Effective energy storage: The essential enabler for affordable Net Zero

Seamus Garvey, professor of dynamics in the Faculty of Engineering at University of Nottingham argues that there is no energy crisis and there never was one.

At any one time, the sun throws down over 10,000 times more energy onto the earth than we humans consume. All we need do is collect a tiny fraction of that energy and deliver it when and where it is wanted.

At COP27 delegates are focussing on the topic of energy, including renewable energy (RE) and energy transformation. As the world continues its journey to net zero, there has been a large push on transitioning from fossil fuel to RE resources – but this is not the full solution.

Most countries have more than enough RE to supply their energy requirements. The UK, for example, could generate five times more electricity from offshore wind in an average year than it consumes, and has potential for solar, tidal, wave and geothermal power too. RE is already cheaper than using fossil fuels, if you only consider the cost per unit of electricity generated. In part this is because energy carried in the wind, sun, and sea delivers itself – whereas fossil fuels don’t. Unfortunately, the times RE resources are available often do not automatically match when we most need energy.

To prepare for a future in which renewables provide a high proportion of our electricity, significant attention needs to be given to balancing supply and demand. Achieving this will account for more cost than simply buying and operating the primary RE harvesters. Collectively, the set of measures delivering this balancing are known as flexibility.

Fossil fuels provide flexibility intrinsically but, in a future free of these fuels, there will be four main contributors – storing energy, adjusting demand to better exploit supply, spreading energy geographically and turning down some available RE.

These broad classes will feature to, at least, some extent in a cost-optimal energy system but the relative prevalence of each will be determined by their costs. For example, if energy storage could be implemented with high performance and low cost, including considerations about locations, then it would naturally dominate as a solution.

We already know that energy storage has a large part to play and will be deployed over a range of times, energy capacities and power ratings, with specific solutions used for different purposes.

Batteries will play a key role in stationary energy storage, providing high power levels for short periods (up to four hours) and being distributed across the grid to alleviate short-term congestion. Meanwhile, hydrogen, made from electrolysis, will play a vital but hugely different role, providing colossal stored-energy capacity in caverns to even-out the long-term (interannual) variations in demand and RE generation.

It is perfectly feasible to develop future energy systems based on batteries and hydrogen alone – and some policymakers are coming to conclusions on this basis. However, such systems would be unnecessarily expensive.

It is not difficult to predict what flexibility will be required in net-zero energy systems. Electricity demand has strong daily and weekly variations, and RE resources differ over timescales between hours and days, periods that are longer than suits batteries and shorter than makes sense for electricity-to-hydrogen-to-electricity. Examine any one plot of Net Demand and you will find that more than 90% of the energy in the positive half-cycles (periods when demand exceeds RE supply continuously) falls within timespans between four and 200 hours.

Most technologies best suited to these timescales are thermo-mechanical in nature. These include compressed air energy storage, pumped-thermal energy storage, liquid air energy storage, gravitational potential energy storage solutions, including pumped-hydro, and some recently developed energy storage systems based on CO2 liquefaction and re-gasification in a closed cycle.

These technologies have lower cost-per-unit energy storage capacity than batteries and have better turnaround efficiencies and cost-per-unit-power than hydrogen. Therefore, it is clear that these systems have a place, but they have been comparatively under-represented in policy and general discussion so far.

A common misconception is that energy storage must be charged from, and discharged to, electricity. It is highly advantageous, in some cases, to store energy prior to generating electricity and in other cases to store what electricity is used to produce rather than storing electricity.

We have major opportunities to unlock when it comes to integrating storage with wind turbines or forming fuel or chemical feedstocks from electricity. Our net zero future isn’t technically difficult – it simply relies on us avoiding unnecessary expense by developing the best energy storage solutions possible.

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