Life Cycle Assessment of a solar thermal system in Spain, eco-design alternatives and derived climate change scenarios at Spanish and Chinese National levels

UPCommons Portal del coneixement obert de la UPC http://upcommons.upc.edu/e-prints Aquesta és una còpia de la versió author’s final draft d'un article publicat a la revista Sustainable cities and society. URL d'aquest document a UPCommons E-prints: http://hdl.handle.net/2117/134599 Article publicat / Published paper: Alberti, J. [et al.]. Life Cycle Assessment of a solar thermal system in Spain, eco-design alternatives and derived climate change scenarios at Spanish and Chinese National levels. Sustainable cities and society , Maig 2019, vol. 47, p. 1-11. DOI: <10.1016/j.scs.2019.101467> © <2019>. Aquesta versió està disponible sota la llicència CC-BY- NCND 4.0 http://creativecommons.org/licenses/by-nc-nd/4.0/ Life Cycle Assessment of a solar thermal system in Spain, eco-design alternatives and derived climate change scenarios at Spanish and Chinese National levels Jaume Albertía,⁎,1, Juliana Raigosaa,b, Marco Raugeia,c, Rafael Assiegoa,d, Joan Ribas-Tura,e, Núria Garrido-Sorianof, Linghui Zhangg, Guobao Songg, Patxi Hernándezh, Pere Fullana-i-Palmera a UNESCO Chair in Life Cycle and Climate Change ESCI-UPF, Universitat Pompeu Fabra, Pg. Pujades 1, 08003 Barcelona, Spain b Innovation Center on e-government, Ministry of Information and Communication Technologies, Bogotá, Colombia c Faculty of Technology, Design and Environment, Oxford Brookes University, Wheatley Campus, Oxford OX33 1HX, UK d ETS Arquitectura, Energy Group-University of Malaga, Pl. El Ejido s/n, 29071 Málaga, Spain e ESCI-UPF Research in International Studies and Economics (RISE), Universitat Pompeu Fabra, Passeig Pujades 1, Barcelona 08003, Spain f Sustainability Measurement and Modeling Lab, Universitat Politècnica de Catalunya (UPC), School of Industrial, Aeroespace and Audiovisual Engineering of Terrassa, Colom st., 08222 Barcelona, Spain g Key Laboratory of Industrial Ecology and Environmental Engineering (MOE), School of Environmental Science and Technology, Dalian University of Technology, No. 2 Linggong Road, Ganjingzi District, Dalian 116024, PR China h Tecnalia, Barrio Lasao, E-20730 Azpeitia, Gipuzkoa, Spain ⁎ Corresponding author. E-mail address: [email protected] (J. Albertí). 1 http://unescochair.esci.upf.edu/. A R T I C L E I N F O Keywords: Renewable energies Building Circular economy National emissions Nationally determined contributions A B S T R A C T Solar thermal energy is considered a ‘clean’ form of energy; however, environmental impacts occur during its life-cycle. The present work compares the environmental performance of two scenarios: a solar thermal system for providing domestic hot water (DHW) used in conjunction with a traditional natural gas heating system, and the natural gas heating system on its own. Weak points are found and different eco-design scenarios are evaluated in order to achieve a more circular economy. In addition, the authors explore what would be the national Greenhouse Gas emission reduction potential of a wider use of domestic solar hot water systems (DSHW) in China’s and Spain’s built environment. In this case, five displacement methods are suggested to show how the emissions reduction vary. Through a review of the state of the art and a Life Cycle Assessment of a solar system the two scenarios are assessed. Some impact categories, such as global warming, suggest a markedly better performance of the solar system (-65%). However, weak points in the solar solution have been identified as there is an increase of impacts in cases such as acidification (+6%) and eutrophication (+61%), mostly due to the metals used. The components with higher environmental impact are the collector, the tank, and the copper tubes. The reduction of national emissions by promoting DSHW depends on the actual displaced technology/ies. The consequences on national emissions reduction depending on these choices are assessed. The potential reduction of emissions, if 30% of the DHW were covered with solar sources, would be between 0.38% and 0.50% in the case of Spain and between 0.12% and 0.63% in China. 1. Introduction and background Since the beginning of the industrial age, human populations have expanded and greatly increased access to natural resources. The exponential rise in human population has been paralleled by increased agriculture, urbanization and energy consumption. In a little over than a century, humans have already consumed a large portion of the existing fossil fuels, which took millions of years to produce (Crutzen, 2002). From the 1970′s, after the oil crisis, renewable energy technologies have been developed in order to supplement, and possibly ultimately replace, oil and other fossil fuels as the main source of energy (Kamp, 2008; Schnitzer, Brunner, & Gwehenberger, 2007). These types of energy are produced in continuous and virtually inexhaustible ways, using energy sources such as: solar, wind, hydro, biomass, and geothermal (Dincer, 2010). 