Abstract:
A continuous power system provides a continuous supply of power to a load in the event that primary power fails or is degraded. The continuous power system includes an electrical machine, a turbine and a flywheel coupled to a shaft. When utility power is present, the machine operates as a motor to drive the shaft. During outages, the electrical machine operates as a generator to provide power to the load. Kinetic energy stored in the flywheel drives the shaft during initial power interruptions. During further short-term interruptions, a thermal energy supply (or thermal storage device) is used to provide vaporized liquid to the turbine so that the turbine drives the shaft. If the power loss or failure is extended, the turbine is driven by vapor produced by an evaporator heated from an external fuel supply. Numerous methods and apparatus are also described for reducing system losses and improving overall performance.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to continuous power systems. In particular, the present invention relates to continuous power systems that provide a continuous supply of electric power when a primary power supply fails, or when deterioration occurs in the power being supplied to the end user. 
     Continuous power systems are often used to insure that, in the event of a loss of power from a primary power supply, such as failure due to equipment malfunction, downed lines or other reasons, electric power continues to be supplied. This is particularly applicable in applications relating to, for example, telecommunication systems because such systems typically include facilities that may be in relatively isolated locations, such as a telecommunication repeater tower. Other applications of the present invention include hospital operating room equipment, computer systems, computerized manufacturing equipment, airplane radar guidance systems, etc. Continuous power systems typically are reliable systems that avoid equipment failures, costly downtime and equipment damage, as well as providing necessary power that otherwise would not be available. 
     Known continuous power systems may employ an uninterruptible power supply (UPS) to provide alternating current (AC) power to the end user or critical load. The AC power may be provided directly to the load, or it may be provided through known switching circuitry that also may be utilized to switch in back-up power when utility power fails. 
     For known continuous power systems, batteries or flywheels may be employed as energy storage subsystems to provide bridging energy while a fuel-burning engine is started. Such flywheel systems may include a flywheel connected to an electrical machine that can operate both as a motor and a generator. For example, U.S. Pat. No. 5,731,645 describes flywheel systems that provide backup power to the load in UPS systems. The electrical machine is powered by a DC buss to operate as a motor when acceptable power is received from the primary power supply. When power from the primary power supply fails (or is degraded), the electrical machine is rotated by the kinetic energy of the flywheel and operates as a generator to supply power to the DC buss. 
     Continuous power systems often use prime movers (e.g., fuel-burning engines) to drive backup generators during prolonged power outages. These prime movers, however, are often costly, complicated, and may require extensive ongoing maintenance. In addition, the engines themselves may fail to start, resulting in a loss of power to the critical load. Moreover, some localities limit the running time or the number of starts per year for backup generator engines, thereby limiting the ability to test and maintain such systems. 
     Other energy storage systems currently used to provide backup power are often expensive and complicated. For example, in typical battery energy storage systems, there is a risk that undetected battery damage or corrosion of battery terminals can result in a failure to deliver backup power when needed. Moreover, batteries have a limited shelf life, in addition to requiring expensive ventilation, drainage, air conditioning and frequent maintenance. Flywheel energy storage systems, while avoiding most of the disadvantages of batteries, can be expensive since they are often mechanically complex and can require complicated power electronics. 
     Some known systems provide long-term power by driving a shaft-mounted generator with a turbine. For example, U.S. Pat. No. 6,255,743 (application Ser. No. 09/318,728) describes an uninterruptible power supply system that includes a shaft-mounted generator and a turbine. These turbines may be open systems, where the turbine is driven by a fuel source that is regularly renewed, such as LP gas, methane, gasoline, diesel fuel. In such instances, the turbine exhaust is allowed to escape into the environment. 
     Other turbine systems, however, may be closed or partially-closed systems. In such systems, some or all of the turbine exhaust is recaptured by the system for later use. For instance, in a partially-closed system that is steam powered, the system may be configured to recapture the steam that is exhausted from the turbine. The system may then condense the steam (using a condenser or through natural cooling) into water prior to reheating, revaporizing and reusing the steam to drive the turbine. 
     An object of the present invention is to provide continuous power systems that are more reliable than conventional UPS systems. 
     Another object of the present invention is to provide continuous power systems that provide multiple sources of short-term backup energy. 
     A further object of the present invention is to provide continuous power systems in which a source of short-term backup energy is angular momentum. 
     A further object of the present invention is to provide continuous power systems in which a source of short-term backup energy is stored thermal energy. 
