Abstract:
Apparatus ( 100 ) comprising a power plant or air motor utilizing compressed air or liquid air for energy storage. The apparatus includes an electrical plant ( 200 ), a mechanical plant ( 300 ), and a pneumatic plant ( 400 ). When operating as a compressor, the plant receives electrical and/or direct mechanical power as an input to drive the plant, compress air, and store its output in the form of compressed or liquefied air. When operating as an engine, the plant consumes the compressed or liquid air to drive a mechanism of the engine and deliver mechanical power and/or electrical power as an output.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     None 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     N/A 
     BACKGROUND OF THE INVENTION 
     This invention relates to power generation; and more particularly to a power plant or air motor apparatus utilizing compressed air or liquid air for energy storage. 
     Fossil fueled power plants have been the mainstay of industry for over 100 years. While internal combustion engines have been firmly entrenched as the main power source for many applications, recent increases in the cost of crude oil have had a particularly devastating effect on industries that rely on engines powered by these fuels. Meanwhile, efforts to develop affordable alternatives to fossil fueled plants have proven challenging. 
     Electricity remains an attractive alternative power source. A reliable generation, transmission, and distribution system for electricity is already in place. Furthermore, electricity is generated from a diversity of sources including coal, nuclear, water (hydro), wind, and solar power. For most parts of the country, the regional cost of electrical energy is largely insensitive to the price of crude oil. Electricity has already been successfully used in certain transportation applications where the energy does not have to be stored—most notably, electric trains, buses, and trolleys. All of these vehicles obtain energy, as needed, from electrified rails or overhead cables. Unfortunately, electricity as a transportation power source has yet to gain widespread use in applications apart from electrified railways. The biggest challenge at this point is not to develop alternative methods of generating electricity, but to develop a means of storing the electrical power in a way suitable for use in the transportation sector. To be useful for common transportation applications, a technology would need to:
         Be suitable for use on non-electrified roadways, waterways, and airways.   Provide sufficient range to the vehicle to make long journeys possible without refueling.   Offer an economically viable alternative to consumers in terms of the acquisition, operational, and disposal costs.   Offer reliable operation, and be easy to repair and maintain.   Be scalable.   Be safe to use.   Be environmentally friendly (by eliminating carbon emissions and minimizing any contribution to landfills.)       

     While batteries are an obvious choice for electrical energy storage, they don&#39;t necessarily satisfy all of the requirements outlined above. There are alternatives to the use of chemical batteries for the bulk storage of electrical energy. One such alternative is to store air (in a compressed gas or liquid form) and use it as needed. Much like batteries which convert electrical energy to chemical potential energy (and back again), so also will a compressed-air energy storage (CAES) system convert electrical energy to compressed-air potential energy (and back again). While a battery generates electricity on demand by a chemical reaction, so also will a CAES system generate electricity on demand by releasing compressed air to drive an electrical generator. Unlike the battery, the CAES system also has the option of using the pressurized air directly to drive an air motor and perform mechanical work (with or without accompanying electrical generation). 
     The use of compressed air as an alternative means of storing energy is not new. U.S. Pat. No. 8,481 (dating from the 1850&#39;s) describes an early air engine. Many developments have occurred since that time; but with the historical affordability of crude oil, and the dependability and effectiveness of the internal combustion engine in performing work, there has been little incentive for industry to develop other technologies, such as CAES, as an alternative power source. While the internal combustion engine became the dominant technology for transportation, air motors have found use in other applications such as for portable tools. In their development, air motors gained a reputation for affordability and longevity; but, unfortunately, also for inefficiency. 
     For compressed air to be considered a viable means of energy storage for transportation purposes, the air motor must be efficient enough to reach transportation design goals in a manner similar to that of the internal combustion engine. Without sufficiently high efficiency, the volume of air that must be transported would be prohibitive for most applications, and CAES will remain an unattractive design alternative. 
