Abstract:
A compressed gas energy storage and release (CGESR) system and method. The system has a gas compressor, a container for storing compressed gas, a pressure engine, and an underground high temperature region. Conduits connect the gas compressor, the container and the pressure engine. The compressed gas drives the pressure engine. An electrical power generating means is operatively connected to said pressure engine to generate electricity. The system further comprises a means to heat at least a portion of one of the conduits utilizing heat from the underground high temperature region to heat the compressed gas therein prior to driving the pressure engine.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates generally to compressed gas energy storage and release (CGESR) systems. More particularly, the present invention relates to a system which provides increased efficiency and reduced system cost compared to known CGESR systems. 
       BACKGROUND OF THE INVENTION 
       [0002]    As is known, energy may be stored in the form of compressed air in underground reservoirs comprising caverns or wells. Air compressors are used to fill such an underground reservoir, ideally during off peak times when the price of electricity is at its lowest (i.e. at night). The compressed air is then converted back into electricity and used during peak times when the price of electricity is at its highest. The conversion back to electricity is with a pressure engine such as turbine or piston engine in a known fashion. 
         [0003]    One problem with the above compressed air energy storage and release systems is that as the compressed air is expanded in the pressure engine it cools rapidly on expansion. Rapid cooling of the machinery results in 1) enbrittlement of metal, rubber or plastic components, and 2) degradation of lubricants leading, both leading to excess wear, increased maintenance costs, and unexpected failure. To prevent the pressure engines from rapid cooling the air is expanded and heated before it reaches the pressure engines. The requirement for heating greatly reduces the efficiency of the compressed air storage cycle. 
         [0004]    There have been various attempts to improve the efficiency of CAESR systems. For example, U.S. Pat. No. 4,281,256 to Ahrens for a “compressed air energy storage system” discloses a compressed air energy storage system in which an internal combustion reciprocating engine is operable as a compressor during slack demand periods utilizing excess power from a power grid to charge air into an air storage reservoir and as an expander during peak demand periods to feed power into the power grid. As an expander, the internal combustion engine utilizes a mixture of air obtained from the air storage reservoir and combustible fuel, which mixture is burned and expanded in the engine. One problem with this approach is that it requires spending a combustible fuel, and entails generation of pollution which is harmful to the environment. 
         [0005]    As another example, U.S. Pat. No. 7,086,231 issued to Pinkerton entitled “Thermal and Compressed Air Storage System”, discloses a system which utilizes an exhaustless heater, such as a thermal storage unit, to heat compressed air prior to it entering a turbine, which powers an electrical generator. The exhaustless heater is described as being any type of heater which does not produce a waste product (e.g., a noxious or toxic emission). However, the Pinkerton system suffers from the same inefficiencies as the Jacoby system in that it requires spending energy to heat the cold compressed air. Depending on what energy source is used to power the heater, the Pinkerton system may also generate of pollution which is harmful to the environment, even if it is not noxious or toxic. 
         [0006]    Another attempt to increase efficiency of CAESR systems is disclosed in U.S. Pat. No. 4,124,805 to Jacoby entitled “Pollution-Free Power Generating and Peak Power Load Shaving System”. Jacoby discloses a method and means whereby during periods of low load demands upon a conventional type electric power generating plant, the excess power then available is employed (at low cost to the system) to pump low temperature ambient air at relatively low pressure into a subterranean cavity in a salt deposit which is in thermal communication via an interconnecting spire or dome of salt with a geological “mother bed” occurring at such depths below the earth&#39;s surface as to constitute a constant height heat source. The air conduit system is intermittently closed, whereupon the heat intake from the earth&#39;s center causes significant storage of heat energy in the entrapped air and substantial increases of the pressure under which it is entrapped. When load requirements upon the generating plant are high, the high pressure/temperature air supply developed within the subterranean cavity is released to operate any suitably responsive turbine so as to contribute to supply of the higher power demand. A problem with Jacoby is that it requires locating a subterranean salt dome and building a generating plant located in proximity thereto. Furthermore, Jacoby requires establishing the cavity by solution mining techniques in a salt deposit which is in thermal communication via an interconnecting spire or dome of salt with a geological “mother bed” occurring at such depths below the earth&#39;s surface as to constitute a constant high heat source, which is a difficult, and expensive procedure. 
