Energy Storage - batteries and beyond

The European Commission has approved €2.9 billion (£2.5bn) of public investment in a research project for batteries. The so-called European Battery Innovation project will cover the whole batteries’ ecosystem from the extraction of raw materials, design and manufacturing of battery cells and packs and the recycling and disposal in a circular economy. 

It is clear that batteries are going to be in increasing demand, for power system backup and also vehicle use, so it is good to see that the environmental and resource implications are being looked at. But they can be severe and may constrain the extent to which electric vehicle use can expand.  For example, in terms of resources, it has been claimed that to replace all UK-based vehicles with electric vehicles (LGVs & HGVs excluded), even using the most resource-frugal batteries, ‘would take 207,900 tonnes cobalt, 264,600 tonnes of lithium carbonate, at least 7,200 tonnes of neodymium and dysprosium, in addition to 2,362,500 tonnes copper. This represents, just under two times the total annual world cobalt production, nearly the entire world production of neodymium, three quarters the world’ s lithium production and at least half of the world’ s copper production during 2018.’ 

There may be ways to avoid these limits, for example via recycling, materials substitution and new resource finds, but hydrogen fuel cell-powered vehicles, with on board hydrogen storage, may be a better bet, at least for some road and aircraft applications, using zero carbon green hydrogen produced by electrolysis powered with renewable electricity. Hydrogen has high energy density by mass but low energy density by volume, so gaseous hydrogen storage is not too attractive an option for vehicles.  However, cryogenic liquid hydrogen storage and chemi-absorbed metal hydride solid state stores are possible options. 

Hydrogen storage of various types, using green hydrogen, could also replace batteries in other key uses. Some domestic PV users are adding batteries for night time power, and, on a larger scale, some utilities are adding big battery arrays to help with grid stability.  But although the power-to-gas and gas-to-power conversion efficiencies need to be improved, hydrogen systems could take over some of this role, and also offer longer term bulk storage options.

Storage duration

For the moment though Lithium Ion batteries rule the roost for short-term storage, although there is a lot of development work going on, some aimed at finding alternatives to Li Ion batteries, including sodium batteries and vanadium binary flow batteries, some of which can be very large. However, even leaving aside the environmental and resource issues, batteries, of whatever sort, are unlikely to be any use for large scale long-term grid support- for example dealing with week-long lulls in renewable generation.  Even pumped hydro, the most common large scale power storage option at present, would find that hard, depending on the scale of the plant and the demand.  

The main long-duration storage options are compressed air and hydrogen gas- stored in bulk in caverns underground. Both clearly are geographically determined- as is pumped hydro. By contrast liquid air cryogenic systems offer a neat intermediate scale/duration option which can be set up anywhere. Pumped hydro storage is well established (over 120GW globally), but all the others are novel ideas, with just a few small projects or test beds, though a lot more are planned or are being developed. For example, Highview has a 5MW liquid air store in Manchester which has worked well and a 50 MW unit is planned, to deliver 250 MWh over 5 hours. RheEnergise have also come up with an upgraded version of pumped hydro, using a heavier fluid than water, pumped up hill to a holder tank in a closed system. Also in the UK, ITM Power is involved with an ambitious PEM cell hydrogen production system using power from an offshore wind turbine, with the resultant gas planned to be stored underground under pressure in a salt cavern.

Although not aiming for long-duration power storage, there also intriguing gravity power system being developed in the UK by Gravitricty, based on dangling weights down a mineshaft- they are winched up and then allowed to fall when power is needed. In case that sounds too oddball, a report from Imperial College London said that electricity released by a typical 10 MW lithium-ion battery would cost £283/MWh over its lifetime, compared to £132/MWh from a similar capacity Gravitricity project. Estimated costs for as yet unbuilt systems have to be treated cautiously, but, for comparison, Highview has said that power from its cryobattery would cost around £110 per MWh, using a 200 MW system, making it one of the cheapest storage options. At £70/MWh, according to one estimate, salt cavern hydrogen storage of hydrogen gas may be cheaper, but that system depends of the availability of cheap, clean hydrogen and at present green hydrogen is not cheap- although it's getting there. 

Storage futures 

Given that, even if we try to reduce energy use and switch to green energy sources, we will still need storage capacity to balance variable supply and demand, and there is still all to play for in the energy storage field, especially since, as can be seen, there are options other than just storing electronic charge in batteries. In addition, heat can be stored very efficiently and at scale. The are some community-scale heat stores using hot water to capture summer solar heat for winter use in Denmark and molten salt heat stores are used with large Concentrated Solar Power plants is some desert areas to allow for continued operation overnight. And more generally, heat storage may be useful for energy balancing in total energy systems, including CHP/district heating systems. 

Whatever the medium can however be expensive, especially post generation storage of electricity.  But, going back to basics, storing energy as methane gas before combustion for lager power generation is still one the cheapest energy options, and if biogas or non fossil syngas/green hydrogen is used, then this route can deliver carbon neutral electricity with no need for CCS. 

However it is done, storage can provide a way to balance variations of supply and demand more efficiently, but it gets even easier if demand can be reduced. Taming transport demand and reducing emissions is a key issue for sustainability, so high efficiency public transport is favoured, but sometimes it is argued that, since there is also demand for private transport, Battery Electric Vehicles (BEVs) are part of the way ahead. Moreover, it is claimed that their batteries can provide a ‘vehicle to grid’ power balancing/storage facility to drawn on. Well maybe, but it will only be significant if there are a lot of EVs and they can supply power to the grid when needed, rather than being a drain on the grid when being charged.  That may not always be the case, and, given the resource issues with batteries, we may have to the limit BEV use, rather than promote its V2G benefits. A heretical thought!