The sector embracing wind turbines up to 50 kW peak power can tick some boxes. It can be useful in certain off-grid applications, bring a level of energy independence within the affordable reach of individuals and small organisations, and may salve consciences with the promise of carbon abatement.
But in terms of economic return, the surprising buoyancy of the small wind turbine market has often represented a triumph of hope over experience. Can this balance be reversed so that favourable experience more often justifies the hope?
Small wind has long shown clear benefits on farms and open spaces where larger wind turbines can be deployed under favourable wind conditions. It is at the micro-generation end of the spectrum that hopes have often been dashed. Hopeful “roof-toppers” who install small wind turbines on homes and offices are frequently disappointed by the trickles of power that result.
According to Greg Price of US, Oregon-based Abundant Renewable Energy, this is often due to mis-selling: “there are manufacturers and installers out there that will tell you just about anything to make a sell — ‘this turbine will produce enough energy to power your entire house!’ I have heard this statement way too many times before the dealer/manufacturer even knew how much their client was consuming, or what their wind resource was, let alone be able to ensure the quality of the turbine”.
For many owners therefore, the wind turbine ultimately ends up making a visible statement about their environmental concern, rather than an economic return.
But domestic-scale small wind should not be mistaken for mere tokenism. As Price alludes to, success with small wind turbines depends crucially on where they are sited, and occasionally someone who has been able to place a large enough wind turbine high enough above the roof-top canopy has cashed in — and even been able to export excess power.
There are also niches in which small or micro wind is supremely useful. Many yachtsmen keep their batteries charged without recourse to engine power or mains connection at marinas. Remote off-grid communications, lighting, security, signage and other stand-alone applications can benefit from this form of micro-generation, sometimes integrated with another form such as solar PV.
Power thus developed is inherently low-carbon (only the embodied, life-cycle carbon has to be allowed for) and a substitute for power that would otherwise be drawn from the grid, or produced by fossil-fuelled generators.
Location, location, location…
The advice estate agents give to house hunters that the three most important factors to bear in mind are “location, location and location”, is equally apt for small wind. Wind turbines react best to rural locations having significant wind along with clear, uninterrupted fetches. These are the most influential determinants of energy capture, thanks to the well-known cube law relationship which dictates that, at least between a turbine's cut-in speed and its maximum rotor speed, power generated rises as the cube of the wind speed.
In a recently published report, the UK's Carbon Trust confirms that, in general, rural locations are best. In a site preference hierarchy, the report rates open countryside or farms as first, followed in order by villages, town edges, small towns and large towns. Small wind typically achieves, says the Trust, a 15%–20% capacity factor in a rural location whereas suburban sites rarely yield over 10%.
When operating down among the rooftops, many wind turbines stand idle much of the time because the wind fails to reach the cut-in speed of small turbines, typically about three metres per second (m/s).
If two-dimensional location is important, the third spatial dimension — height — is equally so. Hill tops are likely to be good sites because of the accelerating effect hills have on local winds. On the flat and even in smooth rural terrain, the ground exerts a drag on wind, so that elevating the turbine above ground shear is helpful. In urban areas, getting turbines up above the roof canopy is critical.
According to a Carbon Trust spokesman, “to be really useful an urban turbine needs to be several metres above the roof. Allowing the height of the top of the turbine to extend 4m above the roof line instead of 3m (as at present allowed under UK planning rules) makes a difference of 30% to 40% in energy yield. Increasing the height to 10m would provide three or four times the power available at 3m.”
Would-be wind turbine owners often expect too much thanks to energy yield projections that are based on annual mean wind speeds given by official bodies (in the UK for example, the Numerical Objective Analysis of Boundary Layer (NOABL) — or the Met. Office's later National Climate Information Centre (NCIC) databases).
Statistics such as these do not, however, take into account the roughness of urban terrain. Applying a blanket correction factor to allow for this is unwise because effects are too specific to particular sites and even buildings and, though attempts have been made to model orographic effects on wind speed, such modelling is still in its infancy and remains an inexact science. It is always advisable, therefore, to take actual measurements of the wind resource at proposed sites, over a period of time.
A related consideration is that urban wind, as well as being low in mean speed, tends to be turbulent. Constant fluctuations in rotor speed and yaw caused by turbulence exacerbate fatigue and shorten wind turbine life, adversely affecting life cycle economics.
