 |
Description  |
|
|
BACKGROUND OF THE INVENTION
This invention relates to a method of converting geothermal energy from
subterranean deposits and more particularly relates to converting
geothermal energy from subterranean salt domes.
After using fossil fuels for thousands of years, mankind has found that
reservoirs of this source of energy are rapidly approaching depletion.
Hence, alternate sources of energy are earnestly being sought. Among these
are "inexhaustible energy sources" which include fusion, solar,
breeder-fusion and geothermal energy. In comparison to the other sources,
geothermal energy appears to be the most accessible source having lesser
technical barriers to overcome to make its use practical.
The most formidable technical problem confronting producing geothermal
energy is that high temperature geological formations are so deep that
they are economically and practically inaccessible. Even if these
formations were accessible by drilling or otherwise, the cost of pumping
fluids into these formations for heat exchange can be astronomical in
comparison with tolerable energy costs.
SUMMARY OF THE INVENTION
It has been found that geothermal energy can be practically utilized by
introducing into a subterranean high temperature formation, a liquid which
will evaporate at the conditions of the formation, recovering the gas at
the surface and converting the pressure-volume energy of the gas to other
forms of energy. The latent heat of condensation of the gas can also be
utilized on the surface before the condensate is re-introduced into the
formation for further recovering geothermal energy.
While the liquid can be introduced into practically any accessible high
temperature formation, it is preferred that the liquid be introduced into
a subterranean salt dome or the like wherein a drill easily penetrates, a
cavity can be easily created and wherein the cavity brine solution can
provide an effective heat exchange medium for an immiscible liquid working
fluid. Additionally, a salt cavity can be effectively sealed to liquid and
gas. Pressure developed from the gas resulting from the vaporized
immiscible liquid can be an aid in preventing cavity roof collapse.
Gas recovered at the surface can be converted to various forms of energy,
but it is preferred that its pressure-volume energy is converted to the
mechanical energy of a turbine which in turn drives pumps, compressors and
the like or produces electrical power. Thereafter, the condensed
immiscible liquid can be returned to the salt deposit. The power generated
can supplement the power required to run a brine field which supplies
brine for other purposes, such as brine for electrolytic cells for
producing caustic soda and chlorine.
BRIEF DESCRIPTION OF THE DRAWINGS
Further advantages and benefits will become apparent from the detailed
description of the invention made below with reference to the drawing in
which a diagrammatical illustration of a solution mine cavity being used
to convert geothermal energy is shown.
DETAILED DESCRIPTION OF THE INVENTION
According to the present invention, geothermal energy is converted to
another form of energy at the earth's surface by introducing into a heat
exchange medium, such as a cavity in heat exchange with a high temperature
formation, a liquid which will evaporate, recovering the gas at the
surface and converting the pressure-volume energy of the gas to other
forms of energy. Any geothermal formation can be utilized as a source of
heat so long as (1) a cavity can be created in the formation, (2) adequate
heat transfer can be obtained across the cavity walls, (3) the cavity does
not collapse under the working conditions of the invention and (4) the
cavity is capable of confining high gas pressures without substantial
leakage. Accordingly, a salt dome or the like, as well as other formations
can satisfy the aforestated conditions.
A salt dome which communicates with a very deep high temperature geologic
structure, such as a salt bed, is especially suited for the present
invention. Since salt is highly heat conducting, the high temperature
geologic structure can be an essentially inexhaustible heat source and an
ideal cavity can be created in a salt deposit. Hence, the preferred
embodiment of the present invention will be described with reference to a
cavity in a salt dome.
A cavity is created in the salt dome by methods well known in the art of
solution mining. Preferably the cavity is made with two thermally
insulated conduits communicating with the surface. This can be
accomplished by drilling two spaced boreholes into the salt deposit and
connecting the boreholes by mining cavities laterally, fracturing or other
methods of the art. It is less preferred that the two conduits are
disposed concentrically or in parallel into one borehole. However, some
degree of insulation is obtainable in this latter case.
The depth at which the cavity is to be established is determined from
economics and the temperature of the dome at various depths. It can be
noted that once underlying rock formations are penetrated by the well
bore(s) and the salt deposit is reached, drilling is relatively easy and
great depths can be reached. Hotter cavities at greater depths need not be
as large as cooler cavities at a lesser depth to produce the same amount
of thermal energy. Hence, it may be desirable to establish the cavity as
deep as practical with existing drilling technology, e.g., about 3,000
meters.
