CA1062026A - Underground storage for cold and hot products and methods for constructing same - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
The present invention provides a reservoir for use in storing a product in a reservoir at a temperature which differs from the natural ambient temperature of the surrounding walls, floor, and ceiling of said reservoir and its surroundings, includ-ing a circulating system comprising a plurality of uniformly distributed and parallel passages disposed along and adjacent the surfaces of said reservoir, means to pass a circulating fluid medium in said system, means to add swelling sealants, substances and water to the circulating system, means to remove water from the same, means to introduce and remove product from storage, means to introduce and remove cooling media from the circulating system, means to detect leaked-in products in the circulating system, and means to recover products out of same and a water drainage system.

Description

lO~'~OZ6 The present invention relates in particular to the underground storage of products whose storage temperature as a rule differs from the natural temperature of the underground s~rroundings in which the storage is located. In one aspect it t relates to a method of controlling the temperature of the walls, floor, and ceiling of said underground storage, this storage often ~j being located in rock, and keeping the temperature of these sections within a determined range or at a stipulated figure, using preferably a circulating stream of gas or in some cases a liquid as a medium, which functions as a vehicle for the trans-portation of heat to or the removal of heat from the mentioned storage walls, floor, and ceiling. As a consequence of said ;~ temperature control this invention provides a possibility of !
establishing a temperature barrier around the area of the circulation system envisaged in this paper, said barrier reducing the ice sublimation process sufficiently for all practical pur-poses. In still another aspect this invention relates to a metho~
of removing water or other substances from or adding the same to the walls, floor, and ceiling of the underground storage, again using the circulating medium in question as a vehicle, at wish applying pressure or vacuum, said medium also picking up water vapors from sublimed ice.
This application is a divisional application of copending application No. 248,925 filed March 26, 1976.
The invention further relates to a method of recover-ing products which possibly could leak out from the underground storage into the said circulating system and further provides a safety system of controlling if and to what extent products, in particular volatile combustible products, are escaping from the storage. In another aspect the object of this invention is to provide a method of utilizing the temperature difference between the suggested circulating medium and some other stream or body .

1062~Z6 with a view to economically recover heat or 'cold' calories.
The invention also provides a new method of supplying sealants with the aid of said circulating system and also suggests new types of sealants which swell upon contact with the stored pro-ducts. At the same time it relates to a safer method of regas-ification of condensed gaseous products. It is an object of this invention to provide for constructing a suitable underground storage for the purposes envisaged, and it therefore incorporates ~, , .
r new types of insulation designs withstanding very low cryogenic temperatures and suitable for the invention presented here.
1 In said copending application there is disclosed and ; claimed a method of storing a product below 0C in a reservoir and at a temperature which differs from the natural ambient temperature of the surrounding walls, floor, and ceiling of said -~ reservoir and its surroundings, which comprises circulating a medium gaseous heat exchange under a slight vacuum with a capacity for absorbing moisture, in a circulating system comprising of a plurality of uniformly distributed and substantially parallel passages, which are disposed about and adjacent the surfaces of said reservoir, supplying of heat through continuous heat exchangc between the medium and areas surrounding the system, said heat exchange being sufficient to keep the areas surrounding the systen~
and thus also the walls, floor, and ceiling of the reservoir within a predetermined temperature range, the temperature of which It~. differing from the temperature of the stored product, and, in addition, constituting a temperature barrier between the wall surfaces of the reservoir and the external environment, which reduces the sublimation rate and at the same time keeping the temperature barrier around the reservoir at a higher level, the circulation of the gaseous medium in the system continuously r removing vapors emanating from any sublimed ice and water.

