Environmental and Thermal Performance of District Heating Pipes

Doctoral thesis, 2002

In order to move towards a sustainable society we need to increase the efficiency of our use of available resources, not only in economic terms but also in ecological terms. District heating, the utilisation of centrally produced heat for space heating and domestic hot water generation, has the potential to contribute to the eco-efficient use of energy resources in the parts of the world where space heating is needed. The introduction of district heating has reduced the environmental impact from local heat production in urban areas. However, in order to fit into a sustainable society, district heating must be continuously developed according to the increasing demands on activities in a society moving towards sustainability. Our technical improvements must result in environmental improvements. This thesis focuses on the environmental performance of the distribution system - the district heating pipes. The purpose of this research is to understand how the distribution system can be improved. The information presented in this thesis can be used to identify possibilities for improving the environmental performance of different life cycle phases of the distribution system as well as a baseline when evaluating new technical developments. The heat losses during use of the pipes are of large importance for the environmental impact of district heating pipes. Such heat losses can be modelled if the mass transfer parameters for gases in the polyurethane insulation and the polyethylene casing are known. The diffusion coefficients, the permeability coefficients and the solubility coefficients and their temperature dependence have been determined for cyclopentane, carbon dioxide, nitrogen, and oxygen. The same parameters have been studied for the blowing agent alternative HFC-365mfc at room temperature. The long-term thermal performance of district heating pipes has been modelled using effective permeabilities for the pipe construction. Environmental consequences of heat losses during the use phase of district heating pipes have been compared with the impacts from production of the pipes and from construction of the district heating network. The use of the district heating pipes is the most important of the studied life cycle phases. Thus, it is very important to minimise heat losses from the pipes. The heat losses will for some district heating pipes increase notably during use due to foam ageing. Regarding the production of the pipes, the environmental performance can be improved mainly by using more material efficient pipe constructions, and for the construction of the piping network mainly by decreasing the excavation work. However, such improvements must be done in a way not jeopardising the insulating capacity. Twin-pipes, using two media pipes in the same casing pipe, will for smaller dimensions give better material efficiency and less need for excavation compared to the use of two single pipes.