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Reuben Lewis


This article is amazing and I love the idea of this. With it though, how would one control the temperature of the house? Would the amount of water in radiators be reduced or increased based on how hot or cool you'd want? Or is it just controlling the flow rate of the water? Sorry for the silly question!

Also, could you reverse the process to cool a house? Instead of having the turbine use the water as a break, you could use the turbine to pump the water through a cooling track and suck the heat out of the house. A gear box could switch between heating and cooling modes. Any thoughts?



Any idea what "LO-FA" is an acronym for?

Even 'Wind Power for the World: The Rise of Modern Wind Energy' doesn't give its meaning.

Also, designing some cavitation into the submersed rotor might increase the rate of heating.



I absolutely hate the "new" math illiteracy.

Efficiency = (work output) / (work input)

"...it would be three times more energy efficient compared to first converting the energy to electricity, and then back to rotational energy."

(note: 3x AS efficient is not the same as 3x MORE efficient)

So, let's say wind -> electric -> heat gives an overall efficiency of 70%. This is saying that direct wind to heat would then be 280% efficient. Of course that's impossible, so what the author wrote is obviously not what he meant.

The convolutions used (and I'm only guessing here) is that 70% efficiency means 30% loss, and that 90% efficient means 10% loss, so 1/3 the loss is the same as a 3x gain. Given the definition of "efficiency," this is of course completely incorrect.

Kris De Decker


@ steve

This is not about math, it's about language. I'm not a native English speaker and I should have written "as efficient". I corrected the sentence, thanks for pointing it out.

@ Reuben

The heat storage tank of a water brake windmill works like any other heating boiler. The hot water is stored in a tank. When the heating is turned on, the water is pumped through the house and then flows back into the tank, where it can be heated again.

I did not look into wind powered cooling but it could be possible. Not sure about your approach though. Could be a topic for a future article.

@ Headakus

No idea what "LO-FA" stands for. There was also a "LO-RA" water brake mill. The writer of the book chapter may know, but he didn't answer my questions. [Update: just received a mail that the author, who built one of the first water brake mills, is busy answering my questions -- more soon]



Interesting article! I am curious about the potential for a hybrid wind turbine (clutchable):

Generate electricity during peak hours (high price)
Generate storable heat during off-peak hours.

Or would it be better to generate e-power all the time if the turbine is capable of it?

Job van der Zwan


What I don't quite understand is: heat will be a byproduct of using electricity anyway, since it is always the waste product in the end. So why not generate electricity that can be put to good use first?



@ Reuben

The system is functionally designed to create and retain heat so would need quite a few modifications to also act as a cooler. what it has going for it is a large volume of water in an insulated box . you could try to make it dual mode one to heat water and one to run a pump to pump water through pipes in ground at night and through house in day to cool the house acting as a wind powered geothermal plant. This would be dependent on a total volume large enough to absorb the whole days worth of heat or dual pump water into house then through ground pipes to recool it.

Kris De Decker


@ Job

Good question. I see a few problems with your approach. First, not all energy loss of an electrical generator is heat, there's also noise and vibration. Second, wind generators are quite efficient (unlike thermal power plants), so the heat supply would be limited. Third, you lose a significant cost advantage because you reintroduce all the electric components.

Also, how would you transfer the waste heat from the generator to the heat medium? It's probably going to be less practical compared to using a water brake.

@ Bruno

That's also an interesting idea, and it could be more practical than what Job proposes.

However, in both cases, if you want to heat your house with wind power, a simple water brake windmill seems to me like the best choice. If you also want renewable electricity, you could use solar PV or a second windmill instead of trying to fit everything into one machine.

Also on a larger scale, I think it makes more sense to have heat generating and electricity generating windmills standing alongside each other, rather than combining them into one machine.

If you only need hot water for washing in an off-grid household then maybe it could be more interesting to combine.



"This old-fashioned approach remains relevant, also in combination with new technology, because it would be three times more energy efficient compared to first converting the energy to electricity, and then back to rotational energy."

The main point was that there's no way for direct heating to be that much more efficient than using electricity as an intermediate energy form. I would believe 30% more efficient. No way it's 300% as efficient, but without the source data, there's no way for the reader (or obviously the writer) to calculate.

kris de decker


@ steve

OK, now I get your point, you are right. I changed it into "more efficient". Note that the statement you quote is about mechanical energy, not about heat.



