if not at the same as rate as fashion. A typical house varies widely depending on the construction period and location. While the results of a specific building might be useful to help make a sound decision in a concrete case, the wider implications are questionable. The data refer to a specific geometry, to a specific window ratio and to a specific floor plan, etc. These parameters influence the energy demand, hence it is difficult to draw general conclusions from this type of analysis. In our opinion, it is risky to choose a few typical buildings, assume a high thermal performance, ideal on-site and off-site conditions, add various renewable energy sources, and justify the future nZEB requirements based on these results. In this case there is a chance that the requirements will be too strict and many new buildings will struggle to comply. On the other hand, requirements that are too soft are also to be avoided, so that the ambitious EU targets regarding renewable shares and CO2-emissions can be achieved. Many experts argue that building geometry is losing its importance with the introduction of highly efficient buildings, which would justify selecting only a few reference buildings. In many MS, current requirements are defined depending on the geometry. This means that higher energy values are allowed for buildings with an unfavourable geometry, for example as a function of the heated floor area or the envelope-to-heated volume ratio (e.g. current Danish, Hungarian, Polish regulations). The notional building method also takes into account the geometry by defining that the notional building must have the same size and shape as the actual building (SAP, 2012). A different approach is to define a threshold independently from the geometry (e.g. passive house requirements) (Feist). This can be justified by the fact that with increasing thermal performance of the building envelope, ventilation losses, which are independent from the geometry, will exceed the transmission losses. However, experience suggests that for single-family houses it is very difficult to comply with the passive house standard in a more severe climate. Others argue that it is unfair to allow higher values for buildings with an unfavourable geometry, since the geometry can be chosen by the investor and architect, especially for new buildings (BPIE, 2011a). They conclude that the requirements should be independent from the geometry, with a possible exception for single-family houses with a very small floor area. However, for the existing building stock they recommend a further analysis of the geometry aspects to avoid extra unfair burdening of the building owners. It is true that compact building shape is an important design factor for new nZEBs. However, there are many constraints which make it necessary to diverge from the ideal geometry. The size and shape of the plot, the rules of positioning the building on the plot are given parameters, just like the layout of the streets, the accessibility of the plot, the surrounding buildings and the topography of the settlement. Another important aspect is that the penetration of daylight is about 5–6 m in the interior at Central European latitude in the winter, which limits the building depth if large unventilated and dark spaces are to be avoided. Also, an architect has no freedom to pick the building category: if the client wishes a single-family house, it is not his responsibility to convince the client to choose an apartment instead, which would have a much more favourable envelopeto-volume ratio.

In this paper, we analyse the influence of building geometry and some basic parameters on the energy use and suggest using a large building sample instead of a few reference buildings when setting requirements or benchmark values.

3. Methodology 3.1. Fundamental considerations for setting nZEB requirement According to the EPBD recast a nearly zero-energy building:


has a very high energy performance,
the amount of energy required should be nearly zero or very low,


the energy required should be covered to a very significant extent by energy from renewable sources (including energy from renewable sources produced on-site or nearby). Regarding the first item, the requirement is obvious and can be guaranteed by a set of elementary requirements: by restricting the U-values of the building envelope elements the heating energy need can be reduced. Nevertheless other factors are to be considered as well: the utilised passive solar gains, which depend on the thermal mass, orientation and solar access of the glazed elements. It is recommended to introduce a sub-threshold that relates to the building as a whole and that is based exclusively on data attributed to the building itself (geometry, U-values, g-values, orientation, solar access, thermal mass, etc.). The second item is difficult to interpret: even if the building has a very high energy performance, the energy need cannot be nearly zero, because this partly depends on the users’ behaviour and expected comfort levels. The better the energy performance of the building shell and the efficiency of the service systems is, the more prevailing the net energy need of the hot water supply will be in the total energy balance. The specific hot water demand depends on the users’ behaviour, the occupant density and the type of appliances and faucets. The input data should be based on statistics. However, there are great variations between the MS in standard input data from about 12 kW h/m2 yr to 30 kW h/m2 yr, which makes it also difficult to compare the energy certificates from different countries (Hermelink et al., 2013; Feist; TNM, 7/2006, 2006). The reasons for the deviations are the floor area per capita values (approx. 25 to 35 m2/person), but also the per capita values, which depend on the user preferences, and also on the use of low flow showerheads and faucets (Hermelink et al., 2013). Obviously, heating and cooling energy need also depend on the users, but nevertheless on one hand the basic input data (temperature, air change rate) are more or less accepted conventional values, on the other hand in residential buildings they represent a smaller fragment of the total energy need. As far as the third item is considered certainly the main goal is not the consumption of renewable energy but to cover a rational part of the needs by renewable energy (pro forma wherever the source is: on-site, off-site, nearby). Nevertheless, the measure of “significant” as well as the reference value is not defined. When determining the renewable energy share, regulations must consider not only the possibilities but the constraints as well. With a high renewable energy share, it is technically possible to reach zero or even positive energy buildings, which produce more energy than what they consume. However, there are some limitations in the renewable energy potential of buildings. It is usually possible to design a free-standing detached house in a way to have favourably orientated, large, unshaded exposed surfaces for passive and active solar utilization. Even in a new urban area, solar access may be maximised by conscious urban planning

