Science Buddies: "Ask an Expert"

Hello,
For my science fair project I have planned to build a modified Crookes radiometer that generates electricity, and then test its performance (assuming it works) against that of a PV solar cell module.
For any who don't know, a Crookes radiometer consists of four panels suspended in a partial vacuum, with each panel having one white side (that is reflective to electromagnetic waves) and one black side (that absorbs the waves), with the given colored panels all facing the same direction around the suspension (i.e. white sides all face counterclockwise direction, and black sides all face clockwise, or vice versa). As light is absorbed more by the darker faces, the air on the black side heats up faster than the air on the white side, causing the air to move from the white side to the black side (as the warmer air creates an area of lower pressure). The air flowing from one side to the other exerts tangential forces on the panels (similar to what occurs in thermal transpiration), causing them to spin.

As currently produced Crookes radiometers are intended as trinkets rather than something useful, I am planning on fabricating my own, as this would also allow for necessary modifications to generate electricity. I am not, however, sure as to what would be the best specifications on several points:
1) What would be the best, within reason, material to use for the panels (mica, aluminum, photography paper, balsa wood?) and what would be an effective way to color it properly?

2) With my design, the panels are attached (not suspended, as in the original Crookes radiometer) on a central shaft which transfers the rotational energy to spin magnets, which induce current in copper wiring encircling them. Any ideas on how to suspend the panels (and accurately maintain balance)?

3) Would having two separate sets of four panels attached to the magnets create more torque than one set?

4) Because all of the forces calculated by Einstein and Reynolds come into play at the panels' edges, would it be advantageous to change the size or shape of the panels (from a small square) or add perforations? And if so, to what shape, or what size perforations at what intervals?

5)With the induction of electricity into copper wire around spinning magnets, coiled insulated copper wire will produce more [controlled] electricity than coiled (uninsulated) copper wire, correct? ("So that the flow of electricity doesn't loop back from one coil to a previous one" being the logic here)

6)In what research I've done, I have found conclusions dictating that 60 mTorr is the pressure yielding peak efficiency for spin. Is there a way to know that I have reached this point when evacuating the encasing chamber other than sealing a pressure gauge in/onto the encasement?

And for the actual data collection:
I currently have a multimeter with functions for measuring the voltage (AC or DC; up to 500 and 600 V, respectively) and for resistance (up to 2 million Ohms). I know that if one knows the voltage and resistance, the flow, and therefore power, can be easily calculated. Before I try this, though, I want to make sure that measuring the resistance of the entire system yields an accurate result (i.e. hooking up a PV module or the generator to the multimeter, once to measure voltage and again to measure resistance to calculate how much power is produced by either).
Also, I know that the generator I plan to make will produce AC electricity, while PV cells produce DC. Is the power conversion between the two something other than 1 (where 100W of AC = 100W of DC, and 55AC > 54 DC), or is there a formula that provides a more objective analysis?

This is a very interesting idea. My best guess, however, is that the forces developed by the vanes will prove too small to generate electricity. Some other peoples guesses can be read at this URL:
http://ask.metafilter.com/29248/Could-a ... lectricity

Still, you could prove that these guesses are in fact correct -- I don't know of anyone who has tried that. I think, just of the top of my head, that the best approach would be to start by trying to measure the actual torque as a function of the intensity of illumination for paddles of varying design. The usual method used to measure tiny torques is a device called a quartz fiber torque balance. One would suspend the balanced vane assembly in a suitable low pressure environment from a thin quartz fiber. At the bottom, just above the vanes, one would mount a small mirror. When the vanes are illuminated, the resulting torque will twist the fiber, which will turn the mirror. By illuminating the mirror with a ray of light (from a laser pointer perhaps) even small rotations of the mirror will produce easily measurable deflections of the spot of light due to the long lever arm of the optical readout. The motion of the spot is, of course, proportional to the small angular deflection of the mirror multiplied by the (long) distance between the mirror pivot point and the readout scale. The beauty of this device is that the light produces virtually no back torque on the system under test. One control the sensitivity by the choice of the size and length of the quartz fiber. If you can't find a quartz fiober, you might try using bare single-mode optical fiber, which is pretty readily available. (I bet you can get a few meters for free from a fiber optics vendor just by asking nicely.) You will need to work out how to calibrate your torque balance. It will be highly linear, i.e. the angle will be proportional to the torque. Look up torque balances on the web for ideas; this technique has a long history in sensitive physics experiments, so there ought to be a good deal of material. If the calibration proves too hard, you might either settle for relative measurements on various designs or for caculating the torque/twist relationship from the already known mechanical properties of quartz (or glass).

