**1. Introduction**

Traditionally, the heating in residential and commercial buildings has been provided by individual systems such as furnaces and boilers. These methods were not only less efficient but also have been responsible for substantial amounts of greenhouse gases. District Heating (DH) systems are simply systems that are powered by a central heat source instead of by multiple individual heat sources for each building. By centralizing the heating in larger systems, it is possible to supply many buildings from one or more sources, such as Combined Heat and Power (CHP), Waste-to-Energy (WtE), and Renewable Energy Sources (RES). Several cities in Europe and throughout the world have begun to shift to DH systems. **Figure 1** provides the percentage of supplied energy with DH and the share of RES in the existing DH systems. In aggregated 28 European countries, there are more than 10,000 DH systems, which provide 9% of heating in the residential sector, 10% in the service sector, and

#### **Figure 1.**

*Share of renewable energy in district heating networks, 2018 [1].*

8% of the heating demands in the industrial sector [2]. District heating technology is less common in the United States and Canada than in Europe. According to the International District Energy Association's (IDEA) database, about 660 district energy systems are operating in the United States, and approximately 80 are working in Canada [3]. District heating is suitable for networks of all sizes, from two buildings up to a community, and even cities.

District heating systems have been evolving with a trend towards lowering supply temperatures and introducing different energy sources. Studies revealed that the 4th or 5th generation of district heating systems, along with thermal storage, is more feasible, fuel-efficient, and cheaper than individual solutions in areas with high urban density [4, 5]. The central concept of fourth-generation is a smart thermal grid. Smart thermal grids are defined as a network of pipes, connecting buildings in a neighborhood, small town, or a large metropolitan so they can be served from centralized plants or distributed heating sources, including individual contributions from the connected buildings [6]. The fifth-generation district heating has a network with temperature as close as to ambient ground temperature. In a recent review article [5], Buffa et al. studied more than forty DH systems that belong to the 5th generation. Most of these reviewed cases use shallow geothermal or groundwater as the heat source. In low-temperature networks, heat loss to the ground is eliminated, and the cost of distribution circuit is radically reduced.

If the DH supply temperature is 25°C and less, it cannot be used directly for space heating or domestic hot water (DHW). An electric heater or a booster heat pump is required to raise the temperature. A heat pump extracts heat from a low-temperature medium (e.g., DH supply) and delivers it to a medium on a higher temperature (e.g., building). In this article, a DH system with a supply temperature less than 60°C is called Low-Temperature District Heating (LTDH); thus, both 4th and 5th generations are categorized as LTDH. **Figure 2** shows the evolution of DH systems.

Several mediums can be used as heat sources of low-temperature DH. However, not all sources are universally available or have the same temperature level. Among the potential heat sources, geothermal heat was identified as the most promising source [7]. Direct use of geothermal energy in the DH system is one of the oldest and also the most common form of renewable energy. Space heating, bathing/swimming, *Recent Progress in District Heating with Emphasis on Low-Temperature Systems DOI: http://dx.doi.org/10.5772/intechopen.94459*

agricultural applications, fish farming, snow melting, and industrial process are examples of direct geothermal energy utilization. Most direct uses utilize geothermal fluids in a low (30–90°C) and medium (90–150°C) temperature. The application of very low (less than 30°C) reservoir temperature has been introduced recently and initiated many types of research and case studies [8]. In a low-temperature geothermal, the thermal energy extracts from a shallow depth either by borehole heat exchangers or with the help of heat pumps. These heat pumps often are called ground-source or geothermal heat pumps (GSHP). According to WGC2020, 88 countries utilize geothermal energy for direct heat applications with significant growth in the GSHP market worldwide. About 6.46 million GSHP units have been installed in 2019, which shows a 54% increase compared to the number of installations in 2015 [9]. The trend on GSHP, as opposed to the other geothermal energy

#### **Figure 2.**

*Historical development of DH systems.*

**Figure 3.** *Direct utilization of geothermal energy [9].*

applications, has been shown in **Figure 3**. The size of installed GSHPs ranges from a couple of kilowatts for residential heating to large units over 150 kW for commercial and institutional installations. However, it is difficult to find out whether these GSHP have been installed in DH systems or not. According to the IEA, the heat supplied through DH increased by 18% in 2019 compared with 2015 [10]. Therefore, it can be concluded that most of the GSHPs were not installed in DH networks. In a comparative analysis, Lee et al. briefly discussed the advantages of DH over individual GSHP [11]. In another study, Shin et al. proposed the integration of GSHP on a shared loop to increase the system efficiency and improve the heat demand control [12].