         [0007]    U.S. Pat. App. Pub. No. 2007/0006586 published in the name of Hoffman entitled “Serving End Use Customers with Onsite Compressed Air Energy Storage Systems”, discloses a system for storing compressed air without the use of combustion in an underground void (such as a cave or mine), a below ground tank, or an above ground tank. Hoffman suggests increasing the output of the system by using solar power to heat the cold air that would enter the expander. Other sources of additional heating of the air are mentioned as well, including waste heat, geothermal and any other source heat available on the site. Although considered environmentally friendly, using solar power to power the cold air heaters is problematic in that the equipment necessary is expensive to acquire and install, and requires constant maintenance. The batteries required by the solar power system are not environmentally friendly and need to be replaced at regular intervals measured in years. Furthermore, some locations do not reliably receive sufficient sunlight to permit solar power as the sole source of power, and in most cases backup power will need to be built into the system, resulting in additional costs and inefficiencies. Using source heat that is available on the site is also problematic in that it severely limits possible locations where the system may be built. 
         [0008]    Attempts by others are described in the following U.S. patents, each exhibiting their own problems: U.S. Pat. No. 4,150,547 (Hobson), U.S. Pat. No. 4,765,142 (Nakhamkin), U.S. Pat. No. 4,872,307 (Nakhamkin), U.S. Pat. No. 4,885,912 (Nakhamkin), U.S. Pat. No. 4,936,098 (Nakhamkin), U.S. Pat. No. 5,671,608 (Wiggs), U.S. Pat. No. 7,178,337 (Pflanz). 
         [0009]    In view of the above, there is a continuing need for improvements in CAESR systems. What is desired therefore, are systems and methods which overcome at least some of the problems associated with prior art CAESR systems. 
       SUMMARY OF THE INVENTION 
       [0010]    Preferably, embodiments of the present invention will address the problem of gas from the compressed gas reservoir freezing a power generating turbine using a geothermal heat source to heat the expanded gas prior to the expanded gas entering the turbine under pressure. More preferably, the geothermal heat source will be accessible across a broader geographic area and a more flexible application than the geothermal heat sources currently being used with prior art systems and methods. 
         [0011]    Accordingly, an embodiment of the present invention utilizes a conduit which extends from a compressed gas reservoir and travels downwards below the earth&#39;s surface to an underground geothermal formation having a region with a high temperature and then back up to a pressure engine such as turbine or piston engine located on the earths surface. The compressed gas from the compressed gas reservoir is heated in the conduit as it passes through the high temperature region underground. The conduit is configured and arranged so that the compressed gas is heated in the conduit to a temperature which results in the expanded gas being above freezing prior to entering the pressure engine. 
         [0012]    Other embodiments of the present invention include a valve which regulates the amount of heated and/or unheated compressed gas entering the pressure engine. 
         [0013]    Yet another embodiment of the present invention utilizes a geothermal loop with one end of the loop positioned below the earth&#39;s surface in the high temperature region of the underground geothermal formation and the other end of the loop positioned in a heat exchanger above ground. A separate conduit carrying compressed gas to or from the underground compressed gas reservoir is coupled to the heat exchanger to permit heating of the compressed gas to a temperature which results in the expanded gas being above freezing prior to entering the pressure engine. 
         [0014]    Therefore, according to one aspect of the present invention there is disclosed a compressed gas energy storage and release (CGESR) system comprising:
       a gas compressor for compressing gas;   a container configured to store said compressed gas, said container being in communication with said gas compressor via a first conduit;   a pressure engine in communication with said container via a second conduit, said pressure engine being driven by expansion of said compressed gas, thereby converting said compressed gas into mechanical work;   an electrical power generating means operatively connected to said pressure engine, said electrical power generating means being configured to convert said mechanical work into electricity;   an underground high temperature region positioned at a level below a surface of the earth;   means to heat at least a portion of said first conduit or said second conduit utilizing heat from said underground high temperature region to form a heated compressed gas therein.       
 
         [0021]    According to another aspect of the present invention there is disclosed a method of storing and releasing compressed gas energy, said method comprising the steps of:
       compressing a gas to form a compressed gas;   heating at least a portion of said compressed gas utilizing heat from an underground high temperature region to form a heated compressed gas, said underground high temperature region being positioned at a level below a surface of the earth;   storing said compressed gas in a container, before or after said heating step;   converting at least said heated compressed gas into mechanical work; converting said mechanical work into electricity.       