Placing the wind turbine away from buildings, where feasible, can help although extra site preparation, cabling and other installation costs should be taken into account when calculating up-front costs and payback.
Despite too many installations having taken place in unpromising sites against unrealistic expectations, small wind remains popular with consumers attracted by the twin prospects of some energy independence and lower bills. Moreover, small wind does not excite the antipathy often felt for large-scale utility wind because its installations are relatively unobtrusive and have a more democratised ownership.
Clearly though, there is a danger that sentiment could turn against small wind if turbine and installation sales are pursued to the exclusion of ultimate customer satisfaction. Conscious of the danger, small wind players are increasingly loathe to market turbines to customers who have not sought appropriate advice, and intend use at poor sites. According to Andrew McMullan, managing director of one leading player, Proven Energy, “we tell quite a few prospective customers that installing a turbine where they first intend is not a good idea, because of the wind conditions. Location is definitely the major consideration.”
Proven markets its 2.5kW, 6kW and 15kW wind turbines mainly through reseller/installers, who take a similar line. One such, Industrial Maintenance Services Ltd (IMS) of Portsmouth, UK, notes growing demand for the Proven and WES turbines that it markets and installs. Says senior executive Steve Squires, “we strongly advise anyone considering a turbine to ask us first. If on investigation of their site we conclude that the mean wind speed will be less than about 5m/s, we say ‘don't; perhaps a turbine is not for you, at least not at the location you propose’.”
Alex Murley, small systems manager at the British Wind Energy Association (BWEA), backs this type of stance. He concedes that the present tide of enthusiasm could turn sour if the industry does not take care to avoid customer disenchantment. Nevertheless, he is broadly confident about the prospects, particularly in the UK which has 40% of Europe's wind resource, and a well-developed small wind manufacturing base: “a small wind installation has to be in the right place, and in that case it can pay back quite quickly. We are aware, for instance, of an 11kW installation in Ayrshire (Scotland) for which the payback is four years — two and a half years if available grants are factored in.”
Murley points out that turbines can pay back in carbon abatement terms rather sooner than they can financially. Embedded energy — especially for micro generation devices — is low and the carbon emitted during production is soon counteracted by that saved by substituting wind for fossil fuel.
“In this respect, micro and small wind can pay back within months,” Murley tells REF. “It's a considerable benefit. Add in the fact that financial payback times are reducing as prices for conventional energy rise, and it is clear why small wind is attracting a lot of attention.”
BWEA figures suggest that the UK's domestic market for small wind expanded by 80% in 2007 and is expected to rise by 120% this year.
While location and height have a huge bearing on small wind success, engineering evolution plays its part too. Wind turbine manufacturers strive by all technical means possible to harness the maximum amount of energy from the available wind.
Proven Energy, for instance, has designed its wind turbines to maximise the band of wind speed over which energy can be garnered. Unhappy with small turbines that are braked and cease to operate above certain wind speeds, company founder Gordon Proven developed a downwind rotor with elastomerically-hinged rotor blades that cone out of a strong wind to shed excess energy but keep turning. This patented concept resulted from Proven's desire to provide a system able to operate in even the strongest winds experienced in Scotland (where the company is based), a country noted for its windy conditions.
When the wind is not “blowing a hoolie”, a simple permanent-magnet generator directly driven without a gearbox ensures minimal electro-mechanical drag, securing a claimed cut-in speed of around 2.5m/s. Thus the turbines achieve high capacity factors, generating electricity across an unusually wide range of wind conditions.
Keeping products simple (i.e. no gearbox, passive blade pitching or coning) benefits reliability, thus minimising downtime incurred through technical failure. And service availability has been an issue with small wind turbines, particularly where manufacturers are not geared up to respond quickly to breakdowns. Prolonged idleness for want of a vital part or a skilled repair technician can waste valuable wind and spoil the energy return statistics for a given period. Today, attention both to design factors, and to maintenance organisation has enabled turbine producers to boast availability levels of 95% plus.
One important area of technical focus is the wind turbine rotor. Designers constantly seek the best compromise between rotor size (hence swept area and power extracted from the wind), resilience, noise levels, weight and inertia. Modelling and empirical experience are combined to improve wind turbine blade aerodynamics. A few manufacturers continue to investigate the prospects for vertical axis wind turbines (VAWT), particularly for urban use. This approach is controversial, many proponents of the conventional horizontal axis configuration dismissing VAWTs as too complex and expensive to produce and operate.
