Modeling of an Ammonia/Water Absorption Heat Transformer to upgrade low-temperature industrial waste heat both at machine scale and at local scale inside the falling film absorber
Résumé
For numerous years, a drastic increase in energy demand has been observed worldwide, mainly originating from fossil fuels, despite an alarming need for a decrease in energy consumption and CO2 emission. Numerous industries need heat at a high temperature while dissipating heat at moderate to low temperature. Answering this need in an environmentally friendly way is required to mitigate the impact of global warming. In this context, sorption systems are a promising solution able to valorise waste energy at a low electricity cost, up to ten times less electrical power compared to an equivalent vapor compression heat pump. Specifically, Absorption Heat Transformer (AHT) are a type of absorption machine able to recover low-level waste heat (~80°C) in order to provide higher-level heat (>100°C) necessary for vapor production or other industrial uses. In this study, a single stage AHT based on the NH$_3$/H$_2$O mixture is modeled and simulated. A thermodynamical equilibrium model has been developed to describe the behavior of the machine as a whole. This model assumes that the solution quickly reaches saturation at the absorber inlet to be able to transfer its heat to the hot source. However this assumption needed to be challenged. To better describe the absorption process, a refined plate heat exchanger falling film model has also been developed by coupling heat and mass transfer equations. Results enable to evaluate the performances of the AHT fed by industrial waste heat and cooled by ambient air. The influence of both the waste heat and ambient air temperatures are evaluated. Different coefficients of performances based on the heat transferred and/or the electricity consumption are calculated as well as the expected output temperature of the hot heat sink. The comparison between both model allowed to asses the validity of the simpler thermodynamical equilibrium model to describe the complex absorption process in the case of an AHT and to assess the need of reaching saturation at the absorber inlet. Finally, the range of valuable working conditions for the system are discussed.
Origine | Fichiers produits par l'(les) auteur(s) |
---|