Saving the World With a Giant Solar Death Ray




So you have finally gone and done it- you've gotten a giant fresnel lens and you have mounted it so you can use the 1000+ square inches of solar collection to really heat things up.

Then what?

Laugh your Maddest Scientist all you want, because it is time to use those powers for good, and not evil.


Step 1: Safety

Be safe! The solar concentrator is not a toy*, and can cause real damage in a matter of moments.

  • The most important thing is good eye protection- the focal point is incredibly bright, and you WILL hurt your eyes if you look straight at it!
    I use a pair of polarized sunglasses underneath a cheap set of welding glasses. This cuts the reflection down to a tolerable level.
  • Keep pets and children away- the focal point for my set up is just the right height to burn a dog- or a baby. It only takes a moment, especially if you are heating metal.
  • Don't leave it unattended- as the sun travels across the sky, the focal point will move. With my set up, I have to readjust it every 10 minute or so. Make sure yours won't accidentally traverse something flammable.
  • When you aren't using it, store it safely. I keep mine in a basement with no windows. If I have to move it, I wrap the lens in an opaque blanket. This protects the lens, and keeps it from starting a fire by accident.

*OK, maybe it is kind of a toy, but it is a dangerous one.

Step 2: Frying Eggs

A obvious choice, and a classic one. Nothing demonstrates more concretely just how hot the focal point is than this, because it relates it to an activity most of us are familiar with. It gives you a good intuitive sense of how hot the focus is, and just how careful you should be.

Would you touch a stove? No, so don't get near the focal point.

My lens will fry an egg in a few minutes on a fall day, or roughly the same amount of time that my stove does. I expect it will cook quicker during the high heat of summer.

Once you have fried an egg, why not try something a little more complex?

Step 3: De-icing Your Steps

Maybe a different frame would work better, but mine was not very good at this. The focal length is fixed, so only one of my steps is getting heated.

Since it is a concentrator, it is effectively acting as a sun shade for every piece NOT in the beam. Fortunately, the heated water flows away from the focal point and melts the surrounding ice. Kind of like this concept, but without the infrastructure.

I tried to use it to melt snowbanks faster, but the frame was too awkward to do it well. Maybe next winter.

Step 4: Making Charcoal, Char Cloth, or Biochar

By putting organic material in a mostly sealed container, you can heat it to a point that drives off volatile gases and water, and leaves just the carbon behind. Depending on what you use, this can be called charcoal, char cloth, or just biochar.

Charcoal is pretty handy to have around. It burns steady and hot. It also has a lot of uses for filtering, and artists use it to draw. Like I said, lots of uses.

Charcloth is the result of heating small pieces of cloth. It is often used by survivalists, because it can catch and hold a spark like few other materials.

Biochar is a catch all term for generic organic material. It is used as a soil additive, and is supposed to be good for the soil.

All of the above tend to be very energy intensive to make. In the old days, they used to use as much as 50% of the wood chopped as fuel to turn the rest into charcoal. How wasteful! Using a solar concentrator means that 100% of your material can be used, instead of burning it as fuel.

I use a very small tin as my heating vessel. It gets hot enough to vaporize the paint off the side, which reveals shiny metal. This is bad, because shiny means less efficient heating. I've tried to stain the metal with the smoke from inevitable fires from the solar death ray.

Step 5 of this instructable shows a good solar char cloth setup.

Step 5: Environmentally Friendly Weed Killing

You don't want to use poisons to kill weeds around your food garden- the garden club bylaws are pretty clear on that. But hoeing is such hard work!

Why not boil water to dump on the weeds while you work? No chemicals, no residue, and no effort on your part except to collect the hot water and pour. Oh yeah, and make sure that no one gets blinded by your setup or burns the garden down.

Here, you can see several weeds that have been boiled next to one that wasn't. The heated water is fast and effective, and leaves no trace behind. Preheating the water you intend to boil makes the process go faster- in my case, I filled a 500 ml water bottle from the hose, and left it in the sun while the pan of water was heating.

The solar Concentrator is hot enough that bubbles will form almost instantly at the focal point, but it will take about ten minutes for the heat to disperse through the water.

Step 6: Sterilizing Soil

Soil for planting is often sterilized, so that any seeds from weeds or parasites are eliminated, ensuring that only the plants you want are free to sprout. Many people use large sheets of black plastic to heat the soil with solar power, but that takes weeks. You can put the soil in the microwave or oven for smaller batches, but why waste energy? The solar concentrator can easily hit the 200F that is recommended.

Here, I have placed the soil out of the focus point, and probably should have placed it even lower. In the video, you can see that it only takes a minute or two for the soil to start smoking- those are the organic components of the soil vaporizing. I understand they can become toxic if the temperature goes too high. Low and slow are the way to go here- no need to roast or melt it.

I've added the cooked soil to my tomatoes, which could use some more dirt to replace some lost to erosion.

Step 7: What Else? You Tell Me.

I look forward to suggestions on other environmentally friendly ways to use a solar concentrator. I'll try out feasible suggestions in the comments over the summer and report back how well they work.

