Things You Can Do With A Giant Fresnel Lens 469
Ant writes "Here is a link where this guy always wanted Edmund Scientific's Giant Fresnel Lens. 'Melts asphalt in seconds!' the ad said. When he went to graduate school he met several other people with the same enthusiasm for aimless destruction through bizarre means, and just enough combined cash to make it happen. Thus the reign of terror began."
Re:Link doesn't work anymore... (Score:1, Informative)
Possible source of free lenses (Score:5, Informative)
Cooking idea: Take a length of thin all-thread and turn it with a slow motor, with a matching nut fastened to a board so that the all-thread and motor are slowly pulled along. Spear a few hot dogs on the all-thread and set the lens to a medium concentration. Spin up the motor, and the sun will cook the hot dogs in a spiral....
Re:Mindless (Score:5, Informative)
But, onto the other point: many geeks like high amounts of kinetic energy. It's true. Often, this love tuns into the irrational lust for wanton destruction of random objects. Sometimes, something is learned by the results, sometimes not. But it's the journey that's important (fun).
Re:$99 for the cheap fresnel... (Score:5, Informative)
- In bright summer daylight, at noon the sun provides 1200W/m2
- This fresnel lens is 80x100, so captures 1200 * 0.8 = 960W at best
- A good steam engine, with a condenser and exhaust reheater provides has an efficiency of about 30%, so it would give 960 * 0.3 = 288W in mechanical power
- A good alternator, going at its preferred RPM (not necessarily that of the steam engine's prefered RPM, but let's assume) has an efficiency of about 90%, so it would give 288 * 0.9 = 260W
So you'd get 260W in the best possible conditions, in the brightest of days, in summer, at midday. Throw some clouds and, assuming the entire thing doesn't stall and stays at its nominal efficiency (not likely, but let's assume), you get about 6 times less power, so about 43W
In short, you're better off with solar panels: perhaps a little less efficiency for the same price, but more surface and a lot less aggravation.
Re:Mindless (Score:5, Informative)
Re:wtf (Score:4, Informative)
Re:Mindless (Score:5, Informative)
Re:Mindless (Score:2, Informative)
Ok, working from memory here, and being lazy, so I'm not going to bother balancing equations:
Chalk (Calcium carbonate) + Oxygen -> Calcium oxide + Carbon dioxide
Calcium oxide + Water -> Calcium hydroxide
Calcium hydroxide is a strong base, so putting your finger in it will probably result in severe alkali burns.
Re:Scaled Up (Score:2, Informative)
Probably not, and yes. A lens of that size would be extremely difficult to build and operate. Instead, you'd use an array of smaller lenses, or even better, mirrors. If you look at large telescopes, most use mirrors in part because of weight issues and manufacturing issues with lenses over say 1-meter.
The size of the lens does increase temperature, or at least the energy density at the focal point. A bigger lens (or mirror) can capture and direct more solar energy.
Not really (Score:2, Informative)
And very possibly the paint they use to put the logo etc... on the can. He also stated that the aluminium can smelled really bad, not the aluminium that the can was made of. So when referencing to the can in that way would mean everything involved that makes it an aluminium can.
Seriously, this article is all about playing with a new destructive toy and not much about using the toy in question to do interesting science-related experiments.
While the expierments they did were fun, then did put some science into it.
The FAQ [umich.edu]
Impressive as destroying a penny may seem, I estimated that we may have only managed to get maybe 10 percent of the available energy hitting the lens (roughly 1kW) into the penny:
* Mass of a zinc penny: 0.0025 kg
* Specific heat of zinc: 390 J/kgK
* Melting point of zinc: 419.58 degC
o Thus 20degC to about 420degC takes 390 J
* Latent heat of fusion for zinc: 1.1x10e5 J/kg
o Thus to melt the penny takes about 275 J
* We heated the liquid zinc considerably as well, but I will ignore that.
o Total energy in the penny: > 665 J
* It takes something under or around 6 seconds to melt a penny:
o 665 J / 6 sec gives us a lower limit of about >= 111 Watts
* Sunlight at the earth: 1365 W/m^2
* Transmission of the earth's atmosphere: maybe 65-70% at this angle with some clouds?
