Micro Wind

Plenty of spin but where’s the power?

Some manufacturers of roof mounted wind turbines are making incredible claims about how much electricity these devices can deliver. One supplier, Renewables Ireland, suggest their Windsave turbine can reduce household electricity bills by up to one third.

Independent researchers strongly disagree. One study carried out in Edinburgh by independent researcher Hugh Piggott indicated that the output of one of these roof mounted turbines may be as little as £10 (€15) worth of electricity per annum.

How to spot dodgy products

Output

The first clue is information on turbine output? Does the supplier provide detailed information about the annual output at different wind speeds? If not, why not?

The dodgy supplier will invent all sorts of reasons as to why they can't provide this information. Usually the excuses are on the lines of 'research is on-going and we'll have more information in the near future'. Another good one is 'there are no standard test procedures in place'. While this is true, it is a very simple matter to put a meter on a turbine to measure output over time, standard test procedures or not.

As the person interested in buying one of these products, it is perfectly reasonable to ask for information about performance. If the supplier can't answer these questions to your satisfaction, look elsewhere.

Claims of miraculous new blade design, higher efficiencies or electronic wizardry

Wind turbines for generating electricity have been around for about one hundred years. There are no miraculous technological breakthroughs wating to happen. Any new innovations will offer no more than marginal improvements in efficiency. Claims of higher levels of efficiency resulting from lower cut in speeds (the speed at which the turbine starts to generate electricity) should be treated with the derision they deserve. At cut in speed, the output of a turbine will be only 1-2 percent of its maximum output, at best...

Debunking the Myths about Micro Wind Turbines
By Andy Wilson (written January 2007)

Wind turbines, love them or hate them, are undoubtedly a symbol of the emerging technology of renewable energy. Micro turbines, small enough to feed power directly into the grid at single phase (domestic mains) voltage, are now available over the counter at DIY retailers like B&Q, and can be ordered on line from an ever increasing number of suppliers. Questions
remain, however, about many aspects of the technology. Can micro turbines significantly contribute to reducing the demand for fossil fuels or are they just another green fashion accessory?

Some proponents of renewable energy seem to believe quantity is preferable to quality. This viewpoint was expressed in a letter to the Local Planetnewspaper: “Sure, micro turbines are small and produce only a little power, but then, why not have more than one micro generator? Why not two, one at each end of the house? One on each corner? Or another at the end of the
garden?”

Unfortunately, this sort of thinking is unhelpful because it completely fails to address the crux issue, namely the need to use diminishing resources more responsibly and efficiently; a wind turbine, which only delivers a few kilowatt hours of energy per annum, may have required more energy to manufacture, transport and install it than it could realistically be expected to deliver during its entire lifetime.

It is important to be realistic about what renewable energy technologies might deliver. When some micro turbines are only capable of delivering a few percent of a typical household’s electricity requirements, it is counter productive to the development and credibility of the micro renewables sector, if suppliers are claiming that their product will reduce our household bills by one third or even one half. The most cost effective method of reducing your electricity bill is simply to find ways to use less electricity.

Output is Proportional to Swept Area

The output of a turbine is proportional to the swept area, which is the area of a circle bounded by the tips of the turbine blades.

Table 1 Comparison of the Output and Size of  different Micro Turbines

*aka Jetstream

Efficiency

Almost eighty years ago, Albert Betz calculated that the maximum percentage of the wind's energy which may be captured by a turbine, is 59.3 percent.

This is known in the wind energy sector as the Betz Limit. No one has ever built a turbine which is capable of this level of efficiency, nor will it ever happen. In practical terms about 40 percent is regarded as the maximum achievable efficiency and even this only occurs at optimum wind speed. Although there is some variation between different models of turbine, the maximum efficiency usually occurs at wind speeds of around 10 meters per second (m/s). In wind energy literature this mechanical efficiency is usually referred to as Cp or Co-efficient of Performance.

Smaller turbines tend to be less efficient than large ones. According to Paul Gipe, one of the United States leading authorities on wind energy, “small turbines seldom deliver more than 30 percent of the wind’s energy for any prolonged period of time... [and] at extremely windy sites, small turbines typically convert only 12 percent of the energy of the wind.”

