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Variability and intermittency – a response to wind power critics

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Critics of wind power often like to claim that wind power suffers from intermittency problems and therefore is an unreliable and costly method of power generation. This claim does not stand up to detailed scrutiny, as many studies concerning this issue have found.
Variability and intermittency – a response to wind power critics

Indeed, the whole question of ‘intermittency’ is in itself misleading, since the wind never ceases to blow. It makes much more sense to discuss wind speeds in terms of ‘variability’ for this reason. That said, wind speeds do indeed vary from month to month and even second to second.

It is therefore true that sometimes a wind turbine will not generate any electricity at all. According to the University of Massachusetts Renewable Energy Research Laboratory (RERL) at Amherst, this does indeed affect the value of the wind power, but not in the manner that many critics like to point out. RERL point out that wind power has been installed in the US for long enough for detailed studies to be made of the impacts of wind variability and that recent studies of installed wind power in the US have revealed the cost of wind variability to be currently in the region of about 2-5 tenths of a cent per kWh depending on the amount of penetration with the higher costs applying to 20% penetration.

This is not insignificant but nevertheless it is still a small percentage of the total cost of generating power (RERL Factsheet). High penetration wind grids, where wind provides almost all the electricity, tend to be located in remote areas.

At present, Denmark and  Northern Germany represent the best examples of high penetration wind grids, producing about 20% of electricity demand. For this reason it is interesting to see how those two countries respond to wind variation. A paper produced by Andrew Smith of London Analytics examined this in detail and found that Denmark is able to use high capacity interconnectors to smooth variations in wind generation at the minute by minute level. Furthermore, Denmark balances demand in neighbouring countries in response to their fluctuating hydro power (Smith, 2010).

In some of these northern European countries it is sometimes the case that wind supplies as much as 50% or even 100% of electricity demand for certain periods. It is most definitely the case then that European wind energy operators have learned how to integrate wind power into the grid in large quantities without the need for large gas or coal plants to provide backup and without incurring the risk of blackouts.

The key to this is building flexibility into the entire system across all types of generation based also on forecasting and scheduling, transmission, demand-side management (DSM) and energy storage (Varrone, 2011).

In essence then, although there is an issue with wind power regarding variability, it is in fact a minor one and certainly something that can be effectively managed with no risk of power disruption or excessive cost.

In order to accurately assess the kind of criticisms levied at the wind industry regarding variability, let’s look at some of these in detail.

Characteristic of the various criticisms of wind power is that launched by the Renewable Energy Foundation (REF) in 2005 examining the role of wind power in reducing carbon dioxide emissions. This report made three claims, specifically that the emissions saved by wind energy are much smaller than claimed, that considerable amounts of backup power will be required and that the performance of wind energy installations is not only worse than anticipated but also expensive.

In response, the British Wind Energy Association (BWEA) published a critique of the REF report in February 2005 entitled Blowing Away The Myths. In its critique, the BWEA point out that the claims made by the REF report to the extent that wind energy is variable, unpredictable and uncontrollable are not supported by a wide number of references and in fact completely omits the Department of Trade and Industry (DTI)/Carbon Trust Renewables Network Impact Study, which includes an Intermittency Literature Survey citing 74 references to studies relating directly to the variability of wind energy (DTI/Carbon Trust, 2004).

About half of these studies were conducted among utility operators. This document in fact states that the variability of wind energy is indeed manageable by system operators and that any additional operating costs are modest. Significantly, a wide-ranging review of American utility studies has drawn a similar conclusion (Parsons et al, 2003).

The UK System Operator, National Grid Transco is one company that is not worried at all by fluctuations in wind power. In 2004 it had this to say on the subject:

“However, based on recent analysis of the incidence and variation of wind speed we have found that the expected intermittency of wind does not pose such a major problem for stability and we are confident that this can be adequately managed…

It is a property of the interconnected transmission system that individual and local independent fluctuations in output are diversified and averaged out across the system.” (National Grid, 2004)

The statement echoed an American study which concluded that:

“A key feature of the present analysis [of the effects of variability] is its integration of wind with the overall electrical system. The uncontrollable, unpredictable, and variable nature of wind output is not analyzed in isolation. Rather, as is true for all loads and resources, the wind output is aggregated with all the other resources and loads to analyze the net effects of wind on the power system. Aggregation is a powerful mechanism used by the electricity industry to lower costs to all consumers. Such aggregation means that the system operator need not offset wind output on a megawatt-for-megawatt basis.” (Hirst, 2001)

The REF further makes the mistake of drawing on a study examining wind speeds in Germany in 2003, thereby completely missing the point that wind speeds in the UK are significantly higher than in Germany (BWEA).

Furthermore 2003 was actually a low wind year anyway, such comparisons are therefore of limited value. The REF also fails to acknowledge that the net impact of wind on the grid should be assessed by taking into account the combined impacts of conventional plant failures and variations in consumer demand. It also claims, incorrectly, that wind capacity factor has never reached 30% - it actually reached 31% in 1998 (DTI, 1998).

