Numerical dimensioning of a pre-cooler for sCO2 power cycles to utilize industrial waste heat

The annual waste heat available from industry in the European Union is more than 2,700 PJ. Consequently, the utilization of the unexploited thermal energy will decisively contribute to a reduced overall power consumption and lower greenhouse gas emissions. Supercritical carbon dioxide (sCO2) power cycles offer a variety of advantages for that purpose compared to established power cycles. Such are a high conversion efficiency and a turbomachinery with high power density. The pre-cooler is one of the essential components in an sCO2 power cycle and the prediction of the flow and heat transfer characteristics is a challenging task. In the present investigation, cycle layouts were developed for one waste heat source: a gas compressor station. The pre-cooler design as well as the boundary conditions of the< numerical simulation were assessed by an analytical model. The most promising design was the printed circuit heat exchanger with inlet temperatures between 209 °C and 352 °C. Subsequently, these heat exchangers were examined in more detail by the numerical code ANSYS CFX for sCO2 mass fluxes between 100 kg/(m² s) and 900 kg/(m² s). The pressure drop along the sCO2 channel was found insensitive to the channel diameter, but increased with the channel length and mass flux. However, the pressure drop of the coolant stream significantly depends on the channel diameter and thus a larger coolant channel diameter is recommended to maintain a reasonably low pressure drop. The overall heat transfer coefficient is limited by the heat transfer on the coolant side. Ultimately, pre-cooler designs were proposed for a waste heat system of a gas compressor station, consisting of compact modular stainless steel plates with an sCO2 channel diameter of 0.5 mm, a coolant channel diameter of 0.8 mm, an sCO2 mass flux of 700 kg/(m² s) and a coolant mass flux of 1029 kg/(m² s). Based on these results more complex channels designs, including internal fins were studied. The fin height was optimized, in order to improve the heat transfer performance.

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