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, Estoniashell and surrounding soil is less than 0.005 °C for twoconsecutive calculations. The‖standard‖ and―advanced‖ model are available online [14]. In the―FEM advanced‖ model we directly implementedequation (1) in the insulation domain, instead. Theresults indicate that in case of low-temperatureoperation, lower total heat losses are calculated if thetemperature-dependency of the insulation lambdavalueis taken into account. Moreover the heat transferbetween the pipes in twin or triple pipes can beproperly evaluated.Total Heat Loss [W/m]7.06.05.04.03.02.01.00.0DN 14 DN 16 DN 20 DN 26 DN 32 DN 40Standard 3.61 4.24 4.62 5.71 6.45FEM Standard 3.34 3.68 4.33 4.80 6.02 6.76Advanced 2.86 3.36 3.69 4.55 5.10FEM Advanced 3.19 3.51 4.14 4.59 5.75 6.47(Aluflex: ≤ DN 26, steel: ≤ DN 50) the best design is toput the supply pipe in the centre of the casing, assuringthe best possible insulation for the supply pipe. Thisstrategy guarantees also the lowest temperature dropin the supply side, which is a critical figure inlow-temperature applications.For bigger sizes (Aluflex: ≥ DN 26, steel: ≥ DN 50) thebest design is achieved by ―moving up‖ the media pipelayout and at the same time by keeping the samedistance between the media pipes as in thesymmetrical case.Double pipesA double pipe consists of a pair of media pipes ofdissimilar size, co-insulated in the same casing. It is afurther development of the twin pipe concept. A sketchof a possible application of the double pipe concept isshown in Figure 7. Though these measures, networkheat loss reduction is possible, in case of operationduring low heating load periods.Figure 6: Comparison of 4 different approaches for steadystateheat loss calculation. Aluflex twin pipe series,supply/return/ ground temperatures: 55/25/8 °C.Asymmetrical insulation in twin pipesThe results show that improvements are possible,thanks to asymmetrical insulation (see Table 3). Weproved that a better design leads to lower heat lossesfrom the supply pipe (leading to a lower temperaturedrop); next, the heat loss from the return pipe can beclose to zero, maintaining isothermal conditions in thereturn line. If commercial available casing sizes arekept, we suggest two design strategies, depending onthe size of the pipes. For small pipe sizesTable 3: Comparison between asymmetrical and symmetrical insulation in twin pipes.The centre of the casing is the origin of the Cartesian system.Size(DN)14Coordinates(x; y) [mm]Heat loss[W/m]86Figure 7: Sketch of the possible application of the doublepipe concept in a simple district heating network.The space heating demand in summer is diminished,except for the energy requirement in bath roomheating. According to the energy balance, the reducedasymm.-symm. [%]Mat. Sup. Ret. Sup. Ret. Tot. Sup. Tot.(0; 0) (0; 27) 3.24 0.01 3.25 -7.6 2.016 (0; 0) (0; 28) 3.56 -0.01 3.55 -5.1 1.120Alx.(0; 0) (0; 30) 4.16 -0.04 4.12 -4.2 -0.326 (0; 0) (0; 36) 4.67 0.00 4.67 -5.1 1.932 (0; -16) (0; 28) 5.54 0.00 5.54 -5.8 -2.550(0; -25) (0; 55) 5.69 -0.03 5.66 -7.7 -2.4Steel65 (0; -36) (0; 60) 6.70 -0.02 6.68 -7.8 -3.2heating load requires lessnetwork flow rate as far as thedesigned building temperaturedrop is sustained. However, thereduction of network flow ratewill increase the supply watertemperature drop along thepipeline due to heat loss. As aconsequence, the supplytemperature at the end usermay lower down below theminimum requirement. Thisproblem is relevant to towenergyDH systems with analready low supply temperature.This design is based on the factthat the supply line acts also asre-circulation line during low