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, EstoniaA desirable design approach is to maintain the by-passsystem as the flow rate adjuster, while avoids themixing of the by-pass water and the return water. Thisdesign concept is schematically shown in Fig. 2, whichis realized through adding a third pipeline for supplywater re-circulation. When the by-pass water is called,the circulation line will transfer the extra supply waterback to the plant where it is re-heated up to the supplytemperature again. On the other hand, the addition ofthe 3rd pipeline provides the possibility to supply waterin two supply lines when the heat demand is high. Thenetwork, therefore, can be designed as two supplylines with reduced diameter together with one returnline.Fig. 3 Annual heating load (blue columns) and durationhours (red curve) at different ground temperatureNETWORK SIMULATIONFig. 1 Schematic for hot water by-pass systemFig. 2 Schematic for by-pass water recirculationHeating LoadThe simulation was performed for a reference area with81 low energy demand houses. The house wasdesigned based on the building standard Class 1,following the Danish Building Regulation. The domestichot water draw-off profile was designed similar to theDanish standard DS439 [5]. Detailed space heatingand domestic hot water heating load simulation can befound from [6, 7]. Figure 4 shows the averaged heatingload and the corresponding duration hours. The annualheating load is divided into 8 intervals, varying as afunction of undisturbed ground temperatures whichranges from 0 to 15 ºC. The summer season lasts 3281hours and the heating load comes only from thedomestic hot water demand. The space heating isrequired for the rest of the year.House InstallationsTwo house installations were considered in this study.Figure 4 shows the instantaneous heat exchanger (HE)in the DH system. Without a buffer tank, the branchpipe which connects directly to the HE installation musthave the capability to supply the instantaneous hotwater demand without causing significant pressuredrop, which otherwise can be compromised byinstalling a booster pump. The HE design load is 32kWper houses at the network supply temperature 55oCand return temperature 22 ºC. On the other hand,simultaneous factors which are the probabilities formultiple users‘ concurrent use of hot water areconsidered for the design of street pipes and mainpipes, as shown in Table 1 [3]. Fig. 5 shows thedomestic hot water storage tank (DHWS) in the DHsystem. The DHWS design load is 8 kW per house. Toavoid the legionella problem, the design temperaturefor DHWS is higher than HE, at 65 ºC /30 ºC for supplyand return respectively.74