Study of a district heating substation using the return water of the main system to service a low-temperature secondary network

The development of district heating (DH) systems is facing the challenge of servicing areas with lower energy demands whose connection might not be either effective or profitable if the conventional DH technology is used. The purpose of this paper is to propose a complementary approach on how to effectively service low-energy building (LEB) areas using the existing DH networks. The proposed solution consists in supplying a secondary low-temperature (LT) network by means of a ‘low temperature’ substation that uses the return water from the main DH network as a substitute for the primary energy source, together with a minor portion of the main DH supply. Two types of LT substations are proposed and compared to a reference substation: First, a one-stage heat exchanger that uses a mixture of the main DH network return and supply flows as thermal energy source. Second, a two-stage heat exchanger that is fed by both the main DH return and supply flows. The system subject to this study consists on the LT substation with supply/return temperatures at 55/25 °C average. The system energetic performance is analysed though thermodynamic simulation. Outdoor ambient temperatures variations throughout the year are considered for two specific locations, assuming full and partial load operation. The results show that it is possible to supply 20-50% of the total annual heat demand of a LTDH network using the return flow from the main DH network. The solution presented in this paper is seen as being of potential interest to deliver thermal energy services to LEB areas.QC 20160616