*2.4. Comparison with Current Sources of Heat in the UK*

Current heat generation scenarios for households in the UK were analyzed and compared with the S-LHTES-PCM system. Data was collected from Ecoinvent 3.7 [24] and adapted to the UK conditions when possible (e.g., UK electricity grid and UK natural gas). The following heat generation scenarios were evaluated: (i) biomass, (ii) heat pump, (iii) natural gas, (iv) S-LHTES-PCM system, and (v) S-LHTES-PCM system with 40 years of lifetime (see Section 2.3). Natural gas was selected because it is the main source of heat in UK households (76% of the total) and biomass and heat pumps because they are considered low-carbon sources [45]. The biomass system includes the production of natural wood from the forest, infrastructure, air emissions, electricity required for operation, and ash disposal. The Ecoinvent database [24] considers wood-fired furnaces for domestic use with nominal capacities of less than 15 kW. For the heat pump, the Ecoinvent database was adapted to European conditions. The system has a heat capacity of 10 kW and a lifetime of 20 years. The heat pump system delivered approximately 20,000 kWh in 2000 operating hours and Ecoinvent 3.7 [24] includes emissions of the refrigerant R134a during operation. For the natural gas system, the Ecoinvent database includes a mix of central and small-scale gas boilers, natural gas production, the energy requirements (electricity, heat, and burnt natural gas), and the emissions of the high-pressure distribution network from the UK.

## *2.5. Impact Assessment*

SimaPro 9.1 software has been used to model the system and the impact categories have been calculated according to the "Recipe 2016 midpoint (H)" methodology [46]. The following impact categories have been considered: global warming potential (GWP), stratospheric ozone depletion potential (ODP), ionizing radiation potential (IRP), ozone formation potential, human health (OFPh), fine particulate matter formation potential (PMP), ozone formation potential, terrestrial ecosystems (OFPt), terrestrial acidification potential (TAP), freshwater eutrophication potential (FEP), marine eutrophication potential (MEP), terrestrial ecotoxicity potential (TEP), freshwater ecotoxicity potential (FETP), human carcinogenic toxicity potential (HTPc), human non-carcinogenic toxicity potential (HTPnc), agricultural land occupation potential (ALOP), mineral depletion potential (MDP), fossil depletion potential (FDP), water consumption potential (WDP), and cumulative energy demand (CED).

## **3. Results**
