Composite reactants of calcium chloride combined with functional carbon materials for chemical heat pumps

Enhancement of heat transfer in the reactor bed is one of the most important subjects for developing gas–solid chemical heat pumps. This study deals with composite reactants combining calcium chloride with expanded graphite (EG) and activated carbon fiber (ACF) to promote the reaction between calcium chloride and working fluid of methanol. We measured variations of effective thermal conductivity as well as volume and void fraction during reaction cycles. Effective thermal conductivity of the EG composite bed was larger than 60% that of untreated calcium chloride bed. Because of the bulky structure of expanded graphite, the overall void fraction of EG composite was larger than 0.8. When the overall void fraction was reduced below 0.78 by compressing the bed, the effective thermal conductivity was sharply increased up to 10 times higher than that of untreated calcium chloride bed. On the contrary, the effective thermal conductivity of ACF composite bed was slightly smaller than that of calcium chloride bed and no remarkable change was brought about by compression. The volume and the overall void fraction of these composite particle beds varied only slightly with the amount of methanol reacted, while those of calcium chloride bed without carbon materials were largely influenced by reaction.