Characterizing a Death Ray… er, Solar Oven

Many of you will probably at some point have looked at a satellite dish antenna and idly wondered whether it would collect useful amounts of heat if you silvered it and pointed it at the sun. Perhaps you imagine a handy source of  solar-cooked hotdogs, or maybe you’re a bit of a pyromaniac.

[Charlie Soeder] didn’t just think about it, he did it. Finding a discarded offset-focus DirecTV dish, he glued a grid of 230 inch-square mirror tiles to it and set to investigating  the concentrated solar energy at its focus. 

Cotton waste, newspaper, and scraps of fabric char and burn with ease. A cigarette is lit almost from end to end, and it burns a hole right through a piece of bamboo. Most of the energy is in the form of light, so transparent or reflective items need a little help to absorb it from something dark. He demonstrated this by caramelizing some sugar through adding a few bits of charcoal to it, once the charcoal becomes hot enough to caramelize the sugar around it the spreading dark colour causes the rest of the sugar to caramelize without further help.

Solar furnace calculations
Solar furnace calculations

To gain some idea of the power of his solar furnace, he recorded a time series of temperature readings as it heated up some water darkened with a bit of charcoal to absorb heat. The resulting graph had a flat spot as a cloud had passed over the sun, but from it he was able to calculate instantaneous power figures from just below 30W to just below 50W depending on the sun.

He records his progress in the video you’ll find below the break. Will we be the only ones casting around for a surplus dish after watching it?

We don’t seem to have had many satellite dish solar furnaces here on these pages, but we have had a more conventional solar oven or two. This dish-based solution would probably benefit from a sun tracker.

35 thoughts on “Characterizing a Death Ray… er, Solar Oven

    1. Wellll spherical comes to several different foci along the same axis, which may not matter so much for a big target you don’t need a coherent image at. So since spherical is easier, and “acceptable” for heating an object that’s not tiny, then by that metric it’s “better”….. Also the tight focus of a parabolic could be considered a drawback if it’s going to laser a hole in the side of your oven instead of heat it somewhat evenly.

    2. Wouldn’t parabolic always be better? In order to not laser the hell out of a single target you just place the target out of focus. For a solar cooker, that just means placing your object to be cooked at focus and your cooking surface slightly out of focus. You heat the cooking surface evenly, which in turn heats your food evenly instead of burning it. I built a crude version of this in elementary school to cook hot dogs. It worked surprisingly well considering it was made of foil, cardboard, bamboo skewers and tape.

    3. I strongly suspect reflector shape is irrelevant.

      The ‘deathray’ relies on lots of energy in a small area. An over utilizes lots of heat in a broad volume (area for a solar stove). The biggest factor is the size of mirror since it controls the minimum size of the effective focus.
      Ideally the mirrors should be sized to the object you’re heating. For a given reflector cross section the energy is fixed ~1000W/sqm of solar irradiance depending on altitude and weather conditions.
      For an oven, building a sufficiently insulated box is also important. So for the shape of the reflector the constraints are available mirror selection, ability to build a given shaped reflector, and cost of materials. Parabolic reflectors may be easier to build due to the prevalence of second hand satellite dishes.

  1. Back in the 70s I consulted on solar energy engineering and helped out on a Boy Scout project for a 36″ parabolic reflector. We provided a pattern for making a frame to be covered with aluminum foil. We recommended crumpling up the foil to avoid having too sharp a focus. Try telling a bunch of Boy Scouts to build something that’s deliberately imperfect. About a month into the project, we heard from a Texas troop that their reflector had worked so well that it burned a hole in a garage wall. The wall was made of cinder blocks.
    Here’s a good cautionary YouTube video: https://www.youtube.com/watch?v=TtzRAjW6KO0

    1. Yes, I thought of those while reading this.

      For lighting a cigarette, or starting a fire, you want a “death ray”, ie a concentrated source of heat. This is no different from using a magnifying glass to start a fire, you concentrate the light from the sun.

      But for cooking, you don’t want just single spot cooked, you want it all cooked. So you want to concentrate the heat, but not o the same effect. I seem to recall more box-like solar cookers from the books I read decades ago on the subject.

      Michael

  2. The quality of the receiver (target) makes a huge difference to the amount of energy absorbed, so although his calculations are a great exercise they are based on a few assumptions and could be misleading.

    http://www.sciencealert.com/australia-scientists-just-set-a-world-record-for-solar-thermal-efficiency

    Also note that glass mirrors will never give you an ideal reflection, polished metal is far more efficient. Is that dish aluminium, if so just polishing it back to a mirror finish would have been better.

  3. When I was a kid, there was an old headlight reflector in Dad’s shed so I pushed a cork into the lamp hole and hammered a nail through the center. Then a “subject” put on the end of the nail would burst into flames when the reflector was pointed at the sun. It was less than a foot across but very effective.

  4. I’m supprised it was such a low figure.

    My pv array generates approx 100w /m2 at its peak with only around 10% efficiency. Which gives around 1000w /m2 solar energy.

    The 50w of heating power from the .6m2 is still down around the 10% efficiency

    I’m think a large part of the loss would be due to the heat loss from the heating chamber. Perhaps an insulated container would yield better results

    1. Yeah but when you make it light enough to track you one day discover you’ve made an only slightly heavy kite. So then you use a crap load of smaller long focus ones pointed at central genny.

  5. There are two paints in the affordable price range that could be used for this. Spaz Stix Ultimate Mirror Chrome and Rustoleum Mirror Effect. Krylon Looking Glass is not reflective enough.

    Sprayed onto glass, the Spaz Stix will produce a mirror shine from both sides of the paint layer. The Rustoleum only produces a mirror shine viewed through the glass. The other side of the paint layer just looks like dull metallic silver paint.

    The Spazr Stix paint was developed for application to Lexan radio control car bodies. It will also produce a mirror shine on smooth styrene. The Rustoleum paint’s solvent attacks styrene. Both also work on urethane resins.

    So, if you could apply a glass smooth base coat of paint, you could use the Spaz Stix paint to make a huge old C band satellite dish into a solar thermal collector. You’d want to over coat it with with the special clear paint from Spaz Stix.

    Alternatively, and possibly less expensive for a large application, is ALSA Killer Chrome. They have the high gloss base coat and clear top coat as a paint system with their chrome.

    1. Wouldn’t epoxying Mylar emergency blankets offer a far better reflective surface than paint? I imagine the price would be very very low. I bought 4 very large blankets for $5.

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