     Most engines (internal combustion engines, jet engines, and the like) burn a fuel which transfers heat into the gas. The resulting rise in gas temperature causes an increase in gas pressure. The design of the engine allows this increase in pressure to be traded for an increase in volume and the force of the gas acting on a movable surface translates to work done by the engine. 
     Air motors, in contrast to internal combustion engines, don&#39;t burn fuel; but instead route air from a high-pressure storage tank into a cylinder to pressurize the cylinder. At high rpm, the compressed gas expands rapidly (following essentially an adiabatic process) and cools dramatically as it expands. The exhaust expelled from the motor can be cryogenic. Such temperatures impose a limit on the efficient operation of the air motor because, at cryogenic temperatures, air loses pressure and consequently its effectiveness as a propellant. 
     U.S. Pat. No. 1,926,463 describes a compressed air motor which warms cold exhaust air using a heat exchanger. This prepares the air for use by a subsequent stage, but does nothing for the air in the original cylinder as it undergoes a power stroke. In a similar manner, U.S. Pat. No. 7,296,405 transfers air from one cylinder to another in an effort to counteract the cooling effect of expansion. A drawback in this process is that, in doing so, the system has to expend energy to compress the ambient air. A similar drawback is found in the systems described in Japanese publications JP02245401 A2 and JP07314315 A2, as well as publications WO9504208 A1 and EP0666961 A1. 
     U.S. Pat. No. 4,311,917 describes a system that consumes liquid air and uses it to generate electricity, charge batteries, and power an assembly. While the invention described herein also connects systems together to create a power plant, it is of a significantly different design than that described in the U.S. Pat. No. 4,311,917. 
     U.S. Pat. No. 4,432,203 describes use of an atomized spray of water to affect the temperature of the working fluid. As described therein, water is sprayed into a hot chamber where it immediately flashes into steam. The resulting hot water vapor transfers heat to air in an expansion chamber. In contrast, the present invention uses water near its ambient temperature to warm air in an expansion chamber which would otherwise be near cryogenic. No fuel is burned in the air motor of the present invention to create heat as is necessary in the U.S. Pat. No. 4,432,203. 
     Modern, large-scale CAES systems are used for the bulk storage of power by electric utilities. As such, the CAES system serves as a peaking power plant to stabilize the utility&#39;s electric grid. The systems are large systems typically requiring an underground cavern in which to store the compressed air. Most such systems also use a process which burns fuel to heat the air as the pressurized air is converted to mechanical energy. The present invention could be used as a CAES system; however, it has the advantage of avoiding the burning of fuel. Also, a major drawback of current CAES technology is that large tanks (or sealed underground caverns) are required to store the pressurized air. A liquid air energy storage (LAES) system such as taught by the present invention avoids this difficulty by storing the bulk of the air as a liquid rather than as a gas. Efficient production of liquid air requires cool, compressed air, and the present invention can be readily used for the production of liquid air. Liquid air is easily converted to a gaseous compressed air by warming, and air engines have been designed to use gaseous air, liquid air, or both. 
     BRIEF SUMMARY OF THE INVENTION 
     The present disclosure is directed to apparatus comprising a power plant utilizing compressed air or liquid air for energy storage. The energy storage system contains an air motor which can operate in a reversible manner as an air compressor. The air motor (compressor) is also very efficient. When storing energy, the system receives electrical and/or direct mechanical power as an energy input. When the air motor operates as a compressor, the system stores the output in one or more tanks of compressed or liquefied air. When releasing energy, the system consumes the compressed or liquefied air previously created. The motor is driven by the air to generate mechanical power. The system can offer mechanical power and/or electrical power as its energy output. 
     Importantly, the present invention uses the environment (atmosphere) as a heat reservoir. That is, it dumps heat generated during its operation as a compressor into the environment, and then draws heat back from the environment when operating as a motor. 
     The energy storage system consists of three main interconnected parts:
     a) an electrical plant;   b) a mechanical plant; and,   c) a pneumatic plant.   

