The building sector is nowadays responsible for a considerable share of worldwide energy consumption, accounting for close to 40% of the overall energy consumption and associated greenhouse gas emissions to the atmosphere [1
]. The prospects are not very hopeful given the current trends of population growth and urbanization, which may lead to an expected growth of 60% in the built environment by 2050 [2
]. Therefore, different energy policies and initiatives at regional, national, and international levels were put in place focusing on the reduction of the environmental impact of the built environment. For instance, European policies aimed in the past few years at a reduction of greenhouse gas emissions to meet previous targets established for 2020 [3
] and new targets foreseen for 2030 consisting of a reduction of 40% of carbon emissions with respect to the levels of 1990. According to the United Nations [4
], sustainable energy production and consumption is one of the goals established to achieve economic growth and sustainable development, and the efficient management of natural resources and the way toxic waste and pollutants are disposed of play an important role in achieving this goal.
The environmental impact of a building is not only related to its service life, but also to the impact of material and system components corresponding to both manufacturing as well as disposal stages. Life cycle assessment (LCA) is a suitable tool that allows the quantification of a set of environmental impacts associated with the different stages of a given product or service, which can assist in achieving sustainable production and consumption from a global point of view. Results from an LCA can be very useful to a wide range of stakeholders, from manufacturers to policymakers, to help in the decision-making process of a new system or technology from an environmental performance point of view. Therefore, one of the main advantages of an LCA study is that it offers the possibility to identify opportunities for the use of better products, materials, or resources that include all stages of their life cycle. LCA methodology estimates the potential environmental impacts of materials and energy flows during different life cycle phases of a product or process, from “cradle to grave” or “cradle to gate”, usually applied in the industrial context [5
Energy efficiency in buildings has been recognized as a priority objective of energy policies in the built environment, as a way to reduce the impact of economic development and population growth on energy consumption [6
]. There are many ways to improve energy efficiency in buildings, such as the integration of renewable energy sources, the implementation of thermal energy storage, the use of innovative technologies and new materials, along with suitable system control and demand-side management. However, an approach for system components’ optimization that is only based on energy efficiency or a pinch/exergy analysis is not sufficient to assess the overall environmental impact of a system or component. Ten research topics regarding the improvement of energy and environmental performance of buildings were addressed in the review by Soares at al. [7
] towards a more sustainable built environment, which included LCA, thermal storage with phase change materials (PCMs), and the importance of improving the use of renewables, among others.
An approach for the evaluation of the energy efficiency and environmental impacts of a modular and integrated system for renewable electricity generation combined with intelligent electrical storage was presented in [8
]. The system allowed for self-production and self-consumption of electricity in residential buildings. The environmental impact along all the life cycle of the system was examined using SimaPro software for three different configurations: (1) a building without production of renewable energy source (RES) and storage, (2) a building with photovoltaic (PV) production without electric storage, and (3) a building with both PV and electric storage. The results showed that the lowest environmental impact was associated with the second system, which had PV production but no storage system. This demonstrated that the storage system had a negative effect in environmental terms, despite providing better results in terms of energy performance indicators, such as self-consumption and self-sufficiency. An LCA methodology was provided by Mousa et al. [9
] to assess the environmental impact of three different solar technologies from cradle through the usage phases. Solar thermal collectors, PV panels, and linear Fresnel collector prototype were assessed through a comparative analysis in 12 locations around the world. Different recommendations were provided based on the results, which should serve as a guidance for manufacturers, policymakers, and future standards. A review of LCA on existing electricity generation systems based on renewable energy sources was carried out by Varun et al. [10
]. The study concludes that there is a clear favor for renewable energy technologies, although some renewable energy systems, such as solar PV, can have significant life cycle carbon emissions that should be accounted for in evaluating carbon credits available from such systems. An LCA of a central solar heating plants with seasonal storage (CSHPSS) for space heating and domestic hot water supply to 500 dwellings of 100 m2
, located in Zaragoza, Spain, was performed by Raluy et al. [11
]. The results showed that the auxiliary system had the highest environmental loads despite of covering only 31% of the heating demand, due to the natural gas consumption. However, the solar subsystem was responsible for a significant NOx and SOx emissions due to the high amount of materials used in the manufacturing phase of the water tank, as well as to the electricity consumed in the pumps. The environmental impact of solar collectors was found to be significantly lower. Therefore, techniques and materials with low environmental impact should be used for the manufacturing of seasonal thermal energy storage systems. The integration of hybrid renewable energy systems with electric and thermal energy storage systems was investigated by Bartolucci et al. [12
] to identify the optimal configuration of a residential hybrid energy system. A multi-objective analysis that considered costs, renewable energy self-consumption, and emission factors was carried out using a rule-based control strategy to develop an energy management system. By means of an LCA applied to PV and electric storage, the effect of production and disposal emission factors on the sizing of the system was evaluated. The results showed that the inclusion of the TES in the system had a beneficial effect in terms of component sizing and energy efficiency and cost performance indicators.
