DE1906787A1 - Process and arrangement for increasing the degree of utilization of electrical power plants - Google Patents

Description

Method and arrangement for increasing the degree of utilization electric power plants

It is known that the generation of electric power can be improved by lowering the cost and increasing the utilization rate in certain places by employing a main generation system and an auxiliary generation system. When an irregular power requirement causes a low load factor of a single generation system, a smaller auxiliary system often becomes used to improve the load factor and utilization rate of the main system that provides the bulk of the power. Such a system provides additional power during peak load times and represents a reserve that is available when required and an auxiliary energy source. Such energy stores work on different principles. For example, hydropower pumped storage plants and storage facilities for compressed air are used. In tents with low power requirements, cheap electrical energy is used to generate the water

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• to pump into an elevated storage tank or to compress Luffc, to store them underground in mines or salt cavities. That on a higher potential energy level Brought water or the compressed luffc are later used as a source of energy during peak periods To drive generators. The resulting auxiliary service is therefore generated at a higher cost because Energy losses in the conversion are inevitable, but it can reduce the total cost of the electrical Electricity generation at such a power plant decreases v / er

P the. So are cost reductions of $ 30 at power plants has been achieved using combined systems in which a thermal power plant is combined with an auxiliary pumped storage plant was. The cost saving results from the improvement of the load factor of the primary generation plant, whereby valuable, quickly available energy is stored, which increases the size of the main power plant reportable 1st. Energy storage devices of this type are also valuable in that they generate excess power absorb during sudden load changes, so that a high frequency stability in the output power and in the distribution system is guaranteed. Another important consideration is the unpredictable value of such

"Auxiliary system as a source of emergency generation in the event of failure of the main power plant .

According to a feature of the invention, underground artesian wells or depleted natural reservoirs are made of liquid or gaseous hydrocarbons, i.e. porous rock formations of relatively high porosity and permeability, used as storage, into the compressed air or another gas pressed in, saved and used for later Use is kept available. The porous spaces

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such reservoirs are usually with \ ϊάγλ- er;.! geiülll-, which is expelled by forcing in compressed gas, the pressure of which is slightly higher than the natural hydrostatic pressure. Reservoirs of this type are known * that are capable of holding billions of cubic feet ^. The gas can be injected and withdrawn at a relatively constant pressure, this pressure being regulated by the natural hydrostatic pressure of the formation. The reservoir resembles a large elastic chamber and expands and contracts in adaptation to the amount of gas stored, namely by the movement of the "water, which is caused by the injection or withdrawal of gas. Thus, the gas, during periods of low electrical power demand or when cheap electrical energy is available, compressed utid can be stored, while the gas is withdrawn under substantially constant pressure during peak periods and drives an air motor which in turn drives an electrical generator or does some other work or chemical The structural parts of such a secondary power plant are reduced compared to the usual methods that use surface reserves, as is the case with pumped storage spillage plants or with underground compressed air, either in salt formations. Power plants constructed according to the invention can jiedoQh only in ε- olcher! Regions are established where suitable fornations are available, but such suitable fornations are abundant and geographically widespread i. Tor · Me. Places which are suitable for pumped storage plants or silo cavities for pressure air systems. If the geological conditions are favorable for the Lego of such a power plant, the invention becomes economical Further improvement 10 :: - Lofit factor has been considerably improved compared to existing systems and the need for additional Vertoä'erJrapazitSt is postponed. Pressures from 2.8 to

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210 kg / cm are suitable for the purposes of the invention.

According to a further feature of the invention, energy is stored during periods of low power demand, with excess Power is available to compress gas e.g. air and put it under high pressure in an underground salt cavity or to press in another gas-impermeable cavity or reservoir. The cavity is in communication with a water reservoir at the surface of the earth, so that the hydrostatic head of the reservoir on the gas in the cavity burdens. When there is a high electrical power requirement, gas is withdrawn from the underground cavity, which is below the hydrostatic There is water pressure and this compressed air is used to drive a compressed air motor or another auxiliary motor, which in turn drives a generator. The gas reservoir is kept under essentially constant pressure.

The invention is described below with reference to the drawing. In the drawing:

Fig. 1 is a schematic sectional view of a system in which separate shafts are used for injecting and withdrawing the gas,

FIG. 2 shows a view similar to FIG. 1 of a system in which only one shaft is used for both injecting and withdrawing gas,

Fig. 3 is a sectional view of a system according to the invention, at which gas is stored in an underground salt cavity under the hydrostatic pressure of a water reservoir which is on the surface of the earth, the reservoir being shown during that period in which air is forced into the cavity under pressure,

4 shows the system according to FIG. 3 in an operating position during the evacuation of gas, whereby several auxiliary air motors are driven, in turn an electric Drive generator.