     When the pneumatic plant is operating as an air compressor to charges its tanks, the apparatus acts to achieve an isothermal compression of the gas (its working fluid) so to minimize the work required to compress a given volume of gas. By maintaining a (near) constant temperature for the pressurization of the gas, the air motor not only minimizes the work input, but also facilitates conversion of the compressed gas (air) to its liquid state. Similarly, when the plant is discharging its tanks (operating as an air motor), the apparatus endeavors to maintain an isothermal expansion of the working fluid in order to maximize work output from the motor. The apparatus improves the pressure during the expansion phase of its power stroke over conventional air motor designs which would otherwise allow an adiabatic expansion of the working fluid. Those skilled in the art will appreciate that the invention can also be applied in the warming of liquid air to create gaseous air. 
     To achieve these goals, a synergist liquid (e.g., an antifreeze laden water) is introduced into the working fluid during its expansion (or compression) and thoroughly mixed with the working fluid. During the expansion phase of the operation, this has the effect of adding heat to the working fluid (air) without the burning of fuel. The thermal mass of the synergist liquid provides heat that is shared with the working fluid. Likewise, during the compression phase of the operation, the thermal mass of the synergist liquid effectively cools the air. This reduces the pressure and therefore the work required for compression. The synergist liquid is an incompressible liquid and does not react in the same way that gaseous air does to changes in pressure or volume. This, in turn, allows the synergist liquid to be easily separated from the working fluid and recovered for reuse. 
     Other objects and features of the invention will be in part apparent and in part pointed out hereinafter. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       The objects of the invention are achieved as set forth in the illustrative embodiments shown in the drawings which form a part of the specification. 
         FIG. 1  is a system block diagram indicating the electrical, mechanical, and pneumatic plants which comprise the apparatus; 
         FIG. 2  is a block diagram of the electrical plant portion of the apparatus; 
         FIG. 3  is a block diagram of the mechanical plant portion of the apparatus; 
         FIG. 4  is a block diagram of the pneumatic plant portion of the apparatus; 
         FIG. 5  illustrates an idealized pressure-volume (PV) curve used to describe operation of the apparatus; 
         FIG. 6  illustrates another PV curve used to describe an alternative operation of the apparatus; 
         FIG. 7  illustrates a PV curve used to describe the compression cycle of he apparatus; 
         FIG. 8  illustrates an exemplary stage of the apparatus with connection between high and low pressure tanks of the apparatus; and, 
         FIG. 9  is a sectional view of an exemplary cylinder assembly of the apparatus which provides a volumetric expansion/compression chamber. 
     
    
    
     Corresponding reference characters indicate corresponding parts throughout the several views of the drawings. 
     DETAILED DESCRIPTION OF INVENTION 
     The following detailed description illustrates the invention by way of example and not by way of limitation. This description clearly enables one skilled in the art to make and use the invention, and describes several embodiments, adaptations, variations, alternatives and uses of the invention, including what is presently believed to be the best mode of carrying out the invention. Additionally, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced or carried out in various ways. Also, it will be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. 
     Referring to the drawings, a multi-mode power plant  100  utilizes Compressed Air Energy Storage (CAES) and/or Liquid Air Energy Storage (LAES). The power plant includes three sections: an electrical plant  200  having a storage capability, a mechanical plant  300  also having a storage capability, and a pneumatic plant  400  also having a storage capability. 
     As shown in  FIG. 2 , in electrical plant  200 , a rectifier/battery charger/inverter module  202  is supplied AC power from a port  204 . DC power is supplied to the module from a storage module  206  which can be, for example, a battery or large (super) capacitor. From module  202 , electrical power is supplied to an electrical engine which functions as a motor/generator  208 . Operation of the motor or generator is provided by an engine control  210  through module  202 . The motor/generator is coupled to mechanical plant  300 . 
     Referring to  FIG. 3 , mechanical plant  300  is connected to motor/generator  208  of electrical plant  200  through a clutch  302  and transmission assembly  304 . Transmission assembly  304  is connected to a main shaft  306  of the mechanical plant. The main shaft, in turn, connects to mechanical plant  400  through a clutch  308 , to a flywheel storage unit  310  through a clutch  312 , and to an external drive shaft  314  through a clutch  316 . 
