Niall McMahon

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Lecture 1 - Introduction to Wind Energy

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Context

You can find out more about me here.

These Notes

The objectives of this lecture are: (i) to outline the historical development and state of the art in wind turbine technologies; (ii) to outline some of the ideas that shape the wind industry; (iii) to outline the main components of a modern wind turbine.

Preliminary Points and Basic Concepts

Opportunities in the Wind Industry

Opportunities in wind include: wind manufacturing and suppliers; wind development (civil and so forth); wind resource assessment; insurance and other financial areas; standards development and testing; electrical utilities - integration and operation; research and development; among others.

What do industrial employers look for? What will this course give you?

Ireland has a lot of wind; it's our primary renewable resource. However, we import a lot of technology and know-how. In the future, if we are to benefit more than superficially from wind, we'll need more workers trained in wind. It's an exciting time for wind energy.

Quantities

When we discuss concepts like energy or power, try to understand what's happening in real life; watch videos, watch real wind turbines, carry out your own experiments. It's easy to concern yourself with defining things that we don't fully understand, e.g. energy; it's better to figure our how something specific is working, e.g. the mechanics of a wind turbine, without worrying about the more general problems at the outset.

Always try to work out things for yourself. See, for example, Sustainable Energy Without the Hot Air, by MacKay, for tips on how to go about this.

Learn off useful numbers and remember approximate values. For example, 1 kW or 1000 W is the continuous rate of consumption of a typical American home, averaged over a year. This is equivalent to leaving a small electric bar heater running continuously all year, night and day. A typical European home uses energy at a rate of about 430 W over a year.

Power in the Wind

Two key concepts to begin with; you will come across these again, many times. The first is that air of density rho, moving through an arbitrary cross-section A with a velocity U has a power content of P, given by:

P = ½ ρ A U3

I'll revisit this in the next lecture. The second key point is that the maximum possible efficiency of a wind turbine with a rotor disc of area A is 16/27ths of this available power P. This ceiling on efficiency is known as Betz's law.



More about these ideas again.

Modern Wind Turbines

There are many kinds of wind turbine but these can be grouped, generally, into two (or three) prototypes: (1) horizontal axis wind turbines (HAWTs) and (2) two families of vertical axis wind turbines (VAWTs).


Wind turbine prototypes. Horizontal Axis Wind Turbine (HAWT) Family. Modified from a public domain image.


Wind turbine prototypes. Vertical Axis Wind Turbine (VAWT) Families; Savonius (left) and Darrieus (right). Modified from a public domain image.

We will only discuss horizontal axis wind turbines (HAWTs) in any detail during these lectures. This is because the HAWT is most common, and uncontroversially, the most efficient design for most purposes.

Horizontal axis wind turbines are classified in different ways, e.g. as small-/medium-/ or large-, by rotor orientation relative to the tower (upwind/downwind), by blade pitch system, i.e. fixed or variable, and by the number of blades. More about these classifications in this lecture and in subsequent lectures.


A small, downwind wind turbine.




Cutaway of a typical large wind turbine. Adapted from a public domain drawing created by the (US) Office of Energy Efficiency and Renewable Energy.

Historical Development

(There is a nice overview of wind development hosted by the Norwegian University of Science and Technology. The photographs are worth studying.)

(There are two well-restored windmills in Skerries in North County Dublin.)

The onset of the Industrial Revolution in the middle part of the 18th century changed the dominance of wind power.

John Smeaton, in a paper published in 1759, observed that:

These are important ideas; as you'll find out in subsequent lectures, these are critical concepts for the correct design of wind turbines.

(You can see some of Smeaton's writing here.)

Multi-blade water-pumping windmills became popular in the American mid-west during the 19th century; there were possibly millions of machines in use across America at the close of the 1890s.

When electrical power generators took off, people noticed that windmills could be easily adapted to turn generators, which generally require a rotating prime mover, i.e. the power input. Windmills could become wind turbines, driving electrical generators. This was attempted by a variety of pioneers starting in the late 1800s, including Charles Brush (USA, 1888) and Poul la Cour (Denmark, 1891). La Cour built many wind turbines between 1891 and 1918, some as large as 35 kW. La Cour believed that wind machines should be used as power plants to harness the maxiumum energy from the wind. La Cour also stands out for his use of a wind tunnel to experiment with rotor configurations.

(You can find plenty of information about, and images of, these machines online.)

Diesel and steam dominated electricity production for much of the 20th century. In general, wind power devloped when fuels were scarce or expensive, for example, during and immediately after the two world wars and in the early 1980s.

After the second world war, the major nations, including Denmark, continued the development of wind. Work by Johannes Juul in Denmark led to the development of the Gedser wind turbine. This machine became the standard by which other machines were judged and the basis of the Danish concept: three-bladed upwind turbine, stall-regulated, asynchronous generator operating at almost constant speed.



State government interventions in California between 1980 and 1985 promoted the rapid expansion of wind. About 1 GW, or 1,000 MW, of rated capacity was installed in this period. Wind farms, as we think of them now, became commonplace. Firms such as Enertech from Vermont sold into the Californian market. European companies such as Denmark's Vestas also sold into California. Many machines were based on the Danish concept.

