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, Estoniacopper or PEX, with the supply and return pipe in thesame casing. The heat losses from twin pipes arelower than from single pipes, considering samedimensions and temperatures.Furthermore commercially available twin pipes, withdimensions up to DN200 for traditional steel media pipeor up to DN50 for PEX media-pipes are usually lessexpensive to install than single pipes [7]. Thistechnology has been introduced in Nordic countries(and it is used in daily operation in many DH networks.Triple pipes might be considered in the near future, dueto flexibility in the way the system can operate andlower heat losses in case of optimal configuration. Thechoice of house connections depends mainly on thelength of the branch pipe, on supply and returntemperatures, building heating load and type ofsubstation. The latter is decisive with regard to energyperformance and thermal comfort. The types ofsubstations are typically divided into three concepts:unit with domestic hot water (DHW) storage tank,where the tank is the secondary-loop and consumerunit with DH water tank, where the tank is placed in theprimary loop. In this paper branch pipe solutions areconsidered for the concept of a consumer unit with heatexchanger and no storage tank. Two possibleconfigurations of user connection to the distribution lineare shown in Figure 1.demand, although a non perfect cooling of DH wateroccurs when tapping of DHW starts. The conceptbased on twin pipes and a substation withinstantaneous production of DHW in a heat exchangeris an optimal solution, if certain conditions arerespected. The first requirement is that the controlmethod gives priority to DHW preparation over spaceheating; the second condition is that the space heatingload during summer, to keep a high level of comfort inbathrooms for example, has to guarantee a sufficientcooling of the return water. As a result media pipes withinner diameters as small as 10 mm can be applied inthe primary loop and the water return temperature canbe kept sufficiently low, even in summer conditions.The triple pipe system is applicable in three differentoperational modes. The first one (mode I) occurs incase of DHW demand, when pipe 1 and pipe 3 both actas water supply pipes; the second operational mode(mode II) is activated when an idle water flow issupplied by pipe 1 and pipe 3 acts as re-circulation lineto the supply distribution line, while the return line (pipe2) is not active: this is often the case when there is nodemand for space heating, but a small amount of watercirculates in the DHW heat exchanger, keeping theloop warm to satisfy the instantaneous preparation ofDHW in the required time. This system avoids anundesirable heating of the water in the returndistribution line. The third operational mode (mode III)occurs during the heating season when there is onlydemand for space heating and no tapping of DHW:pipe 1 and pipe 2 operate as a traditional supply-returnsystem, while there is no water flow in pipe 3. Thedifferent modes are summarized as follows: Operational mode I: DHW tapping, pipe 1, 2, 3active.Figure 1: Sketch of a user connection with heatexchangers: twin pipe connection with/ without boosterpump (1–2) and triple pipe connection (1-2-3).1: supply2: return3: supply/re-circulationA simple and cost-effective configuration is composedof the control system and two heat exchangers for,respectively, space heating (SH) and domestic hotwater (DHW). The main disadvantage of such type ofsubstation unit is that only rather short lengths ofservice pipes can usually be applied; otherwise it wouldnot be possible to assure the required DHWtemperature at tapping points in the required time, dueto the unsatisfactory transportation time. A modifiedunit is therefore proposed and it is equipped with abooster pump which assures quicker response to DHWOperational mode II: supply-to-supplyre-circulation, pipe 1, 3 active; pipe 2 not active.Operational mode III: space heating demand, pipe1, 2 active; pipe 3 not active.METHODSTheory of steady state heat loss in buried pipesIn order to calculate steady-state heat losses in DHburied pipes there are analytical methods [8] andexplicit solutions for the most common cases [9]. Acomplete review of the available literature aboutsteady-state heat losses in district heating pipes hasbeen carried out in [10]. Here the methods arepresented with reference to the present status of thetechnology in the district heating sector. Furthermorekey-points and critical aspects are discussed; finally,improvements in the methodology of how to calculatesteady-state heat losses are proposed, with particularfocus on low-temperature and medium-temperature82