     Mechanical plant  300  interfaces with pneumatic plant  400  through a shaft or hydraulic coupling  450 . The pneumatic plant incorporates a number of stages (labeled “ 1 ” through “N” in the figure) with a separate tank associated with each stage. Tank N is a gaseous air storage tank connected to a port  452 . Tank N is also connected to an insulated liquid-air storage tank  454  through a vent valve  456  and a heat exchanger  458 . Tank  454  has an associated port  460 , as well as a vent  462 . 
     The pneumatic plant  400  portion of the system can operate as an air engine as well as an air compressor. As is known in the art, the compression and expansion of air involves the operation of a volumetric displacement chamber  401  as shown in  FIG. 9 . The preferred embodiment of this chamber utilizes a dual-acting reciprocating piston. For illustrative purposes, a single-acting, two-stroke system is described which contains a piston  402  within a cylinder  403 . Other embodiments are possible (e.g. a fluid piston engine, or rotary engine) within the scope of the invention.  FIG. 9  illustrates an example of a chamber  401  in which the maximum volume is attained when piston  402  is at its Top Dead Center (TDC) position, and when the minimum volume is at the piston&#39;s Bottom Dead Center (BDC) position. Each cylinder  403  has supporting equipment (not shown) to route air and fluids. The resulting assembly forms a “stage.” in order to maximize efficiency, power-plant  100  operates in multiple stages with each stage forming a closed loop cycle for fluids. The fluids are returned to ambient temperature after use within the chamber  401 . 
     As described herein, plant  400  can operate as an air compressor to charge the tanks “ 1 ” through “N.” In doing so, the apparatus affects an isothermal compression of a working fluid (the gas) in order to minimize the work required to compress a given volume of the gas. By maintaining a near constant temperature for pressurization of the gas, the air motor minimizes the work input and facilitates conversion of the compressed gas to its liquid state. 
     When plant  400  is discharging its tanks while operating as an air motor, the apparatus acts to maintain an isothermal expansion of the working fluid so to maximize work output from the motor. The result is a significant increase in pressure during the expansion phase of the air motor&#39;s power stroke over conventional air motor designs which allow an adiabatic expansion of the working fluid. It will be appreciated by those skilled in the art that the invention is also useful in warming liquid air to create gaseous air. 
     Operation as an Engine 
     Referring to  FIGS. 8 and 9 , when an inlet valve  404  to chamber  401  is open, and an exhaust valve  405  from the chamber is closed, a working fluid for the chamber is obtained from a manifold  407  through a flow line  407 L. This working fluid may be either liquid air or gaseous air. Now, piston  402  undergoes the first part of its power stroke (i.e., to point a in the diagram of  FIG. 5 ), and is forcibly moved under full pressure from a high-pressure tank  408  to which manifold  407  is connected. Ideally, the working fluid is maintained at an ambient temperature. 
     At some point, depending on the need for power to serve a load, inlet valve  404  closes. Piston  402  continues to move because the air pressure within cylinder  403  is much higher than the atmospheric pressure. The trapped gas continues to expand within the cylinder which, in turn, results in a drop in temperature. To counteract this effect a synergist fluid (e.g., water mixed with antifreeze and maintained at ambient temperature) is now injected into the cylinder. Valve  416  is opened. The synergist liquid flows from a reservoir  409  through a check-valve  406  to an injector  410 . Atomized droplets of the liquid now thoroughly mix with the expanding air within cylinder  403 . The liquid acts as a synergist to modify the behavior of the expanding gas by interacting with the air molecules and raising the working fluid&#39;s temperature back towards ambient. The liquid also serves to lubricate the sliding surfaces and o-ring seals. 
     Piston  402  continues to move. After a predetermined amount of the liquid has been injected into chamber  403 , valve  416  closes. The piston continues to move until it reaches the ends of its power stroke at its TDC position. 
