I Have The Power!

CDT Solar Provides Excellent Value for Small Panels

I scoured the gambit of solar panel manufactures looking for an affordable solar array in the 10-watt range that was lightweight, robust, and inexpensive. These panels are for use by my freshman this semester for their MEEN 1110 design-and-build project. They must work in teams to create a solar-powered winch that drags a substantial bronze statue 25 meters along a concrete surface.

This project would be difficult for advanced students, even if they were allowed to use batteries instead of solar cells to provide power to their winches. For freshman, it is going to be a battle. The project is made more challenging by extreme budget limitations for materials. If any corporate sponsors out there would like to support UNT’s solar-powered winch freshman design-and-build project, I am happily accepting donations!

With only $1,200 to buy sixteen solar panels, each with enough juice to pull a bronze statue, my choices were limited. Options included 10-watt modules from AEE Solar, Uni-Solar, SunTech, and CDT. Analyzing these products using key parameters including price-per-watt and power-to-weight ratio, the CDT-10W panel blew away the competition at $7/watt and about 3 watts/pound.

The panel’s manufacturer, CD Technology, was a pioneer in the CD-ROM industry, and in 2006, they founded their CDT Solar division. UNT now owns 17 of these panels (16 for the students and one for me to play with), and so far they are an excellent product. They perform as advertised, both under sunlight and artificial light, and have stood up to abuse from students.

For tinkerers or hobbyists interested in getting hold of an inexpensive, small-scale solar panel for projects, the CDT-10W is an excellent choice. For different applications, CDT Solar offers a range of economical solar products in a spectrum of peak-power ratings: 1, 5, 10, 20, 40, 50, 60, 80, 90, 100, 110, 120, and 175 watts.

Simple Trig Explains the Value of Sun Tracking Solar Panels

Sometimes after I stare at a problem long enough, the solution finally jumps out and bites me. I have been staring at the same solar energy problem on and off for seven years, and this week, I finally figured it out.

Before becoming a professor, I worked in the solar industry, and now I am responsible to teach college students about alternative energy. So, I have known for a long time that a sun-illuminated flat solar panel fixed in its orientation with respect to the sky generates power at different rates as the sun traverses overhead. The Ameco Solar Company has posted an excellent example daily flat solar panel output profile on their Web site.

What I could never figure out was the physical mechanism underlying this result. I have heard many explanations, several of which are probably partly true. Some say that when the sun nears the horizon its rays must penetrate more of the atmosphere than when it is overhead. The photon-absorbing properties of the atmosphere reduce the energy reaching the panel. To some extent this observation is true, but it doesn’t explain why a solar panel pointed directly at the sunset collects more energy than a fixed flat panel at a non-ideal orientation.

I have also heard that more sunlight is reflected by a solar panel when the light arrives at an angle. Reflected photons clearly don’t get converted to energy. To some extent, this phenomenon also exists, but it does not explain the rounded profile of the daily solar panel output. If reflection were the main solar profile culprit, the profile would be a linear function of the time of day, not a smooth curve.

So what is the underlying mechanism?

It turns out that Isaac Newton’s inverse square law applies to this situation in a roundabout way. When cast in terms of gravity, electric field, and radiation, the inverse square law states that the intensity of a field emanating from a point source falls off according to the source strength divided by the area of the sphere of radius R from the point source.

In other words, solar energy density is a function of the sun’s output divided by the intersecting area. By analogy, when the sun is not directly overhead, its rays are forced to spread over a larger surface area defined by the angle the sun makes to a flat panel facing straight up. This phenomenon is beautifully illustrated on the Windows to the Universe Web site.

Thus, after a little trigonometry, it becomes obvious why solar panels set up to track the sun can collect so much more energy in a single day than their flat-mounted counterparts. Tracking panels always face the sun; so the absorbed light does not spread out and diminish its energy density.

This point may be trivial, but it is a phenomenon I can finally claim to understand after 7 years of staring at it.

Videos of Nucleate Boiling Under Microgravity Posted on You Tube

I have been searching the academic literature in preparation for a NASA grant I am writing to build a power plant for next-generation deep space probes. Among the primary challenges to be addressed through this design is a means maximize transfer of heat (and therefore energy) into a liquid in zero gravity.

I came across a series of illuminating You Tube videos posted by Dr. Ben Longmier of NASA showing behavior of liquid water taken in on-orbit, zero-gravity experiments. Particularly fascinating to me is the “Nucleate Boiling in Microgravity” video. Watching these movies, keep in mind that there is no gravity to induce buoyancy to pull the vapor bubbles away from the heated surface. Thus, bubbles are expelled by other forces in the liquid that are subsidiary in terrestrial boiling.

A longer video called “Boiling water with a soldering iron in microgravity”, which contains more commentary and technical information is also posted at You Tube. This movie, unfortunately, is highly pixilated and does not show the awing detail of its shorter counterpart.

These videos and the complimentary underlying research demonstrate the ongoing value derived from NASA’s microgravity investigations of fluid thermo-physical properties. Never in a terrestrial experiment could such exquisite video be taken, and this visual data is critical to improving our fundamental understanding of heat transfer (and energy transfer) mechanisms.