**4. Selection of ORC working fluids**

#### **4.1 Working fluid candidates for ORC**

Several studies have been performed to find candidate molecules as alternative working fluids for refrigeration [15], heat pumping, and ORC applications [16]. Considering various criteria (particularly low toxicity, low flammability, low GWP, and high energy efficiency), only a few single-component compounds are suitable for many of the relevant applications. According to the research of Mclinden [6, 15], only a small part of elements in the periodic table would form compounds volatile enough to serve as working fluids. Molecules meeting the screening criteria for working fluids should be limited to small molecules less than or equal to 18 atoms, and the molecules comprise only the elements C, H, F, C1, Br, O, N, or S. Hundreds of substances can be considered as working fluid candidates for ORC application, including hydrofluoroethers (HFEs), hydrofluorocarbons (HFCs), ethers (HEs), unsaturated hydrofluoroolefins (HFO), hydrofluoroiodocarbons (HFIC), fluoroiodocarbons (FIC), fluorinated alcohols, and fluorinated ketones. **Table 2** exhibits pure working fluid candidates for the ORC system.



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


#### **Table 2.**

*Basic properties of alternative working fluids.*

What needs to be emphasized here is that the molecular screening of working fluids requires a trade-off between GWP, toxicity, flammability, and normal boiling point. Taking halogen compounds as an example (**Figure 4**), we can observe rules as follows:


More than 60 million single-component fluids have been screened in a comprehensive project to develop new low-GWP working fluids [22]. Many molecules meet *Utilizing Computational Methods to Identify Low GWP Working Fluids for ORC Systems DOI: http://dx.doi.org/10.5772/intechopen.1003740*

#### **Figure 4.**

*Variation trend of boiling point, toxicity, flammability, and atmospheric lifetime for halogen compounds as a function of element [19]. (a) Variation trend of boiling point of methane-family halogen compounds. (b) Variation trend of toxicity and flammability of halogenated hydrocarbons.*

the requirement of a suitable critical temperature, but the number of molecules decreases dramatically while satisfying both the requirement of low GWP. For each additional screening requirement, the number of optional molecules decreases dramatically. Only 138 molecules with an estimated critical temperature between 47 and 147°C and GWP below 1000 have been identified by Mclinden and co-workers [19]. Because the GWP became an important sector for screening suitable working fluids, some of HFOs and HCFOs have been considered as the optional fourth-generation eco-friendly working fluids for heat pumps and ORC systems [20, 23].

#### **4.2 Assessment of cycle performance**

For a given application (cold source temperature, hot source temperature, cycle structure, etc.), the screening of working fluids is basically such a process: first, build a steady-state simulation model of the ORC cycle under a given heat source and heat sink conditions, and then run this model with different working fluids.

#### *4.2.1 Simulation approach*

We propose an approach to the full modeling of an ORC system. We have only considered a system thermodynamic approach based on the thermodynamic properties of the working fluid. For doing this and as previously commented, some assumptions should be made to simplify the analysis, because geometric parameters of pump, turbine and heat exchangers are initially unknown and have to be designed. These assumptions are essential for different reasons: to ensure the functioning of the cycle according to the thermodynamics laws, avoiding, for example, at some point in the cycle, the fluid temperature exceeds the limits established by the external sources. Moreover, they can also allow a more precise selection of the best working fluid for each application, as it is possible to vary the used fluids and their evaporation and condensation pressures/temperatures. Finally, estimations such as pumping and expansion losses can be made to obtain results that are more similar to what is expected in real processes, in which the heat exchangers are not perfectly isolated

from the environment; therefore, the heat exchangers that occur in the system are not considered perfectly reversible. Moreover, assumptions concerning pinch temperature can also be made to create a more realistic simulation of the ORC and the best comparison of the performance for each working fluid tested.

The proposed architecture for the analyzed cycle in this paper is shown in **Figure 5**.

It is observed that in this architecture, pre- and superheating zones are separated as well as pre- and supercooling zones to simplify the analysis of heating and cooling processes in boilers and condensers. The hypotheses that guide this cycle and make possible calculations for each state are the following:


**Figure 5.** *Schema of the cycle used in the calculation.*

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

on that described in [12], but the value equal to 5 K can also be considered because as the pinch point temperature decreases, the heat transfer effectiveness increases.


In this example, we studied the application of waste heat from annealing furnaces, where the hot source is fixed as water at 363.15 K (90°C), within the temperature ranges mentioned in [25]. On the other side, the cold source is the cheapest and the most common substance: air, at 288.15 K (15°C) and 1 Atm (0.101 MPa). From this, the working fluid evaporation and condensation temperatures are adjusted to include this defined range. **Figure 6** shows a graph created to better illustrate all these cycle processes in the *T*-*s* diagram.

However, to calculate the power produced by the ORC and its efficiency, it is necessary to know the working fluid properties, such as enthalpy, entropy, and density in various states. These values are obtained from the NIST Reference Fluid Thermodynamic and Transport Properties Database (REFPROP) [26], a software whose library includes many substances (including organic compounds studied in this article) and which can be integrated with other calculation environments.

#### *4.2.2 Thermodynamics models*

Section 4.2.1 provides methods for calculating the heat and work quantities associated with various processes, but they are useless without knowledge of the thermodynamic and transport properties. Thermodynamic properties differ from one molecule to another, and the laws of thermodynamics themselves do not provide any description or model of material behavior.

**Figure 6.** *T-s diagram of the modeled cycle.*

An equation of state (EOS) is used to describe the thermodynamic state and the thermodynamic properties of a substance, and the functional formula is used to describe the relationship among pressure, temperature, and volume (pvT) for pure substances and mixtures. Using thermodynamic differential derivation, the thermodynamic properties and phase equilibrium of substances can be calculated from the EOS.

Equations of state can roughly be divided into four categories as follows:
