Abstract:
An apparatus and method for supplying energy to a load includes an energy recharge unit, an energy storage unit, an energy converter connected to the energy recharge unit, the energy converter being capable of transferring energy at a power level from the energy recharge unit to an output node, the power level being determined by a power transfer controller, and a bi-directional energy converter connected to the energy storage unit and to the output node. The bi-directional energy converter is capable of converting energy of varying voltages from the energy storage unit to energy of varying current levels to supplement the transferred energy with energy from the energy storage unit so as to maintain a constant voltage on the output node. The bi-directional energy converter is capable of converting the transferred energy to provide charging energy to the energy storage unit when the transferred energy exceeds a demand level of the load while maintaining the constant voltage at the output node.

Description:
CROSS-REFERENCE TO RELATED U.S. PATENT APPLICATION 
     This application is a continuation application of U.S. application Ser. No. 11/615,074, now U.S. Pat. No. 7,994,657, entitled “MODULAR SYSTEM FOR UNATTENDED ENERGY GENERATION AND STORAGE,” which was filed on Dec. 22, 2006, the entirety of which is hereby incorporated by reference. 
    
    
     GOVERNMENT RIGHTS 
     This invention was made with government support under Contract No. H92222-06-C-0002 awarded by the Department of Defense. The U.S. government has certain rights in the invention. 
    
    
     TECHNICAL FIELD 
     The invention relates, generally, to energy generation and storage systems and, more particularly, to a modular system for unattended energy generation and storage. 
     BACKGROUND 
     Remote sensors, isolated communication devices, distributed wireless networks, and a host of other unattended electrically-operated systems typically require for operation minimum levels of electrical energy. These unattended electrically-operated systems are generally unreachable by conventional electric power or utility grids. As such, alternative energy or power sources such as, for example, solar energy, wind energy, and geothermal energy, have increasingly been relied upon to supply their required electrical energy. 
     While a few of these unattended electrically-operated systems are geographically positioned to benefit from regular access and routine maintenance, others may be unsuitable for maintenance either because of difficult access, highly distributed arrangements, or excessive costs. In a variety of applications, limits on maintenance reflect a vast installation. Consider, for example, a border security application in which thousands of sensors are arrayed along hundreds of miles of frontier. In principle, an access road can be built and regular maintenance can be scheduled. In practice, there may be sections in which road access is problematic and the sensor array may be so vast that dedicated maintenance crews may have to be assigned continuously. 
     Another example is rural broadband access to the Internet or World Wide Web (WWW), in which one approach is a dispersed array of pole-mounted repeaters. If these repeaters can be set up with an “install and forget” strategy, service providers can substantially function without dedicated maintenance crews. 
     These unattended electrically-operated systems typically utilize energy storage devices or units, to maintain power availability at night, to help maintain operation through intervals of bad weather, and to allow the electrical load to draw power in short-term bursts that might exceed the delivery capability of their energy recharge unit generation or recharge units. Moreover, these unattended electrically-operated systems generally need power conversion and regulation to deliver reliable, consistent power independent of conditions on the energy recharge units or in the storage units. 
     In solar applications, conventional remote power systems use combinations of solar panels for energy recharge units and rechargeable batteries for energy storage units. Typically, batteries are connected directly to an output, while the solar panels are connected either through a diode or through a switching power converter. The direct use of batteries typically limits the degree of output regulation and does not provide for the longest possible life of these unattended electrically-operated systems. Thus, the battery terminals serve as the direct power output, in which case the only protection is a fuse. As such, the quality of output regulation is determined entirely by the battery and will follow wide tolerances. 
     These unattended systems typically lack reliability as battery charging processes are not properly managed. Overcharge and undercharge conditions can occur, especially during long periods of cloudy weather. Battery life is relatively limited as a result. If a short circuit blows a fuse, the system will be down until serviced. Multiple battery units may be interconnected, but there is no control mechanism for load sharing or balancing. Further, multiple battery units may be connected to a single output in a modular fashion, and protection and interaction between and among the batteries are not adequate. Thus, these unattended systems need to protect themselves as well as their batteries against output short circuits and other external faults. 
