Magnifying Glass

It's a fine idea...it's just cheaper and easier to use mirrors.
BIll posted this thread about a diy solar collector.

I might try and build a solar kettle this summer so I can have a nice cup of tea .

Keep on dreaming...I always say my job is to have bad ideas...'cos I need 99 bad ones to get to the good one.

Have fun

Del

With lenses, you'd have problems with focus and tracking. Fresnel lenses might be of more use than conventional ones. Mirrors (as Del said) are much better, but you need to keep them tracking or you'll upset pilots.

http://www.trec-uk.org.uk/csp.htm

There's only so much available solar on a given area, so you need to decide how much you want to concentrate/harvest it. For making a cuppa, you'd want a small quantity of high-grade heat, but for heating a bath you'd need a greater quantity of low grade heat. The former sounds better because it's easier to convert it to low-grade than the other way around. I didn't explain that well, but it's a sort of volume versus temperature thing.

It's not possible to focus a magnifying glass when it's not face on to the sun. You can't get a nice pinpoint spot and they won't bend the light sufficiently to concentrate all the beams.

The best you can do is to heat a large area with pinpoints scattered over its surface.

Someone please tell me if I'm wrong.

Yes you are right Vulcan. With lens it is impossible. It is only possible with mirrors.

No Doc needed. In a sense you are right.

But putting many lenses is same as using one big lens. But here also you are right, as making realy big lens is not easy and cheap. Now a days big telescopes are made like this only by using many mirrors (hexagonal) to form effectively one big mirror.

Using more lenses or one big lens you increase the area of collection. Thus more energy.

Principally, you can run the turbine. But keeping the system in sync with moving sun is not so easy.

I believe about 20-25 years ago "The Mother Earth News" had a design for a 10x10 grid of 12" square mirrors on a frame work to focus the solar energy on a small boiler. Unit did have a relatively simple control to move the array to track the sun's position. I think I might have a copy in the garage somewhere.

Output of the particular design was limited to what 100sq ft of solar energy could generate.

It might not be practical, but it can be loads of fun: www.solardeathray.com

In principle, yes. But the question is whether it is practical (favourable cost-benefit ratio). It may be useful to recall that the solar constant is ~1380Watts/sq. meter).That is, if you collect solar energy over a square meter (that is the combined lens/mirror useful aperture), all you get is slightly more tha a kilowatt, after accounting for transmission/reflection losses. Together with specific heat data of liquid,required amount of steam etc., it will help to have an idea of size of collecting optics to get required amount of energy. One may get an idea of cost/ practicality by taking a look at commercially available solar water heaters meant for installation on hoyuse roof tops.

Finally somebody bring in the notion of power available! Bravo.

Basically out of this 1.3KW/sq. meter, only a fraction will be in the infra-red range (heat). This means that you will probably get one or two hundred watts per sq. meter.

You will never reach a useful temperature if your power source has less KW available than what you need for your process. If you want to generate 10KW of electricity to run an efficient home, you will need at least 50 to 100 sq. meter of mirrors properly oriented with a mechanical system to track the sun.

Marcot:

Good point about the IR yield. And my offering of the Neural Network positioning system wasn't intended to be anywhere near powerful enough to move multiple square meters of mirrors (It would be a stretch for it to move a 3 meter dish, a la the older TV network dishes found along the road here in the USA, but I'd sure be willing to try it!)

Micah

Hi IreLandEng,

Nothing scatty about that thought & I did the same thing frightning bugs for miles.

Your idea, was first perfected by French physicist named: Augustin-Jean Fresnel and his Fresnel lens, which reduced the amount of material required compared, to a conventional spherical magnifying lenses, by breaking the lens into a set of concentric annular sections, that he called his Fresnel zones. Conventional convex lenses, were too thick, heavy and expensive to produce and this leap in science, really brought us, the stronger functioning lights for modern lighthouses of that age; actually making them clearly visible, for the first time, an incredible 20+ miles away.

Today, we use them in everything from specialized lighting, to projection televisions. Back years ago a Glass Fresnel lens, was even used as the lighting reflector of sorts, for theater projectors and they did a fine job, though we were on carbon arc. rods then.

Your idea is exemplified, in many demonstrations, of the Fresnel lenses great power, at a web site I saw the other night. Its run by a young salesman & his very attractive wife , at greenpowerscience.com , where they sell lenses and demonstrate the ways of using their stuff , while doing video shows. The shows look like they were intended for weekend broadcast shows and which are now posted over at: youtube.com .

Keep Dreaming Fellow, your on the right historical tract.

Best Regards,

Joe Woodall, Managing Partner

Georgia Adobe Rammed Earth & Renewable Energy

2395 Bowman Hwy. NW.

Dewy Rose, GA 30634

www.georgiaadobe.com

As with so many good ideas, it's already been done:

http://news.bbc.co.uk/2/hi/science/nature/6616651.stm

http://www.nrel.gov/docs/fy01osti/28751.pdf

You have a good memory! I recall seeing a TV show about that. I wonder what ever came from it?

