Ammonia in low capacity refrigeration and heat pump systems

Ammonia has been used as refrigerant in large vapour compression systems continuously since the beginning of the era of refrigeration. In small systems, it has hardly been used at all since the introduction of the halogenated hydrocarbons around 1930. Lately, with the search for alternatives with less influence on global warming, the use of ammonia in small systems has come into focus again.In the present paper, the work done at the Royal Institute of Technology (KTH) with the aim of developing a prototype of a domestic water to water heat pump with a heating capacity of 9 kW is presented. It has been shown that such a system can be designed to operate with about 100 g of ammonia.Crucial problems in the development of the direct expansion system were to arrange for oil return, and to achieve good heat transfer in the evaporator. These problems were solved by use of an oil which is soluble in ammonia.The main obstacle for introducing this technology commercially is the limited supply of components. Particularly, there are no hermetic or semi-hermetic compressors for ammonia available in this size range.