The method of pulling power from the ground’s heat is receiving renewed attention because of new technologies as well as efforts by DOE and other parties to promote this clean, abundant source of energy.
The nation’s politicians and many of its citizens are pushing for more use of renewable sources of energy not only because of environmental issues but to lessen dependence on foreign oil. However, both wind and solar present a problem in that the technology that exists today cannot provide sufficient power for baseload electricity. A third source—geothermal—holds promise, and even though it is not a new source of energy, new types of plant technologies are making this type of renewable even more attractive.
Geothermal energy is actually very efficient compared to other renewables, according to Tim Reinhardt, technology development manager in charge of low-temperature geothermal projects at the Department of Energy (DOE). It can operate 24 hours a day under virtually any conditions, and its capacity factor (the ratio of the actual output of a power plant over a period of time compared to what it could put out operating at full capacity during the entire time) is equal to nuclear and coal-fired plants.
“We’re [DOE] working with the energy industry to make this power competitive for baseload electricity,” he says.
AN UPDATE ON THE INDUSTRY
Today, three kinds of geothermal power plant technologies are used to convert hydrothermal fluids to electricity: dry steam, flash and binary cycle. Which type is used is dependent on the temperature of the geothermal site.
Early geothermal power plants were dry steam plants, which use the steam from geothermal reservoirs as it comes out of wells. This steam is routed directly through turbine/generator units to produce electricity.
More common today are flash steam plants, which use water at temperatures greater than 360° F (182° C) but below 400° F (204° C) that is pumped under high pressure to the generator equipment at the surface. The pressure is allowed to reduce and the water flashes to steam, which is then put directly through a turbine.
The third type of technology is the binary cycle plant, in which the water or steam from the geothermal reservoir never comes in contact with a turbine or generator units. Instead, the technology heats a working fluid that runs the turbine. (This third type of plant is DOE’s Tim Reinhardt’s specialization.)
CHALLENGES FOR GEOTHERMAL
While geothermal energy production holds much promise, it also presents challenges. John W. Pritchett, board member of the Geothermal Energy Association and co-chair of that association’s Science and Technical Committee, shares what he considers some of the most difficult.
First is getting financing for new projects. “The geothermal project developer will obtain no revenue until the wellfield is drilled, the power plant is constructed, a power-purchase agreement has been negotiated with the public utility company, and transmission facilities have been arranged,” he explains.
Meanwhile, “The upfront capital costs of a geothermal project are formidable,” he adds. By comparison, “Financing for natural-gas or coal-fired power plants would be far more challenging if the project developer were required to purchase the entire fuel supply for the project’s lifetime prior to selling any electricity,” he explains.
A second major challenge is locating a suitable underground geothermal resource and obtaining permission to develop it, Pritchett continues. Suitable locations are not plentiful and tend to be located mainly in the western U.S. and in areas that are generally away from population centers, and much of the land is tied up in government ownership.
Also, “Prospecting techniques for geothermal resources are in a relatively primitive state of development and are comparable in effectiveness to those that were in use for oil and gas exploration in the very early part of the 20th century,” he adds.
The geological anomalies of the western U.S. make it possible to have large megawatt geothermal power facilities in Nevada and California, Reinhardt says.
However, DOE has not limited its efforts to that area.
“We [DOE] explore resources spread across the entire U.S., including traditional hydrothermal and low-temperature resources that are outside or below the normal temperatures used to harvest electricity,” he says. They are also working with co-produced resources in locations that have oil and gas production such as the Dakotas, along the Gulf Coast, even Arkansas.
However, these locations are isolated, and isolation creates problems in several ways. According to Pritchett, while geothermal electricity must be generated by surface facilities located above the underlying geothermal resources, these facilities are often situated substantial distances from load centers. A natural-gas plant, on the other hand, can be located almost anywhere (even within the city it serves), though some party still has to install or expand the size of piping. Because of the isolation, geothermal projects ordinarily will require electrical transmission facilities to bring the power to market. Many promising geothermal areas presently lack such means of transmission, he notes.
Another great challenge is technological support and lack of infrastructure, Pritchett says.
