Rapid estimation of geothermal coverage by district-heating systems

An algorithm is developed for the quick estimation of the fraction of annual heat-load that is covered by geothermal energy in district-heating systems. The cases of direct use, heat exchange and heat-pump application are distinguished. The algorithm proved accurate enough for pre-feasibility study purposes and for preliminary considerations with regard to the management of geothermal energy. It is demonstrated that for coverage estimation, detailed heat-supply calculations can be substituted by the use of degree-days, which are quite often available for various base temperatures. Furthermore, the analytic expressions of the algorithm allow the quantitative evaluation of several parameters. Specifically, for low temperature geothermal fluids the coverage is an increasing second-degree function of the temperature of the fluids. Increasing network return temperatures has also a decreasing effect on the coverage. The required geothermal-flow is inversely proportional to the heat exchanger effectiveness, for the same coverage. Application of a heat pump results in a higher coverage that should otherwise require a higher temperature fluid. It is consequently defined as an equivalent geothermal-temperature increase - due to the heat pump - that is proved to be proportional to the compressor capacity. Last, the network supply-temperatures have a rather secondary effect upon the coverage, affecting the flow through the network and so the effectiveness of the heat exchanger.