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Compressing The Wind's Energy

by Keith D. Foote

Wind energy has become one the leading renewable energy resources in the world. While there are complaints about noise and a lack of aesthetics, wind turbines continue generating clean, free energy.

However, as with solar power, the winds do not provide a steady, consistent source of electricity. Wind power is intermittent and is not a reliable source of kinetic energy. An additional problem exists in the engineering of wind turbines. There are design limitations restricting the generation of electricity using wind power. First, the rotors of the turbine can capture just under 60% of the wind's kinetic energy. Second, current designs have a specific capacity corresponding to specific wind speeds. Faster wind speeds will not generate more electricity. The turbine will have ?peaked.?

Jie Cheng, a doctoral student at the University of Nebraska-Lincoln, came up with an idea to help make wind turbines more efficient. He has designed a new type of wind turbine capable of dealing with these limitations. Cheng?s turbine design* is able to produce an extra 16,400 kilowatt-hours per month, normally the amount of electricity used monthly by 18 average households.

The additional energy comes from what is called ?mechanical spillage.? Mechanical spillage is the energy which is lost by the turbines after they peak during high wind speeds, preventing electrical and mechanical damage. Cheng's new prototype comes with an air compression tank, which is used to store the ?spillage.? The energy is stored as compression until the wind speeds lower and the wind turbines are operating at less then maximum capacity. The stored energy, in the form of compressed air, is released, and converted into electricity by the turbines during times of no, or low, wind speeds.

According to Jie Cheng, his prototype would be useful in many parts of the world. He believes it would be useful where there are extreme winds, followed by times of calm.

Cheng's design uses a differential gearbox, which is a gear system allowing for the uneven transfer of power from the main shaft, to 2 ?different? secondary shafts. Cheng used what is called a ?planetary gearbox,? which is a specific of differential gearbox. It is made up of a sun gear in the center, planetary gears in the middle, and a ring gear on the outer gear edges. The shaft on the left is interconnected to the blade shaft. The solid shaft, on the right, is connected to the generator. There is a hollow shaft on the right side, linking to the planetary gears, which are connected to the RVM (Rotary Vane Machine). This design allows positive or negative power to compensate for the difference between input and output.

The Rotary Vane Machine includes an off-center shaft with spring-engaged vanes. The design creates a number of uneven spaces in its housing. The RVM is built into the system using the planetary gearbox as a way to recapture what would otherwise be excess, unused wind power. This unused power then generates electricity when the wind is insignificant.

When the switch valve is turned to the the left, the inlet side of the RVM is open to the atmosphere, and the outlet is hooked up to a compressed air tank. The RVM works in compressor mode while the vanes are being driven by the shaft, and compresses the air to the outlet. When the switching valve is turned to to the right, the tank's outlet air flow becomes reversed. The RVM acts as a crude expansion chamber, with the vanes being driven by compressed air, and turning the turbine to generate electricity.

Air tanks are used as a part of the storage component, working in harmony with the RVM. There are a large number of storage tank designs, with variations in its maximum allowable pressure. For a given volume and pressure, and a specific temperature, the maximum amount of energy stored in a tank can be calculated. The compressed air tank becomes a kind of storage battery.

The air tanks maintain a 95 percent capacity for electrical generation for the first 109 hours. When the wind is blowing in surplus mode, the RVM runs constantly as a compressor until the tank is full, storing the excess energy for later use. The RVM stops turning when there is insufficient wind and the storage tank is empty. Cheng believes his system is quite efficient, and capable of returning its investment costs very quickly. He thinks the savings of electricity purchases, combined with the sale of exported electricity, will accomplish this goal.

Jie Cheng has proposed a unique and novel solution to storing the kinetic energy of the wind for later use. He uses a configuration integrating a compressed air storage system with a traditional wind turbine system. His system recycles the mechanical spillage previously ignored by traditional wind power designers and uses it to generate electricity at a time when it is needed. Compared to traditional systems, Cheng's design increases the capacity of turbines to transform the kinetic energy of the wind into electricity and has the potential to maximize and streamline the overall electricity used by the grid. This design has great potential for improving the efficiency of the wind turbine industry.

* Reference - A Novel Wind Energy Conversion System With Storage For Spillage Recovery, by Jie Cheng Pub. Date: July 17, 2015