Phase Change Material
Phase Change Materials (PCM) are latent heat storage materials. It is possible to find materials with a latent heat of fusion and melting temperature inside the desired range. The PCM to be used in the design of thermal storage systems should accomplish desirable thermophysical, kinetics and chemical properties.
- Suitable phase-transition temperature for the specific application.
- High latent heat of phase transition in order to occupy the minimum possible volume. .
- Melting temperature in the desired operating temperature range.
- High specific heat to provide for additional significant sensible heat storage.
- High thermal conductivity in order to minimize temperature gradient and to assist the charging and discharging of energy of the storage systems.
- Small volume changes on phase transformation and small vapor pressure at operating temperatures to reduce the containment problem.
- High nucleation rate to avoid supercooling of the liquid phase.
- High rate of crystal growth, so that the system can meet demands of heat recovery from the storage system.
- Non-toxic, non-flammable, and non-explosive materials for safety reasons.
- Long-term chemical stability and complete reversible melt/freeze cycle.
- No degradation after a large number of freeze / melt cycles.
- Low corrosivity
Finally, the material must be abundant, available and cheap to help into the feasibility of the use of the storage system.
There are a large number of PCMs, they can be divided into three groups:
- Organic PCMs
- Inorganic PCMs
- Eutectic PCMs
As an example, thermal energy storage can be used in concentrating solar power stations (CSP), in which the principal advantage is the ability to efficiently store energy, allowing the dispatching of electricity over up to a 24-hour period. In a CSP plant that includes storage, the solar energy is first used to heat the molten salt or synthetic oil to store thermal energy at high temperature in insulated tanks. Later hot molten salt is used for steam production to generate electricity by steam turbo generator as per requirement. The use of both latent heat and sensible heat in concentrating solar power stations are possible with high temperature solar thermal input. Various eutectic mixtures of metals, such as Aluminium and Silicon (AlSi12) offer a high melting point (577°C) suited to efficient steam generation, while high alumina cement-based materials offer good thermal storage capabilities.