Life cycle assessment of a solar absorption air-conditioning system

Journal of Cleaner Production 240 (2019) 118206

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Life cycle assessment of a solar absorption air-conditioning system K. SolanoeOlivares a, R.J. Romero b, E. Santoyo c, d, *, I. Herrera e, Y.R. GalindoeLuna c, A. RodríguezeMartínez b, E. Santoyo-Castelazo f, J. Cerezo b
noma de M
Posgrado en Ingeniería (Energía), Instituto de Energías Renovables, Universidad Nacional Auto exico, Priv. Xochicalco, 62580, Centro, Temixco, Morelos, Mexico b
n en Ingeniería y Ciencias Aplicadas, Universidad Auto
noma del Estado de Morelos, Av. Universidad 1001, Col. Chamilpa, 62209, Centro de Investigacio Cuernavaca, Morelos, Mexico c
noma de M
Instituto de Energías Renovables, Universidad Nacional Auto exico, Priv. Xochicalco, 62580, Centro, Temixco, Morelos, Mexico d Universidad Iberoamericana, Department of Physics and Mathematics, Santa Fe, Ciudad de M
exico, 01219, Mexico e
gicas, Av. Complutense 40, 28040, Madrid, Spain Centro de Investigaciones Energ
eticas, Medioambientales y Tecnolo f Tecnologico de Monterrey, Escuela de Ingeniería y Ciencias, Calle del Puente 222, Ejidos de Huipulco, Tlalpan, 14380, Ciudad de M
exico, Mexico a

a r t i c l e i n f o

a b s t r a c t

Article history: Received 14 May 2019 Received in revised form 31 July 2019 Accepted 27 August 2019 Available online 4 September 2019

Environmental impacts of an experimental Solar Absorption Air-Conditioning System based on a LifeCycle Assessment were evaluated for the first time in Mexico. For comparison, a Commercial Air Conditioning System that uses electricity from fossil fuels was also evaluated. In both cooling systems, the construction, operation and end-of-life stages were analysed. Environmental impacts were evaluated using the SimaPro® software and TRACI 2.1 method. Ten impact categories were analysed, among which the ecotoxicity, fossil fuel depletion, and global warming potentials represented the most important issues. A considerable reduction of environmental impacts was achieved by using the solar cooling system in comparison with those large impacts produced by the commercial cooling system. This reduction was due to the use of solar energy in contrast with the conventional cooling system which uses electricity sourced from fossil fuels. The main contribution of environmental impacts in the solar cooling system was the consumption of energy and raw materials for the construction, whereas for the commercial cooling system, the electricity consumption, and the use of refrigerant were the major impacts of the operation stage. An overall emission saving of ~80% of the global warming potential as carbon footprint was obtained from the solar cooling system, whereas, for the fossil fuel depletion and ecotoxicity impact categories, a decrease of 85% and 20% was achieved, respectively. Further details of this environmental sustainability study are outlined in the present work. © 2019 Elsevier Ltd. All rights reserved.

Handling Editor: Yutao Wang Keywords: Renewable energy Solar energy Space cooling Environmental impact Greenhouse gases Global warming potential

1. Introduction The energy portfolio of Mexico is currently dominated by fossil fuels (88.9%), renewable energies (9.5%) and nuclear power (1.6%), which roughly represents a total production of primary energy of ~7,027 PJ (SENER, 2017). This mix of resources may produce some energy supply deficiencies in the future due to the depletion of the

* Corresponding author. , Instituto de Energías Renovables, Universidad Nacional
noma de Me
xico, Priv. Xochicalco, 62580, Centro, Temixco, Morelos, Mexico. Auto E-mail addresses: [email protected] (K. SolanoeOlivares), rosenberg@uaem. mx (R.J. Romero), [email protected] (E. Santoyo), [email protected] (I. Herrera), [email protected] (Y.R. GalindoeLuna), antonio_rodriguez@uaem. mx (A. RodríguezeMartínez), [email protected] (E. Santoyo-Castelazo), jesus. [email protected] (J. Cerezo). https://doi.org/10.1016/j.jclepro.2019.118206 0959-6526/© 2019 Elsevier Ltd. All rights reserved.

hydrocarbon reserves, and to the environmental impacts caused by the greenhouse gas (GHG) emissions. According to the International Energy Agency (IEA), the global CO2 emissions were roughly estimated in 33.1 Gt in 2018, from which about 445 Mt corresponds to Mexico (IEA, 2018a). With the current use of renewable resources, Mexico has avoided GHG emissions of around 29.3 Mt CO2 eq (IRENA, 2016). Among the anthropogenic activities that contribute to the generation of greenhouse gases (GHG), the energy sector produces a large amount of emissions with two thirds of the overall amount of GHG, from which 90% corresponds to CO2. To reduce GHG emissions, and to contribute to the mitigation of climate change, the energy sector must address the use of sustainable and renewable energy sources as key challenges (IEA, 2018a). Worldwide distribution of CO2 emissions by sector indicates

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that electricity and heat generation roughly represent 42% of the global emissions, whereas 11% correspond to the residential sector, which are mostly caused by the energy consumption of Commercial Air Conditioning Systems (CACS): Buyle et al. (2013); Cabeza et al. (2014). The use of CACS has increased during the last decades with a significant impact on the energy consumption of buildings. The space cooling systems still experience a high consumption of electricity, which is associated with high operation costs and significant environmental issues, especially if the primary energy comes from fossil fuels. To face out such challenge issues, clean and sustainable energy sources with low-carbon technologies require to be developed (Franco et al., 2017). By the end of 2016, an estimated 1.6 billion CACS were in use around the world, and this amount represents roughly 11.7 GW in terms of cooling output, from which 50% have been used by the residential sector (IEA, 2018b). The largest commercial market is dominated by China (35%) followed by United States of America (23%) and Japan (9%). These percentages are mainly owing to differences in climate conditions, the size of population and the economic capacity for accessing the space cooling technologies, which are different in developing countries, and reflected in per-capita levels of energy consumption. Space-cooling demand in developing countries is rapidly rising, especially in those places with hot and humid climates. Although, it is recognised that energy efficiency programmes on the use and regulation of these cooling systems are still under preparation (IEA, 2017; Kalkan et al., 2012). It is expected that the large-scale use of CACS will be increased over the next years as the global temperature rises (Davis and Gertler, 2015). From a sustainable energy perspective, the adoption of efficient cooling systems will require electricity consumption coming from renewable sources (e.g., solar, geothermal, wind or hydro). Because solar energy is endorsed for decarbonization and lie et al., 2019), sustainable development of human society (Pr ava space cooling systems based on this renewable source may offer a suitable technological option, and an attractive commercial market for developing countries (e.g., Mexico, India, Brazil and Indonesia). In Mexico, the use of CACS currently represents up to 50% of energy consumption in buildings, depending on the regional climate conditions. According to Oropeza-Perez (2016), an electricity consumption of 11,854 GWh due to the operation of CACS was reported in 2015. After considering an emission factor of 0.527 tons of CO2 eq/MWh, total emissions of 6.247 Mt of CO2 eq have been roughly estimated (Oropeza-Perez, 2016; SEMARNAT, 2018). To minimize this environmental impact, clean and sustainable energy technologies are required to satisfy the electricity demand. Among these technologies, sustainable cooling systems based on the use of solar energy have been proposed both to achieve high levels of indoor thermal comfort, and for lowering indoor temperatures; e.g., the solar absorption air-conditioning systems (Henning, 2007; Zhai and Wang, 2009; Allouhi et al., 2015; Nkwetta and Sandercock, 2016; Beccali et al., 2016). These cooling systems have been especially endorsed for developing countries with a large potential of solar energy (such as Mexico), where the resources are abundant, and widely distributed (Vidal-Zepeda, 2005; OropezaPerez, 2016). The large-scale implementation of solar technologies depends on in-depth knowledge of global solar radiation distribution and intensity levels. According to the Atlas of solar resources of Mexico, the global solar irradiance ranges from 4.6 to 6.6 kWh/m2-d: see
n-Nava et al., 2014; Hernandez-Escobedo et al., 2015; Fig. 1 (Alema ~ a-Ortíz et al., 2015; World Bank Riveros-Rosas et al., 2015; Villican Group et al., 2017). The daily average solar irradiance in Mexico stands at 5.0 kWh/m2-d (Rosas-Flores et al., 2016). With this energy potential, electricity generation projects based on photovoltaic (PV) and concentrating solar power (CSP) technologies have been