1.1. The European Union’s energy characteristics J. Albertí, et al. Sustainable Cities and Society Since 2004, the EU-28′s net imports of energy have been greater than its primary production (EUROSTAT, 2018). Regarding the fossil fuel sourced energy, according to the European Commission (EC) Green Paper published in 2002 (European Comission, 2002), the EU was largely and increasingly dependent on fossil energy imported from non- EU countries. In 2002, the EU was dependant on approximately: 76% for oil, 40% for natural gas, and 50% for coal. After 12 years, the energy import dependency increased, reaching up to the 87.7% for crude oil and the 70.4% for natural gas (EUROSTAT, 2018). As it has been known for many years already (Smithers & Smit, 1997), this accelerated evolution is having important global effects beyond fossil fuels depletion per se, such as climate change induced by human-released greenhouse gases, which is causing negative impacts on society and the economy. Specifically, the building sector is responsible for around one third of the final energy consumption and for around one third of the global CO2 emissions (IEA, 2018; IEA, 2013). Being aware of this situation, the European Commission set a challenge for the year 2020: all new buildings shall be nearly “zeroenergy” buildings (European Parliament, 2010, 2012), i.e., they should produce as much energy as they consume during their operational stage. Additionally, scientists have argued that the definition should be extended to also include other stages of the life cycle of a building (Blengini & Di Carlo, 2010; Hernandez & Kenny, 2010; Kylili & Fokaides, 2015; Li, Yang, & Lam, 2013; Passer, Kreiner, & Maydl, 2012). Two possible approaches to increase sustainability by reducing energy consumption in buildings can be applied: active and passive systems. In a passive building, shell systems such as windows, walls, floors, and roofs are designed to increase the building insulation in order to reduce the energy demand in the use stage, conducting to a lower environmental impact of the building in a life cycle perspective (Passer et al., 2012; Schmidt, Jensen, Clausen, Kamstrup, & Postlethwaite, 2004). However, once a building has been constructed, it is difficult to reduce its energy demand, and active solutions are required. These systems are designed to capture the sun’s energy to convert it into heat or electricity and cover the building energy demand, like solar collectors do. Different alternatives may be used to accomplish this objective and a proper comparative assessment is needed before investment (Assiego De Larriva, Calleja Rodríguez, Cejudo López, Raugei, & Fullana I Palmer, 2014). The European Commission recently issued the Circular Economy Package (European Commission, 2015) to boost global competitiveness, foster sustainable economic growth and generate new jobs. Within this global strategy, the sustainable use of resources is essential, and two energy strategies come to front: energy efficiency and renewable energy (JRC, 2015). On average, the energy use inside a residential building attributed to the operational water heating accounts for 18–25% of the buildings total final energy (EuroACE, 2004; IDAE, 2014). Domestic Solar Water Heating (DSWH) is a well-proven technology used to reduce the nonrenewable energy demand for providing operational DHW, and its potential to reduce domestic energy use is frequently acknowledged (Hernandez & Kenny, 2012). 1.2. The Popular Republic of China’s energy characteristics China’s energy development strategy can be divided into three stages since the reform and opening of the Chinese economy. In the first stage, before 1990, China’s government emphasized energy self-suffi- ciency by adopting policies such as reducing oil burning and replacing oil with coal. The leading position of coal was strengthened in China's energy supply system in this first stage. In the second stage, during the 90 s, heavy industry developed rapidly and the proportion of heavy industry exceeded 50% of China’s total industrial output value. The third stage started with the 21st century, in which the central government emphasizes energy security to meet soaring demand, and pay close attention to energy-related environmental sustainability, such as lowering carbon emissions. Nowadays, China still depends on fossil energy. In 2010, the total energy consumption was 9 × 1011 GJ, of which coal, oil, and natural gas accounted for 68%, 19%, and 4.4% respectively. New energy, which comprise hydropower, nuclear power, and wind power combined, accounted for 8.6% of the total energy consumption (NBS, 2010). According to China’s Energy Development Strategic