001 -- A Simplified Methodology to Evaluate and Optimize the Geothermal Energy Application Using Depleted Hydrocarbon Wells

A simplified methodology was developed to determine the economically optimum plant configuration for a district heating and cooling system using low-grade geothermal fluids from depleted hydrocarbon wells. To accomplish this: • The optimum geothermal fluid flow rate and temperature supplied to the surface end-use system were determined. • The absorption chiller and the desiccant dehumidification and geothermal district heating systems were 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 surface end-use system modeling was developed by combining these heat-operated system models with load profiles of typical buildings. • The inlet temperature requirements and the maximum temperature drops of each system were studied, and possible system arrangements were investigated using the bin method. • Both the site energy load to energy ratio (LER) and the cost LER were introduced and calculated. When building loads are large and the integrated system operates at full capacity, the optimal arrangement is for the desiccant dehumidification and geothermal district heating systems to operate in parallel. The system was evaluated for a case study site where the yearly total energy output is 8572 MMBtu (9044 GJ), 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 uses the absorption chiller system and the geothermal district heating system in parallel and in series with the desiccant dehumidification system. Its yearly total energy output is 3264 MMBtu (3444 GJ) with a cooling LER of 4.72–7.89 and a heating LER of 4.25–7.24. For all the integrated geothermal systems described above, the operating energy use would be less than one-fourth that of a traditional heating and cooling plant meeting the same loads for the case study site. Additional sites should be evaluated to determine the broader application of the methodology and the use of geothermal energy from depleted hydrocarbon wells.