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, Estoniato decrease, and thereby also the temperature dropand heat supply, to be detained at the same level.A test is started by keeping the CV locked for tenminutes. This leaves enough time for the HEX tostabilise. The new level of the difference between theprimary and secondary return temperatures becamestable already after about two minutes in the testedobjects. The CV was maintained locked for tenminutes, which should be sufficient even for very lowflows and most types of HEXs. Subsequently, thecontrol was resumed in order to ensure a constanttemperature drop on the secondary side.The temperature drop was controlled by verifying thecurrent temperature drop, e.g., every five minutes, andcomparing it with the desired temperature drop, i.e., thetemperature that was observed when the CV waslocked. If the difference exceeded a certain value,0.2 °C has been used so far, the set-point for thesupply temperature was updated according toT setpoint = T s,r + T setpoint .Fig. 3 displays a performed test: At 1:00 a.m., the CVwas locked and the radiator flow rate was reduced from0.59 to 0.36 l/s with the result that the secondarysupply temperature rose from 40 to 44 °C. After tenminutes, the temperature drop in the radiator circuitwas automatically controlled (in this case, thetemperature drop was stable and it took more than 15minutes before the CV opening degree requiredadjustment). After ninety minutes, the second flowreduction was carried out, to 0.24 l/s, and thesecondary supply temperature increased to about48 °C.The total primary return temperature varied to arelatively large extent, partly because of tappings ofdomestic hot water (DHW), but also due to the DHWcontrol in this substation being very unstable when notappings were made. However, the return temperaturefrom the radiator HEX was of interest for the tests. Inthis object, the difference between the primary andsecondary return temperatures was very small, andeven for a low radiator flow, the grädigkeit was belowone degree. One can see from the figure that the returntemperature had fallen from just under 32 °C to slightlyover 28 °C during the test. This resulted in, for acurrent outdoor temperature of 8 °C, the set-point forthe secondary supply temperature being changed from40 to 48 °C while the flow should be reduced from 0.59to 0.24 l/s.Temperature [ C]Flow [l/s]%Heat supply [kW]8070605040302010000:30 01:00 01:30 02:00 02:30 03:00 03:30 04:00Time2010010.750.50.250604020CV,heatCV,DHW000:30 01:00 01:30 02:00 02:30 03:00 03:30 04:00TimeT p,sT s,rT p,r,radT p,r,totT s,rT sT oT o,dampFig. 3 Results from a test. The flow was reduced at 1:00and 2:30. The top graph shows temperatures in thesubstation, the next graph presents the valve position forheat and DHW, and the last two display the primary(including DHW) and secondary flow and the primary(including DHW) and secondary heat supply, respectively.An interesting aspect of this test was that the primarysupply temperature fluctuated a lot. Since thesecondary temperature drop was kept constant, it hadno impact on the outcome of the test. One can see thatthe CV generally demonstrated a lower opening degreelater in the night, as opposed to before 1:00, when theprimary supply temperature increased. Without the Tcontrol, the heat supply would have been too highduring the last part of the test.The radiator flow was altered by changing the set-pointfor the pump speed, expressed as a percentage of themaximum speed. It has been found that two flowalterations of ninety minutes each are suitable per test,as this would allow the secondary return temperature tostabilise even at very low flows. The first test for anyoutdoor temperature, as was the case in Fig. 3, meansthat starting conditions include the original controlcurve and flow rate. It is then desirable to perform twofairly large flow reductions since, according to thetheoretical calculations, one can expect to find anoptimum at a relatively low flow. If, however, the flow isalready on a low level, it is reasonable to attempt oneslightly higher and one slightly lower flow rate. Thealgorithm for the adaptive control is illustrated by theflow chart in Fig. 4.m pQ pm sQ sGr210