Here comes the sun
THE EARTH is bathed in a constant stream of energy from the sun. This energy is responsible for moving gigantic masses of air and water around in weather systems and ocean currents, and it provides sustenance (directly or indirectly) for all life on the planet. It is so plentiful that the sunlight hitting Earth in a single hour contains more energy than humans use in a year.
Despite the amount of energy falling out of the sky, human society is still powered mostly by the burning of fossil fuels, which causes immediate environmental damage and dangerous long-term global climate change. Although we have made progress in capturing solar energy, it still meets less than one per cent of global energy consumption.
Ted Sargent thinks we can change that.
“Nature has already solved that problem with photosynthesis,” Sargent says. “Trees and plants and photosynthetic bacteria are incredibly abundant. They construct themselves using solar energy. They repair themselves using solar energy. And they capture about 10 times more energy through photosynthesis every day than humanity consumes.”
Sargent is an electrical engineer who studies photovoltaics at the University of Toronto. He is heading up a new program called Bio-inspired Solar Energy that is entering its startup phase at CIFAR. The program will bring together plant biologists, physicists, chemists, materials scientists and engineers who will apply the lessons of biology to solar energy technology. The goal is to create technology that will allow us to produce enough clean, cheap, carbon-neutral energy to power economic growth without damage to the environment.
“Creating sustainable energy technologies—like solar cells and strategies to generate energy from the sun and store it as fuel—is an incredibly important goal. People have been working on these problems for decades. There’s been progress, but there’s still a need for a breakthrough. We need to understand how to capture the sun’s energy on an incredibly massive scale,” Sargent says.
“This group within CIFAR seeks to bring together those two communities—the photosynthesis community and the solar energy harvesting community—and have them learn from each other and create breakthroughs from that learning.”
A matter of scale
Today, the human population of 7.1 billion people consumes 15 terawatts (trillion watts) of energy every year. By 2050 that amount is projected to double to 30 terawatts as the population grows to 9.6 billion and the standard of living rises. Today, fossil fuels are the source of 85 per cent of our energy production.
There is a broad scientific consensus that fossil fuel use is changing our climate. It’s also clear that demands for energy will continue to increase. One way or another, we’ll need to develop other sources of energy. Fortunately, we are surrounded by enough solar energy to more than meet our needs. On average, 100,000 terawatts of energy reach the Earth’s surface. Just a fraction of that would more than meet our needs—if we could harness it.
Almost all solar energy we use today is captured by solar panels made of silicon semiconductors, the same material used in computer chips. When photons hit a silicon panel, they knock loose electrons, which flow in an electric current that is harnessed to generate power. Conventional solar cells are reasonably efficient, the best converting about 25 per cent of the sunlight that reach them it into energy.
But solar cells have drawbacks. Despite tremendous advances in recent decades, they are still relatively expensive to produce and install, they degrade over time and they produce electricity only when the sun is shining. Taken together, these drawbacks leave a lot of room for improving solar technology.
“Silicon has made huge progress—so much that it’s starting to approach its fundamental limit. The next generation of solar technology will have to become more efficient. There’s still the need for a breakthrough. We need to understand how to capture solar energy on a gigantic scale,” Sargent says.
Luckily, nature has been figuring that out since about 3.4 billion years ago, when single-cell organisms first began to use the energy from the sun. Today, Earth is covered by (and the oceans are full of) plants, algae and photosynthetic bacteria that have become very good at using sunlight.
“Biology is a much more sophisticated machine than the devices we engineer,” says Gregory Scholes, a chemist at Princeton University and a member of the program. “It has feedback and control loops that determine how to change the photosynthetic machinery so that it works optimally every minute of the day. What we can learn from this is different ways of thinking about how you do engineering.”
Even if you’re not a biologist, you probably remember the basics of photosynthesis from high school. Photosynthetic organisms use energy from the sun to turn carbon dioxide and water into glucose and oxygen. But hidden inside that simple description is an amazingly complex series of closely coordinated chemical and physical reactions designed to complete the process as efficiently as possible (see illustration page 24).
Harvesting the light
Researchers are especially interested in learning how photosynthesis has optimized the light-harvesting process, capturing the energy from the photon and converting it into energy that can do useful work. “Plants and algae are amazing at light harvesting. They have things down to a fine art,” Scholes says.
At the heart of photosynthesis in green plants and algae is chlorophyll. Chlorophyll molecules absorb the energy of the photon and convert it into electronic energy that can then be used to synthesize sugars.
One way the process is optimized is by organizing some of the chlorophyll molecules into arrays of antennae. Hundreds of these antennae work together to capture photons, shuttling the energy to a reaction centre where chemical conversion takes place.
The antennae enhance the effective cross-section of the reaction centre and increase its ability to harvest light by a factor of 100. They are especially important for allowing organisms to thrive in low-light conditions—for instance, under the canopy formed by larger plants or deep in the ocean.
