In situ geothermal power

Abstract

A method of generating electricity from geothermal energy utilizing an in situ closed loop heat exchanger deep within the earth using a recirculating heat transfer fluid to power an in situ modular turbine and generator system within a vertical, large bore, deep, tunnel shaft. The shaft length and diameter are dependent on the shaft temperature and sustaining heat flux. The method further includes methods of deep shaft boring and excavating, liner placement and sealing, shaft transport systems, shaft Heating, Ventilation, and Air Conditioning, and operations and maintenance provisions. The method has few global location restrictions, maximizes thermal efficiency as to make power generation practical, has a small site surface footprint, does not interact with the environment, is sustainable, uses renewable energy, and is a zero release carbon and hazardous substance emitter.

Claims

1. A method of producing electric power by operating a geothermal power plant, comprising the steps of: excavating a tunnel shaft that is greater than four feet in diameter, deep within the earth to provide a sustaining heat flux and geothermal energy; installing a plurality of permanently in-situ heat exchanger tubes at the bottom of the shaft contained within a conductive grout to transfer the geothermal energy to a heat transfer fluid; installing a modular turbine in the shaft above the heat exchanger, routinely coupling and decoupling the modular turbine from the power plant and moving said modular turbine to the surface for maintenance; installing a modular electric generator in the shaft above the turbine, routinely coupling and decoupling the modular electric generator from the power plant and moving said modular electric generator to the surface for maintenance; installing a modular condenser in the shaft above the turbine, routinely coupling and decoupling the modular condenser from the power plant and moving said modular electric generator to the surface for maintenance; installing a pump to recirculate the heat transfer fluid; installing connecting conduits for connecting and recirculating heat transfer fluid in a closed loop between the in-situ heat exchangers, the turbines, the condenser, the pump, and back to the in-situ heat exchangers; recirculating the heat transfer fluid through said conduits of the closed loop to drive the electric generator to produce electric power.

2. The method of producing electric power by operating a geothermal power plant as set forth in claim 1, comprising a further step of lining the tunnel shaft with a structurally reinforced, sealed, concrete liner to provide a structural and chemical boundary from the earth.

3. The method of producing electric power by operating a geothermal power plant as set forth in claim 2, comprising a further step of installing a transport infrastructure to the concrete liner to provide an independent engine riding on the infrastructure to transport modular system components, excavated material, equipment, and work crews up and down the shaft.

4. The method of producing electric power by operating a geothermal power plant as set forth in claim 1, wherein the connecting conduits are routinely coupled and decoupled to the in-situ heat exchangers, the turbines, the condenser, and the pump.

Description

DESCRIPTION OF THE DRAWINGS

(1) The drawings are for illustrative purposes. The drawings shown are not restrictive to the design and are not to scale.

(2) The foregoing aspects and many of the attendant advantages of this disclosure will become more readily appreciated as the same become better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein:

(3) FIG. 1 is a cross-sectional illustrative view of a typical IGP plant where all major system components are modular in design and contained within the deep shaft in accordance with the embodiment of the present disclosure;

(4) FIG. 2 is a cross-sectional illustrative view of example in situ heat exchanger configurations where the location specific design is dependent on the shaft's sustaining heat flux and temperature.

DETAILED DESCRIPTION

(5) The detailed description set forth below in connection with the appended drawings, where like numerals reference like elements, is intended as a description of various embodiments of the disclosed subject matter and is not intended to represent the only embodiments. Each embodiment described in this disclosure is provided merely as an example or illustration and should not be construed as preferred or advantageous over other embodiments. The illustrative examples provided herein are not intended to be exhaustive or to limit the disclosure to the precise forms disclosed. Similarly, any steps described herein may be interchangeable with other steps, or combinations of steps, in order to achieve the same or substantially similar result.

(6) In the following description, numerous specific details are set forth in order to provide a thorough understanding of exemplary embodiments of the present disclosure. It will be apparent to one skilled in the art, however, that many embodiments of the present disclosure may be practiced without some or all of the specific details. In some instances, well-known process steps have not been described in detail in order not to unnecessarily obscure various aspects of the present disclosure. Further, it will be appreciated that embodiments of the present disclosure may employ any combination of the features described herein.

