MechE Professors Find Creative Solutions to the Problems of Clean Energy
|Image of the hybrid solar thermoelectric system (HSTE) prototype tested in a vacuum chamber. Research carried out by graduate student Nenad Miljkovic.||Schematic of the hybrid solar thermoelectric system (HSTE). Solar energy is focused by a parabolic concentrator on the evaporator section of the evacuated tube absorber (thermosyphon), which heats the TE hot side. The resulting temperature difference between the TE hot and cold sides produces electrical power while heat carried away (waste heat) by the thermosyphon (adjacent to the cold side) is transferred to the condenser section for the bottoming cycle. Research carried out by graduate student Nenad Miljkovic.|
By now, most people agree that CO2 emissions from “dirty” energy such as fossil fuels will have an increasingly degenerative effect on our planet, if it hasn’t already. We know that the increase in CO2 emissions reduces heat transfer from the Earth outwards, increasing the Earth’s own temperature. We also know that our current sources of energy are limited, and while we may not be able to predict exactly when, at some point in the future, they will run out.
“There is no debate as to whether humans are causing an increase of carbon in the atmosphere because of our energy use,” says Professor Tonio Buonassisi. “We are. You can debate whether or not that’s an issue, but I think once you start doing the numbers, you quickly realize it’s a real problem.”
As a result, in the past few decades, many scientists, politicians, and the public at large have become increasingly concerned with going green and being clean, each with their own ideas about what the most important element is to address. The implications of energy are profound and impact the environment, the economy, and national security.
But what is clean energy exactly? It’s worth defining, because there are many ways to go green. Clean energy refers specifically to energy technologies that either replace or eliminate CO2 emissions in the atmosphere and instead rely on renewable sources of energy such as wind, solar, or biofuels, as well as technologies that reduce CO2 emissions through carbon capture or other similar emerging technologies. Clean energy can be achieved and aided in many ways through energy storage or conversion techniques.
But the solutions – and we need a lot of them – as well as the expertise required to discover those solutions – are complicated and multidimensional.
“Clean energy is a technology that requires a lot of depth in a variety of disciplines in science and engineering, and is the kind of sector that is likely to bring innovation,” says Professor Paul Sclavounos. “It is widely accepted for bringing economic growth and training students and entrepreneurs in the engineering complexities of energy technologies, including the understanding that it’s not only the technology that matters, but also the acceptance of the technology by society both on economic and environmental grounds. For all these reasons, I believe clean energy is going to be one of the most important technologies of the 21st century.”
Clean energy technologies utilizing the sun and the wind aren’t new concepts, but there are still significant hurdles to making them viable – such as the cost and public acceptance Professor Sclavounos refers to, as well as the ever-important element of efficiency.
“When you think about green energy, you have to think about cost,” says Professor Gang Chen. “Passionate people will put solar cells on their rooftop, but ordinary people are worried about putting food on the table and will take the cheapest available option. Research needs a fundamental side that doesn’t concern itself with cost, but if you want to have real-world impact, you have to think about that too.”
One way to decrease costs – and thus increase public acceptance – is to improve the efficiency of technological ideas that have already been discovered. In MechE, our professors are solving the need for efficient and cheap clean energy by looking at it in a myriad of ways, from solar and wind to thermal and chemical, from a myriad of increasingly multidisciplinary perspectives, including design, mechanics, chemistry, physics, materials, architecture, and mathematics. Some are applying old methods to these new problems in creative and innovative ways, and some are discovering entirely new methods altogether.
Gang Chen: Nanostructures and Heat Transfer
One MechE faculty member, Gang Chen, the Carl Richard Soderberg Professor of Power Engineering and director of the Pappalardo Micro and Nano Engineering Laboratories, is tackling the problem with thermoelectrics at the nanoscale. Chen and his team recently developed a new material that conducts electricity well but not heat. By breaking up a piece of material into nanopowders and then reconnecting it, they realized that they could create a multitude of interfaces between the particles to impede heat flow.
Chen foresaw a way to utilize the resulting temperature difference to create a low-cost thermoelectric generator for converting solar energy into electricity. It is approximately eight times more efficient than any previously reported solar thermoelectric device, and, without any moving parts, would be much less expensive to produce.
The device, which is placed inside an airless vacuum chamber, works by collecting heat from the sun that warms up a black copper plate inside the generator. Meanwhile, because of the new material that impedes heat, the other side of the device remains at ambient temperatures, thus creating a 200-degree temperature difference and allowing the captured solar energy to be converted into electricity.
“This is a discovery we’re quite proud of,” says Chen. “It’s an example where innovation in design can lead to eight times better performance than anything anyone has tried before.”
Evelyn N. Wang: Heat Transfer and Energy Conversion
Evelyn N. Wang, Associate Professor of Mechanical Engineering, is similarly focused on heat transfer as a means of increasing efficiency and decreasing costs.
“Heat and mass transfer are one of the critical bottlenecks in making more efficient energy systems,” says Wang. “Solutions in these areas allow us to rely less on fossil fuels and think more about which renewable resources we can take advantage of. When you think about clean energy, there’s an important generation aspect, but I think the conservation side is very important as well.”
