Solar furnace melts rock

In this clip from BBC One's "Bang Goes the Theory," a clip of a high-performance solar furnace that can focus normal sunshine into a heat-ray that reaches 3,500C, hot enough to melt rocks.

Jem Melts Rock Using Sunshine - Bang Goes The Theory - Series 3, Episode 5 Preview - BBC One (Thanks, Fipi Lele!)



  1. I love watching Bang Goes The Theory with my kids; we were watching this episode when it was on in the UK, and all of them were totally blown away by this segment. It may not be the most highbrow science programme on the box, but it’s accessible and helps to foster a sense of wonder – all good things in my book.

  2. the stand which the rock rests on, what material is that? must be able to take quite a lot of heat.

    1. An excellent question. Even if it’s not reaching the 3,500°C (6,332°F, in case anyone’s wondering) of the focal point, conduction alone must deliver quite a high temperature…

    2. Well, the platform isn’t at the focal point so it’s not directly getting any solar heat, it just has to withstand whatever heat is conducted to it from the melting/burning object.

    3. The focal point is tiny, the stand the rock is resting on is not exposed to the power of the fully armed and operational… solar mirror.

    1. There are multiple solar power facilities in the Mohave that could do this. Solar One and Two are tower based systems with, IIRC, molten salt that does the seam generation. There is also another parabolic trough solar power facility that heats a synthetic oil with then heats water to produce steam. There are also plenty of makers out there that have converted old C wave satellite dishes into parabolic collectors with things like crucibles at the focal point to melt aluminum and iron.

  3. Wow. Why is it that affordable solar technology is so far away when it delivers so much frickin’ energy?

  4. Ok, so why can’t this be used to generate electrical power? Why can’t we have a bunch of these suckers powering hydrogen farms and have enough to not use coal or fossil fuels?

    1. It can and is.
      There are solar farms which use multiple parabolic mirrors placed on the ground with their focal points converging at the top of a tower. The thermal energy from this setup is used to boil a type of salt in a chamber in the tower which is then used to spin a generator.

      The problems with this configuration are:

      -Requires a lot of real estate.
      -Heat turns things into fire. Fires make other fires.
      -Mirrors get dirty quickly and can’t safely be cleaned without covering them first.

  5. Just to get a little quantitative, the solar constant is a bit more than a kilowatt/meter^2, so (using the 2 sq. meters mentioned in the clip) the power here is about two kilowatts, or about as much as twenty to thirty light-bulbs. This is not an enormous amount of power, and suggests that the trick is to use a -really- good mirror.

  6. Or the trick is to simply put that 2 kilowatts of power into a space the size of a dime (or less)…

    It’d be like having the big burner on your stove (if it’s electric) shrunk down to the size of a dime as well…I’m pretty sure it’d melt its self in half in short order.

  7. Wow, that’s a fun video.

    But I have so many questions! At what facility is this happening? What does the actual collector look like? Is the sunlight coming into that parabolic reflector from below? Where does the “two square meters” thing come from? Doesn’t that mirror look a lot bigger than two square meters?

      1. Thanks, that’s what I was thinking as well.

        How odd that the largest, most famous (not to mention photogenic) solar furnace in the world doesn’t appear to have a web page, or even a name, apart from “The solar furnace at Odeillo.”

        In any case, if that is the Odeillo facility, the main collection mirrors (heliostats) cover a hillside, in which case “two square meters of sunlight” might be a little misleading. This article says the Odeillo furnace has 63 heliostats; it’s hard to tell from pictures how large they are, but if they’re just 2m x 2m, that’s 252 square meters. If the heliostats are 5m x 5m, they’d total over 1,500 square meters.

  8. The beauty of solar tower tech is that using focussed energy like that you can boil water, driving turbines. But if you start off with seawater, you can actually distil the freshwater out of the process – getting energy AND potable water.

    This plant in Spain can generate 20 MW – so there’s a way to go before you can replace a coal fired station like Fiddler’s Ferry, which can crank out 2000 MW:

    But, there’s a lot of desert in the US – if only you had a decent grid!

