System, method, module, and energy exchanger for optimizing output of series-connected photovoltaic and electrochemical devices

An energy transfer device for solar power systems operates to draw power from high-producing photovoltaic devices and apply that power across low-producing photovoltaic devices. An embodiment is a self-regulating energy exchanger using bidirectional DC-DC converters that operates to maintain uniform voltage across each series-connected photovoltaic device. An alternative embodiment is an energy exchanger that is controlled to maintain each of several series-connected photovoltaic devices at a maximum power point by drawing power from high-performing devices and applying that power across low-performing devices to provide uniform current among series-connected photovoltaic devices.

FIELD OF ART

The present document relates to the field of electronic devices for optimizing output from, and charge to, series-connected devices having mismatches in output capability. The disclosed device is of utility in equalizing output from series-connected photovoltaic devices such as solar cells and solar panels, and charge to and output from series-connected electrochemical devices such as cells in batteries.

BACKGROUND

Photovoltaic cells produce a voltage that varies with current, cell condition, cell physics, cell defects, and cell illumination. One mathematical model for a photovoltaic cell, as illustrated inFIG. 1, models output current as:

I0=reverse saturation current

n=diode ideality factor (1 for an ideal diode)

I=output current at cell terminals

V=voltage at cell terminals

Typical cell output voltages may be about one-half volt for Silicon (Si) cells, which is far below the voltage needed to charge batteries or drive most other loads. As a result, cells are typically connected together in series to form a module, or an array, having an output voltage much higher than that produced by a single cell. Cell voltages differ for other types of cells, for example, Germanium (Ge) cells typically have lower cell output voltage at maximum power point than do Si cells.

Real-world photovoltaic cells often have one or more microscopic defects, these cell defects may cause mismatches of series resistance RS, shunt resistance RSH, and photogenerated current ILfrom cell to cell in a module. Further, cell illumination may vary from cell to cell in a system, and may vary even from cell to cell in a module. Environmental effects that can cause variations in illumination from cell to cell include shadows cast by trees, debris including bird droppings or leaves shadowing portions of a cell or module, dust, dirt, and other effects. These mismatches in illumination may vary from day to day and with time of day—a shadow may shift across a module during a day.

From eqn. 1 and the model, output voltage is greatest at zero output current, and output voltage V falls off nonlinearly with increasing output current I.FIG. 2illustrates the effect of increasing current drawn from a photovoltaic device at constant illumination. As current I is increased under constant illumination, voltage V falls off slowly, but as current I is increased to an output current near the photocurrent IL, output voltage V falls off sharply. Similarly, cell power P, the product of current times voltage, increases as current I increases, until falling voltage V overcomes the effect of increasing current, whereupon further increases in current I drawn from the cell cause power P to decrease rapidly. For a given illumination, each cell, module, and array of cells and modules therefore has a maximum power point (MPP) representing the voltage and current combination at which output power from the device is maximized. The MPP of a cell, module, or array will change throughout a day as illumination, and hence photocurrent IL, changes.

Maximum Power Point Tracking (MPPT) controllers are devices that determine an MPP voltage and current for a photovoltaic device connected to their input, and adjust current drawn from the device to maintain the photovoltaic device at the MPP.

Without added circuitry, all cells in a series string of cells, as illustrated inFIG. 3, must carry the same current.

Variations in photocurrent IL, and variations in effective shunt resistance RSH, between cells of a module or of an array described above may cause the maximum power point output current for one cell Cstrong in a string, to be at a current well above the maximum power point output current Iweak for another cell Cweak in the string (seeFIG. 3). In some arrays under some conditions, if Cstrong is operating at its MPP current, Cweak is subjected to a current above its MPP current and may even reverse-bias, thereby consuming power or blocking current flow from better producing cells in the same string. The net effect is that power output from a panel or a series string of panels is limited by the performances of the poorer-producing cells in the series string.

Prior Photovoltaic Solutions

Some prior solar panels have bypass diodes D1, D2, D3at the module level, at the cell level, or at the level of a group of cells within the module, as illustrated inFIG. 3. The bypass diodes prevent the weak cell Cweak from reverse-biasing and blocking current flow from better producing cells in the string, but, as the low producing cell and any other cells in the same group with the same bypass diode is bypassed, any power produced by Cweak and cells in its group is lost. As illustrated inFIG. 3, while some modules may provide bypass diodes such as D2across individual cells, other modules or systems may provide diodes such as D1across groups of cells, or even across entire modules, instead of across individual cells. Many modules on the market today provide bypass diodes across “6-volt” sections of approximately a dozen cells.

Illustrated inFIG. 4are other systems that use distributed, per-panel, DC-DC converters50or DC-AC microinverters to drive a common power-summing high-voltage bus52as illustrated inFIG. 4. Each converter50receives power from a solar module49, each module having several photovoltaic cells48, at whatever voltage and current that module49is capable of generating and potentially at the MPP of that module, and converts and outputs the power onto the high-voltage power-summing bus52. Since modules are no longer connected in series, low production by one module does not interfere with production by high-performing modules. Further, potential power production by low-performing modules is summed on the bus and not wasted.

An issue with distributed, per-panel voltage converters is that all power produced must pass through the converters, and some power is inevitably lost in circuitry of those converters. Such architectures also help to achieve MPP only at the panel level, but do not work at the individual cell level. For example, when even a single cell of a panel is cracked or partially shaded, the entire panel may not deliver the full potential power from the rest of the cells, particularly if bypass diodes are provided on a per-panel and not a per-cell basis. Cells may also be mismatched through manufacturing variations, differential soiling, and aging as well as damage and shade. Nonetheless, U.S. patent application publication numbers 2009/0020151 and 2005/0121067 propose variations on using local converters to drive DC or AC power-summing buses in parallel.

Yet another alternative is disclosed in U.S. patent application publication number 2008/0236648, in which power from groups of photovoltaic cells is fed into respective MPPT DC-DC converters to produce a current that is constant throughout all DC-DC converters of the array at a voltage at each converter that depends on power available from the attached photovoltaic device. The outputs of the DC-DC converters are connected in series. Once again, all power generated by an array passes through the DC-DC converters such that not only is all array power subject to converter losses, but failure of one or more converters may cause loss of all power from part or all of the array.

It has long been known that different types of photovoltaic cells absorb different wavelengths of light, and absorb wavelengths with different efficiencies. Typically, a photovoltaic cell type has a favored wavelength corresponding to photons of energy slightly greater than an energy gap of the cell. Photons of lower energy pass through the cell, while those of higher energy may be absorbed, but their extra energy provides heat without additional current. Some multijunction photovoltaic devices have been built with two, or in some cases three, junctions of different types stacked vertically. These typically have a top junction made of materials with large bandgap and thus having a relatively short favored wavelength and a maximum power point at relatively high voltage, and a bottom junction device having a lower bandgap and thus having a relatively long favored wavelength of operation and a maximum power point at relatively low voltage.

Cells of multijunction photovoltaic devices are often coupled electrically in series as they are formed, without bringing out a conductor from between the cells. While this construction simplifies connections to the cells, inefficiencies result for the same reasons that output of mismatched series-connected photovoltaic devices may be restricted; effective output current is determined by the lowest-current output of the stacked cells. This situation is aggravated by diurnal variations in color, or wavelength distribution, of received light, and by differences in types and efficiencies of the stacked cells.

Multiple junction photovoltaic devices have been studied, including those having stacked cells with a low-resistance electrical contact to a boundary between junctions, and those having junctions brought out separately. For example, see MacDonald,Spectral Efficiency Scaling of Performance Ratio for Multijunction Cells,34 IEEE Photovoltaic Specialist Conference, 2009, pg. 1215-1220.

SUMMARY

An energy transfer device for solar power systems operates to draw power from high-producing photovoltaic devices and apply that power across low-producing photovoltaic devices. An embodiment is a self-regulating energy exchanger using bidirectional DC-DC converters that operates to maintain uniform voltage across each series-connected photovoltaic device. An alternative embodiment is an energy exchanger that is controlled to maintain each of several series-connected photovoltaic devices at a maximum power point by drawing power from high-performing devices and applying that power across low-performing devices to provide uniform current among series-connected photovoltaic devices.

In an aspect of the inventive concepts, an energy transfer device has a first port coupled to a first interface unit, the first interface unit is coupled to a capacitor. Coupled to the same capacitor is a second interface unit that is in turn coupled to a second port of the transfer device. The interface units are adapted to operation with the first port having a voltage offset from the second port, and to transfer energy between the first port and the capacitor, and to transfer energy between the second port and the capacitor. The interface units are configured to transfer energy from the first port to the capacitor and from the capacitor to the second port when energy available at the first port is greater than energy available at the second port.

In another aspect of the inventive concepts, a system has an energy transfer device, the energy transfer device including a capacitor and having N ports, each of the N ports being coupled to an energy transfer port of one of N interface units, where each interface unit is coupled to the capacitor and is capable of transferring energy bidirectionally between its energy port and the capacitor. In this aspect, each interface unit is adapted to operation with each of the N ports having a voltage offset relative to each of at least one other of the N ports; and the interface units are adapted or controlled to pass energy from a high energy port of the N ports to the capacitor and from the capacitor to a low energy port of the N ports.

In another aspect of the inventive concepts, a solar photovoltaic array has a first and a second series-connected photovoltaic device each having a positive and a negative terminal, the first photovoltaic device being capable of producing a first electric current at a first voltage when illuminated, and the second photovoltaic device being capable of producing a second electric current at a second voltage when illuminated; an energy transfer device coupled to the photovoltaic devices, and capable of receiving energy from across one of the photovoltaic devices and applying that energy across another of the photovoltaic devices.

In another aspect of the inventive concepts, a solar photovoltaic array has first and second photovoltaic devices, the first photovoltaic device capable of producing a first electric current at a first voltage when illuminated, and the second photovoltaic device capable of producing a second electric current at a second voltage when illuminated. The first and the second photovoltaic devices are coupled electrically together in series with a positive terminal of the first photovoltaic device coupled to a negative terminal of the second photovoltaic device. An energy transfer device has a first terminal coupled to a negative terminal of the first photovoltaic device, a second terminal coupled to a positive terminal of the first photovoltaic device and to the negative terminal of the second photovoltaic device, and a third terminal coupled to a positive terminal of the second photovoltaic device, the energy transfer device being capable of receiving energy from its first and second terminals and providing energy to its second and third terminals if a first parameter selected from the group consisting of the first current and the first voltage is greater than a second parameter selected from the group consisting of the second current and the second voltage, and of receiving energy from its second and third terminals and providing energy to its first and second terminals if the second parameter is greater than the first parameter.

