**6. Example of simulation of an ORC for working fluid selection**

This section illustrates the strategy presented on working fluid selection. We propose to discuss an example of selecting a fluid for utilizing available waste heat. In our case study, we have one heat source at the temperature of 120°C and one cold source at the temperature of 15°C.

We remind that the purpose of an ORC system is to generate power from lowtemperature heat sources. Traditionally, water is used as the working fluid in Rankine cycle applications. However, because of its characteristics, water is not suitable for all applications, especially when the power of the heat source is low (unlike in coal, gas, or nuclear power stations). In a study by Dickes et al. [36] various case studies for ORC applications were presented, covering different power levels. The study discussed several working fluids, considering their impact on the environment (zero Ozone Depletion Potential [ODP] and lower Global Warming Potential [GWP]), as well as their inflammability and toxicity. To select the most suitable working fluid, a virtual machine can be used to simulate a simple ORC system. It is important to clearly define the main characteristics of the simple ORC system to conduct the simulation effectively.



#### **Table 3.**

*Properties of the selected working fluids from NIST data base [26] and safety data issued from ASHRAE 34, 20,010 [17].*


The simulation is realized using the Aspen plus software [37]. The Peng Robinson equation of state is considered for all the calculation. Four heater modules have been selected with heat flux connection between REGEN and PRE-HEAT (recuperation of heat). It can also be called Internal Heat Exchanger. GENV is the generator of vapor, and COND is the condenser. Normally, this process contributes to increased work output and so obtains a better overall efficiency. One TURBINE and one PUMP have also been considered.

The thermodynamic performance can be evaluated by considering two coefficients:

1. the energy efficiency given by the work produce over the heat absorbed by the cycle (Eq. 3)

$$\eta = \frac{\mathcal{W}\_P + \mathcal{W}\_T}{Q\_{GENV}} = \frac{(h\_2 - h\_1) + (h\_4 - h\_3)}{h\_1 - h\_{4bit}} \tag{11}$$

2. the volumetric capacity (VC), which corresponds to the work produce per unit of working fluid. If you need an important work, energy, and if the volumetric capacity is low, you need a large flow of working fluid. This means you need to pay attention to the design and sizing of the pipe.

$$\mathcal{V}\mathcal{E} = \rho\_1 \mathcal{W}\_P + \rho\_3 \mathcal{W}\_T = \rho\_1 (h\_2 - h\_1) + \rho\_3 (h\_4 - h\_3) \tag{12}$$

The results of each simulation are presented in **Tables 4**-**8**. **Table 8** presents the comparison of the performances.

Exergy efficiency can also be considered to compare the performance of the fluid (**Table 8**). This analysis is very useful for ORC: exergy can be considered as the quality of energy because it represents the useful part of energy. The definition is given by Eq. 13. *T*<sup>0</sup> is the ambient temperature and *T*<sup>h</sup> is the temperature of working fluid at the outlet of the generator of vapor. For pure working fluid and for our case study, this criterion is not a very different form of energy efficiency, but it can be very


**Table 4.**

*Values of temperature and pressure for R245fa of each point of the cycle.*

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


**Table 5.**

*Values of temperature and pressure for R365mfc of each point of the cycle.*


#### **Table 6.**

*Values of temperature and pressure for R1233zd of each point of the cycle.*


#### **Table 7.**

*Values of temperature and pressure for R600 of each point of the cycle.*


#### **Table 8.**

*Cycle comparison between the 4 working fluids selected.*

useful if a heating power is given (and not a temperature) or a zeotropic mixture is used and if the temperature of the heat or cold sources vary.

$$\eta\_{\text{EX}} = \frac{\text{Exergy recovered}}{\text{Exergy consumed}} = \frac{W}{Q\_h \left(1 - \frac{T\_0}{T\_h}\right)}\tag{13}$$

<sup>1</sup> � *<sup>T</sup>*<sup>0</sup> *Th* is also called the Carnot's factor. Eqs. 11 and 13 are considered in our example (**Table 8**), and we can observe that the most efficient fluid is R365mfc (it will be same if exergy efficiency is calculated but the half of the energy that can be used by the steam generator is transformed into work). R365mfc has the highest critical temperature value. However, its volumetric capacity (VC) is very low, requiring a larger quantity of fluid (due to its low density compared to other fluids). Additionally, it is worth noting that the condenser pressure is lower than the atmospheric pressure,

**Figure 8.** *Structure of the organic Rankine cycle using Aspen plus with recuperator (internal heat exchanger).*

which is not recommended. Based on the ORC performance as well as environmental, hygiene, and safety criteria (EHS), it appears that the most suitable fluid for this example is R1233zd(E). The criteria related to installation volume, environmental protection, and operator safety should play an important role in future studies. For a complete selection of the fluid, it will be interesting to size the different equipment of the ORC and to evaluate if the sizing is normal in terms of the surface of heat exchangers or the speed of fluid in the turbine (subsonic flow) for example.