One of the first LTDH projects with geothermal heat source is the residential area in Berlin-Zehlendorf, with 22 houses, 135 apartments, and a total of 21,000 m2 floor space that was completed in 2016. The network temperature is approximately 10°C so that no heat is wasted, and no expensive pipe insulation is required. Decentralized heat pumps extract heat from the network and supply heat energy to the houses. Heat is provided by a CHP plant and borehole heat exchangers [13].

Another example is the city of Plymouth in the UK that will be adapted to a low-temperature DH. The supply DH temperature in the primary energy sharing network is designed between 2°C and 25°C, and the return temperature will not be higher than 25°C throughout the year. The end-users will equip with DHW booster heat pumps to increase DHW temperature to 50°C. Heat sources of the DH network will be groundwater, sea, and low-grade waste heat [14]. This particular project is one of the HeatNet pilot studies. HeatNet is an EU Interreg project to address the challenge of reducing CO2 emissions across northwest Europe by creating an integrated transnational approach to the supply of renewable and low carbon heat. The project's construction started in 2020, and the first stage of the project is planned to commission in 2021 [15].

DH systems' design requires a case-by-case approach to fully take advantage of the available local energy and identify end-users heating demand profiles. Therefore, DH systems are always site-specific and vary from one location to another, considering the size, climate, heat sources, and technologies. DH system can also be classified by size, which defines by:


Copenhagen is an excellent example of an extensive DH system. The DH system was started with one small local network in 1903, and now 98% of the city is supplied by district heating. The system serves 75 million m2 of net floor area. The annual heat sale is 8500 GWh, and the system capacity is 10,000 GWh. The backbone of the system is a 160 km long distribution network and 3 x 24,000 m3 heat storage tanks [16].

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

Small DH systems are more suitable for residential communities and small to medium size industries with excess heat. In some cases, a small DH system may connect to a large DH grid. However, the general idea is to promote individual piping networks that connect a relatively small number of consumers.

Micro heating grids are a relatively new concept characterized by advanced central control to share the resources and interact with the DH network [17]. One advantage of microgrids is that these systems could be built more straightforward and faster because of the small number of customers, without lengthy procedures.

DH systems can be categorized according to the heat production units' location into centralized and decentralized systems. Most DH systems were designed based on one of a few centralized heat generators in the past. By introducing the 4th and 5th generation, a growing number of decentralized systems use heat from various decentralized facilities. A centralized approach is best suited for upgrades or expansions of an existing district DH system. The distributed approach is recommended for a new and sparse area with relatively low load density. As such, the cost of constructing a new district energy network outweighs the other benefits of a centralized district energy system.

Nonetheless, all DH system encompasses:


Despite the well-known advantages of LTDH, there are a limited number of literature reviews. The majority of the reviews only focus on a specific aspect of the district heating systems, such as modeling [18] or system flexibility [19]. In order to address the lack of a comprehensive literature review, this article provides a preliminary review of LTDH systems. The information was collected through the review of international success stories and recent academic literatures. In the following sections, the progress of low-temperature district heating systems is reviewed with respect to heat sources and distribution networks. Geothermal heat and solar radiation are the most viable types of heat source, therefore both of them are discussed in details. The cost of DH and aspects of network design are carried out by the review of typical LTDH systems.

## **2. Heat sources**

One of the advantages of LTDH is diversified heat sources. Studies and pilot projects have shown that a DH temperature of less than 60°C significantly increases the potential to utilize waste heat of different industrial processes and cooling processes (e.g., supermarkets or data centers waste heat). Heat can be supplied by various sources such as:


Heat recovery from industrial processes is not a new concept for DH. It has been applied in some countries such as Russia, Sweden, and Germany for many years in high-temperature DH networks [20]. An excellent example of the waste heat recovery is MEMPHIS's research project under IEA DHC Annex XII [21]. As part of this project, an open-source map1 has been developed to assess waste heat potential from the industry and business sector and sewer networks. Some studies recommend adapting the industrial process heat recovery systems for LTDH [22, 23]. However, the main barrier is the economic risk associated with these heat recovery systems, if the primary industrial activities close down.

Renewable heating sources, such as solar and geothermal, are emerging in most countries. As an example, the European statistical data shows that the energy supply becomes increasingly renewable. They committed to have 100% renewable resources by 2050 [24]. A review of renewable energy sources for district heating was published recently by Olsthoorn et al. [25]. However, only the two renewable heat sources of solar and geothermal have been discussed here.

#### **2.1 Solar district heating**

Danish district heating is the most innovative district heating sector in the world. More than 1.3 million m<sup>2</sup> solar district heating (SDH) plants are in operation in Denmark<sup>2</sup> . Moreover, more than 70% of the large solar district heating plants worldwide are constructed in Denmark [26]. Since 2009, the European Union has supported three multinational SDH projects regarding solar district heating plants' market development. One of them, called "SDHp2m", addressed market uptake challenges for broader use of SDH [27]. Most of the DHp2m data are all freely available and can provide a basis for SDH feasibility evaluations.

