| Abstract |
Low-temperature geothermal resources (below 150°C), although abundant in the United States as stable renewable energy sources, are typically far away from major energy demands. The low temperature of the resource has limited the efficiency of electricity generation (i.e., with Organic Rankine Cycle) to below 15%, which is too low to achieve economic payback in typical system lifetime. Meanwhile, transporting this low-temperature thermal energy via hot water at an energy density of around 145 kJ/kg in current direct use heating (DUH) systems is not economically feasible over distances longer than 2 miles. Therefore, utilization of low-temperature geothermal energy has been largely limited to remote areas, and the abundant resources have remained quite under-developed so far. To overcome this distance barrier, an innovative two-step geothermal absorption (TSGA) system was proposed in previous studies to store the low-temperature geothermal energy in liquid desiccant at ambient temperature with an energy density of 405 kJ/kg. With TSGA system, strong solution of the liquid desiccant is used to provide space cooling in buildings, and the resulting weak solution is regenerated at the geothermal site. In this study, an enhanced design of the TSGA system using salt hydrate crystals (e.g. LiBr-H2O), referred to as CTSGA, as well as two other alternative technologies (solid desiccant adsorption cooling and absorption ice making/storage) that can store and transport geothermal energy with an energy density higher than that of the conventional DUH were identified. Among all these technologies, CTSGA achieved the highest energy density (915 kJ/kg). However, it requires a higher geothermal resource temperature than that for typical TSGA system (100°C). Thermodynamic models of these technologies were developed and implemented into a screening tool that can calculate the energy consumption and efficiency of each potential technology under user-specified operating conditions. The screening tool can also estimate the investment and operation costs of the investigated systems. With all the costs calculated, the economics of the two-step geothermal cooling systems can be evaluated and compared with the conventional electric cooling system. Sensitivity analysis were carried out using the screening tool in a case study for utilizing an existing hydrothermal resource near Santa Rosa, CA, which has 4,447 L/min flow rate and bottom hole temperature (BHT) of 138°C, to provide space cooling to distant office buildings. The economics of applying the CTSGA and other two alternative technologies were investigated for the given geothermal resource under a range of conditions including distances, electricity rates, and cooling load profile (i.e., peak and total cooling load). The result demonstrated that the paybacks of the two-step geothermal cooling systems would be shorter with closer distance, higher electricity rate, lower peak load, and larger total cooling demand. Among all the investigated systems, the best payback is achieved by the CTSGA system, which yields less than 10-year payback for a 50-mile (80 km) distance between the hydrothermal resource and the office building. |