Making clean energy pay off by storing it as squeezed air.
A stumbling block to increasing our reliance on electricity from cleaner energy sources such as solar panels and wind farms has always been figuring out how to efficiently store the energy for use when the wind isn’t blowing and the sun isn’t shining. Danielle Fong could make clean energy significantly more practical on a large scale by introducing a novel way to use tanks of compressed air for energy storage. “It could radically reorient the economics of renewable energy,” she says.
The idea of using compressed air to store energy is not new. Electricity from solar panels or wind turbines can turn a motor that’s used to compress the air in a large tank, and the air pressure can then be converted into power to drive a generator when the power is needed. The problem is that during compression the air reaches temperatures of almost 1,000 °C. That means energy is lost in the form of heat, and storage in conventional steel vessels becomes impractical.
Fong stumbled on a possible solution while skimming through a nearly century-old book: water spray is great at cooling air. She asked, why not spray water into the air while compressing it, so that the air stays cool? To make the process practical, she developed a technique for separating the heated water from the compressed air and diverting the water into a tank, so the heat can be recaptured to minimize energy loss. The process is about as efficient as the best batteries: for every 10 kilowatt-hours of electricity that goes into the system, seven kilowatt-hours can be used when needed.
Fong founded a company called LightSail Energy in Berkeley, California, to develop the technology. Initially, she planned to produce compressed-air-powered scooters. But backer Vinod Khosla of the venture capital firm Khosla Ventures convinced her to go after the much bigger market of electricity for the power grid.
Batteries are the current state of the art in storing excess wind and solar energy, but Fong says the LightSail system will cost less to purchase and will last for a decade or more. Over the long term, she says, the system could cost as little as one-tenth as much to own and operate as batteries do. A single system, which is about the size of a shipping container plus a car-size unit, will store the energy generated by a one-megawatt wind turbine running for three hours.
Fong and the LightSail team had to come up with a filtering system capable of separating the water from the highly compressed air. Another challenge was to design a system that could handle both compressing the air and expanding it to drive a generator; previous efforts have required two separate systems.
Not only did LightSail meet those challenges, but it managed to find a compound—the company won’t provide details—that can be used more efficiently than steel to make compressed-air storage tanks. Tanks made from this material also don’t need the costly underground installation that’s normally required. And unlike standard systems, LightSail’s doesn’t need the turbine to run at a fairly constant speed to get efficient compression, meaning it is better able to cope with intermittent wind conditions.
Fong says there are no technical barriers to building units large enough to power entire cities. The company plans to manufacture the systems, and she says several renewable-energy developers have already signed on as customers. The first pilot unit is scheduled to ship in late 2013 or 2014—but she is still hoping to see those compressed-air scooters.
Using cell phones to negotiate energy-efficient settings in office buildings.
Office towers and commercial buildings account for nearly one-fifth of all energy consumed in the United States. Burcin Becerik-Gerber has found a cheap way to cut a building’s energy use by a third.
Today’s smart buildings can be programmed to default to energy-thrifty measures, such as turning down the heat or air-conditioning and turning off unnecessary lights—but occupants often just crank everything back up, or even work against the system by plugging in space heaters or opening windows. An assistant professor of civil and environmental engineering at the University of Southern California, Becerik-Gerber has come up with a way to save energy by essentially getting buildings to “negotiate” with their occupants, factoring in the perceptions and desires of each.
The system uses occupants’ smartphones to open up a line of communication. Becerik-Gerber worked with colleagues in social psychology and computer science to design an app that asks people how satisfied they are with the work environment’s current temperature, lighting, air quality, and even noise level. System software then fashions each user’s consumption patterns and preferences into a virtual “agent” that resides in his or her smartphone. “The agent works for you and tries to look after you,” she explains.
The system then works with all the building’s agents to find the most energy-efficient way of adjusting the settings so as to make the greatest number of people happy. To improve the results, it asks those users demanding more energy-intensive conditions if they’d be willing to compromise a bit, and it tells them what the resulting energy savings would be. “If people understand the consequences, they’re more tolerant,” says Becerik-Gerber. The optimized settings are then put in place and monitored automatically.
Finding an optimal solution for as few as five occupants is difficult. Finding a way to coördinate the preferences of hundreds was massively challenging. The problem is especially acute in today’s popular open-plan offices: people with very different preferences often share space, typically guaranteeing that most of them will be unhappy with the environmental settings. But Becerik-Gerber’s simulations indicate that her algorithms could satisfy some 70 percent of occupants—while reducing overall energy consumption by more than 30 percent.
