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, EstoniaADAPTIVE CONTROL OF RADIATOR SYSTEMS FOR A LOWEST POSSIBLERETURN TEMPERATUREP. Lauenburg and J. WollerstrandLund University, Faculty of Engineering, Department of Energy Science, Sweden,patrick.lauenburg@energy.lth.seABSTRACTThe present paper describes how the control of aradiator system connected to a district heating networkvia a heat exchanger can be optimised to provide thelowest possible district heating return temperature. Thiscan be achieved for each operating point by employingan optimal combination of radiator circuit supplytemperature and circulation flow rate. The controlalgorithm gradually creates a modified control curve forthe radiator circuit, enabling it to consistently providean optimal cooling of the district heating water. Sincethe heat exchanger is dimensioned for very low outdoortemperatures, it is oversized for all other heat loads. Inaddition, radiator systems are often oversized due tosafety margins. Such facts render it possible to reducethe district heating return temperature.The objective of the present study was to develop acontrol algorithm and to test it in practice. A descriptionis here given of the algorithm, as well as of field teststhat were carried out to practically verify it. The controlmethod could be implemented in any modern controllogics for adaptive control of a radiator circuit, and theobtained results indicated that one can expect alowering of the return temperature in line with previoustheoretical calculations.oversized for all other heat loads. In addition, radiatorsystems are generally also oversized for safetyreasons, as presented in both Swedish studies [3], [12]and international ones [5], [8] and [10], thus providingfurther potential to reduce the return temperature.ObjectiveThe objective of the study was to develop a controlalgorithm for determining the optimal choice of supplytemperature and flow in an arbitrary radiator system forevery heat load in order to minimise the primary returntemperature.LimitationThe present investigation has dealt with DHsubstations that were indirectly connected to theDH network, i.e., hydraulically separated by HEXs.OPTIMISED HEATING SYSTEM TEMPERATURESThere exist various ways to control the heat output in aheating system. Here, we have dealt with the prevailingcontrol method used in Sweden; an outdoortemperature-compensated supply temperature,ensuring that an adequate amount of heat is suppliedto the building at each outdoor temperature.INTRODUCTIONThe present paper demonstrates how the control of aradiator system connected to a district heating (DH)network via a heat exchanger (HEX) can be optimisedto provide the lowest possible DH return temperature.This is done by always choosing the optimal radiatorsupply temperature and flow rate.Relevance of the topicLow return temperatures are beneficial for theproduction as well as the distribution of DH. A specificadvantage of the control method demonstrated in thispaper, as opposed to, for example, conventional lowflow balancing, is its robustness, enabling the lowestpossible return temperatures to be consistentlyobtained. This is the case independently of the currentoutdoor temperature and heat load, even if the DHsupply temperature changes, the HEX becomes fouled,or the house heating requirements change. The idea isalso to utilise the fact that, since a HEX is dimensionedfor an extremely low outdoor temperature, it is in fact206The benefits with regard to the primary returntemperature from adjusting the flow according to theheat load are known. The idea of using an optimalcombination of flow and supply temperature wasconceived by Frederiksen and Wollerstrand [2], andthis theory has been further studied [13] [11]. Theguidelines from Euroheat & Power [1] state that thelowest return temperature is obtained by varying theflow according to the consumption. If such a variableflow is used, it is controlled by thermostatic radiatorvalves (TRV) either in combination with a constantsupply temperature or with an outdoor temperaturecompensatedsupply temperature. Langendries [4]suggests a central control of the flow rate through thepump‘s rotating speed, but claims that it appears to bea rather difficult and expensive system. Petitjean [9]proposes a lowering of the pump speed at low heatloads, when the TRVs are almost fully open, but finds itproblematic to determine which parameter to use forcontrolling the pump speed.It should be possible to implement the control algorithmpresented in this paper in any modern, state-of-the-art