Please cite this article as: Szalay, Z., Zöld, A., Definition of nearly zero-energy building requirements based on a large building sample. Energy Policy (2014), http://dx.doi.org/10.1016/j.enpol.2014.07.001i

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(but at a cost of the lower density of buildings). However, in an existing dense urban setting, the on-site solar potential will be limited due to the obstruction by the surrounding buildings or sometimes due to the topography. The orientation of buildings must be adjusted to an existing network of streets (Zöld and Szalay, 2013). The potential of biomass (wood, wood chips, pellets), which may be regarded as an off-site energy carrier but used on-site, depends on the location of the building: the emissions may contribute to the development of smog in a dense urban area, and also the large space requirement for fuel storage is problematic. Although there are realised examples for using biomass in an urban area (e.g. the Renewable Energy House in the heart of Brussels), this would not be feasible on a large scale. Regarding heat pumps, the availability of the heat source is a question (ground heat, thermal water, water or air), and the extent of the space in an urban area will put a constraint on the feasibility (e.g. for borehole heat pumps or horizontal pipes). The input energy is usually electric power, the primary energy factor of which varies in the MS. These constraints are gaining importance by the fast increase of urban population. According to UN, (2009), by 2025 in most EU countries the urban population will be higher than 60%, and in 9 MS it will exceed 80%. The potential for on-site or nearby renewables in an urban area is limited and this restriction affects more and more buildings. There are two options for considering this problem in a building energetic regulation: – define the requirements so that also urban buildings can comply with their limited on-site or nearby renewable potential. This way the requirements will be less demanding and the renewable potential may not be fully utilised in favourably located buildings. – include also off-site renewables, for example district heating based on or supported by biomass or wind electricity. Off-site renewables may considerably improve the primary energy consumption; however the availability of such a system is beyond the responsibility of the designer. Obviously, the renewable share in the national electricity mix can be considered, but other off-site renewable energy is a bonus that cannot be guaranteed everywhere. Therefore, in our opinion the requirement cannot be based on the availability of off-site renewables. Also, only physical technical solutions should be considered: contracts on the use of “green” electricity, investments in remote wind turbines and similar ones are not subjects of a technical regulation even if these may be useful measures on a large scale policy. In certain countries suppliers provide green electricity, but these types of contracts can be changed more easily than a built-in equipment. Regarding renewables, in our view the application of one and only one system based on on-site renewable energy source can be expected in Hungary. It is the task of the designer to decide what this source is depending on the local circumstances, the use of the building and the financial issues. Thus any kind of direct prescription (e.g. x m2 collector, y m2 PV array) should be excluded. Certainly the use of more on-site renewable source is appreciated but cannot be expected considering the technical and financial aspects.

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building service system and an expected share of renewables should not exceed the threshold of specific yearly primary energy consumption in order to guarantee the truthfulness of the regulation and to prevent actual fraud in the documents submitted for building permit. (95% is a proposed conventional value, however other figure can be agreed.) When defining the requirements, we recommend analysing a large number of buildings, partly with regard to the energy demand, partly to calculate the available supply from renewable sources. Even if the shape of the building may have a less significant influence on the heating energy demand for nZEBs, some new aspects arise, such as the ratio of the available roof space for active solar energy utilization to the floor area. In this approach, not one or a few reference buildings are selected, but a population of buildings with different geometric features and orientation is analysed. If the average of the primary energy consumption of this population would be taken as the requirement, then half of the sample buildings would fulfill it, the other half would not. Defining the confidence interval of the data at given percentiles, the requirement can be defined so that a given percent of buildings, e.g. 90%, will fulfill it (Fig. 1). The remaining 10% of buildings are either unfavourably located or they are not typical designs, which need extreme energy saving measures or systems. The border of the confidence interval is subject of consideration; the percentiles can be 20% as well as 5%. The population can be based on the statistical data of the existing building stock, but here a different approach was followed according to the principles set in Szalay (2008a). The goal was to determine the requirements for new buildings. Existing typical buildings may not be typical in the future, as described in Section 2.2. Therefore here a large population of “technically feasible” buildings was generated by assigning random values to the basic parameters. The parameters were assumed to have a uniform distribution in a given range (with MS Excel). The analysed population consisted of 6000 buildings, 1000 buildings for each building type. The outline of the method is the following: first the sample was generated considering some basic assumptions. The primary energy demand and renewable share of the sample were calculated for various HVAC and renewable systems, and primary energy requirements were proposed based on these results. The method is illustrated for the case of Hungary, and for the residential buildings category. This method was used to determine the technically feasible requirements. Then a financial analysis was also carried out, which considered the cost-optimality aspects, and finally the two results were harmonised to suggest an nZEB definition. This paper concentrates on the technical study.