Solar irradiance is the amount of solar radiation obtained per unit area by a given surface (W/m2 ) in a location. This irradiance varies month by month, depending on the seasons. It also varies throughout the day, depending on the sun's position in the sky and the weather. The solar efficiency is the ratio between solar heat production and the total solar irradiation on the collector plane. This ratio is a performance measure on how well the system utilizes the available solar radiation. Solar efficiency mostly depends on operating conditions, such as temperature levels and solar

<sup>1</sup> http://cities.ait.ac.at/uilab/udb/home/memphis/

<sup>2</sup> http://solarheatdata.eu/

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

radiation intensity. Hence, low solar efficiency is not necessarily caused by a poorly working system or inefficient collectors [28]. A schematic of the SDH was shown in **Figure 4**. The monthly average solar efficiency and the total heat generated from an SDH in Vojens, Denmark were presented in **Figures 5** and **6**, respectively. This SDH commissioned in 2012 with an effective aperture area of 17,500 m2 , and a 3000 m3 storage tank. The plant went to an expansion in 2014 and 2015, which end up with 5439 solar collectors (area of 70,000 m2 ), and a thermal pit storage capacity of 200,000 m3 for seasonal storing of excess solar heat [29].

The investment cost and the operating costs are the critical factors of the planning. The operating cost depends on the location and system components. As a rule of thumb, an annual rate of 0.54 €/MWh is considered in SDHp2m or 0.0405 €/m2 (collector area) in the Danish Technology Data catalog3 . It is expected that the system capital cost per MWh decreases by increasing the DH size. The capital cost is a combination of equipment costs (i.e., solar collectors, piping system, circulation pumps) and installation. **Figure 7** provides an estimation for solar collectors as per the SDHp2m study and the Danish Technology Data catalog.

Seasonal heat storage is effectively increasing solar heating in an SDH system. The ratio of heat provide by solar collectors in a typical SDH system is around 20%, if there is no seasonal heat storage [30]. The seasonal heat storage can increase the solar heating share to 30–50% [26]. Four different options of long-term or seasonal heat storage are available:


Pit Thermal Energy Storages (PTES) are a relatively cheap storage technology, which has been developed mostly in Denmark (e.g., Marstal 75,000 m3 , Dronninglund 60,00 m3 ) in combination with solar thermal plants. The limitations and advantages of PTES briefly are shown in **Figure 8**. The physical footprint of PTES is significant; therefore, the feasibility of PTES depends on the local conditions. Borehole Thermal Energy Storage (BTES) is a relatively new technology. In a BTES, the heat directly stores underground through vertical boreholes and U-pipes. The thermal flow direction is from the center to the sides to obtain high temperatures in the center and lower at the storage boundaries during the charging period. The flow direction during the discharge is reversed. The upper surface of BTES is usually insulated to minimize the heat loss. The ground can store between 15 to 30 kWh/m3, which is much lower than the PTES capacity of 30 to 80 kWh/m3 [32]. The Okotoks solar district heating system that is located in Alberta, Canada, is an example of BTES. This DH system supplies more than 90% of space heating to 52 detached energy-efficient homes since 2007 [33]. An aquifer is an underground water reservoir. An Aquifer Thermal Energy Storage (ATES) utilizes a mixture of natural water and ground to store the heat. In an ATES, two wells, one warm and one cold, are drilled into the aquifer to extract and inject the groundwater. Another type of thermal storage which is very similar to PTES is called Tank Thermal Energy Storage (TTES). TTES is cylindrical steel or concrete tank placed on the ground and used daily or on a short-time storage basis. A number of guidelines and fact sheets are available through the SHC Task-45 framework. This

<sup>3</sup> https://ens.dk/en/our-services/projections-and-models/technology-data

**Figure 5.** *Input/output plot of monthly measured values of Vojens district heating.*

#### **Figure 6.**

*Solar heat production of Vojens district heating.*

framework was completed in 2014 to assist a sustainable market for large solar heating and cooling system [34].

The investment cost for design, construction, and commissioning of several European thermal storages are available (**Figure 9**). Since the design and construction of thermal energy storage systems are site-specific, **Figure 9** provides an approximate investment per storage capacity. In addition to SHC Task-45, some

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

**Figure 7.** *Investment cost of solar collectors.*

**Figure 8.**

*Seasonal thermal energy storage concepts [31].*

aspects of cost-effective largescale seasonal thermal energy storage for LTDH systems have been studied by Ochs et al. as part of the gigaTES4 initiative [36]. However, the planning and development of seasonal thermal storage require a comprehensive study to identify the project cost.
