Bring me sunshine

By Mark Venables

With the spotlight on renewable energy, the hunt is on to find winning technologies. We look at three solar power projects from MIT.

Ask any scientist to name Earth's most abundant source of energy, and the answer comes quickly: sunlight. In one hour, the Sun strikes Earth with enough energy to power the entire planet for a year.

“There’s nothing that compares to the Sun. Everything else pales in comparison,” Daniel Nocera, the Henry Dreyfus Professor of Energy in MIT’s Department of Chemistry, says.

With gas and oil prices at all-time highs, it’s only logical for scientists to try to harness some of that solar energy. Three projects at America’s MIT are leading research into the area, looking at mimicking photosynthesis, producing a cost-efficient solar power system and finally designs for flexible photovoltaic materials that may change the way buildings receive and distribute energy.

Profiting from Plants

Three MIT researchers are building a device that mimics photosynthesis – the process plants use to capture and store the Sun’s vast energy.

“To turn sunlight into fuel, that’s a chemistry process,” Jonas Peters, MIT’s Keck Professor of Energy and Chemistry, says. “Nature has come up with an elaborate chemical solution, and it looks like we’re going to need an elaborate chemical solution as well, and we need to do it efficiently.”

MIT chemists, including Nocera, Peters and Christopher Cummins, are part of a National Science Foundation-funded solar project in collaboration with Caltech. The researchers are also part of MIT’s recently announced Solar Revolution Project (SRP), which aims to transform solar power into an affordable, mainstream energy solution in the near future.

Professor Ernest Moniz, director of the MIT Energy Initiative (MITEI), says: “Climate change makes the search for more environmentally-benign sources of energy urgent and hugely important. Many experts have concluded that solar energy is a key, if not the key answer to our global energy challenges in the long-term.”

Much more chemical research will be needed to make solar energy technologically and economically viable. When plants photosynthesise, they produce high-energy sugars such as glucose; the chemists aim to produce hydrogen fuel or hydrocarbons such as methanol.

When sunlight strikes the artificial photosynthesis  device, high-energy photons will split water into hydrogen and oxygen. One of the researchers’ biggest challenges is developing inexpensive catalysts that can split water efficiently. Platinum does the job, but it is very rare and expensive, so the researchers are focusing on more abundant metals, such as iron, cobalt, nickel and manganese.

“There are a lot of different approaches that we’re exploring. We’re not putting all of our eggs in one basket,” Cummins explains.

Powering the future

Once water molecules are split into hydrogen and oxygen, the hydrogen can power fuel cells. In Nocera’s dream scenario, within 10 or 15 years houses will have solar panels on the roof that yield hydrogen to power the house or recharge an electric car. “It opens up a future where I see a house that will be its own power station,” Nocera adds.

For solar energy to have a very significant impact on world energy use, it must also yield a liquid fuel that can power cars and other vehicles. To achieve that, hydrogen fuel could be further processed into hydrocarbons such as methanol.

“We want to really emulate photosynthesis, and not just split water into protons and electrons, but turn hydrogen into the chemical currency of liquid fuel,” Peters says

Carbon dioxide can be added to hydrogen to generate hydrocarbon fuels and then released as it's burned, so the entire process is carbon-neutral, notes Peters.

All of the researchers are committed to making solar power a reality, because, as they say, there is no other choice. “We’re never going to get it if we don’t try. We must throw all our resources at this problem because it’s so important to the survival of our habitat on this planet,” Cummins concludes.

Efficient Solar Power

If solar power is to become more widely used the efficiency will certainly need to be increased. And that is the aim of another team of MIT students who have successfully tested a prototype of what may be the most cost-efficient solar power system in the world.

The system consists of a 12-foot-wide mirrored dish that team members have spent several weeks assembling. The dish, made from a lightweight frame of thin, inexpensive aluminium tubing and strips of mirror, concentrates sunlight by a factor of 1,000 – creating heat so intense it could melt a bar of steel.

To demonstrate the system’s power, Spencer Ahrens stood in a grassy field on the edge of the campus holding a long plank. Slowly, he eased it into position in front of the dish. Almost instantly there was a big puff of smoke, and flames erupted from the wood.

Burning sticks is not what this dish is really for, of course. Attached to the end of a 12-foot-long aluminium tube rising from the centre of the dish is a black-painted coil of tubing that has water running through it. When the dish is pointing directly at the Sun, the water in the coil flashes immediately into steam.

Someday soon, Ahrens hopes the company he and his team-mates have founded, called RawSolar, will produce such dishes by the thousands. They could be set up in huge arrays to provide steam for industrial processing, or for heating and cooling buildings, as well as to hook up to steam turbines and generate electricity. Once in mass production, such arrays should pay for themselves within a couple of years with the energy they produce.

“This is actually the most efficient solar collector in existence, and it was just completed,” says Doug Wood, an inventor based in Washington state who patented key parts of the dish, the rights to which he has signed over to the student team.

Wood credits the students, who built this dish as an independent project that started in January, with making significant improvements to his original design to make it a practical and competitive energy producer. “They really have simplified this and made it user-friendly,” he says.

One of the keys to making an inexpensive design was something Wood discovered by accident as he built a variety of solar dishes over the years: smaller really is better. Unlike many technologies where economies of scale dictate large sizes, a smaller dish requires so much less support structure that it ends up costing only a third as much for a given collecting area.

Solar Textiles

Sheila Kennedy, an expert in the integration of solar cell technology in architecture, creates designs for flexible photovoltaic materials that may change the way buildings receive and distribute energy.

These new materials, known as solar textiles, work like the now-familiar photovoltaic cells in solar panels. Made of semiconductor materials, they absorb sunlight and convert it into electricity.

Kennedy uses 3D modelling software to design with solar textiles, generating membrane-like surfaces that can become energy-efficient cladding for roofs or walls. Solar textiles may also be draped like curtains.

“Surfaces that define space can also be producers of energy,” Kennedy, a visiting lecturer in architecture at NIT, says. “The boundaries between traditional walls and utilities are shifting.”

Kennedy, for her part, will pursue her research in pushing the envelope of energy-efficiency and architecture.

A recent project, ‘Soft House’, exhibited at the Vitra Design Museum in Essen, Germany, illustrates what Kennedy means when she says the boundaries between walls and utilities are changing.

For Soft House, Kennedy transformed household curtains into mobile, flexible energy-harvesting surfaces with integrated solid-state lighting. Soft House curtains move to follow the Sun and can generate up to 16,000 watt-hours of electricity – more than half the daily power needs of an average American household.

Although full-scale Soft House prototypes were successfully developed, the project points to a challenge energy innovators and other inventors face, Kennedy says. “Emerging technologies tend to under-perform compared with dominant mainstream technologies.”

For example, organic photovoltaics, an emergent solar nano-technology used by the Soft House design team, are currently less efficient than glass-based solar technologies, Kennedy says.

But that lower efficiency need not be an insurmountable roadblock to the marketplace, Kennedy says, because Soft House provides an actual application of the unique material advantages of solar nano-technologies without having to compete with the centralised grid.

Which brings her back to the hands-on, prototype-building approach Kennedy hopes to draw from in her teaching and work at MIT.

“Working prototypes are a very important demonstration tool for showing people that there are whole new ways to think about energy,” she says.