3.2. Calculation method The requirement system should be demanding: rational threshold values near zero should be approached. At the same time the requirements must be realistic: most (e.g. 95%) of future buildings which fulfill the elementary requirements and have a contemporary

Fig. 1. Illustration of the method, representing the mean and 95% percentile of the sample.

Please cite this article as: Szalay, Z., Zöld, A., Definition of nearly zero-energy building requirements based on a large building sample. Energy Policy (2014), http://dx.doi.org/10.1016/j.enpol.2014.07.001i

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3.2.1. Basic input data Hungary has a typical European continental climate with warm, dry summers and fairly cold winters. The cca 3000 heating degree days lie in the middle range in Europe (BPIE, 2011a). There is only one climate zone due to the small temperature variations in the country. For the calculation, the official Hungarian method was applied (TNM, 7/2006, 2006), which is a seasonal quasi-steady-state method based on EN 13790. The heat losses towards ground were calculated according to EN 13370:2007. An indoor temperature of 20 1C was assumed all day. Internal dimensions were applied, and some assumptions were made according to the Hungarian regulation (e.g. heat losses due to thermal bridges, system losses, auxiliary energy demand). The basic input data are summarised in Table 1.

allowable primary energy demand is a function of the building envelope surface to heated volume ratio in the current Hungarian regulation. Appliances are not included.

3.2.3. Building geometry First, six different residential building categories were distinguished, which are typical in Hungary:









3.2.2. The current Hungarian regulation The current Hungarian regulation has three threshold levels (TNM, 7/2006, 2006). The building complies if all levels are fulfilled. – Thermal transmittance of building elements, U (W/m2 K). Maximum U-values are set for each building element, e.g. external walls, roof, floor, etc. – The specific heat loss coefficient, q (W/m3 K), is the sum of the transmission losses incl. thermal bridge effect minus the utilised passive solar gain for the heating season, for 1 K temperature difference, divided by the heated volume of the building (W/m3 K). This coefficient includes all building-related parameters, and no parameters which depend on the use of the building (infiltration losses are not considered here, as in new buildings fresh air requirements of occupants exceed the infiltration losses). The allowable specific heat loss coefficient depends only on the surface-volume ratio of the building, it is independent from the function of the building. – The primary energy demand, EP (kW h/m2 yr). This is the amount of energy estimated to meet the needs associated with a standardised use of the building, including system losses and self-consumption of the system for space and hot water heating, ventilation, cooling and lighting (neglected for residential), from which the generated own-energy provided by photovoltaics, solar collectors or co-generation can be substracted. Different requirements apply for different uses. The Table 1 Basic input data for residential buildings (TNM, 7/2006, 2006). Air change rate

0.5 1/h

Internal gains Net energy demand for DHW

5 W/m2 30 kW h/m2 yr

one-storey detached houses; two-storey detached houses; three-storey low-rise apartment buildings (one staircase); four-storey low-rise apartment buildings (one staircase); six-storey medium high-rise apartment buildings (one staircase); ten-storey medium high-rise apartment buildings (one staircase).

The relevant parameters that were selected to describe the geometry of buildings:









net area of one floor; number of heated storeys; compactness of the floor plan; ceiling height; window-to-floor area ratio; window frame factor.