In another aspect of the inventive concepts, a solar photovoltaic array has at least a first, a second, and a third photovoltaic device each having a power output port, the first photovoltaic device being capable of producing a first electric current at a first voltage when illuminated, the second photovoltaic device being capable of producing a second electric current at a second voltage when illuminated, and the third photovoltaic device being capable of producing a third electric current at a third voltage when illuminated. The power output ports of the first, second, and third photovoltaic devices are coupled electrically together in series. An energy transfer device has a first port coupled to the power output port of the first photovoltaic device, a second port coupled to the power output port of the second photovoltaic device, and a third port coupled to the power output port of the third photovoltaic device, the energy transfer device capable of receiving energy from its first port and providing energy to its second port if a first parameter selected from the group consisting of the first current and the first voltage is greater than a second parameter selected from the group consisting of the second current and the second voltage, and of receiving energy from its second port and providing energy to its first port if the second parameter is greater than the first parameter; and wherein the energy transfer device is capable of receiving energy from the first port and providing energy to its third port if the first parameter is greater than a third parameter selected from the group consisting of the third current and the third voltage, and of receiving energy from its third port and providing energy to its first port if the third parameter is greater than the first parameter.

In another aspect of the inventive concepts, a system has a first energy transfer device for transferring energy from a high-producing device to a low-producing device, the first energy transfer device having a first port for coupling to a first photovoltaic device, a second port for coupling to a second photovoltaic device, a controller for determining a port attached to a low current producing device and a port attached to a high producing device, at least a first inductor, and a first switching device coupled in series with the first inductor. The energy transfer device is operable with the first and the second ports coupled together in series to transfer energy between the ports. The energy transfer from the first port is performed by a method comprising alternately closing and opening the first switching device at a high frequency, and opening the first switching device disconnects at least one terminal of the inductor from the first port.

In another aspect of the inventive concepts, a system has a first energy transfer device for transferring energy from a high-current-producing junction of a first stacked multijunction photovoltaic device to across a low-current-producing junction of the photovoltaic device. The first energy transfer device has a first port for coupling to the high-producing junction of the photovoltaic device, a second port for coupling to the low-producing junction of the photovoltaic device, at least a first inductor coupled to at least one port selected from the group consisting of the first and second port, and a first switching device coupled in series with the first inductor; and a controller for monitoring voltages at the high-current-producing and low-current-producing junctions and for determining switching of the first switching device to maintain at least an approximate maximum power point for each junction of the multijunction photovoltaic device.

In yet another aspect of the inventive concepts, a subsystem has a multiple junction stacked photovoltaic device with a first DC-DC converter coupled to transfer energy from a high-current-producing junction of photovoltaic device to across a low-current-producing junction of the photovoltaic device. A controller monitors voltages at the high-current-producing and low-current-producing junctions and is configured to determine switching of at least one switching device of the at least one DC-DC converter to optimize power output from the multijunction photovoltaic device.

In another aspect of the inventive concepts, a subsystem has at least a first multiple-junction stacked photovoltaic device having a plurality of photovoltaic junctions coupled electrically in series and at least a first bidirectional DC-DC converter coupled to transfer energy between an output of the first stacked photovoltaic device and a specific junction of the at first stacked photovoltaic device. A controller monitors voltages at junctions of the first stacked photovoltaic device, the controller configured to determine switching of at least one switching device of the first DC-DC converter to optimize power output from the first multijunction photovoltaic device.

In another aspect of the inventive concepts, a subsystem has at least a first multiple junction stacked photovoltaic device having a plurality of photovoltaic junctions coupled electrically in series, at least a first bidirectional DC-DC converter coupled to transfer energy between an output of the first stacked photovoltaic device and a specific junction of the at least first stacked photovoltaic device, and a controller for monitoring voltages at junctions of the first stacked photovoltaic device, the controller configured to determine switching of at least one switching device of the first DC-DC converter to optimize power output from the first multijunction photovoltaic device.

DETAILED DESCRIPTION OF THE EMBODIMENTS

A solar system, as illustrated inFIG. 5, has photovoltaic devices102,104,106, that are coupled together in series to obtain a suitable voltage for a load. In an embodiment, photovoltaic devices102,104,106are individual photovoltaic cells. In an alternative embodiment, photovoltaic devices102,104,106are modules having multiple photovoltaic cells. Similarly, photovoltaic devices102,104,106may each be a group of two or more series, parallel, or series-parallel coupled cells within a module. An energy transfer device or energy exchanger108is operable to transfer power from terminals of higher-producing devices of photovoltaic devices102,104,106to terminals of lower-producing devices of photovoltaic devices102,104,106to supplement current output of the lower-producing devices to maintain output voltages at the lower-producing devices that are sufficient to allow for power production by the lower-producing devices. In certain embodiments, energy exchanger108equalizes voltages across photovoltaic devices102,104,106.

With reference toFIG. 6as well asFIG. 5, the current through photovoltaic device102is I4, the current through photovoltaic device104is I5, and the current through photovoltaic device106is I6. The energy exchanger takes in or provides a current of I1at the point between photovoltaic devices102and104, the energy exchanger takes in or provides a current of I2at the point between photovoltaic devices104and106, and the energy exchanger takes in or provides a current of I3at the top end of photovoltaic device106. Since the sum of currents to each node must be zero, by circuit theory:
I1+I4=I5+I2
I2+I5=I6+I3
I3+I6=Iout.
Next, I9(current on B side of converter134)+I8(current on B side of converter132)+I7(current on B side of converter130)=0 since there is no other charge source connected to energy transfer bus136.
I9+I8+I7=0
Each of the B sides of bidirectional DC-DC level-shifting converter134,132,130are connected together in parallel, so:
V(Bside of 134)=V(Bside of 132)=V(Bside of 130)
The voltage on the A side of each bidirectional DC-DC level-shifting converter134,132,130can be expressed as the voltage on the B side of the converter multiplied by the voltage gain G of the converter as follows:
V(Aside of 134)=V(Bside of 134)*G134
V(Aside of 132)=V(Bside of 132)*G132
V(Aside of 130)=V(Bside of 130)*G130
It is assumed that voltage gain G is the same for each bidirectional DC-DC level-shifting converter134,132,130such that:
G134=G132=G130=G
By substitution:
V(Aside of 134)=V(Aside of 132)=V(Aside of 130)
Therefore, if each bidirectional DC-DC level-shifting converter134,132,130operates with the same voltage gain G (e.g., if each converter has the same topology and operates with the same duty cycle), the energy exchanger forces the respective voltage across each of photovoltaic devices102,104,106to be the same.

Assuming that each bidirectional DC-DC level-shifting converter134,132,130is perfectly efficient, then their respective input and output powers are equal:
V(Bside of 134)*I9=V(Aside of 134)*I3
V(Bside of 132)*I8=V(Aside of 132)*I2
V(Bside of 130)*I7=V(Aside of 130)*I1
Using the relations between the voltages on the A and B sides of DC-DC level-shifting converters134,132,130and voltage gain G, the following relationships can be established:
I9=I3*G
I8=I2*G
I7=I1*G
And by substitution and elimination of G in I9+I8+I7=0:
I1+I2+I3=0.
Now, given that each cell and parallel A port of the related converter are all connected in series with the other combinations of cells with related converters, the same current Iout must flow in the each series branch,
Iout=I3+I6=I2+I5=I1+I4.
Summing yields:
I6+I3+I5+I2+I4+I1=3*Iout
Substituting I1+I2+I3=0 in the above, and dividing by 3 on both sides:
(I6+I5+I4)/3=Iout
Making the same assumptions of 100% efficiency and equal voltage, for M photovoltaic devices:

Iout=∑N=1M⁢Ipanel⁡(N)M⁢⁢or⁢⁢for⁢⁢the⁢⁢case⁢⁢in⁢⁢figure⁢⁢6⁢:⁢⁢Iout=(I⁢⁢4+I⁢⁢5+I⁢⁢6)/3
Thus, as illustrated by the above equations, certain embodiments of the energy exchanger having equal converter voltage gains and negligible losses causes photovoltaic array100output current to be the mathematical average of all photovoltaic device102,104,106currents. In particular, for each photovoltaic device102,104,106, the energy exchanger adds or subtracts current from the photovoltaic device as required to make the sum of the added or subtracted current and the photovoltaic device current equal the mathematical average of all photovoltaic device102,104,106currents. Such characteristic shows that the energy exchanger typically processes only a fraction of the array output power, thereby helping to minimize processing of output power and associated power dissipation.

Assuming that photovoltaic device104is a high producing module capable of providing at its MPP 12 volts at I5=10 amperes (120 watts) or 13 volts at 8 amperes, that device102is a low-producing module capable of providing 12 volts at I4=8 amperes, and that device106is a mid-producing module capable of providing 12 volts at I6=9 amperes or 12.5 volts at 8 amperes. Without an energy exchanger, the system's best output will be about 37.5 volts at Iout=8 amperes because the low ILof device102causes a rapid voltage drop as current rises above 8 amperes. In other words, series connection of photovoltaic devices enforces identical current through a single current path, and this current equals the current produced by the weakest device in the chain.

With these device capacities, energy exchanger108draws I2=one ampere at 12 volts, approximately 12 watts, from across high-producing device104, and supplements current provided by low-producing device102by applying I1one ampere at 12 volts. Device106continues to produce 9 amperes, so I3=0, and the 10 amperes produced by device104is effectively divided into one ampere for the energy exchanger and 9 amperes for the series connected stack. This results in a transfer of energy from across high-producing device104to across low-producing device102, as indicated by the arrow inFIG. 5. The resultant output of the array will be approximately 36 volts at Iout=9 amperes, resulting in a net increase in power output of about 24 watts over the same system operated without energy exchanger108.

In a more dramatic illustration of a benefit of energy exchanger108, consider an example where energy exchanger108is not used and low-producing device102is dirty or damaged and therefore can produce only 1 ampere of current. High-producing device104and mid-producing device106will also be constrained to 1 ampere of current due to their series connection with low-producing device102. Assuming devices102,104,106each have an output voltage of 14 volts at 1 ampere, total system output power is approximately 42 watts. However, if energy exchanger108is used, high-producing device104and mid-producing device106can respectively deliver 10 amperes and 9 amperes, as discussed in the previous example, while device102delivers 1 ampere. Accordingly, system output current will be approximately 6.7 amperes, where energy exchanger108subtracts approximately 3.3 amperes from high-producing device104, subtracts approximately 2.3 amperes from mid-producing device106, and adds approximately 5.7 amperes to low-producing device102. Assuming the MPP output voltage of each device102,104,106under such conditions is near 12 volts, the total system power is approximately 240 watts, instead of 42 watts when energy exchanger108is not used, less losses due to inefficiencies in exchanger108.

