There are many different types of wind turbine on the market, which can be confusing for buyers – each machine claiming its own unique virtues to the detriment of all others.
We have designed our own turbine based on the collective experience of a number of people, including Quentin Gargan and Dougie Blair of Beyond Innovation. It is worth discussing different types of turbines, and the pros and cons with these designs that resulted in the end product we have here. Not everyone will agree with our conclusions – especially our competitors.
Basic turbine types
In relation to turbine configuration, there are three main types of turbine – upwind, downwind and vertical axis. There are further variations in how turbines control over-speed during extreme wind conditions.
This is the most common type of turbine, where the blade faces into the wind. Larger turbines, including mega turbines on wind farms, have a motorised drive to keep the turbine facing into the wind. For small turbines in the region of 3kw, this is a complication probably best avoided.
Smaller upwind turbines usually have a tail vane keeping the turbine facing the wind unless the wind speed gets too high, in which case the blade is allowed to swing out of the wind. This happens because the force on the blades rises considerably in high wind speeds.
This tends to be an unpredictable movement with the turbine suddenly swinging in and out of the wind, making quite a bit of noise in the process.
The alternative is to leave the turbine facing into the wind at all times. This can be done, but you must ensure that there is no danger of the blades bending back and touching the tower, causing the blade to break (rotating at a speed of some 300km/hr). Were this to happen, the turbine would be out of balance, and the vibration could cause catastrophic damage.
There are a number of downwind models on the market with some variations. Some turbines have folding blades on a patented hinge system which causes the blades to cone, reducing their effective area, and controlling output. This allows the turbine to continue running in all wind speeds. However, it also means that there are more moving parts at the top of the tower requiring maintenance.
Our view is that it is better to stop the turbine in extreme winds. This means that the electricity production in such winds is lost, but on most sites, the amount of time when such winds exist is so low that it would not have a serious impact on total energy production.
Downwind turbines have no risk of the blade hitting the tower, but they do suffer from wind shadow, when the blade is behind the tower, and therefore not in clean wind. We decided to minimise this by using a light reinforced pinnacle pole at this point to reduce the shadow area, and by fitting an aerodynamic cowling, the effect is further minimised.
Vertical Axis Turbines
In a vertical axis turbine, the blades rotate around a vertical axis. This means that regardless of what the wind direction is, the turbine is in an optimal position to use this wind. This is seen as an advantage in areas with very variable wind direction as the turbine doesn’t have to yaw to face the wind. The best of these is a Ropatec turbine which is a rugged and well made machine.
However, vertical axis machines have an inherent inefficiency because while one blade is working well with the wind, other blades are effectively pulling in the wrong direction. For this reason, vertical axis machines tend to be larger than their horizontal axis counterparts. It can be more difficult to mount them on a tall enough tower to avail of higher and cleaner winds. However on some sites where there is good wind close to ground level, good quality vertical axis machines will work well.
Turbines need some method to ensure that they don’t over-spin. Because the power in the wind is proportional to its speed, a turbine that produces 2.5kw at 12 m/sec wind speed will produce 10kw at 24m/sec.
Some upwind turbines do this by furling out of the wind as already discussed. Other turbines use a centrifugal mechanism to change the blade pitch to limit the turbine speed but once again, this system involves having complex moving parts requiring maintenance at the top of the tower.
Our preference is to use electronic braking to stop the turbine in extreme conditions. If the coils of a generator are shorted out, the generator becomes extremely difficult to turn because of the increased loading. If we were to do this while the turbine is in full flight, it may well do damage to the coils or magnets in the generator, so we have a two-stage braking system.
We first apply a high resistive load to the coils to slow the turbine down, and then five seconds later apply a short circuit to bring it to a halt. Once this has been done, the turbine will remain almost totally stopped. The wind loading on the turbine is dramatically reduced, and in this condition it can safely withstand wind speeds up to 60 m/sec (140mph).
We can later remove the short circuit and see how fast the turbine rotates under its high resistive load. In this way, we are using the turbine as an anemometer to assess whether the wind speed has fallen sufficiently. If the wind has slowed sufficiently, we can remove the load and run the turbine again.