Thermal Energy Storage Systems

Thermal Energy Storage (TES) may be considered as a useful tool to reduce the number of refrigeration machines by means of spreading the daytime load over 24 hours period. Hence, any type of TES systems can be considered as useful tool to reduce the overall environmental impact for a given cooling application.

Ice TES has the advantage of universal availability, low cost and transport ability through other system components. However, a conventional water based TES system for air conditioning application requires low temperature chillers and therefore standard water chillers must be replaced with low temperature glycol chillers which operate with a lower evaporation temperature.

The disadvantages of a conventional HVAC chiller and ice storage system can be overcome by utilising the latent heat capacity of various "Eutectic" also called Phase Change Material (PCM) mixtures without the need for minus circulation temperatures.  Positive temperature thermal energy storage PlusICE not only enables the designer to utilise existing chiller technology, but also this technique enables charging process to take place possibly by means of free cooling, i.e. without running the chillers.

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.

Unfortunately HVAC & Refrigeration TES applications utilise water ice which can only be produced with low temperature chillers. As a result, the benefits of night time low ambient temperature, existing water chillers and possibly free ambient cooling options cannot be fully utilised. If we can offer designers an ice product which freezes and melts above 0°C (32 °F) this product will open new horizons for environmentally friendly and economical systems for both New and Retrofit type process cooling / heating load shifting applications.

In full storage systems, the entire design load for the design day is generated off peak and stored for use during the following peak period.  In partial storage systems, only a portion of the daily load is generated during the previous off peak period and put into storage.  During the peak period, the load is satisfied by a simultaneous balancing operation of the installed machinery and stored energy in order to satisfy the overall daily design duty.

For HVAC and refrigeration application purposes, water and phase change materials (PCM) constitute the principal storage media. Water has the advantage of universal availability, low cost and transport ability through other system components. However, conventional water based TES system for air conditioning applications require low temperature chillers and therefore standard water chillers must be replaced with low temperature glycol chillers which operate at lower evaporation temperatures.

Positive temperature Eutectic Phase Change Material (PCM) 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). Eutectics are well-known and in fact early applications date back to the late 18th century however the separation and the life expectancy of these mixtures were unpredictable and therefore their wide spread usage was limited.

The disadvantages of a conventional HVAC chiller and ice storage system can be overcome by utilising the latent heat capacity of various "Eutectic" PCM mixtures without the need for minus circulation temperatures.  Positive temperature thermal energy storage "PlusICE" not only enables the designer to utilise existing chiller technology but also enables charging by means of free cooling without running the chillers.

Modern society's reliance on refrigeration and air conditioning indicates that refrigeration and the associated environmental issues will be with us for a considerable time and therefore one has to utilise existing and available alternative technologies with minimum usage of energy.

A Positive temperature Eutectic " PlusICE " PCM  Thermal Energy Storage (TES) 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

The temperature ranges offered by the proposed PCM solutions utilise conventional chilled water temperature ranges for both the charging and discharging sides of the system. Hence, they can be applied to any new or retrofit application with minimal technical and economical impacts.

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.

The task for designers is to explore all available technologies towards achieving improved efficiency regardless of which refrigerant is used, and apply where and when possible diversification technologies in order to minimise the overall CO2 emission related to energy usage. A carefully balanced PlusICE, PCM Thermal Energy Storage (TES) 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