Student Reading: Energy from and to Earth

According to the REN21 Renewable Energy Policy Network, Global Status Report (2016), nearly 20% of global final energy consumption was from renewable energy in 2014. Of that ~20%, about 9% was from traditional biomass burning. Another 4% was from hydropower; hydropower technology has been around a long time, so other newer, renewable energy resources still comprise a small piece of the total energy puzzle. Indeed, about 2% of global energy consumption was from wind, solar, geothermal, biofuels, and non-traditional biomass burning. Where geothermal energy is available, however, its use can satisfy a significant fraction of local energy demand.

Learning Goals: Students will be able to:

1. Determine what components of geothermal energy are renewable.
2. Evaluate the distribution of heat in different types of rock at different depths, locations (e.g., tectonic zones vs granites, limestone).
3. Demonstrate the high heat capacity of water.
4. Explain the operation of heat exchangers.

Learning assessment: Students interpret tectonic plate maps; students construct and analyze the working of a ground exchange system

Energy from Earth

The sun transfers heat radiated to Earth, and this radiated energy keeps much of the surface of our planet warm. Geothermal, however, refers to the heat of Earth originating deep below the surface—this is energy from Earth. Deep down in Earth's interior, it is hot. Volcanoes typically occur along tectonic plate boundaries or in places where Earth's crust is thin, where very high-temperature materials (magmas of molten rock) can rise to Earth's surface. These thermally-active regions are potential sites for large-scale geothermal energy centers, where heat is convected to Earth's surface—this is the transmission of thermal energy as magma expands and transmits heat toward cooler regions. Convective flows of thermal energy are important in these molten rock regions. One can imagine using that concentrated heat to create steam (or harness the steam emitted from Earth) to power a turbine to create electricity. Old Faithful, for example, erupts in a geyser of steam and hot water every 90 minutes.

One cannot easily detect the influence of the heated interior of Earth everywhere on Earth's surface. That hot core is dissipating heat continuously, and in that sense the source of heat at the surface of the planet (i.e., from the interior) is renewable because heat is continuously being generated in Earth's mass via 1) radioactive decay of minerals (e.g., uranium and potassium), and 2) frictional heat of rocks grinding together—at the molecular scale, the kinetic energy of atoms moving in contact with one another at the rock surfaces is converted to thermal energy, generating heat.

Alas, although heat is generated in Earth's interior, the surface area of Earth is so large that the heat conducted through Earth (conduction is heat transferred through rocks via molecular collisions) and dissipated at the surface averages some 50 mW/m² (milliwatts per square meter). That average value varies considerably owing to the different thermal properties of different rock types. Soapstone and marble (metamorphosed limestone), for example, both have high energy density (holds heat well) and high thermal conductivity. Granites do not heat well, but typically exhibit good thermal conductivity. In contrast, basalts hold heat well but have poor thermal transmission properties. With so much variation in available heat, heat storage, and heat conduction by rocks, that 50 mW/m² is really just an average around a fairly large range. Milliwatts, however, is a tiny number of units (think of the heat generated by a 40 watt bulb, which is 800 times more heat than the surface geothermal heat flux per square meter). Thus, 50 mW/m² is somewhat useless because the heat density is small and so diffuse. But the surface soils of Earth do get more heat from the absorption of solar radiation, and we will examine how to exploit that diffuse heat in a little bit.

In addition to letting Earth deliver the concentrated heat to the surface at thermally active areas, one can "mine" the heat located deeper down in Earth where the crust is fairly thin. For example, some wells are drilled deep into permeable rock formations (where there are sufficient spaces in the rock for water flow), and water is pumped 4–12 km down where it becomes heated, then that water (steam) is brought back up to the surface. This can be repeated over and over, although eventually the heat exchange gradient will diminish as the cooler water pumped into Earth is warmed by the rocks. Eventually the rocks will cool, and then other well sites will have to be drilled to extract the heat from Earth. This sort of heat mining is non-renewable, but reserves are very large.

New technologies are being developed called Enhanced Geothermal Systems that will open up these reserves. Many hot rock formations are impermeable rocks with high heat capacity but little water storage. By drilling down to these rocks and injecting water under high pressure, fractures can be created in the rocks, enhancing permeability and water exchange rates. A second well can be drilled to pump out the now-heated water, which can be used to drive a turbine to create electricity. The cooled water can then be re-injected into the original well to close the loop of water use.

The notion of injecting pressurized water deep into the ground may sound familiar, since this is the basic operation used by oil and gas companies to extract fossil fuels from shale deposits. Hydraulic fracturing—or fracking—describes the process of opening up tight rock formations to enhance natural gas and petroleum extraction. The "lifetime" of a fracking well is thought to be one to two decades, so perhaps an opportunity exists to exploit these wells for Enhanced Geothermal Systems once gas extraction has been completed. The wells drilled for fossil fuels might eventually be used for more sustainable energy (heat) extraction.

