Title: It’s Electrifying!
Author: Pete Varney
Publication: The Outcrop, October 2012, p. 14-15
Last month I finished by saying there are other ways to get to electrical power besides nuclear, coal, hydro and petroleum. If we are talking about large scale applications, let’s talk about solar…
The idea of solar power has been around for a long time, possibly 3.8 billion years – it’s what photosynthesis is all about. Photosynthesis is, of course, a process by which plants make tissues out of sunlight, water and carbon dioxide. Plant tissues can include fats, waxes, oils, cellulose, lignins, tannins and other compounds but, in any event, they are composed of C, H and O with N, P and S as primary auxiliary chemicals. On death, plant materials readily oxidize, but preserving plants in an oxygen free environment allows coal to form and hydrocarbon maturation to take place. It’s really all about sunlight – even when the precursor compound kerogen forms, you have stored solar energy.
Petroleum and coal companies are basically in the solar energy industry, though they don’t commonly think of themselves as such.
In the last few years, researchers have been investigating bacteria and algae to convert solar energy directly into liquid fuels that could power vehicles and be the energy source for power plants. Algae contain lipids. Lipids are fats that can be extracted and converted to liquid fuels. Further, algae contain carbohydrates that can ultimately yield such things as ethanol, the active ingredient in good Scotch. An added benefit is that even though algal fuels produce CO2 when burned, growing algae consume CO2 as they grow. There appear to be problems, not the least of which are economic, but algae may end up becoming an important source of synthetic liquid hydrocarbons some time down the road. This is clearly the stuff of the future. Incidentally, if you are interested in biofuel research look up the 23 June, 2011 issue of Nature (Volume 474, issue 7352). Right now, focused solar energy seems to be much more efficient for producing electrical energy and much more economic.
Commercial grade solar converters use some form of circulating medium heated by intensely focused sun rays. Concentrated solar energy heats various salts, sodium, synthetic oils and other materials to very high temperatures under pressure. Circulate those hot, and highly corrosive, materials through a heat exchanger and you can generate steam to drive electrical generating facilities. So, here we are back at steam power – all we’ve done is change out the power source to drive a turbine! I know it’s a long stretch, but I wonder if we could use solar to drive a steam locomotive instead of
burning coal! Yes, yes, I know…
There are a couple of ways to concentrate solar energy with, essentially, mirrors: solar troughs and solar towers. Solar troughs, in which a fluid conducting tube traverses the focal point of parabolic reflectors, represent an older design concept. The most recent design uses a central tower that receives light reflected from a field of mirrors, or heliostats that follow the sun. Regulators have recently approved a 250 MWe solar trough installation in the Mojave Desert. There is some concern, however, that this facility does not use the newer heliostat design because in a solar tower, the circulating fluid is water that is directly converted to steam that can run turbines. The advantage over the older thermal trough design is that there are no caustic or potentially hazardous fluids circulated and there is no need for a heat exchanger. On last check, the approved solar trough installation is under legal review.
Notice that I haven’t said anything about photovoltaic silicon cells. NREL has a map that shows that here in the Denver area we receive between 5 and 6 kWh/m2/day of solar energy. If we could capture all that energy, and use it directly, many energy problems would go away. At the University of New South Wales in Australia, researchers reported in 2008 that they had achieved 25% efficiency in energy capture using silicon solar cells. Commercial multicrystalline silicon cells are approaching 20% efficiency. Looking at this another way, a square meter of commercial-grade silicon solar cells will capture about 1 to 1.2 kWh/m2/day in the Denver metro area. Clearly, there is a use for this technology as evidenced by Xcel energy’s interest in supporting silicon solar cell installation on private residences.
The downside of solar energy is, of course, what if there is no sunlight? What happens if it’s nighttime or if it’s cold and cloudy? There is really no good way to store electrical energy on the big scale, so you have to interlink all of the potential sources in a power grid. Fossil fueled power plants can be online 24/7. The wind blows somewhere all the time. Rivers flow through hydroelectric generators day in and day out irrespective of the weather. These energy sources provide the continuum that can only be supplemented, not replaced, by solar.
So, where are we with all this? There are many ways to produce electricity and a few ways to produce liquid fuels. If our ultimate goal is to provide a power plant for a transportation vehicle such as a personal automobile, what’s a good approach that will minimize pollution and maximize convenience? Enter the fuel cell, next month’s topic.