Claims are sometimes made that wind turbines create electricity intermittently and because of this a) the grid cannot cope and b) back up power stations are needed resulting in no carbon dioxide savings.

A report from the UK Energy Research Centre ‘The Costs and Impacts of Intermittency: An assessment of the evidence on the costs and impacts of intermittent generation on the British electricity network’ (2006) states that ‘it is unambiguously the case that wind energy can displace fossil fuel-based generation, reducing both fuel use and carbon dioxide emissions’.

The report aims to understand and quantify the impacts of variable generation, and therefore addresses the question ‘What is the evidence on the impacts and costs of intermittent generation on the British electricity network, and how are these costs assigned?’ It is based on a review of over 200 international studies, with the review process overseen by an expert group and the final report peer-reviewed by international experts.

Wind energy generation does mean that the output of fossil fuel-plant needs to be adjusted more frequently, to cope with fluctuations in output. Efficiency may be reduced as a result.

The vast bulk of electricity in Britain is supplied through market arrangements via contracts of varying durations between generators and suppliers of electricity. Relatively small, but crucial, adjustments are needed to ensure demand and supply balance each instant – this is known as system balancing.

System balancing entails costs which are passed on to electricity consumers. Intermittent generation adds to these costs. For penetrations of intermittent renewables up to 20% of electricity supply, additional system balancing reserves due to short term (hourly) fluctuations in wind generation amount to about 5-10% of installed wind capacity. Globally, most studies estimate that the associated costs are less than £5/Megawatt hour (MWh) of intermittent output, in some cases substantially less. The range in UK relevant studies is £2 – £3/MWh.

Unless there is a large amount of responsive or controllable demand, a system margin is needed to cope with unavailability of installed generation and fluctuations in electricity requirements (e.g. due to the weather). Conventional plant – coal, gas, nuclear – cannot be completely relied upon to generate electricity at times of peak demand as there is, very approximately, a one-in-ten chance that unexpected failures (or “forced outages”) in power plant or electricity transmission networks will cause any individual conventional generating unit not to be available to generate power. Even with a system margin, there is no absolute guarantee in any electricity system that all demands can be met at all times.

Intermittent generation increases the size of the system margin required to maintain a given level of reliability. This is because the variability in output of intermittent generators means they are less likely to be generating at full power at times of peak demand. The system margin needed to achieve a desired level of reliability depends on many complex factors but may be explored by statistical calculations or simplified models. Intermittent generation introduces new factors into the calculations and changes some of the numbers, but it does not change the fundamental principles on which such calculations are based.

The amount by which the system margin must rise in order to maintain reliability has been described in some studies as “standby capacity”, “back-up capacity” or the “system reserves”. But there is no need to provide dedicated “back-up” capacity to support individual generators. These terms have meaning only at the system level.

The total ‘costs of intermittency’ are made up of additional short-run balancing costs and the additional longer term costs associated with maintaining reliability via an adequate system margin. Intermittency costs in Britain are of the order of £5 to £8/MWh, made up of £2 to £3/MWh from shortrun balancing costs and £3 to £5/MWh from the cost of maintaining a higher system margin. For comparison, the direct costs of wind generation would typically be approximately £30 to £55/MWh. If shared between all consumers the impact of intermittency on electricity prices would be of the order 0.1 to 0.15 p/kWh.

These estimates assume that intermittent generation is primarily wind, that it is geographically widespread, and that it accounts for no more than about 20% of electricity supply. At current penetration levels costs are much lower, since the costs of intermittency rise as penetrations increase. If intermittent generation were clustered geographically, or if the market share were to rise above 20%, intermittency costs would rise above these estimates, and/or more radical changes would be needed in order to accommodate renewable energy.

In summary – yes there are effects from intermittent sources of energy such as wind, however this does not mean dedicated power stations to “cover” for wind energy as every source of energy requires an alternative to share the risk of break downs. Furthermore wind energy is efficient and does result in carbon dioxide savings.

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