Wang’s most recent research has led to the development of a hybrid solar-thermoelectric system to more efficiently collect the sun’s heat, rethinking existing systems that were costly and difficult to implement because of the inefficiency in their design.
“Our goal was to identify the key components of the system that were causing the bottleneck and find a way to fix them. In terms of a heat-transfer perspective, we asked ourselves questions such as, How do you collect solar energy more efficiently? How do you minimize emissive losses from the surface of absorbers? Can you utilize different novel structures or materials such that you can make phase change processes more efficient?”
The answers to those questions can be found in Wang’s hybrid device, wherein a thermoelectric system is placed in the central tub of a parabolic trough so that it produces both hot water and electricity at the same time. A thermosiphon pulls heat from the thermoelectric “cold” side, and the resulting temperature gradient produces electricity. The thermosiphon, designed by Wang, is filled with a fluid that experiences phase change when it’s heated up, allowing it to transport heat efficiently.
generate electricity at an acceptable cost. You might have a new technology, but if it’s produced at twice the cost of fossil fuel, it’s not easy to persuade the public, despite the fact that it’s a cleaner source of energy.”
Paul Sclavounous: Wind Energy Design
Professor of Mechanical Engineering and Naval Architecture Paul Sclavounos went about solving the clean energy crisis yet another way: by using a floating turbine to gather offshore wind. His idea sprung from 30 years of experience as a naval architect and ocean engineer, designing and analyzing ships, high-speed vessels, sailing yachts, and offshore platforms. Sclavounos recognized an opportunity to apply technology used in deep oil and gas exploration to solve the public’s NIMBY (Not in My Back Yard) outcry about turbines that were too close to shore. Even more, he realized he could harness a large amount of offshore wind energy at prices competitive with natural gas because the wind is so strong and steady at such distances.
Sclavounos’s floating turbines rely on buoyancy for support, using steel cables that connect the corners of the turbine platform to a concrete block or other mooring system on the ocean floor.
“The specific design of a floater is a complex process,” says Sclavounos. “The final product may look simple, but it is not easy. The size of the floater, the thickness of the anchoring lines, the type of anchors, how they will weather a big storm, and how much energy they can gather at a low cost are all complicated elements that have to work together.”
Like Chen, Sclavounos is concerned with more than just fundamentals. “At the end of the day, economics matter,” he says. “Not only do you face competition and public acceptance, but you need to demonstrate that you can generate electricity at an acceptable cost. You might have a new technology, but if it’s produced at twice the cost of fossil fuel, it’s not easy to persuade the public, despite the fact that it’s a cleaner source of energy.”
Alexander Slocum: Wind and Solar Storage Design
Where Sclavounos leaves off, Alexander H. Slocum, the Pappalardo Professor of Engineering, dives in designing machines for renewable energy systems. He takes Sclavounos’s design for floating turbines and adds a method of storage to it.
Slocum’s concept places hollow concrete spheres on the ocean floor to act as mooring balls for floating wind turbines and to create a pumped hydroelectric energy storage system. When the wind turbine generates excess electricity energy, the pumped hydro unit begins pumping water from inside the sphere out into the ocean, storing the energy for later use. Then, when the time comes, the water flows back in through the pumped hydro unit to generate electricity.
With one eye on wind energy storage, Slocum fixed his other eye on solar storage. He and his team have created 24/7 solar power potential by developing a hybrid solar thermal receiver and storage device. In place of a tower that gathers concentrated solar power via a field of heliostats around the tower, the team’s “CSPonD” uses a 25-meter-diameter by 5-meter-deep tank mounted to the ground and filled with molten salt. An array of hillside mirrors focuses sunlight on the tank, heating the molten salt inside. As the salt heats, a moving plate gradually descends to accommodate the growing layer of very hot salt and forces the still cold molten salt upward to be heated. The hot salt then flows into a steam generator to power a steam turbine that generates electricity. The unheated salt leaves the steam generator and flows to the bottom of the tank. Two installations could supply enough 24-hour electricity for about 20,000 homes, and could last through one full cloudy day.
Alexander Mitsos: Mathematics and Process Design
Professor Alexander Mitsos takes product designs such as Slocum’s and other ideas from his lab and develops mathematical algorithms coupled with physics-based models to determine optimal ways of implementing such products. In one project, his team employed the math of nature — for instance, that found on the sunflower’s florets — to create an optimized pattern for the placement of heliostats found in any central receiver solar thermal plant.
“Using the Fibonacci spiral as inspiration, my team and I defined and optimized a new pattern to increase the amount of collected heat from the sun and minimize the land area needed,” says Professor Mitsos.
“By applying our accurate yet efficient model of field performance, we can reduce the cost of clean energy by literally moving the heliostats around.”
He calls what he does process-design engineering, and he doesn’t stop with solar. He’s also looking at ways to apply this same approach to carbon capture and sequestration, as well as desalination, honing in on time-variable operation. In order to achieve a large-scale penetration of renewable energy into electricity production, it is necessary to optimally use storage as well as modulate the electricity demand. To this end, Mitsos’s lab again uses mathematical models to determine the best time of day to run desalination and the most crucial times to inject clean energy into the market, increasing the efficiency of the processes while simultaneously decreasing the costs to society.
