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 supply only indirectly and inefficiently.
Solar thermal collectors skip the electricity conversion and supply renewable thermal energy in a direct and efficient way. Much less known is that a mechanical windmill can do the same in a windy climate — by eliminating electricity conversion and by oversizing its brake system, a windmill can generate lots of heat through friction.
Given the right conditions, a mechanical windmill with an oversized brake system is a cheap, effective, and sustainable heating system.
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.  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.  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. 
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  . 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).  
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, more energy efficient, and more sustainable than indirect heat production.
Prototypes of heat generating windmills, built by Esra L. Sorensen in 1974. Photo by Claus Nybroe. Source: 
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 more 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 three times 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. 
The Water Brake Windmill
Heat generating windmills convert 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.   
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 Joule equals the amount of energy required to raise the temperature of 1 cubic centimeter of water by 1 degree Celsius. 
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.
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 and More Efficient to Operate
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, including an electric heat pump. There’s two reasons for this.
First, 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 sturdily built. The Joule Machine has lower weight, smaller size, and lower costs than an electrical generator.  Equally important is that the cost of thermal storage is 60-70% lower compared to batteries or the use of backup thermal power plants. 
A water brake windmill built at the Institute for Agricultural Techniques in 1974. Photo by Ricard Matzen. Source: 
Second, converting wind or solar energy directly into heat (or mechanical energy) is 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.
Furthermore, direct heat generation greatly improves the economics and the sustainability of smaller types of windmills. Tests have shown that small wind turbines – producing electricity – are very inefficient and don’t always generate as much energy as was needed to produce them.  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.  Many were built with used car parts and other discarded materials. 
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. 
A Calorius windmill producing up to 4 kW of heat. Image provided by the Nordic Folkecenter in Denmark.
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. 
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. 
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 result 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. 
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 built for thermal energy generation”. 
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 should 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).  A 2013 study using a prototype obtained similar results, and 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.  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: 
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). 
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: 
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. 
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.   In this case, the windmills no longer make use of the original water brake, but produce heat with mechanical heat pumps or hydrodynamic retarders. 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, involving one less energy conversion.
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, cheaper and more efficient 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: 
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.  
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 the most expensive one. 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: 
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. 
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. 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.  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: 
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 to 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. 
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.
Teaser Illustration: Rona Binay for Low-tech Magazine.