By Norman Quesnel
Senior Member of Marketing Staff
Advanced Thermal Solutions, Inc. (ATS)
If you cross-sectioned the Earth, it would show a multi-layered structure with a solid iron core spinning in a sea of liquid iron and sulfur. You would also find great quantities of flowing heat starting at the very center and moving outward.
The flow of heat from Earth’s interior to the surface is estimated at 47 terawatts, i.e. 47 trillion watts, and comes from two main sources in roughly equal amounts: the radiogenic heat produced by the radioactive decay of isotopes in the mantle and crust and the primordial heat left over from the formation of the Earth 4-1/2 billion years ago.
In a recent study, scientists estimated the temperature of the center of the Earth at 6,000°C (10,800°F) – about as hot as the surface of the sun. 
In fact, more than 99% of the inner Earth is hotter than 1,000°C (1,800°F). Unlike the sun, the Earth is much cooler at its outer crust and on its outside surface. But even at cooler temperatures, the shallow depths of the Earth’s crust provide a geothermal resource for both heating and cooling structures built above the ground.
By drilling deeper down, but still within the crust, much higher temperatures can be harnessed to help generate power that can in turn provide industrial and even community-wide levels of heating and cooling. High-temperature geothermal heat has tremendous potential because it represents an inexhaustible, and virtually emissions-free, energy source. 
Near Surface Heating and Cooling Systems
The ground absorbs nearly half of the solar energy the planet receives. As a result, the Earth remains at a constant, moderate temperature just below its surface year-round. However, air temperature varies greatly from summer to winter, making air source (traditional) heating and cooling least efficient when you need it the most. 
Geothermal heat pumps take advantage of the moderate temperatures typically found at shallow depths to boost efficiency and reduce the operational costs of architectural heating and cooling systems. Unlike conventional heating and air conditioning systems, which use the outside air to absorb and release heat, geothermal systems use heat pumps to transfer heat from below the surface.
The pumps connect to closed loops of plastic pipes buried either horizontally or vertically in the ground below the frost line (about 100-200 meters), where the temperature is consistently between 40-80°F depending on location. Called ground loops, the underground pipes are filled with water and sealed tight except where they are connected to the geothermal heating and cooling system inside the building.
In winter, water running through the loops will absorb heat from the ground and pipe it into the home, while the system will run in the opposite direction to keep things cool during the scorching summer months. The pipes are connected to a heat pump and water heater inside the house and users can control the indoor climate through a smart thermostat.
There are four basic configurations for geothermal heat pump ground loops. Three of them are closed-loop systems that use a self-contained water and an anti-freeze solution. The open-loop system uses ground water or water from a well. 
Newer geothermal systems can be installed at shallower depths, less than 50 feet. Even at these levels, the ground can provide a heat source in colder weather and serve as a cooling heat sink when surface temperatures are hot. 
There are several providers of geothermal heating and cooling systems. Google’s parent company, Alphabet, is among them with its newly created startup, Dandelion. Originally conceived at X, Alphabet’s innovation lab, Dandelion is now an independent company offering geothermal heating and cooling systems to homeowners, starting in the northeastern U.S. 
To put in the ground loops, Dandelion uses its “clean drilling technology” to dig a few small holes in the yard, each only a few inches wide. Then a technician will install the other components inside the house, and the system should be up and running in two or three days. After that, the only regular maintenance is an air filter change every six to 12 months.
Deeper Down Geothermal Systems
The Kola borehole, on Russia’s Kola Peninsula is the deepest mankind has ever drilled into the Earth’s crust. After nearly 15 years of boring, the hole was 12,262 meters (40,230 feet) – over 12 kilometers or 7.6 miles deep. At that depth, the temperature was 185°C (356°F). 
Even at half that depth, there can be much more heat than a near-surface geothermal system can access. The Norwegian company Rock Energy wants to be an international leader in high power geothermal heat and energy. A pilot plant has been planned for Oslo that will collect heat from 5,500 meters deep. The high temperatures from this depth can heat water to 90-95°C (194-203°F) and can be used in district heating plants. 
Rock Energy is planning to drill two wells, an injection well where cold water is pumped down, and a production well where hot water flows back up. Between these will be so-called radiator leads that connect the wells. The water is then exchanged with water in a district heating plant managed by the Norwegian power company Hafslund. 
A hot dry rock system potentially allows geothermal energy to be captured from hot rocks, 3-5 km (1.8-3.1 miles) underground. In operation, cold water is pumped at high pressure down into the very high-temperature, fractured hot rock. The water becomes superheated as it passes through the rock on its way to the extraction boreholes.
In Figure 6, the diagram of a hot dry rock system shows hot water emerging from the borehole and directed through a heat exchanger. After giving up its heat, the cooled water is recycled back down the injection borehole in the hot rock bed. The working fluid, a low boiling point liquid, circulating through the secondary circuit of the heat exchanger is vaporized by the heat extracted from the well water and used to drive the power plant’s turbines. 
At both shallow depths and miles down, the Earth offers thermal energy that can harnessed for heating, cooling and power generation. Compared to most other processes, geothermal energy is cleaner, continuous and, as technology advances, a low cost alternative to fossil fuels or to solar and wind-powered systems.