**2.2 Geothermal**

Geothermal energy is a reliable and secure renewable energy source. A DH supply temperature below 60°C makes geothermal plants more advantageous to

<sup>4</sup> https://www.gigates.at/

#### **Figure 9.**

*Specific investment cost of seasonal thermal energy storage [35].* **\*\*** *including all necessary costs except the design and connecting pipes costs.*

satisfy the baseload in comparison with solar systems. Geothermal energy can be found independent of locations to fulfill space heating demands directly. One example of LTDH based on geothermal energy is Østre Hageby in Stavanger, Norway, where a low-temperature network with boreholes provides heating to 66 dwellings [37]. The project was completed in 2016 and reduced 61% of energy consumption for space heating and DHW (**Figure 10**).5

As a rule of thumb, shallow wells with temperatures between 40–150°C are suitable for hot water DH systems. In contrast, higher temperatures (deep wells) are ideal for electricity generation. The geothermal systems are capital intensive. Drilling can account for up to 50% of the total costs of a geothermal project. It has been shown that lowering the DH supply temperature reduces both the capital and operating costs of geothermal DH systems [38]. The geothermal DH system is a better option than individual geothermal heat pumps. In a comparative study in South Korea, the primary energy use of GSHP was reported higher than district heating systems [11]. Thorsteinsson and Tester have discussed the barriers of large GSHP and provided ten recommendations to overcome the challenges of geothermal district heating system development in the United States [39]. Green Energy Association has compared the DH system and individual heat pump based on the Danish data [40]. The report concludes that a new district heating system's annual operating costs are much smaller than the individual heat pump. Some case studies have been gathered in Pellegrini and Bianchini's literature review [41]. In the light of growing interest towards GSHPs, two concepts of shared GSHPs and centralized heat pumps are discussed here.

The basic principle of a GSHP is presented in **Figure 11**. Heat can be extracted from the ground at a relatively low temperature. The heated fluid is compressed to a higher pressure by a compressor. From there, a second heat exchanger or condenser transfers the heat to the home, via either warm air or circulating water.

One of the essential characteristics of GSHP is that the efficiency of the unit and the energy required to operate are directly related to the temperatures between

<sup>5</sup> https://www.arkitektur.no/ostre-hageby

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

**Figure 10.** *Simple sketch of the Østre Hageby district heating system.*

**Figure 11.** *Process diagram of GSHP.*

which it operates. The temperature difference where the heat is absorbed (the "source") and delivered (the "sink") is called the "lift". Larger lift means greater input power to the heat pump. The heating performance of a heat pump is defined by Coefficient of Performance (COP). The COP is the heating produced divided by the energy equivalent of the electrical input resulting in a dimensionless value. The larger the COP value, the less electricity required to operate. The heat transfer between the GSHP and its surrounding soil is affected by a number of factors such as working fluid thermophysical properties and its conditions, soil thermal properties, soil moisture content, and groundwater velocity and properties.

The GSHP has excellent potential to be one of the primary energy sources in the near future. The ground energy can be tapped in a number of different ways and can be used to produce hot water as well as electricity. It has a broad spatial distribution in all countries concerning the low enthalpy resources available. Geothermal

**Figure 12.** *Illustration of decentralized LTDH based on GSHP in a rural area of Denmark.*

**Figure 13.** *District heating system of City of Kassel, Germany [46].*

energy is a renewable resource that does not rely on specific factors such as the wind or the sun.

A new LTDH concept based on the use of individually adjustable and collectively managed GSHPs, connected to a low temperature non-insulated thermal distribution network has been implemented recently. This concept in the UK is called "Shared Ground Loop6 ," and the related regulations are well established [42]. In Denmark, it is called "Termonet" and is recommended mostly for rural areas. Three

<sup>6</sup> https://www.icax.co.uk/Shared\_Ground\_Loop.html

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

Termonet systems have been commissioned in Denmark during 2017 and 2018, with borehole heat exchangers [43] (**Figure 12**).7

The connected group of GSHP is based on the following ideas:


The other concept is a central shallow geothermal plant. Shallow geothermal plants have less than 400-meter deep boreholes. Since the extracted temperature can have a wide temperature range, it may require to be raised with a heat pump. **Figure 13** depicted the LTDH system of a new community with 131 low energy residential houses on a land area of 115,000 m2 , located in Kassel, Germany. The system includes a centralized GSHP with an LTDH network. Since the buildings require a higher temperature than what was provided with LTDH, especially for DHW, they were equipped with heat pumps. Different aspects of using heat pumps to balance the temperature in a district heating system have been discussed in the IEA Annex 47 project [47]. In another project (i.e., RELaTED), it has been shown that ground source heat pumps fit well with the LTDH concept [48]. However, further research is yet to come in order to fully understand all aspects of shallow geothermal energy [49].
