Many people wish to have a heating system with low energy consumption and/or utilising regenerative energy. This requires a large heat storage capacity to achieve good results. With the integration of a suitable latent heat storage material or so-called phase change material (PCM), compact storage units with high heat storage capacity will be feasible for many applications in the heating and ventilation industry. The crucial question however, is under which conditions a latent heat storage unit provides an advantage over traditional water storage units. The purpose of this publication is to try and define these conditions.
Advantages of latent heat storage
Many heat storage units, like conventional hot water storage units, use sensible heat only, i.e. the temperature change of water. The heat capacity of such a storage unit can be calculated as follows:
Q = m · cp · deltaT
In a latent heat storage unit, the sensible heat is augmented by the latent heat (melting enthalpy) of the phase change material. This latent heat is added when the material melts and released when the material congeals. The storage heat capacity is defined by the following equation:
Q = (m · cp · deltaT)WATER + (m · cp · deltaT)PCM + (m · delta hmelt )PCM
| Q | amount of heat [kJ] |
| m | mass of storage material [kg] |
| cp | storage material specific heat capacity [kJ/kg·K] |
| delta T | temperature difference of storage material [K] |
| delta hmelt | specific melting enthalpy of storage material |
In general, the smaller the working temperature difference, the bigger the advantage of a latent heat storage unit versus a hot water storage unit.
Comparison of latent heat paraffin and water in storage systems
If the heat transfer from the heat transport medium (e.g. water) to the latent heat storage material (e.g. paraffin wax) takes place in direct contact (dynamic storage principle), the unit is known as a hybrid storage unit.
In Fig. 1 the energy storage density in such a hybrid storage unit is shown for mixtures of paraffin wax and water in different ratios over a range of temperature differences.
Fig. 2 shows the ratio of the energy storage density of a latent heat storage unit versus a hot water storage unit compared with the temperature difference deltaT.
From both figures, the following conclusions can be drawn:
- When the paraffin content in the hybrid storage unit is increased, the heat storage capacity also increases and the more constant is the temperature level during phase change.
- The advantages of latent heat storage are more significant when the temperature difference is small, since the cp value has only a limited influence on the heat storage capacity.
- The storage material should contain only a minimum of additives in order to avoid loss of heat storage capacity.
- Small temperature differences and a high content of paraffin will allow very compact storage units (with heat transport materials other than water, such as air for example, the advantage of a latent heat storage unit would be even more significant).
CONCLUSION
In principle, using a latent heat storage unit only makes sense if a minimum of 50 % of the storage unit's volume is filled with latent heat storage material and if the working temperature difference is max. 15-20 K. Examples of such applications are solar energy heating systems and waste heat recovery in ventilation systems.
The above diagrams very clearly show that latent heat storage units are not suitable for storing hot water for domestic use, since such systems exhibit high working temperature differences (heating water from 15 °C to 65 °C). In such an application area latent heat storage units offer neither a technical nor an economic advantage compared with hot water storage units.
| Glossary: • The
Latent Heat Storage Process PCM's in thermal energy storage applications • Bound PCM's • Latent Heat Storage Unit in Heating, Ventilation and Air Conditioning? |