Nature even seems to have harnessed quantum effects to make photosynthesis more efficient. In non-quantum terms, we usually think about the energy from the photon being passed along in discrete “hops” – energy from the captured photon bumps an electron into an excited state, and that excited state is passed along a chain of molecules until it hits the reaction centre. How efficiently this process works depends partly on the wavelength of the energy being passed on, and on how well-tuned each molecule is to accept that wavelength.
Researchers now think that the molecular network makes use of a quantum effect called superposition. While the energy from the electron is in a superposition, it can be thought of as being many different wavelengths at once. The network “chooses” which wavelength will be transferred most efficiently, and then the energy “collapses” to that specific wavelength. This quantum effect may help account for the tremendous speed and efficiency with which chlorophyll passes on the energy it captures.
Fuel from the sun
The details of antenna arrays and quantum effects are just two of the many things that researchers think can help them build better light-harvesting techniques. But there’s another problem they also have to tackle: how to store the energy for later use.
“Storage is really fundamental,” Sargent says. “The fact that the sun shines just about eight to 12 hours a day is something that can only be solved by buffering energy, by finding a way to store it and use it overnight.”
Batteries are one possibility, but even the best are still relatively bulky and expensive, says Curtis Berlinguette, a chemist at the University of British Columbia and a member of the program. “You really need solar fuels.”
A solar fuel solution takes the energy from the sunlight and stores it in a chemical form that can later be converted back to electricity or some other source of power.
One way of doing this is to use solar-generated electricity to split water into hydrogen and oxygen, which are stored away separately. Later, using a fuel cell, the oxygen and hydrogen are recombined into water, releasing electrons that can be captured in an electric current. A system that used about four litres of water and fit in the space of a beer fridge could power a typical family home.
One of the problems right now is that splitting the water into oxygen and hydrogen requires catalysts made of rare and expensive materials.
One of Berlinguette’s interests is designing better catalysts. With his colleague Simon Trudel, he has developed a process that makes efficient catalysts out of normal mixed metal oxides—essentially, rusts—that are as good as catalysts that cost 1,000 times as much to make.
Berlinguette and others are also interested in methods that would bypass the conversion into electricity, instead converting the energy from excited electrons directly to fuel—a process analogous to the way leaves turn photons into sugars. A system like that would be more cost-effective than having to design and build separate devices for generating electricity and creating hydrogen.
All of these lessons are likely to be useful in designing new generations of “thin film” photovoltaics. Unlike the silicon solar cells we’re most familiar with, thin film technologies often look more like a flexible sheet of plastic and can be applied to glass or some other hard surface.
“It’s with these materials that we have a tremendous amount to learn from nature,” Sargent says. “Nature has obviously mastered using available organic materials for solar absorption and for shuttling energy around in the photosynthetic apparatus.”
Although there are many different thin film technologies, one of the most exciting is the dye-sensitized solar cell pioneered by Michael Grätzel, a professor at the École Polytechnique Fédérale de Lausanne and an advisor to the program. This thin film technology makes use of a dye molecule that captures and transfers the energy from the photon in a way that’s analogous to how chlorophyll captures energy in a leaf. It is thin, flexible and potentially inexpensive; it could soon generate electricity as cheaply as fossil fuels do today.
Better than nature
Fully developed, better solar harvesting and fuel storage technologies could change the world. Thin films plastered to existing surfaces could feed vastly increased solar capture. Solar farms would have the capacity to store fuels during the day and feed them to the grid at night. Homes and office buildings could become largely energy self-sufficient. Developing countries could bypass the phase of massive power plants and energy grids, developing clean and local energy solutions instead.
The Bio-inspired Solar Energy program was started thanks to CIFAR’s Global Call for Ideas, launched in April 2013. The call sought proposals that asked foundational questions and were bold, ambitious and complex enough to require sustained collaboration from an outstanding network of interdisciplinary researchers. From the 260 initial proposals, CIFAR selected four to go ahead (see sidebar on the facing page for a summary of the other three).
Bio-inspired Solar Energy is now in its startup phase. An advisory board has been chosen, and members will hold their first meeting later this year. Investigations are under way to understand how the team will engage with key industrial stakeholders.
The CIFAR program will not only lead to advances in individual questions of science and technology, it will also create a new way of thinking and talking that will allow researchers from vastly different disciplines to have meaningful collaborations. With its experience creating global multidisciplinary networks, CIFAR is uniquely well positioned to create the new program. And through industrial partnerships, the program will be able to directly influence the direction of new solar technology.
“The ultimate deliverable of the program will be to use insights from nature to produce more efficient, cost-effective, robust, longer-lasting or self-repairing systems on a mass scale for energy capture and storage,” Sargent says.
“What this program offers is the potential to dramatically enhance what we can do in human-made energy capture processes: to first approach the efficiency of nature and then beat it. Even nature’s not that close to what’s possible in principle from a thermodynamic perspective,” he says.
“We can take insights from nature and use them to make human-generated solar energy better. We can make them more robust, longer-lasting and self-repairing.”
“This isn’t a question of emulating nature. We actually think we can do better.” •
Kurt Kleiner is the managing editor of Reach magazine.
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