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Finally, I have done it. I have bought a large and beautiful four bladed windmill. A smock mill to be precise, really high and all functional. There is the whole ancient machinery in it, completely operational, I have tested it, just put sails on the blades and here I go, I can finally mill my own flour.

But wait, I don't care about milling flour, there are much better ways to do it ! And further more it is almost the winter, and I don't have any radiator to keep me warm, further more the ceiling is really high, it will be hard to heat... Oh I have an idea ! Let's transform this, yet useless, wind power into heating ! No idea how to achieve this though...

I am wondering how to use the rotational mechanical energy that the windmill is giving me into thermal energy.

I would try to avoid doing the mechanical energy -> electrical energy -> thermal energy chain, because it would be too complex. I mean, we are always trying to keep up the energy efficiency by making our transformation systems to make less... thermal energy. But this is what I want to achieve !

Is there a solution to directly transform the rotational to thermal energy ? What would be the cost and the power efficiency of such a device ? Can I heat my windmill only by relying on the outside wind ? What wind would be needed ? What temperature can I have ?

Note : Let's say I have restored my windmill a bit, and it is quite well insulated.

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    $\begingroup$ My gut feeling says that you should actually go and grind something in your windmill since grinding things generates heat due to friction, but you don't want to grind stuff..... $\endgroup$ – Aify Sep 1 '16 at 7:49
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    $\begingroup$ Compressing air generates heat, but I'm not sure it would be very practical. Of course it could be done rather simply with a cylinder and a piston that works like a bicycle pump. And you would want to pump in air from the outside and release it back outside. What you'd be doing is extracting kinetic energy from the air outside and transferring that energy to the air inside, regardless of how cold it is outside, i.e. the air would come in at some temperature and be a bit colder when it goes back out. Then you would probably heat water and pump that through the walls to warm the place. $\endgroup$ – Nolo Sep 1 '16 at 8:04
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    $\begingroup$ Looks like according to Charles Law temperature and volume are directly proportional. This would suggest that as you compress air to half it's initial volume it's temperature will double, but I doubt that is in relation to degrees celcius. Seems there's more to it than that... $\endgroup$ – Nolo Sep 1 '16 at 8:25
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    $\begingroup$ @EngelOfChipolata Let me say though that the notion that it would be difficult to go by an electrical generator: no, there you are wrong. Again: the Law of Energy Conservation is non-negotiable. You are worried about losses in conversion. Well those "losses" are actually exactly what you want: heat. Normally people do not care about waste heat; it is just a nuisance. But you have a special case here: you want that heat. So I would say that you should in fact hook a generator to your mill, along with a big bank of batteries for the not-so-windy days. $\endgroup$ – MichaelK Sep 1 '16 at 11:26
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    $\begingroup$ Grinding does generate heat, potentially rather a lot. To the extent that fires and even explosions from flour dust were a problem in their operation. $\endgroup$ – pjc50 Sep 1 '16 at 13:54
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Stir water

Use the windmill to drive paddles in a pool of water. Have vanes and obstacles for the water so that it is made as turbulent as possible (and does not just set to spinning around with the paddles).

As long as the water acts as a brake for the windmill, the water will heat up, according to the law of conservation of energy.

Using water as a brake is advantageous because usually the braking force of water is the square of the speed of whatever is moving through the water, which means this thing becomes more or less self-regulating.

The only problem you might encounter is if the water becomes too hot and starts boiling. You need some kind of system that always keeps the pool full to the brim.

You can solve that by having a buffer pool next to the hot water pool. The buffer pool feeds into the hot water pool through a narrow opening. If the level in the hot water pool drops, then water will flow from the buffer pool to the hot water pool.

The buffer pool in turn is fed from the nearby natural stream. And that pool has outward flow as well so that when there is too much in the buffer pool, the excess simply drains back to the stream a bit further down.

This concept is really old; it was pioneered by James Prescott Joule, the very scientist we named the standard unit of energy after. In 1845 he published "The Mechanical Equivalent of Heat". The apparatus he was using looked like this:

Joule's Apparatus

Joule's apparatus (Image source)

But a generator is still better

About the notion that it would be "difficult" to go use an electrical generator: no, you are wrong.

Again: the Law of Energy Conservation is non-negotiable. You are worried about losses in conversion. Well those "losses" are actually exactly what you want: heat. Normally people do not care about waste heat; it is just a nuisance. But you have a special case here: you want that heat.

So it is win-win for you, since you get both heat and electricity. I would say that you should in fact hook a generator to your mill, along with a big bank of batteries for the not-so-windy days. I think this would be much easier than to trying to make a swirl-pool.

