The impact of future scenarios on building refurbishment strategies towards plus energy buildings

Accepted Manuscript Title: The impact of future scenarios on building refurbishment strategies towards plus energy buildings Author: Alexander Passer Claudiane Ouellet-Plamondon Patrick Kenneally Viola John Guillaume Habert PII: DOI: Reference:

S0378-7788(16)30243-2 http://dx.doi.org/doi:10.1016/j.enbuild.2016.04.008 ENB 6559

To appear in:

ENB

Received date: Revised date: Accepted date:

27-10-2015 2-4-2016 4-4-2016

Please cite this article as: Alexander Passer, Claudiane Ouellet-Plamondon, Patrick Kenneally, Viola John, Guillaume Habert, The impact of future scenarios on building refurbishment strategies towards plus energy buildings, Energy and Buildings http://dx.doi.org/10.1016/j.enbuild.2016.04.008 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

The impact of future scenarios on building refurbishment strategies towards plus energy buildings Alexander Passer

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• Claudiane Ouellet-Plamondon

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• Patrick Kenneally • Viola John • Guillaume Habert

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1) Chair of Sustainable Construction Institute of Construction and Infrastructure Management ETH Zürich Stefano-Franscini-Platz 5 8093 Zurich, Switzerland

2) Institute of Technology and Testing of Building Materials Graz University of Technology Inffeldgasse 24 8010 Graz, Austria () Corresponding author: E-Mail: [email protected]

3) École de Technologie Supérieure 1100, rue Notre-Dame Ouest Montréal (Québec), H3C 1K3, Canada

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Graphical Abstract

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Highlights  We assess several refurbishment scenarios that meet plus energy standards  Based on an LCA, the optimum refurbishment includes a high-quality prefabricated façade element  The robustness analysis includes future climate change and a shift to a renewable energy mix  The energetic payback time is between two and ten years

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Abstract Buildings account for 40% of total global energy consumption. The International Energy Agency (IEA) and the European Commission (EC) are attempting to achieve an 80% reduction in global emissions by 2050. The objectives of this paper are to identify the refurbishment scenario with the lowest environmental impact using Life Cycle Assessment (LCA) and to assess the scenario’s robustness to future climate change scenarios using a sensitivity analysis. We applied and verified the proposed approach in a residential case study of a reference project located in Kapfernberg, Austria. The environmental assessment included two façade refurbishment proposals (minimum and high quality with respect to energy), onsite energy generation (using solar thermal collectors and photovoltaic (PV) panels), one renewable future energy mix and the effects of climate change according to the Austrian Panel on Climate Change (APCC). The environmental indicators used in the assessment were the cumulative energy demand non-renewable (CED n. ren.), global warming potential (GWP) and ecological scarcity (UBP) over building life cycles. The results indicated that a high-quality refurbishment of the thermal envelope with prefabricated façade elements, including solar thermal collectors and PV panels, represented the optimal refurbishment. In terms of the environmental indicators, the high-quality refurbishment scenario is always beneficial throughout the building’s life cycle. Additionally, the sensitivity analysis of the high-quality refurbishment scenario found an increasing production of surplus electricity with increasing PV area. This surplus of energy provides greater benefit in the short term with the current energy mix. Once the energy from the grid is shifted to renewable sources, the added benefit is decreased. Therefore, it is necessary to find an optimal balance between diminishing returns due to changes in the future energy mix and the financial investment made over the lifetime of the building, especially for plus energy buildings. However the findings from this specific case study need to be evaluated for other refurbishment cases, taking into account future local climate change and energy supply mix scenarios in other regions.

Keywords: Building refurbishment strategies; prefabricated façade elements; future energy supply mix scenarios; future climate change scenarios; Life Cycle Assessment (LCA); embodied green house gas emissions (GWP); embodied energy (CED)

