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, Estoniatemperature, it is desirable for the adaptive control toalso compensate for such short-term variations. Oneway of doing so is to develop a number of parallelcontrol curves for various intervals of the primarysupply temperature. If the temperature is greater than acertain level, an alternative control curve is employed,whereas if it is below a certain level, one utilisesanother. This method has yet to be tested and there isno basis for assessing how much impact one canexpect from normal variations in the supplytemperature or what would constitute reasonableintervals for parallel control curves in this case. Anothervariant could be to perform a linear adjustment for thesecondary supply temperature depending on theprimary supply temperature, according to:Ts, s s,s,0( p,s,0p,s T 1a(T T))(5)where a is a constant that can be determined fromtests.Regarding the measurement of temperatures and flowsRegarding the temperature measurement in thesubstation, supply and return temperatures on both theprimary and the secondary sides are required. Oneshould keep in mind that, on the primary side, thereturn temperature from the radiator HEX is neededsince the total return temperature is affected by theDHW system. This temperature is normally available inmodern substation control equipment.It is desirable to avoid installation of a flow-meter in thesecondary circuit. On the primary side, where theenergy-meter is located, the total primary flow and thetotal temperature drop in the substation are measuredand the energy required for DHW provision is thusincluded. Since the tests are performed at night, DHWtappings can be avoided to a large extent. By closingthe DHW CV for a short time, the primary flow passesexclusively through the radiator HEX. By comparing theaverage level of heat supply with a closed valve to thelevel prior to closing the valve, the flow required forDHW re-circulation can be estimated.In the test objects, indoor temperature measurementswere used to verify that the adaptive control was ableto give the correct indoor temperature. However, onecan in fact be sure that the correct amount of energy istransferred to the system for each operating point,regardless of whether the original control curve or theoptimised curve is used. A possibility is that there is animbalance in the system. For example, the most distantriser may not receive the required flow because of atoo low differential pressure when the pump speed isdecreased. It is, however, more likely that a betterbalance in the system is achieved when the differentialpressure is lowered this since the pressure losses inthe system decreases and all risers receive a moresimilar differential pressure. However, one must be onthe look-out for errors (e.g., short circuits) in thesystems, a problem that is often emphasised inconnection with low-flow systems, as these tend to bemore sensitive to hydraulic imperfections [12].Reduction of the primary return temperatureTo estimate a yearly mean return temperaturereduction (as presented in Table 1) achieved by theadaptive control, an entire, or a major part of the,heating season needs to be evaluated. The controlmethod presented in this paper was developed duringthe winter and spring of 2009, and only a limitednumber of tests were performed during the spring.However, Fig. 7 shows the obtained primary returntemperature that was attained for the tests that wereperformed in one of the houses. Note that these resultswere ―first runs‖ for each outdoor temperature (i.e., theflow was reduced to approximately 40%), signifyingthat no further optimisations were undertaken. Thecurve displaying the original return temperatures wasbased on the average return temperatures from theradiator system prior to any of the modifications (i.e.,for the tests or the constant flow rate change, asdescribed in section 3.1).Primary return temperature50454035302520Tp,r,rad,origTp,r,rad,opt-10 -5 0 5 10 15Outdoor temperatureFig. 7. Primary return temperatures in the radiator systemwhen the flow is reduced (dots), compared to the originalreturn temperatures (curve).CONCLUSIONS AND DISCUSSIONAn adaptive control algorithm was developed in orderto minimise the DH return temperature. The controlalgorithm can be implemented in any modern controllogics for building automation. Some refinement maybe done by compensating for short-term temperaturevariations in the DH network. During the field studies,limitations in the speed control of the circulation pumpshave presented a complication. A modification of thefrequency converter could increase the working range.213