Using The Wind And Magnets To Make Heat

On the face of it, harnessing wind power to heat your house seems easy. In fact some of you might be doing it already, assuming you’ve got a wind farm somewhere on your local grid and you have an electric heat pump or — shudder — resistive heaters. But what if you want to skip the middleman and draw heat directly from the wind? In that case, wind-powered induction heating might be just what you need.

Granted, [Tim] from the Way Out West Blog is a long way from heating his home with a windmill. Last we checked, he didn’t even have a windmill built yet; this project is still very much in the experimental phase. But it pays to think ahead, and with goals of simplicity and affordability in mind, [Tim] built a prototype mechanical induction heater. His design is conceptually similar to an induction cooktop, where alternating magnetic fields create eddy currents that heat metal cookware. But rather than using alternating currents through large inductors, [Tim] put 40 neodymium magnets with alternating polarity around the circumference of a large MDF disk. When driven by a drill press via some of the sketchiest pullies we’ve seen, the magnets create a rapidly flipping magnetic field. To test this setup, [Tim] used a scrap of copper pipe with a bit of water inside. Holding it over the magnets as they whiz by rapidly heats the water; when driven at 1,000 rpm, the water boiled in about 90 seconds. Check it out in the video below.

It’s a proof of concept only, of course, but this experiment shows that a spinning disc of magnets can create heat directly. Optimizing this should prove interesting. One thing we’d suggest is switching from a disc to a cylinder with magnets placed in a Halbach array to direct as much of the magnetic field into the interior as possible, with coils of copper tubing placed there.

53 thoughts on “Using The Wind And Magnets To Make Heat

  1. My parents used to have a windmill that heated the house via a water brake. When it worked, it was very effective, but the system broke down so often and needed so many maintenance, in comparison to the near-maintenance-free solar collectors we also had, they got fed up with it. Then a large storm increased insurance prices on wind turbines which put the nail in the coffin, they deconstructed and sold the thing.
    The pipes no longer running through our garden made mowing a lot easier (I remember taking 1h45 before, then cut it down to 45mn)

      1. The word you’re looking for is effective, not efficient.

        If the magnets were less efficient at converting mechanical motion to heat, it would mean they were producing something other than heat: sound, light, air movements, radio waves…

        Instead, what energy the magnets fail to convert into heat is simply not removed from the spinning disc. It’s nearly 100% efficient at making heat – it’s just not making very much of it because its braking power is very feeble compared to a water brake. Then again, a barrel of water is much bigger: if you were to make a similarly sized magnetic brake, with strong magnets, you could hardly turn it.

        1. You could say that some of the mechanical energy is converted into electrical current, so there is something other than heat, but is all eventually converted into heat.

          A very tiny amount escapes the device as electromagnetic radiation, and sound and vibration etc. but it’s not much.

        2. What about the energy loss to bearing friction and air friction?
          This isn’t operating in a vacuum. The breaking force of the air is significant in comparison to the induction.

          So as long as we aren’t talking about spherical bovines, the efficiency of this induction setup is quite poor.

          1. He’s trying to heat the area this contraption is located in. Friction in the bearings and air generating heat is still an absolute win for that purpose, and doesn’t bring down the efficiency even slightly. That isn’t to say I think this is going to be terribly effective, but it’s efficient by default until you compare it with a heat pump.

    1. It’s not as efficient, but it really doesn’t have to be – this is using harvested wind energy and scrap materials after all. And using a non-ferrous metal makes the construction a lot simpler – don’t have to keep fighting the tendency of the magnets to stick to it!

      1. It’s equally efficient, since the energy cannot go anywhere else except heat.

        The point about iron vs. copper is impedance matching. The load curve of the magnetic brake should match the power curve of the turbine to get optimum efficiency out of the turbine – not the copper or iron material.

    2. Iron vs copper are not really better or worse: they use different heating mechanisms. Copper is heated by eddy currents, and the better conductivity (than, say, tin or lead) allows the use of thinner material. It’s inherently a linear process.

      Iron (and other ferromagnetic materials) is heated by magnetic hysteresis, a non-linear process, where you need a certain minimum magnetic field strength to generate heat.

      If you’re winging it, you’re probably going to find your engineering easier (or lack of engineering luckier) if you use eddy currents as the heating mechanism.

      1. Also, the ferromagnetic heating effect quits working above the Curie temperature, where the material stops being ferromagnetic. In practice this is not significant for cooking, as it’s usually around red heat.

        Incidentally, the Curie effect is exploited in Weller PT series soldering tips: the tip incorporates a piece of ferromagnetic material that becomes nonmagnetic at the programed temperature. A PT700 tip senses when it is at 700F, for example. It’s a bit of genius to make the tip sense its temperature directly. It was even more genius when they stopped using the magnet to toggle the power relay directly, and started using a Triac (around 1980…). That relay was the failure point.