Energy exchanger108can be modified for use with other energy producing devices, such as fuel cells, and energy exchanger108is not limited to use with energy producing devices having particular voltage and current characteristics. For example, each of devices102,104,106ofFIG. 5could be replaced with one or more fuel cells such that energy exchanger108transfers current among individual fuel cells or groups of fuel cells to help maximize system output power. The energy exchanger operation is similar with power supplied to or from battery cells. The energy exchanger equalizes voltage across all cells being charged or discharged, subtracting or adding current to cells with small or large capacity, or with different states of charge. It is also possible to adjust the individual cell voltages for more complex objectives by adjusting the voltage gain of individual converters in the energy exchanger.

Unlike the DC-DC converter or DC-AC inverter with common summing-rail solutions of 2009/0020151 and 2005/0121067, where the full power generated by all of the photovoltaic devices passes through the converters, in the device ofFIG. 5only the difference in power between high and low performing devices passes through energy exchanger108. This enables potentially lower ratings for the power circuits, making them smaller and cheaper. It also minimizes power losses, as losses generally increase with increasing processed power. Further, at least a reduced power output is available from the array100even if the energy exchanger108fails. In fact, if energy exchanger108fails or is disconnected, the system may continue to function but in a manner similar to that ofFIG. 3. Certain embodiments of energy exchanger108may be installed with a new system, or retrofitted to an existing system, with few changes to existing practices. For example, certain embodiments of energy exchanger108do not affect, or may improve by eliminating false peak power points, the voltage-current characteristics of an existing photovoltaic system and may therefore be installed in the existing system without changes to the system's existing charge controller or inverter, whether MPPT or conventional.

An embodiment of energy exchanger108is illustrated in more detail inFIG. 6. In this embodiment, the energy exchanger has interface units having the form of bidirectional DC-DC converters130,132,134, each allowing for bidirectional power transfer between a respective first port A and a respective second port B. Each first port A is coupled across a respective photovoltaic device102,104, and106. Each first port A has a respective first and second terminal140,142. Interface units or bidirectional DC-DC converters130,132,134are adapted to operate with each first terminal140having a voltage offset from each other first terminal140and each second terminal142having a voltage offset from each other second terminal142. Each second port B is coupled to a common energy-transfer bus136. Energy transfer bus136has a filtering and energy storage capacitor138for smoothing voltages on energy transfer bus136. Although energy storage capacitor138is shown connected to the common ground with device102, capacitor138could alternately be referenced to another common return node, such as the top node of device106.

In an embodiment, interface units or DC-DC converters130,132,134of the energy exchanger108are transformer-isolated Cuk converters150as illustrated inFIG. 7. Each transformer-isolated Cuk converter has an A port152and a B port154. Each port is coupled to an inductor156,158for coupling power into the converter, a switching device160,162driven by converter control and driving circuitry that is not shown for simplicity, a blocking capacitor164,166, and a transformer winding168,170. The transformer windings168,170are magnetically coupled to transmit power between the ports, and may also be magnetically coupled to the inductors156,158. The transformer windings168,170may have a turns ratio other than 1:1.

In an embodiment of energy exchanger108using the Cuk converter ofFIG. 7, all switching devices160,162of all interface units or DC-DC converters130,132,134may be driven simultaneously during times that the array is illuminated and generating power. At times when the array is not illuminated, all switching devices160,162of all interface units or DC-DC converters130,132,134may be turned off. In this embodiment, assuming the highest producing photovoltaic device104is producing the highest device voltage in the string of photovoltaic devices, interface unit or bidirectional DC-DC converter132, which is coupled to highest producing photovoltaic device104, will tend to transfer power from photovoltaic device104to transfer bus136. Since the resulting voltage on transfer bus136is higher than the voltage interface unit or bidirectional DC-DC converter130, which is coupled to lowest-producing photovoltaic device102, would be able to apply to energy transfer bus136, converter130tends to draw power from energy transfer bus136and apply that energy in parallel with low producing photovoltaic device102. The net result is a self-regulating energy transfer from one or more high producing devices to supplement output current of one or more low producing devices.

In some embodiments, DC-DC converters130,132,134(which may include capacitive charge pumps as well as bidirectional converters like CUK converters) operate with a constant duty cycle, or a duty cycle that is dependent on a slowly varying parameter such as array illumination, to reduce control complexity and cost while still enabling acceptable system performance. These embodiments do not include a feedback loop, which eliminates feedback loop problems such as feedback loop instability and/or slow feedback loop response. Energy exchangers operating with bidirectional converters at constant duty cycle act to equalize voltage across all cells or modules. An energy exchanger operated in this manner will often provide better performance than a module or array lacking such an exchanger, although some cells or modules may operate near but not precisely at their MPP because MPP voltage may differ slightly between cells of an array. MPP voltages may differ slightly between even identical cells or modules because of temperature differences or differences in obscuring dust across the array.

Duty cycle and/or general operation of DC-DC converters130,132,134can optionally be statically or dynamically modified to achieve additional benefits, such as to adjust for different voltages at energy exchanger ports, to implement true MPP calculation and tracking, and/or to help isolate possible faults. Some embodiments of DC-DC converters130,132,134having inductors, such as those based upon the architectures ofFIGS. 7, 8, and 10, and as discussed herein with reference toFIGS. 14 and 16, have switching devices within each converter that are pulse-width controlled to produce an appropriate voltage gain across each converter to ensure more precise tracking of the MPP of each cell or module. In these embodiments, the processor524,724may use voltage and current monitor506,706to determine a present operating power point of each cell or module by measuring voltage at, and current produced by, that cell or module. Processor524,724may then adjust operating pulse-widths or duty cycles at each converter to alter the power points of individual cells or modules and repeat voltage and current measurements according to a predetermined search and track algorithm embedded in firmware memory of processor524,724. Processor524,724then determines best maximum power points for each of the cells or modules and adjusts converter pulse widths and/or duty cycles to best maintain that power point during operation.

In an alternative embodiment, gain of the of DC-DC converters130,132,134is chosen so that voltage on energy exchanger ports B (effectively on the common bus136) is higher than voltages on the ports A (voltages across cells102,104,106). This respectively decreases currents in the bus136, allowing for thinner and cheaper and lower-cost wires, while capacitance of the capacitor138can be decreased at expense of higher voltage rating.

In an alternative embodiment, interface units or DC-DC converters130,132,134of energy exchanger108are capacitively-isolated Cuk converters180, such as shown inFIG. 8. Each capacitively-isolated Cuk converter180has an A port182and a B port184. Each port is coupled to an inductor186,188for coupling power into the converter, and a switching device190,192driven by converter control and driving circuitry that is not shown for simplicity. Inductors186,188may, but need not, be magnetically coupled by winding on a common core. The converter also has two blocking capacitors194,196for transferring power between ports182and184while providing isolation. The voltage at B port184is of opposite polarity as compared to the voltage at A port182, which makes no difference for energy exchanger operation, as long as all bidirectional DC-DC converters of an energy exchanger according toFIG. 6are implemented in the same way. Other embodiments may have non-inverting energy exchangers.

Another bidirectional DC-DC converter200that may be used as an elementary DC-DC converter for interface units or DC-DC converters130,132,134,332(seeFIG. 11for converter332and energy exchangers302,306,330which will be described in detail below) of energy exchanger108,302,306,330is a capacitively-isolated charge-pump converter as illustrated inFIG. 9. In this device, electronic switching devices202and204close as a pair and are open when electronic switching devices206,208close as a pair. The circuitry enforces a break-before-make to insure that devices of paired switching devices202,204never conduct simultaneously with devices of paired switching devices206,208. Charge is stored in capacitors210,212. The charge-pump converter inverts voltage while transferring energy from the port having the higher absolute value of voltage to the port having the lower absolute value of voltage.

A capacitor equipped with an electronic commutator device may also be used as an energy transfer device. Each interface unit of such a commutator device comprises a pair of switching devices, when closed the switches place the capacitor across the port, when open the switches disconnect the capacitor from the port, and another interface device may close its switches. In such an embodiment, the commutator alternately couples a capacitor across a first port of the energy exchanger, then across a second port of the energy exchanger; energy tends to flow from a port at high voltage to a port at low voltage. With either a charge-pump converter such as that ofFIG. 9, or a capacitor with commutator devices, the switching devices of the energy exchanger may operate at a constant duty cycle whenever sufficient power is being generated that energy exchanger operation is desirable; energy flow will be dictated by voltage differences across the energy exchanger ports.

Yet another bidirectional isolated DC-DC converter that may be used as an elementary DC-DC converter for interface units or DC-DC converters130,132,134,332of energy exchanger108,302,306,330is a SEPIC converter with isolation. A transformer-isolated SEPIC converter220(FIG. 10) has an inductor222coupled to a first, or A port, and a switching device224for coupling current through inductor222and returning current to the first port. As switching device224operates, an alternating voltage is generated and coupled by capacitor226to a first winding228of a transformer, and first winding228magnetically couples to a second winding230of the transformer. Second winding230is coupled through a second switching device232to a second or B port that acts interchangeably as a chopper or as a rectifier depending on a direction of power flow in the converter. Switching device224also operates interchangeably as a rectifier or as a chopper depending on a direction of power flow in the converter.

It is anticipated that several other forms of bidirectional, isolated, DC-DC converters may be used as elementary DC-DC converters for interface units or DC-DC converters130,132,134,332of the energy exchanger. In the interest of brevity, only representative converter types are illustrated in detail here.

With all bidirectional isolated DC-DC converters220,200,180,150, useful as elementary DC-DC converters for interface units or DC-DC converters130,132,134,332of the energy exchanger108,302,306,330, filtering capacitors may be present at one or both ports of each converter. In alternative embodiments capable of handling increased mismatch currents between cells and modules, two or more phases of such converters may be provided for each elementary DC-DC converter and operated on alternating phases.

Two levels of energy exchanger may be used in a photovoltaic array300as illustrated inFIG. 11. In the embodiment ofFIG. 11, one level of energy exchanger302,306is used at the module304,308level for exchanging energy across the multiple photovoltaic cells within each module304,308. For example, energy exchanger302is capable of exchanging energy from high producing to low producing cells across photovoltaic cells310,312,314but, since its energy transfer bus316is local to module304, it is incapable of transferring energy to or from cells320,322,324of module308. An array-level energy exchanger330, however, acts to transfer energy from high to low producing modules304,308, such that current is balanced, and energy production optimized, throughout the array300. In the embodiment illustrated inFIG. 11, energy exchangers having bidirectional DC-DC converters332discussed with reference toFIGS. 5 and 6may be used. However in an alternative embodiment, the MPPT tracking energy exchanger ofFIG. 14may also be used at either module302,306level, array level330, or at both levels.