 
         [0026]    According to another aspect of the present invention there is disclosed a compressed gas energy storage and release (CGESR) system comprising:
       a gas compressor;   a container for storing compressed gas;   a pressure engine;   an electrical power generating means operatively connected to said pressure engine;   an underground high temperature region; and   one or more conduits connecting the gas compressor to said container via said underground high temperature region; and   a conduit connecting said container to said pressure engine.       
 
         [0034]    According to another aspect of the present invention there is disclosed a compressed gas energy storage and release (CGESR) system comprising:
       a gas compressor;   a container for storing compressed gas;   a pressure engine;   an electrical power generating means operatively connected to said pressure engine;   an underground high temperature region;   a conduit connecting the gas compressor to said container;   a conduit connecting said container to a valve;   one or more conduits connecting said container to said valve via said underground high temperature region; and   a conduit connecting said valve to said pressure engine.       
 
         [0044]    According to another aspect of the present invention there is disclosed a compressed gas energy storage and release (CGESR) system comprising:
       a gas compressor;   a valve connected to said gas compressor;   a container for storing compressed gas;   a pressure engine;   an electrical power generating means operatively connected to said pressure engine;   an underground high temperature region;   a first conduit connecting the valve to said container;   at least a second conduit connecting said valve to said container via said underground high temperature region; and   a conduit connecting said container to said pressure engine.       
 
         [0054]    According to another aspect of the present invention there is disclosed a compressed gas energy storage and release (CGESR) system comprising:
       a gas compressor;   a container for storing compressed gas;   a pressure engine;
 
an electrical power generating means operatively connected to said pressure engine;
   an underground high temperature region;   a heat conducting loop having one end positioned in said underground high temperature region   a conduit connecting said gas compressor to said container; and   a conduit connecting said container to said pressure engine, a portion of which conduit is conductively connected to another end of said heat conducting loop.       
 
         [0062]    According to another aspect of the present invention there is disclosed a compressed gas energy storage and release (CGESR) system comprising:
       a gas compressor;   a container for storing compressed gas;   a pressure engine;   an electrical power generating means operatively connected to said pressure engine;   an underground high temperature region;   a heat conducting loop having one end positioned in said underground high temperature region   a conduit connecting said gas compressor to said container, a portion of which conduit is conductively connected to another end of said heat conducting loop; and   a conduit connecting said container to said pressure engine.       
 
         [0071]    According to another aspect of the present invention there is disclosed a compressed gas energy storage and release (CGESR) system comprising:
       a gas compressor;   a container for storing compressed gas;   a pressure engine;   an electrical power generating means operatively connected to said pressure engine;   an underground high temperature region;   said gas compressor being connected to said container to provide compressed gas therein;   said container being connected to said pressure engine to provide said compressed gas to operate said pressure engine;   one or more conduits for passing compressed gas through said high temperature region.       
 
         [0080]    According to another aspect of the present invention there is disclosed a method of storing and releasing compressed gas energy comprising:
       compressing gas and storing the gas in a container;   passing the gas from the container to a pressure engine operatively connected to an electrical power generating means;   heating the gas prior to the gas being passed to the pressure engine by passing the gas through an underground high temperature region.       