(7) Embodiments of the present disclosure relate to methods for the design of an In Situ Geothermal Power (IGP) plant. An IGP plant may be sited anywhere on the planet. Certain locations are less or more desirable. A less desirable location, dissimilar to an Enhanced Geothermal Systems plant, is an area that exhibits elevated seismic and near surface geothermal activity. A more desirable location is an area that may benefit from a low cost electricity supply, for example, an impoverished area that is isolated from an electric grid, that has an average to low seismic activity, as well as an average to high geothermal gradient.

(8) Referring to FIG. 1, a deep tunnel shaft 1 is first excavated. The shaft is not a small diameter well, but rather a vertical tunnel of substantial diameter to accommodate some or all power plant system components. The shaft may be vertical, at an angle, or helical. The major regions of the shaft include a near surface region 9, a geothermal heat exchanger region 8, and a region in between the surface and heat exchanger region 7. These regions 7, 8, 9 are defined by transitions, which are shown as lines 12, 13. The surface of the earth is above the shaft 11. Current Tunnel Boring Machines (TBM) and similar vertical boring technology may be utilized to bore the shaft, excavate the earth, and seal the shaft with a structural liner 10. The liner provides a boundary that inhibits shaft collapse and any of earth's corrosive or other hazardous substances from interacting with the closed loop IGP system. The liner may support an integral transport system such as a rail(s) or conveyor. This integral transport system may negate the use of cranes and rigging that could fail or be inefficient under such long distances. The integral transport system may utilize a modular engine to transport equipment, excavated material, crews, and modular components down and up the shaft. The TBM or similar may be left at the bottom of the shaft upon completion of the excavation.

(9) Completion of the shaft is based on several design parameters. For example, a shaft depth may typically reach 4 to 12 miles dependent on the site location's geothermal properties. The main design parameters are rock temperature and sustaining heat flux. The temperature is simply the temperature of the surrounding shaft rock. Ideal temperatures may be in the hundreds of degrees Celsius. The heat flux is considered as the rate at which heat is replenished by the earth when removed. Referring to FIG. 1, these ideal design parameters are located in region 8. The heat flux rate will be a design parameter in determining the overall heat exchanger(s) dimensions and type. These design parameters will vary based on the shaft site location.

(10) Referring to FIG. 1 for a typical heat exchanger location 8, the deep shaft may reach a rock temperature well above the boiling point of water. However, some shafts may need to stop excavation for local geological reasons and attain a lower temperature. At lower temperatures, a secondary heat transfer system or an alternate fluid or gas with a lower boiling point than water may be utilized in the IGP design. This maintains design flexibility while still being a closed loop system. For a typical location, the heat flux required may be based on the desired design of the plant electricity output. The heat flux minimum value will replace the heat loss removed from the heat exchanger(s).

(11) Referring to FIG. 2, a heat exchanger may have different designs. The heat exchanger type, size and shape are based on the heat flux attained and the desired output of the plant. This maintains design flexibility between the heat flux, heat exchanger design, and output desired. For example, in a location where heat flux is very low, multiple shafts may be bored for the placement of multiple heat exchangers 18 to attain the desired output. Heat exchanger design types are not limited, such as u-tube or once-through types.

(12) Referring to FIG. 2, the heat exchanger 2 is at the shaft bottom and is affixed to the shaft via high conductivity grout 20 or similar. Grout conductivity may be increased with additives such as graphite, aluminum, iron, or similar. Setup inhibitors and/or non-water based grouts may be used due to the high temperature application where water flashing to steam may impede proper setup. Alternatively, coolant may be applied to the heat exchanger during setup to control temperature. Typically, thousands of heat exchange tubes 21 line the shaft supplied by a common header 19 at shaft centerline. U-tube style and other heat exchanger types are also within the scope of design. The individual tube diameter design is also a function dependent on the shaft temperature parameters. Based on the temperature parameters, the total tube length may typically range between feet and miles.