The present invention will be illustrated by way of - --` 106Z~Z6 Figure 1 is a schematic sectional view in elevation , of an horizontal cylindric or rounded type of underground storage , reservoir according to the invention with a plurality of boreholes for the circulation system, drilled near and along the rock ,, '~ surface of the cavity, or cast in a concrete wall inside a cavity , in e.g. silt, clay, or sand (the figure illustrates only the case of a rock cavity).
' Figure 2 illustrates a schematic sectional elevation of an horizontal cylindrical or rounded type of underground storage reservoir according to a modification of the same inventi~, ~, with a plurality of circulation channels between the actual rock storage wall or a cast concrete wall and the inner insulated storage wall (the figure illustrates only the case of a rock ;i cavity).
~,~; Figure 3 illustrates a schematic sectional elevation ~/,.,j of a vertical underground storage reservoir with a round or rectangulàr bottom according to a modification of the same ~ invention, showing the plurality of circulation channels, ducts, 7', or galleries with guiding devices for the circulating medium, placed between the actual rock storage wall or a cast concrete wall and the inner storage wall, all latter surfaces being : . .
equipped with some type of insulation withstanding large temperat-;~ ure differences (the figure illustrates only the case of a rock . cavity).

Figure 4 illustrates a schematic sectional elevation ,~ of an vertical underground storage reservoir according to a ,~ modification of the same invention, the plurality circulation :i - I
channels, ducts, or galleries with guiding devices for the circulating medium, being placed between the inner wall and the concrete wall constructed inside of the actual outer rock wall or `~ surroundings of loose materials such as clay, silt, and sand ~ (the figure illustrates the case of a concrete outer cavity 29, ,, , :

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surrounded by an insulating material).

Figure 5 shows a schematic sectional plan view of one ~ type of insulating design used according to the invention.

'~ Insulation is fastened to a system of rods.

Figure 6 shows a schematic sectional plan view of ;

~, another type of insulating design used according to the invention "s j The insul~at~on is supported by a system of wall laths, which have a repeated regular wave-formed profile, the crest on each vertica~
-~ lath in the figure being at the same horizontal level on every ,~ 10 second lath.
,~ Figure 7 is a sectional elevation along line 1-1 of ; figure 6; and Figures 8a, 8b and 8c are graphs showing temperature ~1 and water vapour pressures at two different operating pressures.
;¦ The principle of this invention offers advantages whet, i storing cold as well as hot products underground. As the most .; prevalent need of underground storage refers to the storage of coid, combustible products such as Liqueied Petroleum Gases (LPG), Liquefied Natural Gas (LNG), Synthetic Natural Gas (SNG), petrochemical products, and industrial gases, I prefer for ,j illustrative purposes to select the underground storage of LNG
as a typical example of the use of my invention though the same principle can for the msot part be applied for all types of products which must be stored at temperatures differeing from the natural temperatures of the underground environment. For the sak-of simplicity mainly the construction of reservoirs in rock is discussed, though the invention also refers to similar storages , .
built of concrete in silt, sand, or a mixture of different materials.
Pipes for the filling and the removal of liquid or gas may be conventional and may not be shown in the drawings. ~'he same goes for some other equipment and instrumentation required.

Corresponding parts have been given the same numerals. The type of insulation or its design used in figures 1-4 has not been -;
denoted, likewise the detailed attaching of it to the outer or inner storage wall.
The petroleum industry produces great quantities of volatile hydrocarbons as a result of processing crude oil and natural gas. Natural gas is being liquefied at ports of ex-portation, stored there, then shipped overseas, and stored at terminals at the port of importation. Stand-by storage facilities 1 10 are located outside consumption centers and along pipelines. Such liquids require enormous storage facilities, particularly during ;~
periods of slack use, for peak-shaving purposes, and on account of requirements stipulated by the authorities for emergency cases such as war and embargos.
Other industrial gases require simllar facilities.
Great quantities of volatile liquids including propane and butane have in the past been dissolved from impervious formations, stored in earthern storage pits, or in mined underground caverns. Loss of product, the difficulty of providing and maintaining an adequat ,.
vapor seal, and excessive heat losses are some of the problems r j encountered.
The general tendency is to locate storage facilities for combustible gases underground of the following reasons:
1. LNG fires and similar fires of highly volatile liquids cannot be extinguished and are therefore left to burn out.
Such fires are also generally accompanied by repeated violent fatal explosions with enormous devastations. When storing such products underground, explosions and other dangers can be prevente~
and fires easily controlled and quickly extinguished. Authorities : 30 are therefore expected to stipulate underground storage location ,~ for such products in the futre, particularly with regard to public opinion and other environmental grounds generally presented.

