Underground pumped liquid energy storage system and method

Disclosed is a system and method for producing electrical power during peak power demand periods and for, in effect, storing electrical power or energy during low power demand periods. Two or more cavities are provided in an underground salt dome or other thick salt deposit, with a first one of the cavities being located above the level of the second. Oil or other liquid in which salt is substantially insoluble, such as saturated brine, is placed in at least one of the cavities for use as a working liquid to produce electrical power. The power is produced by delivering the liquid from the first cavity to a pump/turbine unit located at a level below the first cavity to thereby drive the unit so that it produces electrical power. Such power would be produced, for example, during peak load demand periods when the need for power was greatest. Liquid discharged from the pump/turbine unit is delivered to the second cavity for temporary storage. During low power demand periods, electrical power is supplied to the pump/turbine unit to cause the unit to pump liquid from the second cavity back up to the first cavity to await repeat of the power producing phase of the cycle.

BACKGROUND OF THE INVENTION 
This invention relates to a system and method for utilizing hydrocarbon 
liquids, or other liquids in which salt is insoluble, in underground 
cavities formed in salt deposits for producing electrical power. 
So-called pumped energy storage systems are designed to, in effect, take 
surplus power from electrical power generating systems during low power 
demand or off-peak periods, store the power, and then recover it for use 
during peak power demand periods. This might be accomplished by hydro 
pumped storage systems consisting of a surface reservoir, an excavated 
underground reservoir, an underground powerhouse having an electrical 
power generating unit, and conduits for connecting the surface reservoir 
to the underground powerhouse and also for connecting the powerhouse to 
the underground reservoir. During heavy load periods, water from the 
surface reservoir is directed to the powerhouse to drive the power 
generating units, with the discharge being directed to the underground 
reservoir. During light load periods, water from the underground reservoir 
is pumped back to the surface reservoir to await a heavy load period when 
additional power will again be needed. Such a system is described in "An 
Assessment of Energy Storage System Suitable For Use By Electric 
Utilities", EPRI EM-264, Project 225, ERDA E (11-1)-2501, Final Report, 
Volume 3, July, 1976, prepared by Public Service Electric and Gas Company 
Research and Development Department, Newark, New Jersey 07101. Also see 
U.S. Pat. No. 3,643,426, issued Feb. 22, 1972. 
Some of the drawbacks of pumped energy storage systems such as that 
described above are the unavailability of suitable sites for both above 
ground and underground reservoirs, the possible harmful environmental 
impact of providing above ground reservoirs for energy storage, and the 
high cost and difficulty of hard-rock mining or excavating the underground 
reservoir. 
In the past, underground reservoirs have typically been utilized or at 
least suggested for utilization for storing petroleum products. Thus, U.S. 
Pat. No. 3,385,067 discloses storing petroleum in a pair of interconnected 
underground cavities formed in a salt bed. More recently, an article in 
Newsweek (Aug. 22, 1977 at page 55) described the government's strategic 
oil reserve program in which oil would be stored in cavities formed in 
salt domes. This is considered an attractive alternative to above ground 
storage of petroleum because of the greatly reduced costs of the 
underground storage. Of course, with such passive storage of petroleum, 
the petroleum is not utilized in any way until it is removed from storage 
for use as a fuel, lubricant, etc. 
SUMMARY OF THE INVENTION 
It is an object of the invention to provide a new and inexpensive 
underground pumped energy storage system and method. 
It is also an object of the invention to provide such a system which is 
located entirely underground in salt formations. 
It is a further object of the invention to provide an arrangement which can 
serve to both store hydrocarbon liquids and utilize such liquids for the 
production of electrical power during periods of high power demand. 
It is still another object of the present invention to provide a system and 
method of utilizing petroleum products stored underground for the 
production of electrical power. 
The above and other objects of the invention are realized in a specific 
illustrative embodiment of an underground pumped energy storage system 
which includes a pair of cavities formed in an underground salt deposit, 
with one of the cavities being located at a higher level than the other 
cavity. Liquid in which salt is substantially insoluble, such as 
hydrocarbon liquid, saturated salt brine, etc., is disposed in the 
cavities for use as a working medium for the generation of electrical 
power. Electrical power generating apparatus is disposed at a level below 
the upper cavity and is coupled to the upper cavity by a conduit through 
which the liquid may be delivered to the generating apparatus. Another 
conduit delivers the discharge liquid from the electrical power generating 
means to the lower cavity. 