In an alternative embodiment lacking array-level energy exchanger330, energy transfer bus316of energy exchanger302of module304may be tied to energy transfer bus334of energy exchanger306of module308to provide an expanded single energy exchanger allowing for energy exchange between high and low producing cells across module boundaries.

Photovoltaic systems intended for producing standby power, or for producing power in a stand-alone, off-grid environment, typically require energy storage systems such as batteries to provide load leveling and to provide energy for times when adequate illumination is not available.

Batteries348(FIG. 12) may initially have, or may develop, weak cells354having a reduced capacity relative to other cells of the battery such as battery cell352. Batteries348may also develop charge imbalances where some battery cells356are in a lesser state of charge than others in the same battery. A further energy exchanger350, as illustrated inFIG. 12, may operate during battery charging to transfer charging current from higher-voltage, more fully charged, cells to provide extra charging current to lower-voltage, less fully charged, cells and thereby act as a balancing charger. Energy exchanger350ofFIG. 12may also operate when the battery is under load to transfer energy from higher-voltage cells of greater capacity or charge state to supplement lower-voltage cells of lesser capacity or charge state, thereby serving as a load balancer. Both charger balancing and load balancer operation modes act together to permit more efficient use of battery capacity than may be available with standard battery systems. Basic energy exchanger operation is achieved with a constant duty cycle or constant voltage gain in bidirectional DC-DC converters360,362and364inFIG. 12, regardless of charging or loading mode of operation for the battery cells, however more complex control strategies may be also be used to optimize function. In an alternative embodiment, battery cells428are replaced with alternative energy sources such as fuel cells, the energy exchanger operable to equalize voltages at cells of the system to make up for weak cells when the system is operated under load.

Energy exchanger350has bidirectional DC-DC converters360,362,364, as do the exchangers ofFIGS. 6 and 11, and an energy transfer bus358. All DC-DC converters360,362,364of energy exchanger350may be turned off to reduce battery drain when the battery is neither being charged nor being drained by a significant load.

A solar power system400is illustrated inFIG. 13. In this system, there is a photovoltaic array401within which are modules402,404,406,408,410,412coupled together in series-parallel configuration. Modules402,404are, for example, coupled together in parallel, and in series with the parallel connection of modules406and408. An energy exchanger414is provided to equalize outputs at the module level. In an embodiment, each module, such as module402, may also have an energy exchanger, such as energy exchanger416, to provide for equalization of output currents and voltages at the level of photovoltaic cells, such as cells414,418, of module402and as illustrated inFIG. 6. Array401provides an output420for connection to other elements of the system.

In a grid-tie embodiment of system400, array output420is coupled to an MPPT grid-tie inverter422as known in the art for feeding power from the array into a commercial power grid. In an embodiment for use in stand-alone off-grid systems, grid-tie inverter422may be absent. In stand-alone off-grid systems, systems having energy storage for providing power to a commercial power grid at times of high power cost, and systems for providing standby power to a load, array output420is coupled to an MPPT charge controller424as known in the art for charging a battery system426. In the embodiment ofFIG. 13, array performance with the energy exchangers in each module402,404has sufficiently linear characteristics that standard MPPT charge controllers will work with the system. Battery system426has multiple battery cells428and has, for example, an energy exchanger430approximately as illustrated in the battery system ofFIG. 12.

In stand-alone off-grid systems, battery system426is, for example, coupled to power an inverter432for providing alternating current to a load434. In many systems, inverter432is coupled to load434through a transfer switch436such as may be used to allow powering load434from a standby diesel generator during times of extraordinary load demand or times of severe weather when battery system426has become depleted. In standby systems that are configured to allow power sales to the commercial power grid at high-cost power times, battery system426may also be coupled to grid tie inverter422.

In an embodiment, the energy exchangers ofFIG. 13operate as previously discussed with reference toFIG. 6to allow all cells of all modules to operate efficiently in a balanced manner.

In alternative embodiments, the bidirectional DC-DC converters or interface devices of the energy exchangers ofFIGS. 6, 11, 12, 13, and 15are implemented as a pair of unidirectional DC-DC converters, one carrying power in each direction between the two ports of the converter. In these embodiments, voltages on each port may differ since step-up converters, such as boost and flyback converters, may be used in one direction and step-down converters, such as buck converters, may be used in the other direction; such implementations permit use of higher voltages and thinner wires for the energy transfer bus than practical with equal-voltage embodiments. Further, each unidirectional converter in each interface unit may be separately controlled by a microcontroller, such that power transfers in the energy exchanger may be controlled with considerable precision.

While some types of bidirectional DC-DC converters, such as the bidirectional charge-pump DC-DC converter ofFIG. 9, do not provide for voltage step-up or step-down between the ports of the converter, other types of DC-DC converters can provide voltage transformation. For example, the Cuk converters ofFIGS. 7 and 8can be operated with asymmetrical control waveforms to switching devices160,162to provide a step-up or step-down of voltage between port154coupled to the transfer bus and the port152coupled to the photovoltaic device.

The above-described self-regulating energy exchangers using bidirectional DC-DC converters operate to maintain uniform voltage across each series-connected photovoltaic device. While this will provide operation close to the maximum power point for many devices in many systems, it does not actually detect and operate at a maximum power point for each device.

In some systems having photovoltaic devices of different types, including systems having multiple junction stacked or split spectrum devices where some junctions may predictably have maximum power point voltages differing from those of other junctions in the system, voltage gains of the bidirectional DC-DC converters may be adjusted to provide a predetermined voltage ratio between junctions, cells, or devices of a first type and junctions, cells, or devices of a second type. The voltage ratio is determined such that both the first and second types operate near their maximum power points.

There may be some photovoltaic devices in a system that will produce power with slightly greater efficiency if operated at a true maximum power point. MPPT tracking of individual devices also allows the use of cells of different types and electrical characteristics in the same array while achieving individual MPPT of each cell. For these reasons, an energy exchanger having local MPPT capability may be provided for use in a solar module or array inFIG. 6,FIG. 11orFIG. 13, while individual gain adjustments can be used for different types or capacities of battery cells inFIG. 12.

An alternative embodiment of an energy exchanger500having local MPPT capability, that may be used in systems such as those illustrated inFIGS. 11 and 13, is illustrated inFIG. 14. In energy exchanger500, each port, such as port530,532, for connection to a photovoltaic device502,504couples through an interface device534,536, to an energy transfer bus514. Each interface device534,536is provided with voltage and current monitors506for monitoring at least a voltage across, and in some cases a current through, photovoltaic device502,504. There may be two, three, or more ports530,532for connection to photovoltaic devices and corresponding interface devices534,536. Only two ports are illustrated inFIG. 14for simplicity.

Each port530,532connects to an interface device534,536, for bidirectionally coupling energy between the port and an energy transfer bus, such as bus514. Each interface device534,536has a DC-DC converter510,512of a type that is controllable by pulse-width modulation or pulse-rate modulation of a control signal for transferring power from each port530,532to a transfer bus514and a smoothing capacitor516. Each interface device534,536also has a second DC-DC converter520,522of a type that is controllable by pulse-width modulation or pulse-rate modulation of a control signal for transferring power to each port530,532from the transfer bus514.

A processor524is provided for receiving current and voltage measurements from voltage and current monitors506and for determining a maximum power point under current conditions for each photovoltaic device502,504. The processor524is equipped with pulse width modulators as known in the art, and frequently available on commercially available control-oriented processors, for controlling DC-DC converters510,512,520,522. Processor524has filmware for providing appropriate pulse sequences to DC-DC converters510,512,520,522to draw an appropriate amount of power from a presently high-producing photovoltaic device and for applying that power across a presently low-producing photovoltaic device.

In an embodiment, processor524is equipped with a communication port for communicating with other energy exchangers of a system, and for communicating diagnostic information—such as identification of particularly low-producing modules—to a laptop computer for display to system repair technicians.

While local microprocessor control is an inexpensive way of implementing central control of an energy exchanger, other control strategies are possible. In other embodiments, alternative control apparatus, such by way of example a field-programmable gate array, is fitted in place of processor524. In an alternative embodiment resembling that ofFIG. 11, a field programmable gate array is implemented in each intra-module energy exchanger302,306, while a microprocessor is used within an inter-module energy exchanger330. In this embodiment inter-module energy exchanger330may control the intra-module energy exchangers302,306through a communications link.

Since Cuk converters as discussed with reference toFIGS. 7 and 8, as well as some other types of bidirectional DC-DC converters having inductors, are capable of a controllable voltage transformation between ports, bidirectional DC-DC converters may also be used in an energy exchanging system for use in a solar module or array701that tracks a maximum power point for each photovoltaic device as illustrated inFIG. 16.

An alternative embodiment of an energy exchanger700having local MPPT capability, that may be used in systems such as those illustrated inFIGS. 11 and 13, is illustrated inFIG. 16. The alternative energy exchanger700may also be operable with different firmware as a battery balancer. In energy exchanger700, each port, such as port730,732,733for connection to a photovoltaic device702,704,705couples through an interface device734,736,737to an energy transfer bus714. Each interface device734,736,737is provided with voltage and current monitors706for monitoring at least a voltage across, and in some cases a current through, photovoltaic device702,704,705. There may be filtering capacitors across each photovoltaic device702,704,705. There may be two, three, or more ports730,732,733for connection to photovoltaic devices and corresponding interface devices734,736,737. Only three ports are illustrated inFIG. 16for simplicity.

Each port730,732,733connects to an interface device734,736,737for bidirectionally coupling energy between the port and an energy transfer bus, such as bus714. Each interface device734,736,737has a DC-DC converter710,722,712of a type that is controllable by pulse-width modulation or pulse-rate modulation of control signals for transferring power between each port730,732,733and a transfer bus714and a smoothing capacitor716.

A processor724is provided for receiving current and voltage measurements from voltage and current monitors706and for determining a maximum power point under current conditions for each photovoltaic device702,704,705. The processor724is equipped with pulse width modulators as known in the art, and frequently available on-chip on commercially available control-oriented processors such as those in the Motorola 68HC11® (trademark of Freescale Semiconductor, Inc.) family, for controlling DC-DC converters710,712,722.

