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at the time of usage. Hereby, only a small volume of domestic hot water is held up in the pipes, leading to a higher level of hygiene e.g. in relation to bacteria growth, such as Legionella. As can be seen from figure 4, the concept reduces the number of distribution pipes in the building from five to three compared to the traditional distribution system.
Another example is an existing house area located in Sønderby/ Denmark. The area includes 75 one family houses built from 1997. Before the system renovation, the houses were equipped with domestic hot water storage tanks which were not adequate for low temperature operation. The renovation included new substations including instantaneous heat exchangers for domestic hot water preparation with long thermal length, replacement of the DH distribution pipe network and supply to the area from the existing DH network return pipe. The houses were equipped with floor heating operating at maximum 40°C, fitting well to the low temperature operation. The results showed that it was possible to operate the system at a supply temperature of 55°C, reducing the thermal distribution loss from 41% to 14%. The area was supplied from the existing DH network area return line to the maximum extent possible.
Figure 4: Traditional solution and flat station solution. Heat source can be DH.
A major benefit of the flat station concept is that it makes multi- apartment buildings suitable for operation at low DH supply temperatures, due to the reduced risk of bacteria in the domestic hot water system. The flat station concept is also a good option when renovating older buildings, leading to energy savings due to reduced hot distribution pipes and better match between supply and demand. Discussion The main challenge looking forward is the reduction of DH net temperatures. Looking historically on the temperatures, they have been going downwards and it is a matter of continuous focus and efforts on the short and long term. On the short term, monitoring of the building energy meter regarding weighted return temperature is relevant and widely applied. This temperature is the basis for a bonus system, motivating reduced return temperatures. In case the surface of the radiators is not sufficient for obtaining a low return temperature, the most exposed radiators can be replaced with larger ones. Investigations have shown that for a normal Danish one-family house from 1970, only 8,1% of the heating season duration the radiators need supply temperatures higher than 55°C. In case of installed new windows, this is reduced to 1,8%. In general, the radiator sizing should be sufficient for most of the season. For the preparation of domestic hot water it is a matter of the thermal length of the installed heat exchanger; in case this is not sufficient it can be replaced with a suitable one. By starting already today specifying heat exchanger with long thermal length, the DH system is made ready for reduced supply temperature once the time comes for low supply temperatures.
Figure 3: Low-temperature DH supply for an area in Sønderby/Denmark (75 houses).
The main learnings from the low temperature DH demonstration projects are: the network must be carefully designed to reduce distribution heat losses. This includes specifying pre-insulated DH pipes of a high insulation class and overall optimizing the pressure head utilization in terms of high media speeds. On the building level, the substation should be pre-insulated and domestic hot water should be prepared by instantaneous heat exchanger with a long thermal length. Besides this, the control valves should include automatic hydraulic balance, e.g. by means of combination valves. Hereby hydraulic balancing of the DH network is secured. The cases above focus on one-family houses/row houses. In case the buildings are multi-family houses, the concept of flat stations can be applied. The concept is to have a substation in each apartment, where fresh domestic hot water is prepared
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