The output of an electricity supply with a high proportion of intermittent energy sources can be difficult to predict or control, and can make matching electricity supply to consumer demand problematic.
Electricity supply must be matched to demand minute-by-minute because electricity cannot be stored at volume for any significant time. If supply falls short of demand and no spare generating capacity is available, the National Grid might have to ration electricity.
Wind and solar energy are considered intermittent and therefore unpredictable because their electrical output depends on environmental conditions: the speed of the wind and the amount of sunlight striking a solar panel. This means we need energy sources that can provide back-up, to ensure electricity demand can be met when there is no sun or wind.
Some coal- and gas-fired power stations and most hydroelectric can alter their output rapidly enough to do this. The output of nuclear power stations can also be altered, but these stations are run most efficiently at maximum power.
It is essential to have a secure level of baseload power – the minimum supply of electricity needed to keep the lights on. This is fulfilled by power stations that use gas, coal or nuclear fuel and that excel at producing a continuous and high electrical output. If electricity could be efficiently stored, it could be used to bolster supply when there is a danger that demand might not be met.
Demand is also becoming more responsive (reducing at times of maximum demand) and if this continues it will also help ensure supply is sufficient.
Generating electricity from renewable energy sources like wind and sunlight will help to fulfil our carbon-reduction commitments. But these renewables produce electricity intermittently – we cannot rely on them to meet peaks in demand.
There are two essential features of a reliable electricity supply. First, to be able to meet baseload demand consistently – the supply required all the time. And second, the flexibility to increase output to meet predicted peaks.
Intermittent energy sources are those whose output is variable and subject to factors outside the control of the operating company. Wind and solar energy are considered intermittent (and therefore unpredictable) because their electrical output depends on environmental conditions – the speed of the wind and how much sunlight strikes a solar panel.
The output of an electricity supply containing a high proportion of intermittent energy sources can be difficult to predict or control, which can make matching electricity supply to consumer demand problematic. Energy sources that can provide back-up to ensure electricity demand can be met – if the sun is not shining or the wind doesn’t blow – will also be required.
What's being done?
If electricity could be efficiently stored, it could be used to bolster supply when there is a danger that demand might not be met. Pumped hydroelectric storage is one solution. During periods of low demand for electricity, water is pumped into an elevated reservoir – and when extra electricity is required, the water is allowed to flow downhill past a turbine generator.
This solution is costly, however, and not many suitable sites exist in the UK to develop such storage. Researchers are looking at other ways to store electricity in bulk economically, including advanced battery technology.
What it means for the UK
The UK Government plans to increase the proportion of our electricity that is generated from renewable sources. Renewables, such as wind and solar, are intermittent energy sources. This has implications for the overall predictability of our electricity supply, and back-up sources of generation will be needed.
The Government has signed up to the European Union's Renewable Energy Directive, which commits us to generating 15% of our energy from renewable sources by 2020 – including for electricity, heating and transport. This means that some 30% of UK electricity will have to come from renewables – and a large proportion of this is expected to be as new wind turbines.
Wind turbines are an intermittent method of electricity generation. Their electrical output varies according to wind speed – beyond the control of the operating company. Increasing the contribution of intermittent energy sources such as wind to the UK electricity supply could make maintaining its reliability more of a challenge.
How the UK is responding
The UK Government's commitment to increase how much of our electricity is generated by renewables may increase intermittency of supply. We will need additional generating capacity to provide back-up for when the wind does not blow.
We will also need to ensure there is enough flexible plant in the generation mix to respond quickly to changes in the supply/demand balance. But the overall solution does not lie entirely in building new generating capacity.
Energy efficiency, the development of electricity storage, increased connection to neighbouring countries and demand-side response – where energy-intensive businesses are incentivised to reduce consumption at peaks – can all make a contribution, although none is a solution in itself.
An electricity supply from intermittent energy sources can be more reliable overall if there is diversity in the generating mix. Flexible energy sources can be called upon when less predictable ones such as solar and wind fail to supply.