Patent Application
20060059937
This invention relates to systems and methods for removing heat from a system. More particularly, this invention provides heat exchanging techniques to remove heat from various components and/or subsystems of electrical generation systems such as thermal and compressed air storage (TACAS) backup energy systems or compressed air storage (CAS) backup energy systems. Electrical generation systems may include components and/or subsystems such as electrical machines and power electronics that may require cooling.
Electrical machines such as generators and motors are well known in the art. Such machines are used in thousands of different applications, some of which include the generation of electric power. Electric power is generated, for example, when the rotor of a generator is driven by a prime mover (e.g., turbine) to produce a rotating magnetic field within the machine. The rotating magnetic field induces voltage within the stator windings of the generator that is output as electrical energy.
During operation, heat may be generated by the stator core, stator windings, bearings, rotor, and/or other sources during generator operation. Such heat may be detrimental to generator performance and operation. For example, excess heat can decrease the flux capacity of permanent magnets in the generator and damage generator components such as bearings and generator windings.
Conventional methods for cooling the stator to remove heat include auxiliary cooling fans, circulating water systems, and/or circulating oil systems. Other systems may use compressors to route bleed air over the stator to cool the generator. Though such cooling systems are able to cool generators, they require substantial maintenance and a supply of power to operate. Moreover, such cooling systems are typically unable to maintain an operating temperature of the generator below a desired temperature (e.g., an ambient air temperature).
Generators are often coupled to a turbine by a shaft to form a power generation system known as a turbine-generator. Turbine-generators are highly customizable for a given application such as a micro-turbine system available from Capstone Turbine Corporation of Chatsworth, Calif. This micro-turbine system operates at high shaft speeds, drives a permanent magnet alternator (e.g., a generator rotor), and requires cooling to remove heat from the stator during operation.
During operation, such micro-turbine systems derive stator cooling from shaft-mounted compressor inlet air, compressor bleed air, auxiliary cooling fans, or circulating oil. The turbine is powered by a fuel source (e.g., gas, coal, nuclear) that heats the air being supplied to drive the turbine. Thus, as long as fuel is supplied, the turbine-generator can provide power. Accordingly, such micro-turbines and other fuel-powered turbine-generators can run continuously for thousands or tens of thousands of hours. However, such turbine-generators are subject to several drawbacks, at least one being pollution. Combustion of fuel (or in nuclear applications fission of a fuel) is necessary to drive the turbine. Another drawback is that costly or maintenance intensive bearings (e.g., air bearings, oil film journal bearings, or magnetic bearings) are needed to sustain the long operational lives of these turbine-generators.
Power electronics are often used in electrical generation systems to perform various tasks including, but not limited to, driving electrical machines, conditioning power derived from an electrical machine, and selectively providing power to subsystems (e.g., a flywheel energy backup system or a thermal storage unit). Heat may be generated while power electronics are performing these tasks. Conventional techniques for removing heat from power electronics include using a finned heat sink and a fan. The heat sink transfers heat from the power electronics to the ambient environment (e.g., the air surrounding the heat sink). The fan may be used to force air over the fins to improve the rejection of heat to the ambient environment. Water cooling may be used in power electronics that operate at a higher power density.
These power electronic cooling techniques suffer from many of the same drawbacks experienced in cooling electrical machines. That is, substantial maintenance and a supply of power to operate the cooling mechanism may be required. In addition, such cooling systems are typically unable to maintain an operating temperature of the power electronics below a desired temperature (e.g., an ambient air temperature). Furthermore, the power density is usually limited to a finite power density because conventional cooling systems lack the requisite cooling capacity to prevent the power electronics from overheating if such finite power density is exceeded.
In view of the foregoing, it is an object of this invention to provide improved cooling of components and/or subsystems of an electrical generation system.
It is also an object of the present invention to provide improved cooling of an electrical machine and power electronics used in an electrical generation system.
It is an additional object of the present invention to provide improved cooling that reduces maintenance requirements.
It is still a further object of the present invention to promote electrical machine design flexibility and to reduce electrical machine manufacturing cost.
