Tony Roulstone explores the drivers of change
By 2050 the UK energy system will be completely different from today because of the Government’s commitment to net-zero carbon emissions. The dominant means of providing both space heating and electricity – natural gas – will be outlawed by 2040. Electricity will replace fossil fuels for: transport, space heating and many industrial needs, including producing ‘green hydrogen’. As a result, the size of the UK electricity demand will double to more than 600 TWh, even with greater energy efficiencies.
Zero-carbon electricity is forecast by the Prime Minister to be dominant by 2035 – with renewables providing more than 60% of supply – mostly wind and solar – based largely on our very good offshore wind resources, balanced by smaller amounts of onshore wind and seasonal solar.
These dramatic changes in energy provide an opportunity for other zero-carbon forms of energy, including: bio-energy and gas, both with carbon capture; and nuclear. How will these systems work together with highly variable renewables to keep the lights on and to minimise energy costs?
Renewable energy systems
As the share of renewable energy increases, the inherent variability of renewable supply will be felt across the whole system. Wind and solar output is completely dependent on the weather, its fluctuations and its uncertainties. A completely renewable UK supply for 2050 that generates 600 TWh pa to meet the expected doubled demand would, due to the mistiming of supply versus demand, have an energy gap of 20% of annual demand. When demand is higher than renewable supply, demand will not be met – because there is either little wind or no sun. On the other hand, when supply is higher than demand, energy will be wasted. Increasing the amount of wind and solar above that required to provide the nominal annual demand – overcapacity – reduces the supply gap, but only slowly. Even for renewable supplies twice that of annual demand (1,200 TWh) there remain periods when demand is not met and some form of flexible back-up supply, or energy storage will be required. The current means of compensating for fluctuations in demand and supply are largely fossil-fuelled – mostly CCGT - which are incompatible with the 2050 net-zero emissions target. The current CCGT power plants will be retired, probably before 2040.
As well as supply not always matching demand, the large fluctuations of power delivered by the renewable system will be a problem. This power difference could be more than 120 GW, but occurring very infrequently - less than 0.2% of the time.
All the options for zero-carbon compensating supply have high capital costs. They are cost effective only when run constantly. The alternative is energy storage. It can compensate for renewable supply fluctuations, storing energy when output is high and releasing it when demand is high. The key questions for storage are: how much storage will be required and what will renewables plus storage cost in the future?
Energy storage for the 2050 grid
Energy storage exists today in the UK. Pumped hydro-electricity (current volume 25 GWh with potential for 90 GWh) has been used for many years “peak lopping” the daily power demands. More recently, battery systems (up to 1 GWh) are being used to provide short term grid services, balancing supply and demand for periods of seconds, minutes and a few hours. Though important, these system are much too small (total of 4 GW of power) and store too little energy to address the storage needs of 2050, which will be of the order of 100 GW and 50 TWh.
There is a wide choice of energy storage technologies. Profiles of demand and supply over many years show several different storage needs:
• Short-term – daily with a relatively small volume, ~100 GWh, and 30-50 cycles per year;
• Medium term – weekly with a larger volume, a few TWh, and several cycles per year;
• Long term – monthly/seasonal/multi-year, many tens of TWh, and one or fewer cycles per year.
The size of energy storage required for a largely renewable energy system will be large – many tens of thousands GWh – equal to more than 20 days of demand – and a thousand times the size of planned UK electricity storage, including the batteries in electric vehicles. Also, it will be much bigger than the largest energy stores in the world of the most significant storage technologies.
Because energy storage is a stand-by system, it will be important to minimise its size and therefore cost. The main ways of achieving this include: getting the right mix of solar and wind; providing extra renewable capacity; choosing the right mix of storage technologies - trading-off higher efficiency versus costs and providing a share of baseload supply. Initial studies show for a 70% renewable supply with the balance from baseload reliable supplies, 30% overcapacity cuts the storage requirement in half, from 100 TWh to about 50 TWh – 7.5% of annual demand or 27 days of energy storage. Optimising the power capacity of energy storage is also key.
Economics of highly renewable energy systems
Energy supply technologies are often judged by their stand-alone energy costs (LCOE). This simple cost parameter does not value the dependability of sources of reliable electricity. This was not important when intermittent technologies were a minor part of the energy system and large amounts of flexible back-up supply were available. In the future that will not be the case.
A fairer and more transparent way to compare technologies is to include the cost of compensating for their unreliability, levelling-up the comparison to include the costs of matching reliable supply – levelised cost of shaped energy (LCOSE) that includes:
LCOSE = Renewable energy costs + Cost of overcapacity + Storage energy costs
+ Baseload share – but not including any additional grid costs.
Initial calculations of LCOSE for the UK in 2050 show that storage costs will increase the forecast future renewable energy costs in 2040 from £34/MWh to at least £60-70/MWh. Shaped energy costs could be much higher - more than £100/MWh - if capital costs providing of storage do not fall as hoped and if ways of sharing power between stores are not developed and applied.
2050 energy system design options
Coal will be excluded from electricity generation by 2025. Gas will be gone by 2040. Almost any of the scenarios for 2050 depend on a majority of wind and solar. The grid will need very large amounts – many 10s of TWh – of long-term energy storage for – weekly, monthly, seasonal and year-to-year backup. The means of reducing storage needs by half from 100 TWh to 50 TWh (7.5% or 27 days of mean demand) would include providing: 30% more renewables than required, supported by high power interconnectors from the Continent and 25 GWe of baseload zero-carbon supply.
Alternatively, the large costs of balancing and backing up such a highly renewable system with energy storage could be reduced, but not eliminated, by other possible energy strategies, each of which would have major cost impacts:
1) Reducing the size of the electricity system, by using very large numbers of heat pumps for space heating and large amounts of natural gas to produce hydrogen by steam methane reforming - with carbon capture and storage (‘blue hydrogen’).
2) Increasing the flexibility of the energy system and limiting the size of wind and solar to 50% share, by providing both:
a. A substantial part of the UK’s electricity - 15% 100 TWh, perhaps double what is proposed by the Committee on Climate Change from burning largely imported biomass and capturing and storing the CO2 emitted from combustion (BECCS).
b. A much larger share of energy - 33% 200 TWh, from new more flexible nuclear by constructing 25 GWe (four times current capacity) of modern LWR reactors (including small modular reactors), and providing an economic incentive for them to operate in a more flexible manner.
We can be sure that this will be an energy revolution. The choice of storage and zero-carbon technologies will depend on their ability to radically reduce their capital costs and to ramp-up their production volumes in order to completely transform the UK energy system within the next 20 years.
Tony Roulstone established and teaches on the Nuclear Energy Masters programme in the Department of Engineering at the University of Cambridge.
 Climate Change Committee (2019). Net Zero: The UK’s contribution to stopping global warming. May 2019.
 Cosgrove (2021). RS Working Paper on multi-level, multi-year storage. U of Cambridge. April 2021. http://dx.doi.org/10.13140/RG.2.2.12555.41760
 Roulstone (2021). ). UK need for energy storage. University of Cambridge. September 2021. http://dx.doi.org/10.13140/RG.2.2.32473.03680
 Roulstone & Cosgrove. (2021). Working Paper – UK Multi-year Energy Storage Systems Cost Investigation. April 2021. U of Cambridge. . http://dx.doi.org/10.13140/RG.2.2.12555.41760
 World Energy Outlook (2020) IEA. www.iea.org/weo