The shape of the cavity is important from the standpoint of heat exchange
wall area and from the standpoint of stability against cave-in. While the
cavity may be pressurized according to the present invention, the pressure
may not be great enough to support the roof of the cavity. Hence, a domed
shaped cavity is preferred to minimize the pressure necessary to support
the roof. The area of the cavity walls will be determined in view of the
specific heat of the salt formation, the specific heat of the cavity
solution and the temperature difference between the salt formation and the
cavity solution such that enough heat transfer occurs to produce the
desired amount of energy at surface. Several cavities may be necessary to
supply the requisite amount of heat. A cluster of cavities communicating
with a single station on the surface as taught by U.S. Pat. No. 3,339,979
may be possible, so long as there is immaterial interference or distortion
of the directionality of heat flows into adjacent cavities.
A solution saturated with respect to the salt (i.e., little if any
dissolving of the cavity walls occurs) is maintained in the cavity with a
vapor space provided above the solution level in the cavity. It is also
preferred that changes in cavity solution temperature do not cause
dissolving since a growth migration can cause the cavity to lose the
integrity of its stability and result in roof collapse. The vapor space is
large enough so that cavity solution that may become entrained in gas
escaping the solution surface will not be carried up to the earth surface
along with the power fluid. About one-fifth the cavity volume, for
example, would be a sufficient vapor space. Alternatively, a relatively
large cavity can be developed on the withdrawal conduit and several
hundred feet above the cavity in which the power fluid would be
evaporated. This cavity can also serve as an entrainment separator as does
the vapor space in the former embodiment.
An immiscible liquid (power liquid), such as propane or butane, which is
vaporizable at cavity conditions is introduced into near the bottom of the
cavity. It is preferred that the power fluid is essentially immiscible
with the cavity solution and have a high enough vapor pressure to aid in
maintaining the cavity against roof collapse. Propane is desirable because
it is relatively inexpensive, has a vapor pressure of 34-40 atmospheres at
a temperature range around 120.degree. C., has a critical temperature of
97.degree. C. and is expected to produce a dry superheated vapor around
120.degree. C. Other immiscible power fluids such as butane and other
hydrocarbons can be used. It is also preferred that a vaporized power
fluid is condensable at atmospheric temperature so that air or water
cooling can condense the vapor.
By introducing the power fluid at near the bottom of the cavity, the action
of vaporizing the power fluid creates turbulence in the brine which aids
in heat transfer from the salt through the brine to the propane. Liquid
propane is broken up into small enough droplets to provide a large contact
surface area and thus provide a liquid hold-up which does not allow liquid
propane to accumulate on the surface of the solution. In a preferred
embodiment, the cavity is very long and narrow at its bottom section to
provide a long heat exchange surface. This type arrangement is possible
where an accumulation of insolubles at the bottom of the cavity do not
tend to restrict circulation. A large diameter cavity may be even more
preferred, depending upon the temperature, heat flux, heat transfer
coefficient, and thermal gradient, all of which may be ascertained or
closely estimated by those skilled in the art.
The vaporized power fluid is separated from entrained cavity solution as it
passes through the vapor space. The pressure of the vapor is allowed to
develop to near the formation pressure such as by confining the vapor to a
high pressure, e.g., about 37 atmospheres. This pressure is an adequate
inlet pressure for a turboexpander used above the earth surface to convert
the pressure-volume energy of the vaporized power fluid into mechanical
energy, which can be converted to electrical energy.
The pressurized vapor enters a turboexpander above the earth surface and
exits the turboexpander at lower pressure. The use of these turboexpanders
to convert pressure-volume energy isentropically as well known in the art.
The low pressure exit gas is condensed under pressure such as by a cooling
tower using air or water as the coolant and the pressurized condensate is
returned to the cavity to complete the cycle.
The condensate entering the cavity can be placed in heat exchange with exit
gas from the turboexpander, to preheat liquid injected into the cavity
well and/or also pre-cool gas to be condensed. If low level waste steam is
available, it can be utilized to heat the vapor as it emerges from the
ground and will produce a higher initial heat content. This waste heat can
also be used to preheat liquid power fluid injected into the cavity.
Neither of these uses of the steam has any value unless the steam has no
other use. It only serves to increase the output of the turboexpander.