“Most geothermal development companies are fairly small, thinly capitalized, and severely stressed financially by the high upfront capital requirements of geothermal projects. Their ability to directly support technical research and development activities is very limited,” he says.
In the past, the industry has relied on the government to fill this role, mainly DOE and the U.S. Geological Survey, but government support has been relatively small, highly intermittent and unreliable, particularly in recent years, he adds.
The geothermal industry has had some exciting advances in technology, Pritchett says.
For example, “Substantial progress has been made in recent years in geothermal exploration technology, and many new resources have been discovered and are now awaiting exploitation,” he says. Also, the development of new thermodynamic cycles for geothermal power plants has lowered the permissible threshold resource temperature for efficient power generation, in effect increasing the exploitable national geothermal resource base.
Meanwhile, “New and more powerful techniques for resource modeling and performance forecasting are reducing wastage and lowering the cost of geothermal electricity,” he says.
Finally, the combination of geothermal generation with other energy technologies is making projects more profitable. This includes cogeneration, which combines oil and gas production with geothermal electricity generation using the coproduced hot water and steam from oil/gas wells, and hybrids with other renewables such as the new ENEL Stillwater geothermal/solar project in Nevada, he says.
IN THE FIELD
Some exciting developments have also occurred in the field in the last few years, according to Reinhardt.
For example, DOE is working with Pacific Northwest National Lab on developing better working fluids.
“They’re adding nanoparticles of metal organic heat carriers to the working fluid. The idea is to make the working fluid more heat efficient,” he explains.
Reinhardt says these metallic particles will not create challenges for components such as valves that would process the fluid because “that fluid is contained within the closed circuit of the binary power plant, powering the turbine. It poses no special challenges to the process equipment.”
Another project that holds great promise is in Nevada where “ElectraTherm has its green machine, which uses twin screw expanders as opposed to a traditional turbine to create the electricity,” says Reinhardt.
With more than 3,100 MW of installed power and another 6,000 MW or more in the planning stages, geothermal energy has vast potential. Responsible development could allow geothermal to be a significant contributor to baseload electricity generation in the United States, he says.
Because the DOE realizes the strategic value of geothermal energy production, the agency supports the industry through its own technologies program. DOE is working to advance geothermal energy as part of a broader energy portfolio. To do this, it focuses on research and development projects with universities, national labs and private companies to develop better, more advanced and safer ways to produce electricity, including geothermal technologies. By exploring issues such as geochemistry, drilling and equipment, the DOE is aiming to create more efficient and less expensive means to tap into geothermal.
Reinhardt describes a few current projects, including two in North Dakota that were supported through the American Recovery and Reinvestment Act of 2009.
“We’re going into existing oil and gas fields that are using hot water and re-injecting it into the hydrocarbon layer to enhance production,” he says.
To do so, DOE is putting a binary cycle unit on the front end.
“Since the water doesn’t have to be that hot, we can capture some of that heat and create a portion of the electricity needed for the process itself,” he explains.
For a second project, geothermal fluid, a mixture of hydrocarbon and water, is produced at the end of the process.
“That goes through a separator and the wastewater will be run through a binary unit to produce electricity,” he explains. “With binary units going into an existing oil and gas infrastructure, it is a relatively simple matter to plug into the process.
“It adds no cost to the existing operation and maintenance of the field, yet it can produce electricity for in-field production. It’s especially valuable for those operations that are off the grid, for which the producer has to use diesel generators,” he says.
The result can be significantly cheaper energy, depending on the price of diesel.
“We’re hoping to compete economically with producers who are on the grid as well,” Reinhardt says. “It’s an efficient use of the wastewater and waste heat, so it just makes sense.”
While efficiency and cost make geothermal energy an attractive renewable energy source, some concerns have been raised about the seismic ramifications of drilling into the earth.
“We have taken a look at it [the seismic issue] in the interests of good science and being good stewards of public and environmental safety,” says Reinhardt. DOE has put a seismicity protocol in place to let geothermal developers know the best methods to mitigate or avoid risks.
