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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Since I started writing Common Science in April of 2011, I have averaged 50 columns a year. Most weeks I have my topic selected by Monday, a first draft done by Wednesday and am ready to submit to my editor (she’s not hard to find) by Friday. This routine keeps me on track to publish on Sunday afternoons. Once or twice a year something goes awry during the week, Friday morning rolls around and it’s clear the column is not going to come together. Well, it’s Friday morning now and this is one of those weeks. Maybe I can take a shot at fixing this week’s first effort in the future, but in the meantime, let me tell you about a project I am working on. It should give you a little insight into how engineers spend their spare time, a question that I am sure has been plaguing you.
As I have mentioned in previous columns, I have a “farmette” west of Carrboro where I grow vegetables, generate my own electricity, raise chickens and bees, and am attempting to create a local pollinator refuge. Designing and building projects and systems for the farm is a relaxing and enjoyable hobby for me. Lately I have been working on improved systems for growing tomatoes. I love tomatoes and want to grow my own all year long. However, like most gardeners I know, I find the task of staking and trellising them to be tedious and the solutions offered for this by the local garden shop to be inadequate.
The staking problem can be resolved by growing tomatoes upside down. You can buy expensive planters for this approach if you like. I use inexpensive two gallon buckets hung from a pole with a hole drilled in the bottom. Here is a picture of some tomatoes hanging upside down in my greenhouse (more about the greenhouse below). At this early stage, the plants grow out to the sides of the buckets and then upwards towards the sun. Once the tomato fruits get large enough, the plants are weighed down and the vines hang below the buckets. It’s a wonderful way to grow tomatoes, no staking and no weeding.
The primary flaw in the upside down tomato approach is that the buckets, being suspended in the air, subject the plant roots to much larger temperatures swings that they would experience if planted in the ground. This can quickly kill the plants either by freezing or frying the roots. In order to try to provide an environment with better temperature control, I built the green house pictured here. To keep costs down, it’s built from some 2” by 3”s, a couple of shed wall panels, and the roof and top third of the walls are made from corrugated clear plastic panels. The cost for all of the materials was a little less than $250.
I started my first batch of tomatoes in the greenhouse last April. For a couple of months they looked beautiful and I was feeling like a gardening genius. Then we had a hot spell in June. The temperature in the greenhouse shot up to over 100 °F and the plants began to wither. I bought a thermostat and two exhaust fans which would run on the 12 VDC power coming from my solar electric station (1) to blow the hot air from the top of the greenhouse out while sucking in somewhat cooler air from vents near the ground. This helped a little bit, but by mid July I had lost all of the plants. Next summer I intend to purchase a reflective mesh to put over the roof to see if I can keep temperature below 90 °F while still letting in sufficient sunlight.
Meanwhile, I started a new batch in the green house in August and now have some promising-looking ¾” diameter, green tomatoes. (These are the ones pictured above.) My current concern is that before they have a chance to ripen, we will have a cold snap and the plants will freeze. To prepare for that possibility I started on a new project this week, a solar water-heating system for the greenhouse.
My first step was building the solar collector pictured here. The box is built from treated lumber. On the bottom is a 1” thick sheet of foam board insulation covered with aluminum which I painted black to absorb sunlight. (2) Next I constructed and installed the copper serpentine you see in the picture. Soon after this picture was taken I painted the copper piping black as well.
The next steps are to cover the collector with a sheet of glass – I am using an old window I bought at the Habitat for Humanity Restore (love that place) – face the collector to the south, tilt it to approximately 30° from verticle, and fill the serpentine with a water/antifreeze solution. When the sun warms the liquid in the tubes it will start to rise due to reduced density. The liquid coming out of the collector will flow through an insulated hose to a small tank located high within the greenhouse. From the tank the water will fall by gravity through another serpentine which will be embedded within a concrete block located inside greenhouse. As the liquid flows through the block it will warm the concrete. After passing through the concrete the liquid goes back to the bottom of the solar collector to be warmed again and the cycle continues. This sort of temperature-driven liquid flow is called a thermosyphon. Here is a simple schematic of the system.