installed. The current installed capacity of PV and CSP is around 2,541 MWe and 14 MWe, respectively (IRENA, 2018). The CSP technology uses the most efficient and cost-effective process to generate electricity, which is covered by four main technologies: towers, dishes, troughs, and linear Fresnel. Other applications for these energy resources either at a small- or pilot-scale have been developed in solar domestic water heating (e.g., Ordaz-Flores et al., 2011), solar refrigeration (e.g., Herrera et al., 2010), solar drying (e.g., Silva-Norman et al., 2018), solar sea water desalination (e.g.,
lvez et al., 2009), solar cogeneration (e.g., Pe
rez-Sa
nchez et al., Ga 2019), hybrid solar-geothermal systems (e.g., Lentz and Almanza, 2006), natural ventilation by using solar chimneys (e.g., Zavala
n et al., 2019), solar cooling systems (Galindo-Luna et al., Guille 2018a), among others. To support the development of these solar energy applications, environmental sustainability studies require to be conducted for evaluating their actual impacts. To identify the environmental burdens of all these energy conversion systems, life cycle studies are required. Environmental impact assessment is a crucial methodology to prevent and minimize environmental degradation, excessive pollution, or negative impacts on human health forced by technologies (Nita, 2019). These studies enable the potential carbon footprint and the main environmental impact sources (or hotspots) to be quantified. The environmental impacts of these energy conversion systems may be assessed by a well-established methodology known as Life Cycle Assessment (Santoyo-Castelazo et al., 2011; Santoyo-Castelazo and Azapagic, 2014), which is based on standardised ISO norms (ISO 14040, 2006a; ISO 14044, 2006b). Life Cycle Assessment (LCA) is widely recognised as the most suitable methodological tool for the evaluation of the environmental performances of power generation systems (e.g., TomasiniMontenegro et al., 2017). The LCA studies also open the opportunity to identify the best strategies for mitigating environmental emissions at early stages of development (Parisi et al., 2019). In the last years, the use of the LCA methodologies to evaluate environmental impacts of products, processes, services and technologies have progressed rapidly in new engineering and green energy applications, for example: food engineering and artificial intelligence (e.g., Kaab et al., 2019), energy efficiency and agriculture systems (e.g., Rodríguez et al., 2019), green chemistry (e.g., Delgove et al., 2019), low-energy residential buildings (e.g., Ramírez-Villegas et al., 2019), green building and net zero energy building (Collinge et al., 2018), biogas power energy generation (e.g., Huerta-Reynoso et al., 2019), geothermal energy production and optimisation (e.g., Parisi and Basosi, 2019), net-zero energy communities and renewable energies (e.g., Karunathilake et al., 2019), among others. In the present work, a new application of the LCA for the evaluation of environmental impacts of an experimental solar absorption air-conditioning system (SAACS) is reported. This work constitutes the first LCA study applied in Mexico to evaluate the environmental sustainability of solar cooling systems. The experimental SAACS was installed in Cuernavaca, Morelos (Mexico) at the campus of the “Universidad Autonoma del Estado de Morelos” (UAEM). The solar cooling system consists of a field of parabolictrough collectors as primary heat source (in where ethylene glycol was used as heating working fluid), a field of photovoltaic panels (to supply electricity for the electronic devices of the SAACS), and an air-conditioning-absorption cycle (where an aqueous solution of NaOH is used as an absorption working fluid). For the LCA, the construction, operation and end-of-life stages were evaluated. For comparison purposes, a commercial air-conditioning system (CACS) connected to the local electricity grid was also evaluated. Details of the LCA methodology and the environmental impact results will be outlined.

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Fig. 1. Updated map of solar energy resources in terms of the global horizontal irradiation data for Mexico (Source: World Bank Group, ESMAP, and Solargis, 2017).

2. Previous LCA studies on solar absorption air-conditioning systems (SAACS) In relation to the environmental impacts of solar absorption airconditioning systems (SAACS), some LCA studies have been scarcely reported in the literature (e.g., Batlles et al., 2010; Gebreslassie et al., 2010; Hang et al., 2011; Jing et al., 2012; Bukoski et al., 2014; Hang et al., 2014; Wang et al., 2015; Beccali et al., 2016; Longo et al., 2020). To highlight the novelty of the present LCA study, an in-depth review of the extant literature is briefly presented in this section. Most of the previous LCA studies have been carried out with the following goals: (i) to evaluate the energy and environmental impacts of the SAACS; (ii) to optimize the technical design of these SAACS; (iii) to demonstrate the cost-benefit ratio of these cooling systems; (iv) to reduce the environmental impact of GHG emissions; (v) to evaluate the economic and energy/environmental performance, among other aspects. Batlles et al. (2010) conducted an LCA to evaluate environmental impacts of a solar-assisted heating, ventilating, and airconditioning (HVAC) system, installed in the CIESOL building in Almería (Spain). A comparison between a solar-assisted HVCA and a conventional HVAC system coupled to a heat pump was carried out. Lower environmental impacts were estimated for the solarassisted HVAC, among which a reduction of 80% in GHG emissions was achieved. The main environmental issue detected for the HVCA was the water use for cooling which may impact on freshwater resources. Minimization of water use was identified as a priority for the sustainable development of this technology. Gebreslassie et al. (2010) suggested a decision-support tool based on a mathematical programming for the experimental design of SAACS. The experimental work was focused both for lowering the overall cost of this technology, and to reduce the environmental impacts over the entire life cycle, which were estimated from an LCA study. The design of a solar assisted ammonia-water absorption cycle was performed by considering the weather data of Barcelona (Spain). A significant reduction in the environmental impacts of the SAACS was achieved by installing a subsystem of

