Solar Energy: More than Just Electricity
Most of us, when we imagine solar energy supplying all our energy needs, probably think of electricity as the energy carrier. But that is certainly not the only (and may not be the best) way to use the inexpressively enormous quantity of energy that arrives daily from the sun. For starters, consider the quality and uniqueness of solar energy. The sunlight that reaches earth comes from an energy source whose thermodynamic temperature is 5000-6000˚C. When it arrives, that energy has spread out, but it can still be “reconstituted” to provide very high temperatures to power energy conversion processes. How high is high? Aluminum melts at 660˚C; magma flow temperatures are between 700 and 1600˚C; a flame can reach temperatures between 1000 and 2000˚C and industrial electric arc-furnace processes are typically in the 2000˚C range. Concentrated solar energy can provide temperatures of 3400˚C; the highest (usable) temperature achievable on Earth. At these temperatures, we should be able to convert solar energy to an energy carrier at about 90% efficiency.
The challenge to relying on solar energy is shown below: the sunniest areas are unevenly distributed; and of course the sun sets every day. Both of these difficulties can be solved by converting solar energy into solar fuels that can be easily stored and transported to the industrialized population centers where most people live.
Figure 1. Areas of the Earth that receive the most solar energy per year.
One example of a solar fuels process is hydrogen production. High-temperature solar energy is used to remove the oxygen from metals that can split water to form hydrogen. The end product after water-splitting is the original metal oxide, which can be returned to the solar reactor to create a completely closed cycle. Zinc has been of interest because it can also be used to generate electricity in zinc-air batteries, a promising emerging technology for vehicles. A hybrid process that used carbon to reduce zinc oxide to form zinc in a solar reactor was studied to pilot scale, and the resulting zinc powder was incorporated in commercial zinc-air batteries for testing.
Figure 2. Closed-loop chemical heat-pipe using methane reforming with carbon dioxide to produce synthesis gas. Ammonia dissociation and synthesis has also been extensively studied to provide solar energy around the clock.
Another type of solar fuel system called the closed-loop chemical heat pipe can provide heat to a remote location without any release of climate-altering gases. Highly-concentrated solar energy is used to react methane and carbon dioxide to form hydrogen and carbon monoxide, a mixture called synthesis gas because it may be used as the starting point for the formation of virtually any hydrocarbon fuel. This mixture can be stored at ambient temperatures and transported to wherever a customer needs energy. By the reverse reaction, stored solar energy is released to provide industrial process heat, or heat for electricity generation. A closed-loop 0.5 MW system was extensively tested in 1999.
This type of system can also be used in an open loop, where the product fuel is “solar-upgraded” to a higher energy content using solar energy. For example, the reaction of biomass with water forms synthesis gas. Here, the CO2 released when the fuel is burned is reduced substantially; typically by 30 percent or more. Systems that use solar energy to convert natural gas to hydrogen and solid carbon black have been extensively studied, as have solar biomass, coal and coke gasification.
Someday we may truly power the earth on its two most abundant resources: sunlight and water!
 Visualize an area less than 5% of the Sahara desert: 490 X 490 km. Even at a measly conversion efficiency of 20%, this area receives enough energy from the sun to supply ALL of the energy needs of humanity for an entire year.
 OK-a nuclear bomb produces higher temperatures, but let’s not go there….