A problem in the field of district heating is the oil burners needed to provide power when a peak load occurs. One possible way of reducing the needed amount of oil is to reduce the power demand for space heating in the district-heating substations when the need for district heating water exceeds a certain limit. This can be done by use of a locally working controller function. In this Thesis the options concerning load priority are evaluated. To evaluate the potential for using controller functions concerning peak load priority an experiment was brought out in a chosen district-heating substation. The impact on the indoor thermal comfort during a heat reduction was also taken into account. With simulations and mathematical models the building and the indoor air cool down was evaluated. Also a survey was given to the residents to validate how the indoor thermal climate was affected during the experiment. Possible savings by using these kinds of functions were also accounted for. The result demonstrates that a simple controller function provides a possible way of reducing the power demand, but is not sufficiently reliable. This is mainly due to the used regulating parameter. With modifications or by adding additional regulating parameters such as water flow into the controller, the functionality can be improved. The result from the survey shows that during the experimental period the residents experienced a minor impact on the thermal comfort. Parameters, such as ventilation and heat losses also have a major impact on the building´s thermal inertia, especially at the lowest occurring outdoor temperatures. The simulations confirm the theory regarding the building heat capacity to prevent a negative impact on indoor thermal comfort. In addition, the indoor air temperature can initially decrease faster than the building framework, especially under the influence of ventilation. This must be taken into account when applying functions for load priority. Calculations indicate that the economical benefits by adapting functions for load priority are primarily for the heat-producer, due to reduced oil dependence and also other system aspects. The current ownership structure in Uppsala provides for a possible obstacle when it comes to expanded use of load priority functions. More incentives for the consumer are needed to provide for an increased usage of load priority functions in their district-heating substations.

Europeiska kommissionen pekar ut fjärrvärme som ett viktigt verktyg för att möjliggöra användandet av bränslen och värme som annars skulle gått till spillo. Primärenergianvändningen kan därigenom minskas vilket leder till minskade koldioxidutsläpp och ett mer uthålligt energisystem. För att kunna konkurrera i pris med värmepumpar och privata värmepannor behöver dock energiförlusterna i fjärrvärmesystemen hållas nere och ett ständigt energieffektiviseringsarbete behöver därför bedrivas. Förhöjda returtemperaturer från fjärrvärmesystemets kundcentraler är ett vanligt problem som indikerar onödigt höga hetvattenflöden och leder till ökade värme- och pumpförluster. I detta arbete har en beräkningsmodell konstruerats för att uppskatta hur mycket energi som sparas om satsningar görs för att sänka returtemperaturen ifrån felande kundcentraler. Modellen applicerades på Visbys fjärrvärmesystem och visade att systemets värmeförluster kunde sänkas med 10 procent och dess pumpförluster halveras om 62 procent av kundcentralerna åtgärdades. Detta resulterar i en årlig besparing på över 1 000 000 SEK för fjärrvärmebolaget vilket, tillsammans med den initiala kostanden för åtgärderna och fjärrvärmebolagets investeringskalkylsränta, ger att satsningen genererar en vinst på 4 300 000 SEK sett över tio år. Dessa kundcentralsåtgärder kommer också leda till förbättrad värmeleveransförmåga till Visbys innerstad då flödesminskningen är omfattande i den delen av nätet. / The European Commission view district heating as an important technology for facilitating the use of fuels and heat that would have otherwise gone to waste. In this way the primary energy usage can be reduced which leads to reduced carbon dioxide emissions and a more resilient energy system. To be cost competitive compared to private heat pumps and boilers the energy losses in the district heating network needs to be constantly reduced. Elevated return temperatures from district heating substations is a common problem that indicates unnecessarily high flow of water through the heat exchangers, which leads to increased heat- and pressure losses in the network. In this study a framework of calculations has been constructed to estimate how much energy can be saved if measures are taken to reduce the return temperatures of underperforming district heating substations. The calculations are applied on the district heating network of the historical Swedish city of Visby on the island of Gotland. It is shown that the distribution heat losses could be reduced by 10 percent and the pumping power could be halved if 62 percent of all substations were to be fixed. This will result in annual savings of over 1 000 000 SEK for the district heating company which, together with the initial investment in substation maintenance and the company’s discount rate for investments, results in a total profit of 4 300 000 SEK over a ten-year period. Improved heat delivery capability to the medieval city center is also expected as the mass-flow reductions in that area are considerable.