Mike

Did you see this: apparently old back projection TVs had a Fresnel lens as part of the screen, and, you can pick them up for almost nothing.

http://www.instructables.com/id/Giant-Fresnel-Lens-Deathray-An-Experiment-in-Opti/

Watch out for his warning to ensure you wear suitable protective (dark) glasses.

Your dream may become a fact within a few years. The same dream is how to get energy due to oceans' waves and also from differences in temperatures between upper and lower levels of ocean water.

hi

this is pretty old systems probably early men used this to lit fire and also to coock their food. using lens stand fixed to motor to control the movement of lens so that we get focused sharp light it is possible to acheive what you have thought of

raghunath

...

I don't think early man was into optics!
I think they uses naturally occuring things like flint and nodules of metalic ores, or the did it with friction or actually carried the fire around with them as a piece of smouldering dry fungus or such like.

Del

Ya, I do not remember using optics for making fire... Earlyman

A brilliant idea, just a little late for a patent though , there is an 11 megawatt system based on this idea operating in Spain with bigger ones planned. See link http://www.technologyreview.com/microsites/spain/solar/

There is a project in the French Pyrenees that is using a solar boiler. Here is a url that will help your search:

http://adsabs.harvard.edu/abs/1977Sunwo........7H

The project is for a 1 mW solar boiler using a parabolic concentrator. Here is another interesting site:

http://www.montanagreenpower.com/solar/curriculum/timeline.html

No, you are not crazy - but most kids fooling around with a magnifying glass and the sun usually just use it to burn ants. There are also interesting articles on how to start a fire using ice...you shape it into a lens.

Right now, I am researching infra-red reflective paints to put on the sunny side of my house. There is a compound you can add to standard paint that you can put on the ceilings...it reflects heat back into the room (for winter fuel savings). I am currently researching using a glass bead filled clearcoat that would go onto my roofing shingles...the light hitting the glass beads never makes it past them into the attic, so the house stays cooler in hot weather.

By the way, you wouldn't need to go to the expense of buying a large magnifying glass...an old satellite TV dish coated with a reflective surface would be much cheaper. The other thing you might check out is the idea of using water as a lens...anything to avoid the cost of precision ground optics!

On a smaller scale there is a product called the solar cooker that is being sold to people in small villages where firewood is becoming scarce. It is basically a small umbrella made with reflective material. The lady of the hut places her meal to be cooked on a small stand and points everything to the sun. In a few hours her meal has been cooked. Thus ending the several mile trek for firewood.

Taking into account the length of time that cooking can take, a neural network (modern term for an analog computing/positioning system) and a magnifying glass, WITH an old parabolic reflector, makes a lot of sense.

Set it up this way. Using a cradle capable of movement while supporting all of the components (make it out of salvaged lumber, to keep it cheap), with the ability to move in both East-West and North South directions (one rocking, the other slewing, or one rocking motion riding another rocking motion), use a parabolic mirror to collect the sunlight, focussing its output through a fresnel lens, onto the cooking stage.

This lets the parabolic lens ( a dish lined with aluminum foil) focus the light, solving the problem depicted in post #3, and the fresnel lens concentrate that light, while maintaining a proper focal length (due to the structure holding all the pieces in relation to each other) and delivering the thus focussed beam of sunlight to the cooking stage.

Using two cheap motors (think kids riding toy motors, for plenty of torque and sufficient duty cycle) and a 12 volt battery with a solar recharging panel (NOT under the concentrated beam, You don't want to cook the battery) to keep the battery charged, set beside the cooking stage, mounted ON the structure to track the sun right along with all the other parts, mount the motors so that one will rock the structure right to left, and the other top to bottom. Feed the motors through simple solar cells (CdS cells work well for this, reducing resistance as sun strikes them, increasing it as sun fails to fall on them) mounted under the edge of the platform such that only when the platform is directly facing the sun are the solar cells in shade. Then when the sun is NOT directly hitting the platform, it will tilt in the necessary two axes to put the cell IN the shade, and the platform IN the sun. This keeps the entire assembly following the sun as it moves across the sky, and the center of the cooking stage at its hottest at all times of the day. Of course, at night, the entire system shuts down, neither charging nor discharging, due to a total lack of sunlight to drive it.

Given that the motors will not run constantly, the charging system should keep the battery at high enough charge to make the whole system self-sustaining. Then the only time you CAN'T cook with it will be when there is no sun. AND you don't have to keep moving it to point at the sun.

Micah

The solar cooker http://www.solarcookers.org/ is intended more for underdeveloped parts of the world. I doubt the villagers will be walking to Radio Shack for components to build or maintain your version. Sounds like a great conversation piece at a camp ground though. Set it up next to your tent and I would imagine there would be quite a few people stopping by for a look see.

No doubt of that, on either point. I use it more as a kick-off point for some of my Science Fair competition students. I have yet to get any to do the kinds of math I want them to do in support of the experiments they like to do, though.

Its always fun to blow stuff up. Its a lot less fun doing the planning and paperwork afterward, though. And the same is true of the math involved in physics, in the calculate, predict, experiment, prove, document cycle.

