**1. Introduction**

The main goal of the Paris Agreement is to limit global warming to well below 2 ◦C, preferably to 1.5 ◦C, compared to pre-industrial levels [1]. The UK is one of the 192 signatories and has already taken some steps towards low carbon growth. For example, the UK electricity generation sector produced 48.5% of the electricity with low carbon technologies in 2019 [2]. Nevertheless, to meet the goals of the Paris Agreement and become carbon neutral by 2050 [3], the emissions generated by the heating of homes and industry need to be reduced, as they currently account for almost a third of all the total current UK emissions [4]. As stated by the Committee on Climate Change [5], to meet the mentioned net-zero target, the UK has to move entirely to a low-carbon heating system by

**Citation:** Chocontá Bernal, D.; Muñoz, E.; Manente, G.; Sciacovelli, A.; Ameli, H.; Gallego-Schmid, A. Environmental Assessment of Latent Heat Thermal Energy Storage Technology System with Phase Change Material for Domestic Heating Applications. *Sustainability* **2021**, *13*, 11265. https:// doi.org/10.3390/su132011265

Academic Editors: Fausto Cavallaro and Valeria Palomba

Received: 1 September 2021 Accepted: 7 October 2021 Published: 13 October 2021

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2050, which implies that by 2035, the installation of new gas boilers needs to be phased out and replaced by low-carbon heating systems.

Thermal energy storage (TES) is a form of energy storage that can store heat or cold to be used later [6]. This energy storage mechanism is a possible solution to reduce environmental impacts by balancing the energy demand and supply on a daily, monthly, or seasonal basis [6]. Likewise, the implementation of TES can facilitate the integration of heat pumps or solar collectors into the energy network because the combination of both can reduce the cost of distributed heating for consumers by taking advantage of time-of-use electricity rates. The idea behind this strategy is to store heat in off-peak price periods and release that heat in on-peak price periods [7]. Currently, there are three TES technologies with different readiness levels: sensible heat storage (SHS), thermochemical heat storage (THS), and latent heat thermal energy storage (LHTES). In the case of SHS, heat is stored or released due to a temperature change of the stored material (normally water). The main benefits of SHS are relatively low prices and the use of non-harmful materials. However, the low energy density of this technology causes the use of large quantities of stored material. For example, the energy stored density of an SHS system using water is 84 MJ/m3, whereas for an LHTES system using salt hydrates is 300 MJ/m3 [6]. Hence, the deployment of these systems requires vast areas, making it viable only at industrial scale. On the other hand, THS technologies are promising due to their highest energy densities and lower heat losses. These technologies charge heat during an endothermic reaction and discharge it during an exothermic reaction. Nevertheless, these technologies are still not commercially available [8]. Lastly, LHTES stores energy through the phase change of the storage medium. Its energy density is significantly higher than SHS, meaning the system can be more compact, and it absorbs and releases heat at a constant temperature, which makes the process more efficient and with less thermal loss. The storage media for LHTES are phase change materials (PCM) with high latent heat of fusion, which allows them to store large amounts of heat when the material changes phase [9]. Nonetheless, these materials have low thermal conductivity, making the charging and discharging of the TES system slow, unless high conducting material such as graphite powder is added. Moreover, incongruently melting PCMs (materials that do not melt uniformly), like many salt hydrates, tend to suffer from phase separation phenomenon, which reduces the heat storage capacity over repeated heating and cooling storage cycles. However, this problem can be solved by selectively adding thickening agents, which limit the distance that the phases can separate by increasing the viscosity of the PCM mixture [10]. Finally, "supercooling" prevents the heat of fusion from being released during the discharge process when the melting point of the PCM is reached, and it is usually avoided by using various nucleating agents. However, stable supercooling is sought in long-term LHTES to minimize the heat losses to the environment, as described in Dannemand et al. [11].

LHTES systems with PCM are technologically ready to be implemented commercially [12]. However, the environmental sustainability of LHTES systems with PCM has been scarcely analysed in the scientific literature. Jungbluth [13] performed a life cycle assessment (LCA) on only the production stage of a phase change material (sodium acetate) for energy storage. The study is based in Germany and is focused only on evaluating the global warming potential and cumulative energy demand of sodium acetate. The results showed that the total greenhouse gas emissions for PCM production are about 5 kg CO2 eq. per kg of sodium acetate produced. Moreover, this study concluded that the incineration of the PCM and electricity consumption during its production are the main environmental hot spots in terms of global warming potential. Noël et al. [14] performed another LCA focused on the embodied energy and CO2 emissions of an organic biosourced PCM (Dodecanoic acid) for energy storage. Results showed that this PCM is feasible in energy terms, since less than two years are needed to pay back the embodied energy. Additionally, this PCM could reduce up to 16 t CO2 eq. the greenhouse emissions in a typical house in the USA (over a 10-year period). Hence, to date, there are no studies that consider the whole life cycle of

the LHTES systems, a wide range of environmental impacts (and potential trade-offs) and the comparison with other sources of heat at the house level.

Regarding the assessment of the combination of solar energy and TES systems, Lamnatou et al. [15] reviewed existing literature on the environmental impact of different storage systems used for building-integrated photovoltaic (BIPV) and building-integrated photovoltaic/thermal (BIPVT) installations. The storage systems analyzed were batteries, PCM, and water tanks in different countries such as Canada and the USA. This review concluded that the environmental impact of a configuration depends on the PCM used and the climate conditions, and found that PCM components present a high environmental impact in human toxicity and ecotoxicity categories. Nevertheless, it is worth noting that the studies focused on paraffins and salt-based PCMs used as envelopes of a building and not as the storage medium of an LHTES system, and considered only a limited amount of environmental impact categories. Moreover, none of the reviewed studies analyzed the environmental impacts of the whole heating system of the building (PV panels, heat exchangers, piping, storage system, pumps, valves, etc.).

Therefore, this study investigates, for the first time, 18 environmental impacts and main hotspots of a solar-powered LHTES (S-LHTES) system using sodium acetate trihydrate (SAT) as PCM for short- and long-term heat storage integrated in a UK household combined heating system (hot water and space heating). This approach gives a much more holistic perspective of the environmental sustainability of the S-LHTES-PCM system, observing potential trade-offs between different impacts and proposing improvements based on the hotspots. SAT was selected due to the suitable melting temperature, the high latent heat of fusion and the ability to supercool consistently down to temperatures well below the ambient temperature. The environmental performance of this system is compared, also for the first time, with the most common heat systems in UK households (natural gas, biomass, and heat pumps). Firstly, the methodology used to perform the LCA and calculate the impact categories is described in Section 2. Secondly, the results for the S-LHTES-PCM system, a sensitivity analysis related to the extension of the lifetime of the system and the comparison with other common sources of heat are explained and discussed in Section 3. Finally, the main conclusions are summarised in Section 4.