Action Plan (2014-2020), efforts should be made to optimize the energy mix by increasing the share of low-carbon energy (The State Council, 2014). The statistics newly released by British Petroleum (British Petroleum, 2018) show that China’s natural gas consumption increased by 15% in 2017, compared with to 2016, and reached 31 billion m3 (i.e., 6.6% of 2017 total energy consumption); while solar energy grew by an amazing 76%. By 2020, the share of natural gas will contribute to above 10% according to the planning of The State Council (2014). According to the latest evaluation the China Association of Building Energy Efficiency (CABEE, 2016), the building sector consumed 20% of China’s total energy, which is approximately 15% of the energy consumption by the global building sector, and this percentage is still growing (Xiao, Wei, & Wang, 2014). To curb this rising trend, Chinese government thus formulated a series of polices. The Ministry of Housing and Urban-Rural Development planned to cut 3.4 × 109 GJ of fossilbased energy use during the 12th five-year plan (2011–2015), and 26% of this reduction was achieved from the promotion of renewable energy uses (MOHURD, 2012). In 2017, MOHURD released the “Building Energy Conservation and Green Building Development Plan” to guide energy-saving actions during the 13th fiveyear period (2016–2020). The plan set the goal towards “ultra-low energy building systems” by using cleaner energy as a key avenue. As a major energy consumer of a building, water heating system is especially encouraged to shift J. Albertí, et al. Sustainable Cities and Society to sustainable energy in China. Recently, MOHURD required to add solar systems to over 2 billion m2 when buildings are newly developed (MOHURD, 2017). 1.3. DSHW LCA case studies Life cycle assessment (LCA) and eco-design are comprehensive and integrated methodologies that allow acting in the early stages of the product-supply chain alongside the more traditional, technical, and economic criteria (Lagerstedt, Luttropp, & Lindfors, 2003). Moreover, LCA is considered as an appropriate tool to assess sustainability of products (Ness, Urbel-Piirsalu, Anderberg, & Olsson, 2007). Product design is a critical determinant of a manufacturer’s competitiveness. It has been claimed that as much as 70–80% of the costs of product development, manufacture, and use are determined during the initial design stages (Barton, Love, & Taylor, 2001). The earlier in the product design life cycle a design team considers environmental factors, the greater the potential for cost reduction, and also environmental benefits (Mascle & Zhao, 2008). In that sense, eco-design has been defined as “the systematic integration of environmental considerations into product and process design’’ (Canada, 2003) and its main advantage is that these considerations could be taken into proper account in the early stages of the design process. LCA allows the quantification of environmental impacts and the evaluation of the improvement options throughout the life cycle of a process, product or activity (Jacquemin, Pontalier, & Sablayrolles, 2012). These options could be applied in different stages of the life cycle: process selection, used materials, design, end of life disposal, and system optimization (Azapagic & Cliff, 1999). As detailed in the ISO standard 14,040 (ISO, 2006), LCA addresses the issue of quantifying environmental impacts (e.g., the use of resources and the environmental consequences of releases) throughout a product's life cycle from raw material acquisition, through production, use, end-of-life treatment, recycling and final disposal (i.e. cradle-to-grave), thereby avoiding burden shifting. These characteristics make LCA a relevant and holistic methodology that allows a correct eco-design of products (Byggeth & Hochschorner, 2006; Cerdan, Gazulla, Raugei, Martinez, & Fullana-i-Palmer, 2009). In spite of this, there are often hindrances in integrating eco-design into the practice of small and medium enterprises (SMEs) (Fullana-i-Palmer, Mantoux, Milà i Canals, & Gazulla, 2005; Le Pochat, Bertoluci, & Froelich, 2007). With SMEs, it is essential to apply the so-called Life Cycle Management (LCM) principles (Fullana-i-Palmer et al., 2011), which aim at putting LCA into practice, especially the “Good Enough is Best” principle (Bala, Raugei, Benveniste, Gazulla, & Fullana-i-Palmer, 2010). This is an aim that has been pursued by the LCA community for many years, especially within SETAC Europe developments, where LCA and LCM were even seen so distanced as to call them “two planets” (Rebitzer, Fullana, Jolliet, & Klöpffer, 2001) or, more recently, “Ebony and Ivory” (Baitz et al., 2013). A variety of eco-design strategies exists, including the reduction of the amount and diversity of materials used; the improvement of the energy efficiency during the use phase; or the design