The 2050 UK energy system will be completely different from today because of the commitment to net-zero carbon emissions. The dominant means of providing space heating and electricity –– natural gas — will be a thing of the past. The size of the UK electricity system is likely to double to more than 600TWh even with greater energy efficiencies. Electricity will replace fossil fuels for transport, space heating and many industrial needs, including producing ‘green hydrogen’.

Zero-carbon electricity is forecast to be dominant by 2035. By then renewables will be providing more than 60% of the supply, mostly in the form of wind and solar. The UK has good offshore wind resources to be balanced by a smaller amount of seasonal solar supply.

These dramatic changes seem to provide an opportunity for other zero-carbon forms of energy, including bio-energy and gas (both requiring carbon capture) and nuclear. How will these systems work together to keep the lights on and to minimise energy costs?

Renewable energy systems

As the share of renewable energy increases, the variability of renewable supply will be felt across the whole system. Wind and solar output is completely dependent on the weather. A completely renewable UK supply for 2050 would be generating 600TWh pa to meet an expected demand that is double that of today. But it would have an ‘energy gap’ equal to 20% of annual demand, due to the miss-timing of renewable supply versus demand.

Renewable energy system (GW) 2050 2021
Solar 87 14
Onshore wind 34 14
Offshore wind 68 10

When demand is higher than renewable supply, because there is either little wind or no sun, demand will not be met. 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 supply twice that of annual demand (1200TWh) there are still periods when demand is not met and some form of flexible backup supply or energy storage will be required. The current means of compensating for fluctuations in demand and supply is largely fossil-fuelled — mostly CCGT — which are incompatible with the 2050 net-zero emissions target. Current CCGT power plants will be retired, probably before 2040.

As well as supply not always matching demand, the wide range of power delivered by the renewable system is a problem. This excess in power could be more than 120GW, but occurring very infrequently — less than 0.2% of the time.

All the options for compensating supply that are zero-carbon have high capital costs and therefore are economic only when run constantly. The alternative is energy storage, which can compensate for renewable supply fluctuations, storing energy when output is high and releasing it when demand is high. The key questions are:

-How much storage will be required?

-What will renewables plus storage cost in the future?

Energy storage for 2050

Energy storage exists today in the UK grid. Pumped hydro (25GWh with potential for 90GWh) has been used for many years, “peak lopping” daily power demands. More recently, battery systems (now totalling 1.3GW) are being used to provide short term grid services. They balance supply and demand for periods of seconds, minutes and a few hours. Though important, these systems are much too small (total of 4GW) and store too little energy to address the storage needs of 2050, when levels of storage of 100GW and 50TWh will be required.

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, ~100GWh, and 30-50 cycles per year;

-Medium term – weekly, requiring a few TWh, and several cycles per year;

-Long term – monthly, seasonal or multi-year, requiring many tens of TWh, and one or fewer cycles per year.

Energy storage for a 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.

Options for minimising the size and cost of energy storage 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.

Economics of highly renewable systems

Energy supply technologies are often judged by their stand-alone energy costs (on a levelised cost of energy or LCOE basis). This simple cost parameter does not value the dependability of sources of electricity. This was not important when intermittent technologies were a minor part of the energy system and flexible backup supply was 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 unreliability, levelling up the cost comparison to include the costs of matching reliable supply. That would be a ‘levelised cost of shaped energy’ (LCOSE) calculated from: renewable energy costs + cost of overcapacity + storage energy costs + baseload share. It would not include any additional grid costs.

Initial calculations of LCOSE for the UK in 2050 (Roulstone & Cosgrove. (2021). Working Paper – UK Multi-year Energy Storage Systems Cost Investigation. April 2021. U of Cambridge. ) show that storage costs will increase the forecasted future renewable energy costs from £34/MWh to at least £60-70/MWh. Costs could be much higher, more than £100/MWh, if the costs of storage do not fall as predicted and the ways of sharing the store power requirements are not developed.

2050 energy system design options

Coal will be excluded from electricity by 2025 and gas will be gone by 2040. Almost any of the scenarios for 2050 depend on wind and solar. They require very large amounts – many tens of TWh — of long-term energy storage for weekly, monthly, seasonal and year-to-year backup. Providing 30% overcapacity of renewables, high power interconnectors and 25GWe of baseload would together cut the storage needs to 7.5% of annual demand — 27 days.

Alternatively, the large costs of balancing and backing up an intermittent, highly renewable system by energy storage could be reduced, but not eliminated, by two possible energy strategies for 2050, both of which would have cost impacts:

Reduce the size of the electricity system, by using large numbers of heat pumps for space heating and large amounts of natural gas to produce ‘blue’ hydrogen — ie from steam methane reforming with carbon capture and storage.

Increase the flexibility of the energy system and limit the size of wind and solar to 50% share, by providing up to 16% of the UK’s electricity (double what is proposed by the Committee on Climate Change) from biomass with carbon capture and storage and producing 33% from flexible nuclear. This would require modern light water reactors (including small modular reactors), providing an economic incentive for them to operate in a flexible manner.

Energy storage technologies

Electro-chemical: Li-ion and flow batteries etc.

High round trip efficiency of 90-95%, but with high energy storage costs – ~1,000 times chemical storage;
Physical: pumped hydro, compressed air, liquid air, thermal energy, gravitation etc.

Medium round-trip efficiencies of 45-80%, with lower energy storage costs – ~10 times chemical storage;
Chemical: hydrogen, ammonia, methane, synthetic-gas etc.

Low round-trip efficiencies of 25-42%, with very low energy storage costs.

Largest energy stores World (GWh) UK (GWh)
Li-ion battery Vistra California 1.2 Wiltshire 0.26
CAES: McIntosh Alabama 2.8 LAES: Highview 0.25
Pumped hydro Bath County 28 Dinorwig 9
Hydrogen Moss Bluff Texas 146 Teesside 27

This article first appeared in Nuclear Engineering International magazine.