     In some unattended systems, either the energy recharge unit or the rechargeable storage unit is connected directly to a dc bus, and the other unit is connected through a dc-dc converter. As such, only one dc-dc converter is utilized while having independent control of the energy recharge unit and the rechargeable storage unit regardless of the serviced load. This arrangement supports an improved integration of recharge and storage over the basic solar panel and battery interconnections, but still does not resolve regulation or protection issues. 
     In other unattended systems, the energy recharge unit charges the storage unit, which then charges a capacitor, which is then switched into the load. In this arrangement, power flow from the energy recharge unit to the load goes through a series of device connections: the energy recharge unit, the storage unit, the capacitor, and then the load. This sequence of operations can result in extra power loss, especially during intervals in which the power from the energy recharge unit is well-matched to the load. 
     Therefore, a need exists for a modular system for unattended energy generation and storage that overcomes the problems noted above and others previously experienced for addressing issues of regulation, protection, interconnection, or modularity. These and other needs will become apparent to those of skill in the art after reading the present specification. 
     SUMMARY 
     The foregoing problems are solved and a technical advance is achieved by the present invention. Articles of manufacture and systems consistent with the present invention provide an apparatus or modular system for unattended energy generation and storage for supplying energy to a load. The apparatus includes an energy recharge unit, an energy storage unit, an energy converter connected to the energy recharge unit, the energy converter being capable of transferring energy at a power level from the energy recharge unit to an output node, the power level being determined by a power transfer controller, and a bi-directional energy converter connected to the energy storage unit and to the output node. The bi-directional energy converter is capable of converting energy of varying voltages from the energy storage unit to energy of varying current levels to supplement the transferred energy with energy from the energy storage unit so as to maintain a constant voltage on the output node. The bi-directional energy converter is capable of converting the transferred energy to provide charging energy to the energy storage unit when the transferred energy exceeds a demand level of the load while maintaining the constant voltage at the output node. 
     Articles of manufacture consistent with the present invention also provide a method of providing energy to a load from a power supply system. The power supply system includes an energy recharge unit, an energy converter with its input connected to the energy recharge unit and its output connected to an output node and coupled to a power transfer controller, a rechargeable energy storage unit, a bi-directional energy converter with its input connected to the rechargeable energy storage unit and its output connected to the output node, the output node being connected to an input of the load. The method exposes the energy recharge unit to a corresponding energy source so as to produce electric energy, determines via the power transfer controller whether a power level of the produced electric energy is above a predetermined power threshold in order to activate the energy converter, converts the produced electric energy by the energy converter and delivering the converted electric energy to the load in order to meet at least part of a demand level of the load, monitors a voltage at the output node. The method further converts energy stored in the energy storage unit by the bi-directional energy converter to supplement the delivery of the converted produced energy to the load so as to maintain the output node voltage at a predetermined voltage level, and converts the transferred energy to provide charging energy to the energy storage unit when the transferred energy exceeds a demand level of the load while maintaining the predetermined voltage level at the output node. 
     Other systems, apparatus, methods, features, and advantages of the present invention will be or will become apparent to one with skill in the art upon examination of the following figures and detailed description. It is intended that all such additional systems, methods, features, and advantages be included within this description, be within the scope of the invention, and be protected by the accompanying claims. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate an implementation of the present invention and, together with the description, serve to explain the advantages and principles of the invention. In the drawings: 
         FIG. 1  is a schematic diagram illustrating one embodiment of a modular unattended system for energy generation and storage consistent with the present invention; 
         FIGS. 2A  and B are schematic diagrams illustrating the modular unattended system for energy generation and storage of  FIG. 1  with protection elements consistent with the present invention; 
         FIG. 3  is a flow chart illustrating a method for an initial start-up of the modular unattended system for energy generation and storage of  FIG. 1  consistent with the present invention; and 
         FIG. 4  is a flow chart illustrating a method for detecting and mitigating a fault disturbing an on-going operation of the modular unattended system for energy generation and storage of  FIG. 1  consistent with the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to an implementation consistent with the present invention as illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings and the following description to refer to the same or like parts. 