These and other objects of the invention are accomplished using the expansion of stored compressed gas, which is the same compressed gas used to drive a turbine-generator, to cool components and/or subsystems (e.g., electrical machine, power electronics, etc.) of an electrical generation system (e.g., a TACAS or CAS backup energy system). As gas expands, it cools. Thus, in accordance with this invention, compressed gas is expanded across a valve, the expansion of which cools the gas to, for example, sub-ambient temperatures, and is then routed to one or more components and/or subsystems of the electrical generation system.
One of the components and/or subsystems cooled by the cool gas is an electrical machine, sometimes referred to herein as an electrical generator or generator. The cool gas may be routed through a stator housing and removes heat from the electrical generator, thereby yielding a desired electrical generator operating temperature. The heat being removed by the cold gas may be generated by electrical resistance losses in the stator windings, hysteresis and/or eddy current losses in the laminated stator core, stray load losses on the rotor due to laminated stator core slot harmonics, and/or armature winding current harmonics, rotor windage losses, and friction losses in the bearings located within the electrical machine. In addition, the cold gas may remove heat transferred to the electrical machine by conduction from a turbine, which drives the rotor of the electrical generator.
The cool gas can be routed to other components and/or subsystems of the electrical generation system in addition to, or to the exclusion of, the electrical generator. For example, cool gas may be routed directly to bearings (e.g., thrust-end and non-thrust end bearings) housed in the electrical machine. As another example, cool gas may be routed directly to a bearing housed (e.g., a thrust-end bearing) in the turbine. Cooling such bearings, regardless of whether they are located in the electrical machine or turbine, extends their operational life. As a further example, cool gas may be routed to a power electronics housing to cool power electronics being utilized in connection with the electrical generation system.
The cooling means according to the present invention may be implemented in a CAS backup energy system or a TACAS backup energy system. Such systems may provide emergency backup power in the event of a disturbance in utility power. For example, if utility power fails, compressed gas is drawn from an air reservoir (e.g., pressure tank) and supplied to a turbine. The compressed gas drives the turbine, which in turn powers the electrical generator. Thus, the compressed gas being used to ultimately generate electrical power is also used to cool the components and/or subsystems of the backup energy system.
The cooling means according to the present invention may also be implemented in other systems that use compressed gas. For example, continuously operating TACAS or CAS systems (e.g., systems that do not provide backup power) may be used to provide a continuous supply of power. Such systems may use a compressor to provide continuous compressed gas for use in cooling components and/or subsystems and for driving a turbine-generator.
In TACAS systems, a portion of the cool gas may be heated to a predetermined temperature before being routed to the turbine. The cool gas may be heated by a heating system, such as a thermal storage unit, to increase the operating efficiency of the turbine. If desired, the portion of the cool gas being heated by the heating system may be routed through certain components and/or subsystems (e.g., stator housing and/or power electronics housing) before being supplied to the heating system. Such an arrangement may improve the heating discharge efficiency of the heating system because heat loss picked up by the cool gas passing through the components and/or subsystems is recovered and delivered to the heating system. Thus, a regenerative heating mechanism is built into the operation of the TACAS system which enhances its operating efficiency.
An advantage of the present invention is that the temperature of the gas being routed to the components and/or subsystems may be substantially lower than the heat-exchanging mediums (e.g., ambient air, oil, water, etc.) used by conventional heat exchangers. As a result, this correlates to a lower operating temperature not previously achieved in prior art cooling systems. A lower operating temperature promotes reduced generator sizing (e.g., smaller stator core and stator windings and smaller rotors) and increased generator design flexibility, and thus less cost. Moreover, reduced sizing further decreases spool-up time required for the turbine-generator to start producing emergency power. Another advantage of the present invention is that independent cooling systems, such as fans, compressors, oil circulating systems, are not needed to provide cooling. This correlates to less cost, elimination of a need to power such systems, elimination of maintenance, increased reliability, and a more compact system.
Another aspect of the present invention includes a power electronics housing which routes cool gas in direct contact with, or proximal to, the power electronics of the electrical generation system. The power electronics housing may include a thermally conductive body to which the power electronics are mounted and heat sinks. Cool gas derived in accordance with the principles of the invention may be routed through the thermally conductive body to extract heat generated by the power electronics during an active mode of operation of the electrical generation system. The heat sinks may extract heat from the power electronics during both standby and active modes of operation of the electrical generation system.
