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Geothermal: The Heat Beneath Our Feet

Fig. 1 – Geothermal Heat Resources in the United States.
Fig. 1 – Geothermal Heat Resources in the BUnited States.

Geothermal energy, a technology that takes advantage of the Earth’s ability to supply and transfer heat, is quickly infiltrating the public consciousness. There are, however, some key questions that those who are not familiar with the technology may have about how it works.

The word “geothermal” actually refers to two completely different technologies—one for heating and cooling and another for generating electricity. As people learn more about its potential, and also the limitations of other renewable resources such as solar and wind, geothermal is increasingly presented as a key component of a clean energy solution. But, despite the promise that geothermal energy could provide a constant source of electricity to fill in the gaps when the sun isn’t shining and the wind is not blowing, there are a few stumbling blocks.

The first problem has to do with the fact that geothermal resources vary in intensity, depth and consequent availability with site location. Most of the United States’ geothermal resources that are

Fig. 2 – Planned and Installed Geothermal Power Plants by MW by State.
Fig. 2 – Planned and Installed Geothermal Power Plants by MW by State.

sufficiently close to the surface to tap are in the western half of the country—close to the San Andreas Fault, a major tectonic plate boundary. The increased intensity of heat between 3 and 10 km below the surface in the West is clearly shown in figure 1 from the National Renewable Energy Laboratory (NREL). Figure 2 from NREL gives the number of planned and installed geothermal power plants in the USA.

The lack of geothermal resources east of the Mississippi may seem discouraging, but this only means that geothermal power plants are less likely to be economical there. Geothermal heat pumps, which are used to directly heat and cool buildings, are still a viable option for everyone no matter where they live. While a geothermal heat pump does not produce electricity, it does decrease the amount of fuel needed to heat and cool a building.

The second major stumbling block that geothermal electricity faces is that energy companies that build power plants often choose to pay more per kilowatt-hour as long as that means that they can pay off their upfront investments over longer periods of time. For a number of years now, geothermal electricity has been cheaper per kilowatt-hour than both coal and natural gas. The catch is that the higher initial cost of building a geothermal plant, coupled with the shorter expected lifetime of such a plant, discourages many companies from investing.

In 2014, geothermal energy supplied 0.4 percent of the American electrical power mix. With coal supplying 39 percent and natural gas weighing in at 27 percent, that number is disappointing. Geothermal resources supply a staggeringly small amount of power in this country and the fact that companies are investing in fossil fuels in an effort to reduce and stretch out repayment of initial investments is troubling. Now that the cost per kilowatt-hour is competitive with a number of commonly used fossil fuels, it seems that the last thing left to change is people’s minds regarding what clean energy is worth to them.

Direct Heating And Cooling

When used to directly heat or cool a building, geothermal energy is harnessed by a device called a geothermal heat pump. Beginning only a few feet underground, the earth’s temperature hovers around the mid-fifties year round. This allows tubes buried horizontally four to six feet underground or vertically 100-400 feet underground to be constantly exposed to soil and water in this temperature range.

The tubes can also be placed in a pond, usually at least eight feet below the surface to avoid freezing. Often a mix of water and antifreeze runs through the tubes in these closed systems and moves heat between the air inside of the house and the ground or water that the tubes are buried or submerged in. In contrast to the closed systems that recycle the same working fluid, open systems draw water from a body of water such as a pond or a well. The water is then returned to its source after it transfers heat either into or out of the house.

The way a geothermal heat pump moves heat from one place to another is similar to how a drop of dye diffuses throughout a glass of water until all of the water is the same color—heat always moves from an area of high concentration to an area of low concentration (hot to cold). In the summer, when the temperature of the air above ground exceeds the mid-fifties, the fluid in the tube is heated as the tube passes through the house above ground level. Then as the warm fluid is pumped down below ground level, it releases the heat that it picked up aboveground to the tube’s cooler surroundings belowground.

