Combining Carbon Capture and Sequestration With Geothermal Energy
August 30, 2011
Alternative energy and green industrial technologies and methods are often criticized as too expensive, with the costs outweighing the benefits, say some critics (who have a point, in many cases). The charge is a contentious one: when determining costs versus rewards, often times, opposing sides can't even agree exactly what the long-terms costs and benefits are.
So news of two previously separate green processes – both of which have been criticized as being too expensive – being combined to mutually benefit one another is worth noting. In this case, it's an announcement about an intriguing mix of carbon capture and sequestration processes and geothermal energy generation.
Carbon capture and sequestration (CCS), of course, is the process of retaining carbon released from industrial installations such as fossil fuel power plants or factories and, instead of venting it into the atmosphere to cause widespread damage, pumping it deep underground where it's stored for the indefinite future. The process, which has been in practice for only about a decade, is estimated to be able to capture as much as 90 percent of greenhouse gas emissions from industrial operations. However, it's not cheap. Capturing and compressing the carbon dioxide requires requires a lot of energy and can actually increase the fuel needs of a coal-fired plant by as much as 40 percent, by some estimates. (Which many critics contend cancels out its usefulness as a process that improves the environment.) In the long run, it has been estimated that a new power plant that includes carbon capture and sequestration capabilities would increase the cost of energy by a minimum of 21 percent, with some estimates ranging up to 91 percent. And these numbers apply to newly built power plants with purpose-build carbon sequestration facilities. Trying to retrofit existing power plants with CCS equipment and storage facilities would likely cost even more.
Kind of a hard sell in a recession economy, not to mention in a politically polarized country.
Another major drawback of carbon sequestration is the cost of transportation of the gas. Since most power plants don't have the facilities to pump the CO2 they emit underground and store it, it must be brought to storage facilities elsewhere, usually by pipeline. The result is that while the technology is virtuous, clean and green, it's not particularly cost-effective.
Another drawback of storing carbon deep underground in large reservoirs is the potential for ruptures due to seismic or man-made events. A high-volume escape of carbon dioxide could endanger the local population: the release of a cloud of naturally occurring carbon dioxide – thought to be vented from a volcanic chamber – from under Lake Nyos in Cameroon in 1986 killed 1,700 people and 3,500 farm animals.
Needless to say, proposed carbon storage facilities will likely lead to a host of NIMBY (“not in my back yard”) complaints as utility companies and municipalities hunt for places to store the gas.
Another concern involves proposed carbon sequestration in bedrock under oceans, considered a viable alternative to land-based storage. While it's sometimes considered a way to store carbon further away from human populations, many scientists have expressed concern that leakage of the CO2 could change ocean chemistry irreparably and endanger marine life populations on a wide scale.
Other critics of carbon capture and sequestration contend that it's merely a tool to allow dirty energy “business as usual” to continue: a kind of crutch for the coal and oil-based energy industries to use that allows them to continue burning fossil fuels and dodge the necessity of spending money and time to search for and implement cleaner sources of alternative energy.
But what if the captured CO2 could be used for practical purposes and stored in a safer way?
Researchers from the Lawrence Berkeley National Laboratory (Berkeley Lab) think they've come up with a solution for unwanted carbon dioxide: to generate electricity through a combined geothermal process. By injecting the carbon dioxide about three kilometers (1.86 miles) underground into sedimentary rock layers where temperatures reach a toasty 257 degrees Fahrenheit (125 degrees Celsius), researchers think they can make the CO2 reach a “supercritical” state in which it's neither gas nor liquid, but something in between that can can effuse through solids like a gas, but also dissolve materials like a liquid. The carbon dioxide is then pumped back up to the surface, where it encounters a special turbine that converts some of the liquid and gas properties of the carbon dioxide into electricity. The process is repeated to keep the turbines turning (more carbon dioxide is added as needed) and in the process, some of the CO2 becomes trapped in the sediment underground...which is a good thing.
