The demand for renewable energy has outstripped our technology’s ability to provide it. The introduction of battery storage has greatly boosted the ability of renewable technologies to provide a steady and predictable supply of electricity, but lithium batteries are not a long-term solution. Current wind turbine technology isn’t a long-term solution, either. With wind power providing about 13.8 % of the total renewable energy generation in the UK and 6% of the electricity on the US power grid, we need to hope that research into new technology will bear fruit.
History of wind energy technology
Sailing ships represent the first technology that captured wind power. Sometime before 900 AD, the Persians figured out how to fasten multiple sails around a central axis, the first documented vertical axis windmill. They used it to pump water and grind grain. Vertical axis windmills operate on a force called drag.
Apparently using a water wheel as inspiration, Europeans started to build horizontal axis windmills sometime before 1270, the approximate date of the first known illustration. Horizontal axis windmills require more sophisticated technology, but they are more efficient. They operate on a force called lift.
All important advances in wind energy from that time until the 20th century came from developing horizontal axis technology. But windmills still only pumped water and ground grain until Charles Brush of Cleveland, Ohio built one to supply electricity to his house. The invention of the airplane and development of its propeller provided the final major component of the familiar horizontal axis wind turbine (HAWT), the three-blade rotor configuration.
The development of HAWT technology
From the beginning of the electrical era, burning fossil fuels produced cheaper and more reliable electricity than wind turbines, but research on HAWTs continued. During World War II, wind power temporarily became economically feasible in some parts of the world.
In response to the Arab Oil Embargo of 1973, the federal government in the US spent a lot of money on wind energy research. It explored not only HAWTs, but radically new vertical axis wind turbines (VAWTs).
None of this research, however, produced technologically and economically feasible turbines. As with Thomas Edison’s early attempts to make an electric light bulb, the research provided a vast quantity of useful data and a long list of ideas that didn’t work. But not having Edison’s patience, the government halted funding for further research in 1981.
For some reason, American researchers had ignored the relatively well-understood three-blade design. It was inefficient but worked reliably. Danish manufacturers made such turbines in abundance. No sooner did the federal government stop funding research into wind energy than wind farms, mostly in California, began buying turbines based on Danish models. The only significant advance on pre-war turbines was the substitution of fiberglass blades for the older metal ones.
Federal funding for wind energy resumed in the 1990s. With higher fossil fuel costs, the government had incentive to focus more attention on it. By that time, Europeans and Asians had taken the lead. New research found and corrected the weaknesses in the Danish designs.
As HAWTs became more reliable, they also became less expensive to install. The cost of wind energy, more than a dollar per kilowatt hour in 1978, has plummeted to where it is competitive with fossil fuels.
The trouble with HAWTs
Wind becomes stronger and steadier higher off the ground. So the taller HAWTs rise, the more efficiently they work. Therein lies some problems. The earliest HAWTs were placed atop lattice towers. Once they reached a height of 60 meters, those towers could no longer support the weight of the machinery. So manufacturers developed the familiar tubular steel towers. They are usually made in four sections and trucked to the wind farm.
A steel tower can grow to almost 100 meters, but a taller tower has to be stiffer. The top of a tall tower is more expensive than the bottom. At about 100 meters, manufacturing them becomes too expensive to be practical economically. Some taller HAWTs therefore require a tall concrete base for the steel.
As towers get taller, rotors get bigger. With longer blades, the turbine sweeps a larger area, and the amount of energy it produces increases exponentially. Siemens built a rotor with 246-foot blades that covered an area as large as two and a half soccer fields. The tip of a blade that size travels more than 180 miles per hour. Wind conditions at the top of such a rotor’s sweep are very different from the bottom. Only very sophisticated software and sensors can let the such huge rotors operate safely.
Giant HAWTs bring other problems:
- Transporting huge blades and tower segments is expensive. Special trucks require two lanes of roadway. Delivering the parts can cost nearly $200 per mile. Sometimes it is necessary to cut down lots of trees to make room for them. The loss of trees negates some of the environmental advantages of the turbines.
