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, Estoniaconversionlossesnetw orkheatlossesbuildingheatlossesprimaryenergyheatgeneratorheatflowpumpingelectricitydistrictheatingnetw orkheatflowheatingsurfaceT = 20°Cheatflowpow er plantconversion lossesprimary energyFigure 2. Evaluation boundaries of an energy supply system. The blue (outer) dashed line marks the complete system; theblack (inner) dashed line marks the network subsystem1.2. Integrated system evaluationWhen evaluating a system it is important to specify theevaluation boundaries (cf. fig. 2). It has to be pointedout that an integrated system evaluation is mandatorysince otherwise results are ambiguous and misleading.This can be demonstrated by assuming e.g. evaluationof the building subsystem only. If two systems arecompared, one consisting of a target room equippedwith space heating and the other one with a targetroom equipped with conventional heating, one couldarrive at the conclusion, that the system utilizing spaceheating is more efficient. However, assuming bothsystems are also equipped with an identicalcondensing gas boiler providing the heat, an evaluationcomprising the total system (consisting of heatgeneration and heat transfer to the target) would arriveat a totally different conclusion. In this case, bothsystems possess the same exergy efficiency, which isapproximately 5% for the outlined case. This isbecause a potentially more efficient heating system isnot put to use as the same input and supply flowsoccur in both cases.1.3 Efficiency enhancement potentialsThe complete energy supply system can be dividedinto three subsystems – generation, distribution andbuilding (representing the consumption). Thesesubsystems possess different potentials to enhanceoverall system efficiency.Currently heating demands are met by burning highexergyfuels, great enhancement potentials areavailable within the generation subsystem. Firstly, fuelsshould not be used to directly satisfy thermal demandsat all since this embodies pure exergy destruction.41Instead thermal input flows as industrial waste heat3 orgeothermal energy should be applied. On the otherhand, if combustible fuels are used to meet thermaldemands, at least Combined Heat andPower generation (CHP) with a maximum electricaldegree of efficiency should be utilized. This allowstransforming part of the high-exergy fuel into highexergyelectric current. Heat is produced as ‗wasteproduct‘ of this conversion.Optimization potentials within the distributionsubsystem are basically indirect. At first glance, thedistribution system has no influence at all since thenetwork acts as connection between heat generationand heat consumption. Consequently, no thermal flowsexist that pass the overall system evaluationboundaries. However, two aspects remain and need tobe accounted for. One is heat losses occurringthroughout the network that have to be compensatedby additional heat generation. The other is pumping tomaintain the heat transfer medium circulation, which ismet by an unalterable high-exergy input (electricity).The main problem is that concepts, which lead todecreasing heat losses cause increasing pumpingefforts and vice versa. Nevertheless, heat losses arethe exergetically dominant influence, therefore thefocus should be to confine these losses. Heat lossesdepend on the driving temperature difference betweenmedium and surrounding ground and on surface area.Minimization of the losses can most easily be achievedby reducing the network temperatures since pipedimensions are affixed due to demands so that surfaceareas are not a modifiable parameter. This approach iseven more rewarding since it allows employing low3 Industrial waste heat in this sense is heat that can no more beput to any use within the industrial production process.