Thermal energy storages are essential technologies for the decarbonization of the heating sector. Renewable energy sources are intermittent and this aspect makes crucial the availability of a buffer. The use of Phase Change Materials (PCMs), if properly designed, constitutes a convenient way to store thermal energy, especially in buildings. As suggested by their name, these materials absorb thermal energy in the phase transition from solid to liquid states (and vice versa they release heat during the melting). The energy involved in this physical process is called latent heat, since it occurs with low-temperature gaps, hence this application is commonly known as Latent Heat Thermal Energy Storage (LHTES).

The most appreciated advantage of this technology is represented by its compactness, which is often a mandatory requirement for installations in buildings. Indeed, LHTES units require up to three to five times smaller volumes with respect to hot water tanks, to store the same amount of energy. On the other hand, a proper design of the LHTES unit should be done in order to guarantee appropriate performance despite the poor heat transfer properties of PCMs.

In RE-COGNITION project a shell-and-tube geometry has been selected for the LHTES design. A large tank is filled by a PCM and it is crossed by a series of tubes containing the Heat Transfer Fluid (HTF), i.e. the water of the building heating system. In this application, the LHTES is used to store energy produced at the building level by the renewable technologies installed. When the hot water produced from renewable sources releases its energy to the PCM, this melts inside the tank; the storage is completely charged when the PCM is completely liquid. On the other hand, when the heat is required to the building the storage is discharged and the PCM solidifies.

Concerning the design, the melting temperature should be selected to be compliant with the building applications, thus it is in the range 60-80°C. Furthermore, a crucial aspect concerns the design of fins, i.e appropriate extensions from the surface of the tubes; these allow improving the heat exchange between the PCM and the water flowing the pipes.

In conclusion, the LHTES unit can support the production of thermal energy from renewable energy sources in two ways. At first, it can reduce (or even substitute) the production of thermal energy through fossil fuel during the peak demand when the renewable sources alone are not sufficient (therefore reducing emissions). On the other hand, it allows reducing the renewable capacity installed, avoiding the installation of technology that would be exploited only during the peak demand.