An advantage of cooling the power electronics with the expanded gas is that it increases the cooling capacity beyond that previously achieved with conventional cooling techniques, thereby permitting the power density of the power electronics to be increased to levels not previously sustainable by conventional cooling techniques.
Another aspect of the present invention includes stator housings which route cool gas in direct contact with, or proximal to, the wound stator core of an electrical generator. The stator housing may be machined to fit flush (e.g., air tight) against the stator core to maximize heat exchanging efficiency. Such stator housings may have one or more annular channels for routing cool gas around the stator. Stator housings may include a pressure sleeve to prevent gas from damaging the laminated stator core and/or windings or appurtenances thereof during power generation.
The above and other features of the present invention, its nature and various advantages will become more apparent upon consideration of the following detailed description, taken in conjunction with the accompanying drawings, in which like reference characters refer to like parts throughout, and in which:
Cooling according to the principles of the present invention can be implemented in many different types of electrical generation systems, particularly systems that derive electrical power from stored compressed gas. Such systems include, but are not limited to, CAS systems and TACAS systems. To further facilitate understanding of the present invention, a brief discussion of such a system is provided to set forth a possible framework in which the invention may be practiced.
Backup energy system 100 includes motor 120, compressor 122, one way valve 124, pressure tank 126, valve 128, thermal storage unit 130, turbine 140, electrical machine 150, power conversion circuitry 160, and control circuitry 190. If desired, optional transient power supply 170 (e.g., flywheel energy storage system, ultracapacitor, batteries, etc.) may also be provided. Electrical machine 150 may be a machine capable of functioning as a motor and a generator. During normal operating conditions, utility input 110 supplies power to critical load 180. Utility input 110 may also power motor 120, which drives compressor 122 to charge pressure tank 126 with compressed air. The compressed air may be pushed through one way valve 124 to prevent feedback. Persons skilled in the art will appreciate that pressure tank 126 can be substituted with a different type of air storage reservoir such as a cavern (e.g., underground salt dome).
Although control circuitry 190 is not shown to be connected to any of the components included in backup energy system 100, persons skilled in the art will appreciate that control circuitry 190 can perform control and monitoring functions well known and understood in the art. For example, control circuitry 190 can cause valve 128 to OPEN when utility power fails.
In the event of a power failure, compressed air stored in pressure tank 126 is routed through valve 128 to thermal storage unit 130. Thermal storage unit 130 heats the compressed air prior to being routed to turbine 140. Thermal storage unit 140 may be an exhaustless heater (e.g., a non-polluting heater). Examples of and discussion of the operation of such thermal storage units can be found, for example, in co-pending, commonly assigned U.S. patent application Ser. No. 10/738,825, filed Dec. 16, 2003, U.S. patent application Ser. No. ______, filed ______ (Attorney Docket No. AP-53), and U.S. patent application Ser. No. ______, filed ______ (Attorney Docket No. AP-46 CIP), each of which are hereby incorporated by reference in their entireties. The heated compressed air drives the turbine which in turn powers electrical machine 150. Electrical machine 150 operates as a generator and provides electrical power to power conversion circuitry 160 which conditions the power before providing it to critical load 180.
The foregoing discussion of backup energy system 100 is not intended to be a thorough discussion of TACAS systems, but is intended to provide a general framework of a system in which the present invention may be implemented. For a more detailed explanation of TACAS uninterruptible power supply systems, as briefly described above, and variations thereof, see co-pending, commonly assigned U.S. patent application Ser. No. 10/361,728, filed Feb. 5, 2003, which is hereby incorporated by reference in its entirety. The present invention can be incorporated in other emergency backup power delivery systems such as those described in co-pending, commonly assigned U.S. patent application Ser. No. 10/361,729, filed Feb. 5, 2003, which is hereby incorporated by reference in its entirety.
The CAS and TACAS backup energy systems may be used in the context of industrial backup utility power. Alternatively, the present invention may be used in any application associated with generating power, such as in thermal and solar electric plants. The present invention may be used in continuously operating CAS and TACAS systems to generate a continuous supply of power. Furthermore, the present invention may be used in any other application using stored compressed gas in one form or another.
