Study on Heat Transfer in Heat Exchangers for a New Supercritical Organic Rankine Cycle

Electrical power from geothermal heat is hardly efficient, if the temperature level of geothermal water is low. This situation is predominant in middle Europe, for example, in Germany, where even in hot-spot areas like Norddeutsches Becken, Oberrheingraben, and Molassebecken the temperature of geothermal water is less than 160°C. For this, efforts have to be made in optimizing the power cycle efficiency. Supercritical fluids provide a higher net energy output per unit mass than subcritical fluids but their physical properties strongly depend on temperature variations, especially close to the supercritical point. Based on the requirements of a new modular and mobile supercritical power cycle MONICA (modular low temperature power cycle Karlsruhe), which will be built within the next few years, different heat exchanger types are investigated within this study in order to determine the most compact design with respect to mobility, mountability, and efficiency. Changes in physical properties of propane, the working fluid of the new cycle, are taken into account by iterative, stepwise calculation of heat exchanger types like double-pipe, shell-and-tube, and plate heat exchangers. For this, common Nusselt number correlations are implemented in the stepwise iteration. Influence of geometry on flow conditions and analysis of part load sensitivity are provided.