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    $\begingroup$ +1 I did not know that it is possible to heat (and even boil) water by just stirring it ! Do you think it can be stirred enough with only a windmill to get a significant heat ? $\endgroup$ – EngelOfChipolata Sep 1 '16 at 9:08
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    $\begingroup$ Heh, this physics question actually gives a detailed account of the amount of work required to do this. That along with Mike L.s report of 18kW of power from the mill looks like you could heat up 34 gallons of water by 2 degrees celsius every minute. Seem you could get the heating job done and have some energy left over. $\endgroup$ – Nolo Sep 1 '16 at 9:21
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    $\begingroup$ Actually, this is pretty old concept - mind if I add link and Joule's apparatus illustration to your answer? $\endgroup$ – Mołot Sep 1 '16 at 10:11
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    $\begingroup$ Perhaps a good way to create the turbulence would be to have a single cylindrical container, but 2 rotors submerged in the pool. Gear them in such a way that one spins one way and the other spins the other. Unsure if this will actually create turbulence, or whether it will result in 2 planes. Failing this, create something like a torque converter in a automatic car's transmission and spin the 2 sides in separate directions. That should surely work. $\endgroup$ – Trotski94 Sep 1 '16 at 10:32
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    $\begingroup$ A device designed to maximize wasted input? Mid level managers everywhere rejoice! $\endgroup$ – corsiKa Sep 1 '16 at 14:49
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You could use the windmill to operate a Heat pump. While known heat pumps are driven by electricity it should be easy to build one that uses mechanical energy instead. When done right you could use the same heat pump to cool the mill in summer and you can use surplus heat to generate electricity.

Especially geothermal heat pumps work well for doing this.

Edit: For a basic principle you need two airtight tanks containing a medium that needs to be compressible. Those tanks are connected by pipes with valves that you can open or close. And at least one of the pipes has a pump that is driven by the windmill (many pumps use rotation). Now you open the inflow valve but close the outflow one and you start pumping more of the medium in. As it is compressed it heats up and you can use that heat. Once it cools down again you close the inflow valve and open the other one. The medium flows out, decompressing in the process and by that cooling down even more. Now you let it sit in the second tank, where it returns to ambient temperature. Rinse and repeat.

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  • $\begingroup$ How you would build mechanically-driven heat pump? Question is science-based, so any hint how it is really possible would be nice. $\endgroup$ – Mołot Sep 1 '16 at 10:18
  • $\begingroup$ @molot Thanks for your input, I edited my answer to give the basic principle. $\endgroup$ – Umbranus Sep 1 '16 at 11:20
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    $\begingroup$ If you have a geothermal well, there's no need for the windmill, really — just use the heat from that. $\endgroup$ – mattdm Sep 1 '16 at 14:18
  • $\begingroup$ Or, instead of geothermal, combine this with the "stirring" answer and there you go — build an absorption heat pump $\endgroup$ – mattdm Sep 1 '16 at 14:20
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    $\begingroup$ You want a ground source heat pump, not geothermal. Ground source is also very good at storing heat. Store in summer, use in Winter. $\endgroup$ – josh Sep 1 '16 at 15:27
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Yes, it's extremely easy, just grind stuff. Or even don't bother grinding stuff, just let the millstones scrape against each other with nothing in between them. This will convert the kinetic energy into heat.

But wait, you might say (and indeed do say in the comments), surely this isn't the most efficient way to do it. But in fact it is! The reason is that the only thing stopping the blades from spinning faster and faster and faster in the first place is friction. All of the kinetic energy in the blades gets converted to heat through friction. Some of that friction is in the air outside (you can't do anything about that) but the rest is friction in the millstones and in the gears, shafts etc. in the machinery. All of that's inside the mill already, so just running the mill as usual is an absolutely 100% efficient way to generate heat. You'll have to make sure the mill is well insulated, though, which could be a bit of an issue in a lightweight moving structure that's not designed for it.

There is a better way, though. Better than 100? Yes, actually! By using the energy to run a Stirling engine or some other kind of heat pump to pump heat from outside the mill to the inside, you can get 100% of the kinetic energy (which still turns into heat from friction), plus a bit extra that you can pump in. There is a limit to how much extra you can get of course, but for a heat pump system efficiencies of somewhat more than 100% really are possible. (This is not a violation of thermodynamics and can't be used to generate free energy, before someone gets the wrong idea. It's actually a pretty commonplace way to heat buildings.)

Is any of this actually possible? Well, it says here that a traditional windmill can put out 14kW of mechanical energy, which is more than enough to heat a home, so you probably don't need to worry about the heat pump system. So if you can solve the insulation problem just run the mill normally and you will be nice and warm.

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  • $\begingroup$ Nice answer ! My windwill will be warm and noisy ! Just like back in the old days ! $\endgroup$ – EngelOfChipolata Sep 1 '16 at 11:21
  • $\begingroup$ And dusty with lots of little bits pf stone being ground up. $\endgroup$ – Donald Hobson Sep 1 '16 at 14:25
  • $\begingroup$ 100% plus a bit extra? That's not how energy works... $\endgroup$ – corsiKa Sep 1 '16 at 15:14
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    $\begingroup$ @corsiKa That's not what he means — it's 100% of the kinetic energy, plus whatever other heat energy can be extracted from the air outside of the mill. Not more than 100% of what you put in, of course. $\endgroup$ – mattdm Sep 1 '16 at 15:28
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    $\begingroup$ Heat pumps would generate a good deal more heat than any of the other systems because they multiply the power put into the system by employing it to pull heat from the outside air. I was just going to post pretty much this exact answer. All the other solutions will be about the same so might as well use the power to grind flour and allow that process to generate your heat. $\endgroup$ – Bill K Sep 1 '16 at 18:41
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Why not install a generator? Use the electricity for anything. Don’t constantly wear things as with leaving the brake on.

