Future Efficiency of Solar Cells

----------------- #1212 - How to Make Solar Cells more Efficient?
- Photovoltaic solar cells are still too expensive for the average homeowner. A 10 year payback on your electric bill is too much to justify the upfront costs. But, today’s commercial solar cells are only about 15% efficient. With 1000 watts of sun power incident on the solar panel only 150 watts of electric power is output. The average homeowner uses about 5,000 watt hours. So he needs 33 solar panels on the roof. At an installed cost of $1,000 per panel that is $30,000. Or, if you just cut your PG&E electric bill in half the cost is $15,000. Still a 10 year payback on your electric bill. I have some people brag that they pay no PG&E electric bill at all. Yeah, but for what you paid for that installation it will take you 30 years to get your money back. They did not understand that. They thought paying nothing was a good deal. Please teach math and science in public schools.

- Dr. Zongfu Yu of Stanford University presented some of the technology improvements being studied that would make solar cells cheaper and more efficient. 10-18-10 lecture at SSU “Nanophotonic Light-trapping Solar Cells”. The technology focused on increasing the amount of photon absorption and increasing the electric output per cell.

- The way that a silicon oxide, SiO2, substrate works is that a light photon with a wavelength from 400 to 1200 nanometers incident on the surface is absorbed by the silicon. The photon energy knocks out an electron from the atom creating a free electron and a hole, equivalent to a positive electron. The electron travels down the semiconductor’s energy gap to the n-doped material and negative electrode. The hole travels up the energy gap to the p-doped material and the positive electrode The electron and hole are separated by the voltage potential between the “n” substrate and the “p” substrate. A conductor connected between them will carry a direct current. Solar cells produce direct current. An inverter is added to a home installation to convert the DC to AC for use in the home. The inverter lasts about 10 years and cost $2,000. However, if the solar panels are used to charge batteries, say in an electric car, then no inverter is needed.

- If you plot the light absorption with frequency you will find that the amount of absorption decays from 600 to 1200 nanometers. The goal of a good design is to shift that decay to the right so that absorption continues into the lower infrared frequencies. This can be accomplished by making the silicon substrate thicker. But, the free electron will only travel about 200 nanometers before it recombines with a hole and creates heat instead of electricity. So, make the substrate thinner. The best trade off between thicker and thinner is a function of the refraction index of silicon. The formula for the tradeoff is:

------------------- Absorption = 4*n^2*d = 10
------------------ d is the thickness
------------------ n is the refraction index
----------------- The theoretical limit for silicon is 4n^2d = 50

- Another approach is to create light scattering inside the substrate to cause more absorption inside the cell. This can be done by texturing the surface of the substrate or putting a reflective mirror on the substrate. By putting two mirrors on the substrate you can reflect the photons back and forth increasing the amount of absorption and thus the efficiency of the cell.

- To maximize this light trapping effect of reflective surfaces on the substrate the wavelength separation of the surfaces must match the resonant frequency of the photons. At one particular frequency the resonance curve peaks at that frequency and falls off at the higher and lower frequencies. So, a good technique is to have more than one resonance frequency. To do this a thick layer of amorphous silicon is mounted to a thin layer of crystalline silicon. This has the effect of broadening the resonance curve over a wider range of frequencies. The best broadening occurs when the reflective index of the substrate is much greater then the absorption index of the material.

- Nanotechnology can greatly increase the absorption index by constructing nano-particles of tiny pyramids as the textured surface on the substrate. The silicon surface turns from an shiny blue mirror surface to a totally deep black as all of the light is absorbed. These nano-pyramids greatly improve the amount of photon absorption.
- Another efficiency improvement considers the fact that the Sun arcs across the solar panel from sunrise to sunset. The reflection angle of maximum absorption occurs at high noon. By constructing a half dome lens over the cell the reflection angle can be focused on the center of the cell throughout the day. The period of repeating the half domes across the surface is optimized at 500 nanometers.

- Amorphous silicon is much cheaper to produce than crystalline silicon. The ultimate goal of fabrication would have these layers of transparent half domes -------- nanowire electrodes --------- 200 nanometer amorphous-silicon thick substrate --------- nanoparticle textured surface ------ 5 nanometer crystalline-silicon thin substrate ----- nanowire electrode ----- reflective mirror ---------- solar cells all rolling off in flexible sheets like paper from a printing press.

- Such fabrications have been made in the lab increasing absorption from 4n^2 = 10 to 60n^2 = 119. A ten fold improvement in efficiency. Going from the lab to full production needs a lot of science and engineering. However, super efficient solar cells are in our future. When nanotechnology gets perfected every house will have a solar cell roof, or solar cells windows that are transparent but producing electricity, or solar cell paint, or, who knows what nanotechnology breakthroughs will come from those science and math students that are in schools today.

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home: 707-539-6291 , mobile 536-3272, [email protected] Friday, October 22, 2010

Have you seen this?

https://www.scientificamerican.com/a...ss-solar-roads

Driving on Glass? Inventor Hopes to Lay Down Solar Roads

U.S. roads paved with glass panels encasing photovoltaics and LEDs would double as a national power grid
By David Biello October 6, 2009 53
https://www.scientificamerican.com/m...ar-roads_1.jpg SOLAR HIGHWAY?: This artist's rendering of the solar roadway concept shows how the panels might replace asphalt on the nation's roads. Image: © Dan Walden

A truck tire supporting a 36,300-kilogram load repeatedly traverses an 18-meter stretch of road, day in and day out, rolling up 483,000 kilometers on the odometer at the U.S. Department of Transportation's (DoT) testing facility in Virginia. The goal is to thoroughly challenge any new paving techniques and see how the road surface holds up. Now imagine putting a solar panel under there.