     A still further object of the present invention is to provide continuous power systems that include a closed-loop turbine system that operates at higher reliability than conventional systems. 
     SUMMARY OF THE INVENTION 
     The continuous power systems of the present invention provide backup power in the event of a loss of power or reduction in power quality from a primary power supply—an OUTAGE. An OUTAGE, as defined herein, includes both an interruption in power from a source (such as utility power), as well as a degradation in quality of the power delivered by the source. This includes both short-term—in terms of seconds or minutes, and long-term, or extended OUTAGES (e.g., lasting hours, days, or even weeks). 
     Continuous power systems constructed in accordance with the present invention include a flywheel energy storage device that provides short-term backup power, as well as a source of stored thermal energy in the form of heated working fluid and other material (which may be referred to herein as a “thermal storage device”). In addition, the continuous power systems of the present invention include a turbine that is driven by a closed-loop supply of working fluid to provide long-term backup power to the end user or critical load. 
     These continuous power systems may include an electrical machine that can operate as a motor or as a generator mounted to a shaft that also includes the flywheel energy storage device and the turbine. During STANDBY mode, power from the primary power supply drives the electrical machine as a motor, which rotates the shaft at a predetermined speed. The STANDBY speed of the shaft is selected so that the flywheel can store a given amount of kinetic energy as angular momentum that will be converted into electrical energy in the event of a loss or degradation of primary power. In addition, during STANDBY mode, an accumulator is provided with a supply of liquid that is heated to provide a second source of short-term backup power. 
     During SHORT-TERM OUTAGES, the flywheel drives the electrical machine as a generator to provide the necessary backup power. As the length of time of the OUTAGE continues and the stored kinetic energy is depleted, the flywheel slows down. Once a predetermined speed is reached, the continuous power system activates its second source of short-term backup power—the thermal source. The stored heated fluid in the accumulator is provided to a preheater/evaporator device that adds enthalpy to the fluid that is provided to the turbine. The fluid is evaporated in this process, and the vapor drives the turbine. The turbine then drives the shaft, thereby enabling the electrical machine to continue to operate as a generator that supplies backup energy to the end user or critical load. 
     If it appears that the OUTAGE is going to be EXTENDED, based on a predetermined factor such as, for example, a reduced level of stored thermal fluid in the accumulator, a gas-fired burner is started, and the system enters EXTENDED OUTAGE mode. In this mode, the working fluid used to power the turbine is cycled via a closed loop through the preheater/evaporator to the turbine. Exhaust vapor is recaptured, condensed and resupplied to the preheater/evaporator. Those familiar with the art will appreciate that this process is commonly known as the Rankine cycle. A further feature of the closed-loop aspect of the present invention is the use of the condensed working fluid as a lubricant for the bearings of the shaft-mounted components, such as the flywheel, turbine and electrical machine. This eliminates the need for rotating seals and allows the entire system to be hermetically sealed. 
     Further features of the invention, its nature and various advantages will be more apparent from the accompanying drawings and the following detailed description of the preferred embodiments. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects and advantages of the present invention will be apparent upon consideration of the following detailed description, taken in conjunction with accompanying drawings, in which like reference characters refer to like parts throughout, and in which: 
     FIG. 1 is a schematic diagram of a continuous power system constructed in accordance with the present invention; and 
     FIG. 2 is a schematic diagram of the working fluid delivery system of a closed engine turbine continuous power system constructed in accordance with the principles of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 shows a continuous power system  100  constructed in accordance with the present invention. Continuous power system  100  includes flywheel energy storage device  102 , turbine  104  and electrical machine  106  that may be mounted to a common shaft  108 . Alternately, flywheel  102 , turbine  104  and electrical machine  106  may be mounted to individual shafts provided they are coupled together so that, depending on the operating condition of continuous power system  100 , any of flywheel  102 , turbine  104  or electrical machine  106  may provide the driving force to cause the other components to rotate about their shaft. 
     In addition, continuous power system  100  also includes thermal storage device  110 , controller  112  and electronics  114 , which is shown with a dashed line to indicate that various configurations of electronics may be used with UPS  100  without departing from the present invention. For example, during STANDBY operation, utility supply  116  may supply electricity directly through electronics  114  to load  118 . Alternately, as is well known, electronics  114  may perform a double conversion, so that continuous power system  100  always provides power to load  118  (in which case, power from utility supply  116  is converted from AC to DC and back to AC before being supplied to load  118 ). 