solar collectors. Hang et al. (2011) carried out experimental and simulation works for the economical, energy and environmental assessment of SAACS, designed for a medium-sized office building in California (USA). The results showed a clear trade-off between the economic and energy/environmental performance. The environmental impact analysis was analysed using an LCA methodology. The cooling system reduced the life cycle carbon footprint up to a 70%, which was due to the CO2 emissions caused by the manufacturing and operation stages (as major indicators of the global warming potential). The economic and environmental conclusions were sensitive to some economic and environmental parameters (e.g., the initial cost of absorption chiller and solar collectors, and the conversion ratio of electricity to CO2). Jing et al. (2012) proposed the installation of a novel building cooling heating and power (BCHP) system driven by solar energy and natural gas sources. The BCHP was installed in a typical commercial office building in Beijing (China). The environmental impacts of the solar BCHP were evaluated by using an LCA based on the following electricity (FEL) and thermal (FTL) loads. The LCA was used to estimate the pollutant emissions and the primary energy consumption of the solar BCHP system by considering FEL and FTL operation strategies. The results showed that the contributions of materials, operation and fuel stages were more significant than those related to manufacturing and transportation. From the integral performance point of view, the authors preliminary reported a widespread benefit for the FTL operation strategy. However, when the global warming and acidification potentials were analysed, the environmental benefits of the FEL operation strategy were much better than those referred to the FTL mode. Bukoski et al. (2014) carried out an LCA study for a comparison between the environmental impacts of a solar-assisted absorption chiller (AC) and those caused by a conventional electric-powered vapour compression (VC) chiller system. These systems were proposed to analyse the environmental impacts of implementing a solar/electric hybrid cooling system in a stadium of 15,000 seating capacity. The life cycle cooling production, and the use-phase

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electricity consumption were respectively analysed for the solarassisted AC and the VC systems. Global warming, eutrophication, abiotic resource depletion and acidification potentials were considered as main impact categories over a lifetime of 30 years. According to the general LCA results, the conventional VC chiller system presented lower environmental benefits in comparison with those obtained for the solar-assisted AC system, which enabled to select a better environmentally cooling system. Hang et al. (2014) reported an LCA for evaluating the environmental impacts of double-effect SAACS installed in office buildings in California (USA). Two different configurations of SAACS were evaluated. The first configuration was powered by solar energy and natural gas sources, which were dimensioned with a set of solar collectors and an absorption chiller to cover the peak cooling demand, and an energy backup of natural gas. The second configuration used an energy mix sources consisting by solar energy, an electrical vapour compression chiller, and natural gas. It was dimensioned by a set of solar collectors and an absorption chiller to meet half of the peak cooling demand, whereas the natural gas source was used as energy backup for the absorption chiller. An electrical vapour compression chiller was also configured to meet the rest half of the peak cooling demand. The life cycle economic and environmental assessment was performed by comparing such cooling systems, where the second configuration achieved a better life cycle performance (in terms of a lower present worth cost). The authors specified that both configurations may reduce the life cycle carbon footprint by 35e70%. Wang et al. (2015) optimised the life cycle performance of a hybrid combined cooling/heating and power (CCHP) system by coupling solar energy and natural gas sources. The hybrid CCHP was installed in a hotel placed in Beijing (China). The optimisation was achieved by applying an LCA methodology. The results indicated that GWP was the largest environmental impacts, followed by the respiratory effect potential. The CCHP operation stage played a strong influence on the major emissions generated over the entire life cycle. Beccali et al. (2016) validated a simplified LCA tool for analysing the energy and environmental impacts of solar heating/cooling systems (SHCS) and conventional heating/cooling systems, which were installed in Palermo and Zurich cities. The energy and environmental impacts were expressed as the global energy requirements (GRE in GJ) and global warming potential (GWP in Ton CO2 eq), respectively. Three major configurations of cooling systems were evaluated. A conventional system assisted by a stand-alone photovoltaic (PV) plant, a conventional system assisted by a grid connected PV plant, and a SHCS equipped with a hot backup system. The use phase was identified as the main contributor to life cycle environmental impacts in all systems. For Palermo, the best sustainable cooling configuration (given by the lower GRE and GWP impacts) resulted the conventional system coupled a grid connected PV plant, whereas the SHCS was the best option for Zurich city. As main conclusion, the authors considered that the climate conditions of the installation sites strongly influence the choice of the most feasible cooling technology. Longo et al. (2020) reported a new user-friendly LCA tool named ELISA “Environmental Life cycle Impacts of Solar Air-conditioning Systems”, which was developed for estimating the life cycle energy and environmental benefits/impacts. The new tool was created to simplify the time-consuming tasks associated with users that are non-experts in LCA. As application cases, ELISA was applied to analyse thermal SHC, conventional and PV assisted systems in Greece for identifying the best life cycle performance. The results revealed that the environmental impacts of the PV system were ~60% lower than the other two systems. The main advantage of the

PV system was due to the use of renewable electricity for building cooling in locations with high solar irradiance. To complete the in-depth review, additional technical aspects are summarized in Table 1. This information includes a brief description of the systems, the impact categories, the assessment method, the commercial software used, the functional unit, and the goal and scope of each LCA study. A direct comparison among the results obtained from these studies is not possible to address because there is no a unique standard methodology that meets the same functional units nor other key specifications (e.g., impact categories, assessment method, goal and scope, boundaries system). Notwithstanding the efforts conducted in previous studies, it must be recognised that the technological advances achieved for the design and construction of such solar cooling prototypes still demand a further environmental assessment to be carried out. This sustainability task enables the solar cooling systems to be certified, which will open an interesting innovation market for their future development and commercialization. 3. Work methodology According to the international standards ISO 14040 and ISO 14044 recommended to provide the LCA framework, and the requirements and guidelines (ISO, 2006a; 2006b), five stages were considered in this study as the work methodology (Fig. 2): (i) Description of the space cooling systems; (ii) Goal and scope definition; (iii) Inventory analysis; (iv) Impact assessment; and (v) Interpretation. 3.1. Description of the space cooling systems As was previously mentioned, the LCA methodology was applied for assessing the environmental impacts of a solar absorption air-conditioning system (SAACS). As a comparative analysis, a CACS connected to the local electrical grid was also evaluated. The total environmental impacts associated with both SAACS and CACS have been quantified, and compared for each life cycle stage. 3.1.1. Solar absorption air-conditioning system The solar absorption air-conditioning system (SAACS) under evaluation was installed in the academic building of the Research Centre for Engineering and Applied Sciences (CIICAp) of the main campus of the Autonomous University of Morelos State (UAEM),
xico: which is located at the north of Cuernavaca, Morelos city (Me Latitude: 18.8889 and Longitude: 99.2361): Fig. 3. The climate of this geographic zone of Mexico corresponds to warm-sub humid with the highest ambient temperature between March and May, which causes the peak cooling demand for using air conditioning systems. The maximum ambient temperatures recorded for March, April and May are 32.7, 33.0 and 33.4
C, respectively; whereas for the solar irradiance, the maximum values are typically 5.68, 5.47 and 5.68 kWh/m2-d, respectively. A plot showing the variability of some meteorological parameters is shown in Fig. 4. A full record of ambient temperature and solar irradiance data is available in the solarimetric and meteorological database of the IER-UNAM (ESOLMET-IER, 2018), which is in full agreement with the Mexican National Weather Service (RiverosRosas et al., 2015). Because the SAACS is an experimental prototype powered by solar energy, it was not connected to the local electricity grid. This system has a peak installed capacity of 17.6 kW to potentially supply the peak cooling demand of the UAEM academic building,

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Table 1 Previous LCA studies reported in the literature for similar solar cooling systems. System

Impact categories*

Solar-assisted heating, ventilating, and air-conditioning (HVAC) system equipped with flat-plate collectors.