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In, Fig. 1, reference numeral 1 denotes an underground Reservoir of relatively high porosity and permeability. The reservoir may be a petroleum and dry sand tanker, i. a geological dome or an anticline in which no economic quantities of oil or gas are stored before storage or the reservoir may have been a mined oil field or a natural gas field. Sin an essential characteristic of an Aquifer-Spelcher Reservoir is that a dense cover rock lies over the reservoir prevent gas leakage. A description of suitable oas storage reservoirs and methods by which these can be found in publication no. SPE 162 entitled "Evaluation of Underground Gas Storage conditions in Aquifers through Investigation of Groundwater in Hydrology "issued by the Society of Petroleum Engineers of AIME "during the 50th annual meeting in Dallas, Texas USA., October 196I. The requirements for suitable Underground storage reservoirs result from US. Bureau of Mines Circular 7765 ^ in Section XXV, titled "Underground Storage of Natural Gas in Coal-Mining Areas" by Wheeler and .Eckard, in particular pages 6 and 7. It is preferable, but not essential, that the type of porous Rock reservoirs can be one of these, as is often the case under the designation "water sand" is referred to, i.e. a storage reservoir that is freely bounded with a hydrological system under hydrostatic pressure is limited. The storage of Ga3 in water sand reservoirs (e.g. described by Douglas Ball and Peter Burnett in Mines Magazine, Volume 49, November 1959 * Page 68-72 "Storage of Gas in Water Sands") is particularly useful because of the pressure normalization effect of the hydrological Systems. At reservoir locations where the geological structure is characterized by a thick sequence of layers there may be several suitable reservoir layers

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and simultaneous storage in more than one shift is possible. If multi-zone storage is used, It may be desirable to recirculate gas from one zone to another in order to improve the efficiency of the storage system to improve or maintain. If multi-zone storage is used then air or other suitable will be used Gas is compressed and stored in a low pressure aquifer reservoir under hydrostatic pressure of about 2.812 to

10.545 kg / cm. This air is then compressed and for this purpose excess electrical power is used during the periods of light load. Through this

there is a compression to about 5.625 to 105.45 kg / cm and this air is after a constant pressure aquifer reservoir transferred and stored where it remains under hydrostatic pressure of the same size. The air from the high pressure reservoir is used to drive the air motor and the air expanded by the expansion device becomes In in the same manner as described in US Pat. No. 2,942,411, recycled to the low pressure reservoir. By using there is no need for multi-zone pressure storage reservoirs multi-stage compression and expansion and the installation cost are reduced and the efficiency is increased. Am such a multi-zone underground gas storage system operated by the Natural Gas Storage Company of Illinois at Herscher, Illinois and is described in "Underground Storage of Natural Gas in Illinois "by Alfred H. Bell and published in the Illinois State Geological Survey, I96I Circular 318.

With the reference numeral 3, the cover rock is off in Pig.l Shale or other gas-impermeable rock arranged, which is above the aquifer or the storage reservoir. The reference number 5 denotes the insertion shaft and

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the reference number 7 the drainage shaft. Air or another Gas is fed to the compressor 11 via the line 9 and blown through the shaft 5 into the reservoir, the pressure being high enough to keep the hydrostatic pressure inside of the reservoir to overcome. When the hydrostatic pressure within the reservoir is, for example, 17 * 577 kg / cm, then the air is brought to a pressure equal to the hydrostatic Pressure exceeds by a factor of at least is large enough to initiate the displacement of the water. In general, an amount of $ 10 will be sufficient.

The air compressor 11 is driven by electricity supplied from the power plant such as a hydroelectric plant or a steam power plant. In practice the air only becomes. blown in during periods of low stress, v? enn the electricity demand below the capacity of the water resp. Steam power station is located.