The realistic ranges of these parameters describing the technically feasible buildings were determined based on statistics, functional and architectural considerations (Table 2). The ranges in floor area were based on statistics. The compactness of the plan was described by the proportions of the ‘equivalent rectangle’ (Szalay, 2008a). The equivalent rectangle is a rectangle having the same perimeter and area as the actual floor shape. The depth of the equivalent rectangle mirrors the average building depth on the one hand and the complexity of the plan on the other hand. Excluding atypical circular buildings, a quadratic floor plan can be considered to be the most compact. For example, for a building with a floor area of 81 m2, the maximum depth of the equivalent rectangle would be the square root of the floor area (9 m). The building depth was limited to 14 m to allow for sufficient daylight penetration. The minimum depth of the equivalent rectangle is determined by functional limitations, e.g. economical floor spans and functional limitations. It was assumed to be 6 m in detached houses, and 8/9 m in apartment buildings (Szalay, 2008a, 2008b). The area of the window is given as a fraction of the floor area. According to the Hungarian regulation, the minimum glazed surface is 1/8 of the floor area. A window ratio between 20% and 30% of the floor area was assumed. The area of the glazing differs from the area of the window, and the ratio between them is

Table 2 Classification of residential buildings and the ranges of the main input data. Floor area Astorey (m2)

Number of storeys

Depth of equivalent rectangle (m)

One-storey detached houses

60–180

1

6

Two-storey detached houses

60–120

2

Three-storey low-rise apartment b

120–400

3

Four-storey low-rise apartment b.

120–400

4

Six-storey medium high-rise apartment b

120–400

6

Ten-storey medium high-rise apartment b

120–400

10

pffiffiffiffiffiffiffiffiffiffiffiffiffi Astorey pffiffiffiffiffiffiffiffiffiffiffiffiffi 6 Astorey pffiffiffiffiffiffiffiffiffiffiffiffiffi 8 min(14; Astorey ) pffiffiffiffiffiffiffiffiffiffiffiffiffi 8 min(14; Astorey ) pffiffiffiffiffiffiffiffiffiffiffiffiffi 9 min(14; Astorey ) pffiffiffiffiffiffiffiffiffiffiffiffiffi 9 min(14; Astorey )

Ceiling height (m)

Window-to-floor ratio (%)

Frame ratio (%)

2.7–3.0

20–30

20–30

2.7–3.0

20–30

20–30

2.7–3.0

20–30

20–30

2.7–3.0

20–30

20–30

2.7–3.0

20–30

20–30

2.7–3.0

20–30

20–30

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described by the frame fraction, which is approx. 20–30% for modern windows. Based on these geometric parameters the area of the building elements and the volume of the building could be calculated. Buildings were assumed to have an unheated basement. Detached houses were calculated with an unheated loft, while for apartment buildings flat roof was considered.

3.2.4. Building envelope Nearly zero-energy buildings should have a very high energy performance. The Hungarian Chamber of Engineers proposed maximum thermal transmittance values for the building envelope of nZEBs (Table 3), to be reached by gradual tightening of the requirements by 2019/2021. These values were based on the Passivhaus experiences and policies in the neighbouring countries. According to the analysis of the national minimum energy performance requirements (Severnyák and Fülöp, 2013) as requested by EU, 244/2012 (2012), the U-values currently in force are more than 15% higher than the cost-optimal values (Table 3). The cost-optimal values will enter in force from 1st of January 2015 for publicly subsidised energy-saving investments and from 1st of January 2018 for other buildings (BM, 20/2014, 2014). In this paper, the U-values proposed for nZEB were applied for every analysed building.

3.2.5. Orientation and shading of windows Reference buildings (except for detached houses) were assumed to have an East-West orientation and average shading (Table 4). This is a conservative assumption that can usually be fulfilled even in a dense urban situation. However, if the given building has large North-facing windows or shaded to a large extent, the thermal insulation level may need to be increased to comply with the requirements. For buildings with large Southfacing windows less thermal insulation is needed. Detached houses were assumed to have a more favourable orientation (Table 4), since in a free standing case architects typically have more freedom in the positioning of the building on the plot and the distribution of windows than in an urban situation.