Another model could be to run an air compressor from the turbine. Compression will give you a heater, and then releasing the compressed air will give you cooling.

Thinking about it some more, you should also be able to get kinetic energy out (or electricity) and water, by running a turbine on the compressed air and collecting moisture from the cooled air.

Essentially one of these: https://en.wikipedia.org/wiki/Heat_pump_and_refrigeration_cycle#Gas_cycle - they're not technically as efficient as a phase change, but if it's simpler and there are fewer moving parts, it might be a win.

Davide Honey


@Reuben Lewis

Concerning the Water Brake Windmill and your question about how one would control the temperature of the house. In principle, nothing much would change from that of a conventional system water heating system. Radiators are usually controlled by temperature thermostats preset to the desired temperature for each room. The question is how does one turn the heat producing source (the windmill)on and off. Again, also here things don’t change much from the conventional. Windmills have braking systems that can stop them from turning when you don’t need their energy to impute. This can happen automatically via a temperature limiting switch located on your water storage tank which tells the system when to turn off the windmill. Of course, there are other ways but this is the simplest solution.



> Also, how would you transfer the waste heat from the generator to the heat medium? It's probably going to be less practical compared to using a water brake.

The heat medium would be work done inside the house by the electricity.

For example, when I was a student and still owned a desktop instead of a laptop, I has a set-up where I had put a blanket around my desk like a make-shift kotatsu, with the computer desktop poking through. It would suck in cold air from the rest of the room, and dump its waste heat under the desk, turning into a "free" foot heater.

Davide Honey


@ Anyone and Everyone Interested

Concerning the Water Brake Windmill

I think I would like to experiment with this idea and build a mini Water Brake Windmill with the intent of heating a small off grid house.


1. Has anyone out there done any experimentations of their own?

2. Does anyone know of a source for where to buy a small permanent magnet eddy current heater for water or oil heating generation?

Russ Mattson


Very interested in this concept. It makes so much sense. Much simpler, less material (i.e. metals), no generator, fewer controls necessary (associated with frequency control, transformation etc), turn rotational motion directly into heat (especially if all you want is the heat from the wind). And more efficient.

Question I have is: Does anyone, any companies make these devices for sale? If not where would I find suppliers for this type of turbine? It appears much of the ancillary apparatus could be sourced many places. Or they could be fabricated locally.

Murray Read


The thing about heat pumps is that they move more heat energy than it takes to drive them. For a ground source domestic heat pump, that is typically 4 times more heat energy out than electrical energy in. They move heat energy from outside, making an already cool outside a little colder, to inside your house making it warmer.

Given this, there is far more value in driving a heat pump rather than directly converting the mechanical energy to heat.



The idea of direct conversion of meccanical energy of a windmill into heat is so simple that it seems impossible it has not been applied on a large scale yet.

Too much cheap energy from the fossil fuel for too many years has turned us unable to see these kind of solutions.

Hugh Conway


Well, there is a great problem in that an average wind speed of 11 mps is rare....that is nearly 25mph. That and a 5 meter (16+ ft) diameter rotor......

You will have a difficult time finding a location that reliably supplies the required wind speed and has space for a largish (and noisy) turbine.

Theory fine, practical.....not so much.

BTW, I am off grid, and do have practical experience.

Kris De Decker


@ Murray

The study referenced in note 15 contradicts your claims.

@ Hugh

Obviously a water brake windmill is suited for a windy climate. For example, here in Barcelona, it would be useless. It is much more practical to use solar thermal.

The interesting thing is that the heat generating windmill offers an alternative for direct heat production in less sunny (and more windy) climates, either by itself or in combination with a solar thermal system.

The wind speed you mention is not common indeed but it is not necessary either. Because heat can be stored much more cheaply than electricity, a two-days storm can give heat for a week or longer.

Obviously not everyone has space for a heat generating windmill, but there's no need to put a windmill next to each house. You could build small networks for heat distribution, in which one or a few windmills supply heat to a community of buildings or a factory.

Kris De Decker


@ Davide

> Has anyone out there done any experimentations of their own?

Dozens of Danish tinkerers did in the 1970s and 1980s. They are in their seventies and eighties now, some of them have passed away.

One of these builders, Jørgen Krogsgaard, wrote a manual in the 1970s and I have asked him if it could be published. I'll keep you updated here in the comments section.

@ Russ

> Does anyone, any companies make these devices for sale?

