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, EstoniaDAILY HEAT LOAD VARIATION IN SWEDISH DISTRICT HEATING SYSTEMSH. Gadd and S. WernerSchool of Business and Engineering, Halmstad UniversitySE-301 18 Halmstad, Phone: +46 35 167757henrik.gadd@hh.se, sven.werner@hh.se, www.hh.seABSTRACTIf daily heat load variations could be eliminated indistrict heating-systems, it would make the operation ofthe district heating system less costly and morecompetitive . There would be several advantages in theoperation such as:Less use of expensive peak load power whereoften expensive fuels are used.Less need for peak load power capacity.Easier to optimize the operation that leads tohigher conversion efficiencies.Less need for maintenance because of moresmooth operation of the plantsThere are a number of ways to handle the dailyvariations of the heat load. Two often used are largeheat storages or using the district heating network astemporary storage. If it would be possible to centrallycontrol the customer substations, it would also bepossible to use heavy buildings connected to thedistrict heating system as heat storages.To be able to find the best way to reduce or eveneliminate the daily heat load variations, you need tounderstand the characteristics of the daily variations.This paper will describe a way of characterizing dailyheat load variations in some Swedish district heatingsystems.INTRODUCTIONFor all heat generation/distribution systems, heat loadvariations leads to inefficiencies. You need to designyour system for the peak load even though you onlyneed the top capacity for a very short period of time ofthe year. This is of cause expensive. The solution tothis problem is heat storage. There are a number ofpossibilities to store heat in DH systems:Large heat storages at the heat generation plantsHeat storage in district heating networksHeat storage in heavy buildings in by allowingsmall variation in indoor temperatures[1].If it would be possible to extinguish daily variations itwould lead to several profitable advantages such as:Less use of expensive peak load power whereoften expensive fuels are used.Less need for peak load power capacity.Easier to optimize the operation that leads tohigher conversion efficiencies.Less need for maintenance because of moresmooth operation of the plantsTo do this some questions need to be answered:What input and output capacity to/from the heatstorage is needed?What size of the heat storage is needed?Are the daily heat variations in the specific systemlarge or small during a year?METHODNomenclatureP h = Present hour value [MWh/h]P d = Mean hour value for the present day [MWh/h]P a = Mean hour value for the whole year [MWh/h]S h = Energy transfer capacity [MWh/h]S d = Size of heat storage [MWh/day]S a = Total annual daily heat load variationh= Momentary daily variation [h/h]d= Total daily variation [h/day]a= Total annual relative daily variation [h/year]VariablesMeasured data has been collected from some districtheating systems in Sweden. The collected data is theheat power that is generated and fed into the districtheating network. It is hour mean power that is used, i.e.8 760 data points per year. Only whole years is usedfrom 1 of January to 31 of December. To describe thedaily variation three variables is defined.1. Momentary daily variation ( h)2. Total daily variation. ( d)3. Total annual relative daily variation. ( a)Three system examples are presented in this paper toexemplify the method to characterize district heatingdaily heat load variation:System A: From a city in South of Sweden with anannual heat generation of 200 GWh.199