Abstract:
Batteries are often used for load-following, particularly in combination with generation sources that cannot respond to fast load changes. Batteries that display “memory” (i.e., their ability to operate correctly over their entire depth of discharge depends on their previous level of charge or discharge) cannot adequately follow loads. This invention allows the use of batteries that display “memory” in load-following applications.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to the field of distributed power generation, and more particularly to an improved load follower configuration of a distributed power generation system. 
     BACKGROUND OF THE INVENTION 
     Distributed power generation concerns the movement of power generation devices from central, large scale power plants to smaller units located close to the end-users, thereby enabling significant environmental benefits to be derived from the use of sustainable energy sources. However, various forms of distributed generation are unable to adjust their output power fast enough to follow fast-changing loads. Fuel cells and microturbines are two such examples. Other types of distributed generators such as photovoltaic and wind-turbines produce fluctuating output power as a function of available energy (e.g.—sunlight or wind). Both slow-load-following generators and fluctuating-power generators are typically paired with some type of energy storage to operate as a stand-alone generation system. 
     The most common type of energy storage paired with distributed generators in stand-alone systems is a chemical battery—typically lead-acid battery. During operation of each type of system, the battery has to provide power for instantaneous load changes. For example, in a fuel cell system, the fuel source and the system operating temperature cannot be quickly adjusted. While these and other parameters are adjusted to match the required load, a battery either provides power to or absorbs power from the load, depending on whether the load is increasing or decreasing. In a stand-alone photovoltaic system, the photovoltaic arrays typically charge a bank of batteries and the batteries directly provide power to the load. In all of these systems, the rate of battery charge and discharge can vary considerably with time and is subject to the variability of the load. The batteries can therefore experience frequent cycling, and the level of discharge in that cycling can be shallow or deep, depending on the load and the availability of generator power. 
     FIG. 1 shows a typical power conversion system for a distributed generator system that includes a battery that is used for load-following. The system includes a generator  10 , battery  12 , and first and second power converters  14 A and  14 B (Power Converter 1  and Power Converter 2 , respectively). The generator  10  provides power to the first power converter  14 A. The output voltage of the generator may be dc, low-frequency ac, or high-frequency ac, depending on the output of the generator. The first power converter  14 A is designed to convert the generator voltage to a relatively constant voltage on a dc link. This voltage may then be further processed if necessary to provide the correct voltage to the load. In some applications, for example, telecom power supplies, the dc link may be connected directly to the load. 
     The battery  12  is able to provide power to or draw power from the dc link through the second power converter  14 B. In some cases, power converter  14 B may be replaced by a direct connection of the battery to the dc link. In that case, adjusting the voltage of the dc link could control the charging and discharging of the battery  12 . In other cases, the second power converter  14 B can directly control the charge drawn from or the charge inserted into the battery. 
     When there is a sudden load increase, the second power converter  14 B will draw power from the battery  12  equal to that increase until the generator  10  is able to respond. When the load is steady or decreasing, the generator can provide some additional power to charge the battery. 
     Most types of batteries exhibit shortened lifetimes when subjected to frequent cycling. Furthermore, some types of batteries exhibit a phenomenon often referred to as “memory”. A battery that exhibits a “memory” phenomenon “remembers” its previous cycling history. For example, if a NiCad battery is only partially charged before being discharged, it cannot be fully charged again. This process worsens with partial charge cycles. A NiMH battery exhibits similar phenomenon—it will not provide full power at low charge levels if it was previously discharged only partially before recharging. The worsened performance in NiMH batteries is reversible whereas the worsened performance in NiCad batteries is not. 
     As a result of the “memory” effect, many types of batteries are not used in load-following applications. As a consequence, the most common type of battery for load-following is the lead-acid battery. However, lead-acid batteries have a number of shortcomings that make them undesirable for many load-following applications. Cycling them significantly shortens their service life. Furthermore, they contain lead and therefore offset one of the main benefits achieved by many forms of distributed generation—an environmentally safe solution to generation. Lead-acid batteries also have a relatively inefficient round-trip efficiency. Just keeping them charged can take a significant amount of energy. Since distributed generation is relatively expensive on a cost per kilowatt basis, a high efficiency load-following scheme is crucial to make the overall system cost-effective. 
     SUMMARY OF THE INVENTION 
     The present invention permits the use of batteries that exhibit a “memory” effect in load-following applications. The preferred embodiments use two batteries. At any given time, one battery is only discharged while the other battery is only charged. When one battery reaches full charge and the other reaches full discharge, the role of the two batteries is reversed. In this way, the memory effect does not reduce the performance of the batteries. Other aspects of the present invention are described below. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 schematically depicts a previously known load-follower configuration of a power generation system. 
     FIG. 2 schematically depicts a preferred embodiment of a load-follower configuration of a power generation system in accordance with the present invention. 