No. The only commercial model once made is not for sale anymore. For the moment, there's no other option than to build a water brake windmill from scratch.

Murray Read


@Kris I'm not seeing the contradiction in [15]. The conclusion says that coupling a wind turbine to a heat pump is the most cost effective realization of wind powered heating. The SCOP values in table 3 show heat pumps to have ~3x the performance of retarders or boilers (less than I claimed for GSHP, but consistent with figures I've see for air sourced heat pumps). The CAPEX values are however much higher for heat pumps, so they don't have the same low tech cool as retarders.

In practice I heat my home and hot water with a heat pump powered by renewable electricity, with a bit of solar thermal boost. It's cheaper than my oil fired neighbours, 100% carbon free and practical for many people to install now.

kris de decker


@ Murray

> The conclusion says that coupling a wind turbine to a heat pump is the most cost effective realization of wind powered heating

They conclude that a mechanical heat pump + mechanical windmill is the most cost effective realization of wind powered heating.

Dimitar Bounov


Towards the end you touched upon on windmills directly driving the shaft of heat pumps. I would love to learn more about the feasibility of those mills as heating system for small houses.

For example how much more heat do you get when you use ground loops or ground water vs outside air? Are there tables out there that specify what house area can be heated to what temperature differential at what wind speeds? Or some pointers on the formulas to figure this out ourselves?

If you had any pointers to resources, links to other people's DIY direct wind-to-heat pump setups, or were planning on doing an article on those, that would be greatly appreciated!

kris de decker


@ Job

If you follow that approach, you win indeed. But that's because you heat people, not spaces. Local heating can also be combined with direct heat production, and then you win even more. For example, you could build a desk or a bench with water pipes inside, heated by a small mechanical windmill and heat storage. There is much more to win on the demand side than on the supply side, and the habit to heat the whole space of a building can indeed be questioned.

But this approach doesn't work for industry, where there's also a high demand for heat. And there's also hot water production for washing and cleaning.

@ Dimitar

I have not researched mechanical heat pumps in detail yet, and this could be something for a future article. In the meantime, if anyone else has more info, feel free to share it.

Chuck Hays


Dear Kris,

I thoroughly enjoyed the article on heating with windmills. I went looking for a source for eddy current heaters, however, and came up with nothing.

Is anyone actually making the technology, or is it still in the lab?



Job van der Zwan


> But that's because you heat people, not spaces. Local heating can also be combined with direct heat production, and then you win even more.

Fair points. Purely for the sake of exploring ideas I will pursue my original train of thought anyway ;)

How efficient is mechanical transmission of energy? Like the stangenkunst that you dedicated a few articles too?

Because if most of the energy loss of converting mechanical energy to electric energy is the generator, wouldn't moving the generator inside the house bring any significant benefits?



Once you have a mechanical-input heat pump, it's a simple matter of rerouting the refrigerant flow as to whether the machine extracts heat from the outdoors, and brings it inside, or vice versa.

A heat pump with mechanical input could be run from wind, low-head hydro, a Stirling Engine, or even an electric motor on PV cells.

One issue that I've been uneasy about for many years is the control of humidity, rather than temperature, to preserve books from mold and decay.

Some low-tech cooling approaches, and "just get used to the summer heat", fall short, but a mechanical-input heat pump could also be a dehumidifier.



It all seems to be an interesting concept, but "proof of concept" and "worth doing" are very different.
Does anyone have one for purchase and test? Does anyone have a step-by-step instruction of how to build one that is functional in "actual size", not just a model for display?

[email protected]



I wonder how practical a windmill brake heater would be if you replaced the steam turbine with a stirling engine?

kris de decker


@ Logan

What steam turbine? Water brake windmills don't need steam turbines.

@ Chuck

I don't have the answer but I've sent a mail to some scientists who wrote about the eddy current heater. I'll post their reply (if it comes) here in the comments section.

@ Job

I think that moving the electric generator inside is the worst of both worlds. You introduce extra losses in the mechanical drive train just to get a little bit of waste heat -- and a lot of noise and vibrations -- in the house.



The Joule is defined as the energy equivalent to the work done by a force of 1 Newton over a distance of 1 meter, i.e. 1 J = 1 Nm.

It is the calorie that is defined by the heat it takes to raise the temperature of 1 g of water 1 C. 1 cal is approximately 4.19 J and there are in fact several slightly different calories based on different definitions, see https://en.wikipedia.org/wiki/Calorie. The calorie was defined already in 1824.

