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Salt caverns: the key to storing hydrogen?

Pierre Berret
Pierre Bérest
Emeritus Professor at École Polytechnique (IP Paris)

Pro­jec­tions show that by 2050 the ener­gy con­sumed in Europe will be divid­ed equal­ly between elec­tric­i­ty and hydro­gen – replac­ing fos­sil fuels that cur­rent­ly account for the largest share. It is there­fore crit­i­cal for us to start get­ting a hold of how to man­age this gas on a large scale, now. In par­tic­u­lar, the entire chain hydro­gen ener­gy (pro­duc­tion, trans­port, dis­tri­b­u­tion and end-use) will require sig­nif­i­cant stor­age capac­i­ty, par­tic­u­lar­ly if the pri­ma­ry pro­duc­tion meth­ods are inter­mit­tent, as is the case for wind tur­bines and solar panels.

To do this, we will need a sys­tem capa­ble of con­nect­ing the pro­duc­tion equip­ment, such as elec­trol­y­sers, with the dis­tri­b­u­tion net­work. This sys­tem will need to be able to store hydro­gen en masse when it is pro­duced and con­se­quent­ly make it avail­able when the grid requires. To achieve this, we pro­pose to store H2 in deep saline cav­i­ties, and with the hydro­gen indus­try gain­ing momen­tum, we esti­mate that by 2050 Europe could need sev­er­al hun­dred of them.

An existing solution

Salt cav­erns are wide­ly used in Europe today, par­tic­u­lar­ly for stor­ing methane (‘nat­ur­al gas’). They take advan­tage of the pres­ence of salt lay­ers or domes in the sub­soil sev­er­al hun­dred metres thick that extend over large areas. In France, the total exten­sion of salt cav­erns is cur­rent­ly the order of 20,000 km2. The salt, which is not very per­me­able, can be eas­i­ly dis­solved to cre­ate cav­erns with oil wells to dig the rock down to the salt for­ma­tion. Then, after some time, a cav­ern with a typ­i­cal size of 500,000m3 is avail­able. In the­o­ry, we could sim­ply use these pre-exist­ing cav­erns to store hydro­gen. In the­o­ry, they would be able to stor­ing around 6,000 tonnes of hydro­gen at vary­ing pres­sures of 6–24MPa – oper­at­ed like an oxy­gen tank for diving.

Today in France there are about thir­ty gas stor­age cav­i­ties, spread over three sites. Some have been in oper­a­tion for about fifty years. There are sev­er­al hun­dred in the world, includ­ing a few hydro­gen stor­age cav­erns already used by the chem­i­cal indus­try. Sev­er­al indus­tri­al pilot projects for the pro­duc­tion and use of hydro­gen are organ­ised around some of these exist­ing salt cav­erns. In France, there is the Hyp­ster project led by Storengy in Etrez and sup­port­ed by the Euro­pean Union, Hygéo by Tere­ga, HdF and BRGM in Car­resse-Cass­aber and Hygreen led in Manosque by Storengy-Geo­stock. If these projects exist, it is because there are still some chal­lenges to over­come if we are to achieve large-scale hydro­gen stor­age in salt caverns.

1/ Preventing leaks 

Even for con­ven­tion­al struc­tures, social accept­abil­i­ty is the major chal­lenge posed by the con­struc­tion of new large-scale ener­gy facil­i­ties (nuclear pow­er plants, dams, wind farms). As such, it is up to the design­ers to iden­ti­fy and explain to the pub­lic the par­tic­u­lar­i­ties of the struc­tures from the point of view of their safe­ty and the solu­tions pro­vid­ed. Salt cav­erns are no excep­tion. Since the hydro­gen mol­e­cule is par­tic­u­lar­ly mobile, the major prob­lem is the seal­ing of the met­al access shaft, which is sev­er­al kilo­me­tres long. A num­ber of acci­dents or inci­dents from the past are known and well described in the 2,000 or so salt cav­erns for the stor­age of liq­uid or gaseous hydro­car­bons in oper­a­tion around the world. For the most part, these inci­dents occurred a long time ago, as the indus­try pro­gres­sive­ly adopt­ed a prin­ci­ple known as the “dou­ble bar­ri­er” where­by the pres­sure is con­tin­u­ous­ly mon­i­tored to detect when the gas has breached a first bar­ri­er; there­by avoid­ing the risk of breach­ing the second.

