low-temperature geothermal energy, energy storage and transportation, absorption system, renewable energy, economic analysis
Air conditioning (A/C) systems driven by renewable energy have been studied extensively during the past decade as promising alternatives to conventional electricity-driven vapor compression A/C to alleviate stress on the grid as well as reduce CO2 emissions. Among the possible renewable energy sources to drive A/C systems, low-temperature geothermal heat (<150Â°C/300Â°F) is quite underdeveloped despite its abundance in the United States and the unique advantage of steady output regardless of the weather compared to other renewable energy sources. A major barrier to wider utilization is the typically long distances between geothermal sources and potential end uses. In order to overcome this barrier, an innovative two-step geothermal absorption (TSGA) system was studied. With this system, the low-temperature geothermal energy is stored and transported at ambient temperature with an energy density of 360 kJ of cooling energy per kg of shipped LiBr/H2O solution (about three times higher than hot water for typical space heating applications). Key design parameters of a 900 ton TSGA chiller have been determined based on computer simulations with ORNLâ€™s SorpSim software. A case study for applying the TSGA system at a large office building in Houston, TX indicates that, for a 10-mile distance from the geothermal site to the building, the simple payback of the TSGA system is 11 years compared with a conventional electric-driven chiller. To further improve the density of the transported energy, thereby reducing transportation cost and improving payback, a new system using 3-phase-sorption technology is proposed. In this system crystallized salt solution is used to boost the transported energy density. A preliminary study of this new system shows that the enhanced energy density has potential to significantly improve payback.