One way to avoid making carbon dioxide is to stop burning fossil fuels and rely on solar energy. But in a 24/7 economy, solar -- and its relative, wind -- are serious slackers. Solar, for example, is at its most intense for six hours a day. And some parts of the globe get far less sunlight than others.
When it comes to storing solar energy for use at night or on cloudy days, hydrogen is a very promising alternative. Hydrogen is the simplest atom, with one proton, one electron and no pesky neutrons. It combines readily with oxygen -- in engines or fuel cells -- to form water.
Richard Peterson, National Renewable Energy Laboratory.
If the hydrogen originates in water, hydrogen energy makes a closed loop: the waste product (water) supplies more fuel (water), with essentially no bothersome pollutants and carbon dioxide.
That would do wonders for the threat of global warming, and strip mining, oil spills and acid rain. But how to make hydrogen? The obvious way is electrolysis -- an electric current in a solution -- which has long been used to separate oxygen and hydrogen in water.
But whence the electricity? If it comes from fossil fuels, we're not reducing carbon dioxide one iota. If it comes from solar cells, the efficiencies are dismal, since you need two devices -- one to make the electricity, and one to separate the water. Each device has its own losses, and each has its own costs.
Doin' the ol' one-step
When incoming sunlight strikes the layer of the device made of gallium indium phosphide, electrons are liberated with the right energy level to release hydrogen in water from its bond with oxygen. Light also strikes the gallium arsenide layer, but since this stuff is unstable in water, it must be protected by epoxy. The light makes electron-accepting "holes" in the gallium arsenide, which are transferred to a platinum electrode in the water.
John Turner, a senior scientist at the National Renewable Energy Laboratory, thinks he may have a better idea. This spring, he and post-doctoral student Oscar Khaselev reported on a one-piece solar cell that generated hydrogen and oxygen instead of electricity.
To do this, the researchers sandwiched two semiconductors and exposed one of them to an acid-water solution. They also had to regulate the electricity output, find semiconductors that wouldn't fall apart in a water solution, and take care of some other details, that, frankly, The Why Files couldn't begin to follow. (If you think you can follow, see "A Record in Converting Photons..." in the bibliography).
Loose ends
But there are a few hang-ups, as could be expected from any one-of-a-kind device. The researchers have only made a prototype, and they didn't even bother collecting the hydrogen. Most important, the semiconductor at the heart of the system is way too expensive. To cut costs, Turner wants to try amorphous (non-crystalline) silicon, like what's being used in cut-rate solar cells.
Solar energy, he points out, is essentially a storage game. "Our vision comes from the fact that solar can supply our energy needs." At off-the-shelf efficiency, solar cells on 10,000 square miles of Nevada desert could supply the nation's 24-hour electric needs, but it would all happen during the six hours of peak daylight. (If you haven't heard, sales of photovoltaic cells are booming.)
"If we really want solar to work, storage is the key," Turner says. And given limitations in today's storage options, storing energy in hydrogen made directly from the sun could prove a real winner. But we won't know until a decade or more is spent developing the little gizmos that so fascinate Turner.
Going, blowing, gone. Is wind the answer to harvesting electricity from the sun?
With no moving parts, no energy input (aside from the sun's rays), and no pollution, the little gizmo converts 12.5 percent of the incoming solar energy into chemical energy in hydrogen. That's far better than existing, two-step solar hydrogen generators.