- The larger the HAWTs, the farther apart they must be built to keep them from interfering with each other. Large HAWT wind farms require either very large tracts of land or offshore platforms, which in turn requires that they be built far from the users of the electricity. The very long transmission lines that bridge the distance are expensive and difficult to maintain.
- Such large turbines are noisy eyesores. They destroy scenery and quality of life. Offshore, they threaten to destroy local fishing and tourist industries. Therefore, proposed wind farms often draw fierce local opposition.
- Repairing giant HAWTs is dangerous to workers, who must somehow raise tools and spare parts to the tops of the towers.
Further development of HAWT technology may be reaching a point of diminishing return.
What about VAWTs?
Finnish engineer Sigurd Savonius and French engineer Georges Darrieus both patented vertical axis wind turbines (VAWTs) in the 1920s.
Savonius’ design operates on drag, much like the ancient Persian windmills. While one blade stops the wind and turns the rotor, another is heading back upwind. It contributes nothing to producing electricity. That gives the device very low efficiency. Savonius turbines can generate electricity for small applications, such as providing auxiliary power for a house. They are useless for utility-scale generation.
Darrieus machines operate on lift, like HAWTs. They use one of two basic blade configurations, the so-called H-rotor and something that looks a little like an egg beater.
They have fewer component parts than HAWTs and do not need to adjust to wind direction. All the machinery is closer to the ground, so maintaining it is both less expensive and safer. They can be scaled down better than HAWTs and are therefore more practical for small plots of ground or even rooftops. They can be deployed closer to whomever will use the electricity.
But they have some so-far crippling disadvantages.
For one thing, they can’t start by themselves, no matter how fast the wind is blowing. Mounting a Savonius rotor on the same shaft is one way to make them start.
They are less efficient than HAWTs, because not all the blades contribute to generating electricity all the time. As the wind pushes one blade downwind, another is returning against the wind. That causes a torque ripple that makes a VAWT less sturdy. If wind speed matches a VAWT’s resonant frequency, the turbine experiences violent vibrations that cause it to fail.
As the saying goes, the bigger they are, the harder they fall. So far, it hasn’t been possible to scale a VAWT up for reliable use in a wind farm. Only smaller devices have proved commercially viable.
Researchers have overcome some of the problems with efficiency by placing VAWTs close together. They don’t interfere with each other as HAWTs do. Careful positioning of VAWTs in counter-rotating pairs actually boosts their efficiency. It can theoretically make a VAWT wind farm produce more electricity per acre than a HAWT wind farm, even though each machine is less efficient.
Unfortunately, no VAWT machine now on the market can last an entire season in such a configuration without some kind of failure.
Pursuit of new wind turbine technology
If the standard VAWT designs don’t work, what will? Here are a couple of possibilities that should work in principle. More research is needed to see if they will work as well in practice.
- Instead of attaching blades to a central shaft, one patent proposes supporting the blades on a circular frame. Some kind of central shaft would still have to operate the generator, but it doesn’t have to support the entire weight of the rotor. Such a machine ought to be sturdier.
- Another more radical design called the blinking sail updates the old Persian windmill and the Savonius rotor. Instead of a single piece of fabric forming a sail, each frame of the blinking sail has horizontal bars with separate sheets of fabric on them. Or instead of fabric, plastic or aluminum. The sail blocks the wind on the downwind side and causes the turbine to rotate. But when the same frame returns upwind, the wind blows the fabric open so that it no longer resists the wind.
These are only two of the new ideas for designing a VAWT. Other alternative wind turbines either dispense with towers in favor of some kind of airborne turbine or dispense with rotating blades. The history of invention indicates that most of these ideas will prove impractical. But something is bound to work eventually.
Originally posted 2018-03-26 11:32:14. Republished by Blog Post Promoter