for recycling, among others. The use of these strategies will depend on the type of product or service or the objective of the company (Cerdan et al., 2009; Gazulla, Raugei, & Fullana-I-Palmer, 2010; Lück, 2012; Muñoz, Gazulla, Bala, Puig, & Fullana, 2009; Platcheck, Schaeffer, Kindlein, & Cândido, 2008). The application of these strategies may entail saving raw materials and energy, as well as reducing emissions and waste, leading to a cost reduction, and allowing for a more circular economy. In the case of solar thermal systems, LCA studies have pointed to the implementation of eco-design strategies mainly related to changes in materials and reductions in heat losses. Battisti and Corrado (2005) identified that, for a thermal collector with integrated water storage, most of the environmental impacts were associated to the production phase, specifically the tubes made of copper, leading to a replacement of this material with steel. Related to the use phase, the authors also proposed the use of an additional covering for the collector, a transparent insulating material (TIM) layer, in order to improve its performance for energy production. Also related with the covering, Chaurasia and Twidell (2001) proposed in their study the evaluation of the performance of an integrated collector with and without a TIM layer. In this case, the collector with TIM glazing was found to be more effective than the glass glazed collector by reducing the heat loss factor (UL). Martinopoulos, Tsilingiridis, and Kyriakis, (2013) identified the environmental impacts from the use of different materials in domestic solar hot water systems (DSWH). The net environmental gain achieved by the use of DSWH is influenced, by up to 20%, by the materials and techniques used, among others. In that study, LCAs of a range of typical DSWH were performed. Their environmental impact, as well as the influence from the use of different materials or/and manufacturing techniques on their impact, was identified. As thermal efficiency differs from system to system, their environmental performance is influenced mainly by the conventional energy substituted and, to a lesser extent, by the materials used for their production. A study comparing unglazed and glazed solar thermal panels showed that the performed LCA, using Eco-indicator 99, resulted in 198 eco-points for the DSWH with tradi-steps with a low energy demand). (Piroozfar, Pomponi, and Farr (2016) concluded that, amongst the five solar heater types considered, the one with electric backup appeared to be the environmentally preferable one. The study also stresses the need for a life cycle approach in order to reflect environmental impacts holistically and to facilitate better decision making. Another LCA, carried out by Allen, Hammond, Harajli, McManus, and Winnett, (2010), for a solar hot water system, concluded that the production phase, especially due to the production of aluminium, is a high energy J. Albertí, et al. Sustainable Cities and Society intensive one and produces most of the environmental impacts of the system. The adopted eco-design solution was an increase of the recycled aluminium percentage for the collector frame. The results of the study showed around a 20% reduction in several environmental impact categories. 2. Aim of this work Although solar energy is considered as a’ clean’ form of energy, environmental impacts occur during the manufacturing, transportation, use and final disposal of the solar systems, due to the consumption of resources and the emission of pollutants. The environmental consequences of these transactions include, among others, natural resources depletion, greenhouse gas emissions and acidification. Therefore, it is necessary to evaluate solar technologies accounting for both the direct and indirect environmental impacts caused by the DSHW systems over their whole life cycle (Martinopoulos et al., 2013). These products and systems have been investigated and continuously improved in recent years (Comodi et al., 2014; Martinopoulos et al., 2013; Piroozfar et al., 2016) but there is still margin for further improvement. Some guidelines have been issued to assess the environmental impacts of building components from a life-cycle perspective (Lasvaux et al., 2014). This paper has obtained information extracted from the RENIA project (RENIA, 2012), which aimed at helping Spanish manufacturers of solar (thermal and PV) systems to optimize their products at the design level (Cerdan et al., 2009), reducing their life-cycle environmental impact, as well as to develop Environmental Product Declarations (EPD) (EN 15804, 2008). Although common in other countries, Spain has very little experience in EPDs, with few other projects such as those described in (Benveniste et al., 2011; Gazulla, 2012; Passer et al., 2015). Within this framework, this paper focuses on solar thermal