Something I think would be interesting would be to try generating power- perhaps with a Stirling engine, or a Biolite stove.



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    60 Discussions


    Reply 4 years ago on Step 7

    It occurred to me that the smoke would build up on the inside of the lens so I would recommend adding a plate of regular glass between the lens and the enclosure, that way you can clean the glass easily between batches of smokables.


    Reply 4 years ago on Introduction

    This is a very good and feasible idea! As a vegetarian, I don't often think about this sort of thing very often.

    I've been considering putting some sort of "smoke shield" in front of the lens already, because things burst into flames on a regular basis under it. (it set the DIRT on fire!)

    What is the temperature range for a smoker?

    Bill WW

    4 years ago on Introduction

    Excellent Instructable. There have to be some great applications for using concentrated sun radiation to produce useable energy. The typical solar panel in use currently is only about 15% efficient. So from the 1000 w/m^2 we get from the sun, a solar panel will only produce 150 w max.

    You could try using your lens as the heat source for a Stirling engine, but the engines we hobbyists make are pretty inefficient. If you are interested, however, take a look at my Instructable:

    You would focus your lens on the top plate, and cool the bottom section

    Also, performance of most solar cells (photo voltaic cells) is enhanced by concentrating the sunlight upon the cell - but of course to a limit.

    You said: "The solar Concentrator is hot enough that bubbles will form almost instantly at the focal point, but it will take about ten minutes for the heat to disperse through the water." The bubbles are most likely dissolved air the comes out of water when heated.

    2 replies
    jkimballBill WW

    Reply 4 years ago on Introduction

    That's a very fine Stirling Engine- I could only dream to make one half as nice. I'll probably have to purchase one, since it is unlikely that I will find one in the garbage to salvage.

    Bill WWjkimball

    Reply 4 years ago on Introduction

    A small steam engine might be a better option. You could focus the concentrated sunlight directly on the boiler. I think you could really generate electrical power that way. A steam engine is much more efficient than the kind of Stirling engines we can build.

    Also, to get an idea of the heat at the focus, try melting various materials fo which you know the melting point - try lead (melting point 620F) or 95/5 solder, the solder commonly used for copper pipe, (450F).


    3 years ago

    Excellent titorial j. I'm currently on the path to self-sufficiency and
    energy respect. I've watched a few YouTube videos featuring the Fresnel Lens, nothing as large as yours it must be said, and am aware of the massive potentials of this application. My interest has been reignited... Large THANKS


    4 years ago

    I wouldn't tie up an Arduino building a solar tracker. You could use optical components and a stepper motor. You could build a sauna, heat a tube or coil of water into steam. I was thinking solar oven too. As for a smoker, just use a long tray for wood chips & the moving beam works in your favor. Get a round piece of glass & rotate it through the smoker window to keep it from getting too smoky.


    Reply 4 years ago on Introduction

    I don't feel I have enough control over temperature to roast something usefully. Can you roast coffee in a pan? (I don't drink coffee, so I have never tried to make it.)


    4 years ago on Introduction

    As a scrapper I'm going to try using this idea as a heat source on circuit boards to remove the components. Right now I am using a heat gun and holding the board still. If this can cut down on my electricity consumption then holding the lens still in its frame and moving the board around is an easy enough change to make in my procedure. Thanks.

    1 reply

    Reply 4 years ago on Introduction

    Temperature control will be an interesting problem, as you'll have to keep from setting the board on fire or roasting your parts. Depending on the size of the lens you have, you could try keeping the focal point wide (and thus safely low heat) and preheat the board, then use the heat gun for a controlled heat to pop out the components.


    You could possibly use this to heat air such as an underhouse circulation system for winter that would heat floors or possibly heated air entering a pipe and dispersed into a green house, smoker, dehydrator. Harnessing the energy indirectly rather than directly onto surfaces, I think would be safer and more useful.

    1 reply

    Reply 4 years ago on Introduction

    I think you are correct about "indirectly", and that's why this may not be a good fit for heating.
    Since it is a concentrator, the total heat energy is focused to a point, but the rest of the area is in shadow. A traditional solar heater would be more effective and efficient, I think, because the concentrator would need more infrastructure to handle the "heat dispersal" part.

    Maybe someone with more math can tell us whether it is better to heat a large volume of air slowly, or a small volume of air quickly.

    Kilt Y

    4 years ago on Step 7

    Don't laugh, but I'd be curious how quickly it could sterilize waste for humanure applications: off the grid, in areas without water, and in emergency situations.

    A specific poop recepticale could be designed and placed in the focal point to kill of nasty micro-organisms for quicker composting.

    1 reply
    jkimballKilt Y

    Reply 4 years ago on Introduction

    I would try this, as I have read "The Humanure Handbook" too, but my wife gave me a look so this will not be happening in my backyard anytime soon.

    I think if I tried to defecate in a metal box, and then cook it in the yard, it would be the last straw for the HOA.

    I hope someone else gives it a try and reports back, though.