That's exactly what Scott Brusaw of Sagle, Idaho–based Solar Roadways hopes to do next February. The electrical engineer is currently at work building a prototype of his so-called "Solar Road Panel" with the help of a $100,000 small business contract from the DoT.

"We're building solar panels that you can drive on," Brusaw says. "The fact that it's generating power means it pays for itself over time, as opposed to asphalt."

There are about 260,000 kilometers of roadway in the U.S. National Highway System alone, and thousands more in state highways, suburban thoroughfares and rural roads. Could all that asphalt be replaced with a solar technology that would also double as the nation's power grid?

The key to making this work will be the glass: The solar road panel prototype is 1,024 modules—each containing a solar cell, a light-emitting diode and, someday, an ultracapacitor for storage—sandwiched between a layer of some yet-to-be developed glass and a layer of conducting material. "Nobody's tried to drive on glass long-term," Brusaw says.

In addition to needing strength, this glass will be textured to allow tires to grip and water to run off. It will also be embedded with heating elements—like a car's rear windshield—to melt snow or ice. And it will need to be self-cleaning, coping with the grit and grime of an endless procession of tires as well as dust, dirt and other highway detritus. Needless to say, such glass does not exist yet but Brusaw hopes to partner with researchers at The Pennsylvania State University's Materials Research Institute to develop it.

"Glass theoretically can have a very high strength, provided there are no flaws," says materials scientist John Hellmann of Penn State, a glass expert. But "can you keep the proper optical properties to transmit light to the PV [photovoltaics, or solar cell] and still not weather or change with that traffic going over it? … We make some pretty doggone good glass for structural applications but we're not driving trucks on them."

The engineering challenges are immense, adds materials scientist Richard Brow of the Missouri University of Science and Technology, another glass expert. But glass can be strengthened by compressing its surface using special heating techniques or, at a molecular level, swapping ions in the glass itself. Such enhanced glass is 10 times stronger than the conventional variety and is used, for example, in smart phones to withstand the pressures of texting. "Can you go from a teenager's thumb to a truck? That's a pretty big leap, but 10 years ago we didn't think you could make a 15-micron piece of glass for what's relatively rough handling in a PDA," Brow says.

Glass has been used to build footbridges, such as the Chihuly Bridge of Glass in Tacoma, Wash. And new glass ceramic composites with increased toughness have been developed for the photovoltaics industry, Brow adds—but that might boost the price of the resulting panel.

In the meantime, Brusaw is spending $40,000 of the DoT's money to build a prototype from chemically hardened glass panels that can be purchased today. He will experiment with various types of solar cells, from thin-film to traditional monocrystalline silicon photovoltaics, and he will try to strike the right balance between transparency—so the panel works to deliver at least several thousand kilowatt-hours of electricity each day—and road-gripping texture, which will block some of the light. "If you have perfectly clear glass, you get perfect PV efficiency. But [with] perfectly smooth glass, everybody slips off the road," he notes. "Glass manufacturers can cut grooves into the glass in a hatch-type pattern. We'll try various methods and see what holds up."

Cost will be a factor: "The cost to develop a glass that will hold up in the fast lane of a highway? Fifteen [million] to 25 million dollars over three to five years," Brusaw says. "The cost in mass production? About $1 per square foot." The goal is to produce a 12-foot by 12-foot panel for $10,000 that is capable of producing 7,600 watt-hours of electricity daily; it would take roughly six panels to match the electricity demand of one average U.S. home, which use 936 kilowatt-hours per month, according to the Energy Information Administration.*

In addition to requiring a yet-to-be-invented form of glass, solar roadways would need some form of energy-storage capability—whether batteries or some not-yet-devised ultracapacitor. The goal is to create a cross-country highway system that can also serve as an national electricity generator and power grid. And paired with wind turbines to generate electricity at night, Brusaw estimates replacing the nation's highways with his solar roadways could eliminate the need for fossil fuel–fired power plants. "Based on my calculations, at 15 percent efficiency [from the photovoltaics] we produce more than three times the electricity we have ever produced," he says. Even with cars constantly casting shade over the road surface, along with other challenges, "we think we can make enough to meet the nation's energy needs," he adds.

Other companies, such as the England's Invisible Heating Systems, have developed roads that use embedded water pipes to harvest some of the sun's ample energy that also bathes U.S. roads.

The solar roadway will also offer embedded LEDs to illuminate the road and display information, whether the actual traffic directions, such as lane markers, or messages such as "SLOW DOWN." And, should electric cars become popular, powered pavement could also offer recharging stations wherever such panels are installed.

The first test of Brusaw's crystalline vision will be when the prototype is delivered to the DoT on February 12, 2010. And the DoT's challenges will be followed by some durability testing by the inventor with a pickax, sledgehammer and, depending on the prototype's fortitude, guns. Then it's on to parking lots and perhaps fast food restaurants. "Parking lots are much better than going right out onto the highway," Brusaw says. "You have slow-moving, lightweight vehicles. We can learn all the lessons there before moving into the fast lane."

Get a visual on it:

https://www.youtube.com/watch?v=Ep4L18zOEYI