1. Very Shallow Geothermal Potential (vSGP)

The 5th Generation District Heating and Cooling (5GDHC) networks aspire to connect to renewable energy sources^[1], including geothermal sources^[2]. Shallow geothermal sources are suitable for application regarding 5GDHC networks. They are generally defined by depth and encompass those energy achieved from the sources up to 400 m^[3]. Sources from shallower depths are more practical to exploit as shown in figure 1.

Figure 1:Very shallow, shallow and deep geothermal potential classification^[4]

Very Shallow Geothermal Potentials (vSGPs) are those acquired by depth less than 10 m^[5]. This depth range relevant for very shallow geothermal systems can be divided further into three layers: 0–3 m, 3–6 m, and 6–10 m^[6], which differ by physical soil properties. The thermal properties of soil are the crucial parameters that affect heat extraction of the very shallow geothermal systems. These properties such as thermal conductivity, thermal diffusivity and heat capacity are strongly affected by the physical soil parameters - bulk density, water content, and soil texture defined by grain size distribution^[7][8]. Climate factors, such as solar irradiation and precipitation, also have a certain influence on vSGP. Heat input from the earth’s interior (terrestrial heat flow) is negligible in the shallower soil layers. However, the influence of terrestrial heat flow becomes more important in the regeneration behavior of the soil with increasing depth, while the influence of solar irradiation is the opposite.

The vSGP systems are principally horizontally installed geothermal systems that utilize the heat potential on the earth’s surface rather than at depth. These are horizontal ground heat exchangers, such as geothermal collectors in the classic single pipe installation or tube mats, but also special designs such as trench collectors, heat baskets, and comparable modular collector systems up to a maximum depth of 10 m. Horizontal ground heat exchangers have advantages in lower installation costs and lower drilling technology requirement, therefore are widely used in engineering. When sufficient ground area is available, horizontal ground heat exchangers provide a very effective solution for heating and cooling.

2. Definition Large-Scale Geothermal Collector Systems (LSC)

A very shallow large-scale geothermal collector system (LSC) is distinguished from smaller systems in particular by its size and performance^[4]. Typical large-scale geothermal collector systems supply entire settlements with heating and cooling energy via 5th Generation District Heating and Cooling (5GDHC)^[2] by using the very Shallow Geothermal Potential (vSGP). There are also large-scale geothermal collector systems that are built below the buildings^[9]. The following parameters describe the large-scale geothermal collector classification. At least one of the criteria shown in Figure 2 needs to be fulfilled:

• ≥1.000 m2 area
• ≥25 kW maximum source power
• ≥50 MWh/y source heat extraction

Figure 2: Representation and definition of large-scale collector systems (LSC)^[4]

Large-scale geothermal collector systems differ from small systems, which are usually built for individual buildings, in their size and complexity. They can also be constructed in multiple horizontal layers. Due to the large number of pipelines and distribution shafts, the pressure loss must always be taken into account in detail and optimized for the system, so that all pipelines are evenly flown through with heat transfer medium and thus, an even heat extraction can be guaranteed. Within the large-scale geothermal collector system, the mutual influence of the individual collectors, which affects the heat extraction behavior, also comes into play. This requires more detailed planning, usually with the help of simulation programs.

Previous LSC’s usually consist of modular trench collectors, distribution shafts and an energy center. The advantage of modular trench collectors is that the pressure loss is known and stays the same for all modules within the system. In multilayer geothermal collector systems, the collector layers are usually connected separately. This means that every layer can be controlled separately during operation in order to guarantee targeted use of the advantages of the multi-layer system (seasonal heat and cold storage).

3. Advantages and Disadvantages of Large-Scale Geothermal Collector Systems