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, Estoniaheating load periods; hence by-pass at the criticalconsumers are not necessary and the exergy loss dueto the mixing of warm water into the return line isavoided. Furthermore the water flow in the return linehas the same direction as in the supply line (clockwisein the example), so that the smallest size for the returnpipes are expected in correspondence to the biggestsize for the supply size, and vice versa. This results inlower local pressure differences between supply andreturn lines and savings in operational costs, thanks tolower heat losses. This is shown in Table 4 and Table5, by means of two examples: the first one refers to asmall to medium-size distribution network, the secondone to a bigger one, being capable to supply four timesmore energy than the previous one.Triple branch pipesThe development of an optimized triple pipe solution forlow-energy applications is reported to show thepotentiality of utilizing detailed models for steady-stateheat loss calculation. In this survey focus was given onthe choice of media pipes diameters as small aspossible. The triple pipe geometry is based onmodifications of the 14-14/110 (outer diameters in [mm]of respectively supply pipe, return pipe, casing) twinpipe design which has been reported in [18]. Fourgeometrical variations have been considered (seeFigure 8) and the Cartesian coordinates describing theplacement of media pipes inside the casing are listed inTable 6.Table 4: Comparison between a distribution networkbased on twin pipe (DN40-40 and DN80-80) with adistribution network based on double pipe (DN40-80 andDN80-40). Supply/return/ground temperature: 55/25/8 °C.Size(DN)Heat loss [W/m]Sup. Ret. Tot.Total(system) [%]40-40 -6.24 0.04 -6.20 Twin:80-80 -7.66 0.07 -7.59 -13.7940-80 -5.55 0.05 -5.5880-40 -7.41 0.05 -7.36Double:-12.94Table 5: Comparison between a distribution networkbased on twin pipe (DN100-100 and DN200-200) with adistribution network based on double pipe (DN100-200and DN200-100). Supply/return/ground: 55/25/8 °C.Size(DN)Heat loss [W/m]Sup. Ret. Tot.100-100 -7.83 -0.55 -8.39200-200 -8.92 0.24 -8.68Total(system)Twin:-17.06100-200 -6.4 0.08 -6.36 Double:200-100 -8.07 -0.03 -8.69 -15.056.1[%]11.8We considered an optimal placement of the mediapipes in case of double pipes, thus asymmetricalinsulation is applied. The total amount of insulation isused both in the twin pipe-based distribution networkand in the double pipe-based one, so that theinvestment costs are equal in both cases. Results showthat the heat loss can be reduced by 6% by means ofdouble pipes instead of twin pipes for the low tomedium-size distribution network. Even higher energysavings (around 12%) are possible in the case of thelarge-size distribution network.Figure 8: four different geometries for a triple service pipetype Aluflex 14-14/110.Table 6: placement of media pipes inside the casing forfour triple pipe geometries, type Aluflex 14-14-20/110.VariationPipe 1(Sup.)Coordinates (x, y) [mm]Pipe 2(Ret.)Pipe 3(Sup. orre-circ.)A (14;-14) (0;20.5) (-14;-14)B (10;-14) (0;20.5) (-21;-7)C (3;-14) (0;20.5) (-21;-7)D (0; 0) (0;25) (0;-28)The results of FEM simulations are listed in Table 7 forthe four geometries (A, B, C, D) and the threeoperational modes (I, II, II), previously described. Sincemode II occurs in case of no demand of space heatingand then outside of the heating season, simulationswere additionally performed with a more realistictemperature of the ground during that period(14 °C),considering Danish weather. This gives also aninsight in the effect of ground temperature throughoutthe year.87