Abstract
The efficient recovery of low temperature waste heat, representing from 25% up to 55% of the energy losses in industrial processes, still remains a challenge and even Organic Rankine Cycles (ORCs) experience a strong efficiency decay in such a low temperature operating range (T < 150 °C). In similar heat transfer processes, several nanofluids have been proposed as a solution for increasing heat transfer efficiency, but they produced only moderate enhancements of the heat transfer efficiency in comparison with pure fluids. This paper aims at numerically assessing the potential gain in efficiency deriving from the application of an unconventional type of nanoparticles, the metal-organic heat carriers (MOHCs), in the ORC field. In comparison with standard nanoparticles, these MOHCs make it possible to extract additional heat from the endothermic enthalpy of desorption, with a theoretically high potential for boosting the heat transfer capacity of ORC systems. In this paper a numerical model was developed and customized for considering the adsorption/desorption processes of the pure fluid R245fa (pentafluoropropane) combined with a crystal structure for porous chromium terephthalate (MIL101). The R245fa/MIL101 nanofluid behavior was experimentally characterized, defining proper semi-emipirical correlations. Then, an optimization procedure was developed, combining the numerical model with a PSO algorithm, to optimize the thermodynamic conditions in the ORC so as to maximize the contribution of desorption/absorption processes. The results confirm the increase in net power output (+2.9% for 100 °C) and in expander efficiency (+2.4% for 100 °C) at very low heat source temperature. The relevance of tuning the operating cycle and the nanofluid properties is also demonstrated.
Highlights
One of the pillars of the energy transition of the industrial sector is certainly to make industrial processes more circular and more energy efficient
Tem efficiency and the results achieved by the nanofluid R245+MIL101 were com‐
Fluence of the low grade of the heat source temperature on the system efficiency. To deal with this low grade, nanofluids have been more than once suggested as a promising solution
Summary
One of the pillars of the energy transition of the industrial sector is certainly to make industrial processes more circular and more energy efficient. Nowadays, the process efficiency in this sector is still too low [1,2] and a non‐negligible share of the energy losses is represented by low temperature heat, wasted due to the lack of internal heat demand [3]. ORC) [4,5], their effective and sustainable application is still a challenge due to the strong efficiency decay for low‐grade waste heat (T < 150 °C). One of the main reasons for these poor efficiency values is the strong performance decay experienced by pure organic fluids in the case of heat sources characterized by temperatures lower than 150 °C and especially around 100 °C [9]. Several studies have been carried out to identify a working fluid with characteristics suitable for low grade heat sources. Screening criteria based on the fluid thermodynamic and chemical properties [10,11,12,13], the Jakob number [14] and on tuning the heat source temperature and the working fluid critical temperature [15,16,17] have all been proposed
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