During peak power demand periods, liquid is directed through the conduit to 
the electrical power generating apparatus to cause the production of 
electrical power, and the discharge liquid is delivered to the lower 
cavity. Then, during periods of low power demand, the liquid in the lower 
cavity is pumped back to the higher cavity. 
With the above-described arrangement, hydrocarbon liquids which would 
otherwise be idle in underground cavity storage may be utilized as a 
working medium for the production of electrical power during high power 
demand periods. The use of cavities formed in salt deposits is desirable 
because of the ease with which the cavities may be formed simply by piping 
the water into the deposit to dissolve the salt and make brine and then 
pumping the brine from the resulting cavity. The cavities are 
substantially impermeable to hydrocarbon liquids and require little 
maintenance. Further, since the power production takes place underground, 
there is little impact on the environment. 
Although the invention is especially advantageous for use with hydrocarbon 
liquids as the working medium, other liquids, such as saturated salt 
brine, which do not dissolve salt might also be used for electrical power 
production. Of course, the advantage of combining the storage of liquid 
with use of the liquid for power production would not be present with the 
salt brine arrangement since there is no need to store salt brine. The use 
of cavities formed in salt deposits greatly reduces the cost of the 
arrangement because of the avoidance of the high cost of hard rock mining.

DETAILED DESCRIPTION 
Referring to the drawing there is shown a first cavity 4 and a second 
cavity 8 formed in a salt dome 12 located under the surface of the ground. 
These cavities may be formed simply by supplying water to the desired 
locations of the cavities in the salt dome, dissolving the salt to make 
salt brine, and then pumping the brine to the surface leaving the 
cavities. See, for example, Symposia on Salt (Proceedings), Vol. I-V, 
Northern Ohio Geological Society, Cleveland, Ohio. As indicated in the 
aforecited Newsweek article, and in the U.S. Geological Service Bulletin 
1148, thick salt deposits are quite prevalent in the gulf coast states, as 
well as in midwestern and western states, and would serve as ideal 
locations for practicing the present invention. It has been found that 
these salt formations are quite impermeable to hydrocarbon liquids and 
essentially stable to pressures. 
As shown in the drawing, cavity 4 is formed in the salt dome 12 at a higher 
level than is cavity 8. A suitable liquid 16 is placed into cavity 4 via a 
pipe or conduit 20 which extends from above ground to the cavity 4. 
Although such liquid may be any liquid in which salt is insoluble, the 
discussion hereafter will assume that the liquid is a hydrocarbon such as 
oil or a petroleum product. The conduit 20 serves to allow for subsequent 
storage or removal of the liquid 16 in the cavities. Of course, to remove 
liquid from the cavity 4, the conduit 20 would be positioned to extend 
into the lower part of the cavity. As will be explained further hereafter, 
hydrocarbon liquid may also be placed in cavity 8 either from cavity 4 via 
power producing apparatus or via a conduit 24 extending from above ground 
to the cavity 8. The cavities 4 and 8 may thus be used simply for storage 
of hydrocarbon liquids as part of the "strategic oil reserve" or for both 
storage and production of electrical power during peak load periods as 
will next be discussed. 
Located generally between the cavities 4 and 8 and near the level of cavity 
8 is a chamber 28 formed in the salt dome 12. A tunnel 48 extends from the 
chamber 28 to ground level to provide access to the chamber. Contained in 
the chamber is a reversible pump/turbine unit 32 coupled by mechanical 
coupling 36 to a generator/motor unit 40. A conduit 44 extends from near 
the bottom of cavity 4 to the tunnel 48 and from there downwardly to the 
pump/turbine unit 32. A value 52 controls the flow of liquid through the 
conduit 4. Another conduit 56 extends from the pump/turbine unit 32 to the 
lower part of cavity 8. A valve 60 controls the flow of liquid between the 
pump/turbine unit 32 and the cavity 8. The pump/turbine unit might 
illustratively be a conventional centrifugal power-recovery pump-turbine 
such as those described in Chemical Engineers Hand Book, Perry, R. H., and 
Chilton, C. H., Editors, Fifth Edition, McGraw Hille, New York, 1973, 
pages 24-36 et seq. The generator/motor unit 40 is conventional equipment 
capable of functioning as a generator to produce electricity when driven 
by an external power source, or as a motor when electricity is supplied to 
the unit. The generator/motor unit is connected by power line 64 to a 
power sink or source (not shown). 
