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Here Comes The Sun

A couple of weeks ago I started Neil Gershenfeld's new book, Fab: The Coming Revolution on Your Desktop: From Personal Computers to Personal Fabrication. I had intended to read it quickly and write a review here, but I found that I was bogging down at certain portions of the book.

But bogging down in a good way.

Instead of reading through the book quickly like a novel, I kept finding myself wondering off on tangents that Gershenfeld encouraged. So, a complete book review is delayed. Apparently Glenn Reynolds is also enjoying the book, and is taking his time getting through it as well.

Yesterday Virginia Postrel asked her readers to submit recent scientific breakthroughs. Since she's the person who suggested Gershenfeld's book to me in the first place, it only seems fair that I return the favor by pointing out an anecdote in Fab that, I think, qualifies as a breakthrough on the verge of changing the sunnier parts of the world.

Gershenfeld obviously has a special place in his heart for developing countries. He emphasizes repeatedly that our solutions to problems might not be the best solutions for African countries like Ghana.

Ghana has a huge amount of solar power beating down on it everyday while people struggle to have enough power to do even the most basic things. His answer is to harness the sun, but the method he suggests is truly innovative. When most people think of solar power, they think of capturing solar for electricity. But that's terribly inefficient. On page 86 Gershenfeld states:

If instead of boiling water to cook with, a parabolic dish concentrated sunlight to boil water to produce steam to power a turbine, then the rotary motion would be available for all kinds of mechanical work. It could directly run machinery, pump water or lift weights to store energy, power an electrical generator, or turn an air conditioner's compressor.

...or turn the compressor of a refrigerator or freezer. And Gershenfeld suggests that this power could be used with fabbed vortex tubes. Vortex tubes literally separate hot from cold in compressed air. If solar power is used to compress air, then you could have a solar-powered cold airflow into a home or freezer.

Yeah, I thought, but how much solar power could a person capture with a parabolic dish? I started looking around on the Internet for people who work with these dishes. I soon found the Clear Dome Solar website.

This company makes parabolic dishes for solar cooking, but they also make larger, hotter dishes that they're not selling to the public. These super dishes are being tested by engineers and scientists. So I wrote Clear Dome Solar with a few questions:

  1. Is there a commercially available sun tracking system to keep a pot boiling all day? ...by keeping the dish focused on the pot.

  2. If so, I suppose it would be a simple plumbing matter to keep water in the pot, but has this been done?

  3. Is there a commercially available steam engine that could be run off this system?

The owner sent a very kind and thorough response:

Hi Stephen,

...I've attached a few photos that have not yet been posted to our web page that might interest you. We've had a number of experimental solar heating projects ongoing the last few years and they should interest many of your fellow bloggers.

SFX.jpgThe first photo shows a 4-cylinder Stirling-Steele engine being powered by our SolaReflex 900X parabolic heating reflector (capable of generating over 1300 degrees F). Stirling engines use heat as their fuel source, so the bright light you see on the four round heat absorbers, which are also the cylinder heads comes from our 900X Parabolic solar concentrator.

This engine is made by an inventor in Oregon who has patented this design which generates between 20-40 watts of mechanical power. With full sun, the fan blade turns, but larger models now being made will turn pumps and generators, which will be built into the engine. He has a 100/200 watt engine with a built in generator in the works. His testing shows a 20-25% efficiency.

I purchased this engine jointly with a thermal engineer in India that now has it and is finding ways to reduce the cost of production so they can be sold commercially. The inventors initial design is to use natural gas to provide the heat, and my partner and I are adapting it to solar heating. This will make it a dual fuel Stirling that can be used for pumping, refrigeration or power generation 24 hours.

I've invented a "Solaraxis Sun Tracker Mount", complete with a sun sensor tracker that follows the sun for automatic sun adjustments with the 900X.

SFXb.jpgAnother version of the Solaraxis tracker is the 2x4 SolaReflex trough parabolic. It uses our SolaReflex bendable reflector panels that bend into a perfect parabolic trough that focuses concentrated sunlight on a copper heat collector tube. It is sun tracked and can generate small quantities of low pressure steam and boiling water in about 10 minutes when still water is in the tube.

