Abstract:
A method and system for generating electrical power from geothermal, gas pressure let down, and/or heated waste steam sources utilizes a twin-screw compressor reversed to operate as an expander, wherein the expansion provides mechanical power than can be converted to electrical power utilizing a generator, without the need to utilize dry steam turbines. Multiple stages may be utilized in the expansion process.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to our co-pending U.S. Provisional Patent Applications Ser. No. 61/295,566, filed Jan. 15, 2010, and Ser. No. 61/390,786, filed Oct. 7, 2010, the entirety of which are both incorporated by reference herein. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to generating electricity and, more specifically, to electrical power generating system utilizing waste steam, gas pressure, and geothermally heated water. 
         [0004]    2. The Prior Art 
         [0005]    Despite significant advances in numerous non-thermal power generation technologies, the application of heat to convert water into steam still forms the basis of most power generation worldwide. While coal is the predominant fuel that produces that heat, competing fuels include nuclear fusion, various forms of biomass, garbage and concentrated infrared solar radiation. The exhaust heat of high-temperature air based engines is often used to generate steam in either fire-tube or water-tube boilers. 
         [0006]    Most of the power generated in the world currently is generated utilizing dry steam turbines that drive electrical generators. The dry steam may be generated by heat from nuclear reactors or from the combustion of fossil fuels, such as coal and natural gas. This process has become fairly efficient over the last hundred years. However, there are problems and limitations from this method of electrical generation. Turbines have turbine blades rotating at very high rates of speed, and as a result, they are very fragile. The dry steam that they utilize has to be extremely clean in order to keep from destroying turbine blades. For similar reasons, they cannot utilize wet steam or water. These limitations prevent these turbines from being used in many applications. 
         [0007]    Especially problematic for electric power generation are geothermal applications. At present, heat exchangers are used that heat clean water from heated geothermal water, before the water can be turned into dry steam. This is inefficient and is hard to effectively scale such technology down for use with smaller sources. 
         [0008]    It would be advantageous to be able to generate electricity from geothermal, gas pressure, and/or heated waste steam sources directly without the need to utilize dry steam turbines. It would be advantageous if electrical power could be generated from hot water, gas pressure, and from wet steam. 
       BRIEF SUMMARY OF THE INVENTION 
       [0009]    This utility patent application discloses and claims a useful, novel, and unobvious invention for an electrical power generating system utilizing waste steam, gas pressure, and geothermally heated water. Its major components are: 
         [0010]    1. A Two-Stage Direct Steam and Gas Screw Expander Generator System (DSG) for receiving waste steam, gas pressure, or geothermally heated water and utilizing the energy thereof for driving at least one output shaft; and 
         [0011]    2. A rotary generator coupled to the output shaft for generating electricity. 
         [0012]    One advantage of utilizing a (DSG) in the system is its ability to directly accept waste steam, gas pressure, or geothermally heated water thereby utilizing all of the available energy from waste steam, gas lines, or geothermal wells. A further advantage of the (DSG) is that it is coated with a special polymer coating to protect it from corrosion and abrasion. 
         [0013]    The (DSG) is able to run efficiently over a wide range of power loads at constant speed. Besides being of prime importance to power companies in meeting fluctuations in power demand, this characteristic allows the system to be applied to a wide range of geothermal fluid inlet conditions. As a result, the system of the present invention can operate efficiently in any number of different geothermal and gas pressure let down locations having different pressures, temperatures and flow conditions. The features of the present invention which are believed to be novel are set forth. 