Processor724has firmware for using the voltage and current monitor706of each interface device734,736,737to measure performance of the photovoltaic devices702,704,705attached to the interface device, and from those measurements to periodically determine a maximum power point MPP for each device702,704,705. Processor724has firmware for dynamically determining an appropriate amount of power to draw from across each device702,704,705to maintain the determined MPP. Processor724has firmware for providing appropriate pulse sequences to DC-DC converters710,712,722to draw an appropriate amount of power from any presently high-producing photovoltaic device of devices702,704,705and for applying that power across a presently low-producing photovoltaic device of device702,704,705to maintain each photovoltaic device near its maximum power point MPP.

The Cuk converters ofFIGS. 7 and 8may be used in the embodiment ofFIG. 16as bidirectional DC-DC converters710,712,722. When these converters are used, processor724may control pulses to each switching device190,192,160,162of the converters independently to provide appropriate voltage transformation between the photovoltaic devices and the power transfer bus714and for an appropriate current draw from, or current provided to, each photovoltaic device. For example, processor724may control pulse widths to switching devices190,192,160,162to obtain a desired voltage transformation, and pulse rates to the same devices to control current draw.

In an embodiment, processor724is equipped with a communication port for communicating with other energy exchangers of a system, and for communicating diagnostic information—such as identification of particularly low-producing modules—to a laptop computer for display to system repair technicians. In alternate embodiments, processor724is replaced with or supplemented by another control device, such as an analog control system. In an alternative embodiment, processor724is omitted and exchanger700is controlled by an external control system.

In an alternative embodiment of a solar module or solar array600embodying an energy exchanger or energy transfer device602, bidirectional DC-DC converters604,606are operable between port pairs as illustrated inFIG. 15. In this embodiment, bidirectional DC-DC converter604has a first or A port attached to a first photovoltaic device610and a second or B port attached to a second photovoltaic device612. Bidirectional DC-DC converters604,606may, but need not, be a fully isolated converter as previously discussed. Converter604may be an inverting converter such as a charge pump device similar to that ofFIG. 9or inverting Cuk converter similar to that illustrated inFIG. 8, with reversed polarity connection of either port A or port B to deliver a non-inverted voltage.

In this embodiment, DC-DC converter604serves to equalize voltage across photovoltaic devices610and612by transferring any extra current from the stronger of the devices610,612to the weaker. Second DC-DC converter606serves to equalize voltages across photovoltaic devices612and614by transferring any extra current from the stronger of the devices612,614to the weaker. Since the converters604,606are daisy-chained in energy exchanger602, the composite energy exchanger will also act to transfer excess energy between the pairs610,612and612,614—passing energy from device610to device614will occur albeit subject to circuit losses in both converters604, and606. It should be noted that DC-DC converters604,606individually contain at least one capacitor, and may include an inductor.

Where electronic switching devices are shown as field-effect transistors in the schematic diagrams of various DC-DC converters illustrated for use in the energy exchangers, it is expected that other types of electronic switching devices will also function in many of these converters with appropriate driving electronics. For example, bipolar transistors may be used in some versions of the energy exchanger.

It should be noted that the energy exchanger herein described need not handle the full output current or power of the array or module within which the exchanger resides. In typical applications, the exchanger handles only the differences between currents produced by the various cells or modules in an array. In an embodiment such as that ofFIG. 11, the power handling capacity of each module-level energy exchanger302,306may be substantially less than the output power rating of the module304,308within which they reside, and the power handling capacity of the array-level exchanger330is substantially less than that of the series string of photovoltaic modules304,308it serves.

In some embodiments, the energy exchanger disclosed herein may serve to transfer sufficient power across a shaded or broken cell or module, so that bypass diodes may not be required. It is also anticipated that in some embodiments, auxiliary circuitry may draw power from an energy exchanger bus, such as bus316in exchanger302ofFIG. 11. Such auxiliary circuitry may include timing and control circuitry including oscillators and other circuitry that drive switching devices of DC-DC converters, voltage and/or current monitors506,706, processors724,524, communication circuits for coordinating operation of multiple energy exchangers, and other circuitry that does not directly handle the output power produced by the system.

In photovoltaic arrays, bypass diodes across cells or modules that are coupled to ports of an operating energy exchanger of the type herein disclosed are unnecessary because the energy exchanger will support output of a weak or shaded cell or module sufficiently that the cell or module should never have an output voltage that drops to zero or becomes reverse biased. Bypass diodes may nevertheless be provided to permit operation and prevent reverse-bias of cells and modules should the energy exchanger be disabled, or should power handling capacity of the energy exchanger be exceeded.

In certain embodiments, MPPT is provided on a per-cell basis, such as by using an energy exchanger having microcontroller control similar to those ofFIG. 14 or 16, or by using another type of MPPT device, optionally in conjunction with one or more energy exchangers.

The amount of power processed by each of the energy exchanger's bidirectional DC-DC converters may vary widely depending on performance of the devices (e.g., photovoltaic devices) connected to the energy exchanger. Thus, at times, one or more of the bidirectional DC-DC converters may process little or no power, and under such conditions, converter power losses may outweigh system efficiency improvements associated with converter operation. To promote high efficiency during such lower power operating conditions, certain embodiments of the energy exchanger include bidirectional DC-DC converters that operate in one or more power saving modes, or shut down, when processing little or no power. In such embodiments, a relative or absolute amount of power processed by each bidirectional DC-DC converter is estimated, for example, from the magnitude of current transferred by the DC-DC converter, and the converter's operating mode is controlled accordingly. The operating mode of each bidirectional DC-DC converter in such embodiments is, for example, individually controlled to promote maximum system efficiency, as the magnitude of processed power can vary widely among the energy exchanger's DC-DC converters.

For example, in certain embodiments, each bidirectional DC-DC converter has a topology including an inductor (e.g., each converter is a SEPIC converter or a Cuk converter) and includes a constant frequency pulse width modulation (PWM) mode and a pulse frequency modulation (PFM) mode. As known in the art, PWM operation allows for efficient operation at large current loads, and PFM operation is typically more efficient than PWM operation under light current loads. In such embodiments, each converter, for example, operates in its PWM mode when transferring a current with a magnitude greater than or equal to a threshold value, and each converter operates in its PFM mode when transferring a current with a magnitude less than the threshold value. The threshold value may be dynamically adjusted to provide hysteresis between the PWM and PFM operating modes.

As another example, in certain embodiments, each bidirectional DC-DC converter has a topology including an inductor (e.g., each converter is a SEPIC converter or a Cuk converter) and includes a continuous conduction mode (CCM) and a discontinuous conduction mode (DCM). As known in the art, DCM is typically more efficient than CCM under light current load conditions. In such embodiments, each converter, for example, operates in CCM when transferring a current with a magnitude greater than or equal to a threshold value, and each converter operates in DCM when transferring a current with a magnitude less than the threshold value. The threshold value may be dynamically adjusted to provide hysteresis between CCM and DCM.

The hybrid mode PFM-DCM may provide more efficient DC-DC converter operation than the hybrid mode PWM-DCM under light load conditions. However, current transferred by a DC-DC converter tends to be more constant when operating in PWM-DCM than when operating in PFM-DCM mode. Thus, PWM-DCM may be preferable to PFM-DCM in applications when the energy exchanger is connected to photovoltaic devices, as photovoltaic devices typically operate most efficiently when providing a constant current.

As another example, in certain embodiments, each bidirectional DC-DC converter operates in a “hiccup” mode when the magnitude of its transferred current drops below a threshold value. In the hiccup mode, each bidirectional DC-DC converter is shut down but occasionally restarts to determine the magnitude of its transferred current. If the magnitude of its transferred current is above a threshold value, the converter remains operational. Otherwise, the converter shuts down again. The threshold value may be dynamically adjusted to provide hysteresis. In some embodiments, the bidirectional DC-DC converters include a PFM-DCM mode or a PWM-DCM in addition to a hiccup mode such that they operate in their hiccup mode at very light current magnitudes, in their PFM-DCM mode or PWM-DCM at moderate current magnitudes, and in their PWM-CCM mode at large current magnitudes.

As yet another example, consider the class of embodiments illustrated inFIGS. 14, 16of the energy exchanger having controllable bidirectional DC-DC converters710,712,722with inductors in each converter. Such embodiments may incorporate the converters ofFIGS. 7, 8, and 10. Consider also the class of embodiments having paired unidirectional converters510,520where each of the unidirectional converters has inductors. Consider further that class of embodiments having current monitors506,706, and a controller such as processor524,724for controlling operation of the energy exchanger. Certain embodiments of these classes of embodiments have controllers with the ability to switch operation of some or all of their converters between a low-current-capacity DCM and a high-current-capacity CCM as needed to handle currents in the exchanger.

There is generally a small energy cost whenever a switching device, such as switching device224or232of the SEPIC converter illustrated inFIG. 10, is switched. This energy cost occurs in part because the gate capacitance of each switching transistor must be charged and discharged each time the transistor is switched.

In these embodiments, processor524,724monitors voltages at each photovoltaic device, and currents through each DC-DC converter of the energy exchanger. While doing so, processor524,724computes a desired current through each DC-DC converter and compares this current to a threshold current between low-current operation and high-current operation. This computation is made individually for each converter in the exchanger, and updated regularly.

When processor524,724determines that a particular converter of the energy exchanger need transfer only low currents, those converters are switched to the power-conserving, low-current-capacity, DCM mode. For example, the SEPIC converter ofFIG. 10may be operated in DCM, as illustrated inFIG. 17for low power transfer from A port to B port. In this mode, a quiet time TQT is introduced between a time TCO when inductor222current reaches zero and rectifying switching device232shuts off and a time TON when switching device224next turns on. This reduces the power required to switch switching device224because fewer transitions of its gate occur per unit time than in normal, continuous conduction mode (CCM). Should system parameters change and current-carrying capability be greater than the converter can handle in DCM mode, as indicated when current exceeds the low vs. high current threshold, the controller switches those converters to CCM operation. In CCM the quiet time TQT is reduced to zero such that switching device224turns on at TCO.

Since current transferred by converters in the energy exchanger may vary widely across the exchanger because current produced by photovoltaic cells of the array may vary, each converter may be placed in CCM or DCM independently of other converters in the exchanger.

It should be noted that the energy exchanger's architecture may facilitate simple implementation of power saving modes. For example, absolute or relative power processed by each bidirectional DC-DC converter can be estimated from the magnitude of current transferred by the DC-DC converter, and each DC-DC converter's operating mode can be individually controlled based on the magnitude of its transferred current. In contrast, some prior art power maximizing devices do not lend themselves to simple implementation of power saving modes. For example, in the system disclosed in 2008/0236648, a constant current flows through all DC-DC converters, which prohibits operation of the converters in power saving modes such as pulse frequency modulation or discontinuous conduction modes, and also prohibits estimating processed power from transferred current.