Air that is stored in the reservoir 1 under the prevailing hydrostatic pressure is fed through the line 7 as required deducted. The withdrawal can take place at the same time as the pumping in or only when air is not being pumped in. Ever according to what purpose the extracted air is used. If the air is used for other purposes, e.g. for heat transfer, in steel melting furnaces or in gas turbines for cooling and combustion, the air can be flown off at any time and whenever the load conditions require it. On the other hand, if the air is used to generate additional electrical energy during the Peak load the air is withdrawn during these peak load periods when no air is pumped in through the duct 5, since the electrical device generation capacity corresponds to the electrical energy demand must be adjusted and not available to pump air into the reservoir. When the system is used to

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the output of a V / as power plant, a steam "diesel power plant or another power plant during peak load periods to feed, then the pumping in and removal of the gas can be done through a single shaft *

The air or gas flowing through the pipe; 7 is withdrawn, a drive 13 ' can feed, for example, a turbine or an air rotor, which in turn performs useful work or can be used to drive additional electrical generators 1-5. Suitable air motors for driving electrical generators are described on pages 275 to? Q5 of the book "Compressed Air Plant", 5th edition by Robert Peele, 1930, published by John Wiley & Son, New York.

An alternative to the procedure described is in Pig. 2 ' shown, in which a shaft 16 is used both for pumping in and for drawing off compressed gas, and a Combination of a compressor-air rotor 17 is used to to compress the Qas and zwcoks drive the generator 15 to expand. Such a device, namely a propeller, which is used for compression and works as an air motor «which drives the generator, 1st in" Peaking Power with Air ", Power Engineering, January 1968, pages 50-52 (From Council, Martinez and Whithouse)

A rock layer with a porosity of at least 6 # and up to 40 $ and a permeability of at least 5 and up to 50,000 millidarcies are suitable for the purpose of the invention. The formation should be of sufficient extension and thickness to accommodate the amount of gas required to drive the power plant equipment concerned. Such reservoirs can be porous and permeable sandstone beds, reefs or

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Reff »be breccias, at least above through impermeable Beds are limited. The reservoir should be such that a normal movement of the compressed air or a compressed air Gas is limited so that recovery is possible.

The lateral boundaries can be caused by folds, domes, errors

or or crushing of permeable layers of Reffs / Reff-Ereccias or occasionally formed by horizontal formations without thorns,

As mentioned, the subterranean reservoir according to the invention is able to absorb the amount of gas required are to put the integrated power plant into operation and to operate at maximum efficiency without a significant pressure drop when the air is drawn off or significant losses occur.

The invention has compared to water Pumpspaicherwerke that use electrical energy to pump up mass and converting this back into electrical energy via water turbines, the following advantages

a) the invention is not limited in application to one certain topographical relief that is required in water pump storage plants to ensure that water can drive the turbines?

b) Oven surface water reservoirs are often very expensive, difficult to construct and seal and give rise to ΖΊ Evaporation losses, while underground reservoirs are found in widely distributed areas of the world »Because water is a valuable material, two surface reservoirs generally have to be provided for pumped storage plants, namely one in the height and the other at the valley station, so that the water is preserved and easily available. With Luftspelchersysteraen on the other hand

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only a single reservoir required, because it is atmospheric Air is universally available. Besides, it is without further possible, a reservoir some * distance from the To use the power plant lying down, "as the gas can easily be brought in via pipelines;

c) in certain areas where electrical energy is generated, there is not enough water to operate a water pumping storage plant;

d) because the air or other gas is withdrawn from the reservoir under a substantially constant hydrostatic pressure, substantially all of the stored ftas can be used to power air motors. This makes it possible to use smaller cavities and the system costs are lower than would be necessary to withdraw the gas under gradually changing pressure and therefore only part of the stored gas could be used, because the withdrawal of the gas could be interrupted must be when the di * uok has dropped below the operating pressure of the air motor;

e) in addition to the advantages mentioned above, is the plant according to the invention also for this reason expedient * because a reservoir is created which automatically adjusts to the required Volume expands or contracts without being substantial Pressure fluctuations occur.

Storage in the aquifer is considerably cheaper than storage in washed-out cavities, oil in domes of rock salt, 4 © d ^f joh, storage in rock salt cavities is used according to the invention with a considerable more effective advantage over conventional methods of cave storage, and thus a method is created with which an auxiliary energy is generated in those places where no natural aquifer reservoir is available, but where there are salt beds or salt domes.

