Opponents of wind power like to point out “inefficiency” as a reason against it. “Inefficiency” is a vague term and could mean many things. Here’s an explanation of some of the figures and arguments that are raised.
Energy conversion efficiency
In the strictest sense, energy efficiency is the ratio of how much input energy is converted to useful output energy. When referring to a wind turbine, the efficiency refers to the fraction of the kinetic energy in the wind that is converted to electricity.
The maximum theoretical amount of energy that can be extracted from wind is 59%, known as the Betz law after the physicist who worked it out. It’s not possible to extract all of the kinetic energy in wind because that would mean that the turbine would bring the wind to a complete stop. In reality, the most efficient energy conversion of 59% takes place when 2/3 of the kinetic energy is extracted. That is, the wind speed behind the turbine is moving at 1/3 of the speed of the wind in front of the turbine.
In the real world, wind turbines don’t reach their theoretical maximum efficiency. Sustainability Victoria cites “up to 50%” of wind energy converted, although this would be in optimum operating conditions. In practice, most of the time turbine efficiency is in the range of 35-45%.
This compares to silicon photovoltaic solar panels at maximum 29% theoretical efficiency; a coal-fired power station can reach an energy conversion efficiency of 40% at best (and more like 25% in current Victorian generators). Combined Cycle Gas Turbines may reach efficiencies over 50%.
So using this technical definition of useful energy out compared to total energy in, a wind turbine is relatively efficient.
Resource use efficiency
The concept of efficiency is particularly important when the fuel being converted has a cost. In the case of fossil fuels (coal, oil, and gas), the fuels cost money and create pollution so it’s important to get as much useful energy out as possible. In the case of renewable energies like wind and solar, the concept becomes less important. The “fuel” is free and their use does not cause damage to the environment, so it’s not so imperative to use the fuel as efficiently as possible.
As an example, over 50% efficiency for converting gas to electricity in a combined cycle gas turbine is quite good by power station standards; but if you were to use the gas to supply heating, it would be far more efficient as almost all the energy in it is released as heat, the main energy loss being in transporting the gas by pipeline to the end user.
On the other hand, we can consider the case of a rooftop solar panel with a maximum theoretical efficiency of only 29% of the sunlight energy converted to electricity. As the sunlight is free and would not be used for another purpose anyway, it is not a waste. A wind turbine is likewise without waste, despite not being 100% efficient in a technical sense.
One of the myths sometimes discussed by the anti-wind lobby is that the energy used to manufacture, transport and construct a wind turbine is greater than the amount of energy generated by the turbine. This has no basis in fact; turbines typically operate for over 20 years, but generate enough energy to cover their “embodied energy” within the first year, often with the first six months.
Capacity factor and intermittency
The most common argument against wind turbines is that because they generate power intermittently, they only generate a small fraction of their ‘nameplate capacity’. The term ‘capacity factor’ refers to the average output of a wind turbine as a percentage of the maximum possible output.
The term “intermittent” has been challenged as a large number of wind farms spread over a large geographic area tend to produce some energy all or most of the time, so the term “variable” is often used instead.
On a windy day a turbine might generate 90% of its capacity, but over the longer term quiet days mean that the average is much lower. The long term average capacity factor depends on factors such as the average local wind speed and the model and make of the turbine. Typically, Australian wind farms have capacity factors of around 30-35% of their rated maximum output, although some are more or less than this figure.
You can see daily output and averaged capacity factors for wind farms in southeastern Australia at http://windfarmperformance.info
A capacity factor of 30% is low compared to a coal power station, which (by its nature) tends to run all the time except for scheduled shutdowns; capacity factors are typically around 75%.
A capacity factor of 30% for wind farms seems like an inefficient use of a machine, but it does not mean that it is not working 70% of the time: simply that it runs at less than 100% output. To harvest a variable energy resource like wind, this is inevitable and to be expected.
Peaking gas power stations have a capacity factor of 2-10%, because they are only switched on at peak power use times when the price of wholesale electricity is very high.