Ammonia-water solar assisted absorption cooling system.

Assessment method/ Software

Functional unit

Goal

ADP, AP, EP, GWP, ODP, The Centre of Environmental Studies HTP, FAETP, MAETP, (CML) 2001 method/ TETP, POCP, FEI SimaPro

Five months of cooling and heating services for the CIESOL building

ODP, Ionizing radiation, Eco-indicador 99/ SimaPro** Carcinogen effect, Extraction of minerals, Extraction of fossil fuels, Land occupation, Toxic emissions, AP, EP, REP

Amount of cooling demand satisfied.

The whole life cycle of HVAC systems, namely production of the installed equipment and its transport to the building site, operation, maintenance, and disposal. Construction and disposal of the building was excluded. Common elements of both systems were also excluded. The same number of units for the two systems was considered for the assessment. Manufacture and disposal were also excluded from the study. Cradle to gate analysis that To determine the life-cycle encompasses all the impact of fulfilling a given processes from the extraction cooling demand. of the raw materials required for the construction and operation of the cycle until the energy is delivered to the final customer. LCA study based on a mathematical programming. To measure the life cycle Manufacturing and operating carbon footprint of the whole stages of the system. Raw system for a medium-sized material acquisition and (4,983m2) office building. equipment manufacturing were considered.

Ecoinvent 2.0 database but Not specified. Solar cooling system GWP the method is not specified/ with evacuated SimaPro tube solar collectors and a gas-fired auxiliary heater. Based on computer simulation. Not specified. Solar building cooling GWP, AP, REP and PEC Characterization factors from the emission heating and power pollutants (BCHP) system driven by solar energy and natural gas. GWP, AP, EP and ADP Solar-assisted absorption chilling system (SAACS) or solar/electric hybrid cooling system.

Characterization factors from CML 2 baseline 2000 method/Not specified software.

GWP Solar absorption cooling and heating (SACH) system equipped with parabolic concentrator (XCPC) solar collectors. Solar-assisted hybrid GWP, AP and REP combined cooling heating and power (CCHP) system coupled with solar energy and natural gas sources. GWP and GER Solar heating and cooling (SHC) system.

The Intergovernmental Panel of Climate Change (IPCC) 2007 method/ SimaPro.

Scope

City, Country/ Reference

To compare the environmental performance of two cooling systems (the solar-assisted against a conventional HVAC) for covering the air heating and cooling demand of the CIESOL building.

To make an evaluation and comparison between the electricity loads (FEL) and thermal loads (FTL) in terms of the life cycle environmental and energy performance The generation of To assess the environmental 9,575 RFT-hr. per impacts of SAACS, and to compare it versus an week of chilled equivalent vapour water used for compression system (VC). the heating, ventilation, and cooling. To assess the environmental The amount of impacts of the SACH system, the cooling energy consumed and to compare it versus a conventional system in two by a specific types of office buildings. building

Total Environmental Impact Potential (TEIP) method, according to equivalent factors of pollutant emissions/Not specified software.

Not specified

To evaluate the energy consumption and the TEIP.

IPCC 2013 GWP 100 years and cumulative energy demand and methods/A simplified LCA tool developed in Excel.

The amount of heating and cooling for a building with a peak demand of 12 kW.

To compare different systems integrated with solar energy in terms of energy and environmental benefits.

Almeria, Spain/Batlles et al. (2010)

Barcelona and Tarragona, Spain/ Gebreslassie et al. (2010)

Los Angeles, CA, USA/ Hang et al. (2011)

Beijing, The use of materials China/Jing (exploitation and et al. (2012) transportation of raw materials), manufacture, transport, fuel and operation stages. Cradle to grave stages, including raw material extraction, processing, and unit manufacturing of all system components. The usephase and end-of-life processes were assumed. Cradle to the grave, including raw material acquisition, manufacturing, transportation, operation and end-of-life treatment. All processes and the capital goods during the lifecycle were considered. Cradle to the grave, which involves five phases: raw material acquisition, equipment manufacture, plant construction, operation, and end-of-life.

Bangkok, Thailand/ Bukoski et al. (2014)

Cradle to grave, manufacturing (including supply of raw materials and production/assembly of the main components of each system), operation (including electricity and natural gas consumption) and, end-of-

Palermo, Italy and Zurich, Switzerland/ Beccali et al. (2016).

California, USA/Hang et al. (2014)

Beijing, China/Wang et al. (2015).

(continued on next page)

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Table 1 (continued ) System

Impact categories*

Thermal driven solar GWP and GER heating and cooling (SHC) system.

Assessment method/ Software

IPCC 2013 GWP 100 year and cumulative energy demand methods/ Environmental Lifecycle Impacts of Solar Airconditioning systems (ELISA) tool (developed in Microsoft Excel).

Functional unit

The amount of heating and cooling for a building with a peak cooling demand of 12 kW.

Goal

To asses and compare the environmental performance of three technologies (thermal SHC, conventional and PV assisted system) by using the ELISA tool.

Scope

life, which includes the treatment of final waste. Cradle to grave, including the manufacturing (raw materials supply and the manufacturing of each component of the system), operation (energy generation and consumption) and endof-life stages (treatments of the final wastes).

City, Country/ Reference

Athens, Greece/ Longo et al. (2020)

* Abiotic depletion potential (ADP), Acidification potential (AP), Eutrophication potential (EP), Global warming potential (GWP), Ozone depletion potential (ODP), Human toxicity potential (HTP), Fresh-water aquatic eco-toxicity potential (FAETP), Marine eco-toxicity potential (MAETP), Terrestrial eco-toxicity potential (TETP), Photochemical ozone creation Potential (POCP), Freshwater ecosystem impact (FEI), Respiratory effects potential (REP), Global energy requirement (GER), Primary energy consumption (PEC). ** It is not specified directly in the methodology but appears in the references.

Fig. 2. Schematic diagram of the LCA methodology used in this work.

which consists in five office rooms with a total surface of 78.5 m2. According to Maykot et al. (2018), a comfort thermal environment is achieved when a room temperature is 23.5 ± 0.5
C. The SAACS prototype was designed for operating during the hottest months of the year (i.e., March to May), and during the work weekdays (from Monday to Friday) between 11:00 and 16:00 h. This time period considers both the working hours and the highest increase in temperature inside the building, i.e., the peak of cooling demand. SAACS consists of the coupling of three main operation subsystems (Fig. 5):

(i) a field of 14 photovoltaic panels that generates 1.89 kW as the renewable electricity supply which was used for the operation of the fans, coils, and flow pumps; (ii) a field of parabolic trough collectors that generates 26.44 kW, which was obtained from the circulation of a heating working fluid (ethylene glycol with a concentration of 30%wt); and, (iii) an air conditioning-absorption cycle that generates a space cooling of 17.6 kW with an overall efficiency of 0.67 (using a NaOH aqueous solution as working fluid in the absorption process).