Improving demand forecasting for electric power to save fuel and reduce emissions.
PROBLEM: Many power plants connected to the grid operate well below their full capacity, wasting fuel. If we have no means to store large amounts of electricity or reliably predict power demand, however, maintaining idle capacity is the only way to respond quickly to surges in demand. The problem is particularly challenging in China, a huge consumer of electricity. Its push to add thousands of wind turbines, with their variable, difficult-to-predict output, will make it even harder to efficiently balance supply and demand.
SOLUTION: Software from electrical engineer Qixin Chen of Tsinghua University in China accurately forecasts power demand and helps utilities coördinate power plants. His software is already in use in nearly 200 cities and 10 provinces in China. One province, he says, reported saving $30 million and 240,000 tons of coal in a single year.
Chen found two ways to improve on existing demand-forecasting software. First, he designed the system to better choose the right forecasting approach for particular areas; differences in demand and weather patterns mean that some techniques are much better suited to some locations than others. Then he enabled his system to analyze its own previous prediction errors and adjust its formulas so as to minimize the errors the next time similar conditions occur. The resulting demand forecasts are reliable a month ahead. Other forecasting systems, in contrast, aren’t sufficiently accurate beyond a day or two, if that.
The results are helping utilities dole out electricity more efficiently. Now Chen is working to adapt his forecasting software to predict the power output of wind turbines. His system would take into account wind data gathered for miles around the turbines, providing a sharper picture of which wind shifts are likely to affect them in the coming hours. That means utilities can know when to expect more power from the turbines so they can cut back on conventional power generation.
Pulling hydrogen out of water with the help of concentrated sunlight and an inexpensive material.
While doing his doctoral studies at Caltech, William Chueh showed that heat from the sun can turn cerium oxide—a relatively cheap material—into an effective catalyst for splitting water to yield hydrogen that can be used to make fuel. Most other hydrogen extraction processes rely on expensive catalysts made from precious metals such as platinum. “There’s simply not enough of those metals to make a dent in our fuel needs,” says Chueh, who is now a materials scientist at Stanford University.
His process relies on mirrors of the type that some solar plants use to concentrate sunlight by a factor of 1,500. The sunlight heats the cerium oxide to 1,500 °C, driving out its oxygen. As the cerium oxide cools, steam is fed to it, which then gives up its oxygen to the oxygen-starved material, freeing hydrogen gas. The hydrogen can be collected, and the cerium oxide can be reheated to repeat the process.
Chueh has used the same process to split carbon dioxide. The resulting carbon monoxide can be combined with the generated hydrogen to make hydrocarbon fuel such as methane—a renewable alternative to extracting it from the earth. The technique generates about 100 times more carbon monoxide than previous processes for a given amount of energy.
Chueh’s idea is to use his catalyst in combination with the type of large solar concentrators now used in power generation. Meanwhile, he’s working to make cerium oxide–based hydrogen generation work at lower temperatures, because the only containers that can hold the material at 1,500 °C without melting are made of exotic alloys that cost too much. He’s already developed a hybrid of cerium oxide and another material that shows the potential to work at 500 °C, which would allow the use of stainless-steel vessels.
Making engines super-efficient by getting them to run at extremely high pressures.
It’s hard to radically improve the internal-combustion engine. But Shannon Miller may have done it, by getting one to work at extremely high compression and expansion ratios. Initially designed to generate electricity in homes or businesses, not to power cars, Miller’s engines use 25 percent less fuel than conventional gas-powered generators.
Miller knew that operating engines at high compression and expansion ratios could make them far more efficient, but that’s easier said than done. High compression ratios create extreme temperatures, wasting energy. And high pressure increases friction between the piston and the cylinder.
So she turned to a “free-piston” design, an old idea that allows each piston to bounce up and down independently of any rod or crankshaft. The approach had not been used to operate pistons at very high compression ratios. “To make this work, you can’t just change one or two things,” she says. “You really need to change the whole architecture of the engine.”
Miller cofounded and is CEO of a company called EtaGen, which aims to bring the engine to market. The company has built a prototype that runs for hours at target performance levels. She says the results indicate that upcoming versions of the engine should be about as efficient as large power plants—the current gold standard for energy efficiency—once the energy the plants lose during distribution is factored in.
EtaGen’s first product will be a replacement for conventional diesel and natural-gas generators, allowing businesses to operate a building off the grid or to ride through power outages. Eventually, Miller says, the same basic engine design could be used to make onboard generators for electric cars like GM’s Volt. In either case, the engines would run on common fuels like diesel and natural gas.