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, Estoniaintroduced to avoid the mixing of by-pass water to thereturn water. Double pipe water supply concept wastested to use the recirculation pipe supply water duringwinter season. Two different house installation modeswere considered in the analysis.The by-pass water significantly increases the returnwater temperature in the traditional design. The mixedreturn temperature can reach 35.5 o C for HE and45.6 o C for DHWS. With applying the by-pass waterrecirculation, this return temperature can be maintainedat 22 o C, while the re-circulated by-pass water can bekept as high as 44 o C and 53.5 o C for HE and DHWS atthe plant, respectively. It was found that the doublepipe supply leads to the highest network heat loss.However, the conclusion that whether the concept ofdouble pipe supply is inferior to other network designmethods can only be drawn after further networkthermal-economic optimization.The simulation program simplifies the twin pipe heattransfer prediction as a single pipe, and neglects thereturn pipe heat loss when the return pipe absorbs heatfrom the surroundings. The temperature predictionerrors due to the single pipe assumption were analyzedthrough solving the coupled supply/return pipedifferential energy equations. The prediction errorsincrease with increase the allowable temperature dropin the network. Considerable error was found for thereturn pipe at high ground temperature.NOMENCLATUREc p = specific heat capacity [ J/kg.K]q = Heat transfer rate [kW / m]s = Laplace transform variableT = Temperature [ K]U = Overall heat transfer coefficient [ kW /m.K]U ij = Linear thermal transmittance [kW/m.K]= mass flow rate [ kg/s]Greek Letter = Dimensionless temperatureSubscriptsg = Undistributed groundr = Returns = Supplyu = Upstreamd = DownstreamAbbreviationDH = District heatingHE = Heat exchangerDHWS = Domestic hot water storage tankREFERENCE[1] H. Lund, B. Moller, B. V. Mathiesen, A. Dyrelund, ―The role of district heating in future renewableenergy systems‖, Energy, 35, pp. 1381-1390,2010.[2] Charlotte Reidhav, Sven Werner, ―Profitability ofsparse district heating‖, Appliced Energy, 85, pp.867-877.[3] ―Udvikling og Demonstration af Lavenergifjernvarmetil Lavenergibyggeri‖, EFP 2007.[4] TERMIS Help Manual, Version 2.093,7-Technologies A/S.[5] Dansk Standard DS 439, 2000. Norm forvandinstallationer, Code of Practice for domesticwater supply installations, 3. udgave, www.ds.dk.[6] Otto Paulsen, Jianhua Fan, Simon Furbo, Jan EricThorsen, ―Consumer Unit for Low Energy DistrictHeating Net‖, The 11th <strong>International</strong> <strong>Symposium</strong>on District Heating and Cooling, 2008, Iceland.[7] P. K. Olsen, et.al, ―A new low-temperature districtheating system for low energy buildings‖, the 11th<strong>International</strong> <strong>Symposium</strong> on District Heating andCooling, Iceland, 2008.[8] Logstor. http://www.logstor.com/[9] Benny Bohm, Halldor Kristjansson, ―Single, twinand triple buried heating pipes: on potentialsavings in heat losses and costs‖, <strong>International</strong>Journal of Energy Research, 29, pp. 1301-1312,2005.[10] P.Walleten, ―Steady-state heat loss from insulatedpipes‖, Thesis, Lund Institute of Technology,Sweden, 1991.80