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2. Increased protection against wheather, sabotage, and hostile military operations.
~ 3. Storage at constant and low temperature, generally ,~ in the range 8-10 centigrade; no exposure to sunlight.
4. No space required above ground.
~ 5. Storage under pressure at low cost.
.~ 6. A better and improved understanding lately of the rearl nature of forces in rock makes it possible to avail oneself of less fortunate locations where the underground rock is of inferior quality. Reservoirs can also be built in sand, silt, or clay, which problem, though, will only be mentioned in this paper.
Though storage of LPG in underground rock reservoirs ~i' at temperatures in the range of -40C to -50C for a long time Y1 has been a successful application the same cannot be said about ~1 storage of LNG, SNG, other cryogenic liquefied products like ,~ ethane, ethylene, and other petrochemicals, in such underground caverns because the extremely low temperatures required to store these cryogenic products at substantially atmospheric pressure ~; requires an excessive amount of refrigeration on account of the high heat losses incurred. Another drawback has been the increase ~j product losses at these lower temperature levels. The main reason for the mentioned heat losses is the increased amount of con-traction with subsequent continued incessant cracking of the J; underground rock, developing an ever larger seepage of product wit) time. Further, the intensified rate of sublimation of ice removes the sealing effect of the surroundings and spoils the insulation ''.. ., of the cavlty lf any.
The mentioned c~acking of the rock can continue for years, steadily opening up new cracks and constantly further away ~, out from the storage wall. The natural consequence of the crackin of the rock at these very low temperatures is a continuous increas of observed heat influx from the cavity surroundings to the .. . .

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106Z~26 ; storaged product body, gas seeping out in the environment and causing general nuisance and an explosion danger. There are, of course, always a large number of original cracks in the rock, and these are opened wider up while new cracks are being created, sometimes causing large pieces of rock to fall out into the storage cavity. Conventional reinforcements and precautions are therefore always required.
If the temperature of the rock is controlled within the predetermined limits by the aid of the multi-purpose system I
10 have envisaged and which system is located about the surfaces of -the walls, ceiling, and below the floor of the underground storage such continued cracking can be totally avoided at the same time as the stability of the rock material is ensured. By supplying ,.. ~.
i heat the multi-purpose system thus prevents the temperature of .'~ the rock from falling below a desired critical minimum. Di-;; ficulties on account of the ice sublimation process, such as damag~

~ to the insulation applied, are also halted, which all can be --,~ . .
q achieved with aid of the same multi-purpose circulation system, which constitutes the core of this invention. Water vapors thus moving in the direction of the storage may be carried away by the ~ circulation system with its interconnecting cracks and interspaces g said migration process substantially reduced by the temperature ; barrier established by temperature control around the area of the s - circulating system. By raising the temperautre around the circulation system a change of the temperature gradient in relation to the temperature of the environs is attained, which in the presence of sufficient water in the rock pores towards the outer environs influences the rate of sublimation in the thus created bottle neck between the circulation system and the environ Operation at a temperature below OC also implies generally reduced water vapor pressures and thus reduced sublimation rates, water vapor pressures above ice being much lower than those over ... .

