# Wind Energy: Benefits, Design, and Challenges

## Sarkis Agaian December 3, 2011

### Submitted as coursework for PH240, Stanford University, Fall 2011

 Fig. 1: Wind Farm in the Tehachapi Mountains of California. (Source: Wikimedia Commons)

## Introduction

Energy is necessary for achieving sustainable development among societies. Unlike fossil energies, such as gas and coal, which contain high percentages of carbon, renewable energies consist of sources that are naturally inexhaustible - water, sun, biomass, geothermal heat, and wind. [1] Among these renewable sources, wind is considered one of the most promising types of regenerative energy to reduce fossil fuel imports and greenhouse gases. [1-5] Currently, the United States only supplies 1% of the county's energy supply using wind. [5] However , the federal government has made a goal for 20% of the countrys power to be wind supplied. [4] The following explains provides an introduction into wind energy and describes the challenges associated with its use.

## Design

Wind energy (WE) comes from solar energy: the sun emits solar radiation, which travels to the Earth and causes unequal regions of heating over oceans and land masses. [1,5] Such asymmetrical heating causes pressure gradients to form between regions of high and low pressure. [1,5] By the second law of thermodynamics, these gradients need to be minimized in order to achieve the lowest energy state and maximize entropy; this is achieved through the movement of air in the form of wind from high pressure to low pressure regions. [1] Since air has mass, its movement as wind carries with it kinetic energy, which can be converted into electricity using a wind turbine. [1] Below are the wind kinetic energy, wind power, and electrical power equations that help quantify this process. [1]

Wind Kinetic Energy Wind Power Electrical Power
Ek = ½ mair v2 Pwind = ½ &rhoair A v3 Pgenerated = Cb NgNtPwind
where mair is mass, v is the wind velocity where ρair is air density, A is the area swept by the rotor, v is the wind velocity where Cb ≈ 0.45, Ng ≈ 0.75 generator efficiency, Nt ≈ 0.95 transmission efficiency
Table 1: Energy and Power Equations. [1]

## Challenges

Recent research shows that if wind energy is to replace our current reliance on oil, its conversion into electrical power needs to be more efficient and cost-effective. [1-5] More specifically, WE structure designs require new innovations in fluid dynamics, low wind speed technology, utility grid integration, manufacturing, and electric power distribution. [1-5] WE would require, for instance, moving large amounts of power over long distances - from windy, lightly populated plains to highly populated coasts. [5] Moreover, though WE can be variable over different timescales - seasonally, daily, and even hourly - instantaneous electrical generation and consumption must remain in balance to maintain grid stability. [2,5] Such variability can present substantial challenges in incorporating large amounts of wind power into a grid system, especially when peak wind speeds do not coincide with peak electrical power demand. [2,5] In California, Arizona, and Texas, for example, hot days in summer may have high electrical demand due to air conditioning but low wind speed. [5] As a result, systems of using a combination of different energy sources have been suggested to provide more balance and stability. [2,5] For instance, WE may be relied on more heavily during the winter when winds are relatively strong whereas solar energy may be preferred during the summer when radiation is high. [5]

## Conclusion

Wind energy is a very promising renewable source of energy: it is environmentally safe, requires little maintenance, and doesn't create greenhouse gases or produce toxic or radioactive waste; by using its resources, we can decrease our dependence on oil and protect the planet for future generations. [1-5] That said, there are many challenges to be overcome before WE can replace fossil fuels, especially in terms of its distribution and stability. [2,5] Consequently, WE may have to be combined with other energy sources to become a more robust energy alternative. [2,5]

© Sarkis Agaian. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.

## References

[1] J.F. Manwell, J.G. McGowan and A.L. Rogers, Wind Energy Explained: Theory, Design and Application (Wiley, 2009), pp. 1-52

[2] M. Liserre, R. Teodorescu and F. Blaabjerg, "Stability of Photovoltaic and Wind Turbine Grid-Connected Inverters for a Large Set of Grid Impedance Values," IEEE Trans. Power Electronics 21, 263 (2006).

[3] IEA Wind Energy: Annual Report, International Energy Agency, July 2009, pp. 43-67.

[4] F. Rachidi et al., "A Review of Current Issues in Lightning Protection of New-Generation Wind-Turbine Blades," IEEE Trans. Industrial Electronics, 55, 2489 (2008).

[5] T. Burton et al., Wind Energy: Handbook (Wiley, 2001), pp. 1-37.