*3.3. Comparison with Natural Gas, Biomass and Heat Pumps*

For the scenario analysis, the generation of 1 kWh of heat from biomass, heat pump, natural gas, and two S-LHTES-PCM systems (with regular and extended lifetimes of 20 and 40 years) was considered (see Section 2.3). As illustrated in Figure 5, the S-LHTES-PCM system (20 years) generates the highest environmental impacts in 11 of the 18 categories evaluated (GWP, ODP, TAP, FEP, MEP, TEP, FETP, HTPc, HTPnc, MDP, and WDP). The main cause of these impacts is the emissions from the extraction of raw materials and processes associated with the production of the components of the S-LHTES-PCM system, especially solar collector, PCM, PCM tanks, and heat exchanger (see Section 3.1). In the case of GWP, the S-LHTES-PCM system generates 0.30 kg CO2 eq./kWh. However, when the life expectancy of the S-LHTES-PCM system is expanded to 40 years, the GWP value is reduced to 0.19 kg CO2 eq./kWh, becoming the third-best option and with similar values to the second (heat pumps with 0.17 kg CO2 eq./kWh). Natural gas technology generates

the second-highest impacts in GWP with 0.27 kg CO2 eq./kWh, mainly due to direct emissions from the natural gas combustion process, while biomass technology generates the lowest environmental impacts in the GWP category (0.03 kg CO2 eq./kWh). However, biomass presents the highest value in OFPh, OFPt, ALOP, and CED. Although there is a significant amount of biomass in the UK [47], the indigenous biomass resources and energy crops only could service, in the best scenario, up to 44% of UK energy demand by 2050 [48]. Bioenergy is a key renewable energy technology targeted to provide options for decarbonizing heat, power, and transport energy in the UK. However, there are growing demands for bioenergy for different energy vectors. Therefore, there will likely be growing competition within the bioenergy sector for feedstock [47]. In addition, biomass resources (crop residues, forestry products, waste, and land) also compete with food production, conservation, animal feed, animal bedding, construction material, panel industry, sawmills, pulp and paper industry, among others [47], leading to uncertainty about their future availability in significant amounts for household heating. The heat pump technology has the highest impacts in the IRP category and the second-highest impacts in another ten (TAP, FEP, MEP, TEP, FETP, HTPc, HTPnc, ALOP, MDP, and WDP). The impacts of this technology are mainly associated with the use of energy from the UK electricity system, representing between 60% to 99% of all impact categories evaluated.

**Figure 5.** Comparative environmental impact analysis of different types of household energy sources. For acronyms and impact nomenclature, see Figure 3.

Finally, it is essential to highlight that if the lifetime of the S-LHTES-PCM system increases to 40 years, its use in households can minimize most of the environmental impacts. For example, as mentioned above, an S-LHTES-PCM system with a 40 years life expectancy has significantly lower values of GWP compared to natural gas, which is the primary source of energy in households in the UK [45,49]. This is relevant considering that the S-LHTES-PCM system is a technology still under development with potential room for improvement. In this sense, improving the efficiency of the S-LHTES-PCM system, applying the circular economy principles regarding maintenance and lifetime expansion, and decarbonizing the UK electricity are options that can improve the environmental performance of the system and minimize all the impacts, including climate change.
