Critical point is the point on a thermodynamic diagram (P-h, T-s) where the properties of liquid and vapor meet and become indistinguishable. The temperature, density and composition of the substance are the same for a liquid or gas at this point. The properties at the critical point are referred to as the critical density, critical temperature, critical volume and critical pressure. Depending on the type of the cycle (supercritical or subcritical), the critical parameters should be compared to the set operating conditions of the cycle. In case the cycle is subcritical as in most cases, the critical point should be appropriately above the system evaporating point in order to optimize the heat transfer [1]. For supercritical cycles the heat transfer occurs above the critical point. Attempts to use CO2 in such solar driven rankine cycles were reported recently in Japan [7]. In table 1 are given the critical parameters of few fluids and other data.

2.1. The cycle pressures

The cycle maximum pressure that is the pressure in the evaporator should not be excessive. Moderate pressure is recommended in order to avoid mechanical stress problems. Very high pressure will cause safety problems and increase the cost of the system by requiring additional safety equipments. The cycle minimum pressure or the condenser saturation pressure should be kept above the atmospheric pressure. A low condenser pressure could lead to problems of sealing against infiltration of the atmospheric air into the system. Therefore, for safety and economic reasons, the pressure in the heat exchange units should be kept in the range 0.10-2.50 MPa [1, 6].