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1. Introduction The building sector is a top priority in terms of maximizing energy efficiency because the most cost-effective energy savings can be found in the residential and commercial building sector. Because the building sector accounts for up to 40% of global and European energy consumption, the goal of the International Energy Agency (IEA) is to achieve an 80% reduction in global emissions by 2050. In Europe, the EU Parliament approved a recasting of the Energy Performance of Buildings Directive in 2010 that requires member states to propose measures to increase the number of nearly zero-energy buildings and to encourage best practices for cost-effective transformations of existing buildings into nearly zero-energy buildings [1]. According to the Intergovernmental Panel on Climate Change (IPCC) and related groups around the world (e.g., the Austrian Panel on Climate Change (APCC)) [2], the observed changes and their causes can be described as follows: “Human influence on the climate system is clear, and recent anthropogenic emissions of green-house gases are the highest in history. Recent climate changes have had widespread impacts on human and natural systems“ [3]. Regarding future climate changes, risks and impacts, the IPCC’s AR5 states that “Continued emission of greenhouse gases will cause further warming and long-lasting changes in all components of the climate system, increasing the likelihood of severe, pervasive and irreversible impacts for people and ecosystems. Limiting climate change would require substantial and sustained reductions in greenhouse gas emissions, which, together with adaptation, can limit climate change risks“ [3]. The greatest potential for reducing operational energy is found in unrefurbished buildings built between 1950 and 1980 because many buildings were constructed during this period and have high heating demands [2,4,5]. Such buildings were also found to be the most vulnerable to climate change [6]. Therefore, a strategic refurbishment analysis is required to meet the sustainability targets of the EU and other organizations. Retrofitting wall and roof insulation provides the largest number of opportunities for energy savings in residential buildings [7-12]. For prefabricated modules, the contribution of the embodied energy to the total energy requirement is increased [13].

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The four main categories of building refurbishment are 1) heating and cooling demand reduction (e.g., insulation), 2) installation of energy-efficient equipment and low-energy technologies, 3) installation of renewable technologies and electrical systems, and 4) changes in human factors. The different approaches to energy efficiency in the use phase can be classified as follows: Low-energy buildings are designed to minimize their operating energy [14,15]. Passive houses are low-energy buildings that use passive technology (their very low heating demands are met by controlled ventilation and air heating) [9,16-20]. Net zero energy buildings (nZEB) must establish an overall balance between their energy needs, the excess renewable energy generated onsite and the energy imported from the grid [2123]. Plus energy buildings should be able to deliver more energy to the grid than they consume [8,21,24]. As buildings become increasingly energy efficient, the choice of materials, the methods of construction and end-of-life (EoL) planning are becoming more important [25-27]. When the EoL is included, the assumptions behind the model become important when comparing different construction materials and the use of attributional or consequential modelling approaches [28-31]. Because technical equipment plays a major role in nearly zero-energy passive houses and plus energy buildings, its embodied impact must be considered [32-34]. The method used to assess embodied and operational energy and related environmental impacts is LCA. The application of LCA to the building sector began approximately 25 years ago with an LCA of zero-operational-energy buildings [35] and a comparative assessment of insulation materials based on LCA standards (ISO 14040 and 14044). Various studies have contributed to life cycle inventories (LCI), such as those of energy systems, and to the widely known Ecoinvent database [36]. Recently, the application of LCA to buildings was defined by a European framework for the “Sustainability of Construction Works - Assessment of Buildings” (EN 15643, EN15978, EN 15804). According to this European framework (EN 15978), LCAs of buildings should include the construction stage (modules A1-A3); the preparation of the site for the construction of the building elements (modules A4 and A5); the operational and energy demands; the necessary maintenance, replacement and refurbishment activities (module B); and an end-of-life (EoL) scenario (module C). Recent experiences have shown that environmental product declarations (EPDs) based on these standards are soon to enter the building sector across Europe [37]. Passer, Alexander et al.

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Many LCA case studies and review papers have been published in the literature in the last year [9,21,24,25,28,38-41]. A comprehensive overview of the embodied energy and energy efficiency of buildings can be found in [42]. A comprehensive analysis of the life-cycle energy required by 73 buildings in 13 countries shows that, on average, the operating (80–90%) and embodied (10–20%) phases of energy use are significant contributors to a building’s energy demand throughout its life cycle [43]. With the overall objectives of slowing down climate change and cost-effectively transforming existing buildings in mind, the question concerns whether today’s refurbishment options remain optimal when considering future climate scenarios and changes in the energy supply mix. Climate change is likely to reduce the environmental impact of a building by reducing its operational heating demand, as stated by Berger et al. [44]. However, the environmental impact of its cooling demand will increase, and the net balance is forecasted in, e.g., the UK to produce an increase in emissions due to cooling [45]. In Austria, residential buildings must be designed so that they generally avoid the need for cooling through, e.g., the installation of shading measures and lower transparent openings; therefore, a net decrease in emissions due to a reduced need for heating of 20% is forecasted [2]. In the evaluation of a building’s environmental impact, the implication of the available energy mix is non-negligible due to the long operational phase of a building’s life cycle. The Swiss Federal Bureau of Energy (BFE) has developed future scenarios for the energy mix available in Switzerland [46], which represent the basis of the scenarios examined in this study. The objective of this paper is to evaluate different refurbishment strategies in a manner that considers future energy mix and climate change using a case study. The optimal scenario is the one with the lowest environmental impact, identified using an LCA. A sensitivity analysis is performed to evaluate the strategy’s robustness under future scenarios with a renewable energy mix and considering the effects of climate change. 2. Methods The enormous energy demands of existing buildings resulting from their very high demands for heating and the associated high operating costs signal the need to refurbish buildings in Passer, Alexander et al.

