Thermal performance analysis of triple pipes for district heating systemsA comparison with twin pipes and the literature results

There are a large number of proposals coming every day for the future district heating designs. One of these solutions has been the use of two different supply temperatures, i.e., a high-temperature supply for domestic hot water supply and a low-temperature supply for space heating. For this, apart from the use of two pairs of pipes that was an unreasonable solution economically, employing triple pipes is an interesting idea. Although specific designs of triple pipes are in use for specific district heating applications at some points in Europe, there is not any detailed thermal performance analysis of such pipes yet. This study, by the use of computational fluid dynamic methods, numerically investigates the thermal performance of triple pipes in various configurations. The obtained results are first thoroughly compared with the results obtained for twin pipes when working in the same operating conditions and heat loads and then are compared with those available for specific triple pipe designs used for specific district heating applications in the literature. The results show that a triple pipe for a separate supply of space heating and domestic hot water is better than twin pipes (no matter in which configuration it comes) and more efficient than the triple pipe application already proposed and investigated in the literature.