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, Estoniawith the multipole method in [20] for two examples oftwin pipe (DN 20 and DN 80) and by [10]. For twinpipes of even smaller size, such as in branchconnections, the heat losses occurring in case ofvertical layout are only slightly more favorable than thelosses occurring on horizontally arranged pipes; thisresult is shown with an example in the results section.RESULTS AND DISCUSSIONIn this section we discuss the influence of the soiltemperature on heat losses; next, we present thevalidation of the FEM models; finally we apply themethod to show the potential for energy saving in thecase of asymmetrical insulation of twin pipes, in thecase of double pipes and triple pipes.Temperature field in the soilTemperature conditions in the soil around a typical twinpipe, type Aluflex 16–16/110, were evaluated over a10-year period. Figure 5 shows the all-yeartemperature profiles of the outdoor air and of theground at depth equal to 0.5 m, at three horizontaldistances from the centre of the casing, during the firstyear of operation. No notable differences in the yearlyprofile were noticed in longer periods of time.We found that in state-of-the-art well insulated twinpipes (series 2 or 3) a certain amount of soil is slightlyheated up by the warm twin pipe; nevertheless thelevel of such heating can be neglected because itseffect is not noticeable in comparison to the fact thatthe uncertainties about the thermal properties of thesoil usually have a bigger impact. Considering yearlyaverage temperatures, the magnitude of the soilheating is about 1 °C for distances of around 0.2-0.3 mfrom the centre of the casing, and less than 0.5 °C by0.5 m. The temperature raise is considered incomparison to the undisturbed temperature of theground at a distance of 10 m.Figure 5: All-year temperature profiles of the outdoor airand of the ground at depth equal to 0.5 m and 3 horizontaldistances from the centre of the casing.85FEM model: geometry of the ground and of thepipesWe considered the geometric model of the preinsulatedAluflex twin pipe type 16-16/110; thetemperatures of supply/return/ground are 55/25/8 °C.We calculated the heat losses for vertical or horizontalplacement of the media pipes inside the casing, whichwas embedded in a rectangular or a circular model ofthe ground. The same calculations were repeated forother twin pipe size, up to DN 32 and other mediumpipe materials, i.e. steel and copper. The resultsconfirm that the vertical placement of the media pipesinside the insulation barely affect the heat transfer,being the difference between the two configuration lessthan 2% for the considered cases.Table 2: Heat loss for various placements of the mediapipes and various model of the ground.GroundmodelMediapipeslayoutHeat losssupply[W/m]Heat lossreturn[W/m]Heat losstotal[W/m]A Vert. 3.79 -0.17 3.62A Horiz. 3.80 -0.18 3.62B Vert. 3.84 -0.18 3.66A: Semi-infinite, rectangular (width x depth: 40 m x 20 m)B: Finite, circular (diameter: 0.5 m)Steady-state heat loss in commercial pipesThe model was validated by comparing the results fromFEM simulation to the analytical calculation for preinsulatedpipes embedded in the ground [14].Calculations were carried out for four different sizes ofAluflex twin pipes (size 14–14, 16–16, 20–20, 26–26)and for chosen sets of supply (50, 55, 60 °C), return(20, 25, 30 °C) and ground (8 °C) temperatures. Theselected pipes are suitable to be used as branch pipesin low-energy demand areas. There is a goodaccordance between the two methods, the deviationbeing lower than 1%. Figure 6 gathers the values oftotal heat loss for the Aluflex twin pipe category; fourdifferent approaches are reported. The term ―standard‖is used when the effect of the temperature on thethermal properties of the insulation is neglected and thethermal conductivity of the PUR foam is thus constant.This is in accordance with [21]. The term ―advanced‖ isused when the calculation method takes into accountthat the thermal conductivity of the insulation dependson the temperature. Based on the temperaturescalculated for a number of points in the insulation theprogram calculates an average temperature for thematerial; the lambda-value of the insulation is thencalculated as a function of such temperature. Anaverage temperature of the ground is similarlycalculated. The calculation is repeated until the meantemperature difference for the insulation material, pipe