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The fiaglt er storage according to the invention has the advantage over the conventional methods of using salt mines or washed-out salt cavities in the earth as a storage reservoir that air or gas must be pumped into the reservoir with a fixed volume, which is essentially at atmospheric pressure and as a result, there is a pressure loss until enough gas has been pumped in to build up pressure in the pumped air. Either the pressure in the cavity must be built up to a level which is considerably greater than the pressure required to drive the generator, or only a small proportion of the stored gas can be used because a rapid pressure drop when the gas is withdrawn from the cavity entry. On the other hand, the gas stored against the natural hydrostatic pressure maintains a finite pressure level and drives gas turbines, air motors or an electrical device and when gas is extracted from the rocks, the pressure in the reservoir does not decrease significantly.

The following example shows the costs of setting up a gas storage facility in an underground aquifer for an auxiliary power installation for peak load times ι assumption:

Working pressure 50.270 kg / cm

Peak daily consumption I8.4o8 *; 10 m Then: Specific weight of air - 1.2 kg / nr

Weight from Ι8.4θ8 χ 10 ⁶ nr ⁵ - 18.408 at ΙΟ ⁶ χ 1.2 «22.IxIO ⁶

kg / day

Average required kg air / hour ■ »

22.1 χ 10 ⁶ : 24 - 0.923 χ ΙΟ ⁶ Druok differential during expansion: 49.216 kg / m (Pressure of the gas above atm.} X 10 000 em / in t 1.2

(Weight of 1 rs? Gas) = 0.439 X- 10 ⁶ m gas

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Average metric gas output at $ 70 efficiency

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0.925 χ 1O ⁶ j 60 (min / h} x (0.459 x 1O ⁶ : 45OO) x 0.70

1.051 x
Average power output = * 1.051 χ 10 χ 0.755 = 772,000.

A reservoir can operate safely within a range between 0.5 and 9 to 1 cushion-to-working gas ratio. If this memory is used 5 hours per day to supply auxiliary energy, then 18.408 χ 10 nr i 8 = 2.5 χ 10 nr gas per day are withdrawn and a reservoir with a capacity of 25 x 10 nr is required for this. At a cost of $ 18.77 per 1,000 nr (experience in manufacturing gas storage tanks of this type), the cost of the reservoir would be approximately $ 450,000, or $ 0.58 per kilowatt compared to 40 # from $ 85 to $ 150 / kW (empirical values) for pumped storage plants.

In the following, reference is made to Figures 5 and 4 of the drawing. The reference numeral 21 denotes a shaft which extends from the surface to a cavity 25, the lies in an underground impermeable rock or salt formation 25. The shaft 25 can either be mechanical using mining technology or by dissolving and extracting the salt from the salt bed. The shaft is with a corrosion-resistant lining 26 made of steel or other suitable material and cemented. A pipe 27 passes from the surface through the shaft 21 to the bottom of the cavity 25. There is a reservoir of water on the surface of the earth 29 is provided in such a way that the upper end of the tube 27 opens after the water reservoir or is connected to it is. The capacity of the water reservoir is preferably about the same as the gas reservoir, although it is also could be bigger or smaller. The volume of the gas cavity

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depends on the needs of the assistance.

The upper end of the shaft 21 is closed and over a Tube 51 * in which a valve 55 is switched on, with a motor 55 connected, which can be driven by compressed air. Such a motor is preferably used, which also functions as a pump can serve to press compressed / into the underground cavity or the storage reservoir.

The air compressor 55 is used to drive a motor generator 57 which provides the electricity required for peak loads generated.

During those periods in which the main power plant is operated under partial load, the excess electric cal power is used to energize the motor-generator that controls the air compressor or the reversible air compressor rotor 55 propels to compress air and pass it through ducts 51 and pump the well 21 into the underground reservoir 25 against the hydrostatic water pressure in the pipe 27, wherein the Water within the cavity or reservoir is pressed through the pipe 27 into the reservoir 29, as can be seen from Pig.5 is »During peak load times, as soon as the capacity of the main power plant is exhausted, air is drawn from the reservoir 25 via duct 21, pipe 51 and valve 25 to the air motor or the reversible air compressor air motor 55 to drive, which in turn drives the generator 57, - to the additional electrical power required to be able to provide.

After closing the valve 55, the reservoir system can assume a static condition, with neither air being pumped in nor is deducted.

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The height of the water column in the pipe 27 determines the pressure under which air or another gas is stored in the underground reservoir 23. Gas _# under a pressure of about 2.812 to 103.450 kg / cm could be used, but preferably a pressure between 703 and 84.37 kg / cm is used. The pressure is preferably chosen as high as possible, because the higher the pressure under which air or gas is stored, the smaller the cavity and the surface reservoir need to be, based on a given generation capacity the storage tank can be pumped in at a shorter time interval and this is important if the time in which the power plant operates with low load is short compared to the peak load tents. The <3as can be withdrawn from the accumulator in order to operate the air motor or another drive, either with the accumulator pressure or with a reduced pressure when the valve 33- is partially open.

