Challenges for the Grid
- Power generated by renewable generators is largely intermittent, cannot be relied upon and requires back up to be truly useful, and
- The amount of excess power generation capacity is low and is set to decline further as the old coal fired, gas and nuclear fleet is decommissioned over the next 10-15 years.
The images below show the profile of power supplied by different types of renewables. This illustrates the challenge that renewables bring to managing the National Grid.
Part intermittent supply - hydropower
“Low head” or run of river hydropower (green line) delivers power from the rains in autumn through to late spring/early summer, when river flows slow.
“High head” hydro (blue line) using flows from smaller catchment areas higher in the hills with longer drops, is more “peaky” generating power over the winter.
Hydropower is a good seasonal fit to UK’s power needs but the number and size of rivers on our small island limit scale. Hydropower is a technology established over millennia, is already over 80% efficient at converting potential energy to electricity, and once built is never redundant. Government incentives, such as the “Feed in tariff” are designed to reward new technologies as they become more cost-effective. They do not recognise that hydropower is a mature technology and no reduction in capital costs is possible. In fact the easiest to build hydropower sites have been built – so future hydropower stations are likely to be more difficult and expensive to build.
Daily intermittent supply: Solar PV
Solar PV generates power predictably during the day, especially when aggregated across the UK. It generates much more power in the Northern hemisphere during the summer months (red line) when the angle of the earth is faces the sun more directly and much less during the winter months (blue line). Despite these variations, solar PV is a very dependable source and so is an important source of capacity on which the National Grid can rely.
Random & Intermittent supply: Wind
Wind provides the Grid with the biggest challenge as it is random, intermittent and has sudden changes in output. The graph above shows Wind output across all of Germany over 4 months in 2011 – this isn’t output of a single wind turbine! These characteristics make Wind the hardest form of renewables to integrate into a stable power supply.
UK energy transition to a unmanageable future?
This schematic shows the changing mix of power supply over the last 10 years and where it is predicted to go in the next 30 years (Source: National Grid Future Energy Scenarios 2019)
Coal (light grey) was the main power provider (64%) in 1980, now provides less than 7% and the first coal free day since the 1880’s was on 21st April 2017. The last coal-fired power stations are scheduled to close over the next 7-10 years ending more than a century of dependence on coal.
Oil (dark grey) was second most important in 1980 (18%) has been replaced by gas (green, 40%) which is currently the dominant source of power generation.
Nuclear has stayed roughly constant over the period (14% to 19%) but is likely to shrink in importance as the 1960’s fleet of nuclear power stations is decommissioned before any ‘new nuclear’ is ready, scheduled for 2030.
In this period, the mix of UK power has changed and traditional generators providing steady, stable baseload power now represent only 66% of total power generation capacity.
Since 2000, wind and solar (17%, blue) have both increased capacity dramatically bringing intermittency to the mix of power generation.
The total electricity consumed by UK only grown by around 20% over 35 years from 250TWh to 300TWh (2016); however the “capacity reserve margin” available to the National Grid has declined since the days of the nationalised electricity industry from about 35% in 1980 to only 4% in 2016 – and effectively zero over the winter 2016/17.
This small cushion of excess capacity relies on interconnectors from Europe and intermittent renewables – which need back up to provide security of supply. The UK also has committed to cut carbon emissions by 80% by 2050 and expects to decommission 2/3rds of existing generation capacity by 2030.
National Grid’s Projected Storage Needs
Flexible Technology Comparison
Batteries are the best placed to gain from volatility
There are several types of standby power generation available to the grid, each of which can be classified in terms of its response time, the longevity of its ability to deliver power, the efficiency of a power cycle and how “green” it is.
- Pumped storage. The largest example of pumped storage is the hollow “electric mountain” at Dinorwig in Snowdonia which has 1.7MW of storage capacity. Water is pumped uphill when there is excess power available to the grid and allowed to flow downhill under gravity to generate power during periods of high demand for power. Dinorwig can reach maximum output within 75 seconds, provide power for six hours at an efficiency of 75%, is carbon neutral and can provide ‘black start’ services to the National Grid in the event of grid failure;
- Gas turbines are efficient standby generators but owners need them to be running and earning a return on investment full time. The zero marginal cost of renewable energy, when available, has seriously undermined the economics of gas turbine generation. Gas turbines can respond in 9 seconds if warm, 30 seconds from cold. They are considered to be “low carbon”, and
- Batteries provide near immediate standby power to the grid (less than half a second) and so are useful in reducing volatility and balancing short-term changes in aggregate supply and demand. Batteries can respond for between 15 minutes to several hours depending on how they are configured, are 87% efficient and can provide a range of other services to the grid.