In operation, hydrocarbon liquid 16 is supplied to cavity 4 until a 
sufficient quality is obtained to act as a peak power source liquid. When 
power is needed, valves 52 and 60 are opened so that liquid 16 flows 
through conduit 44 to the pump/turbine unit 32 to drive the unit. The 
discharge liquid flows through conduit 56 into cavity 8. Driving the 
pump/turbine unit 32 causes the unit to drive the generator/motor unit 50 
to produce electrical power which is supplied by power lines 64 to a power 
sink (not shown). Liquid 16 is directed to the pump/turbine unit 32 until 
either the demand for power abates or all of the liquid in the cavity 4 
has been supplied to the pump/turbine unit 32. At this point, cavity 8 
would typically contain more liquid than would cavity 4. As liquid is 
discharged into cavity 8, the gas in the cavity space may be forced out 
through conduit 24 to the atmosphere. Alternatively, conduits 20 and 24 
are capped, as shown, to prevent escape of potentially noxious fumes, and 
a conduit 26 is provided to join conduits 20 and 24 to carry the fumes and 
gas between the cavities as the liquid 16 is moved between the cavities. 
After completion of power production, and preferably during a low power 
demand period, the liquid in cavity 8 is pumped back up to cavity 4. This 
is done by supplying power via power line 64 to the generator/motor unit 
40 to drive the motor and thereby drive the pump/turbine unit 32. The unit 
32 pumps liquid from cavity 8 through conduits 56 and 44 back up to cavity 
4, where the liquid will remain until power from the system is again 
needed, and gas is forced from the cavity 4 through conduits 20, 26 and 24 
back to cavity 8. 
Although a reversible pump/turbine unit 32 and a reversible generator/motor 
unit 40 are shown and are preferred for the present invention, separate 
units could be utilized. Thus, a pump 70 is shown in dotted outline in the 
chamber 28 coupled by a mechanical coupling 74 to a motor 78, also shown 
in dotted line. If a separate pump 70 and motor 78 were used, then the 
unit 32 would simply be a turbine and the unit 40 would be only a 
generator. Then, when liquid was being supplied to the turbine 32, valves 
52 and 60 would be opened and valves 80 and 82, coupling the pump 70 to 
the conduits 44 and 56 respectively would be closed. When pump 70 were 
operating to pump liquid from cavity 8 up to cavity 4, then valves 52 and 
60 would be closed and valves 80 and 82 would be opened. The valves are 
shown schematically and would, advantageously, be remotely operated. Also, 
power lines would be coupled to motor 78 to drive the motor. 
In the manner described above, a pair of cavities formed in a salt deposit 
is supplied with a liquid which will not dissolve the salt and this liquid 
is utilized for production of electrical power preferably during high 
power demand periods. Such high demand periods typically occur during the 
day, whereas at night the power demand slackens off. Similarly, more power 
is consumed on week days than on weekends so that weekends generally 
constitute low power demand periods. All this is well known in the power 
production industry. Since the described system is entirely underground, 
there would be little environmental impact from such a system. Also, since 
plans are already underway to store oil in underground cavities formed in 
salt domes, little additional expense would result in implementing the 
above-described system to increase the efficiency and economy of the 
electric power industry. 
It is to be understood that the above-described arrangement is only 
illustrative of the application of the principles of the present 
invention. Numerous modifications and alternative arrangements may be 
devised by those skilled in the art without departing from the spirit and 
scope of the present invention and the appended claims are intended to 
cover such modifications and arrangements.