Mythbusters Death Ray

DaveC426913

Just saw the Mythbuster's ep where they build a fresnel type reflector that focusses the sun's rays. There's a guy online who built a small version of it: SolarDeathRay

Now, there's two things weird about both the construction mechanisms here.

1] In both cases, they calculated the angles of the mirrors, an arduous and finicky task at best. I don't understand why that's necessary at all. Do it empirically! Shine a light on each mirror one at a time and adjust it manually until its beam is at the focal point. Why is that so hard??? Heck, you don't even need fixed parts. I'm thinkin' (at least in the case of the smaller one) a wad of bubble gum will form your base - infinitely adjustable.

2] The Mythbusters were exploring an ancient myth that has a warring race on the Nile or something using a death ray on a Phoenician ship at sea. They declard the myth busted because they couldn't get the apparatus to concentrate enough light to acheive ignition. But their reasons were all purely logistical - all to do with stability and adaptability of the apparatus as well as the problems in aiming at a moving target.

It seems to me, that you don't need an apparatus at all. All you need is 300 soldiers holding 300 mirrors. They line up on the shore (creating a 2 dimensional fresnel-type reflector), and they each handle their own focussing. This eliminates all the logistical problems. Every lens is now dynamically and intelligently controlled.

OK, so I need 300 volunteers to meet me in the parking lot at the corner of Lakeshore and Brown's Line. Please bring a mirror...

willib

i agree that 300 individally controllrd mirrors would probably catch an old sailing ship on fire..
i suppose that the distance from the mirrors to the ship might have a lot to do with it though..
and mirror size also..

cronxeh

well max you'll ever get is 1.4 kW/m^2

now this number might go down due to air convection and mirror conduction to maybe 1.0-1.2 kW/m^2

so its not how many mirrors you got, its the total area and the type of material you are trying to ignite. For the case of wood you would probably need approximately 300 degrees C constant, so thats about 1 sq m of those mirrors

JamesU

actually, the mythbusters' death ray was flat, that's why it didn't work.
Can you supply mirrors for $1.00?

cronxeh

pretty good idea came up when i was fiddling around with this on my notepad..

if one can harness 1.0 kW/m^2 of heat and transform that into steam and then run through power turbines and generate electricity

as compared to pvc or solar cells, how much more or less efficient the electro-mechanical method is?

cronxeh

OF COURSE!

Sterling Engine!

Edit: the mirror alignment with the Sun's normal vector is a hard task, I dont see how this could be achieved without a computerised controller

morry

Wasnt it archimedes who came up with this idea? I have no idea whether it would work or not.

sid_galt

I don't think you can harness 1 kW. It's more like .66 kW and that too on perfect sunny days. Secondly, the initial investment is high as you need concentrators to concentrate the sunlight in one place. The heating is slower compared to say coal. You cannot use it on cloudy days. The storage methods for electricity in case of night are expensive. That said, I think there is a solar furnace plant somewhere in Europe in the mountains.

Integral

Let's get the history right, it was Archemedes at the siege of Syracuse (Sicily) during the 2nd Punic war.

There is much that is unknown about this, so unless the guys running the Myth Busters show have a mind the quality of Archimedes designing their devices, I do not think that they can honestly say they have busted the myth, only that they were unable to duplicate the feat.

Several hundred trained men with mirrors of the appropriate focal length could, conceivably, put a lot of energy in a small area.

Chi Meson

I tried this with one of my honrs classes a few years back. Can't say much about ancient ships, but 20 mirrors (about 3 x 5 inches each, and approximately 20 feet away) could not set a dried corn stalk on fire. It burned a few retinas though.

willib

I was googling Sterling Engine once and came across a company that used the sun to power a Sterling Engine.
dont have the link offhand, it wasnt cheap, but it was very cool..

LURCH

http://www.nrel.gov/documents/solar_energy.html

About halfway down the page.

Also, this company;
http://www.pureenergysystems.com/new...olar_Hydrogen/
has produced temps of over 1500^oF with an 18-inch mirror! Wood lights up at just 450^o

russ_watters

Sorry, cronxeh - already under development: http://www.solarpaces.org/technology/tower.html No patent for you!

cronxeh

that design is a ridiculous waste of land space

roughly twice as big as it should be really

DaveC426913

The total area of material you are trying to ignite is, ideally, zero. The whole point is to concentrate the light on as small an area as practically possible.

DaveC426913

No, it's supposed to be flat. See the construction of the Death Ray in the link in my initial post.

It's a fresnel lens. The flatness of the superstructure is accounted for by the angles of the mirrors (that's what a fresnel lens does.)

The reason why it didn't work is becasue it was too flimsy, and they couldn't get all lenses focussed on one spot. You can see when they're moving it, that the superstructure is quite flexible and unstable.

cronxeh

no no I meant the area of a focusing mirror. From physics, we know that the maximum amount of solar power we can harness is 1.4 kW/m^2. The area of your telescopic or parabolic mirror is thus the upper limit on how much you can get out of the Sun. The lower limit of course is the clouds and other environmental conditions, plus the Sun's relative angle