In the winter, the exact opposite occurs. The temperature aboveground is lower than the mid-fifties, so the fluid that comes up from the ground is warmer than the air above ground and it releases that heat to the air inside the house. Since the belowground temperature is in the mid-fifties year round, this may not eliminate heating and cooling bills altogether. It will, however, mean that less fuel will need to be directed toward these activities to reach the desired indoor temperature.

Generating Electricity

When used to generate electricity, geothermal energy is captured from hot rocks deep below the Earth’s surface. The heat comes from underground magma chambers and the decay of radioactive elements such as uranium and thorium. The massive amount of heat in these rocks superheats the water to well above its boiling point. At the surface, people can use the heat energy in the water that flashes to steam to turn a turbine and generate electricity.

When water is deep underground in the rocks surrounding the magma chamber, it is subject to high pressures that allow it to remain in a liquid state even though the temperature is well above the boiling point. When the water is brought to the surface, the high temperature combined with the rapid drop in pressure causes the water to immediately turn to steam. This steam turns a turbine, which generates electricity. Sometimes when the temperature of the water is too low to flash to steam, the hot water will be used to heat another fluid with a lower boiling point than the water itself. This other liquid is called a secondary working fluid and it flashes to steam and turns the turbine when heated by the water.

Conventional vs. Enhanced Geothermal Systems

The two basic classes of geothermal technology that will be dealt with here are conventional and enhanced geothermal systems. An enhanced geothermal system (EGS) differs from a conventional one in two key ways. The first is how porous the hot rocks are. The second is the volume of water in the rocks that can be pumped to the surface. In conventional geothermal systems, the hot rocks are porous and contain a large amount of water that can be retrieved easily through a well drilled at the surface.

Enhanced geothermal systems are needed in places where the hot rocks are largely impermeable to water, meaning that there is very little water naturally present in the rock that can be pumped to the surface. In the past, the presence of a large amount of heat energy with no water to transfer that energy to a turbine at the surface meant that that heat was unavailable. Today, with enhanced geothermal techniques, the solution to this problem is to fracture the rock. Once there are spaces for water to collect and heat up, the water can be retrieved from the well in the usual fashion.

Injection Activities

Over the past few years, headlines with names like “Fracking for Geothermal Heat Instead of Gas” and “Is Fracking for Enhanced Geothermal Systems the Same as Fracking for Natural Gas?” have connected enhanced geothermal and hydraulic fracturing in the public consciousness. For the purpose of this article, three potential safety concerns regarding fluid injection including hydraulic fracturing, wastewater disposal, enhanced oil recovery and enhanced geothermal will be examined. The first issue is cement well casing failure leading to saltwater and methane gas leakage. The second concern is induced seismicity, or earthquakes caused by human activity. Finally, the third issue to be addressed is radon leakage resulting from subsurface drilling.

Well Casing Failure Rate Increases With Age

A major concern for all subsurface drilling activities is the integrity of the cement well casings. Cracks in the cement could allow liquids such as radioactive, highly mineralized salt water and gases like methane to percolate up into potable aquifers. Approximately five percent of all new cement casings in gas wells fail, and that rate only increases as the well ages. According to Dina Lopez, a geology professor at Ohio University, cement failure is an inevitability because just about anything can degrade it. In order for cement to be completely chemically stable, it must be in contact with materials having a pH of 14 or higher. For comparison, bleach has a pH of 12 and most soils have a pH in the range of five to seven.

Lopez mentioned that studies have linked aging cement well casings to upper aquifer contamination. It has become clear that there is no way to avoid issues of cement degradation. Since casing failure is inevitable, a reasonable course of action is for scientists to characterize the extent of the risk so that an acceptable level can be determined.

Induced Seismicity

Another concern that has been apparent in many articles is that enhanced geothermal may induce large, damaging earthquakes. The problem, as presented in the New York Times, appears to be that enhanced geothermal wells drill into what is called the crystalline basement—the igneous or metamorphic rock that underlies the sedimentary rock layer where a lot of mineral exploitation is carried out. Also, these sites have a history of seismic activity, which makes them a terrible candidate for technologies that induce even tiny earthquakes. The claims in the New York Times article about drilling too deep were supported by a USGS seismologist named Art McGarr, who cited a famous set of earthquakes in Denver, Colorado that were linked to fluid injection in the local crystalline basement layer.

