Capillary assisted vaporization of water for thermal heat pumps
Adsorption heat pumps often employ the natural refrigerant water, since water combines a high enthalpy of vaporization with benign environmental properties. However, at typical working conditions of adsorption heat pumps, water has a very low saturation pressure, which hinders the evaporation process. Better evaporation of the refrigerant can be obtained by higher wall superheats in the evaporator but at the cost of efficiency losses in the heat transfer process. Therefore, the development of evaporators with high overall heat transfer coefficients is crucial for efficient evaporation of water at low pressures.
To enhance heat transfer, macroscopic structures such as micro-fins as well as microscopic structures such as (porous) coatings have been proposed. These structured surfaces enable capillary action to wet the heat exchanger surface with a thin liquid film, which improves the heat transfer into the refrigerant.
Supported by Federal Ministry for Economic Affairs and Energy on the basis of a decision by the German Bundestag