Separate conduits can be placed in communication with the cavity solution
to control the solution level, to further enlarge the cavity or make other
adjustments in the cavity solution. Particularly, where the evaporator
section of the cavity has been changed through the effect of changing
temperatures or erosion and/or pressure, fresh water or brine can be
injected into the bottom of the cavity to enlarge the cavity. It is known
that water, having a lower density than brine, will rise through the
brine, but the turbulence occurring at the bottom of the cavity will cause
the water to dissolve additional salt at the bottom of the cavity owing to
the agitation effect.
Reference is now made to the drawing which illustrates an embodiment of the
present indication. A well bore is drilled through overburden 1 into salt
dome 2 to a depth of about 3,120 meters. The well bore is cased to a depth
of about 3,000 meters. A tubing is run down to near the bottom of the well
bore and a solvent for the salt is introduced therethrough. An enriched
solution is withdrawn through the annular space between the tubing and the
casing. A cylindrical cavity is mined to about 20 meters radius and
extending from the bottom of the well bore to the casing.
A second well bore is drilled about 50 meters from the first well bore and
to a depth of about 3,020 meters. With a roof insulating blanket in place,
cavities are created in each well bore at a depth of about 3,020 meters
and grown upwardly and laterally by methods known in the art until the
cavities connect at a depth of about 3,010 meters. The cavity is then
grown from both well bores to create a dome-like roof 14, such as by the
method taught by U.S. Pat. No. 2,787,455.
Conduit 3 is run down to near the bottom of the first well bore and conduit
4 is run down to a depth of about 3,005 meters. Solution is withdrawn from
the cavity until it reaches level 6 at about 3,015 meter depth. The
remainder solution is allowed to set and become saturated with respect to
the salt while the cavity vapor space may be pressurized with propane,
natural gas or some other oxygen-free gas to support the roof.
The above earth surface equipment is connected to the conduits
communicating with the cavity. This equipment includes turboexpander 9,
compressor or pump 13 or electric generator 12, and condenser 10 with
coolant inlet 16 and outlet 17. Other equipment necessary to carryout the
present invention is only a matter of expedience by an artisian, e.g.,
valves, pumps, control equipment, etc.
Propane is pumped down conduit 3 into solution 7 and vaporizes in the lower
section 8 of cavity 15. The vapor pressure in vapor space 5 begins to
build until it reaches about 40.8 atmospheres absolute and a temperature
of about 110.degree. C. The vaporized propane exits the earth surface
through conduit 4 at about 40 atmospheres absolute (part of the pressure
is utilized to mobilize the vapor to the surface) and about 104.degree.
C., having a heat content of about 73,700 joules per kilogram at which
condition the vaporized propane enters turboexpander 9. The turboexpander,
having an efficiency of about 70 percent expands this propane
isentropically to an exhaust condition of about 12 atmospheres and about
40.degree. C., and having about 64,609 joules per kilogram. Hence, about
9090 joules per kilogram are utilized from the propane at 70 percent
efficiency. This results in about 123 kilograms of propane per kilowatt
hour energy produced by generator 12. The expanded propane is forwarded by
line 11 to condenser 10 where it is condensed. The condensate is forwarded
by line 21 to heat exchanger 20 where it can be placed in heat exchange
with exit gas from turboexpander 9 by means of inlet line 18 and outlet
line 19.
For a cavity having a cylindrical lower portion which is 40 meters in
diameter and 100 meters deep, about 12,500 square meters wall area is
available for heat exchange. By maintaining the temperature difference
across this surface at about 28.degree. C., it is expected that the rate
of heat flow is about 3.8.times.10.sup.9 joules per hour, based on
somewhat idealized assumptions, which is converted to about 750
kilowatt-hours at generator 12. Hence, about 92,000 kilograms per hour
propane must be circulated through the system.
It can be seen from the above that geothermal energy can be converted to
energy at the earth surface. Advantage is taken of a vaporized gas
developing pressure thereby forcing itself to the surface of the earth
resulting in a savings of pumping costs. Also, its latent heat of
condensation can be expediently utilized. Hence, many modifications of the
present invention will become obvious to those skilled in the art. For
example, old abandoned cavities may also be used for this purpose. Even
though the diameter may be quite large and a significant amount of vapor
passing up through the brine would make increased circulation necessary
within the cavity, it could still provide a large heat source without the
expense of having to drill new cavities. Therefore, the described
embodiment should not be considered limitations except insofar as those
limitations are cited in the claims.
* * * * *
|
|
 |
|
 |
Description |
|