     Electrical machine  106  may be a combined motor/generator, or it may be formed from separate motor and generator components mounted to a single or separate, but coupled, shafts. During STANDBY operations, electric power from utility  116  powers machine  106  as a motor. During any OUTAGE operation, other components provide the energy to drive shaft  108  and machine  106  is operated as a generator to provide the necessary backup power to load  118 . 
     Energy storage device  110  is a container that stores a liquid working fluid (as described more fully below) for use with turbine  104 . The container for device  110  should be a material with desirable thermal and mechanical properties such as steel or other metal, so that thermal energy used to heat the liquid working fluid may also be stored as sensible heat in the container itself. The liquid working fluid, and the container itself, may be heated, for example, by an electrically powered resistor immersed in the liquid working fluid. As the liquid working fluid is heated by the resistor, thermal energy passes from the liquid working fluid to the container, and then may also be passed to other components of the continuous power system as described more fully below. 
     In general, continuous power system  100  operates in accordance with the principles of the present invention as follows. Assuming that power from utility  116  is available and that electronics  114  is configured such that utility power may be supplied directly to load  118 , load  118  is powered by utility  116  during STANDBY operations. In addition, utility power is provided to electrical machine  106 , which operates as a motor to drive shaft  108  at a predetermined speed, thereby storing kinetic energy in flywheel  102 . 
     At the same time, liquid working fluid in thermal storage device  110  is heated, for example, by electricity passing through a resistor, to a predetermined temperature and stored for use during OUTAGE conditions. Moreover, as described above, the container storing the heated liquid working fluid is also maintained at a high temperature, so that the sensible heat in the container is added to the stored thermal energy of storage device  110 . Controller  112  (and/or other circuitry) monitors various parameters to determine whether utility  116  is providing power within certain guidelines (this may be as simple as the presence of power, or it may include determining the quality of the power being provided by utility  116 ). 
     Once controller  112  has determined that an OUTAGE has occurred, continuous power system  100  enters INITIAL OUTAGE mode, and electrical machine  106  ceases to operate as a motor. During INITIAL OUTAGE mode, shaft  108  is driven by the kinetic energy stored in flywheel  102 , and electrical machine  106  is operated as a generator that provides power to load  118  through switching electronics  114 . If the OUTAGE ends prior to the rotational speed of flywheel  102  falling below a predetermined level, utility power is again supplied to machine  106  which switches back to operating in MOTOR mode. After some time has passed, shaft  106  is again rotating at STANDBY speed and continuous power has been provided to load  118  through the entire OUTAGE. 
     If, on the other hand, the rotational velocity of flywheel  102  falls below a certain threshold, continuous power system  100  enters SHORT-TERM OUTAGE mode and thermal storage device  110  is utilized to drive shaft  108  so that electrical machine  106  continues to provide power to load  118 . Thermal storage device  110  includes a heated liquid (which is heated by, for example, a resistive heating element—see FIG. 2) that, upon receiving an activation signal from controller  112  (which may simply be a signal that opens a valve), is provided to a preheater/evaporator device as described more fully below. 
     The preheater/evaporator helps convert the liquid to vapor and add enthalpy to the vapor (because the tubing that comprises the preheater/evaporator also stores sensible thermal energy that radiates and conducts from thermal energy storage device  110 ). It should be noted that, even without the preheater/evaporator, the stored liquid working fluid, if superheated, would turn into vapor once released from energy storage device  110 , which would be pressurized, due to expansion. The vapor is then delivered to turbine  104 , which causes turbine  104  to rotate. 
     Rotating turbine  104  in turn drives shaft  108  and, accordingly, machine  106  (which is driven in GENERATOR mode), to continue to provide uninterrupted power to load  118 . This portion of temporary backup power is based on thermal storage device  110 , the second source of stored backup energy in continuous power system  100 , rather than the flywheel previously described. 
     Controller  112  monitors, for example, the fluid level in thermal storage device  110  during SHORT-TERM OUTAGE mode. When that liquid working fluid falls to a predetermined level (or when a predetermined amount of time passes after SHORT-TERM OUTAGE mode has been triggered), continuous power system  100  begins preparations to enter the next mode by lighting burner  120 , which heats the preheater/evaporator device. When controller  112  determines that the loss of stored thermal energy is imminent (such as by determining that a predetermined amount of fluid is remaining), LONG-TERM OUTAGE mode is triggered. At this point in time, all or almost all of the energy stored in the two different backup devices is depleted, the OUTAGE has not ended, and burner  120  has heated the preheater/evaporator at least to a given temperature. 
