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, Estoniathe temperature difference of the supply network is15 K, and even 2 times, if the temperature difference isonly 10 K. Furthermore, the diagram shows that withrising temperature difference the gradient of the TCEFis lower, which means that the advantage of the PCScompared to water disappear at higher temperaturedifferences. At the point where the gradient of theTCEF is 0, the water system and the PCS system havethe same transport capacity. At that point, the massconcentration of paraffin w has no influence on theTCEF.The use of PCS in energy systems leads to animproved energy transport capacity, which results in areduction of the necessary temperature difference orvolumetric flow rate of the transfer fluid needed totransfer a given amount of heat.Another technical issue of PCS systems is theincreased pressure drop in the pipes due to the higherviscosity of the PCS. A calculation methods andmeasurement data can be found in [6, 7 and 8]. Theviscosity of PCS is related to several influencequantities and can cause an incensement of thepressure drop up 100%. PCS are non-newtonian fluids.2.2. Capillary Tube MatsThe most often used heat exchanger type in heatingsystems is a convective radiator, which is installed inrooms close to the window. The size of a radiatorshould be small, so that also the heat exchangesurface is small and the heating system must beoperated on a high temperature level to ensure theheat transfer from the heating system into the room. Analternative to convective radiators are floor heatingsystems. Floor heating systems consist of a capillarytube mat, which is installed in the upper layer of thefloor. Because of the bigger heat exchange surfacecompared to the convective radiator, the temperaturelevel of the heating system is lower. A new approach torealise heating and cooling of buildings is via CTM,which are integrated in the floors of the building, as wellas in the walls and ceilings. This system offers a bigheat exchange area and allows the heating and thepassive cooling of the building. Due to the increasedheat exchanger area, a low temperature differencebetween the heating system and room is possible. Forthe further discussion, the following simple model isused to describe the heat release of the heating systemin the building. The heating release system isevaluated by the number of transfer units (NTU). Theheat capacity provided by the heating network Q iscalculated by equation (3) with the inlet and outlettemperature T in/out of the supply network, the mass flowm and heat capacity c p of the heat transfer fluid. mcp TinTout(3)QIn view of the heat release in the room, the heatcapacity Q can also be described by equation (4) andis related to the heat transfer coefficient U, the heatexchange area A and the temperature differencebetween the mean temperature of the heat release T mas well as the room temperature T r .Q U AT m T r (4)The mean temperature of the heat release T m iscalculated by equation (5).TT Tin outm (5)TinlnToutBased on the equations (3) to (5), it is possible tocalculate the NTU, which characterizes the heatrelease in the room, according to equation (6), which isonly a function of the inlet and outlet temperature T in/outof the heat supply, the mean temperature T m of theheat release and the room temperature T r .NTUUmAcmrin out (6)pTT T TThe NTU values have been calculated for a convectiveradiator system and a CTM system. The assumedtemperatures for the calculation and the results aregiven in table I.Table I. NTU for both heat release systems:conventional radiator and CTMparameter convective radiator CTM systemT in [°C] 80 37T out [°C] 60 31T r [°C] 20 20NTU [-] 0.4 0.43The NTU value of the CTM system is 0.43 and as highas the NTU value of the convective radiator. Thismeans that both systems have the same heat releasecapacity, although the inlet temperature T in of the CTMsystem is lower and the temperature differencebetween inlet T in and outlet T out of the CTM system issmaller.CONCLUSIONFrom the point of view of the low-ex concept the majortask en route to an exergetically efficient energy supplysystem is the replacement of the combustible fuelboiler by utilization of low temperature thermal input43