Nowadays, building construction is increasingly carried out using modern lightweight materials and large glass surfaces. Lightweight constructions lack thermal-storage capacity, so room temperatures may quickly rise to a level that is equal to or even higher than the temperature outside; moreover, balancing the temperature between day and night time always leads to huge amounts of energy waste. This make scientists search for new solutions. One of the options is to develop energy storage devices.
Thermal storage is an efficient way of energy conservation possible by the incorporation of latent heat (concealed heat) storage in building products. Energy storage in the walls, ceiling and floors of buildings may be enhanced by applying suitable phase change materials (PCMs) within these surfaces to capture solar energy directly and increase human comfort by maintaining the temperature in the desired interval for a longer period of time.
PCMs absorb and release heat when the material changes from one phase to another. Solid-liquid phase change is the main phase change of interest since other types, such as liquid-gas phase change materials, are generally not practicable for most energy storage applications. As a matter of fact, liquid-gas phase changes involve large changes in volume or pressure when going from the liquid to the gas phase, which prevent effective implementation. Some materials exhibit solid/solid phase changes, in which the crystalline structure is changed at a certain temperature. These are available in limited temperature ranges.
How does it work? Initially, unlike conventional energy storage materials, when solid-liquid PCMs reach the temperature at which they change phase (their melting point), they absorb large amounts of heat without a significant rise in temperature. Despite the heat input, the temperature of the material stays relatively constant, while the phase change is taking place (Figure 1). When the temperature around a liquid material falls, the PCM solidifies and releases its stored latent heat.
The simplest and cheapest phase change material is water. The freezing temperature of water is fixed at 0°C. But what if you require this heat at a temperature other than zero? Within the human comfort range of 20° C to 30°C, some PCMs are very effective. They store 5 to 14 times more heat per unit volume than conventional storage materials such as water, masonry, or rock. 
PCMs can be used in a number of ways, such as thermal energy storage whereby heat or coolness can be stored from one process or period in time, and used at a later date or different location for space heating, hot water, air conditioning systems or generating electricity.
For example, a revolutionary new drywall, incorporating one third phase changing material, has the same heat storage capacity as a 23 cm thick brick wall in the critical temperature range for living comfort of 22 to 26 °C. It works by embedding "phase changing microcapsules" from BASF called Micronal into drywall, and is sold in Europe as KNAUF PCM Smartboard. The phase change materials inside the BASF capsules keep a room cool in much the same way that ice cubes chill a drink, by absorbing heat as they melt. Each polymer capsule contains paraffin waxes that melt at around room temperature, enabling them to keep the temperature of a room constant throughout the day (Figure 2). 
All in all, useful PCMs should release and absorb large amounts of energy. To do this they need to have a large latent heat and to be as dense as possible. Having a fixed and clearly determined phase-change temperature is also of great importance which means the PCM needs to freeze and melt as in a small temperature range as possible. In addition, a PCM should remain stable and unchanged over many freeze/melt cycles and should keep its energy storage capability for quite a long time.