Exergy-Rational Utilization of Low-Enthalpy Geothermal Energy Resources and Exergy-Based Limitations

Abstract

Along with continuing the efforts of decoupling CO2 emissions from sustainable development, exergy-rational cities are becoming universally important. This statement does not mean that energy rationality and energy efficiency are ignored. But it is a fact that the mission of the First-Law of Thermodynamics has been almost completed in a pretty good rate of success. Because the Second-Law is especially important and often ignored in geothermal energy, especially for low-enthalpy sources, now it is time to analyze such systems in terms of exergy. Within this context, ORC systems, which are touted as useful systems for utilizing low-enthalpy geothermal resources and ground-source heat pumps, are critically analyzed. In this context, two cases regarding an ORC, which is used only for power generation without utilizing its waste heat and a heat pump operating on grid power, were examined and was concluded that they are not exergetically sustainable, if they operate as individual systems. This study developed an analytical model, which reveals with case studies and examples that a broad hybridization of combining geothermal energy-based ORC technology, heat pumps, absorption units, thermal storage, and other renewable energy resources, like solar and wind provides sustainable and exergetically rational design solutions. New evaluation and rating metrics based on Rational Exergy Management Model were introduced. The option of incorporating low-enthalpy geothermal energy resources at about 80°C to support the model is evaluated. The Hydrogen City model is applied to a new settlement area with an expected 200,000 inhabitants to find that the proposed model can enable a nearly net-zero exergy district status. The results have implications for settlements using geothermal energy and hydrogen energy together towards meeting net-zero targets.Such an enrichment of the multiple systems even in a simplistic manner in an exergy economy cycle analytically reduces CO2 emissions by about 66%, when compared to a conventional district energy system utilizing natural gas.