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Austria from the 1960s. The technical details of the case study were taken from a research project called “e80^3-Buildings,”1 which aimed at highly efficient refurbishments of existing buildings. Examining the future energy and climate scenarios evaluated in this paper appeared necessary for validating the refurbishment concept from the perspective of LCA. 2.1 The case study and the developed refurbishment scenarios A residential building in Kapfenberg, Austria that was built in 1961 and refurbished to a plus energy building in 2014/2015 served as a case study (Figure 1). The four-story building has a length of 65 m (eastern and western façades) and a depth of 10 m (northern and southern façades). The existing building was a typical Austrian building from the 1960s constructed using prefabricated sandwich concrete elements without any additional thermal insulation. The insulation of the basement ceiling consisted of approximately 60 mm of polystyrene, and the ceiling of the unheated attic was insulated with 50-mm wood wool panels. The old roof was a pitched roof with no insulation. The existing windows were double glazed and had a U-value of 2.5 W/m²K; in addition, no mechanical ventilation system was installed. In the existing building, each flat was equipped with a different heating system (e.g., decentralized gas heating, an electric furnace, an electric night storage heater, an oil heater, a wood-burning stove or a coal furnace). For the refurbishment of the case study building, the model includes three refurbishment strategies and five strategies for onsite energy generation for providing heat and hot water. All these options are then evaluated for today’s mix of fossil and renewable energy sources and one future renewable energy mix that includes the effects of climate change specified by the APCC [2] in the sensitivity analysis. To facilitate the comparison, a “scenario encoding matrix” was developed in which the first entry represents the energy generation option (A, B, C, D), the second entry represents the refurbishment strategy (I, II, III), and the last entry represents the energy mix; today’s scenario is denoted by “T”, and “F” represents the future energy mix and climate change scenarios (Table 1). The first scenario (I - “no refurbishment”) tests the environmental impact of the building when it remains in its present state. By renewing some parts of the façade, the building is 1

http://www.hausderzukunft.at/results.html/id5836

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kept habitable in terms of the health and wellbeing for users, and its envelope can be improved by some fraction. The second scenario (II - “minimum refurbishment”) satisfies the “OIB2- Mindeststandard” [47], an Austrian guideline that specifies the requirements for the thermal envelope of a building after refurbishment to improve its efficiency in terms of isolation and energy performance. Therefore, this refurbishment scenario establishes the building’s U-value and end energy demand within the limits set by the OIB. This minimum refurbishment also assumes that the heating system is replaced by central gas and/or district heating. In the third scenario (III - “high-quality (HQ) refurbishment”), the existing building is refurbished into an energy-plus building considering all the above measures, as shown in Figures 1 and 2. The concept of high-quality refurbishment is based on efficiency measures (such as a highly insulated, prefabricated, active energy roofs and energy façade elements with integrated building services), the use of a high percentage of energy from renewable sources and the smart integration of energy supplies for providing heat and electricity. The U-values of the building’s components in each refurbishment scenarios are detailed in Table 2.

The above options (I, II and III) for refurbishing the building’s envelope are then combined with options for technical systems (A, B, C and D). The building’s large roof and south-facing façade mean that a large area is available for active modules such as solar thermal collectors and PV panels, which can produce a large portion or all the energy demanded by the users. Future energy mixtures of 1) conventional fossil and 2) renewable energy are tested in the cases that include onsite power generation. Cases with no onsite energy generation, energy generation using solar thermal area, PV area and both solar thermal area and PV area are evaluated alongside the no and minimal façade refurbishment scenarios. The energy generation scenarios that include solar thermal area and increased PV area are tested alongside high-quality building envelope refurbishment. The increase in the outside temperature caused by global warming due to climate change is expected to primarily affect the heating and cooling demands of buildings. The effects on the domestic hot

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OIB ... Austrian Institute of Construction Technology; http://www.oib.or.at.