 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0084]    Reference will now be made to the preferred embodiments of the present invention with reference, by way of example only, to the following drawings in which: 
           [0085]      FIG. 1  is a diagram of an embodiment of the CGESR system of the present invention, showing a geologic formation for storing compressed gas positioned on a conduit downstream of a high temperature region; 
           [0086]      FIG. 2  is a diagram of another embodiment of the CGESR system of the present invention similar to the embodiment in  FIG. 1 , but which shows the geologic formation positioned on the conduit upstream of the high temperature region; 
           [0087]      FIG. 3  is a diagram of another embodiment of the CGESR system of the present invention; 
           [0088]      FIG. 4  is a diagram of another embodiment of the CGESR system of the present invention; 
           [0089]      FIG. 5  is a diagram of another embodiment of the CGESR system of the present invention; 
           [0090]      FIG. 6  is a diagram of another embodiment of the CGESR system of the present invention; 
           [0091]      FIG. 7  is a detail showing two conduits connected together via a cavity in the high temperature region according to another embodiment of the present invention; 
           [0092]      FIG. 8  is a detail similar to  FIG. 7 , but showing a single conduit having two conduits connected to the cavity in the high temperature region according to another embodiment of the present invention; 
           [0093]      FIG. 9  is a diagram of another embodiment of the CGESR system of the present invention similar to the embodiment in  FIG. 1 , but which employs storage tanks in place of geologic formations to store the compressed gas; and 
           [0094]      FIG. 10  is a diagram of another embodiment of the CGESR system of the present invention similar to the embodiment in  FIG. 1 , but which uses the heated compressed gas to also heat the pressure engine. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0095]    The present invention is described in more detail with reference to exemplary embodiments thereof as shown in the appended drawings. While the present invention is described below including preferred embodiments, it should be understood that the present invention is not limited thereto. Those of ordinary skill in the art having access to the teachings herein will recognize additional implementations, modifications, and embodiments which are within the scope of the present invention as disclosed and claimed herein. In the figures, like elements are given like reference numbers. For the purposes of clarity, not every component is labelled in every figure, nor is every component of each embodiment of the invention shown where illustration is not necessary to allow those of ordinary skill in the art to understand the invention. 
         [0096]    Referring now to  FIG. 1 , there is shown generally at  10  a compressed gas energy storage and release (CGESR) system according to an embodiment of the present invention. A gas compressor  12 , positioned on the earths surface  14 , draws a gas  16 , preferably air, through an inlet  18 , compresses the gas and expels the compressed gas  20  under pressure through an outlet  22  into a conduit  24   a . The conduit  24   a  travels downward under the earth&#39;s surface  14  to an underground geothermal formation having a region of high temperature  26 . The conduit  24   a  passes through the high temperature region  26  and then back up to an underground geologic formation  28 , such as a cavern or cavity capable of holding and storing the compressed gas  20 . The compressed gas  20  is heated as it passes in the conduit  24   a  through the high temperature region  26 . Thus it will be appreciated that the geologic formation  28  will hold and store heated compressed gas  30 . Preferably, the geologic formation  28  will hold and store heated compressed gas compressed by gas compressor  12  at one time, for example an off peak time when the cost of electricity is lower, and release the heated compressed gas to drive the generator  40  at a second time, for example a peak time when the cost of electricity is higher. A conduit  24   b  connects the geologic formation  28  to an inlet end  32  of a pressure engine  34 , such as turbine or piston engine located on the surface  14 . The heated compressed gas  30  drives the pressure engine  34  as it passes therethrough to outlet end  36  which exhausts the gas to the atmosphere. Preferably, the heated compressed gas  30  is allowed to expand in an expander  38  associated with the pressure engine  34  prior to entering the pressure engine  34 . The pressure engine  34  converts the energy in the compressed gas into mechanical work. The pressure engine  34  is operatively connected to and drives an electrical power generating means such as generator  40  which generates electricity which is then transmitted to a power grid through powerlines  42  in a known fashion. Thus generator  40  converts the mechanical work of the pressure engine  34  into electricity. As can now be appreciated the heating of the compressed gas results in an expanded gas used to drive the pressure engine  34  at a temperature above freezing, and so does not interfere with its operation. The fact that the heating of the compressed gas  20  is accomplished by using geothermal heat means that the system  10  is cost effective. Moreover, the fact that the geothermal heat is tapped using an underground conduit, means that the geothermal heat may be accessed in a broader geographical area than is available to known systems. 
         [0097]    Although conduit  24   a  is shown in  FIG. 1  as extending vertically down to the high temperature region  26 , before turning horizontally and looping back vertically upwards towards the surface  14 , it will be appreciated that the conduit  24   a  is prepared using drilling techniques well known in the oil and gas industry. Accordingly, the conduit  24   a  may be formed at other angles than those shown in  FIG. 1 . The ability and technology to drill downwards underground, as well as along a curved trajectory, will be appreciated by persons skilled in the art. Such drilling techniques enable the underground connection of high temperature regions  26  and geologic formations  28  which are not directly below the above ground facilities, but which may in fact be several kilometres horizontally from the aboveground facilities to which they are ultimately connected. The conduits may be simple boreholes, boreholes lined with cement or metal sleeves, or pipes formed from metal or other suitable materials, or combinations thereof. Depending on various factors, including the porosity of the earth at the site of the facility, it may be necessary to have some or all of the conduits formed, in whole or in part, from sections of pipe. In some cases, it may be sufficient to prepare, for example conduit  24   a , by first providing a borehole, then lining only a portion thereof with a cement or metal sleeve near the earth&#39;s surface  14 , and connecting the gas compressor  12  with a pipe. In other cases, it may be possible to omit the cement or metal lining. In other cases still it may be necessary to utilize pipe for the whole of the conduit  24   a , whether or not a cement or metal lining is also used. What is important is that the conduits will be pressurizable. 
