Small scale wind turbines (<10 kW) are an enigma. They have their uses, particularly in off-grid applications. They can be used in remote locations and even offshore on boats.
They can help charge batteries and can be used for pumping water. Due to smaller size, their transportation, installation and commissioning costs are low. The low cost of these wind turbines makes them affordable to small communities as well as individuals. However, there are question marks over the effectiveness of these turbines particularly when compared to their large scale counter parts. The peak efficiency of a very well designed small scale turbine can reach 35% whereas for larger turbines, figures around 40% are more commonly noted. Although the difference might not look very alarming but it should be noted that peak efficiency for small scale turbine is reached for only a fraction of the time when operating in the field. Whereas large scale turbines can reach their peak rated efficiency for durations that are several folds longer and hence the yield is much better.
In the UK, many renowned companies producing these turbines have gone into administration. One renowned company among them was “Proven wind turbines” . The company was an association of stakeholders from the farming community. They pooled resources and ventured into a very successful wind turbine business, at least at the beginning. Proven Turbines were among the first to cater for the small scale turbine market in the UK. Their product and timing of launch was spot on but unfortunately the engineering was not.
The company went into administration only a couple of years after its success. This came after a series of complaints by its customers who reported blade failures. These failures included shearing of blades at the edges as well as their tearing-off from the hub. The latter was deemed a serious safety hazard as the blade could have cannoned anywhere at high speed.
In 2008, just when several small scale turbine companies had mushroomed, research by academia proved that contrary to the manufacturer’s claims, most small scale wind turbines were inefficient. This led the UK government to promptly tighten incentives and grants for many of these machines. The slash of grants shrunk the market which in turn hit the small turbine manufacturers really hard. Many of these turbine makers with smaller pockets did not survive the curb.
Today, only a select few turbines in the UK are applicable for the Microgeneration Certification Scheme (MCS). The companies manufacturing these turbines have had to design and launch their products after rigorous testing. List of MCS approved wind turbines can be accessed from here. A market that initially welcomed the most basic of designs is now only open to products that have risen several notches in technology.
Small Scale Vs Large Scale Wind Turbines
So what are the reasons that make small scale wind turbines feeble compared to their larger counterparts? Listed below are a few of them:
1. Gearing system
Small scale wind turbines have to spin at a certain speed for producing electricity that is feedable to the gird. When spinning at slower speeds, they only produce power that can trickle charge a battery. Because of the lack of a gearing system, the generator shaft cannot be stepped up to a higher speed at the expense of torque. Whereas most large scale wind turbines come with gear box. In fact the majority of the maintenance and insurance costs are due to the presence of gear box.
2. High speed spin
Because small scale wind turbines are designed to spin at high speed, they clock many more revolutions over a period of time compared to larger Wind turbines. This results in a shorter bearing life and quicker sheering of blades. Large turbines on the other hand are designed to spin slow (about 15 to 20 rpm), thus decreasing wear and tear and increasing longevity.
3. Nacelle Yaw System
Horizontal axis wind turbines need to have the blades oriented perpendicular to the wind direction. Yaw systems provide this functionality. In small scale wind turbines, the Yaw systems are passive. A tail blade orients the turbine in the direction of the wind and is a cost effective solution. However such systems have to be designed with care so that they are to some degree insensitive to sudden fluctuations in wind direction as gyroscopic loads increase.
Active yaw systems on the larger wind turbines are controlled by Yaw motors which are in turn controlled by an on board computer. The algorithms for Yaw control can even anticipate wind direction changes through software or measured through LIDARs and can use this information to increase the productivity of turbines.
4. Mast Heights
Small wind turbines are mounted on masts that are about 10 meters long. Although certain small turbines do come with telescopic masts that give them a relatively higher height. A look at the atmospheric boundary layer suggests that wind speed goes up exponentially moving vertically away from the land surface.
Larger wind turbines have nacelles sitting at 50 meters or more, thus cutting into wind at much higher speeds. As power of the turbine has a cubic relationship with wind speed, the output of these larger machines goes up proportionally.
Smaller mast heights also means that a lot more turbulence is faced by the small scale wind turbines due to obstacles near the ground such as trees and buildings.
5. Power Electronics
In industrial terms wind turbines that replace power electronics for gear box are called direct drive. The high power capacitors store energy and release it when wind speed drops. Although small scale wind turbines don’t have a gear box but they also do not have power electronics. Hence calling them direct drive would be a misnomer. Due to the lack of this complex and expensive circuitry, they are either switched-on at rated wind speed or switched off even above cut –in speed.
6. Blade Pitch control
Large scale wind turbines can vary their blade pitch. The altering of blade angle can be used to maximize the lift force on the blades at different wind speeds. The blades angle can also be changed to minimize lift and drag during gales or stormy conditions. This reduction of the angle of attack in high wind conditions is termed as “feathering”. Small scale wind turbines do not have such mechanisms to turn the blades. They normally apply brakes and completely stop at high wind speeds.
Unless the technology of large scale wind turbines is scaled down, the effectiveness of small wind turbines would remain second tier. As for now there is truth in the mantra “The bigger the better”.
Having said that, there have been numerous examples where technology has transcended to small scale wind turbines. For instance the Evance 9000 wind turbine has a response blade pitch control that automatically changes the blade angle with changing wind speed. Similarly, direct drives are now available from power rating as low as 20 kW. The availability of longer masts have also improved the effectiveness.
The presence of a gearbox is hugely beneficial however it should be noted that most of the maintenance issues for the larger wind turbine are due to the gearbox.
Small scale wind turbines despite their shortcomings have their place in the market. Numerous farmers have benefited from them for whom large scale turbines would have been simply inadequate and too costly. The lack of infrastructure (wide roads and high voltage transmission cables) can make large scale wind turbines not only unfeasible but also impractical.
It should be noted that there is an emerging technology that covers turbines with ducts/shrouds. This technology is only available for a small scale. This technology uses Venturi effect to increase the wind velocity, that in turn increases the power generated.
To sum up, small scale turbines may have had a baptism of fire but as the technology trickles down, there is all the more reason for them to prevail.