Energy performance of decentralized solar thermal feed-in to district heating networks

Many papers have been written over the last years addressing the potential of decentralized solar thermal systems in District Heating Networks (DHN), assuming that the network can be used as a virtual storage so that prosumers can release excess solar energy into the network and use it later when it is needed. When looking at the district heating systems in detail, many questions still remain unanswered. Is it possible to feed in all the surplus solar energy into the net? How does the solar input affect the network and the other prosumers? Is it sensible from the energy balance perspective? We have studied an existing DHN in an attempt to answer these questions. A simulation model of the DHN in the Southern German city quarter Ludwigsburg-Sonnenberg was developed and extended to include decentralized feed-in of heat from solar thermal collectors to evaluate their potential contribution to network demands. It could be shown that prosumers supply during summer period significantly more heat than they demand, so that the thermal network could operate as an autonomous micro-grid. By comparing the amount of electrical pumping energy that is needed to feed heat to the DUN with usable solar thermal energy, the benefit ratio of feeding heat into the DUN can be quantified.