Solar Thermal Geothermal Hybrid System With a Bottoming Supercritical Organic Rankine Cycle

Climate change has spurred an interest in renewable energy. Many renewable energy technologies are intermittent, such as solar energy, or are dependent on transient conditions such as the ambient temperature in the case of geothermal energy. While solar thermal energy is able to achieve high temperatures and efficiencies, geothermal energy is limited by its lower temperatures which results in low conversion efficiency. There is an opportunity to create a hybrid system using both solar thermal and geothermal energy to improve their stand-alone performance.\nIn the literature, solar-geothermal hybrid systems are limited by the temperature of the solar field and the cycles and fluids used. In this study, a hybrid solar thermal-geothermal system is studied with a combined cycle operating from two temperature sources: the high temperature source is provided by solar power tower (SPT) and geothermal provides the lower temperature. The innovation lies in the implementation of the geothermal source into the combined cycle and the inclusion of a recuperative supercritical organic Rankine cycle (ORC) as the bottoming cycle to further enhance the system and also capable of operating with the geothermal source only. First, an ORC is optimized for geothermal reservoirs with temperatures between 170 and 240°C. It was found that the optimized parameters result in wet fluids achieving lower expansion ratios. Only two fluids were optimized with a subcritical configuration due to proximity to the critical point.\nNext, the combined cycle was developed and optimized. This analysis was performed assuming only one heat source, such as solar energy, being introduced to the topping cycle. Based on the literature review, a recuperative supercritical carbon dioxide Brayton cycle was chosen as the topping cycle. The pressures of both cycles were optimized as well as the approach temperature difference between the two cycles. The same fluids considered in the first analysis were considered in the bottoming recuperative ORC cycle except ethane and carbon dioxide which performed the worst. The optimized conditions were used for the hybrid analysis.\nThree hybrid configurations were analyzed where the geothermal source was introduced to the combined cycle in various locations. In the literature, the most common solar-geothermal hybrid system analyzed was where the solar energy through parabolic troughs was used to add additional heat to either the geothermal source or directly to the working fluid to increase the cycle temperature and efficiency. In one hybrid configuration, the geothermal source was used to superheat the organic Rankine cycle. The two other configurations used geothermal energy to preheat the carbon dioxide after recompression or to reheat it after recuperation and before being introduced to the ORC. The incremental effectiveness due to geothermal heat, i.e., the additional work that is converted from the additional heat added from the geothermal source, was analyzed.\nFinally, the best performing hybrid system for a maximum cycle temperature of 500°C was selected and analyzed transiently with thermal storage. The superheat hybrid configuration with acetone as the working fluid with a recompression topping cycle was chosen. When solar energy and thermal storage was not available, the ORC was run with geothermal energy. As the acetone has a critical temperature above the temperature considered for the geothermal source, it resulted in a subcritical ORC. Subsequently, the power ratio between the sCO2 cycle and ORC was very low. For this configuration, thermal storage was very beneficial to extend the time of high-power production.

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