         [0098]    Moreover, conduit  24   a , though shown in  FIG. 1  with a looped section  27 , may in fact comprise two separate conduits within the high temperature region  26  and separated by a distance (not shown). For example, as shown in  FIG. 7 , a porous underground field or cavity  29  created by hydraulic fracturing techniques can be used to join conduits  24   a  and  24   b  in the high temperature region  26 . Cavity  29  may be preexisting, or formed when the CGESR system  10  is installed. According to another example, as shown in  FIG. 8 , the looped section  27  may comprise a single pipe  31  defining two conduits  31   a ,  31   b , one for the downwardly moving compressed gas and one for the upwardly moving compressed gas. What is important, however, is that the compressed gas is forced under pressure downward to the high temperature region  26  and recovered by forcing it back upwards, in this case to underground geologic formation  28 . 
         [0099]    Carbon dioxide and helium are alternate gases that could be used instead of air. Other gases may also be chosen by the person skilled in the art on the basis of their heat transfer characteristics, and associated costs, including containment and disposal costs. However, it is contemplated that if a gas other than air were used, it would be desirable to incorporate a closed loop into the system  10  by connecting the exhaust of pressure engine  34  outlet  36  to inlet  18  on the gas compressor. 
         [0100]    For clarity and ease of understanding of the present invention, some of the various components of the system are discussed herein, and shown in the figures, as being separate elements. However, it will be appreciated that some of these components may in fact be combined into a single apparatus which serves the separate functions described herein. For example, although the gas compressor  12  and the pressure engine  34  are discussed, and shown in the figures, as being separate, it is contemplated that they may in fact be combined into one apparatus. Similarly, the pressure engine  34  and the generator  40  may be combined into one apparatus. 
         [0101]    Shown in  FIG. 2  is a compressed gas energy storage and release (CGESR) system according to another embodiment of the present invention, which is similar to the embodiment shown in  FIG. 1 , except that conduit  24   a  travels downward under the earth&#39;s surface  14  to geologic formation  28 , before conduit  24   b  travels to the high temperature region  26 . 
         [0102]    It will be appreciated that in the embodiments shown in  FIGS. 1 and 2 , the preferred CGESR system  10  will include in the gas compressor  12  and the expander  38  means for inhibiting the release of the compressed gas  20  from the geologic formation when either the gas compressor  12  or the pressure engine  34  are not operating. Alternately, valves  37  may be added to the system  10  to accomplish the same result, for example in conduit  24   a  after gas compressor  12 , and in conduit  24   b  before expander  38 . 
         [0103]    Referring now to  FIG. 3 , there is shown generally at  10  a CGESR system according to another embodiment of the present invention. The gas compressor  12 , positioned on the earths surface  14 , draws a gas  16 , preferably air, through an inlet  18 , compresses the gas and expels the compressed gas  20  under pressure through an outlet  22  into a conduit  44   a . The conduit  44   a  travels downward under the earths surface  14  to the underground geologic formation  28 , which as mentioned above is capable of holding and storing the compressed gas  20 . Another conduit  44   b  travels from the underground geologic formation  28  to the high temperature region  26 , passing therethrough and back up this time to a valve  46  on the surface  14 , the function of which will be explained later. The compressed gas  20  is heated as it passes in the conduit  44   b  through the high temperature region  26  and then back up to a valve  46  on the surface  14 . Yet another conduit  44   c  extends from the underground geologic formation  28  upwards to the valve  46  on the surface  14 . Conduit  44   c  connects the valve  46  to an inlet  32  of pressure engine  34  located on the surface  14 . 
