“How low can you go” is a pivotal question for geothermal heating. Plant and installation costs increase the further underground you dig. Yet, deeper digging increases the heat yield. But how deep for geothermal heating to optimally balance cost and opportunity?
The minimal depth requirement for geothermal heating is determined by the nature of the heat resource accessed and the installation’s design. Geothermal HVAC systems require a depth between 5F and 400F. Geothermal energy plants drill between 0.3mi and 4.3mi. EGS enhances the accessible depths.
This post takes a peek at:
- The key types of geothermal heating.
- How heating type determines required depth.
- Differences between geothermal power and geothermal HVAC.
- Drill depth ranges associated with different types of geothermal heating.
Understanding how you’re affected depends on the specifics of your site. Below we’ll look at how site dynamics affect the choice of installation and which depths are associated with the choice.
How Is Geothermal Depth Determined?
By accessing data describing geothermal plants and installation, we can characterize the depth requirements for geothermal. This analysis shows the average depth required for each type of installation.
Analysis along these lines shows how depth requirements are a threefold function of:
- The type of heat extraction technology.
- The situation of the underground heat resource targeted.
- The energy requirement of the consuming geothermal plant or HVAC installation.
In addition, we have regard to the qualitative factors that give rise to variance.
What Are The Types Of Geothermal Heat Sources?
Geothermal heat derives from magma, hot underground water – present in spring and aquifers, dry deep rock, mineral radiation, and absorbing sunshine.
The inner core of planet earth is a searing place. Situated 1,800 miles below the crust (the surface of the earth), it is a center of heat-generating radioactivity. It retains residual frictional and gravitational heat from the origin of the planet.
The heat from the center rises out through magma in the outer core and then fissures in the enclosing mantle. Rocks adjacent to these fissures and the gaps between tectonic plates are particularly exposed to the leaked heat of the core. So too is subterranean water, which flows near or at the earth’s surface in the form of natural hot springs.
A further source of heat is radioactive isotopes that decay in the mantle, closer than the core.
What Are The Main Types Of Geothermal Heat Extraction?
Geothermal heat extraction breaks into two types:
- Geothermal Energy: This is the category of large, utility-scale plants that access thermal resources stemming from the inner heat of the earth.
- Geothermal HVAC: Heating Ventilation And Cooling installations are a form of solar. They access the solar radiation that has been absorbed by the earth. The heat from the sun is more shallowly situated than terrestrial heat.
Each type, in turn, has a set of different architectures, which differ inter alia by drilling depth.
Geothermal Dry Steam Plants
Dry steam plants capture hydrothermal fluids that exist primarily in the form of steam. The steam gets piped to the surface and directed to a turbine, which in turn drives a generator that produces electricity.
Using primary steam eliminates the need to burn fossil fuels for the powering of the turbine. It also obviates the need for transportation and storage of fuels. The only emissions from these plants are excess steam and minor amounts of gases.
The first type of these plants was built in Italy in 1904. The Geysers – a massive suite of plants in Northern California – still uses these plants and constitutes the largest single source of geothermal power today.
Geothermal Flash Steam Plants
These are the most common geothermal energy plants, utilizing a mixture of steam and water from the underlying wells. At a flash plant, hot liquid water from deep underground is kept under pressure, preventing it from boiling. As it surfaces, the pressure is decreased, causing it to boil and “flash” to steam.
The incoming stream is separated from the liquid and used to turn a turbine, which drives a generator as before. Flash power plants require resources at depths in order to tap into temperatures in the 300°F to 500°F range.
Geothermal Binary Cycle Plants
This design departs from Dry Steam and Flash Steam plants in that the propellent steam (or water) from the geothermal reservoir does not make direct contact with the turbine. Instead, two streams of water (hence the name “binary”) are used.
The first is a geothermal fluid heated below 400°F. The second fluid has a boiling point much lower than the primary liquid. They come into proximity in a head exchange module where thermal transfer from the geothermal liquid vaporizes the secondary fluid. This vapor powers the turbine.
These plants tap into the most common naturally-available geothermal resources, all of which are liquid below 300°F.
Geothermal HVAC Horizontal Loop
A horizontal loop is the most common HVAC configuration. It comes in two configurations. In the first, a pipe is buried six feet underground, with a second situated two feet on top of it. The second configuration has two pipes lain side-by-side and buried five feet underground.
An advantage of horizontal looping is that the pipes can be run around obstacles. They fit easily under roadways and obstacles, minimally disrupting the land. This is accomplished by horizontal boring around the obstacles and then fitting the pipes where the bore cables passed.
For retrofitting, horizontal looping is favored because of the ability to work around obstacles and preserve terrain. It does, however, require horizontal space and would not be suitable in buildings where access to outside land is restricted.
Geothermal HVAC Vertical Loop
A vertical loop uses vertical boring to drill two holes, four inches in diameter. The holes are set twenty feet apart and run between one hundred and four hundred feet deep. Each hole houses two pipes that run the full vertical depth of the borehole and are joined at the bottom by a u-bend.
After the pipe has been laid, the hole is filled with grout, which serves two functions. First, it ensures that the pipes have contact with the surrounding earth to promote heat transfer. Secondly, it insulates the bore from a liquid that might seep in from surrounding aquifers.
The vertically looped pipe is connected to a mantle, which consists of a shorter, horizontally arranged pipe lain in trenches. This manifold attaches the ground pipes to the heat pump housed inside the building.
Vertical loops are preferred by commercial buildings, schools, and homes where the land area is too restricted to permit horizontal layout. Their small horizontal footprint minimally disrupts the existing landscape, and vertical drilling recommends them to areas where the soil is not conducive to the laying of horizontal trenches.
Slinky loops are a novel variation on horizontal. Here the pipes are coiled, stacked, and spread – much like the eponymous slinky toy. This configuration allows more earth contact over a smaller horizontal surface area, unlike the standard horizontal arrangement, which picks up the heat only along the periphery.
The Slinky configuration reduces installation time and costs and makes possible installations in offices and residences that would otherwise not have sufficient available stretchable land.
As a variation of the horizontal looping layout, the trench depth is the same. What differs between them is only the length of the embedded pipe – Slinky uses more pipe.
What Are The Depth Ranges For The Types Of Geothermal Extraction?
There’s an array of techniques for accessing the heat resources of the earth – all of which are variously situated in relation to the ground surface. In summary, geothermal depth breaks down as follows:
|Installation||Minimum Depth||Maximum Depth|
|HVAC Horizontal Loops||5f||6f|
|HVAC Slinky Loops||5f||6f|
|HVAC Vertical Loops||100f||400f|
|Binary Cycle Plants||0.5mi||2mi|
|Dry Steam Plants||0.9mi||2.4mi|
|Flash Steam Plants||0.32mi||2.2mi|
|Enhanced Geothermal Systems||2mi||4.3mi|