Phase Change Material Energy Storage For Co-Generation, CHP And Ground Source Heat Pump Systems

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Building services must be designed to provide sufficient flexibility for load shifting and energy usage control in order to achieve the most economical operation. A Thermal Energy Storage technique whereby " Storing High or Low Temperature energy for later use in order to bridge the time gap between energy availability and energy use " can be considered as a useful tool to achieve this aim.


Thermal Energy Storage (TES) may be considered as a useful tool to reduce the number of machinery by means of spreading the total load over a 24 hour period and effectively minimising the number of machinery which runs at constant loads all day long. Hence, any type of TES systems can be considered as a useful tool to reduce not only the overall environmental impact but also it offers considerable economical operational and up front investment capital costs for co-generation applications.


Thermal energy storage in the form of water tanks have the advantage of universal availability, low cost and transport ability through other system components. However, conventional water based TES system which is based on only sensible heat storage requires large volume in comparison with a latent heat energy storage generally called Phase Change Materials (PCM) Eutectic.


Positive Temperature Eutectic Solutions are mixtures of two or more chemicals which, when mixed in a particular ratio, have a freezing / melting point above water freezing temperature of 0 °C (32 °F) and they offer a thermal energy storage facility between +4 °C (39 °F) and +117 °C (242 °F).


Although the term "Eutectic" is widely used to describe the materials we are interested in, a better description would be "Phase Change Materials" ("PCMs"). PCMs can be broadly grouped into two categories; "Organic Compounds" (such as polyethylene glycol) and "Salt-based Products" (such as Glauber's salt). 


Recently a new range of SOLID-SOLID PCM solutions have been developed whereby the high temperature applications such as steam and high temperature oil heat storage applications can be accommodated.


The most common product for large scale installations are the salt hydrate based PCM solutions due to their low cost and widely available materials sources, BUT as they are water based they must be encapsulated in air tight containers.  These containers can be placed either in AIR or WATER circuits and acts as a buffer between the system and the heat source.


A Positive Temperature PCM Thermal Energy Storage not only provides the end user with an Environmentally Friendly design but also the following additional benefits can be obtained:


• Reduced Equipment Size

• Capital Cost Saving

• Energy Cost Saving

• Environmentally Friendly Installation

• Improved System Operation

• Flexibility for the Future Capacities


Furthermore, the possibility of Free Cooling Cycle, Absorption Chillers, Co-Generation, Solar, Hot Water and Heat Recovery TES system combinations offer new horizons for designers to control the energy balance to match the load and electricity demand / consumption of the system as a whole.


In particular, PCM energy storage on both cold and hot side of a ground source heat pump offers technically attractive and economical option whereby while the system requires heat, the other side of the heat pump circuit namely cooling output is stored in the colder PCM energy storage tanks in stead of being damped into the ground.


When cooling required either on its own or simultaneously, this stored normally wasted energy into the ground can be used to cool the system. The storage of the normally wasted energy effectively reduces the number of running hours for the heat pump and system as a whole.


The above operation can be repeated for the heating side of the circuit as well in the same manner and by simply not wasting any energy into the ground as well as not reversing the heat pump which causes a significant energy penalty to handle the system thermal inertia, the running cost of a ground source heat pump can be reduced as much as 50%.


The task for designers is to explore all available technologies towards achieving improved efficiency, and apply where and when possible diversification technologies in order to minimise the overall CO2 emission related to energy usage. A carefully balanced PCM Thermal Energy Storage may be the answer for some of the cooling applications for an Environmentally Friendly and Economical alternative.



Zafer Ure, M.Sc., C.Eng., MCIBSE., MASHRAE., MInstR., is the Managing Director of Environmental Process Systems Ltd. and has 25 years experience in the HVAC industry and has writen many papers for industry conferences and magazines. Details of PCM Products can be found ay www.pcmproducts.net  and Zafer can be contacted on +44 1733 243400 or at z.ure@epsltd.co.uk


 


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