  9. This makes me recall high school science when a team of us built a solar furnace using an old Fresnel lens, from a broken overhead projector. At about 15″ across, the Fresnel lens could achieve nearly 1000 F in short order. With a pivoting frame to hold it aimed at the sun, and a fire brick behind the prime focus, we burned lots of stuff really fast. Good times.

  10. This is going to work great for my plan to take over the world from my secret underground lair. Muhuhuhuhahahaha!

  11. While you might not be able to get to 3500F, you can, relatively easily, get Fresnel lenses anywhere from a few inches to a couple of feet across that can reach heats of well over 1000F. The bigger, the more light you collect. Many many youtube vids are out there of people (with welding goggles and serious gloves) boiling asphalt, melting pennies, and popping sections of concrete apart from thermal expansion pressure.

    Science is exceedingly cool and exceedingly scary. :)

  12. Twice in the past two years, I’ve yoinked fresnel lenses from curbside garbage pickups where they were set because their rear projection TV was busted. Melted the zinc out of a US penny, set fire to things, busted rocks, etcetera.

    Sodium metal is used in production solar steam generator turbines because it has a high thermal inertia; If a cloud passes overhead, steam production will not suddenly cut off. It tends to corrode the pipes, though, and any water contamination is problematic.

    Oil has less thermal inertia.

    If they could get gold to melt, it would be an excellent thermal conduit from sun to water and reacts with almost nothing; Unfortunately gold has a distinctly high melting point, and is currently something like $600 US an ounce and is very limited in quantity on earth.

  13. “Oh my life!”. I like that expression!

    And this is a terrific demonstration of how much energy is all around us, blowing past us almost unnoticed, and how powerful it can be when harnessed. You may think it takes a massive amount of energy to melt a steel bolt, but it’s about the same energy that it takes to give you a good tan over a sunny afternoon. Just think of how much useful energy we’d have if we were much better at harnessing sunlight (or wind, or ocean waves / tides / currents, etc).

    By the way, I’ll be the nitpicky engineer and point out that the temperature you can reach is not dependent on the device. (The device only concentrates sunlight power by some ratio or gain, which is the ratio of the mirror or lens area to the area of the focused beam that hits the subject, minus any inefficiencies). The temperature is indeed a function of how much power you shoot at the subject, but also of how quickly the subject sheds that energy (by radiating it away, conducting it into the base, or ablating it as the really hot parts evaporate away) and of how much hotter the subject gets when it absorbs a given amount of energy. Anything in a beam of concentrated sunlight (or even regular sunlight) will just get hotter and hotter until its heat output matches the sunlight power that it’s absorbing. A conceivable material with a very high boiling point, very poor conduction, very low specific heat, and very low emissivity, could reach 3500°C under a magnifying glass or overhead-projector Fresnel lens, or just outside on a sunny day.

    But yeah, that doesn’t make the capabilities of that mirror any less amazing.

  14. Impressive exercise in physics/optics. I had a hard time watching it with the interviewer messing around far too close to the focal point. I was also getting ready for that rock to explode and spay them with hot shrapnel.
    I’m wondering about the collector geometry.

  15. As the price of oil is driven upwards by Asian demand and American consumption, expect to see more articles like this! Spain has Solar Electric developments worth a Google! Solar, Wind, Wave, Hydro, Geothermal, Nuclear, sources translate into electrical energy. America is starved for liquid oil energy and America is set up to burn only fossil fuels for its survival! Americans are in a slow paradigm shift from oil and fossil fuels to the new electrical situation as we speak! The GM Volt is one small example, the many Hybrid cars makers are selling, another.

  16. Remember the James Bond movie “The man with the golden gun”
    They had a solar collector that was used to collect sunlight and convert to electricity.
    See, even in the 60’s they know this was possible, and even though this was only a movie, they had the right idea.

  17. they did not invent anything it has been discovered a long time ago(i use fresnel lenses since 8 years now wich has been invented 150 years ago by French scientist) but spoiled human being dont use it because they want instant efficiency and not to be dependant of a sunny day to use this free and clean power…
    the fuel companies and goverments won’t allow us slaves to get free energy.

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