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water supply and household electricity demands are neglected in this study because they are expected to be very small. Additionally, according to the Austrian building code, buildings must be designed to avoid the need for active cooling (i.e., by providing passive measures such as shading devices). Therefore, this study focusses on the impact of climate change on the heating demands of buildings. The APCC Austrian Assessment Report 2014 (AAR14) of 2013 [2] includes studies that are based on the impact of climate change on heating and cooling demands in Austria; a 20% decrease in the heating demand due to climate change is predicted for 2050. By the end of the building’s lifecycle in 2070, the decrease in its heating demand will be 30% relative to the year 2010, assuming a linear change. These values are the basis for the future climate change scenarios. The future electricity mix is expected to move in steps towards renewable resources. Therefore, a comprehensive study illustrating a stepwise change in the electricity mix is necessary; such a study could be provided as part of the “Energieperspektiven 2050” of Switzerland [46]. Because there is some uncertainty in predictions of the future electricity mix, two possible electricity mixes are considered in this study to increase the quality of the sensitivity analysis, namely, a fossil-fuel-based electricity mix (matrix encoding scenario 1) and a renewable electricity mix (matrix encoding scenario 2), which are both considered in combination with the effects of climate change. 2.2 Methods for assessing the environmental performance of buildings (LCA) The methods used in this study are based on the European Standards [48,49] written by CEN/TC 350, which provide a system for assessing the sustainability of a building using a life–cycle-based approach. In this system, sustainability incorporates the building’s environmental, social, and economic performances as well as its technical and functional performances, which are intrinsically related to each other. The preferred method for assessing the environmental performance of a building is based on the LCA methodology [50,51]. A model of the building’s life cycle is designed in accordance with EN 15978 [49]. Figure 3 shows the main steps used to assess the building’s environmental performance in accordance with EN 15978. In the first step, the goal and scope of the study are defined, and the boundary of the system used in the LCA calculations is specified. A model of the building’s life cycle that includes all its stages is then established. The amounts of materials and energy used during the building’s life cycle are then quantified in an LCI. Within the modular LCI, the following life cycle stages are assessed separately: the product stage (A1-A3), the construction process stage (A4-A5), replacement (B4), the operational energy use (B6), and the EOL (C1-C4). Benefits and loads beyond the system boundary (e.g., reuse, recovery and recycling potential (module D)) were not considered. Using this LCI data, the environmental impact can be assessed and expressed using impact categories Passer, Alexander et al.

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and environmental indicators. Additionally, a sensitivity analysis is performed, in this case, on the supply considering the energy mix. In the last step, the results are summarized and reported. The goal and scope of this LCA are to compare different refurbishment strategies over a reference period of 60 years, which is equivalent to the building’s service life. Within the sensitivity analysis, the influence of climate and energy mix changes - combining the different refurbishment approaches with future scenarios – is addressed. The objective is to illustrate how sensitive each refurbishment strategy is to changes in the system. The functional unit of the LCA is 1 m2 of energy reference area per year over the building’s lifetime. All the results refer to this functional unit. In the LCI, all the necessary input and output processes, presented in Figure 4 – system boundary – are quantified; the LCI includes all the relevant elements of the building, such as the technical systems, e.g., its solar and PV systems, and operational energy.

The inputs and outputs related to the construction materials were based on the bill of quantities for each building element and are modelled using SimaPro version 7.3.3 with the included database, namely, Ecoinvent version 2.2 [36,52]. The different scenarios were modelled in accordance with the methodological modular approach of EN 15978. Consequently, only materials that are replaced or added to the building, the transportation effort and the operational energy of the building are integrated into the model. In addition, the hoisting crane used in the construction and replacement of the high-quality façade modules is considered. All the building materials are assumed to be disposed of once deconstructed; no recycling options are considered. The existing structure and preparatory work are not included in the calculations because only the impact of the refurbishment is investigated. The LCIA focuses on the non-renewable share of the CED as a critical factor in climate change and on CO2 emissions, which are based on the impact indicator GWP and which have a time horizon of 100 years (GWP 100a). The Ecological Scarcity 2006 method involves calculating the sum of the ecopoints for all the processes, and the result is expressed in UBP. This method evaluates the difference between environmental impacts and legal limits in Switzerland. The summary in Table 3 presents a detailed description of the key parameters of the building. 3. Results This section presents the results of the energy calculations from the LCA perspective and their interpretation. 3.1 Energy demand results

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Following the energy calculation standards [55], we calculated the operational end energy demanded in the different scenarios. Table 4 presents the results for domestic hot water, heating, ventilation, household electricity and PV generation as well as the total energy demanded. The results indicate that scenario III does not reach the plus energy target from an end energy perspective, which is why the authors decided to evaluate another scenario (III-D) with the PV area increased to a maximum of 900 m2. 3.2 The environmental impacts of the energy supply The choice of the energy supply mix has a very significant impact on the cumulative amount of energy demanded from non-renewable sources (measured by the CED n. ren. indicator) in different refurbishment scenarios (Figure 5). For no refurbishment, moving from the worse energy supply (50% coal, 50% oil) to 100% gas offers a 50% improvement in CED n. ren., a 76% improvement in GWP 100 and a 71% improvement in ecological scarcity (ES). The worst-case scenario was not considered in the refurbishment cases. The results clearly show that the first step in refurbishment should always be to change the heat supply (i.e., from fossil-fuel-based to district heating). However, the figure also indicates the strong potential of high-performance refurbishment, i.e., scenario III, which demands less than 30% of the CED n. ren. demanded by the no refurbishment scenario in all cases. Changing from gas heating to district heating improves the energy performance by approximately the same amount. Because district heating is modelled on waste heat and the impact allocated to it, district heating is the best option in terms of environmental performance.

