2.2.1. Raw Materials

The whole S-LHTES-PCM system is divided into three subsections as illustrated in Figure 2: (i) the solar collector unit, (ii) the PCM storage unit, and (iii) the hot water tank unit.

**Figure 2.** Diagram of a solar energy and latent heat thermal energy storage (S-LHTES) system with phase change material (PCM). Adapted from Englmair et al. [19].

The solar collector subsystem includes the solar collectors, a plate heat exchanger, the heat transfer fluid, and a copper piping section. Seven evacuated tubular collectors with an aperture area of 3.2 m<sup>2</sup> each have been considered. Specifically, the Thermomax HP 450 tubular collectors from Kingspan Solar were selected due to their low heat loss coefficient [21]. This low coefficient enables higher temperatures of the heat transfer fluid and, in turn, the achievement of the conditions required for stable supercooling of SAT [11]. According to the data reported by Kingspan Solar [21], the absorber is made of copper with a TiNOX coating reaching 95% absorbance. Each collector is composed of thirty low-iron glass tubes, which maintain a vacuum condition [21]. Additionally, stainless steel and ethylene propylene diene monomer (EPDM) rubber are needed for the mounting frames and clips. A layer of stone wool is also needed to increase insulation. The Ecoinvent dataset [24] for a home solar collector system with evacuated tube collectors was adapted according to the design characteristics mentioned above. The heat transfer fluid considered is TYFOCOR-LS liquid, which is an aqueous solution of propylene glycol [23]. As stated by Kingspan Solar [21], each solar collector needs 1.7 L of fluid, so taking into account the life expectancy stated in Table 1, and the density of propylene glycol, 116.9 kg of propylene glycol have been considered (2.0 g/FU). The plate heat exchanger is composed of 2.5 kg of stainless steel (0.06 g/FU) and 31 kg of carbon steel (0.74 g/FU) [22].

The raw materials for the PCM unit subsystem were obtained from LHTES system studies [10,11,32]. According to these studies, four separate modules of 150 L each, composed the PCM storage unit. Considering the life expectancies in Table 1, 11,742.4 kg of SAT (41.8 g/FU) and 17.8 kg (0.4 g/FU) of carboxymethyl cellulose are needed as PCM and thickening agent, respectively. To store this quantity of PCM mixture, four stainless steel tanks are needed to avoid corrosion [33]. Each tank features an insulation layer of 10 cm of rigid foam to minimize thermal losses, which is the most common type of advanced heating insulation material used in industry [20].

A parallel channel heat exchanger made of steel is used to achieve a high heat exchange capacity rate in the PCM unit, which is an important factor for the system performance. The channels have an internal height of 4 mm and a width of 130 mm separated by 20 mm spacers, as reported by Englmair et al. [32]. In total, there are 16 parallel channels at the bottom of the PCM chamber and 14 on the top. The weight of the heat exchanger (13.8 g/FU) was obtained by estimating the difference between the total weight of each empty unit and the weight of the PCM container.

The dimensions for the hot water tank unit were gathered from Englmair et al. [10], where a "tank in tank" technology provides both domestic hot water and space heating for the house. The outer tank and inner tanks have dimensions of 0.8 m × 1.6 m × 0.002 m and 0.45 m × 1.1 m × 0.002 m, respectively. To obtain the raw materials used in the "tank in tank", data provided by a hot water tank manufacturer called ACV was used [28]. According to ACV, the outer tank is made of steel with a 0.05 m insulation layer of rigid foam, while the inner tank is made of stainless steel. Additionally, the inner tank has a citric acid coating to improve the corrosion resistance properties of the material. Englmair et al. [32] also state that the hot water tank needs a polymeric inlet stratifier (for properly admitting the hot water). It has been assumed that the stratifier is made of acrylonitrile butadiene styrene (ABS) and that it is a cylinder with an inner diameter of 16 mm, an outer diameter of 20 mm and a height of 1.6 m.

The S-LH-PCM system also needs extra components such as piping sections, pumps, expansion vessels, and valves. A total number of 11 valves, five 50 L expansion vessels and three 50 W pumps are needed [32]. For these components, the Ecoinvent dataset [24] was used. Regarding the piping used for this heating system, the weight per unit of length was obtained from the inner and outer diameters and the type of material reported in Aste and Groppi [40] (see Table S1 in Supporting Materials).
