Challenges for the Grid
The United Kingdom needs to modernise and decarbonise its energy system and relies upon the National Grid to manage the network in the most efficient way possible. The two main challenges in delivering cost competitive, stable power to consumers are that:
- 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 electricity supply, demand and excess capacity: 1980 - 2017
This schematic shows the changing mix of power supply over the last 37 years.
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 Infrastructure Commission
The National Infrastructure Commission was asked by the Government to come up with a solution to these problems. The report published in March 2016 had three main recommendations:
- Increase interconnector capacity to Europe, Norway and Iceland,
- Make full use of demand flexibility (essentially reducing or shifting consumption during peak periods), and
- UK should become a world leader in storage systems with a legal and regulatory update to remove barriers to storage by Summer 2017. Ofgem should encourage the use of storage capacity to support intermittent renewables and improve network capacity and resilience.
UK power supply at a glance (real time link)
The Gridwatch website (see www.gridwatch.templar.co.uk) shows the real time mix of sources of power being generated in UK.
On the morning of Monday 27 February 2017, shown above, the main dials (from left to right along the top) show:
- Total power generated = 41.8 GW (gigawatt = 1,000,000 x kW)
- Frequency of the grid = 49.5 Hertz
- Coal = 3.62 GW (9%)
- Nuclear = 7.51 GW (18%)
- Gas (CCGT or Combined Cycle Gas Turbine) = 20 GW (47%), and
- Wind = 3.15 GW (8%)
The profile of daily, weekly, monthly and annual demand is shown in the first column of charts. The pattern is similar most days with peak consumption being between 1600 and 1900hrs.
The interconnectors from Europe are shown in the second row of smaller dials at top right; French 1.5 GW, 3.6% and Dutch 0.9 GW, 2.2%.
These represent an important source of additional power supply to UK, but they also import volatility. For example, when Germany has too much wind, or no wind, supply changes dramatically. When French nuclear has unscheduled maintenance, supply changes rapidly.
The top row of small dials shows the contribution of renewables on this particular day in February 2017:
- Pumped 0.6GW (1.3%)
- Hydro 1.4Gw (3.2%)
- Biomass 2 GW (5%)
- Solar 1 GW (2.5%)
Hovering our cursor over the “Pumped” dial provided a helpful pop-up box.
The last sentence is the most important.
“They represent the nearest thing to “storage” that is attached to the grid”.
The challenge of managing the grid is to have:
- a diversity of sources of low cost power generation,
- that comply with our goal to decarbonise supply by 2050,
- with sufficient flexibility to respond to both changes in supply (when the wind stops blowing for example) and changes in demand.
While “filling in the gaps” of intermittent power generation is simple in theory the cost of building and keeping most types of traditional generation on standby is very high. Renewables have a very low marginal cost of generation and this, while good for power consumers, isn’t helpful in supporting investment in standby generation or storage.
National Grid’s Projected Storage Needs
The National Grid projects that it will require substantial storage and /or standby power generation in order to stabilise the frequency of the grid and to help match the supply and demand for power over the next five years. Today the Grid has procured up to 600MW of Frequency Response capacity, a figure that is expected to grow to 3GW over 5 years. Seasonal variations increase this to between 5GW and 8GW by 2022 as shown in the graph above.
Standby power generation
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;
- Diesel Generator sets, are attached to the grid at many locations and represent the largest part of the National Grid’s 2.25GW Short Term Operating Reserve. They can respond within 2 minutes and run indefinitely, subject to availability of diesel. However, diesel is about the last source of power we would choose to have powering the grid as it is not green at all….
- 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 88% efficient and can provide a range of other services to the grid.