The upper limit of the practical to store air or Another gas selected pressure is determined by the solution of the gas in water or another aqueous liquid. The amount of gas that is dissolved depends on the maturity of the gas-liquid phase, the temperature and the pressure. the Dissolution of gas in saturated sodium chloride salt solution is not nearly as critical as a solution in water, and because of the lower solubility. But if the pressure is too high, gas quantities dissolve in the water or the saline solution and are carried to the surface and under atmospheric pressure an escape takes place, which leads to great energy losses leads. It is therefore important to keep the accumulator pressure below that pressure at which considerable amounts of Air or other gas can be dissolved in the saline solution or in the aqueous liquid. When using air and saline solution

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pressures between 17 * 577 kg / cm and 52.731 kg / cm are satisfactory. During the pumping of compressed gas into the reservoir 23, water or saline solution is pushed up from the reservoir through the pipe 27 into the reservoir 29 and brought to a higher level by the compressed gas, the gas being under the hydrostatic pressure of the water or the brine is inside the tube 27. Of course, a separate well bore for pumping in or withdrawing gas can also be provided after the underground cavity or away from it, and further separate lines for the water flow or the flow of the saline solution between the underground cavity and the surface reservoir. The system works with very small pressure differentials and for all practical purposes can be viewed as a pressure accumulator with variable volume but constant Di ¹ UcIc. The system takes in gas at any pressure above the hydrostatic pressure, but delivers this gas at a constant pressure and speed until the entire amount of gas has been withdrawn from the reservoir. Because of the essentially constant pressure of the gas in the underground volume, the entire storage volume can be used to drive a generator, and for this reason a much smaller reservoir capacity is required than when gas storage is used without hydrostatic pressure.

Another advantage of the invention. fystemn opposite known memories consists in the fact that, weuu have a S & lzhohlraum Used as a reservoir, the periodic moistening of the cavity by the saline solution helps to crack and permeate To seal zones in the rock salt wall, which prevents loss of compressed gas. Also will the likelihood of collapse of the cave ceiling is reduced because there is little or no change in pressure in the Reservoir occurs »

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The invention has significant advantages over conventional pumped storage plants because it saves a lot of money Investment costs possible 1st. In an article entitled "How to Evaluate Pumped Storage for Peak in Generation" by John Pitt, published July 19-64 in Power Engineering, "pages 28-32, it is stated that the cost of a pumped storage plant are over $ 80 per kilowatt. the The costs of creating an underground storage facility in a salt cave are comparatively low, as mentioned on S ".2 of the above Information sheet 7765 ^, section XXV, page 2, is described by Wheeler and Eckard under the title "Underground Storage of Natural Gas in Coal-Mining Areas". By being able to place a relatively small reservoir in the ground rather than a reservoir on top of one Providing a hill, the investment costs are considerably reduced. The combined savings as a result of the smaller gas container size and the location of the water reservoir lead to this considerable reduction in the system costs.

The invention has been explained with reference to the storage of air for driving air motors, which additional generate electrical power, but any other gas such as carbon dioxide or natural gas could be used and liquefied gases such as liquefied petroleum gas under Pressure can be stored in order to either feed power drives or for heating, air conditioning or chemical processes.

As an example of an embodiment according to the second feature According to the invention, a cavity with a volume of 311 487 cubic meters was created in a rock salt formation, by entering at a depth of 260 m below the surface of the earth Wash out occurred. A concrete reservoir was immediately loaded

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Close to the well bore on the earth's surface and this reservoir has roughly the same volumetric storage capacity as the underground © Hohiravjn. The air in the cavity is stored under a pressure of 31.076 kg / cm, which corresponds to a hydrostatic column of a saturated saline solution of 2βΟ m. Under these conditions, the gas storage capacity is about 9 3 ¹ ^ 10 cubic meters, measured at normal pressure and normal temperature. Ss air was pumped into the cavity and thereby the saline solution was displaced to the surface reservoir and swar with a pressure that exceeds the hydrostatic pressure by a few kilograms per square centimeter. Higher pressures could also be used if a high pump-in speed is required. The gas is withdrawn from the cavity at a pressure of 31.076 kg / cm at a rate of 1962 nr / min and fed to the inlet of a reversible compressor air motor which in turn drives a generator capable of producing approximately 67,000 kW to generate a maximum of 79 hours or a total energy of 5,300,000 kWh. The storage unit is used to provide auxiliary energy for peak load times in a daily or weekly cycle.

