Thermal Storage
Thermal storage with PCM (Phase Changing Material) or with water?
In the sector of buildings heating the choice of thermal storage system is a crucial aspect for the overall efficiency of the system. The two most common solutions are water storage and phase change materials (PCM) storage. These are different technologies, each with specific advantages and limitations, which must be evaluated according to the operating temperatures, the available space and the type of heat pump used.
Thermal storage with water
One system using water for heat storage is particularly effective when ΔT (usable temperature difference) is in the order of 30 K. Below this value, the required storage volume increases significantly, resulting in an increased footprint and space occupation.
With heat pumps that work with heat transfer at variable temperature, as in the case of transcritical CO₂ heat pump systems, it is possible to operate, even with ΔT between 25 and 30K while still maintaining high efficiency. A typical example is the heating of water from 30°C to 60°C in a counterflow heat exchanger, the gas cooler of a CO2 system. At 30°C, the heat is still fully usable both for space heating (typically at 20°C) and for the production of domestic hot water, considering that mains water generally enters the heat pump at a temperature below 20–25°C.
The use of water as a storage medium also offers several advantages:
lower cost than PCMs;
operational flexibility, as the storage temperature can be decided and changed at any time;
better heat transfer, without additional temperature differences, since the water itself exchanges heat directly with the heat pump and the heating system.
All this contributes to a higher overall thermodynamic efficiency of the storage process. In addition, by working with high ΔT, it is possible to improve the COP of the heat pump, taking advantage of a lower mean heat transfer temperature, provided that the entire system is carefully designed.
Thermal storage with PCM
The main advantage of heat storage with phase change materials is the limited space occupation. This becomes particularly relevant when the available ΔT is e_qual to or less than 20K_, a situation in which a water storage tank would require very high volumes.
On the other hand, PCMs have some limitations:
higher cost than water;
fixed working temperature, defined during the design phase and not modifiable later;
less efficient heat exchanges, as a further temperature differences is necessary, so reducing the overall efficiency of the system.
Conclusions
In summary, water storage is a more economical, flexible and thermodynamically efficient solution when it is possible to work with high ΔT and heat pumps are well designed and integrated into the system. On the other hand, storage with PCM becomes interesting in cases where the available space is limited and the usable temperature difference is small. The choice between the two technologies is therefore not absolute, but it must be the result of a careful evaluation of the operating conditions, the plant requirements and the overall efficiency objectives.