An average efficiency of anything over 20 percent is very good for any micro turbine. In contrast to these widely accepted figures, data provided by South West Wind Power, the manufacturer of the Air X turbine, implied that the Air X has an efficiency of 29.8 percent at wind speeds of 12 m/s.

Data provided by Renewables Ireland in relation to their Wind Save turbine is even more incredible.  According to the technical specifications on their website, their turbine will deliver 1 kW of electricity at a wind speed of 12 meters per second, implying an efficiency of almost 40 percent.  Albert Betz would be impressed.

The usual glib explanation offered by many manufacturers and suppliers, when challenged about their data, is that the ‘innovative’ design of their particular product has lead to a greater level of efficiency in capturing the energy of the wind. This reality defying explanation, however, does not stand up to any serious scrutiny. The only significant improvements in blade design in recent years have been in relation to durability not to efficiency. In some cases, levels of efficiency have actually fallen very significantly as the micro wind sector has moved more towards budget models imported by the container load from China.

Cut in Speed

Another one of the myths propagated by some turbine manufacturers relates to the cut in speed of the generator. This is the wind speed at which the generator begins to deliver a measurable electrical output and is usually around 2.5-3.0 m/s. It is sometimes claimed that the lower cut in speed of a particular turbine will lead to spectacular increases in electrical output.

The cut in speed however, has very little bearing on the total electrical output of a turbine measured over a period of time. This is because the output of the turbine at cut in speed may be as little as one fiftieth of the output at optimum wind speed. The bulk of energy yield from a wind turbine will not occur on those days of average or below average wind speeds, but on the days when the wind speed is significantly above average.

Choice of Site, Wind Speed and the Quantity of Power

The quantity of power contained within the wind is proportional to the cube of the wind speed which means doubling the speed of the wind increases its kinetic energy eight fold! While it is not possible for any wind turbine to harvest all of this energy, the average wind speed of the proposed site is a hugely significant factor in determining electrical output. In real terms, a doubling of the average wind speed will increase the output from a small turbine by a factor somewhere between three and six.

Average wind speeds for a given location and height can often be accurately calculated from topographical and meteorological data using sophisticated software programs. The data for Ireland can be accessed on the Sustainable Energy Ireland (SEI) website: www.sei.ie

This information, however, does have its limitations. The speeds given on the SEI wind maps are for heights of 50, 75 and 100 meters above ground level. This may be useful for a company interesting in putting up a new wind farm but is of less help to the householder, farmer or small business enterprise wishing to erect a micro turbine on a relatively small mast or tower.

Wind speeds increase with height above the ground. Near to the ground, wind is slowed dramatically by the effects of friction. At 10 meters above ground level, average wind speeds are only 60-75 percent of those at 50 meters, while at 6 meters the corresponding figures are 35-65 percent.  The greater the unevenness of the ground and the more features protruding from it, the greater the reduction in average wind speed. In spite of this clearly understood relationship between height and wind speed, most micro turbines are deployed very close to the ground. Some are attached to buildings even though such locations almost always suffer from the additional problem of turbulence.

Even though much taller masts are used as standard in many other countries, only rarely does one find a domestic turbine at more than 10 meters above the ground in Ireland where micro turbines are often mounted less than 6 meters above the ground. Not only are these turbines unable to take advantage of the stronger winds which blow at greater distances above the ground, there is an additional problem, and that is the quality of the wind.

Wind Quality and Turbulence

Not only should wind turbines be well off the ground, they should be well above anything in the vicinity.  The typical advice found in wind energy literature is to aim for 6-10 meters above anything within 100 meters in any direction. Apart from the lower winds speeds which are found nearer the ground, there are other drawbacks. Close to the ground local topography, buildings and trees cause wind turbulence in much the same way as rocks cause turbulence in a fast flowing stream. The bigger and higher the obstacle, the greater the turbulence. Turbulence is bad news. It significantly increases the stresses on a turbine and  can reduce output by up to 90 percent. Turbulence can also lead to annoying structure transmitted noise.

Almost all turbines mounted on buildings will suffer severely from the effects of turbulence. The typical roof mounted turbine is attached to a gable or chimney, or mounted on a flat roof. Often the turbine barely clears the line of the roof. Such turbines will never deliver the promised ‘one third’ of a households electricity requirements claimed by the manufacturers.

