Phase Change Materials – Eutectic Thermal Energy Storage Products
Ice Thermal Energy Storage (TES) has the advantage of universal availability, low cost, and the ability to transport through other system components. However, a conventional ice TES system for air conditioning applications requires low temperature chillers and therefore standard water chillers must be replaced by low temperature glycol chillers that operate at a lower evaporating temperature and consume more energy.
If we can offer designers ice that freezes and melts above or below 0 ° C (32 ° F), these new products will open new horizons for environmentally friendly and economical systems for cooling / heating load change applications. of processes of a new and modernized type.
Phase Change Material (PCM) Eutectic solutions are mixtures of two or more chemicals that, when mixed in a particular ratio, have a freezing / melting point below or above the freezing temperature of water of 0 ° C (32 ° F) and offer an energy storage facility temperature between -114 ° C (-173 ° F) and +164 ° C (327 ° F).
Eutectics are well known and in fact the first applications date back to the late 18th century, however the separation and life expectancy of these mixtures were unpredictable and therefore their widespread use was limited.
The disadvantages of a conventional HVAC chiller and ice storage system (water ice) can be overcome by utilizing the latent heat capacity of various “eutectic” mixtures without the need for negative circulating temperatures.
Phase change material (PCM) thermal energy storage not only allows the designer to utilize existing chiller technology, but also enables free cooling charging, i.e. without running the chillers.
Although the term “eutectic” is widely used to describe the materials we are interested in, a better description would be “Phase Change Materials” (“PCM”). Unfortunately, very few of the documented PCM solutions are true eutectic and many have to be modified to obtain a suitable material for long-term use.
PCMs can be grouped into two categories; “Organic compounds” and “Salt-based products”. The fundamental requirement for a modified eutectic solution (PCM) can be classified into three categories, namely stable solution (no separation or degradation), minimal supercooling / heating, and finally near freezing and melting temperatures.
There are three fundamental additives that are commonly used to modify eutectic PCM solutions based on hydrated salts. These three components are firstly a nucleating agent that encourages crystal formation and therefore minimizes supercooling, and the second additive is the freeze depressant to achieve a lower phase change temperature and, in third, the gelling / thickening agent to overcome separation and degradation problems.
Once the above components have been carefully applied, a physically and thermodynamically satisfactory eutectic PCM solution can be produced and commercially available solutions offer a range of -114 ° C (-173 ° F) and +164 ° C (327 ° F). ° F).
However, most PCM solutions suitable for HVAC and refrigeration systems are corrosive to commonly used pipe and line components and therefore proper encapsulation techniques must be applied to overcome this problem while provides the best thermal performance. Various encapsulation techniques have been developed, such as tubes, flat containers, or alternatively ball concepts whereby PCM solutions are encapsulated in a plastic and / or metal housing.
Heat transfer between the surrounding medium, flowing around or within the unit and the surrounding PCM solutions, takes place from the outside or the inside or from the combination of both surfaces of the encapsulated PCM unit. These techniques generally eliminate the risk of contamination and offer a new type of TES design that is efficient, cost-effective, practical, and flexible.
Modern society’s dependence on refrigeration and air conditioning indicates that refrigeration and associated environmental problems will be with us for a considerable time and therefore one has to use existing and available alternative technologies with minimal use of energy. .
A positive temperature eutectic phase change material (PCM) thermal energy storage (TES) not only provides the end user with eco-design, but the following additional benefits can also be realized:
o Small team size
o Capital cost savings
o Energy cost savings
o Ecological installation
o Improved system performance
o Flexibility for future capabilities
The temperature ranges offered by the proposed PCM solutions use conventional chilled water temperature ranges for the loading and unloading sides of the system. Therefore, they can be applied to any new or modernized application with minimal technical and economic impact.
In addition, the possibility of combinations of free cooling cycle TES systems, absorption chillers, cogeneration, solar, hot water and heat recovery offer new horizons for designers to control the energy balance to match load and demand. / system electricity consumption as an integer.
The task of designers is to explore all available technologies to achieve greater efficiency of which refrigerant is used, and to apply where and when possible diversification technologies to minimize the overall CO2 emission related to energy use. A carefully balanced eutectic thermal energy storage (TES), phase change material (PCM) may be the answer for some of the refrigeration applications for an economical and environmentally friendly alternative.