As with utility wind, there is a general scaling up tendency in turbine size, in order to mitigate the diseconomies of small scale and reduce costs per watt generated.
Whereas many producers (Ampair, Swift, Windsave etc) founded their businesses on the production of units of several hundred watts, now it can be tens of kilowatts. Proven, which started with a 600 W machine and now has a 15 kW unit as its largest, is in the process of planning a larger wind turbine still. Managing director Andrew McMullan claims that the unit's size has not been decided yet, but says that migration to larger turbines for business parks, out-of-town centres and other municipal sites could ultimately see sizes of up to 100 kW eventually — extending the de facto definition of small wind.
Care will be taken in second-generation turbines, says McMullan, to minimise head mass so that tower steelwork and other installation costs are kept in check. To this end, Proven is developing a high-density generator so that mass per unit of power generated is reduced.
BWEA's Alex Murley cites advances in mechanics, electrics, acoustics and urban airflow modelling as improving the scope for small wind application. He suggests that “massive innovation in electronics to align small wind with the grid” will accelerate present growth in levels of grid connection. This will in turn facilitate the adoption of small wind solutions where large-scale utility wind has proved unacceptable. There is also, he enthuses, a big opportunity to embed small wind solutions in buildings, perhaps as a complement to building integrated PV.
Advances such as these should make small wind a more realistic option for urban applications, giving owners the chance to achieve genuine payback. Improved building-mounting structures to alleviate vibration and other stresses, along with relaxation of planning restrictions, will enhance the outlook for urban small wind. Standards are emerging, giving manufacturers something to work to and consumers a quality yardstick. BWEA last year took a useful step in this direction by publishing a small wind safety and performance standard that is gaining acceptance not only in the UK but also in other countries.
Small wind is a sector of micro generation in which the UK arguably leads, but other countries are active too.
Inevitably, one of the biggest markets is the USA where there is a similar tendency towards larger wind turbines, increasingly grid-connected. According to Trudy Forsyth at the National Renewable Energy Laboratory (NREL), the situation has progressed from when most small wind installations were simply stand-alone direct current battery chargers to the point where 5000 MW of small wind are expected to be on grid by 2020.
Engineering evolution, particularly over the last decade, has transformed turbine productivity. Government-backed research has helped; for instance, the Department of Energy (DoE) and NREL have contributed to the development of the latest Skystream 3.7 residential wind turbine from South West Wind Power.
According to the company's Andy Kruse, Skystream is a direct drive wind turbine with only two primary moving parts. It uses a unique “slot-less alternator that eliminates cogging to improve startup while at the same time provides maximum torque for protecting it in extreme winds. The rotor blades are larger than a typical machine with the same rating giving it “greater energy production in low winds”.
The American Wind Energy Association actively encourages small wind, and of about 80 small wind turbine producers worldwide, 22 are in the USA.
In contrast, small wind in Germany has declined (relatively) since 1995 because utility wind, with its economies of scale and higher conversion efficiency, has taken off in such a big way.
Spain's market is similarly under-developed for the same reason. Ignacio Cruz of Spanish research organisation Ciemat has suggested that better feed-in tariffs would help encourage the sector. Technical improvements in performance and dependability are needed too, he told delegates at EWEC 2007. Wind turbines have to be simple and reliable, Cruz argued, in order to operate without attention in remote locations while surviving high winds and storms. He outlined how the Mini-Eolica Project, on-going between 2007 and 2011, is addressing these issues by focusing on product development. New early-stage designs for turbines of 25kW to 100kW include a VAWT. Cruz drew attention to Ciemat's newly established small wind test facility which, as he spoke, had 9 small turbines under evaluation.
Small wind is also forging ahead in many island nations where distribution systems are problematic, and there is adequate wind resource. Rural populations in much larger countries such as India and China are also benefiting.
Overall, the small wind sector could do more to justify the hopes it generates and the goodwill it attracts. Bringing down costs per installed kilowatt through further technical development is, of course, part of the equation. Just as vital, though, is a disciplined approach to the market in which customer expectations are tempered with realistic advice about site suitability.
This twin-track approach would do more to ensure sustainable growth than short-term focus on immediate sales. Along with the streamlining of planning, and continued rises in conventional energy prices, it should help extend small wind's spread from its present rural stronghold into viable urban applications. Thus, it is to be hoped, buying decisions will be driven more by hard economics than by hope.