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Fig. 3. Photography of the Solar Absorption Air-Conditioning System (SAACS) installed at the Research Centre for Engineering and Applied Sciences (CIICAp) of the UAEM.

Fig. 4. Measurements of some weather parameters (solar irradiance, wind velocity, ambient temperature) collected for the period (MarcheMay 2018) in the ESOLMET station of the IER-UNAM.

A lifespan of 25-years was assumed for this prototype (Beccali et al., 2016). As seen in Fig. 6, the flow diagram shows the inputs of matter and energy, and the outputs generated by the operation of the SAACS (i.e., waste and emissions). Technical and economic information of the SAACS was reported in previous papers, where the building model, the thermal load, the solar collector orientation, and other technical aspects were further reported (Romero et al., 2016; Galindo-Luna et al., 2018a,b).

3.1.2. Commercial air-conditioning system Based on the cooling capacity of the SAACS, a CACS (windowtype) powered by fossil fuel electricity was selected for performing a comparative LCA analysis. To achieve the same cooling capacity conditions of the SAACS, 12.5 units of refrigeration were operated in the CACS by using a conventional vapour-compression refrigeration cycle. The overall weight of CACS was equivalent to 482.5 kg (i.e., 38.6 kg per unit, which provided an installed cooling capacity of

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Fig. 5. Schematic description of the SAACS and its main operation subsystems (parabolic-trough collectors, photovoltaic panels, and the air conditioning absorption cycle).

Fig. 6. System boundaries and life cycle stages considered for the two types of space cooling technologies.

3.52 kW with an average power of 1,250 W). The refrigeration cycle was operated by using a working refrigerant of R134a. According to previous LCA studies, a lifespan of 10-years was assumed for this cooling system (Almutairi et al., 2015). 3.2. Goal and scope The goal of the LCA study was to estimate the environmental impacts of the cooling systems (SAACS and CACS) for providing a suitable thermal comfort in an academic building of a volumetric capacity of 215.9 m3 (14.15 m long, 5.55 m wide, and 2.75 m high). The LCA results will show the major environmental impacts caused by the operation of these cooling systems. The aim of identifying major hot spots, and the improvement of technical design aspects

of these systems will be also addressed for reducing these environmental impacts. 3.2.1. Definition of the functional unit To propose the LCA model for the evaluation of environmental impacts, material and energy balances for the SAACS and CACS technologies were carried out. The functional unit (FU) was defined as the amount of cooling produced by the cooling systems during the lifespan. The FU was quantitatively defined as 114,400 kW of cooling. 3.2.2. LCA model and system boundaries The LCA model developed for the SAACS and CACS is schematically shown in Fig. 6. As shown, the LCA model covers three main

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stages (construction, operation and end-of-life waste management) and the system boundaries. The construction involves the extraction, the raw materials, the consumption of energy fuels, and transportation. The operation includes energy consumption and maintenance, whereas the end-of-life involves decommissioning (or dismantling) and recycling (tasks and materials).

(i) a primary-data source collected in-situ from the design and construction stages of the SAACS experimental prototype which was referred as first-hand information, and (ii) a secondary-data source collected from the commercial technical literature on CACS which was referred as secondhand information.

3.2.3. LCA assumptions According to the LCA model, some assumptions were considered for the three stages (construction, operation and end-of-life stages). For the construction, the energy consumption due to the manufacture of one refrigeration unit for the CACS was assumed as 19.7 MJ/kg (Boustani et al., 2010). For the operation (mainly for maintenance), the replacement of spare parts for the SAACS was estimated according to the lifespan of some components such as flowmeters, pressure gauges, piping and valves. For the CACS operation, it was considered that 50% of refrigerant is lost during the lifespan (Almutairi et al., 2015). For the end-of-life stage of the SAACS, a dismantling and recycling scenario was assumed by considering a hypothetical 100% recycling, whereas for the CACS, all the refrigeration units should be delivered to the manufacturer for their future recycling processing. These dismantling and recycling scenarios are usually assumed in some previous LCA studies (Staffell and Ingram, 2010).

Both primary and secondary data sources were supplemented by combining measured and estimated data for the two refrigeration technologies (SAACS and CACS) during their life cycle stages. For the SAACS, most of data were in-situ compiled and quantified from the listing of components and raw-materials used for the construction (i.e., information collected from the experimental working group, and material suppliers). For the CACS dataset, it was compiled from a database of materials reported in the technical literature for a refrigeration unit of 2.1 kW with a weight of 28.9 kg (Boustani et al., 2010). To have a similar comparison basis between the SAACS and CACS cooling capacities (mainly to fulfil the cooling requirements of the building), the reported specification per refrigeration unit was corrected to obtain a cooling capacity of 3.52 kW per unit (38.6 kg per unit). This weight was used to recalculate the amount of materials used per unit (see Table 2). By considering a lifespan of 10-years and 12.5 units of refrigeration, the total weight of the CACS was estimated as 482.4 kg. The total weight of the experimental SAACS was 2,361.6 kg from which 71.2% correspond to the weight of the parabolic trough collectors, 17.0% to the photovoltaic panels, and 11.8% to the airconditioning-absorption cycle components. A complete listing of the LCI used for the environmental impact evaluation of the SAACS is reported as a supplementary data file, whereas for the CACS, the LCI is summarized in Table 2. The transport and energy consumption data for the SAACS is included in the same supplementary file, whereas for the CACS, the energy consumption data were assumed, and reported in section 2. For the transportation data, a shipping of CACS units (trade and delivery) from the suppliers to the user (i.e., the academic building) was assumed.

3.2.4. Selection of LCA software, impact assessment method and impact categories To evaluate the environmental impacts of CAACS and CACS technologies, the commercial LCA software SimaPro PhD v.8.3 was selected (Goedkoop et al., 2016). As the environmental regulatory framework in Mexico is still under development, the Tool for the Reduction and Assessment of Chemical and other Environmental Impacts (TRACI) was selected as the most suitable impact assessment method (Bare, 2003). TRACI is a method proposed by the Environmental Protection Agency (EPA) of United States (US) to provide characterization factors in Life Cycle Impact Assessment (LCIA), industrial ecology, and sustainability metrics. The environmental impact categories used by TRACI method are categorised at the midpoint level, including a higher level of social agreement about the certainties for modelling in the cause-effect chain of the system. The impact categories used in the TRACI method are the following: ozone depletion potential (ODP), global warming potential (GWP), smog potential (SP), acidification potential (AP), eutrophication potential (EP), carcinogenic potential (CP), noncarcinogenic potential (NCP), respiratory effect potential (REP), ecotoxicity potential (ETP), and fossil fuel depletion potential (FFDP). The TRACI method includes the updates of the US EPA regulations and policy, which are considered as the best existing practice for the LCIA. The TRACI method uses the inventory of stressors, and applies different individual-impact-assessment methodologies that allow the inventory data and the impact categories to be correlated (Bare, 2003). These results will be corrected by using characterization factors. TRACI method was used to evaluate the environmental effects, and to identify the hotspots caused by the operation of SAACS and CACS technologies, which were represented by the impact categories above listed. A schematic representation of the global calculation procedure is schematically shown in Fig. 7. 3.3. Life cycle inventory (mass, transport and energy consumption data) To perform the LCA study, two types of data sources were used for creating the life cycle inventory (LCI) of the two refrigeration technologies under evaluation (SAACS and CACS):

