Rocky Research ::Theral Energy

THERMAL ENERGY STORAGE

Complex compound sorption systems can be used for storage of heat or chill,
or both simultaneously. Complex compound chill storage uses complex
compounds in absorption to provide refrigeration, and is regenerated by
desorbing ammonia from the complex compound by the application of heat,
or by the use of a compressor. Small systems would typically use electrical
power for regeneration. An example flow schematic is illustrated in Figure 1.
Electrical heaters (or other means, such as heat pipes or pumped fluid loops)
are used to heat the sorbers and drive off ammonia vapor, which is condensed
and stored for use during discharge.

Figure 1. Chill storage system schematic.

Cooling is produced during discharge of the chill storage system. Sorbers are cooled to some sink temperature (such as ambient in an environment with atmosphere). At this temperature, pressure in the sorbers is low and they can draw refrigerant vapor from the evaporator to produce cooling. Liquid refrigerant is admitted to the evaporator through a TXV or other suitable control means. A thermostatic valve, possibly combined with on/off control, is used in the vapor suction line to control temperature of the space being refrigerated.

Chill storage systems such as this can be designed for a range of discharge duration and temperatures. Temperature requirements dictate the selection of the sorption media. Different complex compounds provide different amounts of vapor pressure suppression, and thus different evaporator temperatures. A sorption media providing evaporator temperature up to 40 C below sorber temperature requires about 2 grams of sorbent and 13 g total system weight per kJ of chill storage. A different sorbent for up to 75 C temperature lift requires 3.2 g of sorbent and 17 g total system weight per kJ of storage. Thus a typical 2.4 ft3 box operating at -18 C with losses of 0.4 W/ C would loose 310 kJ in 6 hours, and the chill storage system would weigh between 4 and 6 kg.

Regeneration temperature will also depend on media selection. Sorbents which provide lower suction pressures also require higher regeneration temperatures. A condenser operating at 35 C nominal temperature will require »108 C regeneration temperature for the media providing 40 K temperature lift, and about 135 C regeneration temperature for the media providing up to 75 C lift. These temperatures can be obtained by any suitable heat source.

The same system provides heat storage. The exothermic absorption reaction can be used to release heat at some useful temperature. Heat storage temperatures up to 100 C or higher are practical, depending on the media selection and the simultaneous evaporator pressure. Heat storage energy densities are typically twice as high as cool storage energy densities. Systems using both heat and cool storage are possible. As an example, an industrial energy storage system providing refrigeration at -9 C and heating at 60 C has been field tested.