An Optimized Plant For A District Heating And Cooling System Using Low-Grade Geothermal Fluids

Abstract

An optimized plant is developed for a district heating and cooling system using low-grade geothermal fluids from depleted hydrocarbon wells. The optimum geothermal fluid flowrate and temperature supplied to the surface end-use system are determined as 29 gpm and 191°F from inactive wells on the Texas A&M RELLIS Campus. The absorption chiller, the desiccant dehumidification, and geothermal district heating systems are modeled for cooling, dehumidification, and heating and simplified with relationships between the required geothermal fluid temperature and its output at different outside air conditions. The desiccant wheel dedicated outside air system with heat exchanger and condenser water cooling shows the ability to avoid the most cooling coil load and therefore is selected for the surface integrated system. The surface end-use system modeling is developed by combining these three heat-operated system models with load profiles of typical campus buildings. The inlet temperature requirements and the maximum temperature drops of each system are studied, and possible system arrangements are investigated using the bin method. Both the site energy load to energy ratio (LER) and the cost LER are introduced and calculated. When the building loads are large and the integrated system operates at its full capacity, the optimized arrangement is with the desiccant dehumidification system and the geothermal district heating system operated in parallel. When the outside air temperature is lower than 75°F, the geothermal district heating system is operated; otherwise, the desiccant dehumidification system is operated. Its yearly total energy output is 8,572 MMBtu with a total LER of 14.08 – 23.76, i.e. it requires 14-24 times fewer electric Btu than the energy output. When the integrated system matches the building loads well, the optimized arrangement is that the absorption chiller system and the geothermal district heating system are in parallel, and then in series with the desiccant dehumidification system. Its yearly total energy output is 3,264 MMBtu with a cooling LER of 4.72 – 7.89 and a heating LER of 4.25 – 7.24. Thus, all the integrated geothermal systems described above would have an operating energy use less than one-fourth that of a traditional heating and cooling plant meeting the same loads.