The thesis Local district heating systems deals with a new concept for district heating. The district heating system is arranged as one main system and a number of local systems. The production plants and the main distribution lines belong to the main system. The local systems distribute heat power from the main lines through the local networks to the buildings and via substations further to individual rooms. The main network and the local networks operate with different pressures and if suitable with different temperatures. The design of the local systems from the main lines to the room radiator, is the main topic of the thesis.
The new concept is based on a comfortable room temperature in every room. Pleasant room temperatures are reached with a thermostatic valve on each radiator. Good working conditions for the thermostatic valves are created by a controllable jet pump in the substation and a low radiator flow. The jet pump replaces one heat exchanger, the centrifugal pump and the control valve of a conventional substation of today. The jet pump and the pressure separator, which connects the local network to the main network, allow the low return temperatures from the radiator circuits to enter the main network unaffected. The directly connected radiator circuits are made possible by limiting the maximum pressure and temperature to 6 bars and 95°C respectively. Simulations show that the system is robust and comparatively insensitive to deviations from estimated design data. The thermostatic valves and their interaction with the jet pump will automatically adjust the operation to the new conditions. Measurements at Trollhättan Energi AB confirm this behaviour. Altogether the study shows that the new concept will give lower return temperatures, reduced heat losses and increased comfort (closer to preferred values of room temperature). Costs are illustrated with quantities which can be related to investment costs (design values), and to operational costs (quantities of long duration).
low radiator flow