Wind power without wind turbines

April 8, 2013
Electrostatic wind power harvests wind energy with no turbine blades, gearboxes, or other expensive hardware. But there are still challenges to be overcome before the concept is ready for prime time.

Researchers at Delft University of Technology in the Neatherlands have crafted a way of harvesting energy from the wind that elminates the need for turbine blades, gearboxes, generators, and other assemblies normally found in wind turbines. Called the EWICON (short for Electrostatic WIndenergy CONvertor), the device uses principles of electrostatics to produce electrical current.

The EWICON prototype at the Delft University of Technology.A demonstration unit constructed by an architectural firm called Mecanoo is now on the Delft University campus. The EWICON has no moving parts. The only pieces that can wear out are nozzles that spray water droplets which serve as charge carriers. As Mecanoo describes it, the hardware consists of a flowing steel frame in the shape of a squared -0- supporting a framework of horizontal steel tubes. Electrically charged droplets are created within the framework and are blown away by the wind. The movement of the droplets creates an electric current, which can be passed on to the grid.

Many details pertaining to principles of the device's operation can be gleaned from a dissertation developed by Delft researcher D. Djairam. As Djairam writes, allowing the wind to force charged particles against the direction of an electric field boosts the potential energy of these charged particles. The charged particles can then be collected in a charging system that is insulated from earth. Because the charging system starts out electrically neutral, dispersing charged particles causes its potential to rise. Basically, the earth acts as the collector for the charged particles.

Earth acts as the collector in the EWICON system. The system itself is insulated from earth and the dispersal of charged particles will increase the potential of the system.In theory, the charged particles could be anything that could hold an electrical charge. But water droplets turn out to be the most practical way of dispersing streams of charged particles, says Djairam. But there's a limit to the amount of charge that  a liquid droplet can hold before it will break up into smaller droplets, and that charge depends on the natural surface tension of the liquid. Evaporation is also an issue because the charged droplets need to survive until they reach the earth.

One wonders how much water would be necessary in this scheme to generate meaningful amounts of power. Here's how researchers approached this issue: They calculated the work done on a droplet by the wind using some reasonable assumptions and came up with about 1.13×10-8 J. Assuming t a spray nozzle dispersing about 107 droplets/sec or 20 ml/hr (not unreasonable, they say) gives a power associated with this stream of droplets of  113 mW/nozzle. A 30×30 array of these nozzles would produce about 102 W. Assuming water droplets with a diameter of 5 μm charged to 70% of the maximum theoretical charge (given by the Rayleigh limit), the rate of charged droplets is 8.5×107/sec, corresponding to a current through each nozzle of 4.7 μA. This would imply an output power of roughly 0.5 W into an electrical load of 20 GΩ .

In real systems, not all droplets will get completely charged. Researchers assumed a conversion efficiency of 25%, implying a need for 212 nozzles/m2 to produce the current required to feed such a load. If the droplets have a diameter of 15 μm, the current drops to 0.9 μA per nozzle, they say, which implies an output power of 17 mW, implying the system would consist of roughly 5,500 spraying nozzles/m2. In terms of liquid consumption, this equals going from 4.2 l/hr for the former situation to 110 l/hr for the latter.

Another issue: Creating an electric field in a way that the system doesn't consume more energy than it generates. Researchers considered several techniques but found two particularly promising. The first is electrohydrodynamic atomisation (EHDA) often used for applying coatings where a strong electric field focues the meniscus of the liquid leaving a spraying nozzle into a conical shape. Ions in the liquid accelerate toward the cone apex and, consuequently, accelerates the liquid itself. At the cone apex, liquid breaks up into droplets with a high charge density.

Schematic of the EWICON prototype. A reservoir supplies the liquid to the isolated charging system, where charged droplets are produced.The second promising method of creating charged droplets is called high pressure monodisperse spraying (HPMS). It is basically ink jetting: Liquid is forced through small micron-sized pores with equal size creating liquid jets with equal diameter. The high pressure that is applied is usually in the order of 10 to 15 MPa. The liquid jets break up into droplets with the diameter of the droplet proportional to the diameter of the liquid jet.

Another issue is that of remaining energy positive even considering the energy needed to pump meaningful amounts of water to the system. As an example, say researchers, assume water needs to be pumped to a height of 10 m at a flow rate of 400 l/hr. In that case, the minimum required pumping power works out to be 11 W, roughly 1% of the rated power. Currently, the maximum efficiency of water pumps is roughly 90% which means that the required power will actually be 12 W. However, if the charging and spraying efficiency is lower and the output power per nozzle is, for example, 5 mW or 0.5 mW at the same flow rate, then the minimum required pumping power would be 121 W or 1.2 kW respectively. In the latter case, you end up with a net energy debit.

Thus pumping energy is a big issue, and researchers think these systems might make the most sense where they can take advantage of such fortunate happenstances as rain water collection above the height of the wind generation system.

And other issues arise when the EWICON concept is scaled up. Several nozzles ganged together should produce electrical current equal to the sum of that produced by individual nozzles. But to their dismay, researchers found that the current produced by each single nozzle was significantly lower compared to a single nozzle/ring electrode configuration. The maximum current per nozzle was 0.2 μA compared to 0.5 μA measured with a single needle nozzle A nine-nozzle configuration did not yield 2.7 μA as researchers expected based on the individual nozzle result, but slowly reached 1.5 μA as the number of nozzles rose. Researchers say the reason(s) why this non-linearity is present are still a topic of ongoing research and constitute one of the most important issues that must be resolved to make the EWICON system scalable.

All things considered, researchers are leaning toward using HPMS to generate charged water drops. Their best multi-nozzle system result used six such nozzles spraying a water/ethanol mixture, which produced an output power per nozzle of 2.1 mW. A 1 kW version using this configuration would require 4.8×105 spraying nozzles, roughly 700×700 nozzles occupying 14×14 m2 assuming 2-cm spacing. Researchers say this means  spraying nozzles must be more closely packed before a kilowatt version will be economical. For HPMS, even with lower-than-expected charging efficiencies, the increase in produced current should be feasible, they think.

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