Davide Honey


@kris de decker -March 01, 2019 at 08:07 PM

Subject: To build a water brake windmill from scratch.

Thanks for your speedy reply. Great work and service that you are providing! When I asked about anyone out there doing any experimentations of their own, I was thinking more about your "readers" than the Danish tinkerers.

Jørgen Krogsgaard's manual could be inciteful and interesting to look over.

If there are no commercial units of small permanent magnet eddy current heaters for water or oil heating generation currently available, maybe one or more of "us" Low-tech Magazine readers has the capabilities to build one? or knows someone who could?

If more information is needed maybe we could pool our resources and help one another to gather information for a Proto build. Ideas anyone? I am willing and available although I'm not sure of the best way to coordinate such a workgroup.

Kris De Decker


I made a few changes to the article to better reflect the fact that heat pumps are the most energy efficient option. Due to how the article was structured, it sometimes seemed to argue that water brake windmills are more efficient than electric heat pumps.

Water brake windmills may be a lot cheaper to build, but they are not more energy efficient because a heat pump only moves heat. It's mechanical windmills with mechanical heat pumps that are more energy efficient (and cheaper) than wind turbines with electrical heat pumps.

Mario Stoltz


Kris - thanks a lot for this great article, very inspiring! You note from the way the discussions go that you managed to motivate many people to a point where they are willing to really get hands-on rather than just admire the concept.

Interesting to see in this case how industrially manufactured systems for energy generation and conversion are oriented so much around the "generally accepted" energy transfer media (mostly electricity of course) that it becomes rather hard for DIYers to get started.

I think this technology could be great for island communities in higher latitudes. Could be a good opportunity for a smaller manufacturer, though starting a company around this is probably not so straightforward - there must have been a reason why Westrup in Denmark is no longer around. Still, thanks for expanding our horizon of the possible!

Mario Stoltz


Update to my comment above: here's an Island household in Denmark where the owners have posted comparative practical results of the Calorius 37 windmill with solar thermal collectors. All text in Danish, but Google translate is your friend! Seems like they suffered from mechanical and plumbing trouble, probably related to the fact that the system was basically a semi-prototype.

kris de decker


@ Mario Stolz

I'm very curious to read those results, but you did not provide the link you are talking about...

Mario Stoltz


Hi Kris - oh shoot - so sorry. Here it is: http://www.langdal5.dk/energi/info.html

Brendan Howell


Hi Kris,

Very interesting article! Two ideas that might be interesting:

1. What about adding thermoelectric generators to the set-up to generate some electricity for low current devices? They are quite inefficient but this would not matter so much in this context as the unutilized heat would go towards the primary goal of heating an inhabited room.

2. There are quite a few district heating systems which use waste heat from electricity generation or industrial plants (usually nasty fossil or nuclear stuff) but it seems like a Water-brake wind system could be plugged into these to make them greener.

best from Berlin,

Peter Sharp


I built a water brake using a paint can and spun it with my drill press. It took a long time to heat the water and required good insulation. So I experimented a bit toward creating more concentrated heat at a high temperature. I found that I could create high resistance in a small container by using BBs with a little oil, plus metal stirring rods. As the stirring rods are lowered into the BBs, the resistance rises very quickly. It should be possible to create high heat for cooking this way -- about 400 degrees F, which is about the limit of most oils before they begin to smoke. If driven by a windmill, such as my Sharp Cycloturbine, good load matching should be possible. Storing the high heat underground in rocks seems like a cheap option.



I would be interested if a mechanical windmill could be coupled with cryogenic energy storage. Like being commercialised by high view power.


I think they are claiming a round trip efficiency of 60% for electricity storage. This is much better than the 30% of the molten salt.

Maybe a storage system with both hot and cold stores might be interesting.

This adds back an electric generator to the system but it only has to be sized for average power*, not peak windy day power,
the generator is not up on the tower (requiring a stronger tower).

*maybe buffered by an hour or two of battery back-up to allow some flexibility



A direct mechanically coupled heat pump may not be as efficient as it sounds. Beneath a critical wind speed, the compressor will lack the rotational speed to compress the heat transfer fluid back to liquid. A gear box would help, but this adds complexity.