     An exhaust stroke now begins with the opening valve of valve  405 . Valve  415  opens at this time to replenish the fluid in reservoir  409 . Cylinder  403  is designed so that both liquid and air are evacuated from chamber  401  through exhaust valve  405  as piston  402  moves back to its BDC position. Both the liquid and air are warmed as they flow through a heat exchanger  411 . From the heat exchanger, the synergist liquid and air enter a separator tank  412 ; where, because of the pressures involved, the air and synergist liquid readily separate. Exhaust valve  405  closes when piston  402  reaches its BDC position. While the liquid flows back into reservoir  409 , the separated air is drawn from separator tank  412  to a low pressure tank  413  through a low-pressure manifold  414 . 
     The compression portion of the cycle begins with piston  402  moving back to its TDC position. Preferably, exhaust valve  405  is closed such that air pressure within cylinder  403  is approximately equal to the high pressure level of tank  408  just as the piston reaches TDC. The compression stroke is complete when piston  402  reaches TDC, and the 2-stroke engine process described is now ready to be repeated. It will be understood by those skilled in the art that not all of the liquid drawn from reservoir  409  is necessarily completely evacuated from cylinder  403  during each cycle. Further, another approach to thoroughly mix the two fluids, rather than using an atomizing spray, is to have the working fluid bubble up through the synergist fluid. This can be accomplished using any fluid mixing approach to facilitate heat transfer. 
     The diagram of  FIG. 5  illustrates operation of the apparatus in terms of an idealized Pressure/Volume curve. The curve resembles that of a Brayton cycle. However, operation of the air engine of the present invention is quite different than that of a jet turbine engine. In  FIG. 5 , the power stroke of the invention begins at point a, with the opening of inlet valve  404  and follows the curve clockwise under full power (full pressure from high pressure tank  408 ) to point b. At some point, which is based on a target rpm and other engine factors, the inlet valve is closed and the remainder of the power stroke occurs. Under ordinary conditions, an adiabatic expansion would occur, and the PV curve of  FIG. 5  would follow path b-e. However, because in accordance with the present invention, the synergist liquid is injected into chamber  403  through atomizer  410  to warm the gas, the result is that the PV curve follows path b-c instead. This completes the power stroke of the engine. At the start of the exhaust stroke, exhaust valve  405  opens. The PV curve now follows path c-d. At point d the exhaust valve  405  closes, and a partial compression cycle begins. The PV curve now follows path d-a back to the starting point and the cycle is completed. 
     It will be understood by those skilled in the art that the pressurization of cylinder  403  prior to the power stroke may vary.  FIG. 6 , for example, illustrates a PV curve that would result without prior pressurization of chamber  401 . Importantly, introduction of the synergist liquid into the working fluid during the expansion phase, and thoroughly mixing it with the working fluid has the advantage of adding heat to the working fluid without burning fuel. This is because the thermal mass of the synergist liquid provides heat that is shared with the working fluid. Conversely, during the compression phase, the thermal mass of the synergist liquid cools the air, reducing the pressure and therefore the work required for compression. Since the synergist liquid is incompressible, it does not react in the same way gaseous air does to changes in pressure or volume. This allows the synergist liquid to be easily separated from the working fluid and recovered for reuse. It will also be noted that no fossil fuels are burned in the power plant during the process so that the apparatus is non-polluting and environmentally “friendly”. Rather, heat from the atmosphere is added to the working fluid at appropriate times in the process. 
     Operation as a Compressor 
     Before the apparatus of the present invention can operate as an engine, high-pressure tank  408  needs to be charged. Apart from refueling with gaseous or liquid air from another source, the power plant can be operated in a reverse mode to serve as an air compressor. In this regard, operation as an air compressor can be accomplished in one of several ways. Initially, high-pressure tank (or tanks)  408  is empty. Any air pumped into the tanks has a minor effect on tank pressure, but significant heat cannot be generated until significant pressure is present in the system. Use of an atomized liquid is optional until lubrication and/or heat conditions require it. 
     With piston  402  starting at its BDC position, air from low pressure tank  413  through low pressure manifold  414  enters cylinder  403  through valve  405 . When the piston arrives at its TDC position, valve  405  is closed. 
