Substation Types - Low Temperature District Heating

A brief introduction can be rewarding about the substation types while talking about Low Temperature District Heating Systems. 

Source: Tol, Hakan İbrahim, 2012, District Heating in Areas with Low Energy Houses (PhD Thesis)

First thing that comes to mind, in Low Temperature District Heating Systems, is if the low temperature supply at a temperature of 55°C can cause legionella growth or not while producing the domestic hot water. One of the solutions to overcome the legionella growth can be by equipping storage tank before the heat exchanger in the substations with indirect connection. Hence the storage tank (also termed as Heat Accumulator and Buffer Tank) will store the medium circulated in the distribution network not the domestic hot water that is used directly from the tap. According to the German Standard, if the volume storing the domestic hot water medium is less than 3 liter there is no minimal limit for the supply temperature [1]. Hence, keeping the volume of the domestic hot water in the pipeline less than 3 liters can restrain the growth of legionella. Besides, using UV light at the tap points (locations before the usage) can be another solution to kill the legionella [2]. 

Another point can be directed to the reduction effect on the heat demand by using the heat storage tank in the substation. This reduction is due to the low charging rate (from the district heating medium) in the off-peak periods. In detail, the storage tank gets charged in the night time and in the morning time peak-demanding moment comes with the usage of domestic hot water (this time from the heat storage tank) i.e. by using shower. In case of direct connection type for the substation, the heat demand for domestic hot water can be as high as 32 kW (that is directly taken from the district heating network medium). For the case of indirect connection, the charge of heat storage tank is with the rate of 3 kW. Of course, when the consumer opens her/his hot tap, the discharge rate is 32 kW but thanks to the heat storage tank since this high discharge rate of 32 kW is taken from the heat storage tank which was formerly charged at a rate of 3 kW in the long-lasting off-peak period (for example whole night) [3].

There is also connection differences for the space heating connection. Equipping heat storage tank in between district heating network and radiator system is also common. However direct connection is rewarding to avoid heat loss from the heat exchanger. 

References:

[1] DVGW. W551 Trinkwassererwärmungs- und Trinkwasserleitungsanlagen [in Deutch]. - Link

[2] Liu Z., 1995, Efficacy of ultraviolet light on the disinfection of Legionella in a hospital water distribution system - Link.
[3] Tol, Hakan İbrahim, 2012, Phd Thesis, District Heating in Areas with Low Energy Houses - Link (Click!)