U.S. Pat. No. 5,403,404 to Arya, et al. describes a stacked multijunction photovoltaic device having different bandgaps in each absorber layer of the junctions of the cell. In his device the absorbers have different ratios of silicon and germanium. U.S. Pat. No. 6,340,788 to King, et al. describes a stacked multijunction photovoltaic device having a lower junction of silicon and an upper junction of gallium arsenide. Similarly, US Patent Application No. 20100096001 of Sivananthan et al. proposes stacked multijunction photovoltaic devices with as many as five junctions, with silicon, germanium, or silicon-germanium at the bottom of the stack and upper cells of one or more Group II-VI semiconductors include CdTe, CdSe, CdSeTe, CdZnTe, CdMgTe, and CdHgTe. Other multijunction photovoltaic devices have been proposed or manufactured, including some with cells comprising CdInGaSe semiconductor absorber layers.

Another proposal is to use a three junction layered structure having a Gallium Indium Phosphide top cell, which absorbs primarily blue through yellow visible light, stacked on top of a Gallium Arsenide middle cell, which absorbs leaking visible light, as well as red and near-infrared wavelengths, layered on top of a Germanium substrate cell, which absorbs infrared wavelengths. This structure provides utilization of wavelengths from 300 to 1800 nanometers and may reach efficiency levels of 50% or more that are unreachable with prior single junction technology.

An issue with multiple-layer, including three junction layered, photovoltaic devices is that the voltages produced by each junction are different because the band gaps are different. For example, bandgaps of materials Ge: 0.7 eV, Si 1.1 eV, GaAs: 1.4 eV, GaInP2: 1.8 eV; these may translate to maximum power point voltages of approximately Ge: 0.4V Si: 0.55V, GaAs: 1.0V GaInP2: 1.2V. It should be noted that maximum power point voltage of a real cell or panel is affected by actual illumination, electrical resistance of layers and metallization, temperature, and other factors in addition to bandgap differences.

Another issue with three-layered photovoltaic devices is that the current produced by each junction in a layered photovoltaic device is proportional to the number of photons absorbed in the absorber layer of that junction, the number of photons absorbed in, and the number of photons passed to lower layers by, each layer depends on wavelength distributions of photons reaching that layer. Current produced in a layer will therefore depend somewhat on a color—or wavelength distribution—of light received by a panel, a panel exposed to “redder” light may produce proportionally more current in lower than upper layers than will a panel receiving “bluer” light. Color of incident light may vary with time of day, foliage, season, and sky cover as well as aging of encapsulants.

In the interest of simplicity, energy exchangers for multiple junction cells will be illustrated herein with two junctions, however the system is applicable to cells with any number of stacked junctions, and for any number of cells.

A stacked, multiple-layer, photovoltaic device is illustrated inFIG. 18. The device has a substrate802, a back contact conductor layer804, bottom semiconductor absorber806and junction808layers, a first transparent conductor layer810, an upper semiconductor absorber layer812, an upper junction layer814, an upper transparent conductor layer816, a patterned metallic top conductor layer818, and a passivation or transparent protection layer822; these layers are fabricated essentially as known in the art of stacked multijunction photovoltaic devices. Additional layers, such as dichroic reflector layers, barrier layers, and antireflection layers, may be incorporated into the photovoltaic device but are not shown here for simplicity.

In order to achieve a low-resistance connection to the first transparent conductor layer810, and thereby low resistance connection to the boundary between upper and lower junctions of the stacked device, an additional patterned metallic conductor layer824and a patterned dielectric layer826may be added to the multijunction stacked photovoltaic device during fabrication of that device. In an alternative embodiment,FIG. 18A, trenches827are etched through upper absorber layer812, upper junction layer814, and upper transparent contact layer816, to expose first transparent conductor layer810, and a grid of metallic conductors828in contact with transparent conductor layer810is provided. This grid of metallic conductors is interdigitated with but does not contact, and is fabricated on the same layer as metallic top conductor818. The resulting multijunction stacked photovoltaic device is symbolically illustrated inFIG. 19, where the upper cell830typically generates different voltage and current than lower cell832because of the bandgap differences in their materials. Further mismatches in current between upper and lower cell may also arise from differences in effective illumination of the stacked cells.

Similarly, stacked devices may comprise three junctions with low-impedance connections brought out for one or both inter-cell conductive layers. For example, a stacked device suitable for use with the present energy exchanger may have a top junction having a first bandgap suitable for short wavelength light, a middle junction having a second bandgap suitable for medium wavelength light, and a bottom junction having a third bandgap suitable for long wavelength light. In such an embodiment, a top contact makes electrical contact to the top junction, an upper-middle contact brings out an electrical contact between the top and middle junctions, a bottom-middle contact brings out an electrical contact to the boundary between middle and lower junctions, and a back contact makes electrical contact to the backside of the lower junction. Similar devices with stacks of four or more junctions could be constructed, and currents equalized in similar ways.

Stacked multiple junction devices operate by absorbing part of the spectrum of incident light in an upper junction, while allowing light of other wavelengths to pass through into other junctions that provide additional photocurrent at a lower junction. Split-spectrum devices can be constructed by placing a first, upper, single or multiple junction cell fabricated on a first substrate813over a second, lower, single or multiple junction cell fabricated on a second substrate815as illustrated inFIG. 18B, in such an embodiment with a two junction upper cell the layers may be identical to those previously discussed with reference toFIG. 18A. In such an embodiment with a one-junction lower cell, the lower cell may have a back-contact layer807, an absorber layer809, a heterojunction partner layer811, transparent contact layer799, and metal interconnect layers798, where at least the heterojunction partner layer and absorber layer are fabricated of different materials than the absorber and heterojunction partner layers of the upper cell fabricated on the first substrate813. Such multiple-substrate embodiments offer the advantage that high temperature processing of the junctions on the first substrate813does not affect junctions on the lower substrate815, and vice versa.

Split-spectrum photovoltaic devices for use in concentrator applications can also be constructed by placing a first cell817, equivalent to a lower cell and fabricated on a first substrate, at an angle in concentrated light819beneath a concentrator lens821and forming a dichroic, or other wavelength-selective, mirror on the first cell817surface, the dichroic mirror being arranged to reflect short-wavelength light onto second cell823, fabricated on a second substrate, that absorbs the short-wavelength light and serves a similar function as upper cell in a device according toFIG. 18B. Typically, concentrator lens821is a flat Fresnel lens, or equivalently may be a concentrating arrangement of mirrors. Other arrangements for splitting spectra are possible with similar effect.

Split-spectrum photovoltaic devices fabricated on two or more substrates of the types illustrated inFIG. 18Bor C, may have both cells857,859electrically brought out separately, having a symbol as illustrated inFIG. 18D, with outputs825,851separated. In this event, the upper cells may be coupled together electrically in series into an upper string with energy exchangers as heretofore described, and the lower cells coupled together electrically in series into a lower string with energy exchangers as heretofore described. Outputs of the two strings may then be combined at module, panel, or array output by using an MPPT controller and DC-DC up-converter (boost or buck-boost) on the string expected to provide a lower voltage, or by using an MPPT controller and DC-DC down-converter (buck or buck-boost) on the string expected to provide a higher voltage.

Alternatively, split-spectrum photovoltaic devices may have fewer leads brought out, with cells coupled in series and outputs821,851bonded together into a single output terminal, the device also having additional outputs853,855; in a variant, outputs821,851are bonded external to the split-spectrum photovoltaic devices. In this event, the circuits described herein as applicable to multijunction stacked devices with low resistance connection to a conductive layer between cells apply to those split-spectrum devices.

It has proved difficult to match or balance current production in stacked, multiple-junction, or split-spectrum photovoltaic devices, and to maintain matched current production as incident light color changes with weather, time of day, and seasons. Layered devices optimized for high efficiency of each junction may have mismatches of 30% or more in current production between junctions.

An energy-exchanger of the present device may find use in optimizing power output of a split-spectrum device with cells coupled in series or of multiple-junction stacked photovoltaic devices with low-impedance access to intermediate conductors by transferring power from high-current-producing junctions to low-current-producing junctions in the same string or stack. In particular, the multiple junction layered device ofFIG. 18Amay perform well with the energy exchanger ofFIG. 21, 21A, 21B, 21C, 22, 23, 24, 25 or 26. In an embodiment, each multiple junction layered photovoltaic device has an energy exchanger operable across the junctions of the photovoltaic device so that some or all of the junctions of the device may operate at or near their MPP, thereby optimizing current production by each junction of the device. In another embodiment, a second stage exchanger operates across several multi junction layered photovoltaic devices, or several split-spectrum devices, each of which has an energy exchanger operable across the junctions of the device, to equalize current at the device level and optimize current production by the devices.

Several of the multiple junction devices ofFIG. 18DorFIG. 19may be used with an energy exchanger831having multiple, controllable, DC-DC converters as illustrated inFIG. 20. This exchanger831has some similarities to that ofFIG. 16. In the embodiment829ofFIG. 20, exchanger interface cells833,838,840,842are provided for each junction of each multijunction stacked photovoltaic device. Each exchanger interface cell has a bidirectional, controllable, DC-DC converter835and a voltage, and in some embodiments a current, monitoring device834such as a voltage-measuring analog-to-digital converter or a channel of a common analog-to-digital converter associated with processor844. Processor844receives information from monitoring devices834and provides control pulses to each of the controllable DC-DC converters835. The DC-DC converters835pass power from across high-current-producing junctions, such as junctions849,850, of multijunction photovoltaic devices846,848and a local power bus852filtered by storage capacitor854as previously discussed above, to extract power from highly productive junctions or cells, and add power across low producing junctions or cells, to maintain as many junctions in the module or array as possible at or near the maximum power points.

In order to perform similar equalization of output of multiple junction photovoltaic devices with simpler circuitry, an alternative embodiment900inFIG. 21uses a separate energy exchanger902,904for each junction type, such as lower junctions906or upper junctions908in the multijunction photovoltaic devices910. These energy exchangers902,904may be of the type previously discussed with reference toFIGS. 5 and 6, and may use DC-DC converters912of the type discussed with reference toFIGS. 7, 8, 9, 10, or may use commutated capacitors as DC-DC converters. These energy exchangers may run at a predetermined frequency, or may have their switching rate adjusted as discussed with reference to DCM and CCM modes above. Each energy exchanger902,904serves to equalize cell voltage among the junctions906,908of the same type and therefore serves to compensate for poor productivity by any one junction; for example exchanger902serves to equalize cell voltage among all lower junctions906. Each exchanger has an associated energy storage capacitor914,916.