       FIG. 1  depicts one embodiment of a modular system for unattended energy generation and storage or power supply apparatus  100  consistent with the present invention. The power supply apparatus  100  is a modular system that comprises an energy recharge device or unit  10  suitable for long-term operation and expected to provide variable energy, an energy storage device or unit  12  which is charged and recharged by the energy recharge unit  10 , a switching power conversion circuit  14  that draws power when activated from the energy recharge unit  10  in a manner that addresses the needs of a load  16 . The switching power conversion circuit  14  will be hereafter referred to as the recharge unit or source converter  14 . The power supply apparatus  100  further comprises another switching power conversion circuit  18  that manages the energy storage unit  12  and maintains an output suitable for the variable or arbitrary electrical load  16 . The switching power conversion circuit  18  will be hereafter referred to as the storage converter  18 . 
     The power supply apparatus  100  is configured to enable energy delivery to the load  16  from the energy recharge unit  10  while also providing energy to the energy storage unit  12  so as to augment, if necessary, the energy stored in the energy storage unit  12  when sufficient recharge unit energy is available. The power supply apparatus  100  is also configured to enable energy delivery to the load  16  from the energy recharge unit  10  and/or from the energy storage unit  12  when the energy recharge unit  10  is unable to meet the energy needs of the load  16 . The power supply apparatus  100  also prevents energy delivery to the load  16  when the charge level of the energy storage unit  12  falls below a minimum voltage threshold level so that all of the energy generated by the energy recharge unit  10  is supplied or made available to the energy storage unit  12  until the voltage of the energy storage unit  12  reaches this minimum voltage threshold level at which the storage converter  28  will operate at a minimum desired energy conversion efficiency level. As such, the energy recharge unit  10  can build up the amount of energy in the energy storage unit  12  first and then to deliver and maintain delivery of energy to the load  16  as well as to continue building up and maintaining energy in the storage energy unit  12  during periods when the energy recharge unit  10  is producing enough energy to do so. 
     The energy recharge unit  10  may be any kind of power or energy generating system, such as for example solar panel, solar cell, fuel cell, wind generator, rotary generator, etc. For the purposes of the present invention, the energy recharge unit  10  is a solar panel. One important feature is that the energy recharge unit  10  is configured to be controlled to maximize its performance and longevity. In the case of a solar or wind recharge unit, the control draws maximum available power whenever power is available and useful. In the case of a fuel cell or rotary generator, the control acts to maintain operation at an optimum power point, selected for the specific technology. The preferred application is an unattended long-term power supply, so those energy systems that require substantially no maintenance are most suitable. These would include solar photovoltaic panels, wind generators, small water wheels, or devices able to extract energy from the immediate surroundings. Almost all plausible energy recharge units  10  for this purpose have an identified optimum operating condition. As such, the energy recharge unit  10  uses a switching power converter controlled to enforce this optimum. This is intended to make the operation of the energy recharge unit  10  nearly independent from those of the load  16  and the energy storage unit  12 . 
     In the preferred embodiment, the energy storage unit  12  is based on energy storage elements  26  such as ultracapacitors. Among electrical and electrochemical storage devices, ultracapacitors  26  are known to provide the highest number of charge and discharge cycles and to have long life as well. Other energy accumulators, especially secondary batteries but also devices such as flywheels or other rechargeable mechanical devices are applicable. Ultracapacitors  26  or batteries or other rechargeable elements may be used in series strings owing to their relatively low terminal voltages. To ensure long life, the individual storage elements in the series string need to be matched in voltage. Known balancing technology provides this function, which in turn assures the longest possible life. In ultracapacitors and certain battery types, notably lithium-ion rechargeable cells, life is further extended by limiting the charge voltage. Balancing technology allows the voltage to be set to any desired value, and can maintain balance even if a decreased voltage is desired. In the unattended power supply apparatus  100 , the energy storage unit  12  may be controlled to permit operation even when the energy recharge unit  10  is unavailable, and is protected against both under-voltage and over-voltage to prevent failures. 
     The load  16  can be any kind of electric load, which requires electric power at a rate or magnitude that does not exceed the capacities of the energy storage unit  12  and the energy recharge unit  10  over time, or for which periods of nonuse during shortages of such capacities can be tolerated before the energy recharge unit  10  is re-energized and self-started. The energy recharge unit  10  and the energy storage unit  12  need to be suitable for the load  16  that may require either fairly steady or intermittent current. Moreover, the energy storage unit  12  needs to be able to accumulate enough energy capacity to power the load  16  for some periods of time during which the energy recharge unit  10  is incapable of doing so, for example, during the dark of night or during cloudy weather when the energy recharge unit  10  is a solar panel. 
