Geothermal energy systems can roughly be subdivided in three main groups:
Shallow geothermal energy
Shallow geothermal energy is defined as exploiting heat at depths between ~300 and ~1500meter. This implies that the temperature is relatively low (~20-50 °C). In the Netherlands, one shallow system is being developed in 2018. Storage systems, like shallow (HTC: Heat and Cold Storage) and high temperature (HT-ATES: High Temperature Aquifer Thermal Energy Storage) storage, are often included in this category, but they are fundamentally different, because they store and produce heat rather than just produce heat.
Deep geothermal energy
Deep geothermal energy is often just referred to as 'geothermal energy'. Heat is produced from depths between ~1500 and ~4000 meter. The temperatures at this depth ranges between ~50 and 120°C. The produced heat can be directly used for heating greenhouses. City heating through networks is also possible, but the current application in the Netherlands is limited. At this depth range the natural permeability of the rocks allows the production and injection of hot brine without stimulation. About 20 'deep' systems currently exist in the Netherlands.
Ultra-deep geothermal energy (UDG)
A temperature exceeding 120°C is required for most industrial applications (steam), or for generating electricity. UDG refers to depth between ~4 and ~8 kilometers.. At this depth, the natural permeability of the rocks is usually insufficient to produce hot brine at economic rates without stimulation. 'EGS' (enhanced of engineered geothermal systems) produce earth heat from impermeable rocks by stimulation. In the Nedthrlands no ultra deep geothermal systems currently exist.