         [0104]    As above, conduit  44   b  though shown in  FIG. 3  with a looped section  27 , may in fact comprise two separate pipes within the high temperature region  26  and separated by a distance (not shown). What is important however is that the compressed gas is forced under pressure downward to the high temperature region  26  and recovered by forcing it back upwards in this case to the valve  46 . 
         [0105]    It will now be understood that valve  46  may be used to regulate the amount of heated compressed gas  30  that enters the inlet  32  of pressure engine  34 , as compared to the amount of unheated compressed gas  20 . As shown, valve  46  can be configured to mix the compressed gas  20  from the geologic formation  28  and the heated compressed gas  30  passing through the high temperature region  26 . However, as will be appreciated, the gases  20  and  30  can also be mixed in a separate mixing apparatus (not shown) positioned either before or after the valve  46 , depending on the application. For example, in the embodiment of  FIG. 4 , a separate mixing chamber is not necessary since mixing of the gases  20  and  30  occurs in the geologic formation  28 . 
         [0106]    The regulated compressed gas  48  drives the pressure engine  34  as it passes therethrough to outlet  36  which exhausts the gas to the atmosphere. Preferably, the regulated compressed gas  48  is allowed to expand in an expander  38  associated with the pressure engine  34  prior to entering the pressure engine  34 . The pressure engine  34  is operatively connected to and drives generator  40  which generates electricity which is then transmitted to a power grid through powerlines  42  in a known fashion. 
         [0107]    It will be appreciated that in the embodiment shown in  FIG. 3 , the preferred CGESR system  10  will include in the gas compressor  12  and gas expander  38  means for inhibiting the release of the compressed gas  20  from the geologic formation  28  when either the gas compressor  12  or the pressure engine  34  are not operating. Alternately, valves  37  may be added to the system  10  to accomplish the same result, for example in conduit  44   a  after gas compressor  12 , and in conduit  44   c  before expander  38 . Furthermore, the valve  37  before expander  38  (which is used to regulate the amount of heated compressed gas  30  that enters the inlet  32  of pressure engine  34 ) may be obviated by configuring valve  46  to permit complete inhibition of heated compressed gas  30  into conduit  44   c.    
         [0108]      FIG. 4  shows a variation of the system  10  shown in  FIG. 3  which is described above. As can be seen the gas compressor  12 , is positioned on the earths surface  14 , a draws a gas  16 , preferably air, through an inlet  18 , compresses the gas and expels the compressed gas  20  under pressure to valve  46  through conduit  50   a . One conduit  50   b  connects the valve to the underground geologic formation  28 , which as mentioned above is capable of holding and storing the compressed gas. Another conduit  50   c  travels from the valve  46  to the high temperature region  26 , passing therethrough and then back up this time to the same underground geologic formation  28 . The compressed gas  20  is heated as it passes in the conduit  50   c  through the high temperature region  26  and then back up to the underground geologic formation  28 . Yet another conduit  50   c  extends from the underground geologic formation  28  upwards to the an inlet  32  of pressure engine  34  located on the surface  14 . The valve  46  may be used to regulate the amount of unheated compressed gas  20  that enters the underground geologic formation  28 , as compared to the amount of heated compressed gas  30  enters the underground geologic formation  28 . Ultimately, since only one conduit  50   c  connects the underground geologic formation  28  to the pressure engine  34 , the valve  46  regulates the amount of regulated compressed gas  48  which drives the pressure engine  34  as it passes therethrough to outlet  36  before being exhausted to the atmosphere. Preferably, the regulated compressed gas  48  is allowed to expand in an expander  38  associated with the pressure engine  34  prior to entering the pressure engine  34 . The pressure engine  34  is operatively connected to and drives generator  40  which generates electricity which is then transmitted to a power grid through powerlines  42  in a known fashion. 
         [0109]    As above, conduit  50   c  though shown in  FIG. 4  with a looped section  27 , may in fact comprise two separate pipes within the high temperature region  26  and separated by a distance (not shown). What is important however is that the compressed gas is forced under pressure downward to the high temperature region  26  and recovered by forcing it back upwards in this case to the underground geologic formation  28 . 
