The Concept
The present role of thermodynamic cycles.
Thermodynamic cycles constitute the backbone structure of fossil-fuelled and renewable thermal power systems, refrigerators and heat pumps (Carnot, 1824). In thermodynamic cycles, input energies are converted into output useful energy forms (work or heat) by means of an energy carrier, an inert working fluid, which undertakes cycles of thermodynamic transformations. Power cycles have been dominating the global electricity production; and it is expected that refrigerators and heat pumps will represent one of the future major electricity consumers. Improvements of their backbone-thermodynamic structure thus plays a crucial role in the achievement of energy-policy objectives addressing climate change and reducing air pollution, such as the increase of the efficiency of energy use and of the deployment of renewable technologies.
Performance improvement by working fluid selection.
To increase performances of thermodynamic cycles, researches mainly focus on the improvement of their unit operations, on the optimisation of the networking of these components and on the optimal selection of a working fluid crossing the whole cycle. Among these possible measures, the choice of the optimal working fluid represents the core of the process of adaptation of conventional fossil-fuelled thermal engine configurations to exploit lower-grade renewable and waste heat sources, as well as the primary action to reduce the environmental impact of and heat pumps.
Getting stuck on inert working fluids.
It is a fact that only inert working fluids (pure fluids or mixtures) are currently employed and that, despite their optimal selection, the thermal efficiency of these energy conversion systems remains far from the maximum achievable ones, dictated by Carnot limit (Cullen and Allwood, 2010).
Involving chemical energy in thermodynamic cycles: a hint for scientific progress.
A breakthrough idea was suggested by Lighthill (Lighthill, 1957): he proposed to convert the chemical energy like the “large energy change involved in dissociating gases” into work. Practically, he put forward the idea to use reactive working fluids, instead of inert ones, in closed power cycles. Differently from current energy conversion systems for electricity production which are formed by one or more devices being the place of distinguished energy-type transformations, Lighthill’s idea consists in a novel energy conversion process, where the transformation of thermal energy into mechanical one is made possible by the concurrent thermal and chemical conversion of the energetic state of a reactive fluid, all along a closed “thermo-chemical” cycle.
REACHER: why now?
Starting from 1960s, some researchers have investigated the performances of classical power cycles operating with some reactive working fluids, mainly N2O4(g) ⇄ 2NO2(g) ⇄ 2NO(g)+O2(g) (Krasin and Nesterenko, 1971; Bradley, 1976; Angelino, 1979; Sorokin, 1979; Kesavan and Osterle, 1982). Probably due to the fact that, at the time of these studies, the potential of inert fluids was not yet totally investigated, the more advanced idea of using reactive fluids was not fully appreciated and thus remained scientifically unexplored. Now that the use of inert fluids reveals limitations, REACHER intends to revive Lighthill’s idea.
The methodology
WP1 – Development of a computational tool for the prediction of thermodynamic properties of reactive multiphase fluids.
The achievement of REACHER’s targets relies on the realization of a reliable tool to predict thermodynamic properties of reactive fluids. That requires the preliminary selection, improvement and implementation of a predictive equation of state for mixture modelling, and of algorithms for chemical equilibrium calculation. This is the aim of WP1.
WP2 – Establishment of a list of “suitable” reactive fluids.
The objective of this work package is the definition of a list of reactions, considered suitable for the set of studied applications, and the characterisation of their thermodynamics and kinetics. That consists in the implementation of the following steps:
- Definition of thermochemical criteria (ranges of enthalpy and entropy of reaction) for reaction searching and design;
- Search for and design of reactions fulfilling pre-defined thermodynamic criteria;
- Characterization of the kinetics of listed fluids, and further selection of fast reactions.
WP3 – Optimisation of thermodynamic cycles operating with reactive fluids.
Considering a specific application of power, heating or cooling, the optimization of the overall system (fluid and cycle architecture) consists in two steps. Firstly, a process design method will be applied to determine the optimal cycle’s architecture for each fluid of the list defined in WP2. Secondly, a comparison between all these optimized systems, on the basis of their performance indicators, will provide the best fluid-architecture system. Finally, the same process design method will be applied considering some inert working fluids. Optimal solutions obtained with inert and reactive working fluids will be finally compared, to quantify REACHER’s gain.
WP4 – Experimental assessment of the transformation of thermal and chemical energy into work.
This part of the project is devoted to the experimental validation of the energy conversion process undergone by two preliminary selected reactive fluids in the expansion taking place in a micro-axial turbine. The composition change across the turbine will be measured by Raman spectroscopy techniques.
The impact
Main scientific impacts.
- The breakthrough understanding of the fundamental relation between reactive fluid characteristics and their energy transformations opens a new scientific research field on the exploitation of chemical energy.
- The application of cutting-edge methods for the discovery and characterization of new fluids and reactions contributes to address one of the biggest scientific challenges: product discovery and characterization.
- This project contributes to improve the analysis of spectroscopy measurements used to quantify fluid composition, an open research field.
Main societal impact. If the expected performances of analysed thermo-chemical cycles are confirmed, the outcomes of this project will allow the enhanced exploitation of available waste heat and renewable thermal energy sources, by means of small, powerful and efficient machines.
Disclaimer sentence
Funded by the European Union. Views and opinions expressed are however those of the author only and do not necessarily reflect those of the European Union or European Research Council Executive Agency. Neither the European Union nor the granting authority can be held responsible for them.
References
Angelino, G. (1979) ‘Performance of N2O4 gas cycles for solar power applications’, Proceedings of the Institution of Mechanical Engineers 1847-1982 (vols 1-196), 193(1979), pp. 313–320. doi:10.1243/PIME_PROC_1979_193_033_02.
Bradley, W.J. (1976) ‘Recouping the thermal-to-electric conversion loss by the use of waste heat’, in Low-grade heat: a resource in cold climates. Chalk River Nuclear Laboratories, pp. 535–558.
Carnot, S. (1824) Réflexions sur la puissance motrice du feu et sur les machines propres à développer cette puissance. Bachelier Libraire.
Cullen, J.M. and Allwood, J.M. (2010) ‘Theoretical efficiency limits for energy conversion devices’, Energy, 35(5), pp. 2059–2069. doi:10.1016/j.energy.2010.01.024.
Kesavan, K. and Osterle, J.F. (1982) ‘Split-Flow Nuclear Gas Turbine Cycle Using Dissociating N2O4’, in. ASME 1982 International Gas Turbine Conference and Exhibit, American Society of Mechanical Engineers Digital Collection. doi:10.1115/82-GT-181.
Krasin, A.K. and Nesterenko, V.B. (1971) ‘Dissociating Gases: A New Class of Coolants and Working Substances for Large Power Plants’, Atomic Energy Review, 9(1), p. 177.
Lasala, S. et al. (2021) ‘Thermo-chemical engines: Unexploited high-potential energy converters’, Energy Conversion and Management, 229, p. 113685. doi:10.1016/j.enconman.2020.113685.
Lighthill, M.J. (1957) ‘Dynamics of a dissociating gas. Part I: Equilibrium flow.’, Journal of Fluid Mechanics, 2, pp. 1–32. Sorokin, A. (1979) ‘Dissociating Nitrogen Dioxide (N2O4) as a working fluid in thermodynamic cycles’, Nuclear Science and Engineering, 72, pp. 330–346.