## 3 Latent Heat Storage (LHS) Method
### Latent Heat Storage in Porous Medium  
It has been mentioned many times that most pure PCMs exhibit low thermal conductivity, which limits their use in many fields [105]. To deal with this problem, solutions have been proposed:  
* Manufacture of a composite by associating a PCM and a porous matrix (_metal foam or expanded graphite_) [106];
* Addition of metal spheres, fins and wools to form a new MCP with improved thermal conductivity [107; 108; 109];
* Implementation of nanomaterials [110; 111; 112]. Porous supports can be classified according to their pore size into macro-porous (>50 nm), mesoporous (2-50 nm), and microporous (<2 nm) [113]. Figure 15 depicts the most used porous supports [114; 115] that can be used as heat transfer promoters due to their large interstitial surface. Meso and microporous media exhibit excellent guest-host interaction while preventing leakage. In hierarchical porous forms, macro-pores store PCMs in a cavity. As for mesopores, they provide transport pathways, while micropores block the movement of PCMs by exerting capillary forces. The impregnation of PCMs with a porous structure consists of the manufacture of a porous medium (_copper_, _aluminum_, _graphite_, _metallic foam_, etc.) in the form of a solid matrix having interconnected or disconnected voids called pores where the PCM is embedded for the latent heat storage. It has been reported that at the nano or microscale, PCMs can retain their original shape without any leakage during phase transition due to capillary and surface tension forces [28; 74]. Moreover, performance enhancement depends on the morphology and the properties of the structure such as porosity, pore density and thermal conductivity [28; 116; 117]. In this context, [PERSON] et al. [118] numerically studied the laminar phase change process of a PCM in an open-ended porous channel filled with porous metal foam. They found that the melting rate intensifies as porosity decreases and Reynolds number (_Re_) number increases, and they noted that the thermal performance improves significantly at low porosities. Following a similar idea, these authors numerically investigated the effect of pore morphological parameters such as pore per inch (_PPI_) density and porosity of a metal foam/paraffin composite on the heat transfer mechanisms involved during the phenomena of melting/solidification [119]. They demonstrated that increasing _Re_ with high PPI and porosity reduces the melting time, while it is slowed down with low PPI and porosity. Along the same lines, [PERSON] et al. [120] proposed a new LHS system filled with porous metal foams with positive and negative gradients in pore parameters to improve the thermal  
Figure 15: Porous support materials [114; 115], reprinted/adapted with permission from Ref. [114], 2022, Elsevier. performance. They found that an almost 18% reduction in time to complete melting can be achieved with positive gradients porosity design, while an increase of around 36% can be achieved with negative gradients. To cope with the low thermal conductivity issue, [PERSON] et al. [121] developed a physical and numerical model to examine the effect of metal foam porosity and nanoparticle concentration on an LHS device where PCMs are embedded. They pointed out that a 83.7/88.2% reduction in melting/solidification time is achieved with the addition of 95% porous metal foam, while a decrease of 25.9/28.2% is secured by adding 5% copper nanoparticles. [PERSON] et al. [122] investigated the copper metal foams' porosity effect on the melting process and stated that low porosity improves the system performance compared to higher porosity. However, pore size has no effect. For their part, [PERSON] et al. [123] conducted an experimental and numerical survey on the solidification performance of impregnated PCMs into a metallic structure. The effects of porosity and pore density have been targeted.