A good working fluid should have low vapor and low liquid specific volumes [1]. These properties affect the rates of heat transfer in the heat exchangers. The vapor specific volumes relate directly to the size and cost of the cycle components. Moreover, a high vapour specific volume leads to larger volumetric flows requiring a multiplicity of exhaust ends of the expander and resulting in significant losses. The specific volume of the liquid at the condenser pressure should be as small as possible to minimize the required feed pump work.

2.2. Heat transfer properties

The heat-transfer properties of the working fluids are of great importance. Desirable properties are: low viscosity and surface tension, low specific heat of the liquid, high thermal conductivity, high latent heat of vaporization, and so on [1, 8]. The evaporator enthalpy ratio i. e. the ratio of enthalpy of vaporization to the sensible enthalpy required to raise the temperature of the compressed liquid should have a high value. This has the advantage of reducing the amount of heat required for the preheating of the working fluid and allows most heat to be added at relatively higher temperature and the cycle then, can approach more closely the Carnot cycle.

2.3. Molecular weight

The matter of the fluid molecular weight arises when designing of the turbine. From various investigations found in literature [1, 5], the following conclusion can be made: for a higher temperature or higher power output level using multi-stage turbines, working fluids with low molecular weight (<90 kg/Mol) are favorable while working fluids with high molecular weight (>90 kg/Mol) are suitable for low power output plants using single-stage turbines. The illustration is given in Table 2 which shows the comparison of the turbine isentropic efficiencies for different power levels.