**3. Working fluid**

After establishing the parameters of the Rankine cycle, the next step is the careful study of the working fluid. The working fluid plays an essential role in the system of the organic Rankine cycle. Its properties are important to the system's efficiency, and they determine the operation conditions such as operating temperature and pressure. Moreover, given the deterioration of the environment, shortage of energy resources, and extreme climate change, its environmental impact and economic viability should be considered as well.

#### **3.1 Working fluid classification**

Above all, depending on the slope dT/ds in a T-s diagram of each fluid in the state of saturation vapor, as shown in **Figure 3** [7], the working fluids can be divided into three categories:


It is investigated that the dry and isentropic fluids are more appropriate in the application of ORCs because there would be much less formation of liquid droplets compared to the wet fluids in the two-phase region, which could reduce the equipment damage such as erosion in the turbine. Apart from this, to delete the risk of liquid droplets, a superheating process is widely recommended for the wet working *Utilizing Computational Methods to Identify Low GWP Working Fluids for ORC Systems DOI: http://dx.doi.org/10.5772/intechopen.1003740*

**Figure 3.** *Types of working fluids: Dry, isentropic, and wet.*

fluids, at the expense of reduced efficiency and increased installation cost; whereas for dry fluids, superheat is unnecessary and can have an inverse impact on the ORC efficiency. In short, dry and isentropic fluids are always preferable candidates regarding operating and economic viability.

#### **3.2 Criteria for fluid selection**

When we chose the appropriate fluid candidates for the ORC application, the working fluids should be equipped with suitable chemical, thermodynamic, and transport properties and satisfy the requirements of environmental protection standards, cost, and safety.

#### *3.2.1 Chemical property*

Compared with the traditional steam Rankine cycle, organic fluids used in ORCs with high critical temperature, low triple point temperature, and low condensation temperature contribute notably to the recovery of heat from low temperature sources in terms of system performance [8].

#### *3.2.1.1 Critical temperature*

Critical temperature is considered as the primary criteria to select the working fluids by most studies. Liu et al. [8] indicated that the critical temperature should be as high as possible compared with the evaporation temperature in the application of a two-stage Rankine cycle in terms of ORC efficiency.

Some researchers [9] explored that for the pure working fluids, the hot source temperature should be 30–50°C higher than the critical temperature to gain the optimal thermodynamic performance, as for the fluids with similar critical temperature, a positive slope d*T*/d*s* of the saturation vapor line in *T*-*s* diagram is served as the second criteria.

#### *3.2.1.2 Boiling temperature*

Working fluids can be easy to handle in an ambient environment, thus, the boiling temperature is expected to be 02013–100°C.

#### *3.2.1.3 Freezing point*

The freezing point of the fluid must be lower than the lowest temperature of the cycle.

#### *3.2.1.4 Triple point temperature*

The triple point temperature should be as low as possible [7], and the condensation temperature should be below the lowest temperature of the ORC system, in case that the working fluids might freeze at any operating temperature in the cycle. Then, a condensation pressure slightly greater than the ambient pressure is preferred to prohibit air entering the ORC system [8]. In addition, the properties of organic fluids vary with operating conditions, and the thermal stability of working fluids need to be guaranteed at the range of working pressure and temperature for fear of chemical deterioration and decomposition at high temperature [10].

Material compatibility should be considered as well. Noncorrosive and noneroding fluids are preferred for the ORC installation.
