Wind Energy – how it works
Wind energy, currently the cheapest renewable energy source, involves the generation of electricity from the naturally occurring power of the wind.
Wind turbines capture wind energy within the area swept by their blades, proportional to the wind speed cubed, up to the designed maximum blade speed. The blades in turn drive an electrical generator to produce power for export to the grid.
Sites where there is strong, consistent wind, such as Southern Australia, and especially Tasmania are the most appropriate locations for wind farms. An excellent wind site is generally considered to provide average wind speeds greater than 8 meters per second – at ground level, wind of that speed would be described as a ‘fresh breeze.
Greenhouse gas savings.
The amount of electricity generated by the wind annually equates to a saving of 3,256,000 tons of CO2 every year – equivalent to taking 752,000 cars off our roads or planting 4.86 million trees. As an additional environmental benefit, no water is needed for wind farm operation.
In Australia
Australia is blessed with some of the best and most reliable winds on earth. We currently have 42 wind farms in operation with a total of 817 megawatts generating around 2,500 giga Watt hours of electricity annually.
Basics
A wind turbine is a rotating machine which converts the kinetic energy in wind into mechanical energy. If the mechanical energy is used directly by machinery, such as a pump or grinding stones, the machine is usually called a windmill. If the mechanical energy is then converted to electricity, the machine is called a wind generator, wind turbine, wind power unit (WPU), wind energy converter (WEC), or aero generator. Wind turbines are built to catch the wind’s kinetic (motion) energy. You may therefore wonder why modern wind turbines are not built with a lot of rotor blades, like the old “Southern Cross” windmills you have seen dotted all over Australia. Turbines with many blades or very wide blades, i.e. turbines with a very solid rotor, however, will be subject to very large forces, when the wind blows at a hurricane speed.
To limit the influence of the extreme winds turbine manufacturers therefore build turbines with a few, long, narrow blades. In order to make up for the narrowness of the blades facing the wind, turbines rotate relatively quickly. Wind turbines require different speed wind to “Start Up” to start up the turbine must overcome its breaking and begin turning. The speed that this happens at varies depending on the type and size of the turbine.
Vertical turbines start turning well before Horizontal turbines. Wind is either a powerful friend or a dangerous enemy. Ensure you understand the demands of a wind turbine, they do require some maintenance choose ones that are rugged and simple to maintain.
Calculating wind energy.
How much power is in the wind?
P = .5 * AD * (D^2*.7854) * V^3
Where: P = power in Watts
AD = air density ( typically 1.22 at sea level )
D = Diameter of prop ( in meters )
V = Velocity of the wind ( in meters/sec )
So we could say in a 8.9 m/s wind and a 1.8 m dia prop there is …
P = .5 * 1.22 * (1.8^2*.7854) * 8.9^3 or
P= 1094 watts passing through the prop
Unfortunately we cant capture all of it and most blades range in the 20% to 40% range so we
need to add this into our formula…
P = .5 * 1.22 * (1.8^2*.7854) * 8.9^3 * .4
P = 437 watts coming out of our blade at the shaft.
Now there are some other losses we have to deal with… The generator or alternator we are using isn’t 100% efficient so we need to add this into the formula.
We can say that our blades are 40% efficient and our generator is 60% efficient so… Our overall efficiency would be ( .4 * .6 = .24 ) 24%. So now we add that into the total and we get…
P = .5 * 1.22 * (1.8^2*.7854) * 8.9^3 * .24
P = 262 watts
This is the majority of the losses but there are others that we won’t worry to much about at this point. The formulas above will give you a close general idea of what your machine might produce.
Betz Law
Betz’ law was first formulated by the German Physicist Albert Betz in 1919. His book, Wind-Energie’, was published in 1926. The law states that it is only theoretically possible to convert a maximum of 59.3% of the kinetic energy in the wind to mechanical energy using a wind turbine, and that this maximum power output occurs when the downstream wind has 1/3 the speed of the upstream wind. Proof of Betz law is widely published on the internet and in academic papers. Therefore, at 4 m/s, the theoretical maximum power that a turbine can extract from the wind is 59.3% of 38W per square meter of swept blade area, or 22.8W for a 1m2 turbine. The swept area is related to the blade radius by the formula: A = r2
It is very important to consider this data when evaluating the potential dimensions of an urban wind turbine.