collectors and tries to identify their weak points (materials, processes, components) from a life cycle perspective and to generate guidelines on how to optimize these systems in order to reduce their environmental impact. A comparison between two systems for DHW generation is carried out in Section 3. The first system consists of a natural gas boiler (the most common source of DHW in Spain (Institute for Energy Diversification & Saving - IDAE, 2016)), while the second one adds a solar contribution to the gas boiler. Results are described in Section 4. A second exercise is also done in order to understand the potential reduction of emissions at a national level when ensuring a contribution of DSHW of at least 30% of the DHW demand. In this case, a life cycle perspective has not been taken into account because the Spanish na tional glazed panels and in 18 eco-points for the unglazed one. Overall, national emissions inventory does not consider scope 3 emissions. In- 93% of the impact of the traditional DSWH was due to panel production (Comodi et al., 2014). Ardente, Beccali, Cellura, and Lo Brano, (2005) identified that the direct energy used during the production process and installation is only 5% of the overall energy consumption and that another 6% is consumed in transportation along the life cycle stages. The remaining percentage is employed for the production of raw materials, used as process inputs. These results show that the direct energy requirement is much less important than the indirect one (in fact, the production processes consist mainly in cutting, welding, bending and assembling stead, the exercise focuses on the displacement of technologies, when the share of a renewable technology in the mix is increased. This issue is explored, and different results are provided, in Section 5. Although, the life cycle perspective is not included in the characterisation factors, the authors believe that this exercise is a starting point for discussing about different displacement methods and their consequences. The aim is to obtain an estimate of the directly avoided emissions and to check the consequences of choosing one displacement method or another. If a life cycle perspective were adopted with regards to national emissions, these emissions would increase. Likewise, if, in the avoided emissions due to the use of solar thermal, the whole life cycle were accounted for, then the avoided emissions would be reduced due to the emissions generated along the life cycle of the solar thermal system which, as discussed in this article. However, national emissions inventories still only account for direct emissions. Therefore, these are the ones that will be considered for the analysis of emissions reduction in China and Spain. The eco-design measures suggested in the article would contribute to reducing the emissions from solar thermal systems in the indirect life cycle stages. 3. Life cycle assessment of two DHW alternatives 3.1. Product systems This LCA is focused on a product designed and sold by the Termicol Company, which was a partner in the abovementioned RENIA project. This product is a forced circulation solar system used to produce Domestic Hot Water (DHW). The LCA has been performed in line with ISO 14,044 (2006). The study was performed using the LCA software GaBi and the Ecoinvent database as the main source of background data. More specifically, the Energy Systems subdatabase (Dones et al., 2007) was widely used, from which the original model, named Solar System flat plate collector for one-family house – Hot water, was chosen and adapted to be as close as possible to the real system (Termicol, 2011). Table 1 shows the adaptation and the main characteristics of the system. 3.2. Goal and scope de fi nition The main objective of this LCA is to evaluate the environmental impact of a solar system with forced circulation, J. Albertí, et al. Sustainable Cities and Society and to compare it with a traditional heating system that uses natural gas as its main energy source. The results of the study should allow the identification of weak points in the system and the proposal of several eco-design scenarios. A life cycle based eco-design scenario development of industrial systems allows companies to know their products and their potential improvement, giving them an advantage over their competitors and a robust way to communicate to customers in environmental terms. The functional unit (FU) is defined as the production of 1 kW h of thermal energy to cover the DHW demand of a 6 persons house (the same as in the Ecoinvent database), located in Barcelona, Catalonia, Spain. There are two basic energy scenarios: in the first one, the use of solar energy is combined with an auxiliary heating system using natural gas to meet the demand; in the second one, a system that only uses natural gas to meet the entire demand is modelled. For both cases, the life span considered is 20 years. System boundaries are shown in Figs. 1 and 2, respectively. 