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- 106Z~:)26 water except at OC. Frost heaving and front lenses often are consequences of not removing water by a device such as the one described. Said circulating system consists of a multitude of ,. . .
,~ comparatively closely spaced circulation channels along all sur-faces of the storage, the channels carrying a liquid but preferabl j a gas such as nitrogen, carbon dioxide, possibly hydrogen, hydro J carbons or even the stored product itself, or one or several of c its components. In some cases ducts, or galleries with devices l to direct the circulating stream, may partly or completely sub-,; 10 stitute a plurality of channels or boreholes. These circulation ~ systems can also be used to heat or chill the rock, which latter .: ,~ . .
operation also will be required when - as described below - ;
~j sealing the rock at low temperatures in accordance with my propose~
~3 method. The typical operating range for the temperature barrier ~:
of the rock or concrete will depend on the quality of the rock or concrete but will in many cases vary in the range -10C to -50C, ; i.e. about the temperature range used in rock for many current installations. However, also higher temperatures, even above OC, , may be used.
As rock is a good insulator a very long time, years, .:
are required before the temperatures have asymptotically reached ~ their final values. To the extent they can be controlled they ,,~
are selected with regard to a number of factors such as rock characteristics, rock material, drainage problems, distribution of ,, stress in the rock, permeability and porosity of the rock, etc The operation of the rock storage may be controlled electronically ; and in accordance with a predetermined plan.
The controlled movement of water and water vapor in the rock is of great significance for the successful operation of ;~ ; 30 the storage.
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~ Water vapor will move in the direction of areas with :
~ lower water vapor pressures in accordance with known physical laws ;.
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!.' 1~6Z026 If water is partly or totally removed from a particular area the saturated water vapor pressure at a certain temperature there cannot be fully developed, and the water vapor pressure in said area may therefore be lower than in a different area with a lower ~ -: , :
temperature, where there is sufficient water available to develop the full saturated water vapor pressure for the temperature at this last mentioned point. The porosity of the rock will, of course, also involve other physical processes of different nature connected with the migration of water which all influences the general process as put forward. Figure 8a illustrates the approximate distribution of temperatures, plotting only a few points, when keeping the temperature of the circulating system at a high level and at a lower level (dashed curve). The corres-ponding water vapor pressures, unsaturated for the dashed curve except at the far right, are found in Figure 8b. The effect of using a drying gas is shown in Figure 8c. By lowering the water ., ~ vapor pressure curve below OC the amount of water to be removed L,.
the drying gas diminishes on account of the lower water vapor pressures and the reduced water migration rates encountered at these lower temperatures.
There exists a patented method which suggests a con-¦ tinuous sealing of cracks opened up, applying a freezing liquid, which is continuously injected as the rock cracks. I prefer to - start the sealing of natural and potential cracks by first opening ` up these cracks comparatively wide by chilling the rock through my circulating system to a temperature far below the actual futur operating temperature and then apply a sealing material, using pressure and partly distri~uting the sealing agent by said cir-culating system. At the same time conventional injection of the same or similar sealing agents may be carried out after a ~ plurality of auxiliary boreholes have been drilled into the surfa~
c from the cavity. I prefer to select sealants which swell upon ; _ 9 _ !: j '' getting in contact with the stored product, though such swelling sealants not always are imperative. If the product should leak out and get in touch with a swelling sealing material in a crack the swelling sealing agent will automatically close the crack irmer. In some instances the swelling action may be started by injection of water. By first injecting a sealing component and then adding a second component and leave the two to react within a closure the swollen material will act as a very good and elastic seal. The described method of first opening up the cracks by chilling the wall material and then apply the sealant by injection J after which the cracks are closed again by raising the temperature works as well with rock material as with concrete.
~ There exists a great number of chemical compounds or i mixtures hereof which have the propensity of swelling upon contact with fluids or gases, the fluids and gases being absorbed, dis-solved by these materials, or forming new structures with them.
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Some of these materials are polymers, rubbers, or plastics. The sealant must be selected with regard to the product stored, and selection of proper material can be done by the average expert.
I prefer to remove the water in the rock by using a drying gas or fluid, the latter containing a water absorbing ..
component. These media are circulated in the proposed system and then continuously dried by some conventional drying agent. When using a circulating gas, water may also be separated out in con-~, densation, adsorbation, or absorbtion processes, in some cases after compression. The water removal action may be facilitated by first heating the medium. Applying a sealing medium and again taking advantage of the circulating system the sealant is then applied in all cracks and spaces in the environment of the cir-:
culating system proposed in this invention. A conventional water drainage system will always be required in all storage designs ~; discussed.

