12th International Symposium on District Heating and Cooling

The <strong>12th</strong> <strong>International</strong> <strong>Symposium</strong> on District Heating and Cooling,September 5 th to September 7 th , 2010, Tallinn, Estoniasystem. While the total quantity of transferred energyremains the same, the exergy that delivers this energymay vary. Analyzing the required exergy with respect tothe energy transfer equipment will result in an optimumeconomic solution allowing for integration of the systemwith other systems.In this exergoeconomic optimization, the system istreated as an integrated whole together with othersystems. While the (comfort) energy supplied remainsthe same in any given scenario, the exergy required forthis scenario varies and the cost implications of thisvariation are included in the analysis. Therefore, in thisanalysis the consumer of energy does not pay for theenergy, but for the exergy, the real value of the energysupplied.SYSTEM DESCRIPTIONThe system considered here to develop themethodology is a district heating system supplying hotwater for space heating to a 1000 home community inthe Ottawa area in Canada. It was modelled using theRETScreen clean energy project analysis software tool[6] and in-house spreadsheet based models. The hotwater is transported from an energy centre locatedcentrally in the community to the 1000 detachedhomes. Inside the buildings, radiators or cross-flowheat exchangers (water-to-air fan coils) are employedto provide space heating.Energy SupplyThe building temperature set point is kept constant at20°C. The hot water supply temperature is determinedby the outdoor temperature. If the outdoor temperatureis above 5 °C, the supply temperature is 70 °C. Whenthe outdoor temperature drops below -15 °C, thesupply temperature equals 90 °C. Between 5 °C and-15 °C, the supply increases linearly from 70 °C to90 °C. This is a common supply temperature profileused in many European district heating systems. Itprevents excessive flows in the pipes at high loads andpermits smaller heat transfer surfaces in the buildingsdue to the higher temperature difference betweenwater and building air. When the heat transfer surfacearea was varied to reach an optimum solution thesupply temperature was adjusted by a constant valueover the entire load range. The water returntemperature was set at 30 °C in all design calculations,but varied throughout the year according to the offdesigncharacteristics of the heating equipment used.The load of the buildings is related to the outdoortemperature. The annual heat consumption was set at100 GJ per house, a typical value for detached homesin this area. The instantaneous load throughout theyear is simply calculated as a linear relationshipbetween zero and the maximum capacity, when theoutdoor temperature varies between 20 °C and -28 °C,47the design temperature for Ottawa. While this is anover-simplification of reality, it neither hinders thedevelopment of the methodology nor introducesserious errors of consequence.Energy TransmissionThe pipe diameters were estimated using theRETScreen software tool. This means for diametersunder 400 mm the pressure drop is kept below 200 Paper meter of pipe and for larger diameters flow velocityis maximized at 3 m/s [6]. As RETScreen has a limit of13 sections for district heating systems, the 1000homes were assumed to be located along twelve 80-home streets. The final 40 homes were located in aseparate street.The energy transfer fluid is water. The pipes arepreinsulated steel or cross-linked polyethylene (PEX)pipes. The first iteration of the methodology accountedfor heat losses from the pipes. Since it was found thatthis heat loss was a negligible fraction of thetransmitted energy, it was omitted in subsequentversions. For a thorough analysis, it is recommended toinclude heat losses, especially if the piping system isextensive and the supply temperatures reach highlevels.A pressure drop analysis was used to determine therequired pump energy. The electric motor driving thepump was estimated to have 90% efficiency while thepump was assigned an efficiency of 85%.End-UseThe energy supplied to the pipeline was used to keepthe building temperatures at set point. Therefore,regardless of the (size of) building heating systemused, the same energy was used to keep the buildingswarm. However, the exergy used was dependent of thesystem in place and of its size. The larger the size, thelower the required water temperature and hence lessexergy was required to achieve the same end result.Two different technologies were used to model thetransfer of energy into the building space: cross-flowheat exchangers, and radiators. Simulations were donefor both technologies separately, and the technologieswere never mixed. This was done to simplify theanalysis. In reality, mixed systems will occur andshould be analysed as such. While this will increasethe level of modelling complexity, it is not difficult to do.DESCRIPTION OF MODELINGClimateThe local climate has a significant effect on the designof a building heating system. A maritime climate mayhave many degree days but not show the variability indemand that a building in a continental climate with anequal amount of degree days experiences. Even if both