Low-tech Solutions

Heat your House with a Water Brake Windmill

Given the right conditions, a mechanical windmill with an oversized brake system is a cheap, effective, and sustainable heating system.


Image: Illustration by Rona Binay for Low-tech Magazine.

Renewable energy production is almost entirely aimed at the generation of electricity. However, we use more energy in the form of heat, which solar panels and wind turbines can produce only indirectly and relatively inefficiently. A solar thermal collector skips the conversion to electricity and supplies renewable thermal energy in a direct and more efficient way.

Much less known is that a mechanical windmill can do the same in a windy climate — by oversizing its brake system, a windmill can generate lots of direct heat through friction. A mechanical windmill can also be coupled to a mechanical heat pump, which can be cheaper than using a gas boiler or an electric heat pump driven by a wind turbine.

Heat versus Electricity

On a global scale, thermal energy demand corresponds to one third of the primary energy supply, while electricity demand is only one-fifth. 1 In temperate or cold climates, the share of thermal energy is even higher. For example in the UK, heat counts for almost half of total energy use. 2 If we only look at households, thermal energy for space and water heating in temperate and cold climates can be 60-80% of total domestic energy demand. 3

In spite of this, renewable energy sources play a negligible role in heat production. The main exception is the traditional use of biomass for cooking and heating – but in the “developed” world even biomass is often used to produce electricity instead of heat. The use of direct solar heat and geothermal heat provide less than 1% and 0.2% of global heat demand, respectively 4 5. While renewable energy sources account for more than 20% of global electricity demand (mostly hydroelectric), they only account for 10% of global heat demand (mostly biomass). 5 6

Direct versus Indirect Heat Production

Electricity produced by renewable energy sources can be – and is being – converted to heat in an indirect way. For example, a wind turbine converts its rotational energy into electricity by the use of its electrical generator, and this electricity can then be converted into heat using an electric heater, an electric boiler, or an electric heat pump. The result is heat generated by wind energy.

In particular, the electric heat pump is promoted by many governments and organisations as a sustainable solution for renewable heat generation. However, solar and wind energy can also be used in a direct way, without converting them to electricity first – and of course the same applies to biomass. Direct heat production is cheaper, can be more energy efficient, and is more sustainable than indirect heat production.


Image: prototypes of heat generating windmills, built by Esra L. Sorensen in 1974. Photo by Claus Nybroe. Source: 13

The direct alternative for solar photovoltaic power is solar thermal power, a technology that appeared in the nineteenth century following cheaper production technologies for glass and mirrors. Solar thermal energy can be used for water heating, space heating or industrial processes, and this is 2-3 times as energy efficient compared to following the indirect path involving electricity conversion.

Almost nobody knows that a windmill can produce heat directly

The direct alternative for wind power that everybody knows is the old-fashioned windmill, which is at least 2.000 years old. It transferred the rotational energy from its wind rotor directly to the axis of a machine, for example for sawing wood or grinding grain. This old-fashioned approach remains relevant, also in combination with new technology, because it would be more energy efficient compared to first converting the energy to electricity, and then back to rotational energy.

However, an old-fashioned windmill can not only provide mechanical energy, but also thermal energy. The problem is that almost nobody knows this. Even the International Energy Agency doesn’t mention direct conversion of wind into heat when it presents all possible options for renewable heat production. 1

The Water Brake Windmill

The original type of heat generating windmill converts rotational energy directly into heat by generating friction in water, using a so-called “water brake” or “Joule Machine”. A heat generator based on this principle is basically a wind-powered mixer or impeller installed into an insulated tank filled with water. Due to friction among molecules of the water, mechanical energy is converted into heat energy. The heated water can be pumped into a building for heating or washing, and the same concept could be applied to industrial processes in a factory that require relatively low temperatures. 7 8 9


Image; a heating system based on a water brake windmill. Source: 8

The Joule Machine was originally conceived as a measuring apparatus. James Joule built it in the 1840s for his famous measurement of the mechanical equivalent of heat: one calorie equals the amount of energy required to raise the temperature of 1 cubic centimeter of water by 1 degree Celsius. 10

A heat generator based on this principle is basically a wind-powered mixer or impeller installed into an insulated tank filled with water.

The most fascinating thing about water brake windmills is that, hypothetically, they could have been built hundreds or even thousands of years ago. They require simple materials: wood and/or metal. But although we cannot exclude their use in pre-industrial times, the first reference to heat producing windmills dates from the 1970s, when the Danes started building them in the wake of the first oil crisis.


