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, Estoniawhen the supply temperature on primary side droppedtoo low (< 65 °C). The valve allowed a constant massflow (0.015 kg/s) to go past the heat exchanger on theprimary side. This solution helped the situationsignificantly although not without ill effects as can beseen from the heat losses presented below.The use of a by-pass valve to ensure the appropriatetemperature level for domestic hot water also meanhigher heat losses and pumping power and effectivelylower cooling; all of which are undesirable outcomes.One possibility to solve the problem is just to accept theflaw and to use additional electrical heating element toraise the temperature of domestic hot water to therequired level. As the temperature boost needed is formost of the time quite small and is only needed insummertime, the increase in electricity consumption isreasonable.Because of the high capital costs of district heating, thepipes should basically be sized as tight as possiblewhile keeping in mind the future demand for the pipelinein question. As the pipes are small, the volume of watercontained is also low. This leads to water cooling morerapidly than in larger pipes. The Figure 4 illustrates thiswith a simplified example by showing the temperatureon supply side service pipes if there is no flow for threedifferent pipe sizes. The temperature drop of 15 °C, forexample, takes 5 times longer with a pipe size DN 50than with a small DN 15 pipe. The calculations assumea constant return side temperature of 30 °C and aground temperature of 5 °C.Temperature (°C)70605040302010DN 15 DN 25 DN 5000 2 4 6 8Time (h)Figure 4. Temperature drop in three pipe sizes when noflow is introduced.The use of smaller pipes reduces the heat losses inW/m and this is accentuated if the temperature leveldrops as described above. As a result, looking solely onheat losses when designing a low heat density areanetwork on common design principles can lead toreliability issues as the system cannot supply the heatrequired by the consumers.The relative heat losses (that is, heat losses per neededproduction) for the simulated case are 13.8 % in a year.The monthly values can be seen in Figure 5. While the71relative heat losses in the heating season areacceptable, they reached 47 % in the summertime. Thehigh heat losses are partly because of the by-pass valveletting hot water past the heat exchangers. The by-passvalve is also responsible for small cooling, i.e. thedifference between supply and return temperatures,within the system in summertime (Figure 6).Relative heat losses (-)1.00.90.80.70.60.50.40.30.20.10.0I II III IV V VI VII VIII IX X XI XIIMonthFigure 5. Monthly relative heat losses.Cooling (°C)90807060504030201000 50 100 150 200 250 300 350DaysFigure 6. Difference between supply and returntemperatures at the border of the area.The most obvious way to cut heat losses in alreadyreasonable insulated network is to lower the supplytemperature. In the simulated system, this would causeproblems because aforementioned issues concerningdomestic hot water demand in summertime, and duringthe heating season because of the traditional radiatorheating design temperatures of 70/40 °C. However, ifmore significant changes would be possible, a floorheating system and a heat pump coupled with anaccumulator handling the higher temperature levelrequired domestic hot water would enhance theefficiency of the distribution system at a price of a verymodest increase in electricity consumption and higherinvestment costs for the consumer because of theaccumulator, heat pump and floor heating. If thedomestic hot water demand takes 3.75 MWh/year,20 percent of the total consumption of 18.75 MWh/year,the electricity consumption would be a very reasonable