3.3 The environmental impacts of the refurbishment scenarios To facilitate a detailed comparison of the scenarios, the results in terms of each environmental indicator are discussed separately, starting with CED n. ren, GWP and UPB. The results are presented for today’s reference case and the future perspective, considering a renewable energy mix and the effects of climate change. The results presented in Figure 6 (CED n. ren.) demonstrate the influence of both the different construction measures and the different technical systems. Changing from no refurbishment (I) to minimum refurbishing (II) reduces the CED n. ren. by two thirds. The solar thermal area and PV area offer incremental benefits, especially considering that the building façade refurbishment is more sophisticated. Due to its additional embodied impact, the high-quality refurbishment scenario (IIIB+C) is nearly equal to the minimum refurbishment scenario (II-B+C) for both the CED n. ren. and GWP indicators (see Figure 7). From today’s perspective, scenario III-D today shows the lowest overall impact over the building’s life cycle, being primarily due to energy savings and related benefits from energy payback in the use phase. From a future perspective, the scenario remains the Passer, Alexander et al.

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optimal scenario. However, the benefits from delivering energy to the grid are lower, and the CED n. ren. impact increases by a factor of 6. This can be explained by the fact that the grid energy is shifting towards increasingly more renewable sources, and thus, the benefits are lowered.

In the case of the GWP, changing from no refurbishment to minimum refurbishment again provides an improvement of two thirds (Figure 7). In contrast, the high-quality refurbishment strategy (III) offers only minor GWP savings because the 2010 energy mix contains approx. 40% nuclear power. Assuming this to shift towards hydro-power and other renewable sources, this strategy does not have a large influence on the GWP relative to the use of nuclear energy. The ecological scarcity indicator covers substantial more different environmental impacts compared to the single impact indicators CED n. ren. and GWP, which naturally leads to very different results when comparing these two indicators (Figure 8). In terms of ecological scarcity, in today’s term, the optimum strategy is the high-quality refurbishment, followed by the minimum refurbishment with PV area and PV area + Solar thermal strategy. Considering the future scenarios, the minimum refurbishment and the minimum refurbishment with solar thermal represent optimum strategies (II-B). This can be explained by the fact that the ecological scarcity indicator is driven by the higher contribution of the related embodied impacts (construction materials) and related weighting in the assessment method. The plus energy high-quality refurbishment (III-D) of the future scenario represents a 15% increase in the ecological scarcity indicator relative to the scenario with the lower impact (II-B).

3.4 Interpretation of the results The presented results show that the share of the energy supply mix and refurbishment strategies as well as their related embodied impacts are significant influences, which is why we would like to focus on these two issues more deeply in the interpretation of the results. The embodied and operational energies (CED n. ren.) of the different scenarios are presented in Figure 9. Adding onsite energy generation (i.e., scenarios B: solar thermal area and C: PV area) significantly reduces the amount of operational energy; however, adding PV doubled the embodied energy from the scenario with only district heating. The minimum refurbishment reduces the operational energy by more than two thirds compared to the no refurbishment scenario. The highquality refurbishment has the highest embodied energy (123-129 MJ/m2yr), but the operational energy is only 1 MJ/m2yr with solar thermal and PV (III-B+C). When increasing the PV area (scenario III-D), the total CED n. ren. becomes negative due to the benefits provided by the delivered energy produced onsite. Passer, Alexander et al.

