(444f) Thermophysical Characterisation and Drop-in Assessment of Hydrofluoroethers in Organic Rankine Cycles

The United Nations Climate Change Conference (COP 26) held in Glasgow in 2021 underlined the urgency and potential of transitioning to a carbon-free economy and urged for transparency and rigor in climate action plans from both governments and companies. Still, the world remains off-track to beat back the climate crisis; ministers from nations such as the United States and India agreed that additional reductions in hydrofluorocarbons (HFCs) emissions, as well as other climate pollutants like methane and black carbon, are required to keep global warming below 1.50 ºC and avoid millions of early deaths due to air pollution. According to the Climate and Clean Air Coalition (CCAC) approach, HFCs may be nearly eradicated by 2050, with the potential for a 99.5% reduction (from 2010 levels) [1].

Organic Rankine Cycles (ORC) are experiencing a growing interest due to their ability to generate electricity from residual low waste heat sources. HFC-245fa is an example of a representative working fluid for ORC applications, but it has recently been discontinued as a refrigerant in new equipment because of its high global warming potential (GWP). Hydrofluoroethers (HFEs) may be an alternative because they exhibit good thermophysical properties, while some of them also have low toxicity, zero ozone depletion potential, and low GWP. Nevertheless, the availability of the required thermophysical data of these compounds does not cover the full range of ORCs, making intricate an effective design of technological applications. An accurate analysis of the use of HFEs as efficient substitutes of HFC-245fa requires a detailed thermophysical characterization so as to fully understand the behavior of these fluids in the ORC’s operating conditions. While some properties, such as liquid density and vapor pressure, have been reported for most fluids, this information is limited to specific temperature and pressure ranges. In addition, there is a lack of standardized information of other key data, such as enthalpies and entropies. The absence of a comprehensive experimental characterization can be addressed using computational strategies, with the capacity to quickly identify suitable HFCs’ substitutes while meeting environmental and technological constraints in an efficient, cost-saving way with reliable results. In this context, molecular-based equations of state (EoS) such as those derived from the Statistical Association Fluid Theory (SAFT) have become crucial tools for simulating complex fluid thermodynamic behavior and energy calculations.

In this work, the polar soft-SAFT equation of state has been used for the first time to study the use of nine promising low-GWP alternative working fluids in ORC applications using different key performance indicators focused on energy efficiency and consumption. The thermodynamic model has been validated and employed to characterize these fluids by describing some of their key thermodynamic properties, such as saturated densities, vapor pressure, surface tension, and temperature-entropy diagrams, and has been validated by adequately reproducing binary mixtures with ethers and alcohols, the latter ones here described with a combination of polar and hydrogen bonding interactions for the first time. The model's physical foundation allows for the investigation of the molecular properties of the suggested working fluids and their impact on the physicochemical properties influencing their technical efficacy. Then, based on technical criteria focused on the thermal efficiency, and working and auxiliary fluids consumption, the soft-SAFT model has been used to conduct a feasibility study of HFEs as direct substitutes for HFC-245fa in such application. The simulation results reveal that, while any pure fluid can reach higher efficiencies than the benchmark HFC, HFE-356mmz, HFE-7000, and HFE-7100 are promising replacements capable of approaching system requirements and operating at low pressure with low cooling water and heating fluid flow rates while exhibiting significantly lower GWP values [2]. Based on these findings, the optimal operating conditions for the ORC cycle are determined for these fluids using a rigorous and general approach based on the Helmholtz energy function [3].

Acknowledgements

This work is part of the R+D+I project STOP-F-Gas (ref: PID2019-108014RB-C21), funded by MCIN/AEI/10.13039/501100011033/.

References

1. Climate & Clean Air Coalition. Our 2030 Strategy - Climate & Clean Air Coalition n.d. https://www.ccacoalition.org/en/content/our-2030-strategy (accessed December 27, 2021).
2. Jovell, R. Gonzalez-Olmos, F. Llovell, Energy (2022) Under review
3. J. Gonzalez, F. Llovell, J.M. Garrido, H. Quinteros-Lama (2022) Under review