         [0014]    110 Trillion cubic feet of natural gas goes through 3 million Gas Letdown stations each year worldwide. Natural gas is transported for long distances through pipelines at high pressure 1000 psi. The high pressure gas is reduced to a lower pressure by means of Gas Pressure Letdown Stations. In City Gate Stations, the pressure must typically be reduced from 1000 psi to 250-50 psi. Gas pressure reduction is typically accomplished with throttling valves, where the isenthalpic expansion takes place without producing any energy. A certain amount of pressure energy is wasted in that irreversible process of throttling the natural gas and lowering its potential energy. Most gases cool during expansion (Joule-Thompson effect). The temperature drop in natural gas is approximately 1 OP per 15 psi, depending on gas consumption and state. The replacement of the gas-throttling process of expansion with the use of the Langson (GPG) Gas Pressure Generator makes it possible to covert this pressure of the natural gas into mechanical energy, which can be transmitted to a loading device, like an electric generator, thus generating electricity from a previously wasted resource. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]      FIG. 1  is schematic view of an electrical power generating system, in accordance with one embodiment of the present invention. 
           [0016]      FIG. 2  is sectional view of a (DSG) “Two-stage Direct Steam and Gas Screw Expander” utilized in a power generating system, in accordance with one embodiment of the present invention. 
           [0017]      FIG. 3  is front view of two twin-screw expanders connected in series and cascading, which can be utilized in a power generating system, III accordance with one embodiment of the present invention. 
           [0018]      FIG. 4  is a frontal view of a single twin screw expander and generator which can be utilized in a power generation system, in accordance with one embodiment of the present invention. 
           [0019]      FIG. 5  is a side view of another twin screw expander and generator used for gas pressure let down and direct steam expansion and can be utilized in a power generation system, in accordance with one embodiment of the present invention. 
           [0020]      FIG. 6A  is a cross sectional view of an Single Stage, Dry Screw, Gas or Steam Expander, which can be utilized in a power generating system, in accordance with one embodiment of the present invention. 
           [0021]      FIG. 6B  is a cross sectional view of a Single Stage, Oil Flooded Expander, which can be utilized in a power generating system in accordance with one embodiment of the present invention. 
           [0022]      FIG. 7  is a graph comparing the amount of potentially available energy utilized by the system using a Two-Stage (DSG) Screw Expander, in accordance with one embodiment of the present invention. 
           [0023]      FIG. 8  is a block diagram that shows a two-stage gas pressure reduction generator, in accordance with one embodiment of the present invention. 
           [0024]      FIG. 9  is a diagram that shows a two-stage gas pressure reduction system, III accordance with one embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0025]    The present invention is a rugged, continuous-flow, externally heated rotary engine that can operate on low-pressure steam and gas pressure, including saturated or wet steam that may be contaminated with impurities. The rugged design of the engine allows it to be relatively immune to impurities and particles that would erode conventional metallic turbine blades. For equal pressure ratio and power output, the present invention involves a much lower capital cost than a conventional multi-bladed steam turbine intended to operate on low-pressure gas and wet steam. The design of the electrical power generating system which is disclosed utilizes the entire amount of energy available in waste heat steam, gas pressure, or geothermally heated water. The power generating system comprises a source of waste heat steam, gas pressure, or geothermally heated water. One or more twin screw expanders or an all-in-one (DSG) are provided for receiving said waste heat steam, gas pressure, or geothermally heated water and utilizing the energy generated therein for driving at least one output shaft. The (DSG) comprises one or more pair of mating rotors rotate mounted within a housing in a timed relationship. A generator is typically coupled to the output shaft for generating electricity. As the waste steam, gas pressure, or geothermally heated water flows through the expanders, the liquid or gas drops in pressure and a portion thereof may then flash to the vapor phase. The mass flow of vapor continues to increase as the pressure drops through the expanders. This increases the mass flow of the vapor and expands the chambers formed by the rotors to rotatably drive the rotors, and thus the output shaft connected thereto to, for example, a generator to produce electricity. 
         [0026]    Two-Stage Direct Steam and Gas Screw Expander Generator System (DSG). The present invention produces electrical power from waste steam, gas pressure, and geothermally heated water as the motive fluid. The generation of electricity from waste steam, gas pressure, or geothermal water is very desirable for many reasons. Waste steam fumaroles, gas let-down stations, or geothermal wells throughout the world provide a virtually unlimited supply of energy for power generation. Another reason is that fuel-burning power plants can contribute to pollution and possibly global warming through the release of greenhouse gases such as CO2. 
