Nyttan av en dynamisk framledningstemperatur i fjärrvärmesystem : Effekt- och flödesutjämnande körstrategier som tar hänsyn till dygnets varierande värmebehov

Sammanfattning: The heat demand in a city varies a lot over the course of the day because of the outside temperature and social behaviour. The heat demand is at its peak in the mornings when people wake up and are active while it is still cold outside. A common way of fulfilling the heat demand is to use district heating, which uses heated water which is pumped out to costumers, where the delivered energy is used for heating buildings and tap water. A common way of regulating the produced power is to manually adapt the supply temperature to the coldest outside temperature of the day and then let the water flow handle the variations in heat demand. This also leads to variations in the produced power that corresponds to the heat demand. The focus of the master thesis has been to examine the benefits of using a dynamic supply temperature that changes over the course of the day. Two different strategies has been developed with the purpose of equalising the flow and the produced power, to ensure a reliable distribution and to reduce the production costs. Both of the strategies focus on sending out warmer water in advance to peaks in the heat demand while sending out colder water in advance to the troughs in the heat demand, where the foresight needed is a result of the time delay of the system. Besides the actual equalisation of the power and flow, both strategies can result in increases in profit from electricity production, reduced pump costs and lowered emissions. The profit of the power and flow equalising strategies have been evaluated for the district heating system in Linköping between October 2014 and September 2015. The strategies have been evaluated in different cases, both a standard case which is kept within the limits of the district heating system, and also cases where these limits have been stretched. The results of the standard case shows that both strategies increases the electricity production, where the flow equalising strategy can increase the profits from electricity production with approximately 240 000 SEK per year and the power equalising strategy can increase the profits from electricity production with approximately 370 000 SEK per year. The flow equalising strategy leads to both power and flow equalisation. The power equalising strategy provides a better power equalisation compared to the flow equalising strategy but in return leads to large flow variations. For the cases where the limits are stretched, the power and flow equalisation can be additionally improved, which strongly affects the economic results. A more even water flow and supplied heat power is beneficial to the production but their effects are hard to measure economically.