Fig.: Exploded diagram of an electrochemical compressor cell.
To a layman, a heat pump seems to magically draw heat from thin air. But behind that perceived magic is a construction with mechanical compression of fluids, supplying the work needed for useful heating and cooling. This article presents an alternative to the mechanical compression, moving away from the use of moving parts. Instead, the compression is done through a chemical process. The phenomenon, called electrochemical compression (EC), has the potential to be used in applications such as heat pumps, energy storage and dehumidification.
Two different EC technologies are looked into, one including ammonia and one including water vapor. They both make use of electrochemical processes in membranes, based on similar principles that are used to generate electricity in fuel cells. To make the process work, the EC device needs both a membrane and electrodes. An external voltage is supplied across the two electrodes causing the working fluid, i.e., ammonia or water, to chemically react with a carrier gas to form ions. Unlike the original working fluid, the ion is able to traverse the membrane. When the ions then reach the opposite electrode, the chemical reaction goes in the opposite direction, turning the working fluid back to its original state.
Through this process, a gas can be transported from an area of low concentration to an area of high concentration. This means that a compression of that gas is possible, which is how the process can be useful in various energy applications.
Specifically, the ammonia EC could be used to drive a heat pump or refrigeration cycle using ammonia as refrigerant. It might also be possible to use in energy storage technology. Experiments show that it is possible to reach a reasonable energy consumption for the process. The main potential lies in applications that require high pressure ratios, for example liquid storage of ammonia that could then be used as a carbon-neutral fuel, provided carbon-neutral raw materials and process energy are used for the manufacture of the ammonia.
The water EC could be used as a dehumidifier to reduce latent cooling load in air conditioning applications. Traditionally, dehumidification uses the process of condensation. This includes cooling of water vapor below the dew point, a process that requires energy input. With electrochemical condensation, the cooling is not needed, and the energy need is thus reduced. Research will continue so as to determine the effectiveness as compared to the current state-of-the-art in conventional dehumidification.
Joe Baker, Yunho Hwang and Reinhard Radermacher, USA (Department of Mechanical Engineering, University of Maryland)
Longsheng Cao and Chunsheng Wang, USA (Department of Chemical and Biomolecular, University of Maryland)