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In the different refurbishment scenarios, every measure of the evaluated improvement is improved in terms of total environmental impact. The absolute impact of using PV area is substantially greater than that of using solar thermal area (due to the lower installed area and possible gains – assumed symmetric 1:1). The benefit gained in terms of the operational non-renewable CED is much greater than the amount of embodied energy invested in the use of PV area than in solar thermal area. Therefore, a combination of the two types of active modules results in the lowest total impact on the non-renewable cumulative energy demand (and on the other impact indicators) when scenarios using the same construction technology are compared. The other observation behind this comparison is that high-quality refurbishment always has a smaller total environmental impact than the other two types of refurbishment. According to Figure 8, the optimum refurbishment scenarios are the last four scenarios: scenario II-C, scenario II-B+C, scenario III-B+C and scenario III-D. Therefore, only scenario III, namely, high-quality refurbishment, represents a net zero-energy building (nZEB) approach. Similar aspects were observed by Berggren et al. [21], who explained that an increase in embodied energy occurs when a low-quality building is refurbished to an nZEB. When older buildings are considered, the embodied energy increases by a factor of 4 to 5. The use of prefabricated elements in multi-unit residential building retrofitting has also been found to increase the amount of embodied energy [13]. 4. Discussion European climate and energy policies foresee a substantial reduction in the energy consumption of buildings by 2020, e.g., resulting from the implementation of nearly zero-energy buildings [22]. The next logical step is to build plus energy buildings, which produce more energy than they consume on an annual primary energy basis. To evaluate the refurbishment scenarios and the embodied versus operational energy aspects, we used the concept of the payback time (PBT) described by [12,21,40,56-58]. The PBT is the ratio of the embodied energy (blue boxes) to the delivered energy (red and yellow boxes), as shown in Figure 10 and explained in equation 1: PBT = (EEA1-A3+ EEA4-A5+ EEB4+ EEC1-C4)/ OEB6

(1)

Where: EE is the embodied energy in life cycle phases including product stage (A1-A3), construction process stage (A4-A5), replacement (B4) and the end-of-life stage (C1-C4) and OE is the Operational energy (B6).

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4.1 Payback time The different refurbishment scenarios are compared by analysing their energy payback times, and the results are evaluated in relation to the existing situation (scenario I-A), presented in Figure 10, and Table 5 provides an overview of the operational and total embodied energy values for the construction phase. The distance to an intersection point along the time axis indicates the payback time of the lower impact scenario after the intersection in comparison to another scenario.

Both refurbishment scenarios (II and III) pay off after two and three years, respectively, in comparison to the no refurbishment scenario (I-A) due to the no refurbishment scenario’s huge operational energy. Comparing scenario II-B+C to scenario II-C, it can be noted that the additional embodied energy for the B+C scenarios pays off after a period of five years. For the high-quality refurbishment scenario (III), the additional embodied energy pays off after 25 years when comparing scenarios II (B+C) and III (B+C). Hence, comparing it to scenario III-D, the additional embodied energy pays off in less than 8 years due to the additional savings (delivered energy of the PV panels). This scenario is the only scenario with an absolute energetic payback, providing operational benefits after 30 years. High-quality refurbishments are always linked to larger embodied energies and lower operational energy impacts, whereas the exact payback time needs to be calculated individually for different refurbishment strategies. All four refurbishment scenarios have very low payback times in comparison with the impact of the existing building (the continuous black line in Figure 11). The minimum refurbishment scenario (II) pays off quickly due to the lower additional embodied energy and lower operational energies. The high-quality refurbishment strategy (III) requires extra time due to its additional embodied energy. In addition to its environmental payback, the economic payback of refurbishment represents a crucial issue that requires further evaluation. Recently, optimal economic payback periods of two and three years have been reported by Wang et al. [59] and Malatji et al. [60], respectively, based on optimization methods. For Serbian energy efficiency refurbishment public projects, Bećirović et al. [57] reported even longer payback periods, averaging 13 years. Silva et al. [61] presented a prefabricated retrofit module that moves towards nearly zero-energy buildings with an economic payback period of 7-9 years [61]. Because it was calculated as part of a research project, we focus on the energetic payback period. The Authors suggest including the aspects of dynamic LCA in future discussions to enable a more consistent analysis of emission flows and global warming impacts over time. Fouquet et al. [62] Passer, Alexander et al. ENB-D-15-01943_R2 15

argued that prospective LCA could provide more relevant LCA results but increases uncertainty and could be used as sensitivity analysis for long life span buildings, which also applies to refurbishment strategies.

4.2 Sensitivity analysis and comparison of the indicators The sensitivity analysis of the refurbishment options is based on a multiple-criteria decision diagram (Figure 12). The x-axis represents the ratio of the operational to the total non-renewable CED (operational and embodied). The optimum occurs when this ratio is minimized, indicating a positive impact on the payback time. The y-axis represents an absolute value and must remain minimized. Each refurbishment scenario is represented by three points with identical styles (one for the current situation and two for the future scenarios). The robustness of a scenario is determined by the vertical distance (along the y-axis) between these three points. Therefore, the closer together the points are, the less sensitive that scenario is to future changes. Thus, the overall optimal refurbishment scenario is identified by the set of results that are closest to the bottom left corner of the diagram in Figure 12 and have the narrowest distribution of values.