4. Results and interpretation of the life cycle environmental impacts As was proposed in the methodology section, the LCA results were analysed for both SAACS and CACS. Detailed descriptions of these LCA results are outlined in the following sub-sections. 4.1. Environmental impact assessment The total environmental impacts estimated for the SAACS and CACS are summarized in Table 3. As seen in this table, the SAACS provides systematically a lower impact in all the environmental categories analysed than those generated by the CACS (i.e., SAACS/ CACS ratios < 1). On the other hand, the highest environmental impacts estimated for the SAACS and CACS were due to the ETP, FFDP, and GWP categories, whereas the lowest environmental impacts were associated with the CP, ODP, and NCP. For a better visualization, these comparison results have been graphically represented, and grouped as the high (Fig. 8A), medium (Fig. 8B), and low (Fig. 8C) environmental impacts per category. For identifying the emission savings achieved among the two technologies (SAACS and CACS), the LCA results have been graphically represented as percentage values by impact category (Fig. 9). Based on the total contribution of each space cooling technology, and according to the SimaPro output format (Goedkoop et al., 2016), the highest value estimated for each environmental impact category (Table 3) among the two technologies (SAACS and CACS)

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Fig. 7. A schematic representation of the global calculation procedure used by the TRACI method.

Table 2 Data inventory collected from commercial literature for the CACS (Modified from Boustani et al., 2010). Material Metal

Non- Ferrous Ferrous

Plastic

Fluid

Aluminium Copper Iron Stainless steel Steel High-density polyethylene (HDPE) Polypropylene (PP) Polystyrene (PS) Expanded polystyrene (EPS) High impact polystyrene (HiPS) Polyvinyl chloride (PVC) Polyamide (PA-6) Polybutylene terephthalate (PBT) Acrylonitrile butadiene styrene (ABS) Lacquer Rubber EPDM R134a (refrigerant) 280 mL

Mass (%)

Mass (kg)

Total (kg)

6.21 17.00 7.13 1.47 35.12 0.07 0.82 6.55 0.39 16.17 4.04 1.27 0.60 0.21 0.86 0.02 0.15 e

2.42 6.63 2.78 0.57 13.70 0.03 0.32 2.56 0.15 6.31 1.58 0.50 0.23 0.08 0.34 0.01 0.06 0.34

26.10

12.15

0.34

(1)

production of solar refrigeration). With the goal to have a representative environmental assessment for the SAACS and CACS technologies, the impact categories with the largest contribution have been separately analysed. This includes the ETP, FFDP and GWP, which are analysed in detail in the following sections.

As seen in Fig. 9, SAACS systematically provides a better environmental performance over the CACS. According to all the LCA results shown in Figs. 8 and 9, it is possible to infer that the implementation of SAACS contributes to mitigate the environmental impacts mainly due to the use of renewable solar energy (i.e., renewable electricity consumption/generation, and for the

4.1.1. Ecotoxicity potential (ETP) The ETP global category includes the environmental effects on water (fresh or groundwater, river and ocean), air and soil (agricultural and industrial) media or resources. Table 4 summarizes the contribution of the major substances to ETP from the LCA applied to

represented the 100%, whereas the lowest/highest ratio in percentage (%LHR) was estimated as follows:

%LHR ¼

ðLowest valueÞ*100% Highlest value

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Table 3 Total environmental impacts estimated for the SAACS and CACS by using the LCA methodology. Environmental Impact category

Unit

Total SAACS

Total CACS

SAACS/CACS Ratio

Ecotoxicity Potential (ETP) Fossil Fuel Depletion Potential (FFDP) Global Warming Potential (GWP)

CTUe MJ surplus kg CO2 eq

784,779 20,521 15,908

965,115 141,421 79,740

0.81 0.15 0.20

Smog Potential (SP) Acidification Potential (AP) Eutrophication Potential (EP)

kg O3 eq kg SO2 eq kg N eq

1008 112 174

4040 454 404

0.25 0.25 0.43

Respiratory Effects Potential (REP) Ozone Depletion Potential (ODP) Non-carcinogenics Potential (NCP) Carcinogenics Potential (CP)

kg PM2.5 eq kg CFC-11 eq CTUh CTUh

26 0.004 0.033 0.003

163 0.075 0.038 0.004

0.16 0.05 0.87 0.79

Comparative Toxicity Unit for Ecotoxicity (CTUe) and Comparative Toxicity Unit for Health (CTUh). Most of the reported values have been rounded off using statistical rounding rules.

both cooling technologies (SAACS and CACS). As seen in Table 4, major contributions of the ETP impact category were caused by the production of zinc, copper, and vanadium, which may have a direct effect on water resources. After analysing the total impacts of the ETP category, it is observed that the SAACS nearly provides the lower environmental impacts in most of the substance inputs (i.e., SAACS/CACS ratios < 1), except for the vanadium and chromium where the impacts were higher (i.e., SAACS/CACS ratios are > 1). These high ratios indicate that CACS displayed a less consumption of stainless steel and aluminium materials in comparison with those raw materials used by the SAACS. The higher consumption of these metals in the SAACS was mainly due to the manufacture of steel structure parts and 366 stainless-steel pipes. These piping materials were needed to avoid corrosion due to the use of absorption working fluids (i.e., NaOH solutions with a concentration of up to 50 %wt): Abdallah et al. (2012). To face this hotspot, and to decrease the environmental impacts caused by the use of metals, a piping substitute with resistance to corrosion (e.g., high density polyethylene or PVC) needs to be explored as a sustainable mitigation strategy. This piping is recyclable (reliable and environmentally friendly), and suitable for solar absorption air conditioning applications (Whitaker et al., 1979). Another technical option may be the use of sustainable ductile iron pipes by using either green inhibitors or inner coating materials, which may benefit the future life cycle costing of such cooling systems. 4.1.2. Fossil fuel depletion potential (FFDP) The FFDP impact category includes the environmental effects caused by the consumption of fossil fuels (natural gas, crude oil, mineral coal, and others). Table 5 summarizes the amount of fossil fuel substances used during the life cycle of the space cooling technologies (SAACS and CACS), and their characterized fossil fuel profiles in MJ surplus (i.e., FFDP), including the SAACS/CACS ratios. As seen in Table 5, the major contribution to the FFDP impact category were produced by natural gas inputs with the greatest effect on CACS. After analysing the total impact values of FFDP category and the SAACS/CACS ratios, it is observed that the SAACS almost provides the lower impact in natural gas and crude oil substances inputs than those generated by the CACS, except for the impact caused by the amount of hard coal. This behaviour is consistently confirmed by the lower values of the SAACS/CACS ratios (<1). The lower ratios demonstrate that SAACS nearly provide the lower environmental impacts of FFDP in comparison with those caused by the CACS. The SAACS environmental impacts (related to the consumption of hard coal) are also attributed to the manufacturing process of stainless-steel piping and other steel