     Now referring back to  FIG. 1 , the energy recharge unit  10  is connected to the source converter  14  such that energy or power generated or otherwise produced by the energy recharge unit  10  flows through the source converter  14  to the load  16  and to the storage converter  18  via the output node  24 . The source converter  14  is a dc-dc switching power converter, and preferably a boost type dc-dc converter. The source converter  14  incorporates a maximum power point tracker (MPPT) or power controller  26 , which serves to insure that the energy recharge unit  10  generates maximum power without regard to voltage at output node  24  or other conditions. When the energy recharge unit  10  is a solar panel, the source converter  14  (and its MPPT controller  26 ) functions only when solar energy is available, such as during daylight hours. That is, an internal processor (not shown) of the source converter  14  does not consume energy unless solar energy is available. This operational feature of the source converter  14  is accomplished by supplying the recharge unit converter controls from the panel of the energy recharge unit  10 . 
     By incorporating the MPPT controller  26 , the source converter  14  becomes a variable dc-dc converter that may use a control algorithm to force the terminals of the photovoltaic panel to an impedance that produces the maximum power out of the panel of the energy recharge unit  10 . The MPPT controller  26  adjusts the panel operating point to extract maximum power on a moment-by-moment basis. This MPPT controller  26  may act entirely based on panel terminal characteristics so as to function irrespectively of the voltage at output node  24 . As such, the source converter  14  does not disrupt voltage level or interfere with voltage regulation action elsewhere in the power supply apparatus  100 . Many MPPT methods can be used, such as ripple correlation control, perturb-and-observe approaches or constant-voltage-fraction approaches. 
     The MPPT controller  26  is also configured to enforce a voltage limit and permit an external digital command to shut it down. Without such a limit or command, excess solar power may continue to be delivered from the energy recharge unit  10  even when the load  16  is light and the storage energy unit  12  is at full capacity. In this situation, solar energy is not needed, and there is a potential for overcharging the energy storage unit  12  or delivering excessive voltage at the output node  24 . Moreover, when the produced energy falls below a predetermined energy level, the MPPT controller deactivates the source converter  14 . Power tracking subject to a voltage limit and shutdown command is a known practice but has not been used previously with a separate parallel converter, such as the storage converter  18 , which manages a different function. 
     As stated earlier, the energy storage unit  12  is connected to the output node  24  through the storage converter  18 . The storage converter  18  is a bi-directional dc-dc boost converter configured to deliver a consistent or approximately constant output voltage level on the output node  24 , regardless of its input voltage level. That is, even if the input voltage to the storage converter  18  varies, the output voltage of the storage converter  18  on the output node  24 , which is supplied to the load  16 , preferably remains approximately constant, which may be required by the load  16 , especially if the load  16  includes a microprocessor or other logic controller that typically requires such a constant voltage input to avoid inconsistent or incorrect operation that may be caused by voltage fluctuations. 
     Typically, dc-dc voltage converters by nature have different conversion efficiency characteristics for different input voltages. At a low input voltage, the storage converter  18  may be so inefficient that it may drain all the power available from the energy storage unit  12  at a rate faster than the energy storage unit  12  and the energy recharge unit  10  can supply and still not be able to deliver the required constant voltage on the output node  24 . In that condition, the power supply apparatus  100  may not power the load  16 . Therefore, the storage converter  18  is activated only when its input voltage is high enough to operate efficiently and that the energy available from the energy storage unit  12  is sufficient for the storage converter  18  to deliver the required energy at the required constant voltage on output node  24  to operate the load  16 . As such, the storage converter  18  is activated or controlled using a sensorless current mode (SCM) controller  20 , with its primary objective being output voltage regulation. The SCM controller  20  is connected to the output of the energy storage unit  12  and to the output of the storage converter  18 . Alternately, the SCM controller  20 , which may include a processor programmed with operational instructions, may be incorporated into the storage converter  18 . 
     In SCM control, an inductor current is reconstructed from voltage information. The inductor voltage in a dc-dc converter is typically a substantially larger signal than an output of a current sensor and its range does not change as a function of loading. In its simplest form, the SCM approach reconstructs the inductor current directly by integrating the inductor voltage. Alternately, current limiting can be enforced by means of a separate current sensor. As such, current can be limited from the energy storage unit  12  to limit an energy flow when the output node  24  is short-circuited or overloaded. Other control methods, such as voltage-mode or current-mode controls, can be used if they are properly designed to reject energy variations, although SCM has known advantages based on noise rejection and wide operating range. 
