Sustainable Energy

A Solution to Solar Power Intermittency

Converting methane to an alternative fuel using energy from the sun could reduce carbon-dioxide emissions.

The scale of solar is constrained by its intermittency.

Burning natural gas emits about half as much carbon dioxide as burning coal, but it still produces large amounts of the greenhouse gas in the atmosphere. A novel device being developed at the Pacific Northwest National Laboratory (PNNL) could reduce those emissions by 20 percent by using heat from the sun to convert natural gas to an alternative fuel called syngas, a lower carbon fuel.

mirrored parabolic dish
Solar fuel: A parabolic dish concentrates sunlight in a prototype device that could be used to upgrade natural gas.

The process avoids the intermittency problem of solar panels, whose output depends on the weather and the time of day. The fuel produced by the new device can be stored and used whenever it’s needed to generate a steady supply of electricity.

The researchers’ goal is to use the device to produce electricity at six cents per kilowatt-hour of electricity, which is competitive with fossil fuels.

The device uses a parabolic dish to concentrate light from the sun, producing heat. That heat provides the energy needed to transform methane into syngas, which is a mix of carbon monoxide and hydrogen. Water is heated up to produce steam, which is then used in a process called steam reforming, which is used in oil refining and other industrial processes. During steam reforming, steam reacts with methane, the main component of natural gas, to form syngas.

Syngas can be burned in natural gas power plants to generate electricity. The syngas can also be processed to make liquid fuels such as diesel—it’s easier to convert syngas to liquid fuel than it is to convert methane. This could be useful for decreasing oil imports, but the carbon emission benefits aren’t clear. The researchers haven’t yet determined whether the technology will reduce carbon emissions compared to petroleum, in part because carbon dioxide is emitted during the process of converting syngas to liquid fuel.

While it doesn’t completely eliminate carbon dioxide emissions, it’s closer to being practical than some other approaches of using solar power to generate fuels, says James Miller, the head of the Sunshine to Petrol program at Sandia National Laboratory, who isn’t involved in the work at PNNL. Steam reforming is an established process that doesn’t require any breakthroughs in catalysts or materials. “[The PNNL] technology is attractive because it’s something that could potentially be done in the near term to move forward in reducing carbon emissions,” he says.

In contrast, Miller is developing technology that would convert carbon dioxide and water to diesel fuel, which requires much more energy and new materials, and will take longer to develop (see “Turning Carbon Dioxide into Fuel” and “Demonstrating a CO2 Recycler”).

An alternative to the new technology is to use steam from concentrated sunlight to directly produce electricity via steam turbines. In such a system, heat can be stored by heating up molten salts, which can be used to generate steam after the sun goes down (see “Cheap Solar Power at Night”). A potential advantage of the new approach is that the syngas could be used to generate either electricity or liquid fuels, depending on which proves to be more economical, says Robert Wegeng, who heads the work at PNNL.

The process is more efficient than producing electricity from solar panels. The first prototype stores 63 percent of the energy in the sunlight that hits it in the form of chemical energy. Burning the fuel in a power plant will waste about half of that energy as heat, bringing the overall efficiency down to about 30 percent, but that’s still double the efficiency of typical solar panels. Storing solar energy in the form of chemical bonds in syngas could prove cheaper than storing that energy in batteries, at least in the near future.

Yet the system in its current form is still likely to be too expensive to compete with fossil fuels. To lower costs, the researchers are working to improve the efficiency still more, which would increase the amount of fuel each device could make, so fewer would be needed. They are starting by increasing the temperature at which it operates, which requires improving the materials the device is made of. The researchers are also working to improve the performance of a system that preheats incoming fuel—a heat exchanging system that flows the methane through microscopic channels.

Improvements are also being made to the system for concentrating the sun, with the goal of making it cheaper to manufacture. This is being done by a spinoff company called Solar Thermochemical. It’s developing both parabolic dishes, like those used in the first prototype, as well large fields of mirrors that focus light on a central tower, such as the solar thermal plants being built at Ivanpah by BrightSource Energy (see “BrightSource Pushes Ahead on Another Massive Solar Thermal Plant”).

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