The Dark Side of Solar
Sunlight is the world’s largest potential source of renewable energy, but that potential could easily go unrealized. Not only do solar panels not work at night, but daytime production waxes and wanes as clouds pass overhead. That’s why today most solar panels–both those in solar farms built by utilities and those mounted on the roofs of houses and businesses–are connected to the electrical grid. During sunny days, when solar panels are operating at peak capacity, homeowners and companies can sell their excess power to utilities. But they generally have to rely on the grid at night, or when clouds shade the panels.
This system works only because solar power makes such a tiny contribution to overall electricity production: it meets a small fraction of 1 percent of total demand in the United States. As the contribution of solar power grows, its unreliability will become an increasingly serious problem.
If solar power grows enough to provide as little as 10 percent of total electricity, utilities will need to decide what to do when clouds move in during times of peak demand, says Ryan Wiser, a research scientist who studies electricity markets at Lawrence Berkeley National Laboratory in Berkeley, CA. Either utilities will need to operate extra natural-gas plants that can quickly ramp up to compensate for the lost power, or they’ll need to invest in energy storage. The first option is currently cheaper, Wiser says: “Electrical storage is just too expensive.”
But if we count on solar energy for more than about 20 percent of total electricity, he says, it will start to contribute to what’s called base load power, the amount of power necessary to meet minimum demand. And base load power (which is now supplied mostly by coal-fired plants) must be provided at a relatively constant rate. Solar energy can’t be harnessed for this purpose unless it can be stored on a large scale for use 24 hours a day, in good weather and bad.
In short, for solar to become a primary source of electricity, vast amounts of affordable storage will be needed. And today’s options for storing electricity just aren’t practical on a large enough scale, says Nathan Lewis, a professor of chemistry at Caltech. Take one of the least expensive methods: using electricity to pump water uphill and then running the water through a turbine to generate electricity later on. One kilogram of water pumped up 100 meters stores about a kilojoule of energy. In comparison, a kilogram of gasoline stores about 45,000 kilojoules. Storing enough energy this way would require massive dams and huge reservoirs that would be emptied and filled every day. And try finding enough water for that in places such as Arizona and Nevada, where sunlight is particularly abundant.
Batteries, meanwhile, are expensive: they could add $10,000 to the cost of a typical home solar system. And although they’re improving, they still store far less energy than fuels such as gasoline and hydrogen store in the form of chemical bonds. The best batteries store about 300 watt-hours of energy per kilogram, Lewis says, while gasoline stores 13,000 watt-hours per kilogram. “The numbers make it obvious that chemical fuels are the only energy-dense way to obtain massive energy storage,” Lewis says. Of those fuels, not only is hydrogen potentially cleaner than gasoline, but by weight it stores much more energy–about three times as much, though it takes up more space because it’s a gas.
The challenge lies in using energy from the sun to make such fuels cheaply and efficiently. This is where Nocera’s efforts to mimic photosynthesis come in.