Solar Thermal Generator Project

by Chad Bastian.

After all the lively discussion about thermals and their sources lately in Paragliding magazine, the local pilots became interested in an idea that I have long been thinking and talking about. David Wuesher became quite involved in the discussions, and actually went and purchased the materials to begin the testing and experimenting process. He spent many hours at the local hill assembling, rearranging and gathering the stuff when the storms blew it all over the place.This project was a joint collaboration between David and myself, with input from many pilots.

An Uplifting Idea

We wanted to provide a "hot spot" where the thermals could reliably be released from, a trigger that constantly (or periodically) produced the push and flow to get the thermal started. If we could make an artificial thermal factory where we heated the air to a higher temperature than the surrounding area, plus created a smokestack shaped release generator that also heated the air, then we may actually be able to manufacture the kick that the thermals needed to get started.

What we wanted was a more reliable spot where we could go without risking landing out, since the best house thermal is slightly away from the LZ. We were also interested to see if we could actually effect the already great conditions at the site with our ideas.

The Site

The hill where we set up the generator is a 200 foot vertical feet gentle training hill in Santa Barbara where we teach paragliding and hang gliding. The hill is unique because of its southern exposure, year round favorable weather, and the ease with which you can kite your glider back to the top for another flight (or if you're lazy you can ride in the van).

Because of its close proximity to the ocean (1/2 mile), the breeze is typically straight upslope, from the south. Often, in the morning, the wind will start out slighlty SE or from the left as you stand on top, switching slowly over the next 2-3 hours to the SW, producing a period of time where there are unstable convergent conditions.

During this time there is often very little wind, and most of the moving air is thermals releasing. We can see this by watching the Red Tail Hawks climb regularly to a couple of hundred feet over the top of the hill. There are already some areas where we can typically go to reliably get up in these thermals with the birds, and we often spend from 5 to 45 minutes soaring here.

Safety

Because we use the site for training, we wanted the project to be out of the way for new students because of possible turbulence, yet close enough to be able to fly over when launching from the top. We also wanted it to be away from the pattern of our usual landing approach, so that it would not be an obstruction we often had to avoid. The materials would have to be "nerf", that is, unable to harm anyone that happens to fly right into the middle of it.

We decided to put the project just beyond the point where the lower slope meets the large field where we land, off to the left of the typical flight plan. The generator was put close enough to the hill to easily fly to it with maximum altitude, but far enough away that the thermal is released to the air rather than funneled up the side of the hill. We wanted enough room to consider flying in a circle if the lift was found, something not so easy on a 200 foot gentle slope! Remember question #8 on the novice test?

8) Performing multiple shallow banked, circling turns in smooth winds, using less than 1/2 brakes can be dangerous because
    a) A prolonged, spiral turn can lead to excessive speed
    b) After several rotations the glider may spin
    c) You may drift downwind into the hill
    d) You will become dizzy and disoriented
    e) All of the above

Yeah, I got it wrong, too. It didn't mention a HILL. Doh. We have already had our share of eager to circle pilots making a neat little divit in the dirt where their ankle snapped.

A House Made Of Cards

The materials we used may not be the best for the purpose, but we had cost and safety in mind, so we got alot of black plastic to make the base with. 50 x 60 feet seems huge when you're walking around on it, but from 400 feet over the top, it seems rather small.

The "trigger" is made from black tarpaper with wire backing (easily crushed if you land on it), pyramid or conical in shape, and is about six feet tall. It was made with a space around the bottom so that air could enter it from below, with the idea that the stack itself would also heat up, and the hot air would rise up its walls and release upwards.

We hoped that this flow, once started, would continue drawing the surrounding heated air from the surface of the generator and the surrounding field, increasing the release intensity enough to produce a thermal large enough to use.To see the airflow over the generator surface, we put lots of small wind indicators around the surface of the project.

As David was putting some of these flags in place, I noticed the local Red Tail on glide for a taste. It was about 10:30 in the morning, and there was a slight SE flow, maybe 4-6 mph, and she got right over the generator and began to circle! As she topped out about 400 feet higher, we were screaming! Ten minutes later, she was back for more and climbed again before David could even get his glider out.

I was teaching two students at the time, and could not take the time to experiment with it myself, but decided that the conditions were gentle enough to take the third day students out over the project for a taste. Ally was the first to fly over the generator, and immediately began to climb as she approached the black plastic. She had maintained her altitude after launch, and was now climbing nicely. I had her go to minimum sink for a few seconds, then said "Look left, lean left, add left brake, you're going to come all the way around in a 360 degree turn. Hold that turn, dampen with the outside brake slightly, perfect!"

Three circles later, Ally was 300 feet over the top of the training hill in the first thermal used by a human from our project. I had her leave the lift and after a few turns, she landed happily in the LZ. I was able to control my other student in a similar flight, and was really impressed with our results.

While there was not always a useable thermal over the project, there was always lift there on sunny days, extending your glide considerably. Once the seabreeze kicks in, the lift is pushed toward the hill too quickly to be useful, as we could see from the small flags we placed in and around the project. These flags were fascinating to watch, especially when the wind was light or convergent, since you could see the air funneling toward our artificial thermal trigger. Even from the air, they were visible and helpful.

What's Next?

We want to experiment with baffles and fins leading to the trigger from the edges of the plastic, to better protect the air that is heating up on the surface from wind. We figure that cardboard boxes painted black will also trap heated air nicely as well. We'd like to make the project even bigger, but we already have a lot of plastic to gather up when the wind spreads it around! Perhaps some sandbags could hold it together better.

The great thing is that we can experiment with the project just about all year long. Santa Barbara has a mild, Mediterranean climate that allows us to fly all year. In the summer, we typically have an inversion around 1200 - 1500 ft where the marine layer stays quite cool, causing a good lapse rate within the layer. We can't climb higher than the inversion, but hour long flights have been logged at this site, with 10 - 20 minute flights common among the skilled pilots, with a nice top landing if you play your cards right. The hill is not steep enough to produce sustainable ridge lift, so the longer flights are always from thermal lift, although each flight starts out as a "Low Save!"

Winter and spring conditions are more unstable, and we can sometimes get much higher, although the air gets quite punchy. The site record is 3000 ft, with the landing some three miles away at the landing zone for the local mountains. We figure that if we can provide an even more reliable source for useable thermal lift, there will be many more records for the locals to challenge. Best of all, there is a new game in town, and everyone is having fun with it!