Energy cascade connection of a low-temperature district heating network to the return line of a high-temperature district heating network

Heat supply from sustainable low-temperature district heating networks (LTDHN) can be considered as one of the most favourable heat supply option for urban buildings. If buildings are located in an area where heat is supplied by a well-established high-temperature district heating network (HTDHN) then it can be a problem to switch to low-temperature district heating. It can be possible in the case of a new planed buildings or buildings which will be fully renovated. There is an option to design them for LTDHN parameters and utilise the heat from the network return line. If those buildings are located closely, than it can be possible to establish a LTDHN branch connected to existing HTDHN. The goal of the study is to evaluate the technical and economic feasibility of integrating the energy cascade LTDHN into the existing large-scale HTDHN. Additional energy is necessary to raise the HTDHN return heating media temperature to the acceptable low-temperature district heating level because the return water temperature of HTDHN return line is, on average, 40-50 degrees C depending on specific network, when LTDHN maximum supply temperature can reach up to 65 degrees C. Two direct connection options are compared: a mixing shunt and 3-pipe connection shunt. Technical and economic aspects of this solution are analysed, including heat and electricity consumption, water flow rate and pressure, as well as additional investments and heating costs. The most feasible connecting option is compared with reference option applying exergy analysis. Particular emphasis is placed on assessing the decrease in the HTDHN return temperature caused by the integration of the LTDHN. The impact of the temperature decrease on the HTDHN is evaluated, including reduction of heat transfer losses, increase in combined heat and power plants (CHP's) heat and electricity generation efficiency, and flue gas condenser efficiency. Other parameters, such as fuel savings are calculated as well. (C) 2020 Elsevier Ltd. All rights reserved.