Design of a low temperature district heating network with supply recirculation

The focus on continuing improving building energy efficiency and reducing building energy consumption brings the key impetus for the development of the new generation district heating (DH) system. In the new generation DH network, the supply and return temperature are designed low in order to significantly reduce the network heat loss. Meanwhile, the low network operational temperature can make a better utilization of renewable energy and further improve the CHP plant efficiency. Though the designed return temperature is low, it may increase considerably when the heating load becomes low and the by-pass system starts to function. The aim of this paper is to investigate the influence of by-pass water on the network return temperature and introduce the concept of supply water recirculation into the network design so that the traditional by-pass system can be avoided. Instead of mixing the by-pass water with return water, the by-pass water is directed to a separated circulation line and returns back to the plant directly. Different pipe design concepts were tested and the annual thermal performances for a selected residential area were evaluated with the commercial program TERMIS. The simulation program calculates the heat loss in the twin pipe as that in the single pipe. The influence of this simplification on the supply/return water temperature prediction was analyzed by solving the coupled differential energy equations.