So how is geothermal heat energy put to use?

Geothermal heating of buildings

In some regions, geothermal springs emit hot water that can be used directly to heat homes and other buildings. Warm Springs, Virginia, for example, has a local resort (The Homestead) that exploits geothermal for space heating; Thomas Jefferson frequented the resort's pool that uses the hot water. Internationally, almost every home in Iceland is heated geothermally, with water from hot springs piped directly into the homes.

Geothermal heat for making electricity

The first successful attempt of harnessing geothermal generated steam to make electricity in the United States dates to 1922 at the Geysers, located near San Francisco, California. While that first effort was not commercially viable, a nearby location in 1960 supported the first large scale facility of 11 mega watt capacity. It is one of sixty-nine such plants operating on eighteen different locations around the United States, including Hawaii.

In some areas, cool surface water percolates down to hot spots, gets heated, and rises to the surface either as hot water or steam. Alternately, power plants can drill down to areas of heated water and provide controlled venting of steam. In either case, the steam is used to turn a turbine to create electricity, and the cooled water can be returned to the ground and be re-heated. Some "binary cycle" power plants use a heat exchanger to transfer the water's heat to a second liquid with a lower boiling point to drive the turbine and generate electricity. These are fairly closed systems, but older geothermal power plant designs let a majority of the water escape as steam, rather than being re-injected. These plants require continuous supplies of water; if discharge from wastewater treatment plants is used as that water, then the escaping steam will contain some pollutants. A majority of the energy requirements in northern California is supplied by these older geothermal power plants. Watch this YouTube video: Electricity from geothermal energy

As should come as little surprise, the distribution of heat below Earth's surface varies by depth and by different types of rocks. Water is a poor conductor but has high heat capacity, so in sedimentary basins formed with water, the hot water can be mined. Granites have low porosity (no water) but high heat, so heat can be extracted by drilling down and injecting cold water in pipes that return the water warm. Carbonate limestones can also be used.

Energy to the Earth

Although geothermal heat can be used in thermally active regions either to generate electricity or provide heating/hot water directly for residential or commercial use, much of the land surface of Earth does not have access to intense geothermal heat. The land surface is too far from an internal heat source (i.e., not near a tectonically active region) or because economically it is cost-prohibitive to drill to the depths needed to find hot rocks. Nevertheless, almost all regions can exploit the diffuse energy that resides in Earth's surface soils in another way. Just a few meters down below the soil surface, the ground is insulated from seasonal changes in temperature, and the diffuse, downward conduction of solar heat creates a constant, albeit mildly warm temperature of around 50 degrees F. This is energy to Earth. Can that heat energy be exploited?

The answer is yes, but the difference in temperature between the surface and a few meters down is not sufficient to generate usable electricity. This low-grade but constant heat a few meters down can be used to exchange with air or water circulated from the ground surface. Heating/cooling systems that exploit the diffuse heat energy of the ground are variously called geothermal heat pumps or ground source heat pumps and have been in use since the 1940s. For a typical system, extensive loops of tubing are buried at least 2 m underground to act as a heat exchanger, to extract heat during the winter or to dissipate heat during the summer. Water or antifreeze is circulated through the pipes and returned to the building, where the heat is exchanged with refrigerant in a heat pump. The heat pump then circulates heat throughout the building and stabilizes temperatures.

The exchange of heat with the ground can be accomplished in horizontal loop systems that are spread out over a larger area than vertical loop systems that penetrate deeply into the ground. The vertical loop systems are used where space for horizontal loops is not available, or where surface soils are too shallow to allow for horizontal loops. Vertical loops may extend hundreds of feet into the ground.

Watch the video: How geothermal heat pumps work, from Energy.gov.

Another way to exchange heat between a building and Earth is to use earth tubes, large diameter pipes buried next to or below the structure. Air is drawn through the tubes, exchanging heat with the ground, being cooled in the summer or warmed in the winter. That air then enters the building. In the summer, the flow can be generated by a chimney added to the building, or simply by solar warming of a ventilated attic space. That is natural convection. Fans can be added to increase the rate of flow. Earth tubes cool and heat structures ranging from small sheds to huge skyscrapers. View the YouTube video: Earth tubes in operation. Earth tubes work best in drier climates. When warm moist air is drawn into the earth tubes and cooled, much of the moisture may condense on the inside of the cool tubes. If no effective drainage system exists, the tubes will become covered with mold, creating a health hazard for occupants of the structure.

Reflection

How are earth tube systems similar but different from geothermal heat pump systems?

Resources

• How Geothermal Energy Works from UCAR
• Geothermal Heat Pumps article from Energy.gov