A review of low-temperature sub-networks in existing district heating networks: examples, conditions, replicability

Most existing urban district heating networks (DHNs) operate on relatively high temperatures. Typically, resulting from 2nd and 3rd generation of district heating (DH), 90-130 degrees C is used as supply and 50-70 degrees C as return temperature. On the contrary, low temperature district heating networks (LTDHN) with supply temperatures between 30 degrees C and 70 degrees C allow a more efficient operation of the network and can further utilize renewable heat sources such as solar or geothermal energy, low temperature waste heat, or even ambient heat. However, high temperature DHNs cannot easily be modified and transformed into LTDHNs due to established operating conditions and assets. Therefore, the integration of a low-temperature sub-network (sub-LTDHN) and creating an energy cascade is an opportunity to increase the efficiency and sustainability of the overall network. The sub-LTDHN is integrated into the return line of the superior DHN, hence leading to a reduction of the return temperature of the superior DHN. Sub-LTDHNs can be a key enabler for the decarbonization of urban DHNs by enabling an efficient utilization of local energy sources and have the potential to reduce substantially the overall network temperatures. At the same time, a high temperature backup from the main network to the sub-LTDHN is available. Within this study, theoretical conceptualized as well as practical examples of sub-LTDHN are compiled using literature research, expert interviews and the professional international network to DHN operators. Despite the potential advantages of sub-LTDHNs, there are several technical, legislative and economic issues related to their implementation, which is the reason why they are hardly implemented. The review investigates the revealed solutions and conditions of the compiled sub-LTDHNs and tries to generalize them in order to allow its replicability. Therefore, general results on implementations, frameworks, barriers and enablers are derived ("lessons learnt") and good practices are properly elaborated. (C) 2021 The Authors. Published by Elsevier Ltd.