|Credit: Joint Center for Artificial Photosynthesis|
By Andrea Lang, Energy Fellow
By far the most abundant source of potential renewable energy is the Sun, which puts out roughly enough energy in one hour to power all human activity on earth for one year, if only we could efficiently harvest it in large enough amounts. Although the most popular way to convert solar radiation into electricity right now is through the use of solar photovoltaics (PV), a good long-term solution may be to use artificial photosynthesis to produce some kind of solar fuel.
Why aren’t photovoltaics up currently up to the challenge?
The growth of solar PV in recent years has been tremendous for getting the United States to start transitioning off of fossil fuels; there is now over 22,700 MW of installed solar capacity in the U.S., or enough to power 4.6 million homes. However, while we have certainly made huge strides in actively converting solar energy into electricity using solar PV, this existing solar technology is not without its problems. First, photovoltaic cells are not very efficient at turning solar energy to electricity. Even with recent advances in efficiency, the very best cells are less than 50% efficient (meaning that less than 50% of the solar energy hitting the panel is actually turned into electrical energy), while most are far less efficient.
|Credit: Lawrence Berkeley National Laboratory|
Second, and perhaps most significantly, solar energy is highly intermittent, or variable. When there are clouds in the sky or it is nighttime, solar PV panels cannot produce electricity. This intermittency presents real problems for grid operators as more solar power comes online, because as solar output declines in the evening hours of the day, more traditional sources of energy must quickly “ramp up” production to compensate for that loss of electricity production (illustrated left in the so-called “duck” curve). It can take a long time to ramp up production at traditional natural gas and coal-fired power plants, so depending on how steep the curve gets (how much solar is put on the grid), it may be difficult for grid operators to manage higher penetrations of solar PV absent new technology. What is truly needed to prevent this issue is the ability to store large amounts of solar-generated electricity at an affordable cost. Finally, largely due to the issues just discussed (intermittency and a lack of affordable large-scale storage), solar is not currently a great option for vehicles.
Why might artificial photosynthesis be a good long-term alternative for harvesting the sun’s energy?
For billions of years, plants and some bacteria on Earth have been harvesting sunlight to make chemical energy through photosynthesis. Essentially, plants use sunlight, water, and carbon dioxide to make glucose (a form of usable and storable chemical energy) and oxygen. Researchers are currently developing methods of artificial photosynthesis that play on this idea and are trying to turn sun energy (called photons) into some other kind of storable solar fuel that could then be used at will to generate electricity or run cars. The most obvious benefit of artificial photosynthesis is that its end product is not direct electrical current as it is for solar PV, but rather a physical product: solar fuel. This would essentially do away with the intermittency problem with solar energy, enabling it to serve as a source of electric power or transportation fuel whenever it is needed. However, more research is needed before artificial photosynthesis technology can be useable.
Fortunately, this technology has been receiving increased attention from policymakers. President Obama even mentioned artificial photosynthesis in his 2011 State of the Union: “At the California Institute of Technology, they’re developing a way to turn sunlight and water into fuel for our cars….We need to get behind this innovation.”And just a few months ago, the U.S. Department of Energy agreed to provide $75 million in funding to the Joint Center for Artificial Photosynthesis (JCAP).
Solar fuels produced through artificial photosynthesis need not displace solar PV, especially since advancements in battery storage technology may eventually reduce the intermittency problem. At the same time, however, artificial photosynthesis is an attractive alternative. The potential for artificial photosynthetic processes to create a storable solar fuel may make it a good long-term solar energy option. Hopefully policymakers will continue to increasingly recognize the potential of this technology so that it can eventually be deployed on a broad scale.