Renewable energy: the growing need for storage solutions

TRUE – Renewable energies pose new challenges: intermittency and decentralisation.

Didi­er Dal­mazzo­ne. Net­work oper­at­ors, and high-voltage trans­mis­sion in par­tic­u­lar, are facing new chal­lenges with the deploy­ment of renew­able elec­tri­city gen­er­a­tion resources. The first is decent­ral­isa­tion: elec­tri­city pro­duc­tion will move from a few cent­ral­ised facil­it­ies – nuc­le­ar power sta­tions – to a mul­ti­tude of small-capa­city pro­duc­tion facil­it­ies. The second chal­lenge is the inter­mit­tency of renew­able ener­gies: the great­er their deploy­ment, the great­er the risk of an imbal­ance between sup­ply and demand. This risk is sig­ni­fic­ant because it can lead to dan­ger­ous fre­quency vari­ations for the grid, which can even res­ult in power outages.

These two chal­lenges mean that we need to increase our energy reserves, and there are two ways of doing this: on the one hand, by pro­mot­ing flex­ib­il­ity and, on the oth­er, by energy stor­age. Sta­tion­ary stor­age is there­fore essen­tial, provid­ing a vir­tu­ally instant­an­eous response in the event of an imbal­ance between sup­ply and demand. It is destined to be deployed on a massive scale.

FALSE – The proliferation of renewable energies stabilises production.

The pro­lif­er­a­tion of elec­tri­city gen­er­a­tion sources sta­bil­ises pro­duc­tion in the event of weath­er fluc­tu­ations, for example. This applies not only to renew­able ener­gies but also to oth­er pro­duc­tion meth­ods, par­tic­u­larly nuc­le­ar power in France. 

This advant­age can help to over­come some or all the inter­mit­tence prob­lem. But rely­ing solely on abund­ance to pro­tect against imbal­ances in sup­ply and demand means over­siz­ing the gen­er­at­ing fleet, which is not an eco­nom­ic­ally viable solution.

UNCERTAIN – Flexibility is another solution to the challenges posed by renewable energy.

Flex­ib­il­ity [Editor’s note: the abil­ity of an energy sys­tem to adapt to vari­ations in energy pro­duc­tion and con­sump­tion] is a com­ple­ment­ary solu­tion to stor­age. It is based on con­trolling the con­sump­tion of the largest users to pre­vent a supply/demand imbal­ance. But there is still some uncer­tainty: all the fore­casts agree that elec­tri­city con­sump­tion is set to rise, which is essen­tial if we are to meet our decar­bon­isa­tion tar­gets. This increase requires an increase in secur­ity reserves: sta­tion­ary stor­age and flex­ib­il­ity are there­fore essential.

TRUE – The necessary storage capacity cannot be achieved using pumped storage stations (PTS).

To date, two-thirds of the world’s stor­age capa­city is provided by STEPs¹ [Editor’s note: this stor­age sys­tem is based on the prin­ciple of grav­it­a­tion­al energy: the water con­tained in two water reser­voirs at dif­fer­ent alti­tudes is released when needed to power elec­tri­city-gen­er­at­ing tur­bines; it is pumped back to the upper reser­voir when there is a sur­plus of elec­tri­city production].

In France, this fig­ure is as high as 95%, and there is little scope for increas­ing our capa­city. RTE, the French elec­tri­city trans­mis­sion sys­tem oper­at­or, estim­ates that oper­a­tion­al reserve require­ments will be between 4 and 9 GW in 2050 (depend­ing on the pro­duc­tion scen­ario²), com­pared with almost 3 GW today. Accord­ing to the Inter­na­tion­al Energy Agency (IEA), glob­al stor­age capa­city should be mul­ti­plied by 6 between 2023 and 2030³.

FALSE – There are other types of storage, such as stationary batteries, that could rapidly increase global capacity.

The IEA estim­ates that 90% of the growth needed – a six­fold increase between now and 2030 – will come from sta­tion­ary bat­ter­ies. This is a fully developed solu­tion that is grow­ing rap­idly thanks to the fall in its cost over the last ten years or so. Bat­ter­ies do, how­ever, pose the chal­lenge of the scarcity of cer­tain crit­ic­al mater­i­als (cobalt, cop­per, nick­el, etc.).

UNCERTAIN – Other storage technologies exist or are being developed, and certain sectors could develop.

Oth­er means of stor­age do exist. Com­pressed air stor­age, for example, could be developed: this is based on stor­ing com­pressed air in under­ground cav­it­ies. How­ever, this tech­no­logy suf­fers from effi­ciency prob­lems – some of the energy is lost through thermal dis­sip­a­tion of the com­pressed air – and the indus­tri­al sec­tor is not yet fully developed. Elec­tri­city can also be con­ver­ted into hydro­gen, known as power-to-gas. But this meth­od of stor­age is con­tro­ver­sial, not least because the con­ver­sion effi­ciency is low (30 to 40% at best). Finally, elec­tri­city can also be stored in the form of heat. Heat trans­fer flu­ids, phase change mater­i­als or chem­ic­al reac­tions can be used. The advant­age of these tech­no­lo­gies is that they can store energy over the long term, over sev­er­al sea­sons, unlike bat­ter­ies, which dis­charge quickly. A hydrated salt stor­age pro­ject will be launched on the cam­pus of Insti­tut Poly­tech­nique de Paris.

UNCERTAIN – The production of storage resources has an environmental impact.

Of course, the pro­duc­tion of stor­age tech­no­lo­gies has an envir­on­ment­al impact. STEPs require the con­struc­tion of dams: con­sid­er­able quant­it­ies of con­crete are pro­duced, and entire val­leys are drowned, impact­ing biod­iversity and loc­al pop­u­la­tions. Bat­ter­ies con­tain many metals, and we know the envir­on­ment­al impact of extract­ing and refin­ing them, and there is no bat­tery recyc­ling sys­tem yet. But to assess wheth­er this impact off­sets the car­bon emis­sions avoided by repla­cing fossil fuels with renew­able ener­gies, full life-cycle ana­lyses would have to be car­ried out. Wind and photo­vol­ta­ic ener­gies have much smal­ler car­bon foot­prints than coal, oil or even gas. It is highly unlikely that stor­age will reverse this balance.

FALSE – The use of used batteries from electric vehicles for stationary storage has been mentioned as a solution.

When an elec­tric vehicle’s bat­tery falls below 80% of its rated capa­city, it must be replaced. It is pos­sible to give them a second life by using them for sta­tion­ary elec­tri­city stor­age, an oper­at­ing mode that places less strain on the bat­tery. This would improve the envir­on­ment­al bal­ance of sta­tion­ary elec­tri­city stor­age. But this solu­tion remains con­tro­ver­sial because of poten­tial safety prob­lems. We still lack feed­back on these bat­ter­ies at the end of their life.

Anaïs Marechal