A different mixture, also distributed under pressure through the proposed circulating system in a similar manner as in ~ the previous case, contains principally two components, one of ;~ which absorbs water while the other works as a sealant simultan- ;
eously. Such products are commercially available. When using ;! the last mentioned mixture the cracks may all be opened up by chilling and partly closed again by raising the temperature.
The sealing quantities of the storage cavity walls are ;~
:i at times dependant on the water content in the rock. Of this reason this invention also involves the idea of adding water to the circulating stream when necessary~ For the process of sub-~; limation of ice in the rock the possibility of controlling the water content of the circulating stream is of utmost importance, ~ because water from the rock tends to migrate and form ice on the .~ inside wall of the actual storage, thereby having a tendency of pushing away applied insulation or damaging its valuable insulatin

3 characteristics.
It is also an object of this invention to provide a safety system which allows a good control of the proper function-ing of the storage, which fact is of prime importance if the cavity contains a volatile combustible liquid or else a dangerous gas. This can be achieved by keeping the operating pressure of the circulating system somewhat lower than the pressure in the actual storage. If product should leak out from the storage it will enter the circulating system where it immediately can be sensed by a suitable instrument such as a gas chromatograph or mass spectrometer. Such a product may then also be recovered, e.g. through absorbtion or condensation processes. In the case ol combustible gases it would involve a direct danger not to use such a system or a similar device, should the sealing qualities ,~
;~ of the cavity wall prove to be insufficient, needless to mention "~
difficulties which arise at shut-down or maintenance of the stora(, :

106'~C)Z6 The selection of a suitable medium to be employed for the circulation system depends very much upon product stored, its i st;orage temperature, operating temperature range of the circulatin medium, and what type of equipment the medium shall have to pass.
A further point is the question if the circulating medium may ! affect materials contacted in the system. Among the gases, nitrogen, which is inert and of ten employed in starting-up operations, is excellent. Another suitable gas may be carbon ~ dioxide, hydrogen, refinery off-gases, and the product itself, ,~ 10 if volatile. If natural gas is stored, nitrogen is a suitable , medium, and in this case LNG and its components can be separated ~
;~ out completely, if product should leak into the circulating ;
system, as long as the product does not contain hydrogen.
",~ A further point which always arises is if the operat-~;~ ing situation àllows an economical heat exchange between the ,, circulating medium and some other stream or body.
Underground storage offers the advantage of operating at a higher pressure at low cost as compared with storage above ground. This may be important when filling the storage with LNG
liquid, when the specific gravity of the liquid to be filled differs somewhat from the specific gravity of the storage content.
Under such circumstances the pressure in the reservoir may rise suddenly on account of so called roll-over.
With regard to the same advantage to operate at highe~
pressures a further feature of this invention is the suggested use of the reservoir as an evaporation chamber. The heat exchang~
equipment for the evaporation can be located inside as well as outside the reservoir. The corresponding heat exchange equipment for this evaporation of liquid has not been denoted in the drawin~3 and may be conventional.
When storing cryogenic products like LNG the con-.
traction of the plastic insulations used amounts to about one per ' ~ Z~
cent, while the corresponding contraction of the rock for the same temperature interval will be in the order of one per mille.
The contraction differences for these two different materials therefore call for special types of insulation designs to be employed along the cavity walls, on wooden or some other supports along the same walls, or on the walls of a built-in containing vessel. The basic design principle is to prevent the insulation from becoming subject to excessive tensile stress. The insulation designs proposed here are all built up of several layers, e.g. of .. ,~ , .
polyurethane insulation or similar plastics, along with sealing membranes, and a heat reflecting aluminium foil. Suitable seal-s ing membranes and suitable insulating materials are known and ;~ commercially available. The designed final compound insulation layer is formed in such a way that the layer is divided up in equidistant cuplike elements, resulting in regular parallel rows 7 of such element8, where each element is equidistant to any next element. There is an ample amount of insulation material around ~ each element to allow for temperature contractions, which mainly ,',~
result in flexural stresses instead of tensile stresses. The insulation is supported at the centre of each element. The first ; mentioned stresses again can be mitigated or relieved during the initial transition process at start-up by supplying heat to the outside of the insulation layers, using my proposed circulation system as a heat source.
The application of such insulation designs can easily `¦ be carried out on-comparatively even wall surfaces but will otherwise require a system of support rods. When supporting the insulation with the aid of a system of laths with wave-formed profiles the cost will be lower when the cavity surface is even and smooth.
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The support rods are fixed in boreholes, drilled into the rock, or cast in the concrete wall. These boreholes .~ . . . . .
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; form a regular symmetric equidistant pattern, evenly distributed along all walls. Each element of the insulation is thereafter fastened on these fixed rods, leaving a 'valley' around each rod ': to allow for temperature contraction This design makes it possible to leave the actual rock wall in a rough unfinished condition.
The system of wooden laths is fixed to the rock walls in such a manner that the crest of a wave profile on one vertical lath is opposite to the 'valley' of a profile of the next adjacent vertical lath at the same horizontal level. When cooled the ;~. insulating layers will thus through contraction mainly rest on the crests of all laths, the surplus length of the insulation round 1 èach crest allowing for the temperature contraction in conformity i with what also happens when the insulation is fastened on the rods mentioned in the previous parayraph. In the last mentioned design the elements referred to correspond to the crests in the lath system .
The two insulation designs can be fitted with wooden :, supports, if so required, and the circulating medium can be directed between the insulated inner wall and the actual outer .~ rock or concrete wall. The designs constitute a built-in con-;~ tainer. In the figures is the plastic insulation cover over bolts and bolt heads omitted.
Referring now to Figure 1 of the drawing an horizontal rounded type of underground reservoir 10 is shown in cross ~ section. A series of boreholes 11 have been drilled in the rock , 16 along the periphery of the reservoir for the circulation system described, from both ends of the cavity, or, depending on length of storage, also from niches between the ends of the reservoir. If a concrete wall has been cast inside the rock wall or in surrounding loose material like clay or sand, the system of holes are cast. Small size boreholes 12 have also been drille~

