Exergy analyses of heat supply systems for a building cluster with CARNOT

Abstract

In this paper, a model to simulate community heating systems is presented and energy and exergy analyses are conducted for a district heating system with three different heat generation alternatives. The alternatives are a gas boiler system, a system assisted by solar thermal collectors with a seasonal thermal energy storage and a gas boiler as backup, and a system with geothermal borehole heat exchangers combined with a heat pump. The heat supply of a building cluster of 11 buildings is dynamically modeled using the MATLAB/Simulink based toolbox CARNOT. The aim is to match the low exergy heating demand with a low exergy heat source. To cover an energy demand of 263.7 MWh/a, the geothermal system needs 174.0 MWh/a of exergy, the solar thermal system 269.2 MWh/a of exergy and the gas boiler system 324.9 MWh/a. A parameter study of the solar thermal system shows better results for lower supply temperatures and a lower heat loss coefficient k, but the results depend strongly on the chosen storage size. It was found that the use of fossil fuel could be reduced by 43.8 % for the geothermal system and by 17.6 % for the solar thermal system compared to the gas boiler system. 

Keywords

• Exergy analysis,solar thermal collector,seasonal thermal energy storage,building cluster,heat pump,geothermal borehole heat exchanger,CARNOT,dynamic model,LowEx,low exergy

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