System 4 - Deep green heat
Deep well thermal energy extraction is the only system completely independent of climatic conditions and potentially available regardless of the season. In order to achieve the highest possible temperature, ideally around 100°C, the wells must be approximately 3 to 3.5 km deep. This temperature level will allow direct connection to the district heating system without the need for additional temperature increases using a high-temperature heat pump. Artificial fractured underground heat exchanger technology is used to extract the heat. The water injected through one well is heated to the temperature of the surrounding rock as it passes through the fractures and the hot water flows through a second well to the surface where it is used for energy purposes. Part of the energy consumption will be covered by a photovoltaic source and in winter by electricity generated by a fuel cell using hydrogen produced by System 2. System 4 represents the greatest scientific challenge of the whole project and renowned scientists from Europe and overseas are actively involved in its design and related research.
- Deep well WELL
- Heat pumpP
- Fissure exchanger FE
- Fuel cellFC
- District heating system DHS
Deep wells WELL are needed to reach high temperature in the fractured heat exchanger, typically around 100°C. Such a temperature is in Litoměřice at a depth of about 3–4 km. Drilling deep wells is a technologically demanding but manageable process. Its smooth progress is significantly helped by geological and geophysical research, which makes it possible to predict the type of rocks at depth and therefore to properly adapt the design of the well, the drilling method used and the type of drilling tools. In addition to the use of boreholes to collect heat, deep boreholes also have unique scientific significance. They provide an accurate picture of the type and properties of rocks at great depth, i.e. unique information that cannot be obtained in any other way.
Pump P ensures the flow of water in the deep well.
Fissure exchanger FE works similarly to a flow heater, but it is located in a rock environment with a high temperature, i.e. at a depth of the first kilometers below the surface. The natural fissure network of the rock massif is used as a heat exchange surface. Because water has a large heat capacity, it is necessary for the liquid to flow slowly enough to be heated by the rocks. The slow flow in the exchanger therefore requires that a relatively large area of cracks developed in the volume of several cubic hectares of the rock massif be used. Cold water is pumped into the subsoil through an injection well. Heated water is pumped from the massif through a production well, which is several tens to the first hundreds of meters away from the injection well and is parallel to it. For use in the heating industry, up to tens of liters of hot water per second can flow through the production well. With small flows of the natural fracture network, its effectiveness can be increased by the so-called stimulation, i.e. by opening fissures, creating new fissures and preventing them from closing. Stimulation is a controlled and safe process by which pressurized water and sand are injected into the subsoil, thereby opening the cracks and the sand then prevents them from closing again.
Fuel cell FC is an electrochemical device that converts the chemical energy of hydrogen into electrical and thermal energy. The basic element of a fuel cell is two electrodes separated by a membrane or electrolyte. Hydrogen is supplied to the negative electrode and oxygen to the positive electrode.
District heating system DHS supplies heat to buildings in Litoměřice. It is operated by ENERGIE Holding a. s. Currently it burns wood chips and coal. From 2027 it should use geothermal energy thanks to the SYNERGYS project.