Typical Household Electricity Use in Ireland

According to SEI data for 2005, the average Irish household uses about 5,000 kWh of electricity per annum. Although precise figures are not available, it seems reasonable to assume that the amount of electricity used depends on factors such as the size of dwelling, number of occupants, and their ages and lifestyles. Households which have computers, multiple televisions, dishwashers, freezers and two or three electric showers may use ten or twenty times as much electricity as a pensioner living on their own in a   town apartment or rural cottage.

Renewables Ireland, the supplier of the Windsave turbine have claimed their turbine has an output of 1200 kWh per annum at an average wind speed of 6 m/s. Yet for a roof mounted turbine to experience an average wind speed of this magnitude would require the roof in question to be situated on one of the most exposed parts of the coastline in the whole of Ireland.

The average wind speed experienced on the roof of a typical suburban dwelling is likely to be in the region of 1.8-3.6 m/s. This might seem extremely little but an independent study carried out on an Edinburgh rooftop by wind energy researcher Hugh Piggott, discovered the average annual wind speed was under 2 m/s at roof height. This contrasts starkly with the predicted wind speed of 5.2-5.9 m/s at ten meters above ground (at the same location) given in the UK  Department of Trade and Industry data.

Hugh estimated that the output from a Windsave turbine mounted on the Edinburgh roof  in question would be about 100 kWh per annum, or about £10 (€15) worth of electricity at current UK prices (See Hugh’s website: www.scoraigwind.com for further information).  This finding makes the Renewables Ireland claim of a ‘payback period as short as 30 months’ simply laughable.

Mast or Tower Mounted Turbines

Turbines mounted on short masts in suburban or urban locations suffer the same problems of turbulence and poor wind resources as roof mounted turbines. The average annual suburban wind speed, even in towns situated in the windy west of Ireland, is very unlikely to exceed 4.5 m/s at six meters above ground level. Even assuming an extremely optimistic overall annual efficiency of 25 percent, one may expect an annual output of just 232 kWh per m² of swept area. In such circumstances, a 1.4 meter diameter turbine similar to the one sold by the company Surface Power may be expected to deliver only 360 kWh of electricity per annum.  At an average wind speed of 3.1 m/s more typical of suburban sites, the annual output of a turbine of this size might be as little as 100 kWh, or only one to two percent of what a typical family household uses each year.

These predicted outputs differ greatly from claims made by Surface Power on its website: www.surfacepower.com. A product described as Domestic Starting System Solution 4, is advertised as providing“40-60 percent of your electricity needs in a typical house”.  The ‘Solution 4’ comprises one 1.4 meter diameter turbine and 3 solar panels, each rated at 100 watts, plus accessories. The mast or pole is not provided and the only information on the website relating to height stated that the turbine “can be placed outside your bedroom window if required”.

The industry rule of thumb for a sunny south facing location in Ireland is an annual output of 80-100 kWh per 100 watts of solar panel. Three such solar panels might be expected to deliver 240-300 kWh. If we accept the SEI data as being accurate, “40-60 percent” of the electrical requirements of a typical house are in the region of 2000-3000 kWh per annum. Deducting the solar output from the equation, one arrives at the conclusion that the tiny 460 watt Surface Power turbine is expected to deliver 1700-2700 kWh per annum! This really is the stuff of Alice in Wonderland! Even if installed on Tory island, it is unlikely that one of these turbines, deployed at “bedroom window height”, will deliver more than 900 kWh per annum - less than one fifth of the electricity used by a typical household. There is also the question of whether these very lightweight turbines are even capable of surviving a full year on an exposed site. First hand evidence suggests otherwise (please contact Sustainability magazine for more information).

It appears that following the example of British Nuclear Fuels and others who have attempted to hide the inadaquacy of a product or technology by renaming it ( as in Sellafield/Windscale), Surface Power have re-invented the 'Solution 4' as the 'Jetstream' turbine. Pictures on their website suggest it is the same product.

The data in Tables 2 and 3 compares the output of two larger turbines at different locations and heights above the ground. The smaller turbine has a maximum output of 1.4 kW and the larger one 5kW. The blade diameters are 3.12 meters and 5 meters respectively.