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3.2.6. Technical building systems For the technical building system several options were considered (Table 5). Space and water heating with a condensing gas boiler was chosen as the base case, and the energy demand of combinations with heat recovery ventilation, solar thermal collector, biomass boiler, heat pump and photovoltaics were calculated (Table 6). Also the losses due to heat distribution, storage and control and the auxiliary electricity demand were taken into account according to the Hungarian regulation. For the heat pump system low temperature floor heating was assumed for a better system efficiency. No mechanical cooling was considered, but the risk of overheating was checked according to the Hungarian regulation. Energy demand of lighting was neglected. This is allowed for residential buildings in the EPBD. The primary energy factors are egas ¼ 1, ewood ¼0.6, eelectricity ¼ 2.5 and for heat pumps eelectricity ¼1.8 in the Hungarian regulation (TNM, 7/2006, 2006). 3.2.6.1. Solar thermal collectors. In some variants, hot water production was supported by solar thermal collectors. Selective flat plate collectors were assumed, and sized based on a solar share of 60–70% for detached houses and 30–50% for apartment buildings (Naplopó, 2008). The main input data are summarised in Table 7. Solar space heating was not considered, as it is not expected that this will become cost-efficient in Hungary in the near future. A fraction of the total roof area cannot be used for solar utilization, since in practice the typically large number of chimneys, flues, elevator engine rooms etc. make the installation complicated and cause overshading (Zöld et al., 2012). 3.2.6.2. Photovoltaic panels. Energy production from photovoltaic panels was calculated with a simplified method based on the nominal power of the PV panel. This depends on the type and the surface area of the panel. Here multi-silicon PV panels were assumed (approx. 8 m2/1 kWp). According to measured data, in Hungary the yearly electricity generated by an optimally inclined 1 kWp PV system is approx. 1100 kW h/kWp with a performance ratio of 0.75. The yearly electricity output in kW h/yr is DGS LV Berlin BRB (2008): E ¼ PR  g PV  η  APV  k  ð1  Z PV Þ

Table 3 Maximum thermal transmittance of the building elements (W/m2 K) (extract from TNM, 7/2006 (2006) and BM, 20/2014 (2014). Building element

In-force

Cost optimal

Proposed nZEB

External wall Flat roof Topmost floor under unheated loft Floor above unheated basement Window (wooden or PVC frame) Window or curtain wall (metal frame) Door

0.45 0.25 0.30 0.50 1.60 2.00 1.80

0.24 0.17 0.17 0.26 1.15 1.40 1.45

0.20 0.14 0.14 0.22 1.0 1.3 1.3

Detached houses Apartment houses

where PR is the performance ratio of the system, for a well designed system approx. 0.75; gPV is the annual solar radiation on an optimally inclined PV surface in Hungary (kW h/m2 yr); η is the solar panel efficiency (%); APV is the area of the panels (m2); k is the reduction factor due to deviation from optimal orientation and inclination, here 90–100%; ZPV is the shading of panels, here 10–30%. As the panels substitute electricity, this output must be multiplied with the primary energy factor of electricity from the grid (2.5 in Hungary) and this value can be substracted from the total primary energy demand.

Table 5 Main input data for the HVAC systems.

Table 4 Orientation and shading of the reference buildings. Building type

ð1Þ

Orientation of the windows E–W

S

N

20–30 100%

40–60 0

Rest 0

Shading (%)

Condensing boiler heating efficiency (annual average)

97–99%

20–30 20–30

Mech. ventilation, efficiency of heat recovery Mech. ventilation, electricity demand of fans Pellet boiler efficiency (annual average) SCOP heat pump (air source)

0.8 0.45 W h/m3 90% 3.3

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Table 6 Analysed variants.

V0 V1 V1a V2 V3

Space heating

DHW

Ventilation

Photovoltaics

Condensing gas boiler Condensing gas boiler Condensing gas boiler Pellet boiler Air source heat pump

Same as heating Cond. gas boilerþ solar collectors Cond. gas boilerþ solar collectors Same as heating Same as heating

Heat recovery Heat recovery Natural Heat recovery Heat recovery

No On the remaining roof surface On the remaining roof surface No No

Table 7 Input data for solar thermal collectors. Detached house

Low-rise apartment building High-rise apartment building

Location on a South-facing pitch On flat roof in rows, distance between rows b ¼ 1.75a Non-utilised area of the roof (chimneys, elevator stb.) (%) 10–20% 15–30% 20–40% 2 Collector area per users (m ) Max. 2 Max. 2 Max. 2 3 3 Tank volume per daily hot water need (m /m ) 1.2 0.8 0.8 Reduction factor due to deviation from optimal orientation and inclination (k) (%) 90–100% 90–100% 90–100% Shading of collectors (Zcollector) (%) 10–30% 10–30% 10–30%