Image: the heat generator of a heat generating windmill. Source: 8

At the time, Denmark was almost entirely dependent on imported oil for heating, which left many households in the cold when the oil supply was disturbed. Because the Danes already had a strong DIY-culture for small wind turbines generating electricity on farms, they started building windmills to heat their houses. Some chose the indirect path, converting wind generated electricity into heat using electric heating appliances. Others, however, developed mechanical windmills that produced heat directly.

Cheaper to Build

The direct approach to heat production is considerably cheaper and more sustainable than converting wind or solar generated electricity into heat by using electric heating devices. There’s two reasons for this.

First, and most importantly, mechanical windmills are less complex, which makes them more affordable and less resource-intensive to build, and which increases their lifetime. In a water brake windmill, electric generator, power converters, transformer and gearbox can be excluded, and because of the weight savings, the windmill needs to be less sturdy built. The Joule Machine has lower weight, smaller size, and lower costs than an electrical generator. 11 Also important is that the cost of thermal storage is 60-70% lower compared to batteries or the use of backup thermal power plants. 2


A windmill with water brake built at the Institute for Agricultural Techniques in 1974. Photo by Ricard Matzen. Source: 13

Second, converting wind or solar energy directly into heat (or mechanical energy) can be more energy efficient than when electric conversion is involved. This means that less solar and wind energy converters – and thus less space and resources – are needed to supply a certain amount of heat. In short, the heat generating windmill addresses the main disadvantages of wind power: its low power density, and its intermittency.

Mechanical windmills are less complex, which makes them more affordable and less resource-intensive to build, and which increases their lifetime

Furthermore, direct heat generation greatly improves the economics and the sustainability of smaller types of windmills. Tests have shown that small wind turbines – which produce electricity – are very inefficient and don’t always generate as much energy as was needed to produce them. 12 However, using similar models for heat production decreases embodied energy and costs, increases lifetime, and improves efficiency.

How Much Heat Can a Windmill Produce?

The Danish water brake windmill from the 1970s was a relatively small machine, with a rotor diameter of around 6 meters and a height of around 12 meters. Larger heat generating windmills were built in the 1980s. Most used simple wooden blades. In total, at least a dozen different models have been documented, both DIY and commercial models. 7 Many were built with used car parts and other discarded materials. 13


Image: A Calorius windmill producing up to 4 kW of heat. Image provided by the Nordic Folkecenter in Denmark.

One of the smaller early Danish heat generating windmills was officially tested. The Calorius type 37 – which had a rotor diameter of 5 meters and a height of 9 meters – produced 3.5 kilowatt of heat at a wind speed of 11 m/s (a strong breeze, Beaufort 6). This is comparable to the heat output of the smallest electric boilers for space heating. From 1993 to 2000, the Danish firm Westrup built a total of 34 water brake windmills based on this design, and by 2012 there were still 17 in operation. 7

A much larger water brake windmill (7.5m rotor diameter, 17m tower) was built in 1982 by the Svaneborg brothers, and heated the house of one of them (the other brother opted for a wind turbine and an electric heating system). The windmill, which had three fiberglass blades, produced up to 8 kilowatt of heat according to non-official measurements – comparable to the heat output of an electric boiler for a modest home. 7

Further into the 1980s, Knud Berthou built the most sophisticated heat generating windmill to date: the LO-FA. In other models, heat generation happened at the bottom of the tower – from the top of the windmill there was a shaft down to the bottom where the water brake was installed. However, in the LO-FA windmill all mechanical parts for energy conversion were moved to the top of the tower. The lower 10 meters of the 20 meter high tower were filled up with 15 tonnes of water in an insulated reservoir. Consequently, hot water could literally be tapped out of the windmill. 7

The tower of the LO-FA windmill was filled up with 15 tonnes of water in an insulated tank: hot water could literally be tapped out of the windmill.

The LO-FA was also the largest of the heat generating windmills, with a 12 meter diameter rotor. Its heat output was estimated to be 90 kilowatt at a wind speed of 14 m/s (Beaufort 7). This results seems to be excessive compared to the other heat generating windmills, but the energy output of a windmill increases more than proportionally with the rotor diameter and the wind speed. Furthermore, the friction liquid in the water brake was not water but hydraulic oil, which can be heated up to much higher temperatures. The oil then transferred its heat to the water storage in the tower. 7

Renewed Interest

Interest in heat generating windmills resurfaced a few years ago, although for now it concerns only a handful of scientific studies. In a 2011 paper, German and UK scientists write that “small and remote households in northern regions demand thermal energy rather than electricity, and therefore wind turbines in such places should be build for thermal energy generation”. 8

The researchers explain and illustrate the workings of the water brake windmill, and calculate the optimal performance of the technology. It was found that the torque-speed characteristics of wind rotor and impeller must be carefully matched to achieve maximum efficiency. For example, for the very small Savonius windmill that the scientists used as a model (0.5m rotor diameter, 2m tower), it was calculated that the impeller diameter should be 0.388m.