By adding a 12 volt pump to recirculate water, it will heat 10 gallons of 60 degree water to shower temps (100 degrees F) in less than two hours.

By connecting two or more of the troughs, higher temperature steam can be generated. All focused sun reflectors needs to be tracked, which adds to the cost because of the mount, sensor and power to the sensor.

Tandem.jpgIn two other photos we attached two of our 900X offset focus 24" parabolic reflectors to combine heating to a central hot spot. The closeup photo shows a piece of 1/16" thick aluminum that was melted by the tandem 900X reflector. My thermometer only reaches 1800 degrees F, and it melted the probe, so I'm estimating the temps to be well above 2000 degrees F. Aluminum melts at 1200 degrees F.

From years of experimenting with solar thermal energy, I've been attempting to improve on simple, low cost solar thermal product designs and found that concentrated sunlight as a heat and as centrally distributed light source is extremely efficient, up in the 80-85% range.

I've also found that flat plate collectors are quite useful for many applications and are less expensive to make and use, since sun trackers aren't necessary. A good example is solar water heaters and pool heater which are around 30-40% efficient. We are now testing two types of prototype small and portable solar water heater collectors that can be used on RV's, boats and small outdoor applications.

CU-meltedalum-2-900X.jpgOur SolaReflex 900 parabolic cooker is the hottest, fastest 24" cooker made. It's based on the 900X design, but we've diffused the surface to spread the hot spot so it doesn't burn food, and make it safer to use. It will brown meat, chicken and fish, and even bake bread, and they are delicious solar cooked. It's like having a portable toaster oven without a plug....

By the way, water does not need to boil to cook food. Most food is cooked between 155-200 degrees. Boiling eliminates some flavor and nutrition, so solar cooking is actually one of the most nutritious ways to cook food, and all solar cookers brag about how much tastier solar cooking is that cooking with gas or electric power.

Also, unsafe lake or stream water can be Pasteurized at 140 degrees for 30 minutes or 160 degrees for 30 seconds so it's safe to drink. It does not have to boil to kill the deadly microbes. All of our solar cookers and water heaters will Pasteurize water, which is a major issue in developing nations...

The days of the steam engine or similar type heat powered engines like the Stirling will likely return with the use of affordable reflectors that use the sun's clean and abundant energy as the power source.

I hope this is helpful, and stimulates some of your readers. In my opinion, the potential for solar thermal energy is far greater than solar electric (PV), because is is 5 times more efficient, less costly to produce the products, and can be used more efficiently to generate electricity, power pumps, cook food and heat homes (as with our solar forced air heaters that are about 75% efficient.

Best regards,

Deris Jeannette
Owner ClearDome Solar Thermal

In many African countries a full third of the food supply spoils from lack of refrigeration. This innovation could make the difference between living and dying in Africa. Solar air conditioning wouldn't hurt my summertime power bill down here in Louisiana either.



However, I'm afraid I'd leave it outside and it'd be covered with gook in no time and lose efficiency. The problem with high tech is that it requires too much maintenance.

I live at about a mile above sea level. We have to use pressure cookers to get liquid water to the temperature at which it boils at sea level, so I can confirm that it doesn't need to boil to cook food.

Why is it that nobody ever mentions the names of these engine inventors, or how to obtain their products?  Such a Stirling engine would have a hundred and one uses.


I'd bet Deris would be happy to share that info with you. He has contact info at his company's website.

More things:

Vortex (Hilsch) tubes are woefully inefficient; the only reason to use them is if e.g. compressed air is cheap and weight and bulk are expensive.  An expansion turbine will yield cooling many times as efficiently as a Hilsch tube.

World Changing had an article about gobar-gas and solar cookers in India.  Some people don't seem to have trouble keeping them from getting covered in gook, especially if they reduce the amount of gook in the air to begin with.

(aside:  there is a problem with TypeKey cookies getting "stale"; one is told one is logged in until one attempts to post, at which time one is told one is NOT logged in.  Returning to the original page and reloading shows one is still supposedly logged in.  Logging out and logging back in fixes this.  Obviously the two steps are checking different data; they should be the same.)