The simplicity of direct mechanical heat production by wind brake is alluring. If oil or pressurised water could be used as the brake fluid, then much higher temperatures could be achieved. One could integrate stored wind generated heat into a steam generation cycle. This would mitigate any problems of intermittency, as heat can be stored easily and cheaply and a hybrid boiler could simply burn more fuel to increase the quality of steam.



Intresting article for sure! How would it be easily replicable? It would need off the shelf part preferably OpenSource (cost wise it would be better to pay someone most in need to design for instance in Bangladesh with a low fee cryptocurrency to design it). Due to efficiency heatpumps are intermittent energy demand. One option is to use source heat that does not have to be maintained and a storage tank. That way it would be kept simple. Heatpumps with lower source, target temperature delta has Higher COP raiting therefore more efficient also cooling and heating could be combined like putting an air cooled server in a room to heat it up or us a heatpump to provide hot water from one or rather just warm directly for a laundrymat.
Sure could DIY one instead from hydro power, but would be just like a prototype - not extensively tested and very efficient.
Anyway it's not a point - end goal is to reduce our dependence on resources



Very interesting article! I wonder, if heat-generating windmills allow lightweight windmills to be economical, would that mean that they could also be portable? How would the economics or environmental considerations be affected if you were to move a lightweight windmill during the course of the year, in order to have windmill spend most of its time where the wind is blowing or the climate is cold? For instance, what if you moved it to a nearby higher elevation during the summer?

Barton Chen


Happy to see an article related to my PhD research topic!

I would say the biggest challenge of direct wind-powered heating system is: the utilisation of this system would be low because of the mismatch between wind power and heat demand profiles.
Heat storage can solve this issue but it requires the storage capacity to store 'few months' of generation to fully decouple the generation and demand. Of cause it is not economical to have that much storage.

In case someone might be interested, there is a UK-based company called "H2O Turbine", and they have wind turbines using water breaks to produce hot.

In terms of using wind-driven mechanical heat pumps to produce heat. I believe the research institute of your reference [15] - DLR (German Aerospace Centre) do have a project to build and test a prototype of this kind of system.

fell free to contact me if you have any question.
[email protected]

Ian Speer



First, thank you for putting up such an interesting web site. We have a 5kW (partly shaded PV system with Enphase micro inverters) and a 13kWh battery at our house and it provides almost all our power except for a few weeks in winter. We are in Perth in Western Australia.

I just wanted to comment in relation to the article and the use of heat pumps. Heat pumps are certainly the most efficient way to heat either stored water or a house directly however they do not operate on a varying amounts of input power as does simple resistance heating. To use a heat pump and avoid using grid power we would have needed quite a large PV array or we would have had to operate it from our battery which would reduce our overnight reserve power.

We looked carefully at direct water heating via the actual PV panels and found there are two manufacturers (one French and the other Turkish) but they only seem to be effective at lower temperatures such as pool heating plus we have long very hot summers and solar water heaters tend to spend summers venting hot water to avoid overheating in any case. In the end we found a simple device that monitored our PV power production beyond what was being used to charge our battery and instead of allowing this power to be exported it would divert it to heat water no matter how much or how little until the water storage reached a cutout temperature at which time the system would then export surplus power. We receive very little for any exported power so self-consumption is preferred. That said PV panels are so cheap now particularly as many people are upgrading that a simple resistance water heater does the job and probably for rather less than a heat pump.

Not very elegant but it works for us. If you want to see the little relay the following link will take you there. I have no commercial relationship with them other than as a customer.


Best regards,

Ian Speer.

Ps I will be ordering your paper books.

Geoffrey Greene


Could this heat storage via water tank double as a way to store potable water? Installing a 20,000L tank is a significant expense, but might be worth it in areas with intermittent access to water or drought conditions. Would the heat be sufficient to kill pathogens in the water tank for longer-term water storage?

kris de decker


New paper on heat mills: https://asmedigitalcollection.asme.org/energyresources/article/144/4/040802/1120767/State-of-the-art-of-Windthermal-Turbines-A



Hi Kris,

Great article. I'm wondering if the idea of a heat mill would combine well with a tethered balloon or kite wind system? That way you could put it directly above a busy city perhaps.
What do you think?

Or maybe even - to add in another low tech concept - it could then double as a barrage balloon defence system in certain cases; a place like Ukraine, for example, could hypothetically generate heat it needs in winter by way of tethered barrage balloons with heat mills attached...

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