parts. The MJ surplus values estimated for both SAACS and CACS technologies are due to the fossil fuel consumption for the electricity generation, which was required to produce the material inputs like ethylene glycol and PV systems for SAACS and electricity consumption due to the CACS use. 4.1.3. Global warming potential (GWP) GWP is defined as the impact of human emissions on the thermal radiation absorption of the atmosphere that affects ecosystems and human health (Ozbilen et al., 2013). The land surface temperature of the Earth is increased by GHG emissions, which may improve the greenhouse effect). As CO2 is the most abundant gas of the GHG, GWP is reported in units of kg CO2 eq, which is considered as one of the most important environmental impacts reported in LCA studies. A summary of the estimated contribution of GHG to the GWP impact category of the two space cooling technologies (SAACS and CACS) is reported in Table 6. Such GWP estimates were based on the GHG emissions (mainly CO2, CH4, N2O, among others). The total amount of these GHG emissions was calculated as kg of CO2 eq. GHG emissions are far lower for SAACS than for CACS technologies because the former requires low inputs of energy. Taking into consideration the results of GWP for each GHG and the SAACS/CACS ratios, it is observed that the SAACS systematically provides the lower impacts due to the small amount of gases produced during the life cycle, which is consistently confirmed by the lower values of the SAACS/CACS ratios (<1). The GWP results show that the environmental impacts caused by the CACS are significantly larger than those produced from the SAACS, i.e., ~5.3, ~2.0, and ~9.3 times greater for CO2 (73,528 kg), CH4 (90 kg) and N2O (8 kg), respectively. These values highlight the sustainable environmental advantages of the SAACS over the CACS technology. It is also important to keep in mind that the actual environmental loads produced by these GHG, are influenced by the characterization factors (CO2: 1, CH4: 25 and N2O 298). For the impact potential of CH4 is 11.25 times greater than those referred to N2O, where the greater differences among the characterization factors indicate that the impacts of 1 kg of N2O cause an environmental damage ~11.9 times greater than those impacts generated from 1 kg of CH4. Consequently, the N2O constitutes the second more important GHG after CO2. Additionally, and by considering the total amount of cooling produced during the lifespan of the SAACS (114,400 kW), a total amount of 15,908 kg CO2 eq. are produced (i.e., 0.139 kg CO2 eq. per kW) in comparison with a larger amount of 79,740 kg CO2 eq. quantified by the CACS (i.e., 0.697 kg CO2 eq. per kW).

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Fig. 8. Environmental impacts estimated from the LCA study for the SAACS and CACS cooling technologies.

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Fig. 9. Percentages of emission savings obtained from the LCA study for the environmental-impact categories of the SAACS and CACS cooling technologies.

Table 4 Contribution of the main substances (metals) to the ETP impact category for the SAACS and CACS. Substance

Media or Natural Resources

Zinc Copper Vanadium Chromium Copper Vanadium Chromium VI Copper Zinc Others

Total Impact Values of ETP (CTUe)

Water Water Water Air Air Air Soil Soil Soil e

SAACS

CACS

514,661 131,032 31,644 2,335 1,987 1,119 212 101 63 101,624

533,504 231,368 30,697 534 5832 12,400 856 327 390 149,207

SAACS/CACS Ratio

0.96 0.57 1.03 4.37 0.34 0.09 0.25 0.31 0.16 e

Most of the reported values have been rounded off using statistical rounding rules (except the decimal numbers with two significant digits).

Table 5 Contribution of the main substances to the FFDP impact category for the SAACS and CACS. Characterization Factor (MJ surplus/kg or m3)

Substance

3

Natural gas/m Crude oil Hard coal Others

5.75 6.60 0.16 e

Amount of substance used (kg)

Total Impact Values of FFDP (MJ surplus)

SAACS

CACS

SAACS

CACS

1,837 1,312 2,780 e

13,931 8807 1,238 e

10,565 8,660 459 837

80,107 58,127 204 2,983

SAACS/CACS Ratio

0.13 0.15 2.24 e

Most of the reported values have been rounded off using statistical rounding rules (except the decimal numbers with two significant digits).

4.1.4. Emission savings According to the ISO 14040 and 14044 (ISO 14040, 2006a; ISO 14044, 2006b), and as was previously mentioned, it is not possible to carry out a direct comparison of the results among some previous LCA studies reported in the literature on cooling systems because the comparison would require the use of similar LCA parameters, e.g., the same functional units, system boundaries, analysis objectives, impact categories, impact assessment methods, and methodological options (e.g., Tomasini-Montenegro et al., 2017;

Uctug et al., 2017). Nevertheless, a comparison among the space cooling technologies under study may be conducted by evaluating the net reduction percentages achieved per technology in each environmental impact (which are sometimes referred as emission savings). Regarding this, emission savings estimated for the SAACS may be compared with those values corresponding for the CACS. For example, in the case of the GWP impact category, a reduction of approximately 80% in the GHG emissions was obtained for the

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Table 6 Contribution of the main substances (GHG) to the GWP impact category for the SAACS and CACS. Total Impact Values of GWP (kg CO2 eq)

GHG

Characterization Factor (kg CO2 eq/kg)

Amount of GHG emissions produced (kg) SAACS

CACS

SAACS

CACS

Carbon dioxide (CO2), fossil Methane (CH4), fossil Di-nitrogen monoxide (N2O) Others

1 25 298 e

13,789 47 0.9 e

73,528 90 8 e

13,789 1,168 265 686

73,528 2244 2,480 1,489

SAACS/CACS Ratio

0.19 0.52 0.11 e

Most of the reported values have been rounded off using statistical rounding rules (except the decimal numbers with two significant digits).

life cycle of SAACS. This reduction in GHG emissions was similarly reported in a previous comparative study performed for the evaluation of environmental performance of solar-assisted heating, ventilating, and air-conditioning systems (Batlles et al., 2010). 4.2. Environmental impacts by life cycle stage (construction, operation and end-of-life) For a better evaluation of the global environmental impacts produced by the SAACS and CACS technologies per stage, a breakdown of these impacts is shown in Fig. 10A and B, respectively. As seen in these stacked bar charts, the main contribution of the SAACS is largely caused by the construction stage in all the impact categories, whereas for the CACS, the operation is mostly the major source of environmental impacts (except for the EDP, ODP and NCP impact categories). Negative percentages of the environmental impacts were only estimated for the SAACS, which are interpreted as avoided emission impacts because it is assumed that there is no consumption of virgin raw materials (due to recycling). Focusing on the LCA stages that caused the main contributions of environmental impacts in both technologies (i.e., construction for the SAACS and operation for the CACS), a more detailed analysis of each stage was carried out. Table 7 summarizes the major impact categories quantified for the two evaluated cooling systems (SAACS and CACS). For the SAACS case, a breakdown of the main construction subsystems is represented in Fig. 11. Such a breakdown considered the major contributions of the air-conditioning-absorption cycle, parabolic trough collectors, and the photovoltaic field. As observed in the stacked bar plot (Fig. 11), the greatest contribution is mainly caused by the construction of the parabolic trough collectors in most of the impact categories, except for the ETP, EP, and NCP which were generated by the construction of photovoltaic panels. The cycle of air-conditioning-absorption produced a very low contribution of environmental impacts with values less than 10% for all the impact categories. For the analysis of CACS, a breakdown of inputs between the energy consumption from the electricity grid and the use of refrigerant R-134A for the equipment operation is presented in Fig. 12. As seen in the stacked bars, the major contribution is mostly caused by the electricity consumption in nearly all of the impact categories, except for the ODP, which is largely caused by the use of the R-134A refrigerant (although it is considered as a new generation of environmentally friendly refrigerant with no negative effects to ozonosphere). 5. Conclusions Environmental impacts coming from the use of SAACS and CACS were evaluated by using the LCA methodology based on a lifespan of 25-years. Both SAACS and CACS technologies were proposed to satisfy the peak cooling demand of an academic building placed in