         [0027]    There may be 20 times more liquid-dominated geothermal fields in the world than vapor-dominated fields. The vast majority of geothermal energy available in these wells is typically in the form of saturated steam, most of which is typically hot water or brine. Only a limited number of wells throughout the world emit superheated or dry steam. Present day geothermal power systems utilizing steam turbines as their prime mover can typically only operate on dry steam. These turbines simply cannot accept moisture, particulate matter, or dissolved solids. Because of this, present day power generating systems are required to separate the dry steam from the mixture before the steam can be utilized by the turbines. Although the separation and the dumping of this hot water are necessary, this is not very efficient because a vast amount of available energy is wasted. In many geothermal wells, approximately two-thirds of the available geothermal energy is in the form of water, and this energy is wasted with turbine systems that require dry steam. The present invention has succeeded in utilizing waste steam, gas pressure and geothermally heated water as the motive fluid by utilizing (DSG) as the prime mover instead of turbines. 
         [0028]    Heretofore, twin screw machines were utilized mostly as vapor compressors. Few machines were used as expanders and in all of such cases, the motive fluid for these machines was in for form of vapor. In short, prior to the present invention, no one had utilized a (DSG) machine to operate as an expander driven by high temperature, high pressure water, and to drive generators for generating electricity. 
         [0029]      FIG. 1  is schematic view of an electrical power generating system, in accordance with one embodiment of the present invention. The electrical power generating system comprises a source of waste steam or geothermally heated water  10  delivered through a conduit  17  to the DSG  35 . The source of waste steam or geothermal heated water  10  may be a well, and the well may have one or more valves  12 . A filter  14  may be provided for the conduit  17 . A gate valve  27  may also be provided within the conduit  17  for controlling the flow of heated water entering the DSG  35 . A check valve  16  may also be provided. The DSG  35  is connected to the motive fluid from the conduit  17 . The (DSG)  35  includes an output shaft  37  that may be coupled to a rotary generator  40 . 
         [0030]    This portion of the power generating system of the present invention typically operates as follows: The entire flow from the well  10  is preferably kept under pressure to prevent its flashing into steam. A normal condition for the saturated liquid may be I35 psia and approximately 350° F. The liquid passes through the control valve  27  and then into the DSG screw expander  35 . As the liquid enters the expander  35 , it drops in pressure and a small portion of it will flash into the vapor phase. As the pressure continues to drop, the mass flow of vapor continues to increase. This increase in mass flow of vapor is the medium for driving the DSG  35 . The outlet condition for the first stage of the (DSG) may be 75 psia and approximately 300° F. At this point, the majority of the mixture may be a saturated liquid. The vapor mass flow continues to increase to drive the DSG  35 . The outlet condition for the second stage of the expander  35 , again for the sake of example, may be 14 psia at approximately 101° F. 
         [0031]    The mixture exiting from the second stage expander  35  may then be fed into a separator  43 . Some of the functions of the separator  43  are (1) to operate under vacuum to lower the exhaust pressure of the second expander stage thereby increasing the work output, and (2) to separate the liquid from the vapor for having the vapor condensed to a liquid state. After separation, the liquid may then exit the separator  43  through a conduit  45  to a contact condenser  50 . The vapor then may exit the contact condenser  50  through a conduit to a reinjection well  55 . 
         [0032]    There may also be an ejector  18  coupled between the input conduit  17  and the contact condenser  50 . It can also separate out the non-condensable gas  19 . Also, a cooling tower may also be coupled to the condenser  50 , providing additional cooling, should that be necessary. The output from the cooling tower  52  and the condenser  50  may be controlled by a check valve 5151 before being transmitted through a gate valve  54  to the reinjection well  55 . 
         [0033]      FIG. 2  shows an intermeshing (DSG) used as the prime mover  35  in the power generating system. The expander comprises two pair  65  and  67  of intermeshing rotors, each pair preferably rotatably mounted on one shaft  68  within the housing  70 . A timing gear  73  may be connected to the extremities of the shaft  68  and is preferably interengaged to synchronize the rotational speeds of the rotors. The result is that the rotor sets  65  and  67  preferably do not engage in a binding sense during rotation, and form a two stage expander in one embodiment. 
