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, EstoniaENHANCED DISTRICT HEATING AND COOLING SYSTEMS– REALISATION OF THE LOW-EX CONCEPTStefan Bargel 1 , Clemens Pollerberg 1 , Armin Knels 2 , Li Huang 1 , Dirk Müller 2 and Christian Dötsch 11 Fraunhofer Institute for Environmental, Safety, and Energy Technology UMSICHT,Osterfelder Strasse 3, 46047 Oberhausen, Germany,Phone: +49 (0) 208-8598-1276, Fax: +49 (0) 208-8598-1423,stefan.bargel@umsicht.fraunhofer.de, clemens.pollerberg@umsicht.fraunhofer.de2 RWTH Aachen University, E.ON Energy Research Center - EBC,Mathieustr. 6, 52074 Aachen, Germany,Phone: +49 (0) 241-8049-780, Fax: +49 (0) 241-8049-769ABSTRACTSince heating and cooling represent low-exergy energystreams, high efficiencies can be obtained, if theenergy demand is covered by appropriate – meaningalso low-exergy (low-ex) – input energy flows.In order to be able to employ great potentials of lowexergyheat from many different sources, it is importantto develop technologies for the supply and the use ofenergy that allow network temperatures close toambient temperature in return as well as in supplypipes. Two possible technologies are phase changeslurries (PCS) and capillary tube mats (CTM).PCS are discussed as heat transfer fluid, which has anincreased heat capacity compared to water. The use ofPCS in energy supply networks instead of water leadsto an improved energy transport capacity, which resultsin a reduction of the necessary temperature differenceof the transfer fluid. To ensure the transfer of energyfrom the supply network into the building while thetemperature difference between network and building islow, large heat transfer areas are required, which canbe achieved by the use of CTM.This paper discusses opportunities for the realisation ofcold supply networks and low-ex systems and presentsexemplary technologies for their realisation.INTRODUCTIONTemperature levels in district heating and coolingnetworks have long been discussed. During the lastyears a tendency towards low temperature networkscan be observed. From a scientific point of viewanswers to the question for the optimal temperaturelevels can be given using exergy efficiencies as forexample discussed in [1]. The main advantage of thisevaluation parameter is the thermodynamically correctdistinction of thermal (low-exergy) and non-thermal(high-exergy) energy flows.Since heating and cooling represent low-exergy flows,it is of uttermost importance to cover these demands byappropriate – meaning also low-exergy – input energyflows. For example a heating system based on a39domestic gas boiler used to provide space heatingwastes a huge amount of exergy, since the exergyefficiency of such a system reaches only approximately5%! This result is valid for arbitrary heating systems inthe supply target (room) itself. Therefore it ismandatory to use an integral evaluation approach todecide whether an energy system is efficient or not.With respect to district heating and cooling networks asenergy supply systems two findings are important.First, it can be shown that the network subsystem itselfas depicted in figure 2 reaches optimal exergeticefficiency at quite low temperatures since the heatlosses dominate the pumping electricity effort.Secondly the overall energy supply system efficiencycan be greatly enhanced by utilising low-exergy inputenergy flows such as industrial waste heat.In order to be able to employ great potentials of lowtemperaturewaste heat from many different sources, itis important to develop technologies for the supply andthe use of energy that allow network temperaturesclose to ambient temperature in return as well as insupply pipes.Today, district heating and cooling networks use wateras heat transfer fluid. The heat is transported assensible heat and the transport capacity of thenetworks is determined by the heat capacity of waterand the temperature difference between forward andbackward flow. In cold supply networks as well as inlow temperature heating networks, high volumetric flowrates are necessary to provide the required transportcapacity due to the comparably small temperaturedifference between forward and backward flow. Toovercome these restrictions, a new heat transfer fluidwith an increased heat capacity is under developmentas an alternative to water, phase change slurries. PCSare mixtures of dispersed phase change material and acontinuous liquid phase, which can be used as heattransfer fluid in district heating and cooling networks.PCS possess an increased heat capacity due toadditional latent heat of fusion occurring during thephase transition of the phase change material. The useof such a dispersion in energy supply networks leads toan improved energy transport capacity, which in turn