Since lower junctions906operate with longer wavelengths than upper junctions908, voltages and currents developed by upper and lower junctions are typically different. A controllable, voltage-shifting, bidirectional, DC-DC converter918, similar to those previously discussed with reference to510,512,710,712,722, and which may comprise a separate buck converter for one direction and a boost converter for the other direction, or which may embody a true bidirectional level-shifting converter, is provided for transferring power between the energy storage capacitors914,916. In an embodiment, DC-DC converter918is similar to that previously discussed with reference toFIG. 7. Another embodiment uses a DC-DC converter918of the coupled-inductor Cuk type illustrated inFIG. 8, or the transformer-coupled type illustrated inFIG. 10. In an alternative embodiment, where current produced by a particular junction type, such as a lower junction, typically exceeds current produced by a different junction type, such as an upper junction, in the same stack or device, DC-DC converter918is a unidirectional converter such as a buck or boost converter, the converter operable to transfer energy from the junction types producing high current to the junction types producing lower current. In these embodiments, the DC-DC converter adapts for voltage differences between the junctions.

The embodiment ofFIG. 21has a controller920suitable for monitoring voltages at the energy storage capacitors914,916and adjusting bidirectional converter918to transfer energy from whichever capacitor914,916is associated with high-current-production to whichever capacitor is associated with low current production in order to maintain all junctions in order to prevent reverse-biasing any junctions and to maintain all junctions in a power producing mode near their maximum power point. Controller920may also communicate with additional maximum-power-point tracking battery charge controllers and other hardware of the system.

Energy storage capacitors916and914can be combined and converter918can be eliminated, if appropriate different voltage gains are set for the converters in902and904groups to compensate for the voltage differences at the associated junctions.

The embodiment ofFIG. 21may be adapted to photovoltaic devices of three heterogeneous junction types quite readily by providing an energy exchanger similar to those of902,904for the third junction, and an additional controllable, voltage shifting, bidirectional, DC-DC converter, like that of918, for power transfer between the energy storage capacitor of the additional energy exchanger and one of the energy storage capacitors914,916. The embodiment ofFIG. 21may be adapted to greater numbers of junction types by replacing bidirectional DC-DC converter918with a controllable energy exchanger of the type previously discussed with reference toFIG. 16.

As an alternative to the exchanger for each junction type ofFIG. 21, and global energy exchanger ofFIG. 20, an energy exchanger may be provided for each multiple junction stacked or split-spectrum device as illustrated inFIG. 22.

Where a panel901uses a stacked, multijunction, photovoltaic device903that has junctions that have maximum photocurrents that are predictably and significantly mismatched, thrifted energy transfer devices having unidirectional DC-DC converters may be provided in a first level of energy exchanger as illustrated inFIG. 21A, with bidirectional DC-DC converters reserved for a second level of energy exchanger.FIG. 21Aillustrates two multijunction photovoltaic devices for simplicity, while it is anticipated that many embodiments will have more than two such devices. In this embodiment, the lower junction905of each device903,903A is of a type that typically produces significantly more current than does one or more upper junctions907of the same devices under most operating conditions. In a first stage911of energy transfer, a unidirectional DC-DC converter909is provided to draw power from the lower junction905of each device903,903A and to apply an output current across the upper junction907of that device. In some embodiments, unidirectional DC-DC converter909maintains a fixed voltage ratio between its input and its output, the ratio predetermined to position the junction attached to its input and the junction attached to its output at or near maximum power point voltages when the multijunction photovoltaic device operates under load; in some other embodiments, the unidirectional DC-DC converter909has a controller capable of sensing voltage and current flow, and of maintaining junction905at maximum power point. A second level of energy transfer, in the form of energy exchanger913having bidirectional DC-DC converters915,917, provides for any mismatches in current production that may exist between devices903,903A by operating to transfer current from whichever device903,903A is higher producing to whichever device is lower producing within a module or panel. Energy exchanger913has bidirectional DC-DC converters915,917as previously described. In an embodiment, energy storage capacitor919is provided. An additional level of energy transfer may be provided to transfer energy from high producing panels of a system to across low producing panels of the system to equalize current production across a series string of panels or modules in an array. In an embodiment, converters909are boost converters. In an embodiment, each converter909operates to maintain a maximum power point (MPPT) voltage across the lower cell.

In an alternative embodiment, upper cells907have two or more stacked photovoltaic junctions, and in a variation a stage of energy exchanger as herein described is provided to equalize current output between stacked junctions of upper cells907.

Another embodiment931of a photovoltaic subunit for use with multijunction photovoltaic devices is illustrated inFIG. 21B; it is anticipated that each of these subunits replaces one photovoltaic device907and associated boost converter909in the series string with associated energy exchanger illustrated inFIG. 21A. In this embodiment, one junction933is not in series with output terminals935, while remaining junction(s)937are coupled in series with output terminals935. In this embodiment, current provided by the isolated junction933powers a buck-boost converter939coupled to provide current across output terminals935. In embodiments where there is more than one remaining junction937, an additional bidirectional (or unidirectional if remaining junctions937are predictably mismatched) DC-DC converter941is provided to equalize current production in remaining junctions937. The output terminals935of multiple subunits931may then be electrically coupled in series, with additional stages of energy exchanger as described herein, to complete a panel, array, or system. In some embodiments, unidirectional buck-boost DC-DC converter939maintains a fixed voltage ratio between its input and its output, the ratio predetermined to position the junction attached to its input and the junction attached to its output at or near maximum power point voltages when the multijunction photovoltaic device operates under load; in some other embodiments, DC-DC converter939has a controller capable of sensing voltage and current flow, and of maintaining junction933at maximum power point. In some embodiments bidirectional DC-DC converter941has a voltage gain predetermined according to junction types such that it will maintain a ratio of voltages between junctions937that will keep both junctions operating near their maximum power point, in some other embodiments, DC-DC converter941has a controller capable of sensing voltage and current flow, and of maintaining both junctions937at maximum power point.

The embodiments ofFIG. 21AandFIG. 21Bact to transfer power from a high current bottom cell905,933to boost available current from series-connected upper cell or cells907,937. In an alternative embodiment of a photovoltaic subunit949, illustrated inFIG. 21C, a DC-DC converter955can also be used to boost current from upper cells951, at the expense of upper cell voltage, to match current produced by lower cells953, thereby permitting optimum power transfer in a system having multiple-junction photovoltaic devices. In the embodiment illustrated inFIG. 21C, DC-DC converter955is a buck-type down-converter.

Another repeatable photovoltaic subunit for an alternative embodiment with multijunction stacked or split-spectrum devices, as illustrated inFIG. 21D, has a unidirectional step-up, typically a boost, converter957to transfer energy from a high current producing bottom junction967to across lower-current-producing upper junctions963,965, and a bidirectional DC-DC converter959for equalizing current produced by upper junctions963,965. In an embodiment, the repeatable subunit ofFIG. 21Dhave outputs strung in series with an energy exchanger across the subunits to equalize current production by the subunits.

The circuits shown with one and two upper cells, with one lower cell, inFIG. 21, andFIG. 21A-Dmay also be built with other photovoltaic cell designs, including four junction designs having three top cells and one bottom cell. Similarly, the circuits shown are applicable to split-spectrum photovoltaic devices that have one or more junctions in a multiple junction stack forming a circuit equivalent of an upper cell that absorbs some wavelengths of light, and a physically separated cell having one or more other junctions that absorb remaining wavelengths of light. Devices having such physically separated cells may also maintain separate upper and lower strings with separate energy exchangers for each string, with a buck converter to reduce voltage of the higher-voltage string, or a boost converter to boost voltage of the lower-voltage string, at a panel output point where the two strings are tied together.

In an exemplary system the energy exchangers ofFIG. 20, 2121A, or21B are embedded within each photovoltaic module assembly of an array.

FIG. 22illustrates an alternative embodiment of the energy exchanger930for use with multijunction photovoltaic devices932,934, having an integrated exchanger938for each multijunction device. This embodiment has an advantage that the maximum voltage differences handled by each exchanger are small, allowing integration of the exchanger into an integrated circuit. Each integrated exchanger938of exchanger930embodiment has an exchanger subunit940,942,944; where each subunit has a controllable, bidirectional, DC-DC converter948,950,952, and a voltage (and optionally current) monitor device954,956,958, and each subunit is coupled to receive power from one junction of the multijunction photovoltaic device932associated with the integrated exchanger938. Each integrated exchanger has a controller960to ensure that the converters948,950,952are operated in a way that maximizes power extracted from the photovoltaic device932.

In an alternative embodiment, controller960operates to maintain a predetermined ratio of voltages between junctions, the ratio predetermined to provide near-optimum power transfer from each of the junctions of the multijunction photovoltaic devices. Such an embodiment, which maintains an approximate maximum power point, has advantage in that monitoring of voltage ratios is simpler to implement than monitoring current to find an actual maximum power point.

In yet another alternative embodiment, controller960may operate energy exchangers940,942and944at different predetermined constant duty cycles, to accommodate different voltages for each junction inside the cell932, simplifying the circuit and making it less expensive to build. While this arrangement may not deliver MPP in all conditions, it can be sufficiently beneficial to consider for simplicity and low cost.

In this embodiment, each additional multifunction photovoltaic device934is associated with a separate integrated exchanger, such as integrated exchanger936; and balance between devices is obtained by matching devices. In an alternative embodiment, a second level of exchanger is used across the devices within a module, and a third level between modules of an array.

In an embodiment, the DC-DC converters of integrated exchanger938do not have to be isolated, although they could be, rather they have circuitry as illustrated inFIG. 23. The integrated exchanger ofFIG. 23has three junction connections Vjunc1, Vjunc2, Vjunc3each coupled to a junction terminal of an associated stacked multijunction photovoltaic device988, with Vjunc1connected to the lowest voltage terminal, Vjunc2the middle, and Vjunc3the highest voltage junction terminal of the device, and each connection operates through a reversible boost converter to drive storage capacitor980to a voltage equal to, or higher than, the highest voltage of the junction connections. The local ground, Vjunc0, may in embodiments be coupled in series with Vjunc3of another photovoltaic device, and serves as a local return for the converters. The bi-directional boost converter operates as a buck converter when operated to transfer power from storage capacitor980to a junction terminal Vjunc1, Vjunc2, or Vjunc3. Local exchanger ground is coupled to the negative terminal Vjunc0of the stacked photovoltaic device. Each bi-directional boost converter982,984,986has at least one inductor and at least two switching devices as illustrated, as well as additional circuitry including a controller (not shown for simplicity) for such functions as driving the switching devices and controlling the converters to optimize power extraction from the stacked photovoltaic device. The controller may operate in manner similar to that described with reference to controller960ofFIG. 22. In some embodiments, boost converter switching control signals (e.g., PWM switching control signals) are phase shifted from one reversible boost converter982,984,986to another to promote small ripple current magnitude on node970and to reduce the likelihood of switching current induced electromagnetic interference. Additionally, one or more of energy storage inductors971,972,973of reversible boost converters982,984,986may be magnetically coupled to promote low ripple current magnitude in inductor windings, printed circuit board conductors, and boost converter switching devices, thereby promoting high efficiency and low current stress.

