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 required radiator flow for a known temperature drop(and heat supply).where T s,r,n is determined in analogy with T s,s,n ,according to:7070Temperature605040OriginalcontrolcurveModifiedcontrol curveConstant,decreased flowConstant, furtherdecreased flowTemperature605040T Ts,, test s,r,n1T, ,rs r n(3)23030Temperature20-10 -5 0 5 10 15Outdoor temperature7060504030Temperature20-10 -5 0 5 10 15Outdoor temperature7060504030To ensure that the heat supply is kept constant, therequired flow for the new temperature drop iscalculated. Since the flow is inversely proportional tothe temperature drop, it can be determined from thelast used flow and temperature drop, together with thenew temperature drop, according to:20-10 -5 0 5 10 15Outdoor temperature20-10 -5 0 5 10 15Outdoor temperatureFig. 5 A stepwise modification of the control curve. Thesupply temperatures are drawn in solid lines while thereturns are dashed.For the first test to be carried out at a specific outdoortemperature, it is logical to let the results of this testfully replace the original points on the curve. As moretests are performed for the same outdoor temperature,one can proceed in several ways. Since the controlshould be adaptive and thus able to take into accountchanging circumstances both in the DH network and inthe building, the results of new tests should beemployed. However, one may expect that testsperformed close to one another in time, at equivalentoutdoor temperatures, still provide slightly differingresults for varying reasons. A solution would thereforebe to use a forgetting factor, i.e., to gradually ―forget‖old values when the supply temperature curve isupdated with new data. A possible approach for doingso consists in calculating the new supply temperature,T s,s,n , as a mean value of the obtained, T s,s,test , and thelast used, T s,s,n-1 , supply temperature according to:( m T)s n1s,n(4)Ts, nmAs mentioned earlier, the flow rate is set by changingthe set-point for the pump speed. According to theaffinity laws for fluid machines, the flow is proportionalto the rotational speed. The process of letting the lastmodified supply temperature and the result of a newtest form a new modified supply temperature isillustrated in Fig. 6.Temperature60504030OriginalcurvesT s,s,testT s,s,nModifiedcurvesT Ts,, test s,, n1T, ,sss s n(1)220T s,r,testT s,r,n2 4 6 8Outdoor temperatureWhen a new test is performed at the same outdoortemperature, a new mean value is calculated, whichmeans that older values will have less and lessinfluence. To determine the secondary flow associatedwith the new supply temperature, i.e., the one providingthe correct heat supply at the current outdoortemperature, the expected temperature drop iscalculated as:Ts, nTs, s,n Ts, r,n (2)Fig. 6. An approach for modifying the control curve basedon new test results.The proposed method for updating the control curvesindicates that if for instance the DH utility demonstratesa long-term change in the supply temperature in thenetwork, the control system gradually adapts to thenew temperature. However, there are always variationsin the primary supply temperature. This may includeboth unintended and intended variations which may bethe result of, for example, a charging of the network ifthe outdoor temperature is expected to fall. Since theprimary supply temperature affects the primary return212