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, EstoniaNote that the annual flow-compensated meantemperature in Table 2 is based on results from the fieldstudy where measured values for low relative heat loadare missing, which makes the values in the tablesomewhat underestimated.CONCLUSION AND DISCUSSIONBy installing the add-on-fan blower application onexisting radiators the temperature level in the heatingsystem can be substantially reduced. This will also haveimpacts on the DH network and DH production units.The impact on the DH network can be applied based ontwo principles:1) DH supply temperature kept at the same level aswithout the add-on-fan blowers. This will result inreduced primary flow rate.2) Reduced DH supply temperature while primary flowrate is kept constant.The first strategy could be applied immediately, sincethe primary supply temperature is kept as the samelevel as before. This means that not all heating systemsconnected to the DH network need to be modified inorder to apply this method. The lowered secondarytemperature level results not only in reduced DH-returntemperature, but also in a reduction of the DH flow rate.The reduced flow rate could be used to increase thenumber of buildings connected to the DH network, or toavoid bottlenecks in the DH network. The magnitude ofthe reduction of the DH supply temperature is between9 and 12 °C at DOT and at the same time the flow rateis decreased with more than 10 %. On annual basis thepossible reduction of temperature level in the DHnetwork is in the magnitude of several degrees Celsius.In order to apply the second strategy the demand for ahigh temperature level in the DH network needs to bereduced for all the connected buildings. Otherwise theDH flow rate will increase. Calculations based on theresults from the field study in this paper shows that theDH supply temperature can be reduced with about10 °C at DOT without affecting the DH flow rate. At thesame time the DH return temperature will be reducedwith as much as 10 °C at DOT.The performance of the tested add-on-fan blowerscorresponds to the pattern of theoretical calculations.However, the results are not comparable since the airflow in the pilot project has not been measured.The results presented here are an important part in theevaluation of effects of improvements in consumerheating systems on primary energy efficiency in DHsystems including production plants, especially CHP.ACKNOWLEDGEMENTThis work is part of the Primary Energy Efficiencyproject of Nordic Energy Research.NOMECLATUREAbbreviationsCHPDHDHWDOTHEXVariablesCombined heat and power stationDistrict heatingDomestic hot waterDesign outdoor temperatureHeat exchanger (DH substation)α Heat transfercoefficient (W/m 2. K)β coefficient ofexpansion (K-1)δ Thickness (m)λ Conductivity (W/m . K)Gr Grashof number (-)h Height (m)k Heat transfercoefficient (W/m 2. K)L Length (m)ε Emissisivity (-) m mass flow (kg/s)ν Kinematic viscosity(m 2 /s)ζ Stephan-BoltzmanconstantΔθ Logarithmic meantemperaturedifference (K)A Area (m 2 )Nu Nusselt number (-)P Electric power (W orkW)Pr Prandtl number (-)Q Heat output (W or kW)c p Heat capacity (J/kgK) Ra Rayleigh number (-)C Constant Re Reynolds number (-)g Gravity force (N/s 2 ) T Temperature (ºC or K)Subscripts0 Design condition(without fan)Fan Add-on-fan blower inoperationprPrimary (side)Returni indoor rad radiationm Mean rel Relativeout outdoor s Secondary (side) orSupply29