Also, start with you living/sleeping quarters. You complain about the high ceilings: build a small room within the space, further improving the insulation of the finished space and providing a smaller volume to heat.

With electric heating, you can use the least power by using an electric heated sleeping bag.

And you’ll have power to blast Danse Macabre when clients come to visit.

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  • $\begingroup$ Why go from mechanical to electrical to thermal when you can just... go straight to thermal? $\endgroup$ – corsiKa Sep 1 '16 at 15:13
  • $\begingroup$ Why: 1, you don’t cause constant wear and need to constantly adjust the friction components; 2, you can selectively heat a targeted area. $\endgroup$ – JDługosz Sep 1 '16 at 16:22
  • $\begingroup$ 1) not if you do it right.. 2) you can pump the heat wherever you want $\endgroup$ – corsiKa Sep 1 '16 at 17:05
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Well, the simplest way to do it is with a braking system. That converts mechanical energy into heat (plus noise). You'll need some kind of speed governor to keep the brake from stopping the windmill when the wind is low, but that's quite achievable as a mechanical system.

The easy way to distribute the heat through the building is to have a water pump, driven by the windmill, that pumps water through cooling pipes in the brake shoes on the main shaft, and then around the building to radiators, like a conventional central heating system.

The problem with the basic idea is that the heat available isn't closely related to the weather. On cold days with no wind, you need some other kind of heating. Warm days with strong winds are easier to handle: switch the hot water flow to radiators outdoors, or just swing the mill sideways-on to the wind and lock it.

The other problem is that you may not have enough power from the windmill. I can't find power figures for historical windmills, because the Internet is full of pages about modern wind turbines, which are much more efficient, as well as larger. But I'd be surprised if you could get more than 10KW of heat.

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  • $\begingroup$ I believe that pumping the water around is superfluous; you can just rely on convection in a closed loop. $\endgroup$ – Mike L. Sep 1 '16 at 8:33
  • $\begingroup$ 10kW (other answers say 14-18) should be plenty to heat a decently-insulated place. A wooden outer skin, caulked using boat-building techniques, then a gap filled with wool/straw, then more wood, should be pretty efficient. And there must be some pretty big structural timbers running vertically, to form this cavity. Internal ceilings, even partial, would help as well. $\endgroup$ – Chris H Sep 1 '16 at 11:58
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A bit more detail when it comes to friction, hopefully using materials that fit your scenario:

Early (car) brakes were often made of leather pressed against metal. Band brakes (wikipedia) are technologically simple - you just need a metal pulley, a strap and an anchor allowing you to tension it. You could probably use a stone pulley (your grindstone) though it would wear the band quite fast - perhaps sacrifice a few bands with a sand/water slurry to polish the wheel a bit smoother.

The thermal mass of the stone would help you if the wind was variable over the course of a day, but wouldn't be any use over longer periods.

Alternatively, by using the windmill to drive a bellows, you could burn a limited fuel supply more efficiently. If you could work metal to form a heat exchanger, you could push outside air in, warm it with the waste heat from the fire and then feed it under the grate for an efficient, clean-burning fire. You could also get your firewood (or other fuel) nice and dry by using forced air.

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Another option would be to use eddy current. With a windmill that would be rather simple. You take a metal disk and add it to the windmill's axle so that it spins. Perhaps with a gear to make it spin faster. Now you install a magnet very close to the metal disc but without touching it, just as close as you can manage with your level of technology.

While the disk spins within the magnetic field it creates eddy currents which now generate their own magnetic field. Those magnetic fields oppose. By that kinetic energy is transformed into heat.

A positive side of this method is that it is simple to build and needs little maintenance because no moving parts touch and by that little wear occurs.

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Friction.

A windmill reportedly produces 18kW of usable mechanical power. Since you already have in place all the machinery necessary to grind a huge stone against another huge stone, you can just use that.

Change out the gears to get more RPMs, cut down the stones to make them lighter and press them more tightly against one another. Now you have two stones rubbing against each other at relatively high speeds, which is sure to generate friction that in turn generates heat. Submerge them in water to get a pool of warm water you can use for bathing or heating.

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  • $\begingroup$ I would suggest grinding them outside of water to maximise the friction and then dumping them in water as heat is collected $\endgroup$ – Chris J Sep 1 '16 at 8:59
  • $\begingroup$ @ChrisJ That would probably help with the friction, but at the cost of a lot of extra mechanics. I'm of two minds on that. $\endgroup$ – Mike L. Sep 1 '16 at 9:42
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    $\begingroup$ Since you've built a giant stone-polisher, I wonder how often you'll need new stones. $\endgroup$ – Steve Jessop Sep 1 '16 at 12:01
  • $\begingroup$ Lots of dust too. $\endgroup$ – Donald Hobson Sep 1 '16 at 14:05
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    $\begingroup$ @DonaldHobson Not a problem if the stones are submerged. $\endgroup$ – Mike L. Sep 1 '16 at 14:33

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