Latent thermal storage with PCMs

The term PCM is a commonly used abbreviation for ”phase change material", which means a material that changes its state of aggregation. While changing its state, for example from solid to liquid or from liquid to gaseous, the material absorbs a certain amount of heat, called "latent heat", happening at an almost constant temperature. When it is turned around, that is, from liquid to solid or from gaseous to liquid, the process will set free the energy already stored at a nearly constant level of temperature. Out of this range of changing its state, the described material behaves like a conventional, sensible storage device. To be able to meet the requirements of technical application in the low temperature ranges, apart from cycle-stability and other material constants, the volume of the media should alter as little as possible during the phase change. Therefore, in practical applications, only the latent heat of melting/crystallization comes in question. For conventional, sensible heat storage, we have a reachable density of energy that is proportional to product of the density and specific heat capacity of the storage media (.c_p) as well as to the usable temperature difference between charging and discharging. At a usable difference of temperature of about 50 K° by water for example, a volume-specific density of energy of about 58 kWh/m³ will be reached. If a material changes its state in this temperature range as well, also latent heat-storage adds on to the sensible heat-storage.

The conventional heating systems have been using the form of sensible heat-storage since a long time. Indeed, the isolation of storage tanks as well as other components of the heating system have been improved during the latest decades, but the principle of the method has not changed, which means that the storage density is small and the tanks lose their stored energy, caused by heat conduction, in a short time.

Why?

In both cases, that means for sensible as well as for latent heat storage, the energy content of a loaded tank is a function of its actual temperature. To prevent the storage device from undesired discharging, it has to be isolated with a proper thermal insulation material. The best versions are able to store for longer periods of time, but the additional expenditures (for the medium water) are not in proportion to the benefits.  A medium is suitable for long-term storage of heat if and only if the thermal losses are small in comparison to the energy stored over the whole storage period.

This means that for sensible storage huge volumes of the storage medium are needed. When we consider the exhausted possibilities of improving the isolation of the tanks, the pipes and the broiler in relation to the heat storage capacity and space requirements, the development potential of such a system is stagnant.

"High-tech PCMs" on the other hand satisfy the physical conditions for seasonal heat storage through their higher storage density at lower rates of thermal conductivity. If it is desired that phase change, that is, the release and absorption of heat should take place at a definite temperature, then we have here a perfectly fitting material.

Due to the tried and tested developments, the attainable volume-specific heat capacity of the PCMs lies over a manifold that of the water, our most often used storage medium. Another advantage of the PCMs is the less demand on the insulation material.