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in from the storage (only one such borehole is shown in the figure) with a view to tighten cracks throughthe injection of swelling sealants or other materials after the rock has been ~3, cooled down below the future operating temperature. Other cracks~, have been mended with plastics, cement, or similar mixtures, and4 the outer cavity surface, depending on the type of insulation j used, smoothed. After rock bolts and insulation supports have been positioned the insulation 13 is fastened. 14 is evaporation space, and lS withdrawal pipe for gaseous products.
,.~ 10 Figure 2 illustrates how a circulation system of channels 17, placed on the inside of the outer cavity wall, may substitute the circulating system of drilled boreholes in the J rock wall of the reservoir 10 as described in figure 1. The ~ insulation 13 is here fastened in conformity with what has been ;~ outlined in connection with figures 5 or 6. Sometimes it will becheaper to construct galleries with directing devices for the circulating medium.
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,~ The reservoir 10 in Figure 3 is a modification of the. previous two storage types described. This storage may be built according to choice with a rectangular or circular concrete bottom l9, this type of bottom being equipped with circulation channels 17, preferable in block elements of balsa wood 18. The circulation system along the walls consists of a plurality of vertical channels 17 or other gas stream guiding devices which insure a sufficient contact between the streaming medium and the outer and inner wall. While walls and bottom are equipped with the mentioned standard types of insulation 13, the ceiling of the reservoir, resting on a suspended structure 22, is insulated with some pervious loose insulation like rock wool, which permits the vapors to pass and the possibility to use the cavity as an evaporation chamber. Valve 20 is used at start-up.
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Figure 4 corresponds to figure 3 and illustrates how , . .