        1. I have a WTCP (from the 80s) and a WTCPT (bought sometime around 2009), each with TC201 handles. Both use a set of switch contacts in the handle to control the heater cartridge. I’ve never seen the triac design you describe. I did have the contacts stick closed on the WTCP once years ago, but never since then.

          Great, simple design. It’s too bad they’ve discontinued it.

          1. Interesting. I wonder if ours were aftermarket modified? It was important, because the triac ones couldn’t be used on the vehicle (bus) 24Vdc supplies we used in the field — we had to be careful to take the “old” ones for those jobs — we had adapters to plug the irons directly into the bus DC power. Inverters were pretty scarce in that era.

          2. (responding to Paul below)

            I never thought of that, powering the handle straight from 24V DC. Yeah, the TRIAC wouldn’t be very useful there.

            There might have been handles made to be compatible with the TC201 as an ‘upgrade’ option, using a conventional sensor-amplifier-TRIAC circuit all built into the handle.

      2. Iron is also heated by eddy currents, but it exhibits such a strong skin effect that the eddy currents are constrained in a very thin surface layer with a high resistance. At low speeds you get eddy current heating, at high speeds you get the magnetic hysteresis heating.

        1. This is btw. why induction cooktops work with iron pans but not copper or aluminum pans.

          Without the skin effect, the impedance of the secondary coil – the bottom of the pan – would be much lower than the impedance of the primary coil in the base, and most of the heating power would heat the cooktop instead of the pan. The cooktop senses the overload condition and refuses to turn on.

    3. Note sure what you mean by “too conductive.” Heat produced = I^2 * R. Double the resistance, you double the heat, but double the current and you quadruple the heat.

      So whether copper or iron is better for an induction heating vessel (as implemented here) would depend on how effectively you can turn varying flux into current.

      1. The R for copper is much less than the R for iron.

        The eddy currents from the changing magnetic fields are essentially alternating current, and iron is bad at conducting alternating current because it reacts strongly to the magnetic field and that constrains the current to flow in narrow channels or layers inside the metal (skin effect). That means the effective cross-section area of the conductor becomes tiny and the resistance becomes high.

        If you take a piece of iron wire and pass DC through it, the resistance can be a couple Ohms, but if you try to pass AC through it, the resistance can be hundreds of Ohms. For the magnetic brake, this translates into requiring a higher RPM to make more voltage, to push current through the resistance, to produce the heating power.

        The copper magnetic brake, having a lower resistance to eddy currents, applies a stronger braking force at a lower RPM and the braking force goes up more steeply as the speed increases. This may not be optimal for the design of the wind turbine because it overloads the turbine. The iron magnetic brake is weaker as a brake, but allows the wind turbine to spin faster and pick up higher winds without requiring a reduction gear. Which is better depends on what kind of a turbine you use to spin it.

    4. Copper is actually the best because of its low resistance. The reason is, direct eddy-current heating is a little different than induction heating. The inductor in that case is effectively a primary winding of a transformer, with its own electrical resistance. So it makes sense the load has some ideal resistance to generate the heat.
      With permanent magnets, there is no such conversation loss, copper would just drag the rotor more. A lower speed, higher torque impeller is usually its natural mode of operation, and copper would be a good match for it because it would just drag more and demand more force, which there is often plenty unutilized. In short, mechanical to electrical impedance matching is usually more optimal with copper.

    5. Put a thin aluminum or even thinner copper foil on your induction cooktop and turn it on. You’ll be surprised.

      Ferromagnetic material is best for induction cooktops because due to the high magnetic permeability of iron the skin depth will decrease (skin effect). This increases the electric resistance of the bottom of the iron cookware, so providing an optimal impedance matching where the electric energy will heat the pan rather than the coil in the cooktop.

      While copper or aluminum are goid conductors, a thin foil still has a high resistance even without the skin effect. Therefore the foil will heat up.

      I have my doubts about the efficiency of the setup mentioned in the post.

  2. Isn’t this the same as hooking your windmill to a generator and using a resistive heater?

    Wind -> rotation -> generator (spinning magnets induce current) -> heat via resistance (either in the resistive block, or here in eddy currents in the copper pipe)

    You just have no wires and have to heat the water where the windmill is.

    I’d be interested to be proved wrong, but I bet this isn’t even more efficient as a properly built generator is going to be more efficient than this, and the losses in wires are very low.

    A more direct method would be having the windmill drive something like a clutch plate which caused friction and generated heat.

    1. No. This is a mechanical version of inductive heating where the magnets are standing in for electrical fluctuations. He’s extracting heat energy directly from the magnets.

      1. How is it different?

        Magnets induce current in a piece of metal, resistance causes it to heat up. The only real difference is that in using a generator, the inductive pickup and the heating resistor can be separated by a greater distance.

        The practical difference is that generator and a load controller would make more efficient use out of the turbine.

