Wind turbines come in all shapes and sizes and will produce different quantities of energy depending on their type and size. Not only that, but their location and wind conditions also impact the amount of energy a wind turbine can produce.
With a capacity from 100kW to 8.8MW or more, some wind turbines can produce enough power in one turn of their blades to power an average home for one day. In contrast, others contribute to the national electrical supply by connecting to the electricity grids.
We need to look at the following to establish an overview of how much energy wind turbines produce :
- How wind turbines produce energy
- The energy capacity of horizontal axis wind turbines (HAWT)
- The energy capacity of vertical axis wind turbines (VAWT)
Let’s start with understanding how wind turbines produce electricity.
How Do Wind Turbines Produce Energy?
The science behind electricity production from a wind turbine is fairly straightforward. The force of the wind moving over the blades causes them to start moving slowly and then increasing as the wind speed increases.
The spinning blades are connected to a rotor and a drive shaft through a gearbox. The gearbox accelerates the rotational speed and this, in turn, increases the drive shaft RPMs.
The driveshaft is connected to a generator that produces DC electrical current, which is then converted into AC and distributed to supply homes and businesses. Some wind turbines have an AC converter to convert DC to AC and match the grid supply frequency.
What Is A Wind Turbine Energy Rating?
Each wind turbine has a maximum energy rating, and this is the measurement given to the turbine’s maximum generation capacity.
So if a turbine has a 1MW rating, it could produce 1 million watts under ideal conditions and operate all the time.
Ideal conditions are consistent wind speed blowing all year round at consistent speed, but of course, this isn’t going to happen. Another factor in generating capacity is the efficiency rating of the turbine.
The Wind Turbine Efficiency Factor
Because the turbine is a mechanical system, issues around friction and wind speed prevent the turbine from achieving the maximum capacity rating. Most wind turbines operate between 25% and 50% of their maximum capacity.
So if we take the example of the 1MW / 1000kw / 1 000 000 W turbine, if it has an efficiency rating of 25%, it will only produce ¼ of its capacity over 12 months.
According to sciencing.com, there is a formula to calculate the amount of energy a wind turbine would produce over a year, and it looks like this :
365 (days per year) X 24 (hours per day) X maximum capacity X efficiency factor = total kilowatt-hours per year produced.
Applying this to the 1MW turbine, we get the following result:
365 X 24 X 1 000 (kW) X 0.25 = 2 190 000 kWh per year.
To give that number some perspective, if an average home uses around 500kWh per month or 6000 kWh per year, that wind turbine could power 365 homes over 12 months.
This is also why engineers are putting in a lot of research and resources into improving efficiency factors in wind turbines to power more homes using the same level of existing wind power.
How Are Wind Turbines Categorized?
Wind turbines are defined by installed and connected to the grid. There are three wind turbine categories, and they are:
- Land-based wind: wind turbines are installed onshore, and these can range in power capacity from 100 kilowatts to several megawatts. These turbines are usually grouped and provide power through wind farms. They are also the most cost-efficient.
- Offshore wind: These are placed in the ocean off the coast, and these are massive turbines towering higher than the Statue Of Liberty. Because of their size, they can utilize the strong ocean winds and are usually rated from 3MW or higher. Some of the biggest turbines are located off the Scottish coast and are 620ft high and rated at 8.8MW!
- Distributed wind: these are placed near the end-user and are often used with other clean energy sources like solar or geothermal. The distributed wind systems are small turbines often rated at less than 100kW and near smaller residential, agricultural and small-scale industrial or commercial installations.
What Are The ‘Cut-In’ And ‘Survival Speeds’ For Wind Turbines?
The ‘cut-in’ speed is between six mph and nine mph and is the minimum speed needed to get the turbine blades moving. Once they are moving, the gearbox increases the speed from around 13RPM to around 1500RPM so that the turbine can generate electricity.
In generating energy, wind turbines can only operate effectively in a ‘window’ of wind speeds as if the wind speed is too low, it won’t be enough to get the blades moving, and where the wind speed is too high, it will damage the turbine.
The maximum wind speed is usually around 55mph.
Higher than this will cause damage to the gearbox and superstructure, so wind turbines have a safety cut-out that shuts the turbine down once the anemometer measures the maximum wind speed.
How Much Energy Does The Horizontal Axis Wind Turbine (HAWT) Produce?
Whenever you think of wind turbines and the propeller-style design, you think about the Horizontal Axis Wind Turbine or HAWT. This system indicates that the rotational axis of the turbine is parallel to the wind direction.
HAWTs are the most common forms of wind turbines and for a good reason. They are the most efficient and cost-effective, and while there are other turbines designs, to date, none can match the overall efficiency of the HAWT.
The average size of a wind turbine in the USA is around 280ft, with sizes ranging from 120ft to a monstrous 590ft and blade lengths up to 260 ft!
These industrial-sized onshore turbines can produce around 2MW- 3MW, while offshore turbines can reach 8,8MW. Smaller wind distribution turbines can be as small as 100kW and power single homes.
HAWT turbines have the highest efficiency factor of any wind turbine and typically rate between 40% -50% in terms of efficiency. Onshore turbines can produce around 6 million kilowatt-hours per year.