         [0110]    It will be appreciated that in the embodiment shown in  FIG. 4 , the preferred CGESR system  10  will include in the gas compressor  12  and the gas expander  38  means for inhibiting the release of the compressed mixed gas from the geologic formation  28  when either the gas compressor  12  or the pressure engine  34  are not operating. Alternately, valves  37  may be added to the system  10  to accomplish the same result, for example in conduit  50   a  after gas compressor  12 , and in conduit  50   c  before expander  38 . Furthermore, the valve  37  after the gas compressor  12  (which is used to regulate the amount of heated compressed gas  30  that enters the inlet  32  of pressure engine  34 ) may be obviated by configuring valve  46  to permit complete inhibition of heated compressed gas  30  into conduit  44   c.    
         [0111]    These latter two embodiments shown in  FIGS. 3 and 4  offer a greater degree of control over the pre-expansion and post expansion gas temperatures, but at the expense of greater capital costs as well as operating and maintenance costs. 
         [0112]      FIG. 5  shows another embodiment of the CGESR system  10 . As can be seen the gas compressor  12 , positioned on the earths surface  14 , draws a gas  16 , preferably air, through an inlet  18 , compresses the gas and expels the compressed gas  20  under pressure to the underground geologic formation  28  via conduit  52   a . As mentioned above, the underground geologic formation  28  is capable of holding and storing the compressed gas  20 . Another conduit  52   b  extends from the underground geologic formation  28  upwards to the surface  14  through heat exchanger  54  and terminates in inlet  32  of pressure engine  34 . Heating loop  56  comprises a closed loop conduit filled with a heat conducting medium  58  such as, for example, air, water, carbon dioxide, or any type of thermal conductor in liquid or gas phase known in the art or later discovered. The heating loop  56  travels underground to the high temperature region  26 , passing therethrough and then back up to the surface  14 . A pump  60  may be used to cycle the heat conducting medium  58  inside the heating loop  56 . Inside the heat exchanger  54 , conduit  52   b  is heat conductively connected to a portion of the heating loop  56 , to provide heat for the compressed gas  20 . However, it is also contemplated that the heating loop  56  may be arranged so that a portion thereof heats the pressure engine  34  and/or expander  38  instead of, or in addition to the compressed gas  30 . 
         [0113]    The heated compressed gas  30  drives the pressure engine  34  as it passes therethrough to outlet  36  before being exhausted to the atmosphere. Preferably, the heated compressed gas  30  is allowed to expand in the expander  38  associated with the pressure engine  34  prior to entering the pressure engine  34 . The pressure engine  34  is operatively connected to and drives generator  40  which generates electricity which is then transmitted to a power grid through powerlines  42  in a known fashion. 
         [0114]    Heating loop  56  though shown in  FIG. 5  with a looped section  27 , may in fact comprise two separate pipes within the high temperature region  26  and separated by a distance (not shown). What is important however is that the heat conducting medium  58  is forced downward to the high temperature region  26  and recovered by forcing it back upwards in a loop. 
         [0115]      FIG. 6  shows a variation of the system  10  shown in  FIG. 5  which is described above. According to this embodiment, the position of gas compressor  12  relative to heat exchanger  54  is swapped with the pressure engine  34  and generator  40 . Thus the gas compressor  12 , positioned on the earths surface  14 , draws a gas  16 , preferably air, through an inlet  18 , compresses the gas and expels the compressed gas  20  under pressure to the underground geologic formation  28  via conduit  52   a . As mentioned above, the underground geologic formation  28  is capable of holding and storing the compressed gas  20 . Heat exchanger  54  is positioned on the surface  14  between the gas compressor  12  and the underground geologic formation  28 . Inside the heat exchanger  54 , conduit  52   a  is heat conductively connected to a portion of the heating loop  56 , to provide heat for the compressed gas  20  before it passes into the underground geologic formation  28 . 
         [0116]    As described above, heating loop  56  comprises a closed loop conduit filled with a heat conducting medium  58  such as, for example, air, water, carbon dioxide, or any type of thermal storage comprising liquid or gas known in the art or later discovered. The heating loop  56  travels underground to the high temperature region  26 , passing therethrough and then back up to the surface  14 . A pump  60  may be used to cycle the heat conducting medium  58  inside the heating loop  56 . 
         [0117]    Another conduit  52   b  extends from the underground geologic formation  28  upwards to the surface and terminates in inlet  32  of pressure engine  34 . The heated compressed gas  30  drives the pressure engine  34  as it passes therethrough to outlet  36  before being exhausted to the atmosphere. Preferably, the heated compressed gas  30  is allowed to expand in the expander  38  associated with the pressure engine  34  prior to entering the pressure engine  34 . The pressure engine  34  is operatively connected to and drives generator  40  which generates electricity which is then transmitted to a power grid through powerlines  42  in a known fashion. 
