State-of-the-art low-temperature DH substations

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• Maximum length of DHW pipes, based on the maximum allowed volume of 3L

• No storage of DHW, based on the maximum allowed volume of 3L

In addition to temperature DH, the same concept can be used for other low-temperature heat sources, such as solar-thermal collectors or heat pumps.

The state-of-the-art DHW HEX with a temperature drop between primary and secondary sides of 3°C and an additional 2°C temperature drop as an effect of cooled DHW pipes at the beginning of tapping means that the first requirement defines the minimal supply temperature of low-temperature DH as 50°C.

The second requirement gives the maximum length of DHW pipes. It is suggested that the DHW fixtures should be individually connected with PEX pipes with an inner diameter of 10 mm, which allows 38 m of pipe in total. In the case of steel pipes with DN15 or DN10, the maximum length is reduced to 15 or 25 m respectively, which is still seen as enough for a single-family house if the location of all DHW tapping points is planned during the design phase of the house. Figure 3.2 shows an example of the design in the pilot low-temperature DH project in Lystrup, where the total length of the DHW pipes is 12.6 m. Since the DHW pipes have an inner diameter of 10mm, this means only 1 L of DHW. Proper location of the tapping points also means there is no need for DHW circulation, which is another source of energy losses.

DHW fixture

nominal flow [L/min]

length to fixture

[m]

volume in pipes

[L]

velocity [m/s]

transportation delay [s] for:

nominal

flow flow 0.2L/s

shower 8.4 2.2 0.17 1.8 1.2 0.9

basin 3.4 4.1 0.32 0.7 5.8 1.6

kitchen 6 6.3 0.49 1.3 4.9 2.5

Figure 3.2 – Example of location and connection of DHW tapping points designed in proximity of DH house substation based on the instantaneous principle of DHW in Lystrup. The table shows the transportation delay for nominal and expected flows and lengths of individual feeding pipes (inner diameter 10 mm)

The last requirement for no storage of DHW water leads to the development of a low-temperature house substation with a buffer for DH water (discussed in the next chapter).

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Usually, the DHW part of the house substation consists of the HEX and controllers, connected together with pipes and fittings, controlling the heating of the DHW to the desired temperature.

Traditional high and medium-temperature DH house substations can be divided to two groups:

• Substations based on the instantaneous principle of DHW preparation (IHEU

= Instantaneous Heat Exchanger Unit), typical design heating power 32.3 kW [22]

• Substations with a DHW storage tank, design heating power depends on size of the DHW storage tank

A substation based on the instantaneous principle of DHW heating produces DHW only when needed (see Figure 3.3 left), whereas in a substation with a storage tank the DHW is heated slowly and stored to be ready for use. DHW storage tanks are generally used to reduce the design heating power needed for DHW preparation. In the case of DH, they mean that the diameter of pipes in DH network can be reduced, leading to reduced heat loss and also reduced peak heat power for DH heat sources.

The DH house substation can also provide a building with SH, either through an additional HEX (indirect SH) or the DH water can be used directly in the SH system (direct SH). House substations with a direct SH connection can also be equipped with a mixing loop to reduce DH water temperature, often controlled by the outdoor temperature and known as weather compensation. Design temperatures for the SH part of the substation are more a question of the SH system than the substation, so here the focus is on the DHW design temperatures.

Figure 3.3 – Low-temperature DH substations; left: instantaneous DHW principle, i.e. IHEU [35], right:

storage tank for DH water, i.e. DHSU [25], [36]

The DHW HEX in traditional DH substations are designed for minimum DH supply/return temperatures of 60/30°C (summer conditions of medium temperature DH), whereas a low-temperature DH substation should work at the temperature levels of 50/25°C and produce DHW of at least 45°C.

DHW 1

2

T₂₂

T₂₁

T₁₂ SH

T₁₁

T

bypass 35°C

T₁₂

DHW 1

2

ON/OFF

SH

T₁₁ T₂₂

T₂₁

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The use of low-temperature DH therefore requires some modification to:

• The DHW HEX

• The DHW controller

• The DHW storage tank

DHW HEX

The key component of a low-temperature DH substation is a highly efficient HEX with Micro Plate™ design of plates [37], specially developed for low supply temperatures by Danfoss (see Figure 3.4). Compared to traditional HEX for high and medium temperatures, the HEX for low-temperature DH should be more efficient because the temperature difference between the DH water supplied and the DHW produced is for design conditions only about 3°C (50°C/47°C) while in traditional HEX it is as much as 10°C (60°C/50°C). Such a low temperature difference in the case of low-temperature HEX is possible thanks to the special “dimpled” pattern of the HEX’s plates, which in comparison with traditional fishbone corrugated plates increases the heat transfer area and the overall heat transfer coefficient while maintaining high cooling of primary water (i.e. low return temperature).

Figure 3.4 – The new Micro Plate™ design compared with the traditionally used fish bone design (courtesy of Danfoss A/S)

Moreover changing the corrugation pattern reduces the pressure drop down to 65% of traditional HEXs, making possible closer installation of individual plates and thus a more compact size. An example of such a HEX is the XB37H or XB06H+

implemented in a low-temperature DH substation such as Akva Less II TD [35] or Akva Les II S [38].

DHW controllers

The state-of-the-art DHW controller is a combined proportional-thermostatic DHW controller with an integrated differential pressure controller and e_save^TM function, which ensures that the heat exchanger is cold during standby (period without DHW tapping), e.g. PTC2+P [39]. At the first sight, it may be surprising that the controller is a simple self-acting mechanical controller without any electronics, but the reason is

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to make the product as simple as possible to reduce the cost, extend operation time and eliminate possible malfunctions.

A DHW controller with a combined proportional-thermostatic function ensures that when the customer asks for DHW, the DH flow is set to the maximum value until the DHW reaches the desired temperature, when the DH flow drops to the value needed to maintain the desired DHW temperature. This feature is very important at the beginning of DHW tapping, when the DH water in the service pipes (pipe connecting the DH pipe in the street with the DH substation in the building), the HEX and other parts of the substation can be cold and a low flow could increase the waiting time for DHW considerably. The differential pressure controller maintains constant differential pressure across the control valve and thus enables the control valve to operate on whole stroke (lift) giving the full control range.

DHW storage tank

To follow the German standard DVGW 551 [33], the water volume in the DHW system cannot be more than 3L. This requirement will be not met by traditional substations with a DHW storage tank, usually accounting for 100-150L. The solution is to “move” the storage of DHW water to the primary side and store DH water instead [25]. DHW is then prepared on the instantaneous principle in the HEX (see Figure 3.3 right) as in the case of a house substation based on the instantaneous DHW preparation principle. This solution is called District Heating Storage Unit (DHSU).

The unit with the buffer tank for DH water was originally designed to reduce the pipe dimensions in the DH network to further reduce the heat loss, but [7] documented that heat loss saved due to the reduced size of pipes in the DH network is lost by additional heat loss from the DHW storage tank, so this solution, with higher investment cost and higher space requirements, is suggested for use mainly on the outskirts of DH networks experiencing capacity problems.