Cuernavaca, Morelos (Mexico), where the maximum ambient temperature is about 33
C. SimaPro software and the TRACI method were used as LCA tools to evaluate the environmental sustainability of the SAACS and CACS by considering ten impact categories (ETP, FFDP, GWP, SP, AP, EP, REP, ODP, NCP, and CP). The LCA results highlighted the major contribution of the construction and operation stages in the environmental impacts quantified for both SAACS and CACS technologies. The best environmental performance results were mostly obtained for the SAACS, which showed large benefits due to the use of solar refrigeration (from a solar assisted absorption cycle) and renewable electricity (from the PV panels). During the construction stage, it was found that SAACS needs a larger amount of inputs (energy and raw materials) than those required for CACS, which explains the higher environmental impacts estimated in some impact categories (ETP, SP, AP, and EP). The consumption of vanadium and chromium was the major source of these environmental impacts in the case of ETP, in contrast with the lower consumption of these metals for the CACS. Further research is still needed to determine how these small environmental impacts of the SAACS should be mitigated. In particular those identified for the manufacture of stainless-steel pipes, which are required to avoid corrosion problems for handling the NaOH absorption solutions. To mitigate these impacts, screening-level LCA should be conducted in the future to evaluate the environmental performance between the use of stainless-steel pipes and other green materials (e.g., inner surface coatings or inhibitors). The use of alternative materials will be necessary not only to protect the SAACS for the corrosion, but also to obtain a lower environmental impact. For the operation stage, lower environmental loads were obtained for the SAACS in comparison with those impacts caused by the CACS, which were mainly due to the electricity consumption coming from fossil fuels. The large environmental benefits obtained from the operation of SAACS become evident, surpassing by far, the environmental performance of the CACS. It was also demonstrated that the use of the SAACS reduces both the demand of fossil fuel energy, and the GHG emissions about an average value of 80%. Lower environmental impacts due to the fossil fuel depletion and ecotoxicity potential were additionally decreased by 85% and 20%, respectively. GHG emissions are far lower for SAACS than for CACS technologies because the former requires low inputs of energy, and their waste materials may be almost 100% recycled. The main difference arises from the environmental benefits of the renewable energy source used for electricity generation. The global warming potential of SAACS was lower than CACS, where the former presented a total emission of 15,908 kg CO2-eq., whereas the latter exhibits an emission of 79,741 kg CO2-eq over the life cycle. On the other hand, the major contributors of the ETP impact category were the production of zinc, copper, and vanadium (which may affect the water resources), whereas for the FFDP was attributed to the consumption of hard coal.

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Fig. 10. Breakdown of environmental impacts per stages of both SAACS and CACS cooling technologies.

Table 7 Major impact categories quantified for the two evaluated cooling systems (SAACS and CACS). Impact category

(Tons of Substance - equivalent) Cooling System

Construction

Operation

End-Of-Life

Total

GWP (CO2 eq)

SAACS CACS SAACS CACS SAACS CACS SAACS CACS SAACS CACS

19.13 9.47 1.15 0.80 0.12 0.08 0.18 0.11 0.03 0.01

0.42 70.23 0.02 3.20 0.002 0.37 0.00 0.29 0.00 0.15

3.65 0.04 0.16 0.05 0.01 0.001 0.01 0.0003 0.01 0.0001

15.91 79.74 1.01 4.04 0.11 0.45 0.17 0.40 0.03 0.16

SP (O3 eq) AP (SO2 eq) EP (N eq) REP (PM2.5 eq)

All the integrated LCA results show the advantages or environmental benefits obtained from the use of the SAACS, as a sustainable proposal for the substitution of the CACS. The hot climate of the locations where the cooling systems were installed also

contributes to this proposal. The implementation of SAACS instead of the most commonly used commercial air conditioning systems (CACS) represents a more environmentally friendly option to mitigate most of the environmental impacts identified in the

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Fig. 11. Environmental impacts caused by the construction stage of the SAACS cooling technology.

Fig. 12. Environmental impacts caused by the operation stage of the CACS cooling technology.

present LCA study. This technical solution is highly efficient and technically viable for places with a vast amount of solar resources. It is expected that the LCA studies encourage a more assertive development of renewable energy projects in Mexico, especially those technologies based on solar energy sources, which include the solar cooling systems. These LCA studies provide a suitable and efficient methodology for the eco-design of new innovative cooling technologies. A more intensive use of renewable energy sources to satisfy the future energy demand constitutes one of the main crucial tasks of the government for promoting technological projects and for establishing public policies for the residential sector. With these innovative solar cooling systems, the transition towards the use of sustainable and environmentally friendly technologies would be supported. Finally, it is very important to remark that this LCA study only analysed the environmental aspects of the SAACS, which is not the unique evaluation parameter in a comprehensive sustainability study. Life cycle costing and social life cycle assessment are also needed for complementing the sustainability study which will be carried out in the future before the technological transfer of the

SAACS technology to the industry for its commercialization. Conflicts of interest We wish to confirm that there are no known conflicts of interest associated with this paper. Acknowledgments This work was supported by the CEMIE-SOL-P09 and CB-167434.
tica SUMAS” (CONThe authors also want to thank to “Red Tema ACYT Project No. Ref. 271614). The first author acknowledges the scholarship grant provided by the funds CONACYT-SENER. The authors also express their gratitude to J. DíazeSalgado and M.A. CruzeChavez for the useful comments on the construction and operation of the SAACS. Finally, the corresponding author acknowledges the CONACyT, the CIICAp-UAEM, and the Universidad Iberoamericana (Department of Physics and Mathematics) for the financial and infrastructure support in two sabbatical leave programs carried out from 2016 to 2017, and from 2018 to 2019,

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List of Abbreviations TRACI: Tool for the Reduction and Assessment of Chemical and Other Environmental Impacts LCA: Life Cycle Assessment GHG: Greenhouse Gas Emissions GWP: Global Warming Potential ETP: Ecotoxicity potential FFDP: Fossil Fuel Depletion Potential SAACS: Solar Absorption Air-Conditioning System CACS: Commercial Air-Conditioning System FU: Functional Unit LCI: Life Cycle Inventory LCIA: Life Cycle Impact Assessment