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, EstoniaDynamics of heat storageMost of the daily fluctuation in heat demand wassmoothed with heat storages and electric heat boilersin all heat areas. If CHP units were operated, they wereusually operated at maximum heat output.The investment cost for heat storage was assumed tobe 1840 €/kWh. With the assumed ratio of 12 betweenstorage capacity and heat capacity this translates to153 €/kW. In comparison the capacity cost of electricheat boilers was assumed to be 40 €/kW and 50 €/kWfor natural gas heat boiler. This means that investmentinto heat storage capacity was not driven by need fornew capacity since heat boilers were cheaper. Therehad to be operational benefits from the use of heatstorage to cover the additional investment costs.Heat storages create operational benefits by movingconsumption from more expensive sources of heat toless expensive by shifting demand in time. In allheating areas whole operating ranges of heat storageswere extensively utilized. During most 168 hour periodsheat storage reached both the minimum and maximumstorage capacities. In the ‗Rural‘ area heat storage was2.1% of the time either full or empty. With a largerstorage capacity this could have been reduced, but itwas not worth the investment.The size of the heat storage in ‗Industry‘ area waslarger than in other areas in relation to daily heatdemand (Fig. 8). In ‗Industry‘ area charging of heatstorages took place over several days during higherpower prices, when wood waste CHP units wereproducing extra electricity. Storing the extra heatrequired larger heat storage capacity. On the contrary,in ‗Rural‘ and ‗Urban‘ charging and discharging wasmore balanced and smaller heat storage was enough.Heat (GWh)180160140120100806040200Heat storage sizeMax daily heatMin daily heatAverageRural Urban IndustryFig. 8. Heat storage size compared to maximum, minimumand average daily heat demands.In the ‗Rural‘ area during winter time, charging of heatstorages is mostly based on the use of electric heatboilers. They create large amount of heat in relativelyshort time during periods of low power prices. Duringsummer time, heat storages are charged by turning onwood waste and forest residue CHP units. Duringspring and fall CHP units operate more often, since theheat load is larger, but still the heat storage helps toshut them down for periods of some hours.‗Urban‘ area has similar dynamics, but during summertime the adjustment is made by heat pumps instead ofCHP. In the winter during high power prices old naturalgas CHP units are less expensive to operate than theheat pumps.CONCLUSIONSDistrict heating systems offer good possibilities forincreasing the flexibility of the power system, if thepenetration of variable power like wind power increasesgreatly in the future. According to the results, mainvessels to increase flexibility are the use of heatstorages, electric heat boilers and flexible operation ofCHP units.Investment in electric heat boilers in district heatingsystems is driven mainly by periods of very high windpower production. The resulting cheap electricity isconverted to heat and to some extent stored in heatstorages for later use. Investments in heat storage inturn are driven by the same mechanisms, but also tocreate flexibility in the electricity production when pricesare higher. To enable this, the operation of CHP unitsand heat pumps is altered with the help of heatstorages. Heat pumps mainly compete against CHP asa source of heat. They succeed in replacing coal CHP,but are not very competitive against wood residues.This is naturally due to assumed costs where coal hasa considerably penalty due to CO 2 cost. Heat pumpsare not very important as a source of flexibility, sincethey require lot of full load hours due to theirinvestment cost.While the research has been conducted on districtheating, similar dynamics could be achieved inhousehold heating not connected to district heatingnetworks. However, the costs are likely to be largerunless there is an existing hot water tank. Flexibilitycould also be gained from district cooling or airconditioningunits with the addition of a cold storage.Further research should also address some of theshortcomings of current study. Sensitivity analysiswould be important, especially concerning the costestimates of the analysed heat measures. Heat storagemodel was very simple and this should be improved.Heat grade, especially in the industrial environment,can vary and the model should take this into account.197