The researchers then ran simulations over a period of fifty hours to calculate the windmill’s heat output. Although the Savonius is a low speed windmill which is ill-suited for electricity generation, it turns out to be an excellent producer of heat: the small windmill produced up to 1 kW of thermal power (at wind speeds of 15 m/s). 8 A 2013 study using a prototype obtained similar results, and calculated the efficiency of the system to be 91%. 9 This is comparable to the efficiency of a wind turbine heating water through electricity.

A 2013 study using a prototype calculated the efficiency of the system to be 91%

Obviously, it’s not always stormy weather, which means that the average wind speed is at least as important. A 2015 study investigates the possibilities of heat generating windmills in Lithuania, a Baltic country with a cold climate that’s dependent on expensive fuel imports. 14 The researchers calculated that at the average wind speed in the country (4 m/s of Beaufort 3), generating one kilowatt of heat requires a windmill with a rotor diameter of 8.2 meters.


A heat generating windmill with a water brake, placed inside the bottom of the tower. The mill was built by Jorgen Andersen in 1975, and stood in Serritslev. Photo by Claus Nybroe. Source: 13

They compare this with the thermal energy demand of a 120 m2 energy efficient new building, heated to modern comfort standards, and conclude that a heat generating windmill could cover from 40-75% of the annual heating needs (depending on the energy efficiency class of the construction). 14

Heat Storage

The average wind speed is not guaranteed either, which means that a heat generating windmill requires heat storage – otherwise it would only provide heating when the wind blows. One cubic meter of heated water (1 ton, 1,000 liters) can hold up to 90 kWh of heat, which is roughly one to two days of supply for a household of four persons.


The same windmill as the one pictured above, seen from below. Source: 7

Providing enough storage to bridge a week without wind thus requires up to 7 tonnes of water, which corresponds to a volume of 7 cubic meters plus insulation. However, energy losses (self-discharge) should also be taken into account, and this explains why the Danish heat generating windmills usually had a storage tank holding ten to twenty thousand liters of water. 13

A heat generating windmill can also be combined with a solar boiler, so that both sun and wind can supply direct thermal energy using a smaller water tank.

A heat generating windmill can also be combined with a solar boiler, so that both sun and wind can supply direct thermal energy using the same heat storage reservoir. In this case, it becomes possible to build a pretty reliable heating system with a smaller heat storage tank, because the combination of two – often complementary – energy sources increases the chances of direct heat supply. Especially in less sunny climates, heat generating windmills are a great addition to a solar thermal system, because the latter produces relatively less heat during winter, when heat demand is at its maximum.

Retarders and Mechanical Heat Pumps

The most recent and extensive studies to date are from 2016 and 2018, and compare different types of heat generating windmills with different types of indirect heat generation. 1 15 In this second type of heat generating windmill, heat is produced with with mechanical heat pumps or hydrodynamic retarders, not with a water brake.

A mechanical heat pump is simply a heat pump without the electric motor – instead, the wind rotor is directly connected to the compressor(s) of the heat pump. This innvolves one less energy conversion, which makes the combination at least 10% more energy efficient than an electric heat pump driven by a wind turbine.

The hydrodynamic retarder is well known as a brake system in heavy vehicles. Like a joule machine, it converts rotational energy into heat without the involvement of electricity. Retarders and mechanical heat pumps have the same advantages as Joule Machines, in the sense that they are much smaller, lighter, and cheaper than electrical generators. However, in this case a gearbox is required to achieve optimal efficiency.


Different types of direct and indirect heating production compared. Source: 15

The study compares heat generating windmills based on retarders and mechanical heat pumps with indirect heat production using electric boilers and electric heat pumps. It compares these four technologies for three system sizes: a small windmill aimed at heating an off-the-grid household, a large windmill aimed at supplying heat to a village, and a wind farm producing heat for 20,000 inhabitants. The four heating concepts are ranked based on their yearly capital and operational expenditures, assuming a lifespan of 20 years. 1 15

Directly coupling a mechanical windmill to a mechanical heat pump is cheaper than using a gas boiler or the combination of a wind turbine and an electric heat pump.

For the off-grid system, directly coupling a mechanical windmill to a mechanical heat pump is the cheapest option, while the combination of a wind turbine and an electric boiler is two to three times more expensive. All other technologies are in between. Taking into account both investment and operational costs, small-scale heat generating windmills with mechanical heat pumps are equally expensive or cheaper than conventional gas boilers when assuming the typical performance of a small windmill (which produces – over a period of one year – 12% to 22% of its maximum energy output).