     FIG. 3 schematically depicts another preferred embodiment of a load-follower configuration of a power generation system in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     First Embodiment 
     FIG. 2 depicts a first presently preferred embodiment of the invention. The generator  10  can produce either ac or dc voltage. The power converter  14 A transforms the voltage from the generator to dc voltage at the dc link, and also transforms the voltage from the dc link to the desired output voltage at the load. A charger  16  and discharge controller/converter  18  are also connected to the dc link. While switch S 2  is in the UP position, the charger  16  charges a first battery (Battery 1 )  12 A until it is fully charged while the discharge converter  18  draws power from a second battery (Battery 2 )  12 B to provide power to the dc link as needed to make up the shortfall in available power from the generator  10 . While switch S 2  is in the DOWN position, the roles of batteries  12 A and  12 B are reversed. Some types of generators (e.g.—some microturbines) might also require a braking load  20  connected to the output through a switch S 3 , as shown. It is possible to connect the braking load to the DC link rather than to the output. When a negative load step occurs, some generators (such as fuel cells) can reduce their output almost instantaneously, whereas other generators (such as microturbines) cannot immediately reduce their output. Generators that cannot immediately reduce their output can then put some of the extra energy into the battery being charged, and some of it into the braking load. If the battery being charged has become fully charged before the battery being discharged has become fully discharged, then all of the extra energy coming from a negative load step would be transferred to the braking load. 
     Switch S 3  may be either a mechanical or a semiconductor switch. When switch S 3  is a semiconductor switch, it can be pulse-width-modulated to control the amount of energy going into the braking load. If switch S 3  is a mechanical switch, then the amount of braking energy cannot be controlled. Effective power control during braking would then be obtained by closing switch S 3  and then controllably discharging the battery that is currently in discharge mode. Switch S 3  can also be replaced with a power converter that can be used to control the braking energy. The most preferably embodiment is to connect a braking resistor in series with a semiconductor switch across the dc link. Braking energy could then be controlled by pulse-width-modulating the semiconductor switch. 
     The system of FIG. 2 may be delivered with battery  12 B mostly (or completely) charged and with battery  12 A completely discharged. Operation begins with switch S 2  in the UP position. After the generator  10  is started, the generator provides power to the load and also some charging power to battery  12 A through the charger. When a positive load step change occurs, battery  12 B provides the shortfall in power until the generator is able to produce enough power to support the load. It is also possible to reduce the rate of charging to battery  12 A to provide some of the shortfall in power (the decision of whether or not to do this will depend on the expected level of load step changes and the type of generator). 
     The batteries are preferably sized such that battery  12 A will be completely charged before battery  12 B is discharged. Sizing of the batteries to fulfill these conditions is no different from sizing of the batteries in prior art implementations of load-following. The size of the batteries will depend on the size of the maximum load step, the amount of time required for the generator to respond to a step-load change, and the expected frequency and magnitude of load changes. 
     When battery  12 B is completely discharged, switch S 2  is repositioned to the down position and the cycle repeats, this time with battery  12 B charging and battery  12 A discharging. When battery  12 A is completely discharged, switch S 2  is moved back to the UP position and the cycle repeats. 
     Since battery  12 A and battery  12 B are preferably run down to complete discharge before recharging them, the memory effect will not be present, or will be minimal. It is therefore possible to use batteries that normally exhibit a memory effect. 
     Second Embodiment 
     Another preferred embodiment of the present invention is shown in FIG.  3 . In this embodiment, a second power converter  14 B and a third power converter  14 C can each function as either a charger or a discharge controller. Operation of the circuit is similar to that of the circuit shown in FIG. 2 except that the function of switch S 2  is replaced by changing the direction of power flow in power converter  14 B and power converter  14 C. For example, in the initial state, battery  12 A is completely discharged and battery  12 B is fully or mostly charged. Power is drawn from the dc link through power converter  14 B to charge battery  12 A. When there is a shortfall in available power from the generator  10 , power converter  14 C draws power from battery  12 B and sends it to the dc link to make up for the shortfall in power. When battery  12 B is completely discharged, then power converter  14 C will begin to charge battery  12 B while power converter  14 B draws power from battery  12 A as needed to provide for shortfalls in generator power. Power converters  14 A and  14 B may share components such as output filter capacitor, internal house-keeping power supplies, and digital signal processors. It is also possible to operate power converters such that their PWM signals are 180 degrees out of phase. That will reduce the ripple current in the shared output filter capacitor and might allow a reduction in size of that capacitor. 
     Conclusion 
     The presently preferred embodiments described above operate load-following batteries in such a way that the battery is discharged completely before being recharged. This allows the use of batteries that display a “memory” effect in load-following applications. It may be noted that these embodiments require twice as much energy storage for load-following as the single-battery prior art system depicted in FIG.  1 . However, many distributed generation systems require a significant amount of stored energy at startup. The startup energy may be more than twice the energy required for load following. In these applications, it is also possible to deliver systems with both batteries fully charged. At least one battery will be completely discharged during startup, so the system can operate normally after startup. Furthermore, the generator can be programmed to remain on long enough to fully charge both batteries before shutting down, thus assuring enough stored energy for startup. Such a system would contain the same amount of energy storage as prior art systems. 
     Persons skilled in the art of power generation will appreciate that the components described herein as making up the preferred embodiments of the present invention are well known and may be implemented in many different forms. Thus, the present invention is by no means limited to any particular form of generator  10 , battery  12 , power converter  14 , charger  16 , discharge converter  18  or braking load  20 . Moreover, it is apparent that the present invention may be practiced without necessarily using all of these components, or by replacing some or all of these with functional equivalents. For example, if generator  10  is a dc generator, then in some cases, power converter 1 .  14 A might not be required. Accordingly, the scope of protection of the following claims is not intended to be limited to the presently preferred embodiments described herein.