If my calculations are correct, the heat stored in the concrete block should be sufficient to keep the air inside the greenhouse above freezing on even a cold Chapelboro night, and we’ll be having fresh homegrown tomatoes for my birthday in January. I’ll let you know how it works out.
Have a comment or question? Use the comment interface below or send me an email at firstname.lastname@example.org.
(1) Consistent readers will have noticed that I have been using more footnotes of late. As I write these columns a large number of tangential topics come to mind. I used to weave these into the narrative of the column which created for a fair bit of clutter. So I am trying to put more of those thoughts down here. In this case, I wanted to remark on the absolute joy of running equipment from electricity you make yourself. This is not difficult to do, and if you like tinkering around with things, I highly recommended it. Unfortunately, there is not a supplier of solar electric equipment in the area, nor is it easy to find equipment, thermostats, pumps, etc. which run on DC power.
(2) If you are installing a solar water collector to heat water for your house, you want to use expensive solar absorbing paint rather than just black spray paint.http://chapelboro.com/columns/common-science/how-engineers-spend-their-spare-time
White Cross Farm
CHAPEL HILL – The sun’s rays now cannot only brighten your days, but also your home.
In early July, Strata Solar teamed with Orange County to begin construction on a 35-acre, six-megawatt solar farm on White Cross Farm in Chapel Hill.
After about twelve weeks of construction to install more than 26,000 solar panels, the sun’s light will provide power to about 750 homes in the area, says Vice President of Sales and Marketing at Strata, Blair Schoff. Schoff says, though more difficult, the builders even continued to make progress during the heavy rains of early July.
He says the construction can take up to 120 workers to get the land ready, drive posts, mount solar panels, and wire the electrical work.
Schoff says the farm generates power, which is sold to utility companies, and businesses and residents close to White Cross Farm will consume most of the solar power.
“One of the things that we’re trying to do is add additional green power to the grid,” says Schoff. “It helps the utilities because a lot of their peak demand is during the day when businesses and air conditioners are being used at their peak and we’re providing them the alternative of having energy generated by the sun added to this mix.”
Schoff says Duke Energy will receive the solar power from White Cross Farm, and then provide it to local residential and commercial clients.
He says using solar power benefits the environment and eliminates 4,224 tons of carbon dioxide from being emitted annually by other power sources, like coal.
“There’s no moving parts,” says Schoff. “There’s no noxious fumes. There’s no liquids being dropped into the soil. It’s very clean, renewable. It’s an excellent source of renewable generation.”
Solar power relies on electrons for power.
“Solar panels, the type that we use, it’s essentially silica that’s cut into wafers,” says Schoff. “One of them has a treatment on it with an element that is interested in yielding electrons. Another wafer has a treatment on it with an element that is interested in gaining electrons. Those two wafers are essentially sandwiched together. The photon, from the sun, of enough energy catalyzes the reaction where that electron from one wafer is liberated and is finding its way toward the electron on other and we try to harness as many of those electrons as we can and use them as power.”
Many farms have the necessities for a solar farm, but Schoff says White Cross met the standards.
“There’s a certain criteria that we look for in any of our farms,” says Schoff. “Having a farm that’s flat with access to the appropriate infrastructure are the key components. White Cross and that property just happened to pick all the boxes to make an appropriate solar farm.”
Headquartered in Chapel Hill, Schoff says Strata Solar is excited to build so close to home.
Strata Solar is in the midst providing solar power to many other places across the state. It is developing eight farms in the Triangle and two in Orange County.
Schoff says everywhere could benefit from solar power.
“All counties have a need for solar power,” said Schoff. “It’s not just Orange County. It’s the state of North Carolina. It’s the country.”
For more information on Strata Solar, click here.http://chapelboro.com/news/development/orange-county-welcomes-solar-power