According to this robustness analysis, scenario III-D is the most strongly influenced by the future energy mix even though it obtains the best absolute performance (i.e., the lowest values). This is due to the surplus (PV) electricity production of the building when it uses renewable resources, thereby producing negative credits during its operational phase. Because the proportion of renewable energy resources in the future electricity mix increases significantly, these savings decrease over time. Therefore, it can be stated that the extra investment in PV area is not creditable over the building’s lifetime because of its sensitivity to changes in the electricity supply mix. In contrast, scenario III-B+C is the most robust because of the smaller differences in the total impact, which are due to the building’s improved ability to adapt to uncertain future conditions. Therefore, the authors recommend further evaluation of the primary energy factors and LCA allocation rules for delivered and exported energy, as is also recommended by [63,64]. 5. Conclusions The LCA method can facilitate the assessment of potential impact shifting between different life cycle stages and support the choice of construction material selection and design choices. Because the operational energy is constantly being reduced due to new energy efficiency regulations, embodied energy and related impacts are becoming increasingly more relevant. Based on our results, the optimal type of refurbishment is high-quality refurbishment of the thermal envelope using prefabricated façade elements, solar thermal collectors and PV panels. In terms of environmental Passer, Alexander et al.

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indicators, this type of refurbishment is always beneficial because it has the lowest environmental impact from a life-cycle perspective. In addition, a change in the energy supply to district heating is very beneficial under all construction standards and should, therefore, be rapidly implemented as a first and rather simple step. The installation of a solar system and PV provide further incremental benefits. Additionally, the sensitivity analysis of the high-quality refurbishment scenario showed that producing surplus electricity by increasing the PV area is not always feasible because the predicted operational benefits are sensitive to shifts in the future district electricity mix towards renewable resources. Therefore, especially for plus energy buildings, it is necessary to find the optimal balance between diminishing returns due to changes in the energy mix and financial investments and to embodied impacts over the lifetime of the building. Because there is a significant need to refurbish a large number of buildings due to their high energy consumption – at least in temperate climatic zones – another significant advantage provided by the high-quality refurbishment is the reduction in construction time due to its high degree of prefabrication, as shown in our case study. The use of prefabrication technology is not limited to any specific region and could include other technical building systems in other climatic zones. In any case, these façade elements are designed to ensure minimal disturbance of the inhabitants during the construction and replacement phase. However the findings from this specific case study need to be evaluated for other refurbishment cases, taking into account future local climate change and energy supply mix scenarios in other regions. The authors would appreciate it if the façade elements and refurbishment scenarios were further evaluated in terms of the choice of construction materials and their embodied impacts as well as using a dynamic time-dependent LCA. Last but not least, one must consider that high-quality refurbishment also substantially increases the comfort and acceptance of a building, which automatically results in greater acceptance from the building’s users. Therefore, decisions concerning construction measures should include LCA and aspects of resource scarcity, e.g., as presented by Wallbaum et al. [65]. In addition, such decisions need to be well-balanced and follow a holistic approach, i.e., following a building certification scheme (e.g., DGNB, BREEAM or LEED) including LCA [66,67].

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Acknowledgements The authors wish to thank all partners and Dr. Karl Höfler from AEE INTEC and for his excellent collaboration

as

the

lead

project

manager

of

the

research

project

e80^3-Buildings

(http://www.hausderzukunft.at/results.html/id5836), which serves as the basis for the case study described in this paper. This project was funded by the Federal State of Styria and the “Building of Tomorrow” program of the Austrian Federal Ministry of Transport Innovation and Technology (BMVIT) via the Austrian Research Promotion Agency (FFG). The authors deeply appreciate support from ETH Zurich (Chair of Sustainable Construction) and the Swiss National Science Foundation’s (SNSF) international co-operation programme (Grant number: IZK0Z2_154373), which made the collaboration for this paper possible. The authors would also like to acknowledge the fruitful input of all the participants in the IEA EBC Annex 57 project (http://www.annex57.org/). The preparation of this manuscript was part of the Austrian contribution to the IEA EBC Annex 57 project, which is financially supported by FFG (4069100) and BMVIT.

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Figure Captions

Figure 1: Case study residential building; refurbished building with the high-quality strategy (right) and before refurbishment (second part – left).

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Figure 2: The energy concept in scenario III (the plus energy standard). Source AEE INTEC and TU Graz

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Figure 3: The main steps in the environmental assessment workflow according to EN 15978.

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Figure 4: System boundary of the case study

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Figure 5: Impact of heating system on the CED n. ren. in each scenario

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Figure 6: CED n. ren. from today’s and future scenarios

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Figure 7: Global warming potential from today’s and future scenarios

Figure 8: Ecological scarcity from today’s and future scenarios

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Figure 9: Operational and embodied CED n. ren. of the scenarios from today’s perspective

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Use!stage!