Low capacity factor is not considered an issue when discussing other common machinery. For example, the capacity factor of a car might be around 1%. It could be transporting you around 100% of the time however it is usually idle. A mobile phone could be used to make calls 100% of the time but its capacity factor would be a great deal lower than this too.
Some people incorrectly suggest that integrating a variable energy source like wind into the grid causes problems for the grid as a whole and that additional fossil-fuel generators are required for backup.
Of course, the electrical grid is designed to deal with significant variability all the time as demand fluctuates depending on time of day and the weather. In addition traditional large generators sometimes go offline unexpectedly for for scheduled maintenance.
Currently, wind energy makes up close to 2% of Australia’s electricity supply, and fluctuations are easily balanced with the other existing generators.
Much, much higher wind penetration is quite manageable in the grid. The UK Sustainable Development Commission stated in 2005 “up to 20% wind capacity penetration is possible on a large electricity network without posing any serious technical or practical problems.” Larger proportions still may be integrated with some work on the energy grid to accommodate this.
Spain averaged 17% of its electricity from wind over the first two months of 2011. Spain does not rely on large interconnections with the rest of the European grid to compensate for the variability of this energy, and is still successfully integrating it with the rest of its grid.
To complement a variable energy source such as wind, other flexible sources can be dispatched when required. Coal power plants are quite inflexible, but even they can reduce their output to allow wind power into the grid, and gas and hydro generators are more flexible still.
We support the development of concentrating solar thermal (CST) power stations that store heat in order to be able to run 24 hours a day. These plants operate simply using mirrors to focus sunlight on a central heat receiver. The heat is stored in a fluid, typically molten salt. Wikipedia gives a basic overview of the technology.
CST provides rapidly dispatchable power that is a perfect complement to a variable resource such as wind and has zero carbon emissions. These power stations are currently quite expensive as they are new technology, but are predicted to be cost competitive with gas (and coal, with a carbon price) by the time only 8GW of capacity is installed worldwide according to the US Department of Energy. This is less than Victoria’s total power generation capacity which is over 10GW.
In a competitive (privatised) electricity market, the bottom line for any power generation plant is cost effectiveness, or dollars spent per unit of electricity generated.
Wind energy has reduced in cost dramatically over the years and is still getting cheaper due to economies of scale as world manufacturing capacity increases. The cost per unit of electricity output has dropped 80% in the last 25 years. A 2004 study published by the Australian Wind Energy Association predicted that wind energy costs would converge with gas generation somewhere between 2008 and 2015, and with coal fired generation from 2016 on.
Wind and other renewable energy generation technologies do not have fuel costs. Gas prices fluctuate a lot, and may increase significantly as oil prices go up. Relying on gas for future power supply could actually lead to very large increases in power bills.
Currently brown coal is not traded internationally so its price is low, although the Victorian government has promoted investigations into ways to export it. However, the price on carbon dioxide pollution will add to the cost of these fuels as well and the environmental costs are likely to see brown coal plants closed down regardless of operating costs. Wind and solar face no such constraints.
We have separately discussed the issue of subsidies for renewable energy vs subsidies for fossil fuels. We concluded that for a new technology it is reasonable to subsidise, but that currently fossil fuels receive far greater subsidy in various ways.
Traditional large power stations in Victoria were built by the state government, not on commercial principles, simply because they were considered necessary infrastructure. As we move towards a cleaner future, we hope that wind farms will be treated the same way.
Wind farms: the facts and the fallacies
Andrew McIntosh and Christian Downie, The Australia Institute, Oct 2006
Wind Energy myths and facts
Sustainability Victoria, May 2007
Greenhouse solutions with sustainable energy
Mark Diesendorf, UNSW Press 2007
Renewable Energy: power for a sustainable future
Godfrey Boyle (ed), Oxford University Press 2004
Thanks to Alicia Webb and Dave Clarke for useful feedback in preparing this article – BC