**3. Distribution network**

The distribution network's role is the transmission of heat generated in centralized or distributed locations through a system of pipes for residential and commercial heating requirements. The DH heat supply must provide sufficient energy at the appropriate temperature and pressure to meet end-user demands. In LTDH planning, the system design starts with the identification of demands (**Figure 14**). The demand can be calculated on a case by case basis or estimated for a group of buildings. Several tools for heat mapping are developed in order to facilitate DH planning such as:

1.PlanHeat: http://planheat.eu/

2.THERMOS: http://www.thermos-project.eu/home

3.HotMaps: http://www.hotmaps-project.eu

<sup>7</sup> https://termonet.dk/

<sup>8</sup> https://www.kensaheatpumps.com/shoebox-ground-source-heat-pump/

### **Figure 14.**

*Basic principles of traditional DH and LTDH planning [50].*

The heating capacity of a DH network can be defined by the following three parameters, which are all related to each other.


After determination of heating demand, four factors influence the optimum design of an LTDH network:


Several pipe types are available, and the selection of them depends mainly on operating conditions and cost. The different kinds of pipes are ranging from rigid steel pipes to flexible plastic pipes manufacture with pre-insulated bonded. The pre-insulated flexible single or twin pipes are the standard choices for LTDH. A twin pipe integrates both the supply and return lines within one casing. Depending on the insulation thickness, both single and twin pipes are categorized in series 1, 2, or 3. The two types of single and twin pre-insulated pipes are shown in **Figure 15**.

The required pipe length is calculated by the linear length between all buildings within a hectare. Each pipe section must accommodate the peak heat loads.

The pipe diameter defines by heat density and must be carefully selected. The project capital cost and network heat loss are directly related to pipe size. In order to determine an optimum pipe diameter, different techniques have been proposed in recent publications [52–56]. Increasing the pipe size, improve the linear heat density of the DH system and, on the other hand, increases the project cost.

*Recent Progress in District Heating with Emphasis on Low-Temperature Systems DOI: http://dx.doi.org/10.5772/intechopen.94459*

Linear demand density is calculated from the flow velocity and network temperature. Different guidelines recommend different thresholds for the design velocity. The impact of different design guidelines on the DH network cost has been evaluated by Best et al. [57]. They compared the guidelines of Sweden, Germany, and Austria and concluded that allowing high specific pressure drops of ≥300 Pa/m at maximum heat load leads to transportation pipe investment savings of 6–8%. An increase in the pipe diameter without additional insulation thickness increases the

**Figure 15.** *Pre-insulated district heating pipe (a) single (b) twin [51].*

**Figure 16.**

*Heat loss data from the existing DH networks in Denmark [58].*

**Figure 17.** *Prices and maximum fluid velocity of network pipes.*

**Figure 18.** *Typical values of heat loss coefficients and external diameters of pre-insulated pipes.*

lateral heat loss. However, this heat loss increase is minimal when compared with the heat density increase. **Figure 16** shows the magnitude of heat loss versus linear heat density.

As part of the effort to expand the LTDH networks, a research project under European Commissions, Horizon 2020, provided a useful pre-design support tool. In this project, which is called FLEXYNET<sup>9</sup> , an Excel tool has been developed to carry out preliminary feasibility studies on the implementation of LTDH. The following cost data has been selected from this publicly available tool [59] (**Figures 17** and **18**).

The efficient operation of DH is based on complicated interactions of different heat sources with different consumers. Appropriate control of such a complex system is another challenge of LTDH systems. The control logic is a combination of head/pressure control, temperature control, and distribution optimization. Inadequate control of pressure in the DH network would lead to more water flow through the consumers close to the DH pumps and insufficient water flow through the consumers located far away. Since the supply temperature of LTDH is low, a small unpredicted variation in the demand will impact the system operation and

<sup>9</sup> Fifth generation, Low temperature, high EXergY district heating and cooling NETworkS: http://www. flexynets.eu/

*Recent Progress in District Heating with Emphasis on Low-Temperature Systems DOI: http://dx.doi.org/10.5772/intechopen.94459*

efficiency. Some DH network design approaches are recommended to improve system flexibility. One of these solutions is the ring network [60]. Unlike the traditional designs, ring topology equalizes the pressure differences between the supply and return pipes. The rink network reduces the risk of pressure spike in case of malfunctioning of any control valves.

Further review of different controlling strategies has been provided by Vandermeulen et al. [61]. One of the recent efforts to improve the DH controls is the STORM initiative. In this project, a new control algorithm has been developed and successfully applied in two demo sites [62, 63].