Micah

Perhaps you might concider making a big reflector from cob with your students? I have come up with a way of making a parabolic reflector that needs no math. (practically none anyway). It is called the mechanical mathematician and it is at solar cookers international. You and your students could make one pretty easily. It would be a good introduction to alternative building (cob) and alternative energy too. I used a 4ft by 4ft one last summer to steralize soil to kill weed seeds and bugs so I could start my vegetables. I worked all day so I wasn't there to use it for cooking. It worked good. This year, I am going to make one that follows the sun. Brian White BC Canada

I am currently working on a reflector to do just what you. It is a section from a parabolic dish covered with kitchen foil. It is to be mounted on "equatorial mount" so that all you need to do is point the reflector at the sun and the focus (also on the equatorial mount axis) stays at exactly the same spot all day as long as the reflector turns at 15 degrees per hour. Once or twice a week, it will need to be adjusted to repoint at the sun (due to seasonal change in the sun path). I still have not got anything that goes slow enough and strong enough to do the 15 degree per hour turning but everything else is almost ready. It is called the tracking solar barbecue and it can be found at the solar cooker international wiki. Brian White

Your fabulous..thank you.

How simple to think in the negative. Well done.

This solves the SunFlower problem I've been having.

Yours

Ric

I'm not completely sure from your description about the geometry of your setup, but it sounds like you are contemplating an array of (convex) lenses, packed densely together in a 'honeycomb' pattern. The problem with this is that conventional lenses can't redirect the light sideways-they only concentrate it. The optical axis of the lens is parallel on both the object (incoming) and image (outgoing) sides. Even if you tilt a lens so that the 'incoming' light strikes the lens at an angle, it will exit the lens in a cone of converging light on the other side, but the axis of that cone will be parallel to the 'incoming' (incident) angle. So no matter how you angle the lenses you would still end up with one hot spot for each lens. You would need a system of carefully angled mirrors (or prisms) to bend the light sideways to bring all the hot spots together in one place.

Since the system requires mirrors, the lenses don't really serve any purpose. A parabolic mirror will concentrate all the light and heat, without the use of a lens. I think that the current solution to this problem is to use an array of long troughs with parabolic cross sections, made from highly polished metal to focus the light and heat on a pipe running along the trough at the focus. The pipe is filled with a suitable heat conducting liquid (sodium) which is then pumped to a heat-exchanger/turbine to run a generator.

Your idea would work if you used cylindrical rather than spherical lenses. This would accomplish the same thing as the parabolic troughs. Using long cylindrical fresnel lenses could reduce the cost, but I doubt that it could be competitive with polished metal. Both systems require some maintenance to keep the surfaces clean, but I think the metal would be more durable, especially if you expect hail storms.

Somebody tell me I've lost it - what would happen if you have a funnel shape, lined with reflective coating ? Would light enter the wide end, and bounce all the way down and thru the small end ? Can I make a death-ray if the funnel has an acute enough angle ! It's late in the day and I need some coffee, OK !

If it works*, KrisDel^TM have the usual worldwide rights on it. When it's raining we'll use it to broadcast advertising slogans (for a fee, and only if they meet our high standards).

* This has no similarity to a light-tube whatsoever !

Thought you might find this interesting....

http://en.wikipedia.org/wiki/Surface_plasmon_resonance

Thanks electromagnetic1, it looks kind of complex ! I'll keep it in mind though, you never know when things will come in handy. CR4 has some good stuff on Solar systems, all the way from detailed analysis/hi-tech to home systems based largely on judgement/experiment.

When it is raining, you may use it for collecting the rain water

I thought that too but when you test it out, the light actually bounces out again after just a couple of bounces. There ARE solar funnel cookers which are very simple indeed to make (just stick foil to a 4ft by 2ft piece of cardboard and bend it into a funnel shape and tape a 4 ft side to a 2ft side to make the funnel. (You can leave the "wings" on to collect more light and heat). Then you stick your food in a 1 liter preserving jar in water, screw on the lid and sitck it in an oven bag about 6 inches high close to the middle of the funnel, point the funnel at the sun and let it cook. I add a little molasses to darken the water. The funnels cook surprisingly well!

A series of glasses in a circle will not work unless using a mirror or a special designed glass to bend the light to get more than one magnifying glass to heat up the same point. mirrors are better for this purpose. If you have access to an unused satellite disk in your garage or somewhere , you can cover its inside with aluminium foil and place your magnifying glass where the lnb is, this way you can produce much more heat in one spot. But this can hardly be "practical power" for many reasons. To run a tubine connected to alternator I think you will need a dozen of satellite disks in your garden to get enough power to light up a diode. If you are looking for a practical power, build a nuclear power plant. :)

This way you are going to melt the magnifying glass.

Wit dish and mirrors/ al foil, you don't need magnifying glass

You can make a water lens, by just using polythene sheet fixed on round support and filling the water in it. The focal length ca be adjusted by adjuasting the slackness of the sheets. Keep this assembly in open Sun.

The rays will focus and obviously go on shifting with position of Sun. Go on shifting your target with this focused ray.

With this method, you can make a big enough lens.

is there construction drawings?