The primary energy demand per floor area has been calculated for the statistical population, considering buildings that fulfilled the elementary requirements (U-values) as well as the specific heat loss requirement. The reference HVAC system was the condensing boiler with balanced heat recovery ventilation, but without renewables (v0). This population consisting of 6000 buildings is shown in Fig. 2 as a function of the surface-tovolume ratio. Fig. 3 shows the box plot diagram of the population as a function of the number of storeys. It can be observed that larger buildings have a lower surface-to-volume ratio, hence lower heat losses and primary energy demand. In the next versions, on-site renewable energy sources were applied (v1–v3). Net zero energy or energy positive buildings can be achieved in case of low rise buildings if solar collectors and PV arrays are mounted on the roof (Figs. 4 and 5). For larger apartment buildings the roof area is satisfactory for solar collectors, but there is no remaining space for PVs. In this option our previous statement regarding the A/V ratio and the primary energy demand is not valid any more: the lowest primary energy demand belongs to detached houses with a high available roof area to heated floor area ratio. This ratio is a new geometric parameter, which will be of high importance for nZEB buildings. In this paper, all electricity generated was assumed to be fed to the grid. In many MS the energy transported from the PV arrays to the national grid must not exceed the self-consumption of the building for a given period (but this is a man-made regulation rather than a technical constraint). Biomass systems (v2) achieve a considerable primary energy saving compared to the reference system (Fig. 6). Air heat pump solutions have a low primary energy demand (v3), less than 40 kW h/m2 yr for apartment buildings. As described previously, the application of only one on-site renewable energy source is expected in the Hungarian regulation. The least favourable option for every building category is the upper envelope of the 95% percentile curves of v1, v2 and v3 in Fig. 6, which is the v3 curve (biomass), where the number of storeys was chosen for the abscissa. The proposed primary energy requirements based on this curve are summarised in Table 8. On request, the number of categories could be reduced, for example to detached houses and apartment buildings.

Primary energy demand (kWh/m2y)

4.1. Primary energy demand

120 100 80 60 40 20 0 0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

1.3

A/V (m2/m3) 1 storey

2 storeys

3 storeys

4 storeys

6 storeys

10 storeys

Fig. 2. Primary energy demand of the population with condensing gas boiler and heat recovery ventilation (v0) as a function of the surface-to-volume ratio.

120

Primary energy demand (kWh/m2y)

4. Results and discussion

100

80

60

40

20

0 1 storey

2 storeys

3 storeys

4 storeys

6 storeys

10 storeys

Fig. 3. Primary energy demand of the population with condensing gas boiler and heat recovery ventilation (v0) as a function of the number of storeys on a box plot diagram.

For detached buildings, it is possible to comply with the proposed requirements even without heat recovery ventilation, if the available roof area is fully utilised for solar energy production (collector and PV). Nevertheless, to avoid the problems associated

Please cite this article as: Szalay, Z., Zöld, A., Definition of nearly zero-energy building requirements based on a large building sample. Energy Policy (2014), http://dx.doi.org/10.1016/j.enpol.2014.07.001i

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Primary energy demand (kWh/m2y)

60

9

Table 8 The proposed requirements for Hungary.

40 20 A/V (m2/m3) 0 0.3 -20

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

1.3

-40

Number of storeys

Maximum primary energy demand (kW h/m2 yr)

1 2 3–4 5 and more

72 60 53 50

-60 -80 -100 -120 1 storey

2 storeys

3 storeys

4 storeys

6 storeys

10 storeys

Fig. 4. Primary energy demand of the population with condensing gas boiler, heat recovery ventilation, solar collectors and PV (v1) as a function of the surface-tovolume ratio.

buildings with 4 or more storeys MVHR is essential for compliance. Up to 5 storeys, the highest primary energy demand values correspond to the biomass system. Above 5 storeys, the options with biomass and solar have about the same primary energy demand. 4.2. Recalculated results

Primary energy demand (kWh/m2y)

60 40 20 0 1 storey

2 storeys

3 storeys

4 storeys

6 storeys

10 storeys

-20 -40 -60 -80 -100

Fig. 5. Primary energy demand of the population with condensing gas boiler, heat recovery ventilation, solar collectors and PV (v1) as a function of the number of storeys on a box plot diagram.