Power Curve
A turbines performance will be determined by the wind regime and by its power curve.
A typical power curve for a wind turbine plots wind speed on the x axis against power output on the y axis. It will generally consist of a cubic curved section intersecting the x axis at cut-in speed, and flattening out or dropping at rated wind speed.
Calculating Urban Wind
Despite the fact that wind resource is the most important factor in calculating the economic viability of a wind turbine, it is very difficult to predict in an urban area. This is because modeling such a diverse and dynamic environment is infinitely complex and actual wind speed data is only accurate very close to the monitoring station.
Urban Wind Mapping
A typical wind map indicates annual mean wind speeds for a given geographical region at a given height. These can be very useful for gaining a general picture of a large area. These maps are put together using computer models which take into account topographical data (the shape of the land), roughness models (the nature of the land, eg: is it smooth pasture or urban area or forest) and wind data from a number of point sources. The resulting map is most accurate closest to the wind data sources. Wind maps are useful for deciding where to site large commercial wind farms because the turbines are at such a height so ensure that surface roughness is a less significant issue. Closer to the ground however, and in urban environments, roughness becomes more variable and conditions are much more difficult to generalise.
According to Garrad Hassan Pacific Pty Ltd, experts in wind energy, an urban wind map for Melbourne would give useful data for an elevation of 25m and higher. Below that, the terrain is too complex for accurate computer modeling. Unfortunately 25m is an impractical height for most urban dwellings as a mast that high would make it difficult to get planning permission. As such, wind maps have limited application in urban areas.
Lightning Strikes
Lightning strikes do occur and can cause significant damage to the electrical components of a domestic wind turbine. Some domestic wind turbines come with lightning protection systems although it is very difficult to completely protect a turbine from lightning strike. Lightning protection generally involves grounding the turbine’s tower and attaching a lightning rod which is higher than the turbine and mounted on the tower at a lower point than the turbine. An alternative method is to build a secondary separate tower which is taller than the wind turbine although this could have the adverse affect of attracting lighting strikes to the area. In an unprotected system, lightning will hit the turbine casing and induced voltages and currents will cause damage to the electrical components inside. In a protected system, it is hoped that the lightning will skip over the turbine and run to ground. According to the British Wind Energy Association’s web site, insuring the turbine against lightning damage may be a wise precaution.
Maintenance
Maintenance is not a significant cost to domestic wind turbine owners.
The frequency of major turbine maintenance ranges from twice a year to never.
Turbines require general maintenance every 5 years for the duration of its 20 year life. The maintenance is expected to be a 2 hour job and may or may not involve a cherry picker.
Structural Design
Most small turbines use fibreglass-reinforced composite material. An example is a pultruded blade embodying a solid cambered plate or a multi-cell airfoil. More recently, injection moulding has also been used to fabricate small blades. This method involves injecting resin, such as polypropylene containing short glass fibers, into an aluminium mould of the blade geometry. Strength and thickness considerations for this method limit the blade length to less than two meters. This approach requires a large production volume to justify the high tooling costs. Most small blades are bolted directly to the hub plate with or without an outer plate. A root joint having an outer plate is the preferred approach because it provides two shear load paths and helps avoid submitting the bolts to high bending loads.
For large turbines, blade weight and cost become much more important. Larger blades mean lower rpm, less centrifugal stiffness, greater relative weight, and greater edgewise root-bending fatigue caused by the 1/rev gravity load. For large turbines, the edgewise root bending load becomes a significant design load. To minimize weight, the small-turbine, solid-blade construction gives way to a lightweight, hollow blade design approach with one or more load paths.
I Want Energy use composite fiberglass blades on our horizontal turbines and aluminium on our verticals.
