Solar Electricity - Method Based on Natural Updraft

Many Birds Know How to Circle to Fly without Flapping, from Thermals

Dynamic Soaring

As a young man, many years ago, I noticed that many birds fly in circles during hot sunny days. The Physics of why birds do this is quite impressive. They somehow know how to capture solar energy in order to fly without needing to use up any birdfeed. Glider Pilots know that we humans are as smart as birds so they do the same circling to sometimes fly for hours without even having any engine in their Glider airplanes. I have discovered an amazingly simple device which produces an impressive amount of electricity, essentially directly from solar energy. It is based on fairly simple Physics. Solar heat gets absorbed into the black ground soil surfaces, which absorbs the sun's heat and gets warmer. This localized heat creates vertical air convection cells where a lot of warm air rises upward above the black ground, which we now sometimes call a Thermal. I added a little Technology, such as an enclosing large 'chimney' and a horizontal-oriented farm-style windmill inside the chimney.

The device is simple and inexpensive to build. Except for possible damage from severe windstorms, the new device also seems to be virtually indestructible! My prototype devices have each cost less than $100 to build and they each spun a standard (used) car alternator to produce around 700 Watts on a nicely sunny day (50 Amperes at 14 Volts DC to supply to a car battery).

I had recently (2015) been reviewing some experiments I did around 1970 regarding a fairly standard paper airplane, and I realized this new device. At that time (fifty years ago) I had chosen a newly paved black asphalt Mall parking lot for my experiments. I already knew that many birds use black fields and black parking lots and other similar locations to circle above for the thermals or updrafts. Birds know that they do not need to use up energy flapping their wings, if they slightly tilt so they fly in a circle. Each time around the circle, they get a few feet higher, never having to flap at all! This entirely complies with the Conservation of Energy, by first taking sunlight and getting it absorbed into black ground to heat it up, and then using that warm surface to create vertical Convection Cells in the atmosphere (which is Kinetic Energy) and then using the circling process to convert that into Potential Energy (of gradually getting higher and higher.)

I made my generic paper airplane (circa 1970) slightly asymmetric to circle in about a 50-foot diameter circle (so it stayed over the hot asphalt parking lot!) My paper airplane circled for more than 9 and a half minutes, before a slight wind blew it sideways and it was no longer over the hot asphalt. It had gotten about a hundred feet high by then and it continued to fly for about another minute in coming back down. Altogether, it flew more than 5,000 feet, a mile! Is that a Guinness World Record, of around an 11-minute flight for a paper airplane?

This new invention is somewhat different, but it is based on similar Physics. I will first describe a Prototype demo-sized unit to prove that this works as well as the math says it should. It does! Larger units are obviously easy to make and the production of electricity is proportionally greater by the square of the diameter. It does not have to be very large to produce significant amounts of electricity.

I drove eight standard metal fence posts into a circular pattern, two feet apart. The resulting circle was about five feet in diameter (where the circumference was about 16 feet). I installed them in an odd way, where the flat side of the fence post was inward, and the rib was outward. I bought two sheets of 4x8 countertop (or similar) veneer which was 1/8" thick. They had to be thin enough to bend fairly easily.

I used small bolts to mount the panels to the inside of the circle of fence posts. I jacked up the panels four inches from the ground. The smooth surface of the panels were pointed inward. I had to chop off two feet at the end of one of the panels so I could use it to make myself a door to get inside the cylinder, and I bought some hinges and a latch to be able to close it. So I now had a 4-foot 4-inch tall cylinder, 5 feet in diameter. It then acted as a large diameter chimney.

In an assortment of experiments, I tried removing all grass and weeds on the floor of the cylinder to leave just black dirt exposed; I also tried covering the floor with black tarpaper (which did not last very long) and various other black materials. The most durable was probably a bucket of black driveway asphalt.

In all cases, the point was to absorb sunlight into the floor. On a bright sunny day, roughly 100 Watts of sunlight radiation hits every square foot. The black materials I put on the floor generally had an absorbtivity of around 90%, so about 90 Watts might get absorbed down into each square foot of the floor, if the Sun was directly overhead. My floor was about 19.6 square feet, which mathematically indicated that an overhead Sun should supply about 1750 Watts of heat into the floor.

That is a pretty impressive number for such a small device! But near Chicago, the Sun is never actually directly overhead, and so part of my walls blocked some of that heat from hitting the floor. Early in the morning or late in the afternoon, very little sunlight hit the floor! But we will now just discuss the hour near noon in June (near Chicago) when the Sun is about 60 degrees above the horizon. Using the sine of 60, we see that around 1500 Watts then actually gets absorbed into the floor of the cylinder. Still a decent amount!

The floor gets quite hot from all this sunlight heat! It is really just a small sample of the black asphalt of a very hot parking lot that burns bare feet! And the cylinder then acts as a chimney for that heat! The 4-inch high slots at ground level let some new air get into the cylinder and over the floor so it can heat up and rise. A basic law of science is the Conservation of Energy. All that 1500 Watts of solar heat that comes into the device and gets absorbed into the black floor, cannot disappear. Nearly all of that heat energy becomes vertical kinetic energy of the air rising inside the cylinder.

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Now, build a 59-inch diameter transparent fan blade, that resembles what old-time farm windmills looked like. It will not be mounted like that old windmill, but instead it will be on a vertical axle. It will block off the entire area of the top of the cylinder, just a couple inches below the very top of the cylinder. No warm air could escape from the cylinder without spinning this blocking fan! The majority of the vertical kinetic energy in the rising air has to spin this fan. Actually, a researcher named William Rankine discovered long ago that some remaining kinetic energy must remain in the rising air after it has given up torque in rotating the fan, but still about 59.3% of the kinetic energy can be extracted here. This indicates that we should be able to extract about 900 Watts of power spinning that central vertical axle. If we drive a standard car alternator by that shaft, we can get around 700 Watts of electricity, around 50 Amperes at 14 Volts to charge batteries.

The production of electricity tapers off in morning and afternoon. A sunny day might produce around 4 KiloWatt-hours, a decent production!

This Prototype version has one major disadvantage. Even if the rotating fan blade is made out of transparent Lexan plastic blades, it still blocks a significant amount of the incoming solar energy.

Many variants are possible. One is to make the chimney much taller, maybe 12 feet tall, and to make the south-facing side of the chimney out of transparent Lexan plastic. Instead of the solar energy having to go through the fan blades, it enters through the south-facing walls. This actually also improves the time of good performance, and the fan blades could be made of sturdier material such as metal. This same sized Prototype unit can produce around 7 KiloWatt-hours of electricity.

Another variant is to make the chimney structure like the Prototype but to add a separate 'solar collection area' adjacent to it and directly south of it. Instead of having the entire bottom perimeter open to accept new air, only the side toward this collection area is open, so it accepts all its new air as warmer air from the solar collection area. This eliminates the need for Lexan walls or rotor blades. The solar collection area is 'dug away' so it is tilted at around a 40-degree tilt toward the south, to more efficiently collect the solar heat. It is not tall, only a few inches up from the ground, but it is covered by transparent Lexan. The cover makes sure that all the warm air created (over the tilted black floor) gets collected and directed to go into the bottom of the chimney cylinder.

Many other variants are possible, including much larger variants.

This presentation was first placed on the Internet in May 2015.

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Carl W. Johnson, Theoretical Physicist, Physics Degree from University of Chicago