Thermal-Hydraulic Performance Analysis of Twin-Pipes for Various Future District Heating Schemes

Abstract

Future energy systems will come with a 100% share of renewable energy and high integration of energy systems. District heating and cooling systems will be undeniable parts of the future energy systems, as they pave the bed for high-efficiency, low cost, and clean production. District heating systems may come into a wide range of designs in the future. Currently, most of the world’s district heating systems are based on the third generation design while everything in this framework is on the verge of a transition to the fourth generation. A large number of technologies for the future district heating systems has been proposed so far, among which low-, ultralow- and variable-temperature systems seem more of qualification. This study employs computational fluid dynamics to make a comprehensive examination of the compatibility of regular twin-pipes with various potential district heating schemes for future energy systems. The results show that both low- and ultralow-temperature systems could efficiently use regular twin-pipes commonly used in the third generation district heating systems, though the insulation of the pipe could be proportionally strengthened based on a techno-economic trade-off. In contrast, the results show that the thermal inertia of the pipe does not allow the variable-temperature district heating system to effectively operate when the transmission pipeline is longer than a limited length. Therefore, a regular heat distribution network may not be an appropriate host for a variable-temperature district heating scheme unless decentralized heat production units come into service.

Reduction of Heat Losses in a Pre-Insulated Network Located in Central Poland by Lowering the Operating Temperature of the Water and the Use of Egg-shaped Thermal Insulation: A Case Study

“…A change in the geometry of the thermal insulation reduced the heat flow under the supply line, while, on the left and on the right, there was a slight increase in the heat flux. The trend of heat flux density distribution in a pre-insulated duct with circular thermal insulation(Figure 3a,b)is consistent with the results of the calculations in Reference[12].…”

“…Numerical studies of pre-insulated heating networks can be made on the basis of one-dimensional [17][18][19][20], two-dimensional [15,21], and three-dimensional [12,22] analytical or numerical models. For the purpose of this work, in order to perform numerical calculations, a calculation program of stationary two-dimensional heat conduction written in Fortran using the boundary element method was used, the verification of which was presented in References [10,14].…”

“…The presented results of research on double pre-insulated networks in Reference [11] showed that, in standard pre-insulated district heating networks with increasing pipe diameter and mass flow, heat losses increase. An increase in the operating temperature of the heating network also causes an increase in heat losses [12]. A reduction of heat losses can be obtained by changing the geometry of the position of the pipe in the thermal insulation or by changing the geometry of the cross-section of the thermal insulation.…”

Reduction of Heat Losses Using Quadruple Heating Pre-Insulated Networks: A Case Study

“…Another method of reducing heat loss is to place a few pipes in common thermal insulation. This method is mainly used in twin pipes, where the supply and return pipes are located in circular insulation [15,16]. One of the methods to reduce heat loss in pre-insulated twin pipes is to change the cross-sectional shape of the twin pipe thermal insulation from round to oval [17], elliptical [18] or egg shape [19,20] while maintaining the same cross-sectional area of thermal insulation.…”

“…One of the methods to reduce heat loss in pre-insulated twin pipes is to change the cross-sectional shape of the twin pipe thermal insulation from round to oval [17], elliptical [18] or egg shape [19,20] while maintaining the same cross-sectional area of thermal insulation. Heat losses in pre-insulated twin pipes can be determined by simplified analytical methods [12,21,22] and numerical methods [16][17][18][19]23]. Heat losses in pre-insulated double pipes with round thermal insulation are about 30% lower than heat losses in pre-insulated single pipes [19,20,22].…”