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, EstoniaDESIGN OF LOW TEMPERATURE DISTRICT HEATING NETWORK WITH SUPPLYWATER RECIRCULATIONHongwei Li 1 , Alessandro Dalla Rosa 1 , Svend Svendsen 11 Civil Engineering Department, Technical University of DenmarkABSTRACTThe focus on continuing improving building energyefficiency and reducing building energy consumptionbrings the key impetus for the development of the newgeneration district heating (DH) system. In the newgeneration DH network, the supply and returntemperature are designed low in order to significantlyreduce the network heat loss. Meanwhile, the lownetwork operational temperature can make a betterutilization of renewable energy and further improve theCHP plant efficiency.Though the designed return temperature is low, it mayincrease considerably when the heating load becomeslow and the by-pass system starts to function. The aimof this paper is to investigate the influence of by-passwater on the network return temperature and introducethe concept of supply water recirculation into thenetwork design so that the traditional by-pass systemcan be avoided. Instead of mixing the by-pass waterwith return water, the by-pass water is directed to aseparated circulation line and returns back to the plantdirectly. Different pipe design concepts were tested andthe annual thermal performances for a selectedresidential area were evaluated with the commercialprogram TERMIS. The simulation program calculatesthe heat loss in the twin pipe as that in the single pipe.The influence of this simplification on the supply/returnwater temperature prediction was analyzed by solvingthe coupled differential energy equations.INTRODUCTIONIn European Union, one of the major energydevelopment targets is to reduce the building energyconsumption and increase the supply of renewableenergy. The introduction of European EnergyPerformance of Building Directive (EPBD) posesstringent requirement for the member countries toeffectively reduce their building energy consumption.According to the national energy policy, the buildingenergy consumption in Denmark will drop to 25% ofcurrent level by the year 2060, while the renewableenergy share will increase from 20% to 100% at themeantime [1].District heating (DH) benefits from economic of scalewith mass production of heat from central heatingplants. The significant reduction of building energyconsumption and wide exploitation of waste heat andrenewable energy, however, makes the current DH73technologies become barriers to further increase themarket share [2]. In order to sustain the economiccompetiveness and realize the long term sustainabledevelopment, the concept of design and operation ofDH system needs to be re-examined under the newenergy regulation and development trends. This is themain impetus for the development of the newgeneration DH system. Based on previous studies, in aproperly designed in-house substation system, thenetwork supply temperature at 55oC and returntemperature at 20oC can meet the consumer spaceheating and domestic hot water demand [3].The low return temperature has the advantages toreduce the network heat loss, increase CHP plantpower generation capability, and utilize direct flue gascondensation for waste heat recovery. However, thereturn temperature can become much higher than thedesigned value when the heating load becomes lowand the by-pass system at the critical user starts tofunction. In this paper, the influence of by-pass wateron network return temperature was examined for areference residential area. The concept of supply waterrecirculation was introduced to avoid the mixing of bypasswater and the return water. Three network designmethods were tested. The annual thermal performancewas evaluated with the commercial district heatingnetwork hydraulic and thermal simulation softwareTERMIS [4]. The simulation program calculates theheat loss in the twin pipe as that in the single pipe. Theinfluence of this simplification on the supply/returnwater temperature prediction was analyzed by solvingthe coupled differential energy equations.SUPPLY WATER RECIRCILUATIONThe solution to overcome the excessive temperaturedrop along the supply pipe due to reduced flow rate isto install by-pass system at the critical user in thenetwork. Figure 1 shows the principle of supply waterby-pass. Extra flow is called based on the temperaturemeasurement at the critical user until the minimumsupply temperature requirement is met. This extra flowis then ―by-passed‖ and sends back to the return pipe.As the by-pass flow rate may be considerable and itstemperature is high, the mixing with return water willsignificantly increase the return water temperaturewhich causes both increased heat loss in the returnpipeline and decreased power generation capability inthe CHP plant.