         [0034]      FIGS. 6A and 6B  show examples of different embodiments of pairs of intermeshing rotors  69 ,  71 . Thus, the DSG  35  shown actually has four rotors—a male  69  and a female  73  rotor in the first stage  65 , and a male  69  and a female rotor  73  in a second stage  67  set of rotors. This is illustrative, and other numbers of stages are also within the scope of the present invention. However, it has been found that a two stage system as shown here provides good results in many situations. 
         [0035]    Suitable shaft and thrust bearings  77  are preferably provided to adequately support the rotors  65  and  67  within the housing  70 . As the motive fluid enters the inlet  22 , pockets formed between the rotors and the casing wall typically begin to form. As the rotors  65  and  67  turn, these pockets are further separated and increase in volume permitting the motive fluid to expand. As pointed out above, the (DSG), is capable of accepting waste steam, gas pressure, or geothermally heated water. It expands directly the steam or gas that is continuously being produced therefrom as the water, gas, or other fluid decreases in pressure through the machine. Thus, as the mass flow of steam, gas, or other fluid increases as the pressure drops through the expander, the inherent energy is more fully utilized and not wasted. 
         [0036]    U.S. Pat. No. 7,637,108 titled “Power Compounder” issued Dec. 29, 2009, and U.S. Patent Application Number 2006/0236698 Al titled “Waste Heat Recovery Generator” published Oct. 26, 2006, both by the Applicant herein, disclose single and dual rotor expanders applicable herein, and are incorporated herein by reference. 
         [0037]      FIG. 3  is front view of two twin-screw expanders connected in series and cascading, which can be utilized in a power generating system, III accordance with one embodiment of the present invention. In this illustration, the twin-screw expanders drive the electric generator with a belt. This is illustrative, and other methods of transferring power from the twin-screw expanders to an electric generator are also within the scope of the present invention. Moreover, other uses than for generating electricity are also within the scope of the present invention. 
         [0038]      FIG. 4  is a frontal view of a single twin screw expander and generator which can be utilized in a power generation system, in accordance with one embodiment of the present invention. In this illustration, the single twin-screw expander drives the electric generator with a belt. 
         [0039]      FIG. 5  is a side view of another twin screw expander and generator used for gas pressure let down and direct steam expansion and can be utilized in a power generation system, in accordance with one embodiment of the present invention. In this illustration, a DSG  35  is coupled by a shaft  37  to an electric generator  40 . While this embodiment shows an electric generator  40  being driven by the shaft  37  from the DSG  35 , it should be understood that this is illustrative, and other uses of the power transferred by a drive shaft are also within the scope of the present invention. 
         [0040]      FIG. 6A  is a cross sectional view of a Single Stage, Dry Screw, Gas or Steam Expander, which can be utilized in a power generating system, in accordance with one embodiment of the present invention.  FIG. 6B  is a cross sectional view of a Single Stage, Oil Flooded Expander, which can be utilized in a power generating system in accordance with one embodiment of the present invention. 
         [0041]      FIGS. 6A and 6B  show twin rotor expanders that have a male rotor  69  interfacing with a female rotor  73 . The male rotor  69  may have four lobes  71  which are adapted to extend into six flutes  72  formed in the female rotor  73 . A housing  70  may also be provided with an inlet  22  extending into the one end of the rotor chamber  15  and an exhaust  23  leading from the other end. A timing gear may be connected to the extremities of the shaft  68  and is preferably interengaged to synchronize the rotational speeds of the rotors. The result is that the rotors  69  and  73  preferably do not engage in a binding sense during rotation. Indeed, it is preferable that, through timing and tolerances, that the two rotors  69 ,  73 , never actually touch, but rather the tolerances between them are sufficient that there is no binding between rotors or between rotors and the sides of the housing  70 , depending on the expected work material for a particular DSG. 