Primary energy demand (kWh/m2y)

120 100 80 60 40 20 No of storeys

0 1

2

3

4

5

6

7

8

9

BPIE (2011a) recommended that the primary energy demand should be below 40 kW h/m2 yr. We proposed higher values for Hungary. One reason is the high net hot water demand (30 kW h/ m2 yr). The Hungarian data are based on statistics, but it is likely that these will decrease in the future with the widespread use of water saving faucets. Another reason is that some Hungarian primary energy factors are higher than in other MS. For example, in Hungary the primary energy factor is 0.6 for biomass to take into account that forests are slowly renewable sources, while in other MS typically 0.2 is applied. Also, we calculated with current primary energy factors for electricity, but the BPIE studies considered a future projection. To validate the method against the BPIE values, we recalculated the results with the basic input data from BPIE (Table 10). We assumed a net hot water demand of 15 kW h/m2 yr for detached houses and 20 kW h/m2 yr for apartment buildings (Hermelink et al., 2013). For the renewable shares and primary energy factor for electricity, the 2011–2040 average values were considered based on the renewable energy projections of the Energy Environment Agency and ECN for EU 27 (BPIE, 2011a, Table 9). The renewable share was calculated according to the REHVA principles (Kurnitski et al., 2013) as the produced renewable energy on-site (including the renewable share of heat pumps), nearby or off-site, divided by the delivered total primary energy (minus the exported total primary energy). The reference system (v0), as expected, would not comply with the proposed thresholds. Solar houses (v2) show very good results up to 3–4 storeys, but for 6–10 storey buildings the renewable share is only about 20%. Biomass systems (v3) comply with all the requirements with the lower primary energy factors. For heat pumps, CO2 emissions are acceptable in smaller buildings, and the other results are good.

10

-20

5. Conclusions and policy implications

-40

Fig. 6. 95% percentiles of the primary energy demand of the building population as a function of the number of storeys for the analysed variants.

The paper analysed the nZEB definition from various aspects and proposed a methodology for setting ambitious but realistic primary energy requirements considering a large sample of buildings. Our method consists of the following steps:

with airtight buildings, it is recommended to apply an MVHR system or another ventilation solution for a good air quality. For 3–4 storey apartment buildings, it is possible to reach lower values than the proposed requirements even with a condensing gas boiler if both solar collectors and photovoltaics are applied. For 3-storey buildings MVHR could be abandoned in some cases. For

– basic building parameters are defined, e.g. geometry, orientation; – realistic ranges of parameters are set and a distribution is assumed; – the building sample is artificially generated; – results are statistically analysed and requirements proposed.

-60 v0

v1a

v1

v2

v3

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Table 9 Primary energy factors, CO2 emissions and shares of renewable energy of energy carriers (BPIE, 2011a).

Current primary energy factor Hungary Projected primary energy factor EU average Projected CO2 factor Projected renewable share

Unit

Off-site grid electricity

Natural gas

Wood pellets

On-site electricity

– – kg/kW h %

2.5 2.0 0.252 35

1.0 1.1 0.202 0

0.6 0.2 0 100

 2.5  2.0  0.252 100

Table 10 Results with the current Hungarian and the projected values (95% percentiles, colour codes according to BPIE (2012a).

Primary energy

Primary energy

demand HU

demand, projected projected

2

(kWh/m y)

2

CO2 emissions,

Renewable share, projected (%)

2

(kg/m y)

(kWh/m y)

v0, 1 storey

104.01

83.82

14.67

0.02

v0, 2 storeys

85.89

65.75

11.50

0.02

v0, 3 storeys

69.91

59.30

10.51

0.02

v0, 6 storeys

63.34

52.70

9.28

0.02

v0, 10 storeys

62.09

50.94

8.96

0.02

v1, 1 storey

-35.90

-23.38

0.23

2.00

v1, 2 storeys

15.03

13.98

4.12

0.81

v1, 3 storeys

37.75

34.36

6.44

0.35

v1, 6 storeys

45.14

39.26

6.88

0.22

v1, 10 storeys

46.46

39.88

6.93

0.20

v2, 1 storey

76.01

29.36

1.97

0.80

v2, 2 storeys

63.62

24.35

1.72

0.80

v2, 3 storeys

52.99

21.19

1.40

0.79

v2, 6 storeys

48.59

19.42

1.34

0.79

v2, 10 storeys

47.42

19.11

1.34

0.79

v3, 1 storey

62.55

51.18

6.41

0.54

v3, 2 storeys

50.88

39.90

5.03

0.54

v3, 3 storeys

40.60

34.41

4.32

0.55

v3, 6 storeys

36.89

30.67

3.87

0.55

v3, 10 storeys

35.95

29.84

3.75

0.55

In this paper the population of technically feasible new buildings was analysed, but the method can also be applied to the existing stock. The advantage of this method is that instead of a few reference buildings, the results are based on a large building population. The analysis of a large building sample will help to eliminate uncertainties resulting from the evaluation of a few typical buildings and the conclusions will have a more general validity. The method may have many applications. Here it was used to define the nZEB requirements with a bottom-up approach, instead of validating pre-set targets. The method can also assist modelling policy scenarios for the improvement of the building stock. For example, certain statistical data are available about the existing building stock, such as number of buildings in a building