     Using the SCM controller  20 , the storage converter  18  can be controlled to produce a substantially constant voltage at the output node  24 . In effect, the storage converter  18  can deliver whatever current is needed to maintain the substantially constant voltage. If the current of the energy recharge unit  10  exceeds the current amount required by the load  16 , the storage converter  18  takes the excess and acts to deliver it to the energy storage unit  12  at the actual capacitor voltage level. If the current of the energy recharge unit  10  is insufficient for the load  16 , the storage converter  18  acts to take current from the energy storage unit  12  and to maintain the load with constant voltage output. Thus, the storage converter  18  manages the current to take on whatever value is needed (positive or negative) at any given moment to maintain the constant voltage at output node  24  and support the load  16 . 
     Moreover, the SCM controller  20  can monitor energy buildup in the energy storage unit  12  and detects when the voltage in the energy storage unit  12  reaches or exceeds a minimum threshold level. More specifically, the SCM controller  20  may prevent any energy generated by the energy recharge unit  10  and any energy in energy storage unit  12  from being delivered to the load  16  until such time as the energy storage unit  12  has reached a minimum threshold of energy storage or a minimum charge level. 
     As stated above, in  FIG. 1  the energy storage unit  12  uses the ultracapacitors  26 , which can deliver extreme cycle counts, 100,000 or more, thus supporting decades of daily or even more frequent cycles. These ultracapacitors  26  have limited voltage ratings, typically 2.3 to 2.7 V each, so they are used in series combinations in most storage applications. In series combinations, a charge imbalance is reflected as a voltage imbalance, and it is imperative to make sure the highest voltage in the series string stays below the operating limit. Furthermore, life extension can be obtained if an even lower voltage limit is enforced. Similar limitations based on voltage apply to batteries, which can be used in place of ultracapacitors  26  with only minor circuit modifications. 
     In the embodiment of  FIG. 1 , the energy storage unit  12  is coupled to an equalizer  22 . The equalizer  22  can guarantee that each of the ultracapacitors  26  has a corresponding voltage that is a correct fraction of the total string voltage. Thus, if there are 5 ultracapacitors  26  in series and the total voltage is 10.0 V, then each individual ultracapacitor  26  is within a few millivolts of 2.0 V. The use of the equalizer  22  enables the monitoring and enforcement of capacitor voltage limits and the management of the energy storage unit  12 . The inclusion of the equalizer  22 , especially to facilitate reduced-voltage operation, is an advantageous aspect of the invention. 
     As shown in  FIG. 1 , the power supply apparatus  100  can be subdivided into two subsystems  11  and  13  and the load  16 . Namely, an energy recharge unit or recharge subsystem  11  which includes the energy recharge unit  10  and the source converter  14  with its MPPT controller  26 , and an energy storage subsystem  13  which includes energy storage unit  12 , the equalizer  22  and the storage converter  18  with its SCM controller  20 . 
     An advantageous feature of the power supply apparatus  100  is that the system-level power converter control, which includes controls incorporated into the source converter  14  and the storage converter  18 , decouples the action of the individual subsystems  11  and  13 . This allows the two subsystems  11  and  13  to interact in a simple and parallel manner without introducing control challenges. The basic action is that the energy recharge unit  10  is controlled for power flow, the output node  24  is controlled for fixed voltage, and the current to and from the energy storage unit  12  is controlled to make up any differences. 
     As both the energy recharge subsystem  11  and the energy storage subsystem  13  supply the load  16  in parallel, the solar energy produced does not go through multiple power processing stages on its way to the load  16 . Moreover, as the output node  24  need not be connected directly either to the energy storage unit  12  or to the energy recharge unit  10 , each of the intervening dc-dc converters  14  and  18  manages the needs of the corresponding subsystems  11  and  13  while delivering regulated output power to the output node  24  that serves the load  16 . This arrangement of the power supply apparatus  100  can be termed a “virtual bus” structure. 