(13) For an example using water as the fluid heat transfer medium; feed water is pumped downward into the top of the heat exchanger header, circulates upward through the grouted tubes, absorbs geothermal energy, and exists as steam to supply the steam turbines(s). Referring to FIG. 1, steam dryers, pumps, Heating, Ventilation, and Air Conditioning (HVAC), and other support systems are not shown.

(14) As shown in FIG. 1, typically, the modular high pressure turbine 3, low pressure turbine(s) 4, and generator 5 share the same rotor 16, and are placed relatively close to the heat exchanger 2 within the shaft to minimize thermal energy dissipation. The closed loop system is defined by the connective piping 14 from the heat exchanger that is sequentially connected to the turbines and condenser, and then piped 15 back to the heat exchanger by pump 17. The modular components may be located in a region 7 away from the heat exchanger region 8 where harsh ambient conditions exits. The typical electric generator 5 is modular and sized for the design parameters and desired plant output. When the shaft is many miles deep, the energy in the fluid will dissipate while traveling to a surface turbine. Thermal efficiency is increased when placing modular plant system components as close to the heat exchanger(s) as the shaft ambient conditions allow. These in situ modular components are designed to remotely couple and decouple for transport to and from the surface 11 for maintenance.

(15) The shaft may or may not be pressurized dependent on the local site design parameters. A typical shaft is not pressurized or sealed. The shaft is structurally lined and sealed from the earth. The ambient air within the shaft, therefore, is not naturally pressurized and may only be a few atmospheres at depth. Shaft ambient air temperatures and chemistry may be maintained as designed with the use of an HVAC or similar system. The HVAC system may make use of the above ground atmosphere to maintain cooling and chemistry.

(16) FIG. 1 shows a condenser 6. The condenser may be placed in the shaft region 9 where the shaft temperature and shaft length provide enough cooling to make it possible to return the spent fluid back to the heat exchanger. Based on the design parameters and the availability of conventional surface cooling options specific to the site location, an above ground or enhanced shaft condenser cooling system may be utilized.

(17) A typical IGP plant may have the electric generator connected to an insulated, high voltage, output line that conveys electricity to a standard surface transformer(s) prior to connecting to a standard switchyard and grid. Depending on the length of the shaft and other design parameters, a modular transformer may also be placed in the shaft prior to the output line exiting the shaft.

(18) The modular component design used in a typical IGP plant allows for periodic decoupling and conveyance to the surface for maintenance or replacement.

(19) Multiple IGP shafts at one plant location may serve to smooth power transmission outages from both planned maintenance and unscheduled maintenance. Plant output is dependent on location and associated site parameters, but may be designed at the typical fossil fuel plant MWe range per IGP shaft. This output is significantly higher than other renewable power plants like solar and wind farms. The typical IGP plant surface footprint is considered small as compared to fossil, nuclear, solar, or wind power plants of similar MWe output.

(20) The IGP fuel source is the geothermal energy from the earth and is considered renewable and sustainable. The carbon emissions from an IGP plant are near zero. The IGP plant system is a closed loop system that does not directly interact with the earth itself, therefore, once the grouted heat exchanger(s) is in place, there are no generating sources of manmade seismic activity as with fracturing, or conditions where toxins and fluids are released into the environment.

(21) IGP is unlike any current geothermal process in that it requires the excavation of a deep vertical shaft of sufficient diameter to place a closed loop, grouted, in situ heat exchanger that supplies a power system of in situ modular components to maximize the plant's thermal efficiency.

(22) The principles, representative embodiments, and modes of operation of the present disclosure have been described in the foregoing description. However, aspects of the present disclosure which are intended to be protected are not to be construed as limited to the particular embodiments disclosed. Further, the embodiments described herein are to be regarded as illustrative rather than restrictive. It will be appreciated that variations and changes may be made by others, and equivalents employed, without departing from the spirit of the present disclosure. Accordingly, it is expressly intended that all such variations, changes, and equivalents fall within the spirit and scope of the present disclosure, as claimed.