Direct (deep/ high grade) geothermal energy
Theoretical explanation.
The thermal conductivity of formation rocks can be derived in large part from the prevailing thermal gradient. Geologic formations with low thermal conductivity will typically exhibit a high thermal gradient of 3.5 degrees or more per 100 meters of depth. In contrast, geologic formations with high thermal conductivity will exhibit a thermal gradient of 2.5 degrees or less per 100 meters of depth.
The conductivity of energy in rock formations averages between 1.3 - 8 Watts/mK
By inducing convection in formations with a permeability of 1x 10-e-17 m/s or better, the conductivity can be multiplied at the outside diameter of the wellbore.
Schematic overview of the single-hole well
Bovenstaand een schematische weergave van de werking van het NotusPid systeem. Zichtbaar gemaakt is de warmte uitwisseling door convectieve stroming in de formatie binnen de zogenaamde convectie peer. Tevens is te zien hoe de warmte stroming zich verdicht aan de boorwand.
Schematic representation of ring room essential in the operation of the NotusPid system
Ring Room
As a result of the water saturation of the formation rock and by creating a temperature difference in the well bore, a circulation is created inside the formation. The size of the circulation is called the ring chamber. Convection, in turn, causes the energy density within this ring chamber to increase. Tidal currents present throughout the formation (4 times per day) enhance this process, aided by the density and weight differences of the formation fluid. The energy absorbed by the flowing back fluid at the wellbore wall will be "harvested" at the heat exchanger at the end of the circulation cycle and the process will begin again.
The flowing back fluid will absorb the energy through the thermal conductivity of the formation rock, assisted by the ever-present pore water in this rock. Eventually, the reflowing fluid stream will be heated to the point where it is equal to the surrounding temperature of the formation. Extensive research by the universities of Heidelberg, Bochum and Zurich has confirmed that the processes taking place in the ring chamber remain stable over a long period of time (>25 years)
System Components
The main components of the NotusPid® installation are:
The suction line (from the lower well bore) is made of a special alloy that is resistant to pressure, temperature, chemicals and corrosion. The alloy is manufactured and extensively tested by our exclusive partner
Return lines are manufactured in the same way and (number of lines, alloy, diameter, positioning are all included and protected by the patent) The suction lines and return lines are joined together in a fixed module and will be installed along the entire length of the borehole. The void space will be filled with gravel with a special grit to disperse the return flow and increase the working area by increasing the exchange surface.
At the bottom of the well, the suction line will contain a filter to prevent the suction of small particles that could damage the lining and equipment. (Heat exchanger, pumps). Length, size and grain of the filter will be determined upon completion of the drilling process.
At ground level, a basement will be constructed with a diameter of approximately 2 meters and a depth of up to 200 meters (depending on the total depth of the well). This basement will be used for the installation of the circulation pumps. The construction of the basement will be gas/liquid and pressure resistant using steel and concrete. The return and suction piping will be fully insulated. The basement will be sealed with a removable cover.
The installed pumps will have a capacity of between 20 - 130 m³/h. Temperature resistant up to 150 °C and made of corrosion resistant material.
The electric motor for driving the pumps will be installed outside the wet room. On request the well can be designed with the installation of 2 circulation pumps. The reason why two pumps will be installed is the continuity of the production. During production, one pump will always be in the standby mode and will automatically activate when the pressure at the active pump drops. The installation will be such that each pump can be replaced (maintenance, etc.) without interrupting production from the well.
The suction and return lines will be routed to the surface through the sealing cover.
The above diagram gives a comprehensive overview of the possibilities of direct (i.e. without heat pumps etc.) geothermal energy. Electricity production is not included in this and refer to the block geothermal and steam production for that.
Direct Geothermal Energy
A direct-use geothermal system consists of several main components, including:
production facility
o Downhole and circulation pumps and wells
mechanical system
o Transmission pipelines and distribution networks
o Heat exchangers
o Heat convectors
o Cooling systems (refrigeration)
Peak or back-up systems
Drainage system
o Storage tank
o Injection well.
Production Facility
A well is drilled into a favorable geologic layer that, determined by the geothermal gradient at the site, has a high ambient temperature at that depth. Formation fluid that has the ambient temperature can then be brought to the surface. A production plant is used to bring the water to the surface, and a mechanical system delivers the heat directly for its intended use. The fluid, after decreasing in temperature, is cooled by heat exchangers back to the borehole.
A downhole pump is a device used in a well to bring the fluid to the surface. Unless the well has an artesian (natural) source, downhole pumps are required, especially in a large-scale direct-use system.
Mechanical System
The fluid state in transmission pipelines of direct-use projects may be formation liquid or steam vapor, or a mixture of two phases (i.e., steam and liquid). These pipelines transport fluids from the source to the application site, or to a steam-water separator. Thermal expansion of metal pipelines that are rapidly heated from ambient temperature to the temperature of the geothermal fluid (which can range from 100°C to 300°C) causes stresses that must be accommodated by careful engineering design. The cost of transmission pipelines and distribution networks in direct use projects is significant. This is especially true when the geothermal resource is located a long distance from the main load center.
Heat Exchangers
Heat exchangers are typically used to transfer heat from geothermal water to a secondary fluid. The main heat exchangers used in geothermal systems are the plate, shell-and-tube, and downhole types. The plate heat exchanger consists of a series of plates with gaskets held in a frame by clamping rods. The countercurrent and high turbulence achieved in plate heat exchangers provide efficient thermal exchange in a small volume. Compared to shell-and-tube exchangers, they also have the advantage of taking up less space, being easily expanded when additional load is added, and typically being 40% less expensive. The plates are usually made of stainless steel, but titanium can be used when the fluids are particularly corrosive. Plate heat exchangers are commonly used in geothermal heating systems in the United States.Shell-and-tube heat exchangers can be used for geothermal applications, but are less popular because of problems with fouling, the higher inlet temperature (the difference between the incoming and outgoing fluid temperature), and the larger size compared to the plate type.
Heat convectors
Heat convectors are used to transport heat from the heat exchanger to the application area. These can be residential buildings but also industrial applications.
It is also possible to supply cooling using the geothermal heat
Cooling systems
Cooling using geothermal energy can also be achieved using lithium bromide and ammonia absorption cooling systems.
Geothermal cooling installation. The heat pump in this setup is driven by a solar panel to form a completely "green" solution.
The lithium bromide system is the most common because it uses water as the refrigerant. However, it is limited to cooling above the freezing point of water. The main application of lithium bromide systems is to supply chilled water for space and process cooling in single-stage or two-stage units. The two-stage units require higher temperatures (about 160°C), but also have high efficiency. The single-stage units can be powered by hot water with a temperature as low as 82°C. Lower geothermal water temperatures result in lower efficiency and require higher flow rates. Generally, a condensing tower or cooling tower is required, increasing costs and space requirements.
For geothermally driven cooling below the freezing point of water, the ammonia absorption system should be considered. However, these systems are usually used in very large capacities and are still in limited use. For lower temperature cooling, the thrust temperature must be at or above 120°C for reasonable performance.
Peak System
A peak system may be required to meet the maximum load. This can be done by increasing the water temperature or by providing tank storage. Both options mean fewer wells need to be drilled. When the temperature of the geothermal water is warm (less than 45°C), heat pumps are often used. The equipment used in direct-use projects consists of several operating units.
Return of geothermal fluid
The removal of the fluid is done by return to the borehole.