Optimising pipe sizing and operating temperatures for district heating networks to minimise operational energy consumption

Abstract

Reducing heat losses is an important factor in improving the performance of district heating networks. High heat losses are sometimes experienced in UK district heating networks and this is often as a result of poor design, partly due to oversized pipes. To provide additional guidance to designers on this issue, this paper proposes a set of maximum pipe diameters to be used for a given maximum flow rate to reduce the risk of oversizing and high heat losses. It was found that the optimised velocities are higher than those given in CIBSE CP1 Heat Networks Code of Practice and adopting these will result in smaller pipe diameters and lower operational and capital costs. The methodology used in this paper is based on minimising the total operational energy. The flow and return optimised pipe diameters are calculated for different scenarios which include variations in the pipe material, insulation thickness, water temperature and ambient environment. It was found that the optimised velocity in every scenario remains relatively constant with a varying pipe diameter. This paper shows that the operational cost of a DH network can be reduced when selecting the recommended maximum pipe diameter, compared to commonly used sizing criteria. Further savings can be achieved if the flow and return pipe are sized separately; to minimise operational energy of both flow and return pipes, the return pipe should be sized for a lower velocity. This paper also explores particular questions of interest: A range of insulation thicknesses. Benefit from using a variable flow temperature and to supply the flow at 90℃ at peak times instead of a constant temperature at 70℃. A comparison between steel and plastic pipes which have different roughness coefficients. Optimised differential pressure drop per unit length in both flow and return pipes. Maximum distance occurring before measuring a temperature loss and pressure drop of 1℃ and 1 bar, respectively, at peak load. However, this paper does not take into account any potential benefits that a lower flow temperature might have in reducing heat production costs or a lower return temperature might have in enhanced heat recovery, or that lower temperatures may enable use of plastic carrier pipes. Such matters are site specific but should be included in any similar pipe sizing analysis and operating temperature optimisation carried out for an actual scheme. Practical application: Optimising maximum pipe diameters is necessary to improve the viability of district heating systems. The current guidance for designers is not considered optimal and this paper suggests a new methodology to size pipes to reduce both the heat losses and the operational cost. This paper also gives guidance to sustainability engineers to assess the performance of 90/40℃, 70/40℃ and 60/30℃ district heating networks; 70℃ was assumed to be the minimum acceptable flow temperature in the network to enable domestic hot water to be heated to a safe temperature of 60℃ for storage; 40℃ was the used return temperature to be in line with the CIBSE Code of Practice CP1 stating in Objective 3.5 that best practice should achieve return temperatures below 40℃ for a scheme supplying heat to new buildings. A 90/40℃ DH network performance when increasing the flow temperature was then compared to the performance of a 70/40℃ system. Finally, a 60/30℃ system was also analysed to investigate the benefit from operating a DH network complying with the 4th generation of DH network temperatures that encourages minimal exergy destruction from the network to the end user.