Production!&! Construction!

Energetic!amorsation!

End!of!Life!

Harvest!period!

Payback!ratio!

Construction,!operation,!disposal!! [CED]!

Energy!generation! [CED]!

!

Start!operation!

Replacements!

Deconstruction!

Start!of! construction! works!

Figure 10: The concept of the energy payback time

! PBR!=!! !

!

!!!!!+!!!!!!!!!!+! !

!

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Figure 11: Comparison of payback times of selected scenarios in terms of CED n. ren.

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Figure 12: Optimal refurbishment scenarios – Interpretation of the results of the sensitivity analysis

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Tables Table 1: The change in energy supply scenario encoding matrix

Refurbishment scenario

Code

Scenario energy supply

A: Basis

100% Gas

B: Solar thermal area

54% District heat + 46% Gas

C: PV area

54% District heat + 46% Gas

B+C: Solar thermal area + PV

54% District heat + 46% Gas

All scenarios D:

Solar

thermal

area

+

54% District heat + 46% Gas

increased PV area

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Table 2: The U-values used in each of the three scenarios.

Refurbishment scenario I None

II Minimum

III High quality

U-value [W/m2K]

Building element External wall

0.87

0.31

0.14

Window

2.50

1.33

1.00

Top floor ceiling

0.74

0.19

0.10

Basement ceiling

0.39

0.39

0.30

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Table 3: Key building parameters. Parameter

Case study description

Location/climate

Austria/moderate climate

Building/usage type

Residential home, refurbishment

Energy standard

No, minimum and plus energy refurbishment

Gross floor area/net floor area

2 845 m²/2 240 m²

Gross volume

8 673 m³

Surface/volume ratio (m-1)

0.37 m-1

Construction method

Minimum thermal insulation system and prefabricated timber elements

Thermal insulation

Insulation of external walls, ground floor and roof

Ventilation system

None and mechanical ventilation with heat recovery (85%)

Heating and cooling system

Heating: district heating supported by an onsite solar thermal installation (140 m²) with a 7500 litre storage tank (buffer), a 2-pipe system (flow and return), and radiators in the flats Cooling: n/a in Austria

PV system Final demand for electricity

600 m² (900 m2 in the case with increased PV area) Calculated with GEQ (www.geq.at) v2012 according to ÖNORM H 5055; the results are listed in Table 2

Final demand for heating and hot Calculated with GEQ (www.geq.at) v2012 according to water ÖNORM H 5055; the results are listed in Table 2 Final demand for cooling

None

Reference study period (Tref)

60 yr

Included life cycle stages

From cradle to grave

Included parts of the building

-

Construction stage

-

Use stage

-

End-of-life stage

External walls (only refurbished parts considered) Roof (only refurbished parts considered) New doors and windows

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Installations (ventilation, equipment)

heating,

sanitary

equipment,

electrical

Scenarios and assumptions used

3 different refurbishment scenarios (none, minimum and high quality)

Electricity mix

Based on future energy supply scenarios

Database used

Ecoinvent v 2.2 [53]

LCA software used

SimaPro 7.3.3

Environmental indicators assessed

Global warming potential (GWP) [54] Cumulative energy demand (CED)[54] Ecological scarcity (UBP 06) [54]

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Table 4: The end energy demands under the different scenarios

Energy demand End energy demand [kWh/yr] Energy supply

Scenario I

Scenario II

Domestic hot water Electricity Heat from waste (54%) Gas (46%)

47,881 0 0

180 33,803 28,795

324 11,371 9,686

324 11,371 9,686

Heating Gas (100%) District heating Electricity

429,538 0 0

0 167,538 121

0 86,259 886

0 86,259 886

0

0

4,858

4,858

46,743

46,743

46,743

46,743

(-40,280)

(-40,280)

Ventilation Electricity

system

–

Household – Electricity

Scenario III

Solar system – heat (included in domestic hot water) PV system – Electricity End energy demand (total)

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Scenario III-D

0

0

1,103 -80,640

1,103 -120,960

524,162

277,180

80,590

40,270

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Table 5: Energy payback values based on CED n. ren. Stage phase Product stage [A1 - A3] Construction process stage [A4 - A5] Total embodied Operational energy use per year [B6]

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I-A 129 3 131 929

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II-C 1,957 11 1,968 66

CED n. ren. [MJ/(m2)] II-B+C III-B+C0 2,053 3,111 11 24 2,064 3,136 42 1

III-D 3,295 24 3,319 -107

43