     Now, a compression cycle begins. As pressure in cylinder  403  approaches the pressure level of high-pressure tank  408  (via manifold  407 , inlet valve  404  is opened. As the piston reaches its BDC position, this valve is closed and the cycle is complete. 
     This technique can be used for short periods of time (such as for regenerative braking). However, to operate for extended periods of time, occasional lubrication is required. An engine controller  415  (see  FIG. 4 ) can modify the ordinary compression cycle described above to inject liquid to both cool and lubricate the piston/cylinder mechanism. The liquid is routed to cylinder  403  during the intake stroke by briefly opening reservoir  409  and allowing liquid flowing from the reservoir to flow through check valve  410  into the cylinder. The need for a liquid to serve as a coolant in chamber  401  increases as a function of the pressure level in high pressure tank  408 . 
       FIG. 7  illustrates a PV curve for the compression cycle. When the temperature is properly managed, the cycle follows the path a-d-c-b-a. When operating in a mode in which the cylinder temperature is not managed, the cycle tends to follow the less efficient a-d-c-e-b-a path. 
     The various valves are controlled such that their timing varies in response to engine requirements. While this can be accomplished in several ways, including electromechanically, the preferred embodiment is to operate the valves mechanically. Adjustments can be made to the configuration of the injection mechanism in response to engine requirements. In this regard, the cam for each lifter will have a three dimensional profile and the cam is maneuvered along its axis of rotation to present a shape to the lifter that corresponds to the amount of “fuel” to be dispensed. 
     Multiple Tanks 
       FIG. 9  shows an example of a single stage of mechanical plant  400 . As noted,  FIG. 4  shows that plant  400  can have multiple stages with each stage having an associated tank. Tanks are positioned so that one serves as a high pressure tank  408  (labeled “ 1 ” through “N”) and one serves as a low pressure tank  413  (labeled “atmosphere” through “N−1”). For example, stage  1 &#39;s high pressure tank  408  is labeled “Tank  1 ” and its low pressure tank  413  is labeled “atmosphere”. Stage N&#39;s high pressure tank  408  is labeled “Tank N”, and its low pressure tank  413  labeled “Tank N−1”. Each stage of plant  400  functions to convert mechanical energy to or from energy using compressed air. The respective stages can be connected in parallel, in series, or in a combination thereof. 
     Tank N is a tank of gaseous air at the upstream end of the highest pressure stage (stage N), and tank  454  is a tank of liquid air also at this upstream end. As such, both tank N and tank  454  are the highest pressure tanks. When the system is operated as an air compressor, the pressure in the final gaseous-air tank, tank N, will increase. When its pressure exceeds a predetermined value, vent valve  456  opens to relieve the pressure. The air is routed to insulated tank  454  where it rapidly expands. The adiabatic expansion results in rapid cooling and conversion of the air to a liquid. When the system is operated as an air engine, and the pressure in tank N is inadequate, liquid air is brought over from tank  454 , through pump/valve  464 , and allowed to warm in the heat exchangers. The fluid mixing apparatus, as described above, now acts to efficiently convert the liquid air to gaseous air. 
     Power plants of many different sizes are conceivable within the scope of the invention. When the power plant is small, it might be called an “air motor.” When the power plant is large, it might be called an “air engine.” While the term “air” is used throughout this description to refer to the working fluid, the term encompasses any, or all, components of air including nitrogen, carbon dioxide, etc. The working fluid (i.e. air) may be in a gaseous or liquid form with the preferred embodiment having gaseous-air flowing in and/or out of each stage as the working fluid. Further, it is important to note that the power plant of the invention burns no fossil fuels during its operation either as an engine or as a compressor. Accordingly, the power plant is a “green” plant and does not add to atmospheric pollution. In this regard, power plant  100  releases heat generated during its operation as a compressor into the environment, and then draws the heat back out of the environment when operating as a motor. 
     In view of the above, it will be seen that the several objects and advantages of the present disclosure have been achieved and other advantageous results have been obtained.