3.3. Inventory analysis In the Life Cycle Inventory (LCI) Analysis, data were listed for each of the components and stages for both systems (Figs. 1 and 2). Due to the fact that the studied solar system includes an auxiliary heating system to meet the yearly demand of DHW, some calculations were done for the use stage in order to calculate how much of the total energy was covered by each source (solar and natural gas). Literature containing real data on use stage of solar systems is scarce. However, use and maintenance stages are relevant as they have great influence in the performance of the solar system (Hernandez & Kenny, 2012). According to the regulation established in Spain (CTE, 2013), building engineers must consider that each residential building’s user consumes approximately 40 L of DHW at 45 °C every day, which means that a household with, for instance, six inhabitants has a yearly demand of DHW of 3180 kW h (Table 2). To know how much of this demand can be covered by the solar system, also called the solar contribution, two basic parameters should be taken into account: the collector thermal efficiency FR (UL), related to the thermal losses (UL); and the optical efficiency FR (τα), related to the light transmission capacity of the covering (τ) and the absorption capacity of the copper surface of the collector (α) (Duffie & Beckman, 2001). Producers of this type of systems usually provide values for both of these parameters. For the assessed system, the thermal efficiency is 4.086 W/(m2 K) and the optical efficiency is 0.77. Therefore, and also considering the tank capacity and the area of the collector (78.9 L/m2), the solar system under study is able to cover 75.6% of the yearly demand of DHW. The auxiliary heating system that uses natural gas should cover the rest. Table 2 reports values for each month and for the yearly total. 3.4. Impact assessment In order to describe the environmental impacts of the system throughout its life cycle, some categories were selected following the recommendations of the EN 15804 (2011), which contains the core rules for developing Product Category Rules (PCR) of construction products. The selected categories for emissions taken from the CML 2001 method, due to its problem-oriented perspective (Monteiro & Freire, 2012), are those included in the EN 15,804: • Acidification Potential • Eutrophication Potential • Global Warming Potential • Ozone Layer Depletion Potential • Photochemical Ozone Creation Potential J. Albertí, et al. Sustainable Cities and Society Fig. 1. System boundary for the solar system with forced circulation. Fig. 2. System boundary for the natural gas heating system. J. Albertí, et al. Sustainable Cities and Society Table 2 Yearly DHW demand and solar contribution. (Source: CTE-2013). In the case of the traditional heating system, the entire demand is covered with natural gas: 8781 m3 of gas over the 20 years of life span. The studied system is a high energy transformation product and, due to this fact, further impact metrics are used, related to the cumulative amounts of both non-renewable and total (renewable plus nonrenewable) primary energy, which is directly and indirectly transformed over the system’s lifetime. Both of these indicators, respectively named “non-renewable cumulative energy demand” (NR-CED) and “cumulative energy demand” (CED) are calculated including the indirect energy demand for the provision of materials. In some older literature, NRCED is sometimes also referred to as “gross energy requirement” (GER). This type of metric is a standard requirement by the EN (1580)4 (2011), and it has been widely used in the scientific literatures for energy analyses (Gürzenich & Wagner, 2004; Slesser, 1974; Thiaux, Seigneurbieux, Multon, & Ben Ahmed, 2010) including previous LCA studies co-performed by some of the authors (Ulgiati, Raugei, & Bargigli, 2006; Puig, Fullana-i-Palmer, Baquero, Riba, & Bala, 2013; Raugei, Bargigli, & Ulgiati, 2007; Ulgiati et al., 2011), in spite of not being a standard LCA metric. 4. Results and discussion 4.1. Environmental profile of the system Table 3 reports the results for the emission-related categories described above and shows the percentage of relative change that the solar system produces when it is compared with a traditional natural gas heating system. The comparison between the two systems can also be seen in Table 4, where the cumulative primary energy demand is listed. When the solar system is compared with a traditional system that uses natural gas for DHW production, a substantially environmental improvement is obtained due to the reduction of the Global Warming Potential (-65% in Table 3). This is often identified as one of the most relevant environmental indicators nowadays. This result is