        1. There are friction and eddy current losses in the generator, which would heat it up, rather than heating the water/home/whatever. There is also loss in the cabling from the generator to the heater, which is also not captured for the intended purpose. There are additional up-front and ongoing maintenance costs for the generator and electronics.
          If you want electricity as well as heat, you could simply have both systems connected to the windmill(s), and size everything appropriately.

  3. I think spinning physical magnets to make a very quickly alternating field is always neat, but any solution that’s neither cheaper nor better than a regular generator, even if only in one way such as at matching itself to the current output of the turbine, is a limited solution.

    In the options for wind heat, there’s an older article full of discussion where it was argued that if someone was going to do that, they should probably build an adjustable pump or at least an adjustable fluid brake. The max temperature depends on the fluid, but it would be massively cheaper and able to make the most of varying wind. Plus, it takes no real effort to make a poor efficiency pump rather than to make a pure fluid brake, but the pump extracts useful work before it turns the rest into heat. Mechanical heat pumps were also an option.

    1. Fluid brakes are a great option. My family built a few wind-to-heat contraptions for our rural house back in the 1970s that were similar to the ones in the Low Tech Magazine. Of them all, the best was a simple arrangement of large paddles spinning and churning water in a repurposed oil drum. A heat pipe with antifreeze and buried below the frost line fed the heat to the house. It was built from plans in a charming little book that I still have. The thing was cheap, but durable and thrown together from steel scraps. It kept the chill out of the house. A neighbor up the road built one with magnets and a huge aluminum plate in the mid-1980s. Even with step-up gearing, it barely worked in low winds. Both the neighbors and us ended up swapping them out for the more direct solar air heater panels in the early 2000s. Power is still rather expensive and spotty out in the sticks, but the sun (generally) shines regardless.

  4. I like this simple, bare-bones system. My ideal system, however, would drive a heat-pump with the windmill, and use it to pump heat out of one insulated tank of (salt?)water, and into another. Water heating, and heating the house in winter, would use a coolant loop in the hot tank, while refrigeration and summer cooling would (obviously) use a loop in the cold tank.

  5. >or — shudder — resistive heaters.

    Why the hate? Resistive heaters are, literally, 100% efficient. They are several times more efficient than heat pumps for heating.

    1. “several times more efficient than heat pumps for heating”… for certain definitions of “efficient”, or if misapplied, maybe.

      But if you define heat pump efficiency as (work output to the load) / (work input), any decent heatpump operating in its design envelope will have an efficiency several times unity.

      If you’re finding a heatpump that isn’t making more heat than the energy put into it, well, you’re using it wrong.

    2. Because heat pumps are usually much greater than 100% efficient.

      If you want to heat up your room and run a 1000watt resistive heater for an hour, it gives you 1kwh of heat for 1kwh of electricity.
      A heat pump might pull 500wh of heat from outside for 500wh of electricity, but because you combine those, you get 1kwh heating for just 500wh of power, which is 200% efficient.

    1. Thanks for posting that link.

      In reference to this Hackaday hack, it says:
      Because high temperatures are needed to produce electricity efficiently with a steam turbine, these systems can’t make use of joule machines or hydrodynamic retarders, but instead rely on a type of retarder called an “eddy current heater” (or “induction heater”). These are comprised of a magnet mounted on a rotating shaft, and can reach temperatures of up to 600 degrees Celsius. Using eddy current heaters, windmills could provide direct heat at higher temperatures, making their potential use in industry even larger.

      However, using the stored heat for electricity production is considerably more costly and less sustainable compared ro using heat generating windmills for direct heat production. Converting the stored heat into electricity is at most 30% efficient, meaning that two thirds of the wind energy is lost due to needless energy conversions – and the same is true when solar thermal is used for power production. 15

      and links (9) to this paper about a eddy current generator

  6. Anyone who understands thermodynamics will recognize that turning high grade mechanical energy into low grade heat is very wasteful of a renewable energy resource. If the wind energy were instead used to drive a heat pump to heat your home, you’d need perhaps 3X less wind energy.

  7. If a copper pipe were run around in a loop around the circle of magnets, you could pass water through, gather the heat, and distribute it in your house to heat up the air in your house.

  8. I’m not entirely sure, but I think the design could be improved significantly by adding bars of ferrous material between the back sides of adjacent magnets. This would shorten the magnetic path length between North and South poles. Since magnetic field intensity falls off as the CUBE of distance, the field intensity in the copper pipe would be much higher.

    I think this would result in higher efficiency. At the very least, it would enable lower rotation speed and/or weaker – and therefore cheaper – magnets.

  9. This setup will generate a lot of EMF all around… Coupling magnets with an iron yoke is a good way to limit that and create a much concentrated magnetic field (about 3X more with the same air gap and magnets): Put a semi-circular pipe in the middle of the magnets and you will have a system that won’t trash your phone if it goes too close.

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