category or the total floor area. Mean values and deviations may be available for certain parameters. However, building energy calculations require a lot of additional information that are not available in the statistics, such as the compactness of the building, the window ratio, orientation, shading, etc., which may be very time consuming to compile. The usual approach is to select a reference building, for which all the relevant information is available. Our method defines realistic ranges and distribution of the parameters based on architectural and functional considerations to carry out the energy calculation, which better describes the building stock than just one specimen. This approach allows energy strategies to be set, benchmark values calculated and projections to be made for future changes.

Please cite this article as: Szalay, Z., Zöld, A., Definition of nearly zero-energy building requirements based on a large building sample. Energy Policy (2014), http://dx.doi.org/10.1016/j.enpol.2014.07.001i

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In this paper, our method was applied with steady state energy calculation, which may provide less accurate results for one building, but works well on a stock level. The same approach could be further developed for dynamic calculations and combined with parametric design tools. We highlighted that although the theoretical potential for renewable energy utilization is large, there are technical constraints of renewable energy production. Especially buildings in a dense urban environment have lower potential for on-site renewable energy due to limited space and shading. At the same time, due to the population density these areas have a good potential for off-site renewables, for example a renewable based district heating. In our opinion off-site renewables should be supported on an urban scale, however, their availability is beyond a technical regulation on the building scale, at least in the near future in many MS. Hence a building energy regulation cannot be based on these sources. The analysis proved that besides the surface-to-volume ratio another parameter, the available surface for active solar energy utilization to total floor area is becoming significant for nZEB buildings. With the assumptions considered in the case study, favourably oriented one-storey detached houses easily achieved plus energy, and two-storey detached houses net zero energy balance. It is a question whether the primary energy requirements should be based on the surface-to-volume ratio or the number of storeys or should be a fixed number for all buildings of identical use. The requirements proposed for Hungary may not seem tight enough for the first sight. To prove the compliance with the EU targets, we recalculated the results with projected primary energy factors, as due to the future decarbonisation of the electricity production it is likely that primary energy factors will decrease in the EU. The new results show that the proposed requirements are in line with the EU goals. Some countries have already introduced qualitative targets to use renewable energy, especially the Southern countries. In our opinion it is important to define a minimum renewable share but it should not be specified which energy sources are expected. It is the task of the designer to determine which renewable source is ideal in a given situation. Last but not least: the requirements do not limit the possibility to build better buildings—the better quality can be approved in the certification, these prestigious buildings may be subsidised, but the requirement should not be excessive. Advanced solutions (e.g. micro CHP, heat pump driven by piston engine running with gas or with biofuel) may be used (examples exist) and they are worth promoting in buildings or centres of building complexes where professional service is guaranteed. However, the requirements should not be based on the best available technology, since at present these technologies are not affordable for the majority, may have technical constraints and may have high human resource need for the operation.

Acknowledgement The publication is supported by the TÁMOP-4.2.2.A-11/1/ KONV-2012-0041 project. The project is co-financed by the European Union and the European Social Fund. References Aste, N., Adhikari, R.S., Manfren, M., 2013. Cost optimal analysis of heat pump technology adoption in residential reference buildings. Renewable Energy 60, 615–624.

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Please cite this article as: Szalay, Z., Zöld, A., Definition of nearly zero-energy building requirements based on a large building sample. Energy Policy (2014), http://dx.doi.org/10.1016/j.enpol.2014.07.001i

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Zöld A., Szalay Z.s., 2013. Nearly zero-energy requirements and the reference buildings. In: International Conference Proceedings of Envibuild, Bratislava, Slovakia. 978-80-227-4070-8. pp. 123-128. Zöld, A., Csoknyai, T., Kalmár, F., Szalay, Z.s., Talamon, A., 2012. Requirement System of Nearly Zero Energy Buildings Using Renewable Energy (in Hungarian). University of Debrecen, Debrecen.

Zöld, A., Csoknyai, T., Kalmár, F., 2013. Adjusting the nZEB Requirements to the Costoptimality Calculation Results (in Hungarian) (2013). University of Debrecen, Debrecen.

Please cite this article as: Szalay, Z., Zöld, A., Definition of nearly zero-energy building requirements based on a large building sample. Energy Policy (2014), http://dx.doi.org/10.1016/j.enpol.2014.07.001i