     Now referring to  FIGS. 2A  and B, another embodiment of the power supply apparatus  100  of  FIG. 1  is illustrated with protection elements consistent with the present invention. As shown, the subsystem  11  now includes an inductor element  30  connected at the output of the energy recharge unit  10 , a unidirectional element  32 , such as a diode, connected in series between the inductor element  30  and the load  16 , and a switching element q 4    34  connected in parallel with the energy recharge unit  10  and the inductor  30 . The switching element q 4    34 , which can also be referred as the recharge unit switch q 4    34 , is coupled for switching operations to the source converter  14 . 
     As shown in  FIGS. 2A  and B, the subsystem  13  includes an inductor element  36  connected at the output of the storage unit  12 , and a switching element q 1    38  connected in parallel with the storage rechargeable unit  12  and the inductor element  36 . The switching element q 1    38  may be a field-effect transistor (FET) or metal-oxide semiconductor FET (MOSFET). The recharge unit switch q 1    38 , is coupled for switching operations to the storage converter  18 . Additionally, as shown in  FIG. 2A  two other switching elements q 2    40  and q 3    42  are connected in series between the inductor element  36  and the output node  24 , and coupled for switching operations to the storage converter  18 . The switching elements q 2    40  and q 3    42  may be field-effect transistors (FET) or metal-oxide semiconductor FETs (MOSFET). Alternately, the switching elements q 3    42  may be positioned between the output node  24  and the load  16 , as shown in  FIG. 2B . Moreover, a current sensor  44  is used to supply a value of a current I bat  of the energy storage unit  12  to the storage converter  18 . 
     Now referring to  FIG. 3 , a flow chart  300  illustrates a method for an initial start-up of the power supply apparatus  100  of  FIG. 1  consistent with the present invention. The energy storage converter  18  is configured with its SCM controller  20  to manage the start up procedure. At an initial start-up of the power supply apparatus  100 , the energy recharge unit  10  being a solar cell is exposed to available sunlight in step  302 , and the energy storage unit  12  is in this initial condition has no stored energy charge or only a very low amount of stored energy or charge. Upon initial start up of the power supply apparatus  100  in this configuration, the storage converter  18  prevents any energy converted by the source converter  14  from being delivered to the load  16 , at step  304 . That is, the storage converter  18  ensures that all of the energy generated by the energy recharge unit  10  is flowing to the energy storage unit  12 , at step  306 . The storage converter  18  manages this power flow and prevents excessive starting current by means of a hardware limit on the duty ratio of recharge unit switch q 1    38 . Once the energy storage unit  12  reaches a minimum threshold for its energy level, at step  308 , the storage converter  18  allows the energy generated by the energy recharge unit  10  to be delivered to the load  16 , at step  310 , while a remainder of the energy generated, if any, is supplied to the energy storage unit  12  so that the energy level in the energy storage unit  12  continues to increase. That is, once the voltage of the energy storage unit  12  has exceeded the minimum allowed threshold (minimum operating energy level), then the power supply apparatus  100  may supply the load  16  (i.e. enter normal operation), at step  312 . Moreover, as stated above during normal operation the energy storage unit  12  can supply the energy required by the load  16  when the energy recharge unit  10  cannot generate sufficient energy to operate the load  16 . 
     Moreover, when the energy storage unit  12  is discharged to below the minimum allowed operating energy level, that is, its terminal voltage is low, The recharge unit switch q 1    38  is turned off and a diode (not shown) allows energy to flow into but not out of the energy storage unit  12 . Conversely, when the energy storage unit  12  is completely charged (maximum allowed operating energy level), that is, its terminal voltage is high, the storage converter  18  sends a disable signal to shut off the source converter  14 . In this circuit condition, the switching element q 2    40  is also turned off and the diode (not shown) allows only energy to flow out of but not into the energy storage unit  12 . 
     Now referring to  FIG. 4 , a flow chart  400  illustrates a method for detecting and mitigating a fault disturbing an on-going operation of the power supply apparatus  100  of  FIG. 1  consistent with the present invention. As shown in  FIGS. 2A  and B, the output protection switching element q 3    42 , which may be provided in alternate positions, allows the energy storage subsystem  13  to be completely disconnected if a problem occurs. This is an unusual arrangement in a dc-dc converter and may be a unique aspect in this particular application. Further, an automatic recovery procedure programmed in the processor of the storage converter  18  to restore power if a short-term output problem appears during normal operation, at step  402 , is described below. 