Offshore turbines rated at around 3.6 MW could easily double that as the wind is more consistent offshore. Using the formula to calculate how much energy an 8,8MW turbine rated at 25% could produce, we would get the following result :
365 X 24 X 8800 (kW)X 0.25 = 19 272 000 kWh, and that’s enough energy to power more than 3200 homes in the USA for a year!
What Are Some Other HAWT Designs??
A few other HAWT concepts could increase the efficiency of the turbines. Still, at the moment, the cost of this technology is prohibitive, and the standard HAWT configuration remains the dominant turbine.
Those designs included the shrouded turbine, where the blades are utilized in a ring-shaped aerofoil system. While this could increase the efficiency by more than 500%, the cost of this system is too high to be viable or practical.
Another concept that may have merit is the direct-drive system, which removes a gearbox. Gearbox repairs are the largest maintenance cost on HAWT systems, and the direct drive uses rare earth magnets instead of the gearbox – but again, the cost of these is too high to be practical.
How Much Energy Do Vertical Axis Wind Turbines (VAWT) Produce?
The VAWT system is where the turbine rotation is perpendicular to the wind, and these are also known as the ‘eggbeaters.’
The HAWT systems always require a system of wind orientation to utilize the maximum windspeed, while VAWT systems do not require this as the ‘S-shaped’ blades will capture the wind from any direction.
The main issue with energy efficiency in the VAWT design is that only a small number of blades produce torque as the other blades are merely passengers.
The result of this is much lower energy efficiency and power generation. When power producers consider the production capacity of wind turbines, the energy production capacity is calculated over 20 years.
Right now, there are no VAWT systems that can compare with the volume of power a HAWT system can generate over 20 years, and this is why most wind turbines are HAWT design.
What Is The Efficiency Rating Of VAWTs?
This is where the VAWT systems fall short compared to their HAWT counterparts. The typical efficiency rating for HAWTs is between 40% and 50%, while VAWT systems with similar power ratings only come in between 30%-40% efficiency.
This is because they produce less torque as only a few blades simultaneously deliver torque to the driveshaft, reducing their capacity to generate power. To illustrate this better, let’s compare the generation capacity between the two using the power generation calculation.
HAWT vs. VAWT – Power Generation Capacity
The turbine will be rated at 2MW (2000 Kw) for both calculations and use a 40% efficiency rating for the HAWT and 30% for the VAWT.
HAWT @ 2MW = 365 X 24 X 2000 X 0,40 = 7 008 000 kWh Per year.
VAWT @ 2MW = 365 X 24 X 2000 X 0,30 = 5 256 000 kWh Per year.
This is a difference of 1 752 000 kWh or 292 fewer homes per year supplied with renewable energy. Over 20 years, this would equate to more than 35 million kWh or 5840 homes supplied with power.
You can now see why the HAWTs are currently the preferred turbine systems.
What Are The Main Types Of VAWTs?
The most common VAWT design is called the Darrieus, and this is the classic ‘eggbeater’ design with 3 ‘C’ shaped blades that capture the wind. The Darrieus is the most efficient of the VAWT systems and is also an AC system rather than a typical DC system.
The Savonius VAWT is another design that uses an ‘S-shaped system of blades to capture wind energy. However, the lower rotational speed and torque only give these an efficiency rating of just 10%-17%.
There are some other designs, like the Giromill, Cycloturbine, and Flapping Panel turbine. Still, these tend to suffer the same issues as all VAWTs and are not cost-effective, resulting in lower energy generation and operational efficiency.
What Are Other VAWT Generation Problems?
There are some other issues with VAWT systems that influence their ability to compete on power generation.
- Most VAWT systems are installed lower to the ground, and while this makes maintenance easier, it reduces their access to high wind speeds that the taller HAWT systems can utilize.
- VAWT systems experience higher levels of component wear and tear as they experience more turbulence and vibration, leading to more pressure on the bearings between the rotor and the mast.
- HAWT blades only require adequate wind speed to start moving, while many VAWT systems require a small mechanism to provide the initial torque.
Why Could VAWT Windfarms Produce More Energy Than HAWT?
A study done by Stanford University shows that the efficiency of VAWT turbines in wind farms could produce significantly more power than the HAWTs. This is because the HAWTs require more space between them due to their size.
The more compact VAWTs could be closer together, providing greater generation capacity per square meter than the HAWTs could.
This study showed that HAWT configurations would produce around 3W per square meter, with the VAWT configuration creating a staggering 30W per square meter. This addresses the issue around the space required for generational capacity.
HAWT turbines need much more space to install huge HAWT systems offshore, but onshore, the limitations of available space would make VAWT windfarms a better option.
Even though the VAWTs would still have the same lower efficiency issues per turbine, the ability to fit more VAWTs in a smaller space could allow them to generate far more energy and provide higher levels of energy to the grid or wind distribution installations.
For the time being, the HAWT systems provide measurable greater energy production and consistency than their VAWT cousins.
Still, as technology evolves in the quest for super-efficient turbines to reduce the dependency on fossil fuels, we may well see VAWT or hybrids between the two evolve over the next twenty years.
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