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, EstoniaTable 7: Steady state heat losses of triple pipes typeAluflex 14/14/110 for 4 geometries and 3 operationalmodes. Temperature supply/recirculation/return/ground:55/55/25/8 °C.ModeI(DHWtapping)II(supply-tosupplyrecirculation)III(spaceheating)Table 8: Steady state heat losses of triple pipes typeAluflex 14/14/110 for 4 geometries and operational modeII. Temperature supply/recirculation/ return/ ground:55/55/25/14 °C.II(supply-tosupplyrecirculation)Geom.Geom.Pipe1Pipe1Heat loss [W/m]Pipe2Heat loss [W/m]Pipe2Pipe3Pipe3Tot.A 2.67 -0.08 2.67 5.30B 2.91 -0.29 2.75 5.38C 2.52 -0.22 2.74 5.06D 2.46 0.05 2.74 5.24A 2.67 / 2.67 5.34B 2.69 / 2.85 5.55C 2.48 / 2.70 5.18D 2.49 / 2.75 5.25A 3.46 0.48 / 3.95B 3.39 0.43 / 3.83C 3.41 0.35 / 3.76D 3.53 -0.01 / 3.53Tot.A 2.35 / 2.35 4.70B 2.37 / 2.51 4.88C 2.39 / 2.63 5.02D 2.20 / 2.42 4.62We conclude that an absolute best design for theservice triple pipe does not exist, but it depends on theoperational mode that is chosen as critical. In fact theresults reported in Table 7 and Table 8 show thatgeometry C gives the lowest total heat loss foroperational modes I and II, while geometry D has thebest thermal performance for operational mode III andfor operational mode II, if a temperature of the soil of14 °C is considered. It has to be underlined that,considering the operational mode III, geometry Dshows no heating of return water; this is a situationalways desirable, although it has a slightly higher heatloss from the supply pipe than the other geometries. Itis proved that usually operational mode I occurs forless than 1 h/day [20]. Moreover the temperature dropin the supply pipe to the DHW heat exchanger is criticalin low-temperature applications, so that it is stronglyrecommended to minimize the heat loss from thismedia pipe. Considering all this and the fact that modeIII is the most likely during the heating season and88mode II is the most likely outside heating season, theconclusion is that geometry D is preferable.CONCLUSIONSThe soil temperature at 0.5 m below the surface variesbetween 2 °C in January-February and 14 °C inJuly–August, for Danish conditions. This knowledgecan be used to better predict the winter peak load andthe temperature drop in the distribution line duringsummer.The slab-model for steady state heat loss calculationscan be replaced, in case of small sizedistribution/service pipes, by a model where the effectof the soil is represented by a circular soil layer aroundthe district heating pipe.The results confirm that the vertical placement of twinmedia pipes inside the insulation barely affects the heattransfer, in comparison to the horizontal placement; thedifference between the two configurations is less than2% for the considered cases.We proposed a FEM model that takes into account thetemperature-dependency of the thermal conductivity ofthe insulation foam; in this way we enhanced theaccuracy of the heat transfer calculation among pipesembedded in the same insulation.We applied the model to propose optimized design oftwin pipes with asymmetrical insulation, double pipesand triple pipes. We proved that the asymmetricalinsulation of twin pipes leads to lower heat loss fromthe supply pipe (from -4% to -8%), leading to a lowertemperature drop; next the heat loss from the returnpipe can be close to zero.It is possible to cut the heat losses by 6–12% if anoptimal design of double pipes is used instead oftraditional twin pipes, without increasing the investmentcosts.The development of an optimized triple pipe solutionwas also reported. It is suitable for low-energyapplications with substations equipped with heatexchanger for instantaneous production of DHW.REFERENCES[1] S. Froning, ―Low energy communities with districtheating and cooling‖, 25 th Conference on Passiveand Low Energy Architecture, Dublin (2008).[2] S. F. Nilsson et al., ―Sparse district heating inSweden‖, Applied Energy 85 (2008), pp. 555–564.[3] F. Schmitt, H.W. Hoffman, T. Gohler, Strategies tomanage heat losses – technique and economy,IEA-DHC ANNEX VII, (2005).[4] P.K. Olsen, B. Bøhm, S. Svendsen et al., ―Anew-low-temperature district heating system for