REHEAT

Une conversion efficace de la chaleur résiduelle industrielle en chaleur à haute température avec pompes à chaleur à fluide de travail réactif

L’REHEAT aims to contribute to the decarbonisation of Industry with the development of a new vapour compression heat pump, allowing the recovery of available industrial waste heat and its efficient conversion into heat having a temperature level suitable for industrial processes, that is up to 200°C. The novelty of the proposed heat pump is the fact it will operate with working fluids being the seat of a reversible reaction evolving along the thermodynamic transformations occurring in each of its unit operations. Indeed, these reactions being extremely fast in both the forward and backward directions, they evolve as dictated by the chemical equilibrium. More precisely, we propose to study two fluids, namely formic acid and acetic acid. These carboxylic acids are capable of dimerising or, conversely, monomerising, in their vapour and liquid phases, by the formation or breaking of hydrogen bonds. We have preliminarily proven that the resulting simultaneous conversion of thermal and chemical energy of the working fluid leads to: (1) a significant increase the efficiency of the heat pump, since the reactive fluid tends to release more heat (useful effect) and to consume less electricity (input energy) and (2) a reduction in the outlet temperature (critical today) of the compressor, due to an endothermic (i.e., cooling) reaction that occurs along the compression of the reactive fluid. Such a cooling effect allows increasing the heat sink temperature of the heat pump, and therefore the application at higher emperatures, without running the risk of reaching the thermal stability limit of the fluid.

REHEAT aims to prove these potentials, by proposing a modelling and an experimental approach organised around the following four main axes. 1) Theoretical investigation of the efficiency and operating conditions of different applications of high-temperature heat pumps for waste heat recovery, operating with carboxylic acids as reactive working fluids, and comparison with a typical inert fluid operation. 2) Measurement and modelling of the phase equilibrium, volumetric and calorimetric properties of carboxylic acids and their mixture with carbon dioxide and water-impurity. 3) Experimental study of the compatibility of carboxylic acids with the materials potentially used in high temperature heat pumps, considering different levels of commercial purity of formic and acetic acids. 4) Design, realization and testing of a TRL4-HP prototype operating between 80°C and 150°C, to validate the performances and operation of the developed technology.

Some of the tools needed for REHEAT, such as the thermodynamic properties calculation program, are developed in a related ERC-funded project, the REACHER project. The consortium is currently formed by CNRS-LRGP (the coordinator), and 5 partners: Mines Paris PSL- CEEP, responsible for phase equilibrium measurements of formic and acetic acid, and of their mixture with CO2; UCA-ICCF, responsible for the calorimetric measurements of formic and acetic acid, and of their mixture with CO2; UL-IJL, responsible for the analysis of the compatibility of heat pump materials – corrosion behaviour with carboxylic acids; UL-LIEC, responsible for the Raman spectroscopy analysis carried out within the project, UGE-MCD responsible for spectroscopic developments and instrumentation. The coordinator is responsible for the theoretical thermodynamic investigations of heat pumps, the design, the realisation and the test campaign carried out on the prototype, and will participate in the modelling of the experimental thermodynamic data obtained by CEEP and in the realisation and testing of the pilot by ICCF.

The expected improvements introduced by the developed technology, and to be validated in REHEAT, concern: 1) efficiency, which is attested at 70% -instead of current 40-50%- of the Carnot efficiency; 2) the possibility of reaching higher temperatures (150-200 °C) compared to state-of-the-art high temperature heat pumps.