Patent Application
20230194049
The present invention is in the field of devices for storing compressed gas. More specifically, the present invention discloses a new apparatus for storing large quantities of compressed gas at high pressure underground.
The ability to store large quantities of compressed gas such as air, natural gas, and carbon dioxide is instrumental in transitioning to renewable energies and to combating climate change. Using natural gas as transition fuel, as well as the feasibility of compressed air energy storage technologies, require sizeable storage facilities that can safely withstand large internal pressures. In addition, a compressed gas storage facility can provide a viable solution for sequestering carbon dioxide if the size of the storage facility is sufficiently large to handle the quantity of carbon dioxide involved at a given location.
Currently, the only really viable means of storing large quantities of compressed gas are natural underground caverns, cavities, and voids that have formed in various geological formations over time. These are particularly prevalent in formations bearing fossil fuel, which become available for compressed gas storage once the crude oil has been extracted. Others are formed in porous limestone over time by the action of acidic groundwater or underground rivers that dissolve away the limestone. However, these opportunities are highly location-specific and are hence not widely available where needed.
There are two attributes of natural underground cavities that make them suitable for large-scale compressed gas storage. One is the sheer size of the cavities, which are very large, and the other is their ability to withstand large internal pressure because of the structural strength of the confining ground formation and the weight of the soils above. These conditions are unique to the underground environment and cannot be created at scale aboveground.
Therefore, there is a need to create underground voids with sizable volume that can withstand large internal pressure to store compressed gas. Such an ability would greatly facilitate the transition to renewable energies and provide a means for sequestering carbon dioxide, which is urgently needed to combat climate change.
The present invention provides a solution for the above stated need with an underground tunnel designed and purposely built to provide a certain volume of void space that can withstand a target level of internal pressure. These are the main requirements that determine the dimensions of the tunnel and its location and alignment in a given geological formation underground.
The present invention is essentially a certain length of a tunnel with circular cross section having a design diameter to provide a desired volume of void at a particular location underground. It may be bored substantially level at a design depth within a particular geological formation and depth below ground, which possesses the necessary geotechnical properties and structural strength to enable the tunnel to withstand the design internal pressure that results from storing compressed gas inside. Alternatively, it may be bored with variable slope and alignment to be located within target geological layers underground.
The design internal pressure depends on the purpose of the compressed gas tunnel. For compressed air energy storage, it is desired for the tunnel to have as high a pressure rating a possible since this maximizes its energy storage capacity. For natural gas and carbon dioxide storage consideration must be given to the liquid-gas equilibrium per their respective phase diagrams in order to determine the maximum design pressure for the prevailing temperatures.
The tunnel of the present invention may be bored underground using conventional tunnel boring techniques and technologies, as well as future improved techniques. Conventional techniques use a launch shaft to lower and locate the tunnel boring machine down to the invert level of the tunnel, plus a receive shaft at the other end to retrieve the tunnel boring machine. In addition, the plan alignment of the present invention can be a closed loop circle such that it is possible to use a single shaft for both launching and receiving the tunnel boring machine where a circular plan alignment is used. For multiple tunnels, it is possible to use the same launch and receive shafts with tunnels vertically stacked with certain vertical spacing, which may or may not be connected. The tunnel of the present invention may also be constructed using the recently developed surface-launch and receive technologies for the tunnel boring machine, which eliminates the need for a shaft. However, using this technology would require adequately plugging the tunnel with structurally sound material at both ends down to the design geological formation such that the internal pressure rating of the tunnel is not compromised.
The key requirement for the tunnel of the present invention is to locate it at the correct depth underground where internal pressure forces in the tunnel exerted on its perimeter are adequately and safely resisted by the confining geological formation. The correct location for the required internal pressure rating may be determined by conducting geotechnical investigations that reveal the mechanical properties of the underground formations and using them in calculation procedures disclosed herein to determine the tunnel pressure rating. Another requirement is air-tightness, which may be provided by internal lining with a suitable gas-impermeable membrane, as necessary.
The tunnel of the present invention may be used for compressed air energy storage, for both adiabatic and isothermal processes. In particular, the long slender shape of the tunnel of the present invention has similar thermodynamic heat exchange properties with the confining ground as the vertical shaft compressed air storage vessel disclosed in U.S. Pat. No. 9,787,161 B2, which is incorporated in its entirety herein. This makes the present invention particularly suitable for utilization for isothermal compressed air energy storage in the form of a large hydraulic accumulator, disclosed herein as one embodiment of the present invention. In this embodiment, the compressed gas tunnel may be filled with compressed air up to a certain pressure to contain a design mass of air. Water may then be pumped into the compressed gas tunnel to further compress the air inside and thus store energy. The process may then be later reversed to recover the stored energy. The slow rates of air compression and expansion provide adequate time for heat exchange between the tunnel and surrounding ground to keep the temperature substantially the same resulting in an isothermal process.
The water needed for the process may be stored at atmospheric pressure housed either at ground level or in a conventional tunnel underground. In the case of the latter, consideration must be given to the net positive suction head required at the water pump, meaning that the elevation difference between the pump and the water tunnel at atmospheric pressure cannot exceed the water vapor pressure. This limitation may be overcome by also pressurizing the water storage tunnel to provide adequate back pressure for the pump at the desired pump elevation. This will reduce the energy storage capacity of the pressurized gas tunnel, but will not result in any energy loss over the cycle.
It is an object of this invention to provide a practical facility to store large quantities of compressed gas underground.
It is an object of this invention to provide a facility for compressed air energy storage and recovery technologies, particularly for an isothermal compressed air energy storage facility in the form of a hydraulic accumulator.
It is an object of this invention to provide a facility that can safely and permanently sequester large quantities of carbon dioxide.
It is an object of this invention to provide improved elements and arrangements by apparatus for the purposes described thereof, which is comparable in cost with existing systems, dependable, and fully effective in accomplishing its intended purposes.
These and other objects of the present invention will become readily apparent upon further review of the following specification and drawings.
Referring to
The corresponding mass of CO2 stored can be determined from gas law equations that equate the product of gas pressure and volume to the product of number of moles, universal gas constant, and temperature. Accordingly, one mile length of the pressurized gas tunnel of the present invention, which furnishes a void volume of 16,914 m3 (596,850 ft3), can store about 39,500 metric tons of carbon dioxide at 70 atmospheres. This is equivalent to the average annual carbon dioxide emissions of about 8600 average size internal combustion engine cars, which is significant. Assuming a total cost of $10.0 Million per mile for the tunnel of the present invention, the corresponding unit cost of CO2 sequestration is about $250 per metric ton, which is significantly less than the current cost of underground CO2 sequestration using present art of about $400 per metric ton.
The present invention is susceptible to modifications and variations which may be introduced thereto without departing from the inventive concepts and the object of the invention. Configurations other than those described may be used to construct the compressed gas tunnel of the present invention. Also, the term belowground implies confinement by geologic formation and includes tunnels located in mounds, such as mountains and hills, which may be at higher elevation that the surrounding ground. Such modifications and variations do not depart from the inventive concepts and the object of the present invention.
While the present invention has been described in connection with what is considered the most practical and preferred embodiments, it is to be understood that the present invention is not to be limited to the disclosed arrangements, but is intended to cover various arrangements which are included within the spirit and scope of the broadest possible interpretation of the appended claims so as to encompass all modifications and equivalent arrangements which are possible.