Concerning your explicit questions:
1.The vane material must be a poor conductor of heat, stiff, easy to work with, and stable under vacuum. Glass, mica, or plastics might work.
2. As I have already said, I don't think you can make a mechanical system that will transfer torque from the vanes to a generator with sufficiently low friction to work. If I were to try this I would look into using the innards of an old mechanical pocket watch with jewel bearings.
3. Yes, but there is a lot of value in keeping everything as simple as possible.
4. That's a really good question that you can study. My guess: perforations. But I don't think anyone knows. You could be the first.
5. You are correct. Use varnished magnet wire.
6. Why not make the pressure one of the variables you study. I'd guess you would need to buy, or better, borrow a pressure gauge for the appropriate range of vacuum. Check out eBay.

Thanks for all the input.
I've realized while working on preparing this project that there are so many unknowns and different variables that would each need an experiment, if not several, to find the ideal values (such as for the size/shape, perforation, vane material, suspension/design, pressure, etc.).
Unfortunately I didn't/don't feel I have enough time to go through all of these (my teacher wants the data for the project collected w/i the next two weeks), and decided (somewhat unknowingly when I initiated this project, and greatly to my own detriment, in all probability) to attempt to skip ahead to building the entire unit. Your ideas for other tests are good ones, of course, which would have been good for me to do earlier, assuming I'd thought of them and had time (like testing torque would've been a better experiment, as it would allow one to see if mine has even the ability to succeed).
Anyways. . .

Do you know if it would be better to use larger gauge (physically thinner) or lower gauge (thicker) wiring, or a specific middle ground, as thinner wires have higher resistance but obviously allow for more turns around the rotating magnets (which is supposed to increase voltage induced)? Or would rectangular wires work better for induction, and under what orientation?

Also, do you have any ideas for how to silver/make reflective one side of a plastic or glass vane?

Thanks again

Do you know if it would be better to use larger gauge (physically thinner) or lower gauge (thicker) wiring, or a specific middle ground, as thinner wires have higher resistance but obviously allow for more turns around the rotating magnets (which is supposed to increase voltage induced)? Or would rectangular wires work better for induction, and under what orientation?

Just off the top of my head, I would think that 1) the number of turns for a given total cross section for the coil will be inversely proportional to the area of the cross section of the wire, 2) the electromotive force (EMF) induced in the coil will be proportional to the number of turns and, hence, inversely proportional to the cross section area of the wire, 3) the resistance of the wire per unit length will be inversely proportional to its area, 4) the resistance, R, of the coil will be proportional to the resistance per unit length times the length of the wire, which length will be proportional to the number of turns and hence inversely proportional to the cross section area of the wire: the product of the resistance per unit length and the length will then be inversely proportional to the square of the cross section area of the wire, and finally 5) by Ohm's law the current, I, will be equal to (EMF)/R, which will be proportional to
(1/A)/(1/A)^2 = A, where A is the cross section area of the wire. This reasoning would lead one to use thick wire rather than thin wire to maximize current. Note, however that the power produced will be I times EMF so that the power will go as A(1/A), i.e. it will not depend on the cross section area of the wire. However, I worry that there may be a flaw in my reasoning, because in actual real world applications electro-magnets tend to be wound with very thin wire. Perhaps in most applications either the voltage is fixed (say, if one uses a battery to energize an electro-magnet); or one wants to maximize the voltage rather than the current (e.g. a dynamic microphone feeding a high input impedance amplifier), and by point 2 above, the voltage is maximized by minimizing the cross sectional area of the wire.

Also, do you have any ideas for how to silver/make reflective one side of a plastic or glass vane?

It would be easier to make it have a high albedo (diffuse reflectivity rather than specular reflectivity). Titanium white paint will provide a high albedo coating. Titanium white can be found in small amounts at art supply stores; in gallons, at paint stores. The more expensive paints will generally have more titanium dioxide in them and as a result the higher albedo.

Good luck with your deadline!!

Hello,

Did you manage to build a generator using Crookes Radiometer Principles?

It seems to be a really nice project and I am curious about the result of your researches.

