Salt impregnated carbon fibres as the reactive medium in a chemical heat pump: the NH3–CoCl2 system

Some of the best heat transfer results with chemical heat pumps have been obtained using chloride salts intercalated into graphite. As an alternative to intercalation, this work examined an impregnation technique that produced cobalt chloride salt crystals dispersed on carbon fibres. It is a less complex preparation procedure than the intercalation technique that has been used in other studies. Ammonia was used as both the reagent for the inter-conversion of CoCl2•2NH3 and CoCl2•6NH3 compounds and as the refrigerant. The performance of the salt–fibre material was evaluated in experiments using a bench-scale chemical heat pump reactor. The reactor was weighed continuously during the decomposition reaction, thereby providing an instantaneous measurement of the reaction rate that was independent of heat transfer measurements. The power produced by the system was measured while simultaneously monitoring temperatures, pressures, and flow rates throughout the reaction system. At a decomposition pressure of approximately 1000 kPa, power densities of 280 kW/m3 were recorded. The results obtained in this work are within the range of power densities reported previously using more complex intercalated graphite materials. This indicates that the simpler impregnation preparation method may produce materials that are suitable for practical chemical heat pumps. Consistent with other work that has been reported in the literature, degradation in the performance of the salt–fibre material was observed as a function of the cumulative time on stream. Temperature transient measurements in the reactor indicated that heat transfer was the rate-determining step for these decomposition experiments.