Latent heat storage

Gerhard Faninger

13.2.1 The physical principle of latent storage

Latent heat storage uses the principle of the change of phase of a material to absorb or release heat. When a material is heated and changes its state (between a solid, liquid or gas), it will store much more heat than would occur from just 1 Kelvin temperature increase. When the material cools down and reverses back to the original state, this heat is then released. This heat of fusion is typically 80 to 100 times larger than the heat required for heating a material 1°. The storage capacity is equal to the phase-change enthalpy at the phase-change temperature + sensible heat stored over the whole temperature range of the process. This process is illustrated using the example of water in Figure 13.2.1.

Latentheat Storage

Source: Gerhard Faninger, University of Klagenfurt

Figure 13.2.1 Heat absorbed and released by phase change

Source: Gerhard Faninger, University of Klagenfurt

Figure 13.2.1 Heat absorbed and released by phase change

13.2.2 Materials for storage

As thermal storage media, phase-change materials (PCMs) such as paraffin and eutectic salts offer an order-of-magnitude increase in thermal storage capacity, and a big advantage is that their charge and discharge occurs almost at a constant temperature.

The classic example for phase-change materials are salt hydrates. One common type is Glauber salt (Na2SO4.10H2O), discovered during the 17th century by Johann Glauber. Sodium sulphate decahydrate has an ideal melting point of 32°C for building heating applications.

13.2.3 Phase-change materials in building constructions

Phase-change materials can be incorporated within building materials and thus contribute to lower energy consumption and power demand by storing solar energy during the day in winter or storing cold air at night during the warm summer season.

Since the PCM has a sharp change in the heat storage or release rate at the phase-change temperature, it can be used for temperature regulation. For example, mixing PCM into the building material could increase the thermal capacity of a wall board. To illustrate this great capacity, consider a concrete wall, in comparison. When it is heated or cooled 10K to 15K, it can absorb or release approximately 10 kWh/m3. This is about one fifth of the heat storage capacity of paraffin, a classic PCM. Mixing two different PCMs in a suitable proportion theoretically gives the possibility of matching the phase-change temperature exactly with the temperature of the application.

The PCM concept is particularly interesting for lightweight building construction. Figure 13.2.2 shows some examples. Phase-change applications in buildings typically involve liquid/solid transitions. The phase-change material is solidified when cooling sources are available and melted when cooling is needed. PCMs have two important advantages as storage media: they can offer an order-of-magnitude increase in heat capacity and, for pure substances, their discharge is almost isothermal.

Phase-change material implemented in gypsum board, plaster or other wall-covering material would permit the thermal storage to become part of the building structure. PCMs have an important advantage as storage media: they can offer an order-of-magnitude increase in thermal storage capacity. For example, 30 per cent of the recent BASF Micronal PCM mixed with plaster allows a 0.5 inch thick plaster layer to act as a 6 inch thick brick wall in terms of thermal capacity around 26°C. This allows the storage of high amounts of energy without significantly changing the temperature of the room. As heat storage takes place inside the building, where the loads occur, rather than externally, additional transport energy is not required.

Extended storage capacity for night storage of cold air during the summer obtained by using PCM wallboard is able to keep the room temperatures close to the upper comfort limits without using mechanical cooling.

Cooling of residential buildings in milder climates contributes significantly to electrical consumption and peak power demand, largely due to very poor load factors. Thermal mass can be utilized to reduce the peak power demand, to downsize the cooling systems, and/or to switch to low-energy cooling sources.

The use of PCM wallboard coupled with mechanical night ventilation in office buildings offers the opportunity for system downsizing in climates where the outside air temperature drops below 18°C at night. In climates where the outside air temperature remains above 18°C at night, the use of PCM wallboard should be coupled with discharge mechanisms other than mechanical night ventilation with outside air.

So far, few samples of PCM-treated wallboard have existed. There are several approaches to treating wallboard with PCM material. Thanks to the recent technology of micro-encapsulation of PCM, new products were released in 2004. Capsules of Micronal from BASF can be mixed with plaster to enhance the thermal properties of a wallboard.

13.2.4 Phase-change materials in tanks

PCMs can also be included in containers of different shapes. One common container is the plastic capsules or nodules (SLT) that are put into a tank where the heat transfer fluid (usually) water melts or solidifies the PCM. Figure 13.2.3 shows some examples. Several different PCMs with melting points ranging from -21°C up to 120°C are commercially available. Phase-change materials and chemical reactions are also used for heating and cooling purposes in small applications such as hand warmers (sodium acetate trihydrate).

Les Capsules Micronal Basf

Source: Gerhard Faninger, University of Klagenfurt

Figure 13.2.2 Phase-change material in building constructions

Source: Gerhard Faninger, University of Klagenfurt

Figure 13.2.2 Phase-change material in building constructions

Output

Output

Storage lank

Input

Capsule

Storage lank

Input

Capsule

Source: Gerhard Faninger, University of Klagenfurt

Figure 13.2.3 Phase-change materials in tanks

Recent research work has tried to incorporate PCM into a solar tank in order to increase stratification and/or heat storage capacity. A good candidate is sodium acetate with some additives to increase thermal conductivity and reduce super-cooling. Commercial products could reach the market within three years.

Micro-encapsulation of PCM in a fluid (called a slurry) can increase the ability of the fluid to transport and store heat. Some research work is also heading in this direction to enhance the solar loop of a solar combi-system.

Small PCM storage units have been sold mainly for special applications. PCM storage still requires research and development efforts to be practical. The main goal of a new international research project in the framework of the International Energy Agency Solar Heating and Cooling Programme (IEA-SHC Task 32) is to investigate new or advanced solutions for storing heat in systems, providing heating or cooling for low energy buildings.

References

Faninger, G. (2004) Thermal Energy Storage, available at www.energytech.at

Stetiu, C. and Feustel, H. E. (1997) Phase-Change Wallboard and Mechanical Night Ventilation in Commercial Buildings, Lawrence Berkeley National Laboratory, Berkeley, CA

Websites

International Energy Agency (IEA): www.iea.org;www.iea-shc.org

+2 -3

Post a comment