Salt caverns: the key to storing hydrogen?

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.

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.

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.

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.

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