The term ‘capacity factor’ refers to the ratio of the actual energy produced in a given period to the hypothetical maximum possible, i.e. running full time at rated power. All technologies have capacity factors and these vary considerably according to the resource, technology and purpose. Essentially, wind turbines have a much lower capacity factor but a much higher efficiency level than most fossil fuel plants. A higher capacity factor is not an indicator of higher efficiency or vice versa. Neither does it make much sense to compare capacity factors across technologies, since the economics of both production and capacity are very different among different technologies. The capacity factor is therefore just one factor among several to bear in mind when judging when a particular technology is feasible or not.

Typical capacity factors for wind are between 20% to 40%. A number of Scottish wind farms have reported capacity factors greater than 30% leading Scottish Power to predict between 35% and 40% for its best sites (Scottish Parliament Enterprise and Culture Committee, 2004). nPowerhave also reported long term capacity factors of between 36% and 40% for five of their wind farms (Warren et al, 1995).

The output from these has been, at times of peak electricity demand, above average thereby contradicting the idea that wind power is not available at times of peak demand. On this basis, the BWEA predict that, with the increasing number of wind farms being built in Scotland and offshore, UK average capacity factors are only going to increase.

Some critics claim that winter anti-cyclones calm the country thereby potentially causing problems for the system operator due to the absence of wind power. It is worth pointing out here that the REF, who you would expect to make such a claim, have not provided any evidence of this being a regular occurrence and none of the references cited in their report mention the matter either.

The BWEA therefore point out that wind energy does a ‘capacity credit’ in that it can displace thermal plant. Admittedly, it cannot do so on a megawatt for megawatt basis, such that around 1000MW of wind power will displace about 350MW of thermal power. However, this ratio declines with increasing wind energy penetration (Milborrow, 1996; Swift Hook, 1987; and various others).

Energy consultant David Milborrow published a comprehensive report in 2009 entitled Managing Variability, the findings of which correlated with two other extensive reports in that year from the National Grid and Helsinki-based Poyry Energy Consulting respectively. The Milborrow study looked particularly at the UK market and concluded that there are no serious reasons why wind power shouldn’t provide a major contribution to the UK energy mix. In an interview with Renewable Energy Focus Milborrow adamantly explained “there are no fundamental technical reasons why a high proportion of wind energy cannot be assimilated into the system. Steady growth of wind power worldwide shows that this renewable energy source is now seen as a robust choice for reducing greenhouse gas emissions.”

So wind power is most definitely on the up, and if there was any further evidence needed consider this: in Spain this year wind energy broke a record back in February with wind turbines generating their highest ever electricity output in the country at a total of 4,890GWh, according to an article appearing in Renewable Energy Magazine  in March. Figures from Red Eléctrica de España (REE) revealed that wind energy represented the third largest source of power in Spain in February after coal and nuclear reaching a demand coverage of 21.7%.

The more the subject of variability is studied in depth, the more it becomes apparent that it simply isn’t an issue given the right response from governments and power generators, thereby making criticisms of the industry largely redundant.  

References and further information:

Carbon Trust & DTI (2004) Renewables Network Impact Study, The Carbon Trust

DTI , Digest of UK Energy Statistics, 1998, The Stationery Office

Hirst, E (2001) Interactions of wind farms with bulk-power operations and markets, Prepared for Sustainable FERC Energy Policy, Virginia

Milborrow, D (1996) Capacity credits – clarifying the issues, Proceedings of British Wind Energy Association 18th Annual Conference, Exeter, Mechanical Engineering Publications Ltd, London

Milborrow, D. (2009) Managing Variability WWF-UK, RSPB, Greenpeace UK and Friends of the Earth EWNI

National Grid UK (2004) Seven Year Statement

Parsons, B, Milligan, M, Zavaldi, B, Brooks, D, Kirby, B, Dragoon, K and Caldwell, J (2003) Grid impacts of wind power: a summary of recent studies in United States, Proceedings of European Wind Energy Conference, Madrid, European Wind Energy Association

Renewable Energy Research Laboratory (RERL), University of Massachusetts at Amherst, Community Wind Power Factsheet 2a ‘Wind Power: Capacity Factor, Intermittency, and what happens when the wind doesn’t blow’.

Scottish Power (2004) Submission to Enterprise and Culture Committee, Scottish Parliament, for “Renewable Energy in Scotland” Inquiry

Smith, A. (2010) ‘Quantifying Exports and Minimising Curtailment: From 20% to 50% Wind Penetration in Denmark’, London Analytics

Swift-Hook, DT (1987) Firm power from the wind, Proceedings of British Wind Energy Association 9th Annual Conference, Edinburgh, Mechanical Engineering Publications Ltd, London

Varrone, Chris, (2011), ‘Why wind intermittency is not a big deal’, Clean Technica

Warren, Hannah, Hoskin, Lindley and Musgrove (1995) Performance of wind farms in complex terrain, Proceedings 1995 BWEA Conference, Warwick, Mechanical Engineering Publications Ltd


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