Anoth­er essen­tial check is the “leak test”. This con­sists of low­er­ing a col­umn of nitro­gen a lit­tle above the roof of the cav­ern and mon­i­tor­ing the evo­lu­tion of the gas-brine inter­face: a rapid rise is a sign of poor seal­ing. On the scale of the cave, the well is a very thin cap­il­lary, and the sys­tem resem­bles an extreme­ly sen­si­tive barom­e­ter or ther­mome­ter. The chal­lenge is to track down small leaks, of the order of 10-4/year of the stored vol­ume. An abun­dant lit­er­a­ture, to which the Sol­id Mechan­ics Lab­o­ra­to­ry (LMS) has made a major con­tri­bu­tion, is devot­ed to this test and we can expect, with hydro­gen stor­age, impor­tant devel­op­ments con­cern­ing the method, peri­od­ic­i­ty, and accept­abil­i­ty criteria.

2/ Controlling the behaviour of salt

Over large time scales, salt is a vis­cous liq­uid and there­fore any cav­i­ty will grad­u­al­ly close. The result­ing annu­al loss of vol­ume must remain below one per­cent to pre­vent the stor­able vol­ume from decreas­ing too quick­ly, espe­cial­ly for the deep­est cav­i­ties. But also because it can cause dam­age to the wall or the salt-access shaft inter­face. The descrip­tion of the behav­iour of salt is old but has been pro­found­ly renewed; Rock Physics estab­lish­es that there is a spe­cif­ic mech­a­nism of defor­ma­tion under very low stress­es, which is dif­fi­cult to mea­sure because the defor­ma­tion speeds involved are of the order of 10-12/s. How­ev­er, salt is also a brit­tle mate­r­i­al, which is sus­cep­ti­ble to break­age – par­tic­u­lar­ly under the effect of a sud­den change in mechan­i­cal load. Sud­den vari­a­tions in stock and there­fore in pres­sure can be expect­ed if the cav­i­ties are fed by high­ly inter­mit­tent hydro­gen pro­duc­tion and must sat­is­fy a demand that is also discontinuous.

3/ Understanding gas thermodynamics

The uses of hydro­gen for mobil­i­ty requires extreme puri­ty. How­ev­er, the gas will have to remain for a long time in a cav­i­ty at the bot­tom of which there are thou­sands of m3 of brine. The brine con­tains sul­phates from the anhy­drite (H2S) fre­quent­ly asso­ci­at­ed with under­ground salt. The gas is wet and loaded with var­i­ous impu­ri­ties includ­ing H2S, which is par­tic­u­lar­ly harm­ful for down­stream gas uses. Purifi­ca­tion can be a major expense.

Final­ly, giv­en its large size, the cav­ern is a com­plex ther­mo­dy­nam­ic machine – one can speak of cave mete­o­rol­o­gy with rain, snow, and tem­per­a­ture inver­sion, made very exot­ic by the very large changes in pres­sure. The dri­ving force behind this machine is the pres­ence of a nat­ur­al tem­per­a­ture gra­di­ent in the sub­soil; it gen­er­ates intense con­vec­tion of gas in the upper part of the cave, coun­tered in the low­er part by the main­te­nance of a rel­a­tive­ly cold tem­per­a­ture of the brine result­ing from the suc­ces­sion of episodes of vapor­i­sa­tion and con­den­sa­tion of the water. These phe­nom­e­na, high­light­ed by the LMS with Storengy, have con­se­quences for the puri­ty of the gas extract­ed that have yet to be ful­ly explored.

Interview by James Bowers


Pierre Berret

Pierre Bérest

Emeritus Professor at École Polytechnique (IP Paris)

A graduate of the Ecole Polytechnique and the Ecole des Mines de Paris, where he obtained his doctorate, Pierre Berest was head of the underground safety department at the French Ministry of Industry. From 1981, he became a researcher at the Ecole Polytechnique, at the Solid Mechanics Laboratory (LMS), of which he was the director and then associate professor in structural mechanics at the Ecole Polytechnique. Pierre Berest was also an advisor to the CNRS Science for Engineering Department (SPI) and chaired the Scientific Committee of the LCPC (now Ifstar). From 2011 to 2014, he was coordinator of the SACRE consortium dedicated to adiabatic compressed air storage (CAES) and supported by the French National Research Agency (ANR). Expert of the Scientific Council of the French Petroleum Institute (IFPen) for 10 years, Pierre Berest is the author or co-author of 200 articles in the field of continuum mechanics applied to underground works in mines, tunnels, underground storage of gas, oil, CO2 or nuclear waste.

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