If you paid attention to the news last spring, you may remember that solar roads were experiencing their 15 minutes of fame. There were newspaper articles, TV reports, viral Facebook messages, and at least one Kickstarter campaign to fund this ambitious project. The concept was to start covering our roads with small, modular solar panels to generate electricity. The implication of most of the reports was that generating electricity from our roads was going to usher in a golden era of plenty, and generally included an undertone suggesting that we had somehow been blind in not perceiving this veritable cornucopia of energy before. Then suddenly the 15 minutes were over. Let me explain why.
The electricity-generating part of a solar panel is made from a semi-conductor, usually silicon. A semi-conductor is just what it sounds like: something that will conduct electricity, but only a little bit. When sunlight shines on a solar panel, electrons migrate away from one of the surfaces and towards the other. If I now connect these two surfaces of the panel to a circuit, electrons will flow through the circuit. Voila, electricity! The type of electricity generated by a solar panel is direct current (DC), which means that electrons flow through the circuit in the same direction all of the time.
In contrast to the DC electricity generated by a solar panel, the electricity in our homes is alternating current (AC). In an AC circuit, the electrons move back and forth, switching directions 60 times every second. In order to understand why our houses have AC rather than DC electricity, we have to go way back to the 1880s and the War of the Currents.
By the 1880s, Thomas Edison had accumulated a number of patents for the generation and distribution of DC electricity and was collaborating with George Westinghouse to commercialize them. At about the same time in Europe, a number of inventors and engineers were developing AC technologies. In order to get a better understanding of these developments in Europe, in 1884 Edison hired Nikola Tesla, an electrical engineer who had been born in the Austro-Hungarian Empire in 1856. Not long after arriving in the United States, Tesla learned that due to Edison’s ego and personal attachment to DC electricity, he would not have a real chance to continue his work on AC technologies. So Tesla found some investors and struck out on his own.
Tesla soon had several patents for AC technologies, several of which he licensed to Edison’s former partner, George Westinghouse, and the War of the Currents was on. It was a nasty public battle filled with misinformation and personal invective. In an effort to “prove” that AC current was far too dangerous to unleash on the American public, Edison inadvertently invented the electric chair. In what may be considered a bit of macabre foreshadowing for our current difficulties and controversies over executions and the death penalty, the first use of the electric chair for an execution in 1890 went very poorly and several cycles were needed. George Westinghouse quipped “It would have been better done with an axe.”
Despite his valiant efforts, Edison was doomed to lose this battle before it started. Physics was against him. For reasons of which I will spare you the explanation, when you transmit direct current over any reasonable distance, much of the electricity is lost. Let me give you an example from personal experience. I have written previously about the solar power system that I built at my farm, which uses a single 220 Watt solar panel and four batteries. Approximately two feet away from the panel, I have a refrigerator which I use to store chicken eggs, vegetables and cold beer. It runs like a charm. About 150 feet from the solar power station is my tool shed. I ran a heavy gauge wire over to the shed – the thicker the wire the less the electricity loss – and I can just barely run a radio, a device that needs far less power than a refrigerator.
The beauty of AC current is that it can be transmitted over long distances with limited loss. This technical advantage explains why Tesla won the War of the Currents despite Edison’s head start and considerable fame. It is also the primary reason that electricity generation and distribution in the United States came to rely upon a relatively small number of very large power plants which deliver electricity over great distances.
The issues which impacted the War of the Currents are the same ones which determine the viability of solar roads. We could certainly cover the highways of sunny places in the U.S. such as New Mexico and Arizona in solar panels and generate quite a bit of DC electricity. However, given that highways are generally far away from homes and businesses, trying to transmit power from the roads to where it is needed would waste nearly all of it. One could consider using streets in cities and towns, but as you can see from my example from my farm, even the short distance from the street to your house would result in a noteworthy loss of power.
A far better place to install solar panels is where we generally already do install them: on roofs, which is as close as possible to lights, TVs, and appliances. Furthermore, rooftop solar panels are getting so efficient now that they are beginning to compete with your local electric company on a cost per kilowatt hour basis.
Sadly, I think the brief flurry of interest in solar roads actually hurt the alternative energy movement. Getting everyone’s hopes up on an unrealistic project and then letting them down creates negative feelings. In the end, as is often the case, the best approach is not always the sexy one. Rooftop solar panels are a key feature in helping us to become less dependent on fossil fuels, and we should just keep making steady progress in installing them.
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