As the compressed gas passes through valve 220, it decompresses (e.g., expands) to a lower pressure than that of the compressed gas stored in air source 210. This HIGH-to-LOW pressure drop results in a Joule-Thompson expansion of gas that results in a substantial drop in gas temperature. For example, in a controlled environment, expansion of gas from 4500 PSIA to 400 PSIA can generate gas temperatures below 30 degrees centigrade. Such cool gas temperatures are much lower than temperatures achieved using conventional fans and water or oil cooling systems.
This cool gas is then used to cool various components and/or subsystems of an electrical generation system 230. It will be appreciated that the cool gas may be used to cool components and/or subsystems 230 in a variety of different ways. For example, the cool gas may be routed to components and/or subsystems independently of each other. That is, a separate path may route cool gas to each component and/or subsystem. As another example, the cool gas may be routed to components and/or subsystems in combination with each other. That is, a single series path or multiple parallel paths may be used to route cool gas to two or more components and/or subsystems.
If desired, the cool gas may be routed to a particular component and/or subsystem and be immediately exhausted to the ambient environment. In other configurations, the cool gas may be routed to other components and/or subsystems of an electrical generation system after cooling a desired component or subsystem. For example, cool gas used to cool an electrical machine may be routed to a thermal storage unit and then to a turbine.
It will be appreciated that the present invention has a number of different applications, but to keep the discussion from becoming too abstract, and to provide better comprehension and appreciation of the invention, references will frequently be made to specific uses of the invention. It is emphasized that these examples merely represent a few of the many possible applications of the invention.
Valve 330 regulates the pressure of the gas provided from air source 328 as the gas is delivered downstream to turbine 350. As the gas reaches valve 332, valve 332 directs a portion of the regulated air to path 336 and the balance of the regulated air to path 338. The gas in path 338 is routed to electrical machine 360 and then routed to thermal storage unit 340, which heats the gas. The gas in path 336 bypasses thermal storage unit 340, but is recombined with heated gas exiting thermal storage unit 340 before being supplied to turbine 350. This combined gas then drives turbine 350, which in turn drives electrical machine 360 to produce electrical power.
The particular valve (e.g., valves 330 and 332) and gas routing configuration (e.g., paths 336 and 338) shown in
During an emergency mode of operation, valve 330 regulates the expansion of the compressed gas being supplied by air reservoir 328 to a predetermined pressure. This creates a HIGH-to-LOW pressure drop, resulting in a Joule-Thompson expansion of gas that results in a substantial drop in the gas temperature.
After the gas expands, the cool gas is routed to electrical machine 360. More particularly, the cool gas may be routed to a stator housing (not shown), such as those shown in
As the cold air passes through the stator housing, heat generated by electrical machine 360 during operation may be absorbed by the cool gas. Thus, the stator housing functions as a heat exchanger and the cool gas functions as the heat exchanging medium. After the cool gas absorbs heat from electrical machine 360, the partially heated gas may be routed to thermal storage unit 340.
An advantage of routing the partially heated gas to thermal storage unit 340 is that it increases the heating discharge efficiency of thermal storage unit 340. Thus, thermal storage unit 340 may not have to impart as much heat energy into the gas being supplied to turbine 350 to discharge the gas at a predetermined temperature. Moreover, recovering the heat losses of electrical machine 360 in the gas being supplied to thermal storage unit 340 may enable thermal storage unit 340 to be sized smaller and/or operate at a lower temperature.
The cooling methodology according to the principles of the invention can prevent other heat sources from adversely affecting the operating temperature of electrical machine 360. For example, the cooling of the stator can extract heat from bearings housed within electrical machine 360. Such bearings may include a non thrust-end bearing, a thrust-end bearing, or both. An added benefit of cooling bearings is that it prolongs their operational life. Stator cooling may extract heat from the rotor of electrical machine 360. Other sources of heat removed by the cool gas include resistance losses, eddy current losses, and hysteresis losses.