         [0042]    Since the (DSG) is a positive displacement machine, it is typically able to run efficiently over a wide range of power loads at constant speed. Besides meeting the fluctuations in power demand, the system can be applied to a wide range of steam, gas pressure, and geothermal fluid inlet conditions. Thus, one system can efficiently cover a multitude of different pressures, temperatures and flow conditions. 
         [0043]    As steam, gas, and liquid enters the machine and drops in pressure, a fraction thereof flashes to a vapor phase. As the pressure continues to drop, the mass flow of vapor increases. Similarly the enthalpy drops. 
         [0044]    In contrast, a turbine installation on the same fluid input must first reduce the pressure to an optimum point where the flashed steam is separated. Then only this fixed amount of steam is utilized. As a result, the amount of the power potential utilized by the turbine is approximately one third of the full potential energy utilized by the (DSG). 
         [0045]    The surface of the screw and the interior surface of the screw housing may be coated with a special polymer coating to prevent corrosion and excessive wear by chemicals, solids, and minerals. This may be a version of Teflon, or other material, depending on the type of fluid or gas being expanded. 
         [0046]      FIG. 8  is a block diagram that shows a two-stage gas pressure reduction generator  90 , in accordance with one embodiment of the present invention. Natural gas may enter  82  the system at, for example, 600 psia and 100° F. A direction control valve  84  may be utilized to selectively direct the natural gas through either a gas pressure reduction valve  86 , or the two stage pressure reduction generator  90 . If the natural gas is directed towards the two-stage pressure reduction generator  90 , it first enters a first stage DSG  92 . Then, when it leaves the first stage DSG  92 , it enters the second stage DSG  94 . When the gas leaves either the second stage DSG  94  or the gas pressure reduction valve  86 , it will typically be at a significantly lower pressure and temperature. For example, the gas may leave the system  96  at 50 to 200 psia and 60° F. In this embodiment, a two-stage gas pressure reduction generator is shown. This IS exemplary, and other numbers of stages are also within the scope of the present invention. 
         [0047]    Natural gas is typically transported long distances at a much higher pressure than is utilized for delivery. Currently, the energy inherent in that high pressure is lost when the pressure is reduced so that the gas can be utilized. The gas pressure reduction valve  86  shown in this FIG. is a typical mechanism for accomplishing this pressure reduction in the prior art. One of the advantages of utilizing the present invention in this way is that this energy can be efficiently captured and turned into electrical power. 
         [0048]      FIG. 9  is a diagram that shows a two-stage gas pressure reduction system, in accordance with one embodiment of the present invention. Natural gas may enter the system at, for example, 600 psia and 100° F. on a main gas line  101 . A reducer  102  controls the flow of natural gas from the main gas line  101  into a first high pressure line  103 . The first high pressure line  103  feeds into a gas heater  104 , the output of which may be fed into a second high pressure line  105 . In a prior art portion of the system, the high pressure gas line  105  feeds into a Let Down Station  106 , and its output is fed into a low gas line  107 . Alternatively, a portion, if not all, of the gas from the second high pressure gas line  105  may be fed through a ball valve  110 , followed by a pressure regulator  112  into a feed gas line  113 . The gas in the feed gas line  113  is then fed to an additional gas heater  114  if necessary, and thence by a pressure gauge  116  and temperature gauge  118  into a two-stage twin-screw expander  120 . The output gas from the twin screw expander  120  is fed to a return gas line  129  which passes a pressure gauge  126  and temperature gauge  128 , and into a check valve  108  and ball valve  109 , and back into the low pressure gas line  107 . The twin-screw expander  120  may drive a generator  122 , which may produce electricity  123 . It may also be coupled to a temperature gauge  124 . 
         [0049]    In summary, the power generating system of the present invention has unique qualities which enable the efficient use of waste steam, gas pressure, and geothermal energy. This system is simple, low in maintenance and long-lived. 
         [0050]    Those skilled in the art will recognize that modifications and variations can be made without departing from the spirit of the invention. Therefore, it is intended that this invention encompass all such variations and modifications as fall within the scope of the appended claims.