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, EstoniaAs the pipe diameters are quite small, twin pipes withinsulation class IV are used in the area asrecommended by Energy Industry [1], [2] in Finland.The pressure drop design principle used here is roughly~1.5 bar/km.Pipe lenght (m)700600500400300200100015 20 25 32 40 50 65Pipe size (DN)Figure 2. Pipe size distributionThe linear heat density is 0.44 MWh/m which makes thearea a low heat density area by definition [3].The heat demand around the year is presented inFigure 3. The peak demand for the area is 507 kW. Asexpected, in the summertime the load consists almostsolely of domestic hot water consumption.Total heat demand (kW)60050040030020010000 50 100 150 200 250 300 350DaysFigure 3. Heat demand of the simulated area.SIMULATION MODELA node-and-branch type simulation model [4] was usedto study the case in hand. The model calculatestemperatures and pressures for the nodes and flowsand heat losses for the pipes, i.e. the branches. Fromthese results pumping power can also be calculated,although a constant efficiency of 0.5 is used for thepump. The pressures are calculated separately fromtemperatures. The temperature calculation is dynamicwhile the flow and pressure calculation is not. Aminimum 0.6 bar pressure difference over a consumeris assumed.When defining the network, each pipe is given a startand an end node, a pipe type (twin, single), aninsulation standard (class I to IV) and length.The consumptions for both heating and domestic hotwater use were given as hourly time series as well asthe radiator supply and return temperatures on thesecondary side.The heat exchangers were modelled with logarithmictemperature principle in a design point (described inTable 1) after which the conductance in W/K isassumed to be constant. When heat demand, bothsupply and return temperatures on secondary side andsupply temperature on primary side are given as input,the primary return temperature and district heating massflow can be calculated.Table 1. Design point for heat exchangers.DescriptionValuePrimary side temperatures 115/45 °CRadiator heating 70/40 °CDomestic hot water 55/10 °CDesign heating loadDesign DHW load8 830 W2 060 WThe design loads for domestic hot water are lowcompared to a real life design load of a heat exchangerin normal detached house in Finland, 50 kW is acommon choice. This is due to the simulation modeltaking hourly data originally calculated for a multifamilyhouse as input so the domestic hot water demand isalso flatter than it really is. However, from the networkdesign point of view hourly data is considered accurateenough.Other input data used were the undisturbed groundtemperature of 5 °C, assumed to be constant, and thesupply temperature from the main district heatingnetwork as a function of outdoor temperature. Theoutdoor temperature time series used described atypical year in Southern Finland. The supplytemperature reaches its maximum value of 115 °C in anoutdoor temperature of -26 °C and its lowest value of75 °C in 5 °C. Between these two points, the relation islinear.SIMULATION RESULTSThe most interesting results concern the heat lossesand the temperature variations within the network. Thepumping needed (less than 1 MWh) in a network of thissize is quite low and thus negligible.In the initial simulation runs it was noted that the systemwas struggling to maintain high enough temperaturelevel in the summertime when the load consist solely ofdomestic hot water demand. This problem was met bydefining a flow through valve at the consumer, opening70