“For most of our work,” says Professor Mitsos, “the challenge is to identify the design variables that optimize the performance or minimize the cost. We’ve come up with a new metric that finds the best combination of technologies to identify tradeoffs between objectives.”
Yang Shao-Horn: Energy Conversion and Storage
It’s not all design and process in the Department of Mechanical Engineering though. Yang Shao-Horn, the Gail E. Kendall Professor of Mechanical Engineering, is sharply focused on the development of fundamental science for solar energy conversion and storage. She is working to increase the efficiency of electro-catalysis, a poorly understood catalyst process that promotes electro-chemical reactions. In particular, she and her team are looking at the naturally occurring process of oxygen reduction, an electro-chemical reaction that happens in our cells all the time as our body converts the oxygen we breathe. During this process, oxygen is split from water molecules, creating chemical bonds that can store energy and be distributed later to produce electricity.
“The reaction times for oxygen electro-catalysis are among the slowest and most difficult to promote; and because they are slow, it limits the efficiency of many devices that rely on this process,” says Shao-Horn. “We are trying to understand how these reactions occur at the molecular level, and then how to enhance them to make them faster.”
One of the best ways to increase the reaction time is to use a highly reactive catalyst, but it’s been a mystery until now which characteristics make catalysts the most reactive.
Shao-Horn and her team recently discovered that it was the configuration of the outermost electron of transition metal ions that best predicted their level of reactivity and, through this discovery, also found one of the most effective catalysts yet for water splitting – composed of cobalt, iron, and oxygen, with other metals. Now that they know what to look for, Shao-Horn and her team will continue the search for even yet more efficient catalysts.
Tonio Buonassisi: Solar Cell Performance
SMA Assistant Professor of Mechanical Engineering and Manufacturing Tonio Buonassisi’s interest in clean energy began when he was 16 years old and living in the then-polluted city of São Paulo, Brazil. One day on his bus ride home from school, he vowed to make it his career to help clean up the world.
And indeed he did. Today, Buonassisi is focused on identifying the limitations and defects of solar cells and using that information to increase their efficiency and decrease their cost. First, he and his team developed a unique device in partnership with a synchrotron facility, which uses a beam of highly focused X-rays to peer inside solar cell materials and identify the performance-limiting defects therein.
He gives the example of iron. “Iron is a big problem in silicon-based solar cells. Only a few nano-grams of it are needed to corrupt the efficiency of silicon devices, but to detect something that small is very, very hard. Fortunately, we are able to conduct these defect analyses using an X-ray beam that’s a thousand times thinner than your hair, then engineer a way to move the iron toward innocuous places where it doesn’t affect charge transport within the device.”
With this knowledge, Buonassisi and his team have developed a model that determines final solar cell efficiency based on the input material quality and the time-temperature profile used to process the material.
“We want to provide a tool that industry can use to improve the quality of the cells based on the materials used without spending precious resources on a ‘trial and error’ approach,” he says.
|Graduate student David Fenning mounts a sample onto the sample stage at the synchrotron (see close up on upper right). Underneath, a sample measurement reveals micron-size impurity precipitates clustered along a grain boundary in multicrystalline silicon solar cell material.|
Ahmed Ghoniem: Efficient Systems for Carbon Capture and the Deployment of Renewables
While many faculty members are busy devising new methods of creating clean energy, Professor Ahmed Ghoniem, the Ronald C. Crane Professor of Engineering, is developing ways to utilize fossil fuels while reducing or eliminating their negative impact on the environment. Since almost 85% of the worldwide energy production comes from fossil fuels, Ghoniem’s focus on the high-efficiency separation and affordable oxy-combustion processes that enable carbon capture is crucial in the search to solve the world’s energy crisis.
A near-term solution is one Professor Ghoniem has recently demonstrated: the superior efficiency of pressurized oxy-fuel combustion, which separates carbon dioxide from burning coal by turning it into a concentrated, pressurized liquid stream for storage in deep geological formations. Another solution his group is currently working on: combustion systems that burn natural gas and other light fuels in pure oxygen while mitigating the extreme temperatures and instabilities resulting from the lack of nitrogen in the oxidizing stream.
Ghoniem’s group is also working on a novel ion transport membrane system to enable the separation of oxygen from air and the burning of fuel in the same reactor, thereby reducing the energy penalty and complexity of the process. In parallel, they are also exploring solid oxide fuel cells that use heavy hydrocarbons instead of hydrogen to produce electricity directly, while separating the CO2 stream in the products.
Lastly, Ghoniem’s group is looking at the integration of renewable energy with conventional sources to reduce the cost of renewables and improve their dispatchability. First, they are exploring the co-gasification of coal and biomass to produce syngas for use in the production of liquid fuels or electricity, while removing a fraction of CO2. Second, they are researching approaches to integrating concentrated solar energy with fossil fuel heat to power electricity plants while allowing for solar energy storage in a chemical form.