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, Estoniarequirements, and the different flow requirements, theexergy used by each system was unique.Capital and Exergy Cost AssessmentsAll cost numbers reported in this paper are in 2009Canadian Dollars. An in-house costing tool was used toestimate cost for the district heating energy centre,containing the pumps and boilers, and the burieddistribution piping. For the two heating systemsconsidered, the costs were assigned as shown inTable I. For water-to-air fan coils (cross-flow heatexchangers) an installed cost of $250/m2 wasconsidered representative, and for radiators $200/m2was selected as a typical value.Table I. – Cost for heating technologiesCross-flow heatexchangerRadiative system$250/m 2 of heat transfer surface$200/m 2 of exposed panelFuture cash flows were discounted at a rate of 8% andsystem lifetime was set at 40 years. Annual operatingand maintenance (O&M) cost other than cost for heatand electricity were set at a fixed fraction of 1% of totalinvestment cost.To compare traditional optimization with exergoeconomicoptimization three types of analyses wereperformed. The ‗classical analysis‘ applies thetraditional optimization where heat is valued based onenergy content, at a rate of $5/GJ, which is consideredrepresentative for heat from natural gas combustion.Electricity cost has been set at $17/GJ (just over$60/MWh).In the exergoeconomic analysis heat and electricity arepriced based on the exergy content. The exergy chargewas determined at $30/GJ for thermal energy, basedon the above mentioned $5/GJ for heat, assuming a 1to 6 ratio of exergy to energy content (applies to atemperature around 80 °C). The electrical energy toexergy ratio was taken as one, resulting in an exergycharge of $17/GJ for electricity. At first glace it mayseem erroneous to charge more for exergy from thethermal source than that for the electricity for the pump,but it must be remembered that the (thermal) exergy isa fraction of the thermal energy.The third type of analysis is a classical analysiscorrected for the difference in value of low- and hightemperatureheat, by assuming energy under 60°C isavailable free of charge (as waste heat from a nearbyprocess). For energy over 60 °C the charge is still$5/GJ.To assess the influence of carbon taxes, two sets ofresults are presented. One assumes no carbon taxesare in place and the other assumes a carbon tax of $3049per ton CO2eq. Carbon intensity factors of 0.050 tonCO2eq/GJ were used for natural gas and 0.054 tonCO2eq/GJ for electricity (taken from RETScreen [6] asrepresentative for Canada). This results in a $6.5/GJenergy charge for heat, a $38.9/GJ exergy charge forheat and an $18.6/GJ energy (or exergy) charge forelectricity.Thermoeconomic FactorThe exergoeconomic or thermoeconomic factor ―f‖compares two sources contributing to cost, investmentrelatedcost and exergy destruction cost. It is definedhere as the ratio of Capital Cost Rate (CCR, whichincludes O&M cost, but excludes heat and electricitycost) and the sum of Exergy Destruction Cost Rate(EDCR) and CCR. The CCR equals the cost per unittime for the installation, depreciation, maintenance, etc,while EDCR is the cost of exergy.fCCREDCR CCR (4)Since CCR and EDCR have the dimensions of $/time,―f‖ is dimensionless.A high value for ―f‖ indicates that the capital andmaintenance costs are dominant. Also, a high f – valueindicates good use of the exergy in the fuel. On theother hand, a low value for ―f‖ indicates an inefficientuse of fuel resources. For each heating systemvariation, the average annual thermoeconomic factorwas calculated.MODELLING RESULTSBase case designTable II shows the main information for the base casedesigns for both the radiator and cross-flow heatexchanger systems. As expected, the distribution pipediameters, required pump capacity, annual space heatconsumption and annual heat cost are the same forboth systems.As the water return temperatures throughout the yearare generally lower for the radiator system, the requiredwater flows and consequently the annual electricityconsumption are lower for the radiator system. As bothsystems have a design supply temperature of 90 °C(and thus also the same off-design supplytemperatures throughout the year), the annual exergyconsumption is the same for both. The lower returntemperatures for the radiator system also show in thehigher fraction of energy provided under 60 °C. Interms of cost, the radiators are clearly more expensiveresulting in higher annual investment and O&M cost,which is not offset by the somewhat lower electricitycost. Overall the more capital intensive radiator systemhas a higher f-factor than the cross-flow heat