Their financial payback time is much longer than their life expectancy and in urban areas they will not even deliver as much energy as was needed to produce them. Sad, but true.

A small windmill on your roof or in the garden is an attractive idea. Unfortunately, commercially available micro wind turbines deliver hardly enough energy to power a light bulb.

Small windmills have been around for some decades, but in recent years the focus has shifted on developing their potential use in an urban environment (where most of us live). It’s difficult to keep track of the numerous proposed new designs, meant to be placed on the roof or on a mast in the garden.

Small windmills in built-up environments are a remarkable trend. Through the ages, windmills have always demanded a free flow of strong wind. They are preferably placed on an open plain, as high as possible, with no obstacles around. In cities, however, this is not the case. Yet, the designers of urban windmills all claim to have invented a “revolutionary” windmill, especially created for the low wind speeds in those environments.

Most of these windmills are not yet commercially available, which makes it hard to verify whether or not the claims of the designers are justified. The few that are include the “Energy Ball”, a product from the Netherlands which is also sold in the neighbouring country of Belgium. The windmill is made by a company named “Home Energy” (read a snippet of their website in English, French and Swedish).

The Energy Ball, which can be placed on a roof or on a mast in the garden, is said to deliver more energy than a traditional windmill, and to generate electricity at a very low wind speed of 2 metres per second (Beaufort 2).

The secret of these results is the “Venturi-effect”, inspired by river currents. Thanks to the “unusual and exceptional aerodynamic characteristics”, the machine creates a wind flow pattern that “converges first and is then accelerated through the rotor”. Furthermore, Home Energy labels the Energy Ball as "beautiful" and "noiseless", addressing two important objections against urban windmills: noise pollution and visual intrusion.

Energy output: 100 kWh per year

All this sounds promising, but do the numbers add up? Home Energy states that the Energy Ball can deliver 500 kilowatt-hours of electricity per year, or 15 to 20 percent of the electricity use of an average Dutch household (which consumes 3,567 kWh per year). But these claims are based on an average wind speed of 7 metres per second (Beaufort 4) – highly optimistic.

If you look at the wind map of the Netherlands below (this data is nowhere to be found on the Home Energy website) you can see that the average wind speed on land (at a height of 10 meters) is only 4.3 metres per second. Holland is also a notoriously windy country. Only a small part of the coastline receives an average wind speed of 7 metres per second. In Belgium, the average wind speed at the coastline is nowhere higher than 6 metres per second.

At an average wind speed of 4 metres per second, the yearly electricity output of the Energy Ball only amounts to 100 kilowatt-hours (this figure comes from their website). This is not 15 to 20 percent, but just 3 to 4 percent of the yearly electricity use of an average Dutch household (100 kilowatt-hours corresponds to a continuous power consumption of 11 watts). Obstacles like trees and buildings can make the yields in specific locations even lower than that.

Payback time: 50 to 750 years

The very low power output of the Energy Ball would not be such a problem if the machine was cheap. After all, as Home Energy states, the windmills can be placed in series. However, the price of one Energy Ball, everything included, is around 5,000 euro (7,300 dollar). If our average Dutch household wants to cover 15 percent of their energy use by wind energy, it needs at least 5 Energy Balls. Total cost: 25,000 euro (36,500 dollar). If the household wants to cover all its needs by wind energy, it needs to buy 30 Energy Balls for a price of 150,000 euro (219,500 dollar).

"The energy output of the Energy Ball is based on an average wind speed of 7 metres per second, which is unrealistic in cities"

How much time does it take to earn back the initial investment of an Energy Ball? Home Energy is careful enough to state on their website that the payback time depends on “the initial investment, the yearly yield and the prevailing price per kilowatt-hour”. However, it would be fairer to state that the Energy Ball will never pay itself back.

Rates per kilowatt-hour of electricity fluctuate largely around the world and even within countries, but let’s assume a price of 0.20 euro, the relatively high average electricity price in the Netherlands (that’s 0.29 dollar – three times the price of electricity in the US). If you also assume Home Energy’s optimistic average wind speed of 7 metres per second (which corresponds to an output of 500 kilowatt-hours) then the payback time is 50 years. Take a more realistic average wind speed of 4 metres per second and the payback time becomes 250 years. At the average US electricity price, payback time is 750 years.

Warranty of 2 years

Of course, electricity prices may rise, and the Energy Ball may become cheaper to produce. If you assume an electricity rate of 1 euro (1.46 dollar) per kWh, then the Energy Ball pays itself back in 10 years (at the most optimistic wind speed of 7 m/s) or in 50 years (at a more realistic wind speed of 4 m/s). If Home Energy can also cut the selling price in half, then we are talking about a payback time of 5 years (at high average wind speed) or 25 years (at realistic average wind speed). Even in these hypothetic cases, however, payback time is speculative.

