Solar thermal systems use mirrors to focus sunlight, generating temperatures high enough to produce steam to drive a turbine. One of the advantages of the solar thermal approach, versus conventional photovoltaics that convert sunlight directly into electricity, is that heat can be stored cheaply and used when needed to generate electricity. In all solar thermal plants, some heat is stored in the fluids circulating through the system. This evens out any short fluctuations in sunlight and lets the plant generate electricity for some time after the sun goes down. But adding storage systems would let the plant ride out longer periods of cloud cover and generate power well into, or even throughout, the night. Such long-term storage could be needed if solar is to provide a large share of the total power supply.
BrightSource Energy has become the latest solar thermal power company to develop a system for generating power when the sun isn’t shining. The company says the technology can lower the cost of solar power and make it more reliable, helping it compete with conventional sources of electricity.
BrightSource is using a variation on an approach to storage that’s a decade old: heating up a molten salt—typically, a combination of sodium and potassium nitride—and then storing it in a tank. To generate electricity, the molten salt is pumped through a heat exchanger to generate steam. BrightSource CEO John Woolard says one big factor in making this technology economically attractive is the use of power towers—in which mirrors focus sunlight on a central tower—that generate higher temperatures than other solar thermal designs. That higher temperature makes it possible to store more energy using a smaller amount of molten salt. “It’s a much more efficient system and much more cost effective, overall”
The alarming rise of carbon dioxide in the atmosphere has led a numerous proposals on how to capture and store CO2 in order to mitigate the damaging emissions from fossil fuels. Popular proposals, some already being tested on a large scale, involve carbon sequestration and subsequent storage in geological formations (geo-sequestration). Other ideas revolve around recycling captured carbon dioxide, for instance by converting it into hydrocarbons that can be re-used to make fuel or plastics. While these solutions would result in removing some carbon dioxide from the atmosphere, their disadvantages are that most of them are expensive, technologically challenging, or energy-intensive.
Researchers have now presented the first experimental evidence of a new solar conversion process, combining electronic and chemical pathways, for carbon dioxide capture in what could become a revolutionary approach to remove and recycle CO2 from the atmosphere on a large scale. Rather than trying to sequester or hide away excess carbon dioxide, this new method allows it to be stored as solid carbon or converted in useful products ranging from plastics to synthetic jet fuel.
“The STEP (Solar Thermal Electrochemical Photo) process proactively converts anthropogenic carbon dioxide generated in burning fossil fuels, as well as eliminates carbon dioxide emissions associated with the generation of metals and bleach,” Stuart Licht, a professor in the Department of Chemistry and Solar Institute at George Washington University, explains to Nanowerk. “Our new STEP carbon capture process is the culmination of over 20 years of ongoing research, starting with developing solar energy to drive chemical, rather than only electronic, energy (“A description of energy conversion in photoelectrochemical solar cells”). In 2003, we set the theoretical basis that solar visible and solar thermal sunlight will provide a synergistic enhancement of solar energy conversion efficiency, and in 2009 the theoretical basis for STEP carbon capture (“STEP Generation of Energetic Molecules: A Solar Chemical Process to End Anthropogenic Global Warming”).”