A Preliminary Study on Innovative Absorption Systems that Utilize Low-Temperature Geothermal Energy for Air-Conditioning Buildings

摘要

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 CO_2 emissions. Among the possible renewable energy sources to drive A/C systems, low-temperature geothermal heat (<150°C/300°F) is currently underutilized despite its abundance in the United States and the advantage of steady output. 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, low-temperature geothermal energy is stored and transported at ambient temperature with an energy density of 349 kJ of cooling energy per kg of shipped LiBr/H_2O solution, which is about five times higher than directly transporting geothermal fluid itself for space heating. Key design parameters of a 900 ton TSGA chiller have been determined through computer simulations using 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 three-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 increased energy density has potential to shorten the payback of the TSGA system by 50%.