Gershenfeld mentioned that Vortex tubes aren't terribly efficient, but he was interested in them because they could be easily and cheaply fabricated by the fab labs he is working on. When an air conditioner costs several hundred dollars and a vortex tube can be fabbed for 30 cents, it begins to matter less that you need more power to cool a (small) home with vortex tubes.

You said:

"the only reason to use them is if e.g. compressed air is cheap and weight and bulk are expensive."

The expense of bulk is not what makes vortex tubes interesting for Africa - plenty of room for the bulk (a vortex tube could be a real space/weight saver on aircraft or spacecraft).

But, compressed air might be especially cheap if you are using parabolic dish solar. Gershenfeld said:

"If instead of boiling water to cook with, a parabolic dish concentrated sunlight to boil water to produce steam to power a turbine, then the rotary motion would be available for all kinds of mechanical work. It could directly run machinery, pump water or lift weights to store energy."

Instead of lifting weights to store power for later why not compress air? Many tools and some appliances are already made to be powered by compressed air - more could be engineered if there was a market for it. You could compress air all day and have power in that form for the night. Cooling with a vortex tube would be quite sensible under that circumstance.

And, of course, vortex tubes have zero moving parts - low or no maintenance.

Airliners use expansion turbines fed by engine bleed air to get their air conditioning; they use as little as they can, to reduce fuel consumption for the sake of compressing the air (bleed air reduces engine thrust and raises fuel requirements).  You can rest assured that they wouldn't use Hilsch tubes no matter how much lighter and simpler they are, because the additional fuel required to run them would cost far more than any savings.

The same is true of refrigerators.  Solar energy is fairly expensive, and a Hilsch tube at 20-30% of the efficiency of mechanical refrigeration is going to require 3.3 to 5 times the collector area to run it.  It would probably be cheaper to use ammonia absorption refrigeration, which can be built to incorporate storage.

Stephen wrote:

Instead of lifting weights to store power for later why not compress air?

Or how about compressed water...essentially a hydraulic battery? Or maybe we could store kinetic energy by winding an easily fabbed spring?


Water isn't very...elastic (I guess that's the right word). It doesn't easily compress like air.

The spring idea would work. Both the weight idea and the spring idea have long histories providing power to precision instruments:


The problem with both compressed air and springs is that their specific energy is rather low, and they have catastrophic and dangerous failure modes.

Batteries are popular for good reason.

I suppose this is related. Here's an article that explains how a pendulum stores energy - by explaining a child's swing set.


"What your little girl is doing is to generate angular momentum with her legs at the ends of her oscillation, which she transfers to the swing by means of forces she applies with her arms and seat. Since these two forces do not act in line with each other, they create a torque on the swing. This torque, applied mostly at the ends of the oscillation, will add energy just like a push. Since the swing will store energy, these little additions will pile up, and the swing goes higher."

Unfortunately, the explanation is wrong.

1.)  Angular momentum is not generated by the legs, it's made by the pull of gravity.  Lowering the legs at the peak of the swing increases the angular inertia of the swing, so gravity has to pull longer to bring it back to center.

2.)  At center, the swinger brings her legs up.  This decreases the angular inertia of the swing while leaving its angular momentum unchanged, increasing the energy and the angular velocity.  Thus the swing goes to a higher peak.  The energy comes from the swinger doing work against the centripetal force on the swing pivots.


I think you're right.

It has to be a center-of-gravity thing. That's the only thing that changes. Throwing your legs out in front isn't going to add energy (equal and opposite force is applied).

But...when you change the center of gravity the pendulum seeks to return to the middle. If you keep oscillating the center of gravity in the right rhythm, then energy will added to each swing.


Madsci.org is written, supposedly, by science profs. Oh well, nobody's perfect.

"Center of gravity" is a misleading term in this context, because two pendulums with identical pivots and centers of mass can have very different characteristics.

I'd try writing more but I've got a backlog of stuff I've been trying to blog and I really need to finish more of it - just got one piece posted but I'm out of energy (no pun intended).

To make use of Solar Power, Fresnel Lens is the ideal one. When sun rays are focussed to a point, experiment shows that even iron can be melted.Fresnel lens have been tried to generate even power up to 1000s of watts.

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