In separate seminars, two MIT Energy Initiative (MITEI) Clare Boothe Luce Fellows described advances in monitoring underground carbon dioxide (CO2) and developing higher-efficiency solar cells.
Storing CO2 in saline aquifers and other underground formations is an attractive way to keep it out of the atmosphere, but monitoring it is a challenge, said petroleum engineer Carolyn Seto on April 27.
A new material that could double the efficiency of solar cells may one day be a game changer for the entire solar industry, physicist Bonna Newman said on May 11.
The Clare Boothe Luce Postdoctoral Fellowships for Women in Energy at MIT program has supported these two female scholars working in disciplines where women are typically underrepresented, said MITEI Deputy Director Robert C. Armstrong. Funding for the fellows is a result of the generosity of the Henry Luce Foundation’s Clare Boothe Luce Program, which aims “to encourage women to enter, study, graduate, and teach in science, engineering, and mathematics.” The Luce Postdoctoral Fellows have played an integral role in the energy community at MIT through their research activities and mentorship of students interested in the energy field.
Seto, who was hosted by the Gregory J. McRae research group in the Department of Chemical Engineering, is researching model-based methods to integrate data and enhance monitoring capability in CO2 injection. She also is investigating the role of technology in unconventional gas recovery as part of an ongoing MIT study on the future of natural gas.
Newman, who completed a PhD in physics at MIT in 2008, used the two-year fellowship as an opportunity to switch from atomic physics to exploring the limitations of inorganic solar cell materials. Working with Tonio Buonassisi, assistant professor of mechanical engineering, at the MIT Laboratory for Photovoltaics Research, she studied the impact on conductivity of tiny defects deep within the atomic structure of photovoltaic materials.
At an April 27 seminar, Seto said risks associated with large-scale CO2 sequestration could be reduced with a properly designed monitoring program that integrates multiple sources of information with uncertainties about the system, while balancing the risks and operational concerns of the project.
The need for CO2 sequestration is great, she said, because greenhouse gas-emitting fossil fuels provide 85 percent of the world’s energy and are expected to continue to do so for several decades to come.
One 500-megawatt coal power plant produces 3.6 megatons of CO2 a year. There are more than 500 such plants in the US alone. Furthermore, cheap, abundant coal supplies in regions of high energy demand such as Asia and North America, coupled with continued deployment of coal-fired power plants in China, mean that coal will remain a dominant energy source. Meanwhile, climate change experts recommend stabilizing atmospheric concentrations of CO2 between 450-500 parts per million to limit mean global temperature rise to 2-3 degrees Centigrade.
“Scalable solutions are needed now, and carbon sequestration is one of those solutions,” Seto said. Target formations for sequestration include unmineable coal beds, oil and gas reservoirs, and deep saline aquifers. These formations could be hundreds of kilometers in size and will need to be monitored for hundreds, if not thousands, of years.
For sequestration to play a role in climate change mitigation, injection volumes will need to be scaled up one hundred fold over existing pilot projects, Seto said. The biggest challenge in deployment of large-scale injection is that “we don’t know how the Earth is going to respond to such large volumes of matter in the subsurface.”
Potential sequestration dangers include water contamination, threatened ecosystems, and property damage. In addition, sequestering sites face challenges such as underground faults that could be disturbed, abandoned boreholes that could create an escape route for the gas, and induced seismicity that would affect the integrity of surface structures. To avoid potential worst case scenarios, such as the Lake Nyos event, where a crater lake in Cameroon released a large, natural accumulation of CO2, killing people and livestock in nearby villages, models coupling subsurface and surface processes can be used to design effective monitoring systems to limit the possibility of catastrophic events.
Models and monitoring design have a synergistic relationship. Combining models with observational measurements, the predictive power of process models can be enhanced, reducing uncertainty and limiting the risks associated with CO2 leakage. Models simulating the expected response of the system from injection provide guidance on optimal deployment of sensors for specified design constraints of resolution and cost.
Seto has demonstrated that a more expensive monitoring program with a high resolution and sensitivity for leak detection is not always the most accurate. She presented evidence that a lower resolution, less expensive program could locate a leak that wasn’t visible using the higher cost design.
Even though “models have uncertainty, through parameters, measurements, and decisions about physical representation,” Seto said she believes that risk can be mitigated with a model-based monitoring system that accounts for these uncertainties. “Monitoring design is not a one-size-fits-all problem,” she said. “We need to consider multiple decision points and trade off uncertainty, cost, and risk.”
The fact that defects occur naturally within all semiconductor systems seems like a drawback. But at her May 11 seminar Newman said she uses high concentrations of impurities to actually engineer better silicon solar cells. “Defect engineering of inorganic photovoltaic materials is one way of enhancing solar cell performance,” she said.
“Solar is interesting to me because it generates 10,000 times the amount of power we currently use as humans,” she said. “We have effectively unlimited solar resources—if we can figure out a way to directly tap into that resource economically. This is no small task.”
Currently, solar technologies provide electricity at around two to 10 times the cost of grid power. Newman said solar cells today have a thermodynamic limit of around 33 percent efficiency. Among solar cells’ stumbling blocks is that they don’t absorb the entire spectrum of available sunlight.
A new material for solar cells, black silicon, is made by shining a series of very short, very intense laser pulses at a silicon surface in a chamber filled with sulfur-rich gas. Irradiating the flat, mirror-like surface in the presence of the gas creates a forest of microscopic peaks. The resulting “doped” material absorbs nearly all light from the ultraviolet to the infrared.
By pushing solar cells’ absorption capabilities into a broader range of wavelengths, Newman hopes to boost electrical output to as much as 63 percent.
In what Newman called the first direct probe of doped silicon, she and colleagues heat silicon to scorching temperatures before using X-rays to inspect its molecular structure. They’ve determined that the state of the doping agent’s atoms seems to be strongly correlated to the amount of absorption. “The annealing step actually decreases the density of the defect, which we think leads to infrared absorption.
“Our study is helping us understand the role the molecular structure of the material plays in enhanced absorption,” she said.
Understanding these materials on much more fundamental levels may lead to potential applications ranging from superconductors to solar cells to batteries for energy storage, Newman said. New materials such as black silicon may lead to higher efficiency, lower cost solar cells in “as early as five to 10 years,” she said. “It’s not around the corner, but we could see it having an impact on helping meet growing global energy consumption.”