     During LONG-TERM OUTAGE mode, shaft  108  is driven by turbine  104 , which continues to receive vapor from the heated preheater/evaporator. The working liquid may be, in accordance with the present invention, in a closed loop system such that the exhaust vapor is condensed back to liquid and collected in a reservoir or tank, as described in detail below, for recycling to the preheater/evaporator. Electrical machine  106  continues to operate in GENERATOR mode so that continuous power system  100  continues to provide power to load  118  throughout the entire OUTAGE. Controller  112  continues to monitor the availability and quality of power from utility  116  so that, when the OUTAGE ends, continuous power system  100  can be switched from LONG-TERM OUTAGE mode to STANDBY mode. 
     During the transition from LONG-TERM OUTAGE to STANDBY mode, while power is again being provided from utility  116  to electrical machine  106  (so that it may return to MOTOR mode), liquid working fluid is returned to thermal storage device  110  and reheated to its standby, superheated temperature. At that point, because flywheel  102  is already rotating at or near its standby speed from turbine  104 , both of the sources of backup power in continuous power system  100  are fully recharged and ready to perform in the event of another OUTAGE. The return and reheating of liquid working fluid to thermal storage device  110  may also be fully or partially accomplished during LONG-TERM OUTAGE mode. 
     FIG. 2 is a schematic diagram showing a closed engine (or closed loop) continuous power system  200  constructed in accordance with the principles of the present invention. Continuous power system  200  may, in fact, be the same continuous power system as continuous power system  100  of FIG. 1, thereby showing a particular embodiment of a closed engine continuous power system (continuous power system  100 , however, need not be a closed loop system). For clarity, the controller and its connections to the flywheel, turbine, electrical machine, thermal storage device and switching electronics have been omitted (as well as connections to the control valves and pumps shown in FIG.  2 ). While various working fluids are known to be used with turbines, it may be preferable to utilize toluene, refrigerants, water, or other substances with advantageous thermal and fluid properties, as the working fluid. 
     As shown in FIG. 2, closed engine continuous power system  200  includes flywheel  102 , turbine  204  (numbered  204  to show that it may be different than turbine  104  of FIG. 1, but need not be) and electrical machine  106  mounted to shaft  108 . Feed pump  230  may also be mounted to shaft  108 . Shaft  108  is itself mounted so that it may rotate within bearings  109 . The thermal storage device in continuous power system  200  is accumulator  210 , which is a vessel or container that includes a heater  211  mounted therein for heating the liquid working fluid to its standby temperature. Heater  211  is controlled by controller  232 , which is powered by utility  116 . 
     As briefly described above, continuous power system  200  also includes preheater/evaporator  234 , which may be heated during LONG-TERM OUTAGES. Under those circumstances, preheater/evaporator  234  is heated by gases that have been heated by burner  120 , which burns fuel from fuel supply  238 . Burner  120  may be controlled by burner controller  236  (as shown), or it may controlled by a central controller that controls continuous power system  200 . 
     Continuous power system  200  also may include recuperator  240  to transfer heat from the vapor exhausted by turbine  204  to the working fluid entering preheater/evaporator  234 . Moreover, as described above, the exhausted vapor is eventually passed through condenser  242  which converts the vapor back to a liquid. The liquid is then collected in liquid tank  244  (commonly known as a hot well). 
     The collected liquid is fed from tank  244  back into the system by boost pump  246 , although the final destination of the liquid supplied by boost pump  246  depends-on the current mode of operation. For example, if continuous power system  200  is in STANDBY mode, boost pump  246  supplies liquid to accumulator  210  through charge pump  248  until a predetermined level is reached. Under other circumstances, boost pump  246  provides liquid to feed pump  230 , which pressurizes it and provides it to preheater/evaporator  234 . 
     In another aspect of the present invention, boost pump  246 , in all modes of operation, provides the working fluid in liquid form to bearings  109  for lubrication. Lubricating liquid is returned to tank  244 , as shown by return lines  250 , for reintroduction into continuous power system  200 . 