Another example of heat being removed from electrical machine 360 may include heat that is imparted to electrical machine 360 by turbine 350. In some applications, turbine 350 may be directly coupled to electrical machine 360. Thus, the heat of the inlet gas being supplied to drive turbine 350 may be transferred to electrical generator by way of conduction or convection, or a combination thereof.
Removing heat from electrical machine 360 according to the principles of the invention may result in a lower operating temperature than that achieved with conventional air, water, or oil cooled machines that reject heat to ambient conditions. This lower operating temperature allows current and flux density of the stator to be driven higher than the current and flux density that can be achieved at higher temperatures. This results in a greater current and flux carrying capacity for a given volume of the stator, resulting in a reduction in the volume of stator material (e.g., iron and copper) needed to construct a generator.
A similar advantage is realized for use of magnetic materials such as a rotor. With magnetic materials, the lower operating temperature results in increased flux density for a given volume of magnetic material. Thus, the volume of the magnetic material can be reduced, yet still provide the same electrical carrying capacity. The reduction in the volume of stator and magnetic materials reduces generator manufacturing costs and promotes increased generator design flexibility.
A further benefit of the reduction of rotor materials is that the polar moment of inertia is decreased. This may result in faster turbine-generation spool up time, thereby decreasing a time lag in providing backup power in the event of a power failure. Moreover, by bringing the turbine up to speed faster, less power may need to be drawn from a transient power source (e.g., flywheel backup energy system) during the transition period between utility power failure and the time it takes for the turbine generator to get up to speed and supply backup power.
It will be appreciated that the cooling methodology being used in accordance with this invention can use regenerative heating, which may result in a more efficiently operating and cost effective backup power supply system. Regenerative heating may be realized by redirecting heat picked up from electrical generator 360 (while being driven by turbine 350) to thermal storage unit 340. Thus, the same gas that cools, yet ultimately drives the generator may be used to improve the operating efficiency of thermal storage unit 340 and turbine 350.
In an alternative arrangement, heat extracted from bearings 464 may be fed back into path 426 instead of being vented to atmosphere. However, such an arrangement may require path 439 to be connected directly downstream from valve 432, as opposed to being connected downstream from a valve such as valve 434—that is, valve 434 is omitted. Moreover, this alternative arrangement permits heat extracted from the stator jacket to be combined with heat extracted from bearings 464 prior to being routed to thermal storage unit 440. As discussed above, recovering this heat energy may improve the heating efficiency of thermal storage unit 440, thus permitting increased flexibility in sizing the thermal storage unit and adjusting the operating temperature of the thermal storage unit.
Alternatively, instead of re-directing heat extracted from bearing 554 back to thermal storage unit 540, the extracted heat may be exhausted to the ambient environment. In such an arrangement, it may be necessary to add an additional valve in path 539 to step down the pressure of the cool gas supplied by valve 532 to prevent excessive loss of compressed gas.
An advantage of cooling bearings in accordance with the invention, coupled with the fact that backup power systems 300, 400, and 500 spend a majority of their operational lives in a standby mode of operation, is that low cost bearings such as grease lubricated bearings can be used. The cool gas may sufficiently cool such bearings when the backup power system is in an active mode of operation, thereby obviating the need to use conventional cooling techniques such as oil cooling.
Power electronics 662 may continuously operate regardless of whether backup energy system 600 is operating in a standby mode, transient mode, or active mode. Therefore, power electronics 662 may continuously emit heat regardless of the mode of operation. Power electronics 662 may include rectification electronics (e.g., AC to DC converters), inverting electronics (e.g., DC to AC converters), capacitors, inductors, control circuitry, and other components known to those skilled in the art.
During a standby mode, power electronics 662 may power thermal storage unit 640 and transient power supply 670. Thermal storage unit 640 may be powered so that it is heated to a predetermined temperature suitable for heating gas passing therethrough during an active mode of operation and to overcome heat losses such as, for example, losses due to the environment. Transient power supply 670 may be powered so that it can instantaneously supply power to load 680 when utility power 610 fails.
Power electronics 662 may condition power supplied to load 680 by transient power supply 660 during a transient mode of operation. Power electronics 662 may also condition power supplied to load 680 from electrical machine 660 during an active mode operation. It is during the transient and active modes of operation that power electronics 662 emits the most heat. This heat is removed using cooling in accordance with the principles of the invention.