     If a short circuit is detected occurring at the output node  24 , at step  404 , the protection switch q 3    42  disconnects immediately, at step  406 . Switch ratings are chosen so the protection switch q 3    42  can interrupt a maximum short-circuit current. As such, the energy stored in the ultracapacitors  26  will not dissipate into the short circuit. When this switch q 3    42  is open the storage subsystem  13  can charge when sufficient voltage is available, at step  408 , because a reverse parallel diode (not shown) connected with switch q 3    42  permits inflow but not outflow. The storage subsystem  13  may not discharge until a fault recovery sequence is completed. The protection switch q 3    42  also opens if the capacitor voltage falls below a lower limit or threshold, indicating possible undercharge so as to protect the energy storage subsystem  13  against total energy loss if a fault occurs. Energy remains available, and allows the power supply system apparatus  100  to recover normal operation immediately when the fault is gone and the energy recharge unit  10  is once again delivering power. After determining if a predetermined amount of time has elapsed, at step  410 , and once the fault clears, the storage converter  18  enters a recovery mode. At step  412 , the switch q 3    42  is turned on and the storage converter  18  begins providing energy to the output node  24 . At step  414 , a check is made as to whether the fault is still present. In the affirmative, switch q 3    42  turns off again. Otherwise, the output voltage at output node  24  starts to come up and normal operation resumes, at step  416 . 
     Another advantageous protection feature of the invention is the positioning of the unidirectional element or diode  32  in the recharge unit subsystem  11 . This diode  32  is provided at an output of the energy recharge unit  10  to prevent current from flowing toward it. That is, the diode  32  permits current outflow but not inflow toward the energy recharge unit  10 . In this embodiment of the power supply apparatus  100 , no additional protection switching element is provided between the energy recharge unit or solar panel  10  and the load  16 , because solar panels are inherently current limited. That is, a short-circuit current is typically only slightly higher than the rated output current of the power supply apparatus  100 . As such, short circuit conditions do not result in excessive current flows, and the source converter  14  and its MPPT controller  26  may not need to be shut-off because as discussed above a separate process programmed in the storage converter  18  manages the recovery process from a short circuit condition. 
     In summary, the combination of power-based tracking control for the source converter  14  and SCM control for the storage converter  18  is an advantageous feature of the invention. These two controls work together in a highly useful manner. The MPPT controller  26  acts to maximize power from the energy recharge unit  10  no matter what voltage is present at the output node  24 . The SCM controller  20  acts to regulate the voltage no matter what the power. As a result, undesired dynamic interactions are avoided. When the energy recharge unit  10  is able to deliver extra power, this power flows into the energy storage unit  12  without any extra intentional effort. When the energy recharge unit  10  is insufficient or absent, energy flows from the energy storage unit  12  to maintain the desired voltage at the output node  24 . The two controls can be designed independently, and work together in a manner that is highly beneficial for the modular arrangement of the power supply apparatus  100 . The only added aspect needed to avoid undesired interactions is to address the condition when the energy storage unit  12  reaches its “full” state or charge. In this case, extra power input to this modular system is not useful. Under this condition, which is sensed easily by confirming that the energy storage unit  12  has reached its voltage limit, the energy storage converter  18  sends a signal to shut off the source converter  14 . The energy recharge unit  10  will not turn back on until its power is again useful to the modular system or power supply apparatus  100 . As such, the energy storage unit  12  is managed with both upper and lower voltage limits to prevent overcharge and undercharge. A limited charge range is essential for high reliability. A current limit is enforced both on the energy storage subsystem  11  and at the output node  24 . If the output current becomes excessive, then the protection device q 2    40  temporarily shuts off the output. 
     This control combination is advantageous because it delivers excellent output regulation no matter how power is flowing in the power supply system apparatus  100 . Since the power flow is uncertain and highly variable, it is essential that the regulation function be carried out in a manner independent of flows, directions, and other factors. The SCM controller  20  is able to carry out its function even when short-term load peaks are applied to the power supply system apparatus  100 . An advantage is that highly variable loads that require intermittent power can be handled without special problems with this power supply system apparatus  100 . Loads with consistent, constant power needs are also handled without trouble. 
     While various embodiments of the present invention have been described, it will be apparent to those of skill in the art that many more embodiments and implementations are possible that are within the scope of this invention. Accordingly, the present invention is not to be restricted except in light of the attached claims and their equivalents.