     The operation of closed engine continuous power system  200  may include the same four modes as previously described, namely, STANDBY, INITIAL OUTAGE, SHORT-TERM OUTAGE and LONG-TERM OUTAGE. During STANDBY mode, flywheel  102  is rotated within a predetermined range of velocity and accumulator  210  is full of fluid heated by heater  211  within a predetermined temperature range. Switch valves  252 ,  254  and  256 , respectively located between: accumulator  210  and preheater/evaporator  234 ; turbine  204  and preheater/evaporator  234 ; and feed pump  230  and boost pump  246 ; are CLOSED. Power from utility  116  is provided to machine  106  (which operates as a motor), heater control  232 , burner controller  236  and load  118 . Boost pump  246  provides pressurized liquid working fluid to bearing  109  to properly lubricate the bearings. 
     Once an interruption in power from utility  116  has been determined (which may include a complete disruption or a degradation in power quality), continuous power system  200  enters INITIAL OUTAGE mode, at which point flywheel  102  becomes the driver of shaft  108 . Due to the OUTAGE, power is no longer supplied to machine  106 , which begins to operate as a generator instead of as a motor. The generator provides power to load  118 , burner controller  236  and pump  246 . At this point in time, valves  252 ,  254  and  256  remain CLOSED. During INITIAL OUTAGE mode, the controller monitors the rotational velocity of flywheel  102  until it falls below a predetermined level. 
     Once flywheel  102  has slowed down to the predetermined level, SHORT-TERM OUTAGE mode is triggered. In this mode, the second supply of stored energy in continuous power system  200  is utilized to provide backup power to load  118 . Valves  252  and  254  are opened, while valve  256  remains CLOSED. 
     The superheated vapor passing through valve  254  is injected into turbine  204 , which causes turbine  204  to apply torque to shaft  108 . Rotating turbine  204  drives shaft  108 , which enables machine  106  (operating in GENERATOR mode) to continue generating electricity that is provided to load  118  and all operating valves, pumps and controllers of system  200 . The system controller monitors the level of liquid in accumulator  210  to determine when to begin transitioning to the next mode. 
     Once the liquid in accumulator  210  falls below a first predetermined level (or, if a different triggering event is used, such as when a predetermined amount of time has passed since SHORT-TERM OUTAGE mode was triggered), burner controller  236  ignites burner  120 , which begins burning fuel from supply  238  and heating preheater/evaporator  234 . After the liquid in accumulator  210  falls below a second predetermined level, LONG-TERM OUTAGE mode is triggered and valve  252  is closed, while valve  256  is opened. Once valve  256  is OPEN, liquid working fluid from tank  244  is pumped by pumps  246  and  230  through recuperator  240  and into preheater/evaporator  234 . 
     Once the liquid is vaporized and superheated, vapor is injected into turbine  204  causing it to continue driving shaft  108 . In this state, valve  252  remains CLOSED, pump  248  remains OFF, and valves  254  and  256  remain OPEN. The working fluid continues to be cycled in a closed loop from the tank, into the preheater/evaporator, through the turbine, condenser and back into the tank. 
     In accordance with another aspect of the present invention, recuperator  240  provides additional thermal efficiency during this mode by exchanging the heat from the vapor being exhausted by turbine  204  to the liquid being provided to preheater/evaporator  234 . During LONG-TERM OUTAGE mode, excess power may be used to refill accumulator  210  and reheat it. 
     After the OUTAGE ends, continuous power system  200  begins the transition back to STANDBY mode. Flywheel  102  is already spinning at least somewhat close to its standby speed (from being on shaft  108 , which is being driven by turbine  204 ). Valves  254  and  256  are CLOSED and charge pump  248  is turned ON, causing liquid working fluid to be provided to accumulator  210 . The filling of accumulator  210  continues until the level of liquid in accumulator  210  reaches a predetermined level, at which point charge pump  248  is turned OFF. Power from utility  116  is also provided to heater controller  232  and heater  211 , which heats the liquid working fluid in accumulator  210  to its STANDBY temperature. Burner  120  is eventually turned OFF. 
     From the foregoing description, persons skilled in the art will recognize that this invention provides effective, uncomplicated, battery-free, low maintenance, and relatively inexpensive ways of providing an uninterrupted and continuous supply of electrical power to a critical load. It will also be recognized that the invention may take many forms other than those disclosed in this specification. Accordingly, it is emphasized that the invention is not limited to the disclosed methods and apparatuses, but is intended to include variations to and modifications thereof which are within the spirit of the following claims.