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Energies 2020, 13, 4014 7 of 18 Obviously, not all of the fluids listed in Table 2 can actually be proposed as working fluids. For example, hydrogen sulphide is extremely toxic and flammable (see Table 3), while perfluorobutane has a high GWP. All of the fluids in Table 2 were primarily selected to elucidate the impact of the main properties of the different fluids on the final thermodynamic performances of heat recovery Rankine cycles. Other fluids can be considered for more detailed and more specific analyses aimed at selecting the optimum working fluid. Table 3. Some environmental and safety properties of the considered fluids. Fluid Instability/Reactivity (a) 0 Toxicity (a) 2, SA (b) 2 Flammability (a) 0 0 0 4 0 4 1 4 1 3 GWP (d) ODP 1 0 0 1 0 0 0 0 0 13 0.02 0 10 Carbon dioxide Perfluorobutane R134a Propane (R-290) Water Hydrogen sulphide Ammonia Chloromethane (R-40) Dichloromethane (R-30) Methanol 1 1 1 2 0 4 3, COR (c) 2 8600 0 1370 0 ≈20 0 0 3 na 0 0 na na 10 (a) according to NFPA 704, Standard System for the Identification of the Hazards of Materials for Emergency Response, https://www.nfpa.org/codes-and-standards/all-codes-and-standards/list-of-codes- and-standards/detail?code=704 . Toxicity 0–4, Flammability 0–4, Instability/Reactivity 0–4. For a concise codes description, see https://en.wikipedia.org/wiki/NFPA_704 NFPA 704 Codes. (b) Asphyxiant gas. (c) Corrosive. (d) over a 100 year period. Another primary and decisive property that must be taken into account in the selection of the correct working fluid is its thermochemical stability at the devised maximum working temperatures. Assessing the long-term thermochemical stability of a compound is generally very difficult to achieve, given the several variables involved and the different possible operating conditions: the metallic surfaces and their surface/volume ratio, the presence of contaminants, the final acceptable level of decomposition and the corrosion problems. As an example, dichloro-methane (methylene chloride) in a quartz vessel starts to decompose at 450 ◦C and according to a sigmoid curve, after a reaction period of about 350 min, the decomposition reaches a maximum value of about 34% [30]. In a stainless steel (AISI 430F) cylinder, an appreciable decomposition starts at 350◦C [31]. Thus, in general, the presence of different materials and contaminants (air, water, oil) has a strong catalytic effect on the decomposition of the working fluid. However, as the thermochemical stability is strictly correlated to the chemical stability (at least in an inert environment), we take carbon dioxide (a compound with a well established high chemical stability) as a reference; in Figure 5, values of the ratio ∆HB = ∆f H0/nB are reported as a function of the number of chemical bonds in the molecule nB. As the standard enthalpy of formation, ∆f H0 is proportional to the energy required/released during the formation of the molecule, while the parameter ∆HB may be a rough indicator of the relative thermal stability of any working fluid. Certainly, the thermochemical decomposition of a fluid occurs through many complex reactions and—as in all chemical reactions—the activation energy plays an important role. Thus, compounds that are apparently chemically unstable (with a positive ∆f H0) can nevertheless show an acceptable chemical and thermochemical stability. Even so, the parameter ∆HB can give an approximate useful indication for the first classification of working fluids. In Figure 5, all flammable compounds have a value of ∆HB close to zero.PDF Image | CO2 Mixtures as Working Fluid for High-Temperature Heat Recovery
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