         [0118]    As above, heating loop  56  though shown in  FIG. 6  with a looped section  27 , may in fact comprise two separate pipes within the high temperature region  26  and separated by a distance (not shown). What is important however is that the heat conducting medium  58  is forced downward to the high temperature region  26  and recovered by forcing it back upwards in a loop. 
         [0119]    It will be appreciated that in the embodiments shown in  FIGS. 5 and 6 , the preferred CGESR system  10  will include in the gas compressor  12  and the expander  38  means for inhibiting the release of the compressed heated gas  30  from the geologic formation  28  when either the gas compressor  12  or the pressure engine  34  are not operating. Alternately, valves  37  may be added to the system  10  to accomplish the same result, for example in conduit  52   a  after gas compressor  12 , and in conduit  52   b  before expander  38 . 
         [0120]    The diameter of the conduits will typically depend on the depth at which the geothermal formation is located and whether air is used as the gas. If a liquid is used as the heat conducting medium  58  for example in the embodiments shown in  FIGS. 4 and 5 , and the geothermal formation is deep underground, a small diameter pipe such as 2⅞ inch may be used and a shallower geothermal formation would more likely require a 3½ inch diameter pipe because of the pressures involved with a 10,000 to 20,000 foot column of fluid. If the heat conducting medium  58  is air the pipe diameter may be greater such as for example 7 to 12 inches, and even greater for shallower geothermal formations. 
         [0121]    It is contemplated that since the compression cycle typically occurs in a different time period from the expansion cycle, another variation on the above described embodiments of the present invention is to heat the just compressed air just before it is put into the underground geologic formation so that the gas that is later brought up for expansion does not need to be heated at the last moment. 
         [0122]    As will be appreciated by persons skilled in the art, the underground geologic formation  28  is an underground cavity which can be used to hold and store compressed gas. One example of an underground geologic formation which may be suitable is an abandoned potash solution mine where water was pumped down into a potash formation and dissolved potash in solution was pumped out. Once all of the economically available potash has been obtained the mine is abandoned and a well contained, stable, cavernous formation is left behind. 
         [0123]    The region of high temperature  26  may be any underground region having a temperature sufficiently above ambient temperature at the surface that the expanded gas will be above freezing when it enters the pressure engine  34 . Preferably the expanded gas will be warm enough to avoid continual condensation of moisture from the ambient surface air. Preferably, temperature of the region of high temperature  26  will be between 30 and 50° F. or higher above the temperature of ambient air at the surface. However, the temperature may be much greater depending on the economics of the heat transfer process and the gas used if not air. An approximation of the temperature gradient is 1° F. fα every 100 feet of depth. Of course this depends on a host of factors including geology, whether it is a geothermal resource zone, etc. A depth of 2,500 to 5,000 feet for example would be a shallow depth that would be sufficient with a good geothermal resource, whereas 10,000 to 20,000 feet might be used under other circumstances including the availability of disused oil or gas wells. 
         [0124]    A well known example of a geothermal formation having a region of high temperature closer to the surface is magma having come closer to the earths surface thereby providing heat energy to a large underground region. 
         [0125]    As another example, it is not uncommon in the oil and gas industry to encounter brine filled formations deep underground where the temperature is as high as 300° F. In this case the brine may be used as the region of high temperature  26 . 
         [0126]    While reference has been made to various preferred embodiments of the invention other variations, implementations, modifications, alterations and embodiments are comprehended by the broad scope of the appended claims. Some of these have been discussed in detail in this specification and others will be apparent to those skilled in the art. For example, the underground geologic formation  28  may be replaced with one or more above ground compressed gas storage tanks  62  as shown in  FIG. 9 . As another example, the heated compressed gas  30  may, in addition to driving the pressure engine  34 , may be used to heat expander  38  as well as the pressure engine  34  itself, as shown in  FIG. 10 , with a heated sleeve  64  about the exterior thereof. Those of ordinary skill in the art having access to the teachings herein will recognize these additional variations, implementations, modifications, alterations and embodiments, all of which are within the scope of the present invention, which invention is limited only by the appended claims.