“We actually want some of the CO2to become trapped,” said Barry Freifeld, a mechanical engineer in Berkeley Lab’s Earth Sciences
Division. “Our approach relies on this gradual loss as a way to store a power plant’s CO2 underground rather than emitting it into the atmosphere. Our planned demonstration is the first attempt at proving that we can simultaneously mitigate greenhouse gas induced climate change and generate clean baseload power using geothermal energy.”
In essence, the carbon dioxide, combined with the earth's heat, creates energy as a byproduct, offsetting the high costs of carbon storage. These costs have been the primary barriers involved in large-scale deployment of carbon sequestration.
Noted Freifeld, “Carbon storage takes a lot power — large pumps and compressors are needed. We may be able to bring down its costs by generating electricity on the side.”
Of course, the process hasn't actually been used yet, in practice. Next year, Lawrence Berkeley Lab researchers – with the help of a number of other interested parties and $5 million in funding from the U.S. Department of Energy – plan to begin building and then testing the technology at an existing carbon sequestration installation in Cranfield, Mississippi, with the help of a group called the Southeast Regional Carbon Sequestration Partnership. The Cranfield site was chosen because it has been operating a high-rate CO2 injection program since 2009. As a result, a lot of the infrastructure needed for the experiment – such as injection and production wells – is already in place, as is the supply of CO2, which will be brought in by a pipeline operated by a company called Denbury Resources, a Texas-based oil and gas company that owns the largest reserves of carbon dioxide east of the Mississippi River.
During the course of operations at the site, researchers will be able to evaluate whether the process is suited to be replicated at other sites: both existing carbon sequestration facilities and potential greenfield sites.
But before researchers even begin on-site testing, they need to demonstrate the availability of potential resources, design the heat extraction cycle, and then assess the potential environmental risks, which will be no easy process.
However, if it could be made to work, the benefits are many. Not only can carbon sequestration be made to be practicable by turning it into a kind of fuel for clean alternative energy, but it can also help solve some of the problems inherent in traditional geothermal installations.
While geothermal has a great deal of potential as a clean energy source, it's hard to justify in areas that are short on water. Traditionally, it's water rather than carbon dioxide that's injected into the hot sediment layers underground and brought back up heated to drive turbines, and often, about 10 to 20 percent of the water used is lost during the process: trapped in the “pore spaces” of the rock and necessitating the addition of more water. This makes geothermal energy a pretty hard sell in countries that are short on water, even if their clean energy needs are great. By using CO2 instead of water, some parts of the world may be able to begin generating energy from geothermal sources without endangering limited and precious water supplies.
Lawrence Berkeley Labs researchers won't be alone in the ambitious endeavor. In addition to Denbury Resources, which will provide the carbon dioxide, an Akron, Ohio-based company called Echogen Power Systems that specializes in waste heat recovery will be designing the special turbine that can handle the supercritical carbon dioxide along with the hydrocarbons and residual water that is forced up from the underground sediment layers. University of Texas at Austin scientists will be part of the project as well, analyzing the environmental impact of the whole operation (which will ultimately help determine its viability). Researchers from the Australian Commonwealth Scientific Industrial Research Organization (CSIRO) will be present to observe and determine the feasibility of the process in their own country.
The project will be funded, in part, by the U.S. Department of Energy, which is currently led by former Berkeley Lab head Steve Chu. The U.S. DoE has given a total of $11.3 million to eight geothermal energy projects in five states, including the Lawrence Berkeley Project. The goal of the grant money, says the DoE, is to help push forward projects that will help reduce the costs of geothermal energy and make it competitive with conventional sources of electricity.
The Lawrence Berekely Lab project could ultimately kill several birds with one proverbial stone: bring down the costs of carbon capture and sequestration, improve its safety and make it more practically and politically popular; increase the cost-effectiveness of geothermal power generation; and save water.
Researchers caution that it's too early to determine how much electricity the process will be capable of generating. They say that it will depend on a number of factors, including how much carbon is being sequestered and how deep the heat and storage reservoirs are. Still, it's a step forward for carbon capture and sequestration, which...with the help of these researchers...could ultimately become a revenue generating process rather than a revenue drain, and turn down the volume on some of its critics.