Image: Water brake windmill developed by O. Helgason (left), water brake with variable load system (right). Images from “Test at very high wind speed of a windmill controlled by a water brake”, O. Helgason and A.S. Sigurdson, Science Institute, University of Iceland. Source: 7

On the other hand, the combination of a small wind turbine and an electric heat pump requires a windmill with a “capacity factor” of at least 30% to become cost-competitive with gas heating – but such high performance is very unusual. Larger systems present the same rankings – the combination of mechanical windmills and mechanical heat pumps is the cheapest option – but they have up to three times lower capital costs due to economies of scale. Larger windmills have higher capacity factors (16-40%), which result in even larger cost savings.

Due to the large energy losses for heat transportation, the heat generating windmill is at its best as a decentralised energy source, providing heat to an off-the-grid household or – in the optimal case – a small city.

However, larger systems also reveal a problem when scaling up the technology: storing heat may be cheaper and more efficient than storing electricity, but the opposite holds true for transportation: the energy losses for heat transportation are much larger than the energy losses for electricity transmission. The scientists calculate that the maximum distance that is cost-achievable under optimal wind conditions is 50 km. 15

Consequently, the heat generating windmill is at its best as a decentralised energy source, providing heat to an off-the-grid household or – in the optimal case – a relatively small town or city, or an industrial area. For even larger systems, energy needs to be transported in the form of electricity, and in that case direct generation of heat – with all its benefits – becomes unattractive.

Blinded by Electricity

Heat generating windmills are also investigated for renewable electricity production, mainly because they offer a better solution for energy storage compared to batteries or other common technologies. 16 In these systems, the generated heat is converted to electricity by the use of a steam turbine. The storage system is similar to that of a concentrated solar power plant (CSP), and the solar concentrators are replaced by heat generating windmills.


An “eddy current heater”. Source: 9

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

Direct heat production thus offers the possibility to save three times more greenhouse gas emissions and fossil fuels using the same number of windmills, which are also cheaper and more sustainable to build. Hopefully, direct heat production will be given the priority it deserves. Despite a warming climate, the demand for thermal energy is as high as ever.

Kris De Decker

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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. Posted by: steve | February 28, 2019 at 12:36 AM

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?


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

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 Stolz

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, -Brendan

  1. Nitto, Dipl-Ing Alejandro Nicolás, Carsten Agert, and Yvonne Scholz. “WIND POWERED THERMAL ENERGY SYSTEMS (WTES)“. 

  2. Integration of Thermal Energy Storage into Energy Network, Sharyar Ahmed, 2017 

  3. The bright future of solar thermal powered factories, Kris De Decker, Low-tech Magazine, 2011 

  4. Solar Heat Worldwide, edition 2018, International Energy Agency (IEA). 

  5. Renewables 2018, Heat, International Energy Agency (IEA). 

  6. World Bank: Renewable electricity output

  7. The Rise of Modern Wind Energy: Wind Power for the World. Pan Stanford Publishing, 2013. See chapter 13 (“Water brake windmills”, Jørgen Krogsgaard) and chapter 16 (“Consigned to Oblivion”, Preben Maegaard). These seem to be the only English language documents on Danish water brake windmills. 

  8. Chakirov, Roustiam, and Yuriy Vagapov. “Direct conversion of wind energy into heat using joule machine.” Fourth International Conference on Environmental and Computer Science (ICECS 2011), Singapore, Sept. 2011. 


  10. Joule’s experiment: An historico-critical approach, Marcos Pou Gallo Advisor. 

  11. Okazaki, Toru, Yasuyuki Shirai, and Taketsune Nakamura. “Concept study of wind power utilizing direct thermal energy conversion and thermal energy storage.” Renewable energy 83 (2015): 332-338. 

  12. Real-world tests of small wind turbines in Netherlands and the UK, Kris De Decker, The Oil Drum, 2010. 

  13. Selfbuilders, Winds of Change website, Erik Grove-Nielsen. 

  14. Černeckienė, Jurgita, and Tadas Ždankus. “Usage of the Wind Energy for Heating of the Energy-Efficient Buildings: Analysis of Possibilities.” Journal of Sustainable Architecture and Civil Engineering 10.1 (2015): 58-65. 

  15. Cao, Karl-Kiên, et al. “Expanding the horizons of power-to-heat: Cost assessment for new space heating concepts with Wind Powered Thermal Energy Systems.” Energy 164 (2018): 925-936. 

  16. Okazaki, Toru, Yasuyuki Shirai, and Taketsune Nakamura. “Concept study of wind power utilizing direct thermal energy conversion and thermal energy storage.” Renewable energy 83 (2015): 332-338.