Buildings and district heating - contributions to development and assessments of efficient technology

Sammanfattning: The objective of this thesis is to contribute to improved performance of district heating systems by focusing on three items. Item 1: Study of three methods for improving the performance of sub-systems within buildings connected to the distribution network. The methods can be considered as more or less innovative. Results from two of these measures are quantified in terms of so-called Primary Energy Factors. Item 2: Investigation of the extent to which space heating supply to buildings connected to a district heating network can be maintained in the event of a failure in the electric power supply. Item 3: An experimental investigation of the cavitation phenomena within primary side substation control valves. More specifically, with respect to item 1, three methods were investigated; a) Deriving substation connection schemes that will improve overall thermodynamic efficiency, taking into account a complete range of load cases, including variations in hot water load, b) Adding small fan blowers onto the surfaces of existing radiators, with the aim of increasing convective heat transfer, thereby making it possible to lower the operating temperatures of the hydronic space heating system such that supply and return temperatures on the primary side of substations can be lowered, c) Advanced control of hydronic space heating systems such that for all heat loads an operational mode is automatically selected that optimises the performance of a substation in terms of the lowest possible return temperature on the primary side. The thesis shows how Primary Energy Factors for district heating can be affected when the heat is produced in a combined heat and power station and in a heat-only boiler. Calculations of the factors were performed according to the EU standard, EN 15603:2008. The results of the analysis show that the size of the factor depends strongly upon the particular production mix of the electric power system to which the combined heat and power plant is assumed to be connected. Concerning item 2, district heating in case of power failure, both numerical analysis and field experiments showed that a significant share of the heat supply to buildings can be maintained even if a circulation pump is no longer running. This is due to the fact that gravity driven circulation takes over to variable extent. Generally, 20-80% of the initial heat load can be supplied, provided that circulation is maintained in the district heating network. Further improvements are possible if some minor modifications are made. Item 3 pertains to a small laboratory study of cavitation in modern state-of-the art control valves. It was found that cavitation primarily occurred under conditions that in practice only seldom occur, such as a large differential pressure across the valve combined with a situation when the valves are close to being fully opened.

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