A review of the existing research on the use of PCM for thermal applications from an environmental perspective, by means of LCA methodology, and economic performance criteria, was carried out by Kyriaki et al. [13
]. The LCA analyses were focused on PCM implementation in building envelope as well as in heating and cooling systems. The environmental impact of including PCM in three experimental cubicles with a typical Mediterranean building construction system was assessed by de Gracia et al. [14
]. Their results showed that the addition of PCM in the building envelope does not produce a significant reduction of the global impact over the entire lifetime of the building. Castell et al. [15
] carried out an evaluation of the environmental impact corresponding to the manufacturing and operation stages of an alveolar brick construction system incorporating PCM. The study concluded that the environmental impact reached thanks to energy savings during the operational stage compensated the higher environmental impact due to the inclusion of PCM in the manufacturing stage. An LCA study based on the EcoIndicator 99 of a ventilated double skin facade with PCM in its air chamber was performed by de Gracia et al. [16
]. The environmental impact of the PCM was assessed in comparison to the same system without PCM. The results of the LCA showed that the use of PCM reduced by 7.7% the environmental impact of the whole building considering a lifetime of 50 years. The study also revealed that the environmental payback of this system is significantly lower than systems that incorporate PCM in the building envelopes. Aranda-Usón et al. [17
] applied LCA methodology to determine if the energy savings due to the use of three commercial PCM in buildings located in five different Spanish weather climates compensated the environmental impact of the PCM manufacture and installation. The results showed that the implementation of PCM was able to reduce the overall environmental impacts, and that climate conditions and type of PCM used had a strong influence on that reduction.
A simplified LCA methodology for solar cooling systems with adsorption chillers in different European climates for residential applications was presented by Longo et al. [18
]. Their results indicated that the life cycle step with the most impact was the manufacturing stage, whereas during the operational phase, the solar systems outperformed the reference. The useful life of the system proved to be a key parameter and only for at least 15 years of expected lifetime the environmental benefits of using a solar system during the operation step counterbalance the additional impact generated during the other life cycle steps.
The present study analyzes the environmental effects through a comparative LCA of an innovative system aimed at providing cooling, heating, and domestic hot water (DHW) in residential buildings in Mediterranean climate regions. The system includes a heat pump fed by a DC bus connected to a PV system and an electric storage, in cascade with a sorption chiller connected to linear Fresnel collectors to enhance the energy performance of the heat pump. Moreover, an innovative PCM storage is used in the low-pressure side of the heat pump to store the surplus of energy produced during high solar radiation availability. The environmental impacts of the system are compared with a standard system that is used as a reference.
A detailed LCA was carried out for an innovative system aimed at providing cooling, heating, and domestic hot water (DHW) in residential buildings in Mediterranean climate regions. The LCA was performed comparing the results to a selected standard reference system. The LCA was carried out for a functional unit, 1 m2 of living floor, and two different indicators were used, ReCiPe and IPCC GWP (20 years and 100 years).
The inventory of the systems was carried out collecting data from components manufacturers or using data from the literature. The operational data was obtained through simulations carried out to estimate the energy consumption of both systems using a single-family house located in Athens as reference building.
When using the ReCiPe indicator:
The overall impact of the innovative system is higher than for the reference system. This is mainly due to the higher complexity of the system;
The damage category with a higher impact is the eco-system quality one, and within this, the urban land occupation withstands among all others;
When evaluating the life cycle stages, it is clear that for the reference system, the operation has a higher impact than the manufacturing and disposal, while in the innovative system, it is the other way around. This is due to the lower use of electricity from the grid in the innovative system.
When using the IPCC GWP indicator:
The overall impact of the innovative system is lower than that of the reference system;
In this case, the impact of the operational stage clearly makes the difference, since although in the manufacturing stage the impact of the innovative system is higher than that of the reference system, the decrease in the operational stage in the innovative system clearly compensates it.
Contribution of the different subsystems:
In the innovative system, the sub-systems with higher contribution in the overall impact are the latent TES system (29%), the sorption storage (27%), and the solar field (21%);
The other systems have much lower contribution, i.e., electrical storage (14%), heat pump (7%), PV panels (1%), and sensible heat storage (1%).
Given that it is the sub-system with a higher contribution to the overall impact and that it also has an impact on the energy performance of the system, the influence of the latent storage sub-system in the overall impact was evaluated;
When changing the PCM-aluminum ratio in the storage component (the two materials with higher impact in the energy performance and in the environmental evaluation), the contribution of this sub-system to the overall impact changes from 21% to 34%, showing that this is a target to study to improve the innovative system.
Overall, the results of this study show that although complex and innovative systems for cooling, heating, and DHW supply may reach high efficiency and achieve a relevant reduction in the energy consumption during the operational stage, the entire life cycle of the system should be considered to assess its real environmental impact. This is because the manufacturing and disposal stages of complex systems may have too high of an environmental impact that could not be compensated by the operational stage. The main limitation of this study is the difficulty of obtaining accurate values of some of the system parameters, such as the amount of material used in some components, or energy consumption in real conditions. Therefore, future research is needed to study the influence of the most impactful parameters’ variability through an error propagation analysis using, for instance, a Monte Carlo approach.