* Area of the lens is about 1.1m^2
o Power on the lens: approx. 1000 W
o Power to the penny >= 10.0%
Re:Would this be possible? (Score:4, Informative)
Re:Ideas (Score:5, Informative)
another place you can get one.. (Score:5, Informative)
since I repair them for a living, ive actually done this.
its quite fun, but do be careful!
I like using the fresnel from a 60" projection tv the most
I have burned up phone books in no time with it, and I have tried cans, I got one to melt.
next time you see a projection tv in the trash, get the lens.
the lens will be the innermost of the 2 (or 3, if there is a protective screen)
have fun!
build your own spiral fresnel reflector (Score:3, Informative)
popping concrete et al (Score:4, Informative)
As would burning tar, or any other heavy petroleum derivate.
Concrete doesn't contain the slightest amount of petroleum. You're thinking of -asphalt-, which is entirely different.
What smoked was contaminants on the surface of the concrete, and possibly some stabilizers. It popped because of the moisture in the concrete expanded- concrete doesn't handle much except external compression very well.
Aluminium doesn't smell bad when it burns. I suspect whatever soda pop chemicals remaining in the can do.
No, more likely the label ink.
It's being done! (Score:5, Informative)
Overall it works better than solar cells because it's so simple and you harness the heat energy rather than the light itself, but therer's only economy to it on a large scale, you need enough space to get a huge amount of water to constantly boil. Also, it's significantly harder to get this thing working on less-than-ideal days; solar cells still collect juice on slightly cloudy or overcast days, but this method doesn't work nearly as well.
Still, a good way to apply solar energy when in conditions that permit. I'd like to set up a small unit with a fresnel lens and 'boiling globe' to generate hot water (which I'll pump through a radiator) for my house in the winter. The problem I see is with safety, that beam has to be EXACTLY where I want it or I'll burn the house down.
Watch Your Eyes (Score:5, Informative)
One thing that cannot be overstated is the use of eye protection. And, whatever is selected for that application must handle IR as well as visible light. (Nearly all of the UV is absorbed by the plastic the lens is made out of, so it is not much of a factor.)
Using such a lens, to focus solar radiation, can produce power densities equivalent of a Class-IV laser; where the warnings typically read "avoid exposure to direct or scattered radiation". Even if focused to a spot size of 4cm^2--at an estimated 1kW--the power density would still fit 2.5W/mm^2. This is the same level as a 10W laser, with 2mm beam focus.
Granted that the focus is only at one point, it is easy to overlook when scattered radiation--from a "point" source--can be dangerous.
As the article states, use very heavy welding goggles, and maybe have some sunglasses on under those! It is also recommended to ensure that the goggles cover the infrared parts of the spectrum effectively.
Also note: laser safety goggles would be ineffective for this application, due to the fact that they typically use dichros, which typically are not very "wide-band". They reflect very specific wavelengths--very efficiently. But, since solar radiation is very wide-band, a lot of it will still get through.
Surprised no one mentioned... (Score:5, Informative)
Cool, but if you want destruction on the cheap (Score:3, Informative)
Re:Laser Communications (Score:2, Informative)
To keep down the S/N ratio, most long-range laser comm experiments are done at night. I suppose safety is another good reason for working after the sun goes down!
Re:It's being done! (Score:5, Informative)
Regular solar arrays need to have panels or mirrors that track the sun - lots of stuff to break over a large area, fancy focusing algorithms and sensors and motors...
Often simple is best with stuff like this - that's how many home solar heating systems work - forget completely replacing the grid, just pipe some water through panels on the roof to heat it up...
Re:Possible use as power source? (Score:2, Informative)
Re:Watch Your Eyes (Score:4, Informative)
http://yarchive.net/metal/welding_filter_glass.
Re:Edmunds Scientific is gone, gone (Score:2, Informative)
Re: Melting Glass (Score:1, Informative)
Re:Burnination (Score:2, Informative)
The temperature you can achieve is mainly determined by the ratio of the diameter of the lens to the focal length. A larger lens with the same ratio lets you heat up larger objects, but it won't get small objects much hotter. A penny was much too large for my lens to have a visible effect on.
solar cell concentrator? (Score:3, Informative)