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106'~'Z6 a reservoir can be built in sand, silt, clay or similar loose materials, or elsewhere where only inferior rock is available.
The reservoir, including the walls with the built-in circulation system, is cast in concrete 29, using a travelling mould. Another method is to construct the storage using prefabricated elements and pre-stressed concrete. Construction in earth is generally j preceded by freezing the surrounding soil before excavation.
:~ When required insulating material, impervious insulating material, s~ or foamed insulating material 27 may be filled in round the 1 10 structure.
.~ Figure 5 shows how the insulation 13 is fastened '.!; after a regular pattern of equidistant support rods has been ;~ positioned in the rock wall. 23 are elastomeric membranes, 24 i:? polyurethane foam, 25 aluminium foil, and 26 support rod, which has been fixed in a hole drilled in the rock or in the concrete.
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28 is an optional support of wood, plywood, or plastic.
Figure 6 illustrates the utilization of a system of laths 30 with a configuration of regularly repeated wave-like profiles.
, 20 Figure 7 is a sectional elevation along line 1-1 ¦ in figure 6.
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Claims (14)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. An underground reservoir located in rock for use in storing a product below 0°C at a temperature which differs from the natural ambient temperature of the surrounding walls, floor, and ceiling of said reservoir and its surroundings, including a circulating system comprising a plurality of uniformly distri-buted and parallel passages disposed along and adjacent the sur-faces of said reservoir, means to pass a circulating fluid heat exchange medium in said system to supply heat through continuous heat exchange between the medium and the area surrounding the system and keep the area surrounding the system and thus the walls, floor and ceiling of the reservoir within a predetermined temperature range which differs from the temperature of the stored product and constituting a temperature barrier between the wall surfaces of the reservoir and the external environment which reduces the sublimation rate and at the same time keeping the temperature barrier around the reservoir at a higher level than that of the reservoir and thus reduces cracking in the walls of the reservoir, means to produce a slight under-pressure in the system in relation to the pressure in the reservoir, means to add swelling sealants, substances and water to the circulating system, means to remove water vapour from the medium so that the medium is capable of absorbing moisture during circulation, means to introduce and remove product from storage, means to detect leaked-in products in the medium and means to recover products from the medium.

2. A reservoir as claimed in claim 1 including means to introduce and remove cooling medium from the circulating system.

3. A reservoir as claimed in claim 1 including a water drainage system.

4. A reservoir as claimed in claim 1, in which the passages are boreholes disposed along and below the surface of the reservoir which reservoir is insulated.

5. A reservoir as claimed in claim 3, in which the circulating system comprises an arrangement of substantially parallel and uniformly distributed channels along the walls, floor, and ceiling of said reservoir without insulation and located between the reservoir and an inner insulated storage container.

6. A reservoir according to claim 4, wherein a plur-ality of ducts and galleries with stream guiding devices for the fluid stream comprise the passages.

7. A reservoir according to claim 4 or 5, wherein the circulating medium is a gas.

8. A reservoir according to claim 4 or 5, wherein the circulating medium is a liquid containing a gas.

9. A reservoir according to claim 4 or 5, containing normally gaseous liquids.

10. A method of storing a product below 0°C in an underground reservoir located in rock at a temperature which differs from the natural ambient temperature of the surrounding walls, floor, and ceiling of said reservoir and its surroundings, which comprises circulating a fluid heat exchange medium contain-ing a gas in a circulating system comprising of a plurality of uniformly distributed and substantially parallel passages, which are disposed about and adjacent the surfaces of said reservoir, supplying of heat through continuous heat exchange between the medium and areas surrounding the system, said heat exchange being sufficient to keep the areas surrounding the system and thus also the walls, floor, and ceiling of the reservoir within a predeter-mined temperature range, the temperature of which differs from the temperature of the stored product, and in addition, constitut-ing a temperature barrier between the wall surfaces of the reservoir and the external environment, which reduces the sublima-tion rate and at the same time keeping the temperature barrier around the reservoir at a higher level than that of the reser-voir.

11. A method as claimed in claim 10, in which the medium is a gaseous medium.

12. A method as claimed in claim 10, in which the medium is a liquid containing dissolved gas.

13. A method as claimed in claim 10, 11 or 12 in which the passages are boreholes disposed along and below the surface of the reservoir, which reservoir is insulated.

14. A method as claimed in claim 10, in which the system comprises substantially parallel and uniformly distributed channels along the walls, floor, and ceiling of said reservoir and located between said reservoir and an inner storage container one of said reservoir and said container being insulated.