Thanks,
Kamel

Kamel,

As of yet, I have not succeeded in building this generator.
The reason for this, however, is not because it is necessarily impossible, but rather that, during construction (which is ongoing) I've had several obstructions, the latest of which being creating and holding a vacuum. (I had tried using a couple different boxes, and have now switched to a cylindrical body for the apparatus, due to the fewer number of seams to seal and superior strength of cylinder to hold over that of butt joints.)
Most of the obstacles I faced in fabricating the generator were not knowing what to do; when I began this project, I did not realize quite how many variables there are and the amount of research that would need to be put in to find the best answer. I did not fully realize how ambitious this project was until the time was about half over. To me, that did not seem like near enough time to continue spending the majority of my time doing research (paperwork also suffered), so I ended up making a multitude of assumptions and missing key aspects while thinking through what I was doing and thinking ahead--too many of my decisions were arbitrarily based, depending on what materials I had (access to/resources for) and what "logically" seemed to make sense, or even just what I thought of at the time (though , rather than from conclusions based on in-depth research and complete analysis.

So yes, all of the basic principles that this should work on seem sound, but I wasn't prepared for the planning part.

Let me know if you have more questions or some suggestions.

"...when I began this project, I did not realize quite how many variables there are and the amount of research that would need to be put in to find the best answer. I did not fully realize how ambitious this project was until the time was about half over..."

Sounds so familiar to me. I've had the same experience many a time during my scientific career. Research seems to take longer and cost more than one ever imagines, even after a lot of experience. It's probably for the best -- if we knew just how hard projects would be, we wouldn't have the nerve to start any. In my view to be happy in a research career you must be able to enjoy the process not just the end results, because you will spend 90% of your efforts struggling with problems and only 10% using the finished product. For me the answer was yes, I really did like messing around on the way to my goal as well as (sometimes) achieving it. Research can be the best job in the world!

Best of luck, whatever career path you take.

So are you still working on this project? Your project was due the 24th of February. Maybe are you doing this on your own and not for school anymore?

Also did you find any relevent information (internet, books...) to build a Generator using Crookes Radiometer Principles?
Do you know any similar project that has succeded?
Or maybe did you find any relevent information indicating that this project cannot be done?

Thanks

Thanks for the empathy, John. It is still interesting, even if not so successful.

And yes, I am still currently working on the project. I'd received an extension on the deadlines, but at this point in time the fair is in a couple weeks, so there are now non-negotiable headlines this week so there will be time left for fine-tuning the written portion.
Although I might continue to work on this project after the fair (and after I'm required to for school), but it will be much slower than the pace I'm trying right now with a lot more emphasis on the research before further construction attempts.

As to relevant information, I've used the internet mostly (for ease of access):
right now I have over 50 different pages bookmarked just for this project, although many are Wikipedia or other pages mostly about specific aspects involved in a generator or Crookes radiometer (also includes a bunch of original papers by Crookes and his contemporaries).
For pages that discuss/propose a generator using radiometer forces, I've only found a couple with finalized/realized plans for building one, while most or all of the rest contain lengthy forum discussions with seemingly rather opinionated arguments and lots of physics equations that I currently lack the education to understand.

Here are a couple of the design pages:
http://www.ece.drexel.edu/pix/cmf2.pdf
http://www.freepatentsonline.com/0685958.pdf
spiedigitallibrary.org/proceedings/resource/2/psisdg/7787/1/77870P_1?isAuthorized=no
(This last one is the paper on the experiment shown in the first link, I believe--haven't bought it)

Among the discussion boards there are many reasons proposed as to why an efficient/effective radiometric generator might be impossible to create. They mostly reference the inefficiency of conversion of sunlight into kinetic energy and the problems with scaling it up:
The (known) forces that spin it all act around the perimeter, but area will increase by the square of the increase of perimeter scale factor, causing an greater increase in inertia and friction than in force for rotation. There is also the problem of the energy investment in evacuating containers large enough to hold up-scaled panels; larger panels also require higher evacuation of the container to keep the mean free path of molecules comparable to the size of the panels (this might be counteracted through perforation of panels or other modification to decrease the distance between edges).

Thank you very much for sharing all these info.

I wish you the best!

Mr J6Taylor (I couldn't find your name),

Thank you very much for this article:
spiedigitallibrary.org/proceedings/resource/2/psisdg/7787/1/77870P_1?isAuthorized=no

I guess you didn't read it!

However there is a lot of information that could be vey helpful for your project.

For more info contact me in private [Information deleted by moderator]
K.

Please note that Science Buddies does not allow anyone to post personally-identifying information on the board.

If interested, in this article you can build your own radiometer generator. everything is explained step by step and corresponds to the pic you sent me:
http://www.ece.drexel.edu/pix/cmf2.pdf

I guess this is exactly what you are trying to do..