In addition to what the DOE does internally, there are also existing National Environmental Policy Act (NEPA) regulations that govern seismicity compliance.
With respect to NEPA, “Generally speaking, geothermal projects are usually relatively environmentally benign and have a fairly small footprint relative to the amount of electricity that is generated,” Pritchett says.
However, permitting delays (NEPA and otherwise) can have extremely adverse effects on geothermal projects, mainly because of the long lead-times and high up-front capital costs of geothermal development,” he says.
Also, the industry is currently finding both technical and nontechnical ways to mitigate the risks of geothermal exploration.
Both DOE and private industry are conducting research “to improve our ability to image the subsurface prior to drilling, to improve the chances of drilling successful steam production wells and to reduce the per-foot average cost of geothermal drilling,” Pritchett says. Meanwhile, a variety of approaches have been proposed for ameliorating the financial impact of “dry-hole” failures, including proposed federal cost-sharing programs and risk-sharing (insurance) pools.
While many renewable resources cannot provide baseload power, geothermal has the potential to produce enough reliable megawatts to provide that power. With technological innovation, driven by private enterprise partnering with DOE scientists and engineers, this resource could become an important part of the energy mix needed to help the country achieve its clean, independent power goals.
VALVES AND CONTROLS USED IN GEOTHERMAL
The valves used in the geothermal energy process are standard to any turbine-generated system.
“We use basically the same valves used in any power steam application,” Reinhardt says. “Butterfly, gate and globe valves are common. There really isn’t anything special although there are situations where we’re dealing with highly corrosive environments, and the pressure is high, so valves are chosen accordingly.”
Butterfly-type control valves are used to regulate inlet pressure to maintain evaporator outlet vapor pressure. Poppet-type main stop valves are commonly used to assure accurate shut-off of the steam flow when a turbine is stopped. In some systems, an internal bypass valve, assembled in the main stop valve, stabilizes control in the low-steam flow range.
On DOE’s wish list for geothermal projects are valves that are reasonably priced but offer more corrosion and pressure resistance.
“A reliable, inexpensive, automated control valve that could fully seal even in highly corrosive environments would be valuable to us,” says Reinhardt. Heat and fluid loss need to be prevented as much as possible, so anything including superior packing materials that can make that happen are desirable components of the valves used in these systems.
With respect to automation, most of the systems use standard hydraulic or electric actuators and have control panels (even on the smaller units). “You can remote-control some of the more advanced electric units and some are even coming up with an iPad or iPhone application,” Reinhardt says. “Remote control is becoming the norm, and monitoring is 24/7,” he adds.
The total electricity consumption of the United States presently averages about 470,000 megawatts (MW). Of this demand, 45% is now being met by coal-fired generating plants, 24% by natural-gas plants, 20% by nuclear plants and 6% by hydroelectric projects.
Most of the remaining 5% comes from biomass, wind, solar and other renewables, including geothermal. The total installed capacity of geothermal electrical projects in the U.S. is presently about 3,200 MW, representing only 0.7% of the national electricity demand. The U.S. Geological Survey has estimated these existing geothermal projects represent less than 10% of what geothermal is capable of contributing, and that geothermal projects could provide as much as 8% of the U.S. electricity demand (which is more than hydroelectric does today) using only existing, present-day technology.
There are no important technological obstacles to substantial short-term expansion of the industry, although estimates of the longer-term possibilities using various kinds of advanced geothermal technologies vary widely. A 1999 study that used fairly conservative assumptions about the resource base and technology concluded that geothermal resources using existing technology have the potential to support between 35,448 and 72,392 MW of worldwide electrical generation capacity. Using enhanced technology, the geothermal resources could support between 65,576 and 138,131 MW of electrical generation capacity.
Regarding U.S. potential, the National Renewable Energy Laboratory released a report in 2006 that estimated 26,000 MW of geothermal power could be developed domestically by 2015, and that by 2025, more than 100,000 MW of geothermal power could be in production. Currently more than 4,500 MW of geothermal power projects are under development in California, Oregon and Nevada.
—Courtesy of the Geothermal Energy Association