*Utilizing Computational Methods to Identify Low GWP Working Fluids for ORC Systems DOI: http://dx.doi.org/10.5772/intechopen.1003740*

This type of cycle consists of a boiler/an evaporator, a condenser, a pump, and a turbine. **Figure 1** presents the cycle's elements as well as each numbered step. Then, **Figure 2** symbolizes the cycle (fluid) in a thermodynamic diagram of temperatureentropy (T-s diagram).


In the analysis, it is considered that the cycle operates in a steady-state, which means that all the state variables as well as the fluid flow do not change in time. Furthermore, the variation of the kinetic and the potential energy of the fluid are neglected. Thus, it is possible to calculate the efficiency linked to the ideal Rankine cycle by knowing the properties of the working fluid at different states/conditions. The ideal case occurs when the pump and turbine can guarantee isentropic conditions.

$$\eta = \frac{\mathcal{W}\_{turbine} - \mathcal{W}\_{pump}}{\mathcal{Q}\_h} \tag{1}$$

**Figure 2.** *Rankine cycle represented in the T-S diagram.*

Eq. (1) states that the efficiency is equivalent to the difference between the work generated in the turbine and that needed to pressurize the fluid in the liquid state divided by the heat provided by the boiler. To find the value of these energies, it is necessary to apply the first law of thermodynamics in each cycle component (control volumes). In this case, the efficiency can be written in terms of enthalpy in each cycle point, as described in Eq. (2) below.

$$\eta = \frac{(h\_3 - h\_2) - (h\_4 - h\_1)}{h\_3 - h\_2} = 1 - \frac{h\_4 - h\_1}{h\_3 - h\_2} \tag{2}$$

There are some aspects that directly influence the efficiency of Rankine cycles. A detailed study of these aspects is carried out by Mclinden et al. [6]. Basically, they are related to the increase of the temperature difference between the heat supplied and the heat rejected. For this purpose, some modifications in the cycle architecture are proposed and developed with the objective of taking advantage of the increase in performance that occurs at higher pressures but avoiding high steam humidity levels at the end of the expansion. These solutions include the use of intermediate heat exchangers and/or modifications of the flow rate of the hot source, the system's energy source.