According to the manufacturer, the life expectancy of the Energy Ball is 20 years. That’s just a promise. The machine comes with a warranty of only 2 years. Solar panels have a warranty of at least 20 years. Contrary to solar panels, windmills consist of mechanically moving parts, which means that there are more breakage possibilities.

Embodied energy

Advocates of personal windmills sometimes admit that yields are not very impressive. But they state that buying a small windmill can still be a good choice from an ecological viewpoint, even if it is crazy from a financial perspective. This sounds rather reasonable, but something is forgotten here: the energy needed to manufacture and install these machines.

Urban windmills are not as energy-intensive to produce as solar panels, but since they also have much lower yields and much lower life expectancies than solar panels, their energy footprint is even worse. According to a recent report by the UK Carbon Trust, windmills in urban environments will almost always have an energy payback of more than 20 years.

In other words: small windmills in cities will never deliver as much energy as was needed to manufacture and install them. Installing an urban windmill will actually harm the environment. On the other hand, the energy payback time of a large windmill is less than one year.

Wind physics

Ecotech boffins will be fast to reply that the Energy Ball might be a failure, but that it does not mean that other concepts can not do better. Unfortunately, the problem is not the windmill – it is the wind. The Dutch have a long tradition of designing windmills, so there is a big chance that the Energy Ball does better than its competitors.

"Doubling the wind speed increases wind power 8 times. How you design a windmill hardly makes any difference"

Wind speed has a much larger influence on energy output than the design of a windmill. To calculate wind power you have to multiply the density of air, the swept area and the cube of wind speed. Doubling the rotor radius of a wind turbine increases wind power 4 times.



Doubling the wind speed increases wind power 8 times. At an average wind speed of 7 metres per second, a windmill delivers 5.36 times more energy than at an average wind speed of 4 metres per second. How you design your windmill hardly makes any difference.



Cut-in speed

At lower average wind speeds, even very small changes can make a huge difference. According to the Carbon Trust the cut-in speed of a small wind turbine (the moment it starts producing energy) is between 3 and 4 metres per second. This is close to the average wind speed on land in rather windy countries like Belgium and the Netherlands.

A test by the Carbon Trust (see graphics below) showed that a windmill receiving an average wind speed of 4.5 metres per second produced 7 times more energy than a windmill receiving an average wind speed of 3 metres per second – because the latter is not operating most of the time since it does not reach its cut-in speed. While large wind turbines have an average capacity factor of 28 to 35 percent, small windmills only achieve 15 to 20 percent of their capacity in rural areas and only 10 percent in urban areas.

Home Energy claims a lower cut-in speed of 2 metres per second, but that’s just bogus. And they know it: in the Q&A section of their Dutch website they admit that the windmill starts rotating at 2 metres per second, but only starts delivering energy at 3 metres per second.

Height

An important factor to do with wind speed is height: wind speed rises and is more constant the higher you go, which is the reason why traditional windmills are built ever larger and why the concept of floating windmills is attracting so much interest. It is also the reason why the potential of small windmills is generally overestimated. When you are shown a wind map, the chance is big that it concerns a map of wind speeds at a height of 75 metres or more, indicating the potential of traditional windmills. Wind maps showing the wind speed at a height of 10 metres are much harder to find.

Urban windmills are - by definition - located close to ground level, where wind speed is as low as it can be. Of course, you can place your urban windmill on a mast of 100 metres, but such a construction would make the carbon footprint of the machine even worse. Placing windmills on a skyscraper is of not much help either: in this case you have much higher wind speeds but the roof is way too small to set up a windmill for every household that lives in the building.

What we need

The fundamental problem of urban windmills is that they harvest electricity from an inferior energy source. On a rooftop in a built-up environment, wind speed is low and freakish. And while you can think of a thousand ways to change the design of an urban windmill, it is impossible to change the wind itself.

"Most wind maps show wind speed at a height of 75 metres or more"

Installing one big windmill will therefore always be a better choice (economically as well as ecologically) than installing many more small wind turbines instead. Sad, but true. This is not the case with solar panels. Other buildings (or trees) might cast shadows on solar panels in a city, but if that can be avoided, capturing solar energy from your roof is not less efficient than capturing solar energy from a larger solar plant.



Since it is impossible to substantially improve the power output of urban windmills, the only hope for decentralised wind energy is to produce machines which are much cheaper and more importantly leave a much smaller carbon footprint and/or have a much longer service life.

Kris De Decker (edited by Shameez Joubert)

Update : Small windmills put to the test

It seems that we have been too friendly for the Energy Ball. A real-world test by the Dutch province of Zeeland (a very windy place) confirms our analysis that small windmills are a fundamentally flawed technology.

The Energy Ball has the worst score of all, with an energy yield of only 73 kWh per year, even less than our worst case scenario. Other micro wind turbines did not do much better. Read the article here.

Related articles :