The utility of HP (heat pump) in winter is limited by the problem of frost. The authors discuss a method for preventing HP from frosting, i.e. air is dehumidified by solid adsorbent before entering the evaporator. Not only does air humidity reduce, but also air temperature rises for absorbing adsorbing heat. As a result, the frost problem is resolved, and performance of the HP is improved in winter. The adsorbent bed can be fixed behind the outdoor unit of HP. It is made of zeolite plates, with an active carbon coat. Both sides of each zeolite plate are coated with a very thin layer of a mixture of active carbon and sodium silicate. The active carbon coat (black adsorbent material) can absorb sunlight as a supplementary source of energy for electrical heater for desorption, thus saving electrical energy. Air pressure loss through the bed is so low that the HP fan need not be changed. Using a mathematical model simulation for the sorption process, the method is proven to be valid. (C) 2004 Elsevier Ltd. All rights reserved.