| Abstract |
Low-temperature geothermal resources (less than 150癈, or 300癋) are abundant in the United States. Although higher temperatures are preferred for power production, low-temperature resources can directly provide buildings with space heating, or drive an absorption chiller for space cooling. One of the barriers to wider utilization of the low-temperature geothermal resources is the typically long distance between geothermal sources and potential end uses. An innovative two-step geothermal absorption (TSGA) system was recently proposed. With this system, the low-temperature geothermal energy is stored and transported at ambient temperature with an energy density significantly higher than transportation of hot water. A conceptual design of the TSGA system has been developed based on the single effect absorption cycle. LiBr/H2O solution is selected as the working fluid pair. Key design parameters of a 900 ton (3,165 KW) two-step absorption chiller have been determined based on computer simulations. Energy density of the transported solution is 349 kJ of cooling energy per kg of shipped LiBr/H2O solution, which is about 5 times higher than that of transporting hot water for typical direct use applications. Technical challenges identified include: (1) minimizing the required volume and the associated transportation cost of the working fluid; (2) maintaining appropriate vacuum levels at various components of the absorption cycle; (3) retaining good quality of the working fluid during transportation and storage; and (4) harvesting heat from geothermal wells sparsely located and with varying production rates. A case study for applying the TSGA system at a large office building at Houston, TX indicates that, for a 10-mile distance from the geothermal site to the building, the simple payback of the TSGA system is 10.7 years compared with a conventional electric-driven vapor compression chiller. It is found that the payback of the TSGA system is highly sensitive to the distance, building size (cooling loads), transportation cost, and the electricity rate. It is also found that, for a 10-mile distance, transporting the working fluid with tanker trucks leads to lower life cycle cost than a pipeline using high-density polyethylene pipes. Transportation cost is the most significant contributor to the life cycle cost of the TSGA system. One approach to reducing transportation cost is to increase the energy density of the transported solution by enlarging the concentration difference between strong and weak solution. It is possible to enlarge the concentration difference by integrating the TSGA system with a desiccant dehumidifier to enable separated sensible and latent cooling, or utilizing crystals of the solution to enable local regeneration. |