Most Californian machines were much smaller than contemporary wind farm machines, with power ratings of up to 200 kW, with rotor diameters/tower heights up to about 30 m. Many were smaller, with rotor diameters/tower heights of about 20 m or less and power ratings of about 50 kW.



After the Californian expansion ended with the state government's policy reversal, coincident with falling fuel prices, most wind expansion occured in Europe.

The Danish Wind Industry Association has a nice summary of this Great Wind Rush.

Security of energy supply, environmental concerns and the recognition of climate change, peak oil and the finite supplies of other fuels, as well as the recent difficulties of nuclear power, have all contributed to strengthening demand for wind power and other renewable energy solutions.


Oil prices from 1987 to 2012

This file is licensed under the Creative Commons Attribution-Share Alike 3.0 Unported license. Retrieved from:
http://en.wikipedia.org/wiki/File:Brent_Spot_monthly.svg.


Oil prices from 1968 to 2005

Created by the United States Department of Energy. Public domain.


Wind turbines and farms have been built at scales scarely imaginable in the 1970s and 1980s.

(There's an interesting Popular Mechanics article archived online about giant wind turbines of the future.)



Some Facts and Figures

The standard commercial machines have increased, incrementally and steadily in size since the 1980s. This is fundamentally because it's better to have a few larger machines than many small machines for utility scale generation.

(Since power capture scales with rotor swept area, or rotor diameter squared, a doubling of the rotor diameter quadruples the power capture, i.e. a small increase in rotor diameter can lead to a very large change in power capture.)

You can see a nice image of the change in wind turbine size with time here.

The incremental change is down to knowledge and technology. If you can build a machine with a 100 m diameter rotor, you might have confidence enough in your technology and manufacturing capability to build one that's 110 m in diameter next year. You'll probably not want to jump to 200 m!

At present, see the Vestas range includes:

Installed Rated Capacity Worldwide

2002: 31,180 MW
2003: 39,431 MW
2004: 47,620 MW
2005: 59,091 MW
2006: 73,938 MW
2007: 93,889 MW
2008: 120,624 MW
2009: 158,975 MW
2010: 198,001 MW
2011: 238,126 MW
2012: 283,194 MW
2013: 318,105 MW
2014: 369,597 MW
Now: 456,000 MW
End 2016: 500,000 MW (Projected)

The source for these numbers is the Global Wind Energy Council. Rated capacity is the nameplate power output of a wind turbine. Machines are rated at a particular wind speed. The power output from the turbine at this fixed wind speed is the rated power.

America has the most turbines in the world, followed by Germany, China, Spain, India and then other European countries (see GWEC or EWEA for more information.)

See also this overview of wind capacity in Ireland. As of September 2015, Ireland has a total installed wind capacity of 2,375 MW. This represents, on average, about 18% of our electricity demand. The maximum output of 1,967 MW happened in January 2015.

Technical Challenges of Wind Development

See:

Some of the big technical challenges facing large-scale wind include:

Commercial Challenges of Wind Development

See:

The costs associated with wind turbines and wind farms change with time. For example, the cost of electricity is generally increasing. This favours wind. Additionally, the cost of manufacturing with time. For example, some wind turbines use permanent magnet generators that depend on the availability of rare earth elements. The costs associated with turbine size change as technologies improve.

Wind turbines must be online, i.e. available to generate, at least 95% of the time. This is a commercial pressure that presents particular engineering challenges. Gearboxes have traditionally been among the most problematic and costly of components. Some manufacturers, for example Enercon, use gearbox free designs.

Different countries and economic areas have different market conditions; these are complex but are affected by government regulation. Some countries have set targets for wind capacity in order to meet international climate-related obligations, e.g. Kyoto. Additionally, electricity market mechanisms differ from place to place.

About 75% of the money spent on a wind turbine installation is for the machine and its tower. The remainder covers civil engineering works, electrical works and related consulting.

Finally, and perhaps most importantly, the rise of solar creates uncertainty about the future scale of wind energy generation. It's likely that in future, electricity generation will move towards a distributed model, with several complementary technologies meeting demand. Wind will always have a role, but its future extent is yet to be determined.

Social and Environmental Challenges of Wind Development

The external costs of wind energy are low; take a look at Wind Energy: The Facts. Externalities are the social costs that are not normally reflected in the economic cost of producing something, e.g. pollution and damage to health. Attempting to cost these effects, which are ultimately paid for by society, is a more honest way to go about economic comparison.

Environmental considerations include emissions reduction and the visual impact of wind turbines. The emissions reduction depends on the technology that wind replaces. For example, coal creates about 1 kg of CO2 for each kWh, oil creates 0.75 kg for each kWh and gas creates 0.5 kg for each kWh.

Other social/environmental issues include noise, television/radar interference, collisions with birds and shadow flicker. Careful implementation will eliminate most of these effects. Noise levels have dropped in recent years and regulation has improved. The noise emanating from a typical large wind turbine is of a low intensity.

Conclusions

References

Consult the introductory chapters of Wind Energy Explained. The Danish Wind Industry Association has a nice introduction to wind here. There are many other useful sites online. See also the resource lists here.

Legals

Please see here.

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