The reservoir is always at high pressure when it is filled and therefore a large amount of energy is consumed to fill the reservoir during short periods when over- ' Loss capacity (low load) is available. This leads to the stabilization of the load on the system.

In addition, during storage, the gas becomes saturated with water vapor and, as a result, the generated power increases than that required to pump relatively dry air into the formation. ...- ■ ·

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From the foregoing description, it will be seen that the invention provides a method and system «whereby performance with very much lower costs than previously possible can be delivered «because the storage costs are low and an increased output» performance of the stored gas is achieved.

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Claims (1)

«F f < Patent claims: 1. Method of storing electrical energy using compressed gas in an underground Reservoir is stored, the storage being through electrical power during times of low electrical power Power requirement takes place and the compressed gas is withdrawn in the peak load times and used to feed a Drive motor is used, which in turn has an electric Generator drives, characterized in that · that during pumping in, during storage and while the gas is being withdrawn from the reservoir, the latter is kept under a hydrostatic pressure which increases does not change significantly and is sufficient to drive the engine. 2. The method according to claim 1, characterized, that the underground reservoir is an aquifer with a gas impermeable rock cap is. 3. The method according to claims 1 or 2, characterized in that that the gas is pumped into the reservoir through a first line and out of the reservoir through a second line is deducted. ^. Method according to claim 2, characterized , that the aquifer has a porosity of not less than 10 # and a permeability of not less than about 4 millidarcies. 909849/0791 5. The method according to claims 1 to 4, characterized in that the gas is subterranean in more than one subterranean reservoir different pressures that the gas is stored from the reservoir at lower pressure due to excess electrical power during the relatively low periods Power requirement is compressed to the pressure of a higher pressure reservoir and stored therein, that the gas from the higher pressure reservoir is used to drive the engine; and that the gas drives the engine leaves under a pressure that is above that in the reservoir with lower pressure is available and returned to the reservoir with lower pressure without compression will. 6. The method according to claim 5 » characterized in that the gas is initially in a reservoir with a lower ρ ρ Pressures of approximately 2.812 kg / cm to 10.546 kg / cm are stored and then transferred from this reservoir to a reservoir for higher pressure and at about 5 * 625 kg / cm up to 105.46 kg / cm is stored. 7. The method according to claim 1, characterized in that the subterranean reservoir is substantially gas impermeable and that the gas is under a substantially constant hydrostatic pressure by means of an aqueous liquid is held within a reservoir placed on the surface and above an enclosed column of water communicates with the aqueous liquid within the subterranean reservoir. 909849/0791 • t «I • ill »I 8. The method according to claims 1 or 7 *, characterized in that that the underground reservoir is a washed out cave in a bed of salt. 9. The method according to claims 1 to 8, characterized in that that the gas is air. 10. Plant for storing electrical energy for carrying out the method, according to claims 1 to 9 » with at least one underground storage reservoir, an electrical generator, a gas compressor that can be driven by the generator-motor with a line extending from the reservoir to the surface for pumping gas into the reservoir, with means for withdrawing gas from the reservoir and with means by which the withdrawn gas is supplied to the drive motor,
characterized in that means are provided to keep the reservoir under a pressure which is higher than atmospheric pressure and which does not change significantly when the gas is pumped in, during storage or when the gas is withdrawn. 11. Plant according to claim 10, characterized in that the means by which the pressure in the reservoir is maintained at a pressure above atmospheric, from an aquifer of relatively high porosity and permeability, which has a gas impermeable rock cap. 909849/0791 12. Plant according to claims 10 or 11, characterized in that the aquifer has a porosity of not less than about 10 # and a permeability of not; less than about Owns 5 mlllidarcies. 15. Plant according to claim 10, characterized in that the means by which the reservoir is maintained below a pressure above atmospheric pressure is a reservoir of aqueous liquid located on the surface and through a column of liquid with the liquid within the underground reservoir is in communication. 14. Plant according to claims 8 or 15, characterized in that the underground storage reservoir is a cave washed out of a salt * bed. 90 9 8 49/0791 13th _f Lee rse ι te