While some enhanced geothermal experiments have resulted in damaging earthquakes, it is possible that not every case will end this way. A theme that Douglas Green, a geology professor at Ohio University, said that seismic activity is extremely difficult to predict until something happens.

According to the United States Geological Survey, “It is probable that the duration of injection, the magnitude of the fluid pressure increase, and the size of the region affected by injection will strongly influence whether earthquakes will be induced and how large they will be.” So scientists have broad outlines, but no definitive system, to indicate when an activity will or will not induce an earthquake. With each passing study, scientists inch closer to a complete understanding of how induced seismic events are triggered and how we can avoid causing them in the future.

 

Radon Exposure

Finally, the third issue associated with subsurface drilling is radon emissions. Radon is a chemically inert, but radioactive gas that forms when uranium that naturally occurs in soils breaks down. Radon is the number two cause of lung cancer behind smoking and tends to seep into cracks in buildings’ foundations and settle in basements that are not properly ventilated. As the EPA radon zone map shows, most of Ohio is in Zone 1 with high levels of radon. In contrast, Athens is in a small pocket of Zone 2 with lower levels. This simply indicates that the soils in southeast Ohio naturally contain less radon than those in most of the rest of the state.

According to Lopez, when rock is fractured, the greater surface area increases the risk of radon mixing with groundwater, venting to the atmosphere and settling in basements. The key question with issues like this is always one of dose. Maybe radon levels increase with increasing subsurface drilling, as a 2015 report found; but are the increased levels enough to pose a significant health risk?

It is important to note that the report contradicts a 2013 Pennsylvania Department of Environmental Protection (PADEP) report that did not find elevated radon levels around hydraulic fracturing wells, and because of this has made a lot of people skeptical. Also, the PADEP report found that the radon levels that they measured did not present a significant risk to human health. That being said, the PADEP report also did not take measurements inside of houses like the 2015 study did.

As with the induced seismicity issue, the radon question is far from being cut and dried and more research will be needed to come to conclusions that are solid enough to base any kind of policy on.  Since radon emission is an issue for hydraulic fracturing, which breaks open shale where uranium is present and enhanced geothermal does not, radon may turn out to be a non-issue for enhanced geothermal. Although not strictly pertaining to the enhanced geothermal discussion, radon is seldom mentioned in association with hydraulic fracturing. If nothing else, hopefully this will spark that discussion.

Where To Go From Here?

Enhanced geothermal is a technology steeped in controversy—and for good reason. Injecting large volumes of water at high pressures to fracture rock is risky business, especially if there happens to be a fault that was not anticipated. As time goes on and scientists learn more about the distribution of subsurface faults, maybe at least the seismic risk can be mitigated. It seems that cement well casings can only be monitored and repaired and the only way to fight radon exposure is to get your basement tested and ventilated.

So what about Ohio? In Ohio, hot rocks are too far below the surface for geothermal power generation to be feasible, so enhanced geothermal is off the table.

“Geothermal doesn’t have a tremendous potential in the eastern U.S., unfortunately,” said Green. “Maybe that’s fortunate—it also means you have less tectonics.”

Although there is little potential to generate electricity from Ohio’s underground heat, it can still be harnessed directly.

Lopez’s suggestion for Ohio geothermal was documented in a study she recently completed regarding the use of abandoned underground mines. Her recommendation is that Ohio’s abandoned mines be used for direct geothermal heating. According to Lopez, the water in flooded mines can act as a heat exchanger in the same way the ground can in a typical geothermal heat pump, and they also must be close to the building they are going to heat. Given how many abandoned mines exist in Ohio, Lopez is confident that it could be a way to gain some benefit from the mines that are certainly not going away and otherwise would be providing nothing but acid mine drainage.


 Andrew is a recent graduate of Ohio University’s Environmental and Plant Biology Department who makes his own deodorant and chocolate. You can usually find him out in Waterloo or in Porter Hall helping with ecophysiology research. On the weekends, he works at The Village Bakery

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