Methanol-based heat pumps for storage of solar thermal energy. Phase I

ABSTRACT:
The reaction of CH30H vapor with a solid-state inorganic salt substrate to produce a solid methanolated salt can be used as the basis of a combined solar heat pump/thermal energy storage system. Such a system should be capable of storing heat indefinitely at ambient temperature, and can be

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used both for heating and air conditioning. In solar heating, the coefficient of performance should be in the range 1.5-1.8, thus reducing the size of the solar collector required. In solar air conditioning, the coefficient of performance should be in the range 0.5-0.8, comparable with other solar air conditioners; however, the built-in long term storage feature should eliminate the need for resistive electrical backup, thus sharply reducing operating costs. Experiments carried out in Phase I of this program indicate that the reaction of CaC12 with CH30H vapor to produce CaC12·2CH30H is well suited to both the heating and cooling applications. The heat of reaction is about 20 kcal per mole of CH30H, and the kinetics and thermodynamics of the reaction appear adequate down to a CH30H pool temperature around -10°C. The required solar collector temperature should be about 150°C. Analytical and experimental work on the design of the heat exchanger for the reacting salt bed indicate that high rates of heat transfer can be accomplished in a reasonably compact system; the indicated energy density for the solid-phase reactant is in excess of 13,000 BTU/ft3 , assuming that the void fraction for anhydrous CaC12 is 85% or less. Vapor pressure losses through the bed can be minimized by the use of pelletized CaC12. Furthermore, this material appears to react without apparent degradation of the rate of methanolation or demethanolation through 300 complete cycles, and does not appear to corrode aluminum. No major technical obstacles to the development of a CH30H-based solar heat pump/storage system are apparent, and Phase II work is now proceeding on the development of a prototype system.

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