ECN: Working principle
How does a thermochemical heat pump work?
The operation principle of a thermochemical heat pump is based on the reversible absorption and desorption of a vapour (water, alcohol, ammonia, hydrogen) in a solid substance (salt, ceramic, metal, etc.). The absorption of a vapour is an exothermic process which delivers heat, while the desorption of vapour requires heat (endothermic). The specific solid/vapour couple determines the temperature range in which the heat pump operates. It is possible to construct a heat pump by using two different solids. Pure condensation and evaporation can also be used instead of a second solid.
A schematic view is given below which shows the use of heat effects that occur during absorption/desorption of a vapour in a salt to construct a cooling machine. The picture shows vapour equilibrium lines belonging to two different salts. The low temperature salt (LTS) has a higher vapour pressure than the high temperature salt (HTS) at the same temperature. A heat source of, for example, 140?C (Th) is used to desorb vapour from the HTS. This vapour is absorbed by the LTS that delivers heat at a temperature level Tm of 30?C. This proces continues until enough vapour is released from the HTS and has been transported to the LTS by the pressure difference between the two salts. This process is called ?charging?. Following this process is the ?discharging?. Heat is supplied to the LTS at a temperature level Tl of, for example, -40?C. This heat is supplied by the process stream that has be cooled. The vapour that is desorbed by this heat is transported by the pressure difference to the HTS. The HTS absorbs the vapour at a temperature level of Tm. This ends the ?unloading?. The overall effect is that the temperature difference Th ? Tm in the HTS is used to pump heat from Tl to Tm in the LTS.
The heat flows in this figure are reversed when a heat pump instead of a cooling machine is intended. Heat is then supplied at a temperature level TM and released at the ambient temperature TL and as useful heat at temperature level TH.
Since at some point in time all vapour has vanished from the LTS, such a system can only be operated in a batch way. The heat flows have to be reversed in order to return the vapour to the LTS and regenerate the salt. This batch process can be made continuous by doubling the amount of reactor vessels and have them run in parallel.