Table 2

Table 3

n/a = not applicable

One can see the huge difference in output between turbines on a high mast on a good site and ones on a low mast at a poor one. It is assumed that no one in their right mind would want the Montana turbine (with its five meter diameter blades) on a mast only 6 meters in height, or in a suburban garden!

 Conclusion

Does this mean that this technology is a waste of time? Some people involved in the renewable energy sector certainly seem to think so.  While one hopes that the official position might be changing, only a couple of years ago a representative of the Department of Communication, Marine and Natural Resources stated that he saw “no significant contribution” being made by micro renewables towards meeting our national  electricity requirements “for the foreseeable future”.

I would be inclined to disagree; I believe there is certainly a role for micro wind, albeit this role may be confined predominantly to good exposed sites in rural locations. This, however, could easily include as many as 100,000 households, or about seven percent of our total population.

Just as our rural post offices are being wound down and closed, this is also a likely scenario for the out lying regions of our national electricity grid, in the years which follow privatisation and the well flagged demise of the Celtic Tiger. It will probably not be cost effective to maintain a national grid network out to every last rural household.  In such circumstances, off grid power supplies or small localised grid networks make perfect sense.

There is much we can do to improve the viability of the micro wind sector. Perhaps the first thing is to carefully evaluate what is already available on the market. Unfortunately, there is very little independent research into micro wind so it is very hard for the consumer to distinguish between manufacturer’s sales spin and hard fact.

Durability is a big issue with all micro turbines. In the past, much effort has been directed into trying to develop a maintenance free product. This is a pipe dream: when moving parts are involved, things will always wear out. Instead, research and development should concentrate on ease of maintenance, building up local skills bases and repair shops, and on the interchangability of spare parts. In spite of the bewildering range of micro turbines on the market, there is massive room for improvement right across the board. Attempts to gain funding from the State sector for badly needed research into micro wind have so far been unsuccessful.

Update

Another recent turbine entrant onto the Irish market, the Air Dolphin sold by AirEn, has also been advertised using misleading information. The company website speaks of 'up to 40 percent saving' on electricity bills. According to Paul Muldoon at AirEn, the Air Dolphin (with a rotor diameter of 1.8m) is expected to deliver 1440 kWh of electricity per annum at an average annual wind regime of 6m/s . This seems very optimistic. Our own estimate would be about half that figure.

But even were this figure somehow true, it is only some 25-28 percent of the average annual domestic electricity requirement of 5000-6000 kWh, and as this average figure includes many pensioners living on their own it would be reasonable to assume that more typical figures for potential wind turbine owners would be 7000-10,000 kWh per annum, or 5-7 times the claimed output.

Also on the website, the claim was made that the turbine will reduce household CO2 emissions by 4 tonnes per annum. Perhaps the decimal point got misplaced here as a more realistic scenario would be 0.4 tonnes per annum.

There is no mention of the inevitable electricity surpluses which will be generated at night-time or when the household is away from home, which is simply spilled into the grid or into heat sinks.

Even with battery based off-grid systems such as those used by the Sustainability Institute - which come reasonably close to best practice, it is difficult to utilise much more than two thirds of the turbine output for displacing fossil fuel generated electricity. In other words, for a turbine with a given output of 1440 kWh per annum, the usable electrical resource is likely to be under 1000 kWh owing to the inefficiencies of the battery and inverters, and unavoidable spillage of electricity when the batteries are fully charged.

We wish AirEn well with its product, but feel that the consumer deserves accurate information. We also feel that the Irish Government should not provide financial assistance for companies sellingrenewable energy products unless the productshave been first been independently tested and evaluated over a minimum of a 12 month period.

References and Sources of Information

Hugh Piggott’s website: www.scoraigwind.com. One of the best sources of information available

Articles:

“Can We Harvest Useful Wind From the Roofs of Our Buildings?” by Nick Martin, Building For a Future magazine Vol.3 No. 15 Winter 2005/06
This is an excellent article which covers many of the issues raised here.

Books:

Wind Energy Conversion Systems Resource Book, Brisbane Institute
(excellent reference book for anyone requiring technical information)

Choosing Wind Power, Hugh Piggott, CAT Publications

Wind Power, Paul Gipe, Chelsea Green Publications 2004
(the definitive micro wind text)

Other Sources of Information:

There are a number of independent energy consultants working in the micro renewables sector, including the author of this article

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