Wind energy is one of the fastest growing energy sectors. In some countries, it already covers a remarkable fraction of the total energy supply. According to recent studies, this resource could satisfy the global energy demand although its main problem lies in the instability of production.
Future challenges include more efficient installations and adequate storage systems. In Europe, some experts have proposed an intercontinental electricity network that incorporates Africa.
Wind energy technologies have improved significantly over the last four decades and many experts are impressed by solutions being introduced on the market. Between 2005 and 2011, global wind capacity increased by 75 percent. In the United States, two recent reports published by the Department of Energy show the rapid transformation in the sector.
Significant progress is being made thanks to the development of wind turbine farms featuring vertical axis wind turbines. This undoubtedly creates the best renewable wind energy resource.
Reports show that wind energy is the main source of renewable energy for electricity generation in the United States. Up to 43 percent of new electricity installations involve renewable sources. On a global scale, the country ranks fourth in terms of increased wind power capacity. China leads the way with a 98 percent increase in wind capacity followed by France (88 percent), Canada (87 percent) and finally the United States (80 percent).
Over the last three decades, the size of wind turbines has increased by a factor of 10 to 12. The capacity of an isolated turbine has risen from 100 kilowatts (kW) to 2 megawatts (MW). In the future, scientists estimate they can increase this capacity to 12 MW per turbine.
Currently, horizontal axis wind turbines are commonly used for electricity production. They come with a rotor mounted at the top of a mast, perpendicular to the direction of the wind and the axis of rotation is horizontal. The rotor generally comprises three blades, which are driven by the wind, thus converting the kinetic energy of the wind into a mechanical energy before it is converted into electrical energy.
The inclination of the blades is adaptable according to the wind speed. This is aimed at regulating the propeller’s speed of rotation and ensure the safety of the installation. On the other hand, vertical axis wind turbines are fitted with a rotor mounted vertically at the top of the mast. Its axis of rotation is vertical. Recent trends show renewed interest in this design.
According to data from the World Wind Energy Association (WWEA), an organization charged with promoting the development of this energy source, wind turbines connected to the network today have an installed power of 215 gigawatts and cover about 2 percent of the world energy consumption. Wind electricity production also represents a rapidly expanding economic sector, with a minimum annual growth of 20 percent per annum.
Vertical axis wind turbines
Studies conducted in the 1980s concluded that these turbines were less adaptable to large-scale energy production and their development was much slower. However, vertical axis turbines are aerodynamically more efficient. Thus, this technology has recently been reconsidered and some researchers claim that these units are better suited for large-scale electricity generation than traditional turbines.
One of the strengths of these turbines stems from their versatility. They do not require a particular orientation of the rotor, unlike horizontal axis turbines. Thus, the rotor does not need a control system. In addition, the wind speed is not regulated and its direction is rarely uniform. Such an omnidirectional system is largely favorable. All the main components are placed at the bottom of the mast, which allows easier access when compared to horizontal axis turbines.
Improving efficiency with innovative technologies
To improve the energy production of wind turbines, manufacturers introduce additional components to the basic structure: mast, blades and nacelle. The purpose of these structures is to increase efficiency and power output since the overall output of a turbine is proportional to the wind speed. The addition of a flange diffuser reduces the pressure at the outer flange.
The depression accelerates the surrounding wind, which boosts wind speed on the diffuser. As a result, the turbine’s power output is increased four to five times. Recently, a team from the California Institute of Technology (CIT) focused on adding a deflector upstream of a vertical axis wind turbine. This approach is particularly simple and allows a sharp increase in the power output of the turbine.
The presence of the deflector serves as a shield against the currents created by the returning blades. The components exert negative torque while directing the incoming wind toward the opposite blades. In turn, the deflector helps boost positive torque, thus the axis of the turbine must be placed outside the wake near the deflector. In addition, the height of the baffle has a significant influence on the power output.
The team intends to continue its study by checking if these concepts are applicable to other types of turbines. The presence of the deflector increases the instabilities locally of the flow, an effect solely related to a confined testing environment. Researchers are now interested in studying flow velocity in relation to the deflector and the turbine. The same applies to the effect of the upstream deflector on the aerodynamic load of the downstream blade.
Coating the blades to protect them from the weather can significantly boost performance. The use of nanostructured coatings makes it possible to achieve interesting hydrophobic, anti-frost and anti-gel properties. A study conducted at the Harvard University focused on producing a material that is highly frost repellent. It is based on porous surfaces that are slippery due to the infusion of liquid in the pores.
The researchers focused on the nanostructured surface, which they chemically functionalized. This was aimed at ensuring that the material has a high affinity for the liquid that will be infiltrated into its pores. This liquid is immiscible with water and remains infiltrated in the pores of the material thanks to its high affinity. It constitutes an upper lubricating layer that is ultra-smooth and stable.
The study shows that the material prevents the accumulation of ice and frost by effectively removing moisture condensation. It also has a gel adhesion that is lower than standard materials used to manufacture blades.
Portable wind turbine
A study conducted at the Virginia Tech University studied theoretically and experimentally the viability of a portable wind turbine. The experiment was conducted in a low-speed tunnel. The results showed that the turbine can produce an output power of up to 2.2 watts for a wind speed of 5.5m/s. When comparing these results with similar projects, this turbine is one of the most efficient miniature turbines (diameter less than 19 inches).
The unit is designed to operate in low wind speed conditions (less than 5 m/min). Up till now, small turbines required high wind speeds for their operation (above ten m/s), which prevented their use on the ground. The turbine applications developed by the Virginia Tech team are wide-ranging. The applications are relevant to both developing and developed countries.
A direct application that could be envisaged would be, for instance, powering wireless sensor networks for highways and bridges. The cost of wiring or replacing batteries is not viable in the long term.
Researchers at Georgia Tech University, and the Institute of Nano-Energy and Nano-systems in Beijing, recently presented a device based on the triboelectric effect, which uses wind energy to function.
Triboelectricity is an electrostatic phenomenon, which occurs when two materials come into contact. The convergence induces the transfer of electrons from one material to the other. This transfer is retained even after the materials have been separated. The effect can be increased by adding mechanical energy or rubbing the two materials against each other. Researchers have taken advantage of this effect using wind energy as a trigger.
The device comes with a fluorinated propylene-ethylene film placed between two aluminum sheets, which are connected to ground via an external charging circuit. The vibrations induced by the wind on the film cause a transfer of electrons between the two aluminum electrodes and the mass.
The electrical output power created is sufficient to light up to 10 commercial light-emitting diodes. Based on this architecture, the researchers also proposed to introduce a self-powered sensor. The component is designed to measure wind speed and direction. The wind direction is determined by the real-time analysis of the output voltage.
The future of wind energy
As more countries invest in the best renewable wind energy resource, the sector is witnessing an impressive array of new technologies. Some of the innovative wind energy solutions include flying wind turbines, floating wind generators, freeway turbines and bladeless wind turbines.
A number of companies developed wind turbines that float in the air thanks to rotors. The helices behave like dynamos that transmit the generated energy through a cable to the ground. They are designed to climb hundreds of meters above the ground to take advantage of intense currents.
Floating wind generators are turbines installed at sea. This technology reduces the costs of power generation by eliminating the need to install clamping towers. The generators only require the installation of steel lines that work as anchors. In addition, they have a stabilization system to prevent tipping.