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

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.

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.

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