When backup energy system 600 emerges from a standby mode of operation, compressed gas from air source 628 is expanded across valve 630, the expansion of which cools the gas, and is directed to power electronics 662. More particularly, valve 632 may direct cool gas to a power electronics housing (not shown) to remove heat generated by power electronics 662. After the gas exits the power electronics housing, it may be directed to thermal storage unit 640 before being routed to turbine 650. The heat picked up from power electronics 622 “pre-heats” the gas before it is supplied to thermal storage unit 640, increasing its discharge efficiency.
During standby mode, power electronics 662 generates heat, but generally not as much heat that is generated during the transient and active modes of operation. Because heat losses are less, a natural convection heat sink (not shown) may function as the primary cooling mechanism during standby mode.
Mounting brackets 710 and 714 may secure natural convection heat sinks 730 and 732 adjacent to gas-cooled heat sink 724 and to capacitors 712. Natural convection heat sinks 730 and 732 can assist gas-cooled heat sink 724 in removing heat generated by power electronics 762 during transient and active modes of operation. Also, during standby modes of operation, natural convection heat sinks 730 and 732 may remove heat generated by power electronics 762. Air currents that are naturally present due to the differences in temperature at different heights of the system may enable heat sinks 730 and 732 to remove heat from power electronics 762. Natural convection heat sinks 730 and 732 may be made from materials such as, for example, plastic, aluminum, copper, gold, iron, steel, any alloys thereof, or any other material with suitable thermal conductivity properties.
Removing heat from power electronics 762 according to the principles of the present invention results in a greater cooling capacity than that achieved with conventional air, water, or oil cooled techniques. One benefit derived from the increased cooling capacity may be that the operating temperature of power electronics can be decreased to temperature levels lower than that achieved with conventional cooling techniques. For example, in one embodiment, the operating temperature of the power electronics may be maintained below or near ambient temperatures.
Another benefit is that increased levels of power density can be sustained for a long period of time without risk of overheating power electronics 762. This may enable the power electronics to operate in a “saturated” power density mode. A “saturated” power density mode may be an operating condition in which cooling according to the present invention permits the power density of the power electronics to be increased to levels above and beyond that which can be sustained by conventional cooling systems (e.g., forced air, water, or oil cooling). That is, if such increased levels of power density are demanded of power electronics being cooled with conventional cooling systems, the power electronics may cease to function, or if it can sustain operation, such operation may be momentary (e.g., a few seconds).
The power electronics may operate in a normal power density mode when lower levels of power density are required. Examples of normal power density mode include standby modes of operation and modes in which conventional cooling techniques, if such techniques were to be used, may sufficiently cool the power electronics.
Cool gas derived in accordance with this invention is supplied to inlet 812, which is connected to an annular channel 816 that permits the cool gas to flow proximal to and around the stator to outlet 814. Annular channel 816 may be a ring of predetermined depth and width that is built into stator housing 810. Further note that gas may split as it enters inlet 812, with a portion of gas passing through a first half of annular channel 816 and the remaining half passing through a second half of annular channel 816. As the cool gas passes through annular channel 816, it may absorb heat from stator 820 and other components associated with electrical machine 860.
The stator housing shown in
Persons skilled in the art will appreciate that the illustrations shown in
Manifolds (not shown) may be coupled to one or both ends of stator jacket 1410, depending on how gas is being routed through axial flow channels 1420. For example, only one manifold may be used if gas is being routed in and out of the same end. Persons skilled in the art will appreciate that if the single manifold arrangement is used, a return path for re-routing the gas back to the manifold is needed. Two manifolds, positioned on opposite ends, may be used if gas is routed from a first end to a second end of stator jacket 1410.
Persons skilled in the art will appreciate that other arrangements of piping, tubing, and integral cast tubing (
Thus it is seen that the same compressed gas being used to drive a turbine can also be used to cool components and/or subsystems of an electrical generation system. A person skilled in the art will appreciate that the present invention can be practiced by other than the described embodiments, which are presented for purposes of illustration rather than of limitation, and the present invention is limited only by the claims which follow.