directly related to the use of non-renewable primary energy (primary energy from non-renewable resources, Table 4), of which the solar system uses 64% less than the natural gas system (and, correspondingly, as expected, much more renewable energy). Improvements can also be seen in other impact categories of high relevance such as ozone layer depletion (- 69%) and photochemical ozone formation (-41%). Month Temperature cold water (ºC) DHW demand (kWh) Solar contribution (kWh) Solar contribution % Jan 9 311 157.5 50.6 Feb 10 273 174.4 63.9 Mar 11 294 222.2 75.6 Apr 12 276 233.6 84.6 May 14 268 242.5 90.5 Jun 17 234 222.5 95.1 Jul 19 225 219.4 97.5 Aug 19 225 216.4 96.2 Sep 17 234 209.7 89.6 Oct 15 260 198.6 76.4 Nov 12 276 163.9 59.4 Dec 10 303 142.8 47.1 Total 3,181 2,403 75.6 J. Albertí, et al. Sustainable Cities and Society Table 4 Values for non-renewable cumulative energy demand (NR-CED) and cumulative energy demand (CED). Unit Natural gas system Solar system Relative change NR-CED MJ 4.63 1.65 − 64.00% CED MJ 4.65 5.30 +13.90% Table 5 Components with high environmental impacts in the solar system. Category Unit Collector Water tank Copper tubes Acidification kg SO 2 33.8% 25.0% 8.3% Eutrophication kg PO 43 − 23.8% 28.2% 1.8% Global Warming Potential kg CO 2 9.5% 14.6% 0.3% Ozone Layer Depletion kg CFC11 5.9% 6.6% 0.2% Photochemical Ozone kg Ethene 17.8% 24.6% 3.0% Formation Categories like acidification and eutrophication (especially the latter) are weak points of the environmental profile of the solar system throughout its life cycle, instead. These results can be associated with the intensive use of metals in the production phase, causing an increase in the environmental levels of acidic gases or phyto-nutrient discharges (such as nitrogen or phosphorus). However, acidification and eutrophication are local categories that need a more detailed analysis due to their radii of emission. For instance, in the case of the natural gas system, most of the emissions that come from burning gas are produced in a smaller radius, focusing their impact on the local community. On the other hand, the emissions from the solar system are mostly due to the production of components and extraction of materials, activities which can be carried out in different locations and possibly far from each other, making the emissions more scattered. A full analysis of the above mentioned aspects is highly relevant in the analysis of local impact categories, but falls outside the scope of the present paper. After comparing both systems, and as a second step of the analysis, the solar system was disaggregated into its components (Fig. 1) in order to find the ones that contribute the most to each of the impact categories. As a result of this disaggregation, the collector, the tank and the copper tubes of the secondary circuit were identified as the components with the highest environmental impact in the system (Table 5). 4.2. Potential system improvements: eco-design scenarios Based on the detection of weak points in the analysed solar system and guided by the previously commented analyses in Section 2 (Methods), the following eco-design scenarios were established and evaluated: (1) Production phase: replacement of copper tubes with galvanized steel tubes in the secondary circuit of the system. (2) Use phase: replacement of the glass covering with a multi-wall polycarbonate covering. (3) Production phase: increase of the percentage of secondary (recycled) aluminium for the collector frame. The described changes do not affect the system durability or its need to any additional maintenance. 4.2.1. Eco-design scenario 1: galvanized steel tubes The main objective of this material substitution is to reduce the impact within the acidification and eutrophication categories by using a material that is widely used in Spain for tube production (galvanized steel). The virtual change of material was carried out taking into the consideration of the dimensional and functional equivalence between tubes, changing from 7.14 kg of copper to 16.5 kg of galvanized steel. The use of galvanized steel would yield a reduction of 5.77% in the acidification category for the solar system (Table 6) and a small reduction in the photochemical ozone formation potential, too. The reduction of these impacts is a positive result that could help to improve the environmental profile of the solar system. The values for primary energy demand (Table 7) would increase by a small proportion, demonstrating that the heavier steel tubes would be slightly more energy-intensive than the existing copper ones. 4.2.2. Eco-design scenario 2: polycarbonate covering The main objective of changing the covering material from glass to a multi-wall polycarbonate layer is to reduce J. Albertí, et al. Sustainable Cities and Society 11 EN 15804 (2008). 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