In an alternative embodiment illustrated inFIG. 24, since converter subunit982fromFIG. 23can be designed to have a voltage gain of one, and converters984and986a voltage gain near the ratio of a normal voltage at Vjunc3to a voltage at Vjunc2or Vjunc1respectively, converter subunit982is replaced with a wire. The junction terminals Vjunc3, Vjunc2, and Vjunc1, couple to a stacked photovoltaic device in a manner similar to that ofFIG. 23. The remaining converter subunits985,987, operate as boost converters in the forward direction conveying power to the capacitor981, or as buck converters in the reverse direction as described with reference toFIG. 23. In this way, a functional energy exchanger for three junctions can be constructed with only two DC-DC converters, while for N junctions only N−1 DC-DC converters are required, where N is greater than or equal to two. Actual power transfers between the junction terminals are determined by switching patterns of the switching transistors as determined by control circuitry, not shown inFIG. 24for simplicity. Typically, the control circuitry operates by monitoring voltages at each of the junction terminals and adjusts switching patterns of the switching transistors to maintain a ratio between voltages at each junction that has been determined to optimize power output from the photovoltaic device. Each converter inFIGS. 23-26has a switching device in series with an inductor as shown.

An integrated energy exchanger for use with stacked multijunction devices can also be constructed from buck-type converters as illustrated inFIG. 25instead of the boost configuration ofFIG. 23. The integrated exchanger ofFIG. 25has three junction connections Vjunc1, Vjunc2, Vjunc3each coupled to a junction terminal of an associated stacked multijunction photovoltaic device (not shown for simplicity) in manner similar toFIG. 23. Each junction connection Vjunc1, Vjunc2, Vjunc3couples through a buck-configured converter990,992,994to transfer power to a capacitor996; the converters990,992,994are reversible and therefore capable of operation as boost-configured converters to transfer power from the capacitor996to one or more of the junction connections Vjunc1, Vjunc2, Vjunc3.

By setting voltage gain of the lower converter994of the integrated exchanger ofFIG. 25to one, and controlling the voltage gain of the other converters appropriately, a functional energy exchanger for three junctions can be constructed with only two DC-DC converters, while for N junctions only N−1 DC-DC converters are required, where N is greater than or equal to two, as illustrated inFIG. 26, where converter991corresponds to converter990ofFIG. 25, and converter993corresponds to converter992ofFIG. 25.

In certain embodiments of the energy exchangers ofFIGS. 25 and 26, buck converter switching signals are phase shifted among buck converters990,992,994and among buck converters991,993to promote low ripple current magnitude on nodes998and999, thereby promoting low ripple current magnitude through the energy storage capacitor and possible corresponding photovoltaic device junctions electrically coupled to these nodes. Low ripple current magnitude through photovoltaic device junctions promotes maximum power transfer from the junctions. In some embodiments of the energy exchangers ofFIGS. 25 and 26, two or more buck inductors (e.g., buck inductors975,976,977) are magnetically coupled, in addition to being phase shifted, to promote low ripple current magnitude in inductor windings, printed circuit board conductors, and buck converter switching devices, thereby promoting high efficiency and low current stress.

A module may be assembled by attaching an integrated exchanger of the type illustrated inFIGS. 23 through 26to each of several multijunction photovoltaic devices as illustrated inFIG. 23, with the integrated exchangers isolated from each other. The multijunction photovoltaic devices are then stacked in series as illustrated inFIG. 22. An additional energy exchanger may be provided at module level, where each converter of the module-level exchanger couples to the top junction of each multijunction device. An array may be assembled from several such modules connected in series, and an array-level exchanger may be provided as illustrated inFIG. 13.

A thrifted energy exchanger1000is illustrated inFIG. 27, having isolated DC-DC converters or commutated capacitor DC-DC converters, and derived from that ofFIG. 6by effectively placing capacitor138in parallel with a photovoltaic device1008to obtain a capacitor1010. This energy exchanger therefore has N−1 DC-DC converters, for N photovoltaic devices1002,1004,1006,1008. The DC-DC converters1012,1014,1016of the thrifted energy exchanger1000may be free-running converters having a fixed voltage gain of one, or may alternatively be controllable converters. If the converters1012,1014,1016are controllable converters, the converters1012,1014,1016operate under control of controller1022having analog-to-digital voltage monitoring apparatus1020for monitoring voltages across each photovoltaic device1002,1004,1006,1008. In principle, the thrifted energy exchanger1000is applicable to any number N photovoltaic devices in a module, or in a panel. For relatively low voltage applications, non-isolated converters can be used. In the embodiment ofFIG. 27, the converters1012,1014,1016essentially cooperate to regulate the voltage at the capacitor1010.

Occasionally, it may be found that the sum of maximum power point voltages for series-connected photovoltaic devices in an array is less than a desired array output voltage or system battery voltage, requiring a voltage boost to optimally drive the load. In this case, a voltage-boosting, or series-connected, energy exchanger1000resembling that ofFIG. 27may be used, in a configuration where one photovoltaic device, typically top photovoltaic device1008, is omitted. This embodiment may use DC-DC converters1012,1014,1016operating under control of a controller1022and voltage monitor1020. In an alternative embodiment, voltage monitor1020also has current monitoring capability, the output current port Iout from the exchanger is coupled directly to a battery pack in a photovoltaic power system, and controller1022has firmware adapted to locate and maintain photovoltaic devices1002,1004,1006at their maximum power points. In an embodiment, controller1022has a configuration switch to set a battery voltage, and to thereby configure a voltage gain for the DC-DC converters. In this manner, for example, an energy exchanger having two DC-DC converters and coupled to two, series-connected, photovoltaic panels each having an open-circuit output voltage of 18 volts and maximum power point of 13 volts may be used to charge either a 36-volt or a 48-volt battery. It should be noted that the higher the DC-DC converter1012,1014,1016voltage gain from photovoltaic device1002,1004,1006to capacitor1010, the greater percentage of system power flows through the converters1012,1014,1016, and the greater potential for power loss in the system.

Omitting photovoltaic device1008from theFIG. 27system may advantageously allow achieving maximum power extraction in applications where a number of energy exchangers1000with corresponding cells1002,1004,1006and capacitors1010are electrically coupled in series. In particular, voltage across each capacitor1010in these circuits connected in series can be regulated, and the voltage across capacitor1010typically can have a wide range of values without affecting maximum power extracted from photovoltaic devices1002,1004,1006. Thus, total output voltage, and therefore total output current Iout, can be adjusted by varying voltage across capacitor1010without affecting power generated by photovoltaic devices1002,1004,1006. However, when multiple instances of energy exchanger1000are electrically coupled in series, output current Iout must be the same for all energy exchanger1000instances. In such applications, the ability to adjust voltage across capacitor1010allows for each instance of energy exchanger1000to maximize power extracted from its respective photovoltaic devices1002,1004,1006even though output current Iout magnitude cannot be varied due to the series connection. Similarly, if a number of instances of energy exchanger1000are connected in parallel, each instance must have the same output voltage, which will be achieved by independently varying the voltage across capacitor1010in the exchangers. Then each individual output current of parallel connected energy exchangers Io magnitude can be set to maximize power extracted from photovoltaic devices1002,1004,1006that are coupled to different energy exchangers connected in parallel.

In certain embodiments, the magnitude and/or polarity of voltage across capacitor1010is regulated. Such feature permits arbitrary selection of total system output voltage fromFIG. 27, thereby enabling energy exchanger1000to operate in applications that constrain either output current Iout or total output voltage. For example, such embodiments may maximize power extracted from photovoltaic devices in applications where energy exchanger1000is electrically coupled either in series with an external system that constrains Io or in parallel with an external system that constrains total output voltage.

For some applications, including some low voltage applications, non-isolated step-up converters can be used instead of the isolated converters illustrated inFIG. 27. An alternative embodiment of the energy exchanger1030using non-isolated boost converters1031,1032,1033is illustrated inFIG. 27Awith return wire of each connected to the negative pin of cell1031instead of the negative pin of the capacitor1034. Similarly, an alternative to the embodiment ofFIG. 28has non-isolated step down converters, such as buck converters. Semiconductor devices for such non-isolated converters can be integrated on a single chip, which may also include driver and control functions.

It is understood also that alternative embodiment1038(FIG. 28) of the energy exchanger previously described with reference toFIG. 27, with or without one photovoltaic device1008omitted, may be constructed such that polarities are reversed, such that capacitor1010is relocated to be a capacitor1036at the negative end of a string of photovoltaic devices1040,1042; such an exchanger has multiple DC-DC converters1037and operates in manner previously described with reference toFIG. 27.

In alternate embodiments of the systems ofFIGS. 27 and 28, an energy exchanger capacitor is electrically coupled between photovoltaic devices in a series string of photovoltaic devices. A respective photovoltaic device may or may not be electrically coupled in parallel with the capacitor.

In the embodiments1000,1038, ofFIGS. 27, 28, it is not necessary for all power generated by the photovoltaic devices1002,1004,1006,1008,1040,1042, to pass through the DC-DC converters1016,1014,1012,1037because some current can flow directly from photovoltaic device1002into series connected photovoltaic device1004. For purposes of this document, configurations where only part of system power flows through DC-DC converters because at least some current flows directly from one photovoltaic device, or battery cell, to another, are known as partial-power energy exchangers; those with a photovoltaic device, or battery cell, that produces at least some photocurrent at each step of the series-connected chain of photovoltaic devices or battery cells are further also known as partial-current energy exchangers. Similarly, configurations such as those voltage-boosting embodiments described with reference toFIG. 27 or 28with a photovoltaic device removed, where only part of system power flows through DC-DC converters because at least some current flows directly from one photovoltaic device, or battery cell, to another, but where the full output current of the system is provided by DC-DC converters, are known as partial-power, full-current energy exchangers.

Embodiments where the full current and full power of all photovoltaic devices in the array pass through DC-DC converters are known herein as full-power, full-current, exchangers. In the full-power exchanger embodiment1045ofFIG. 29, in block1046, for example, all power produced by photovoltaic devices1058,1060, passes through DC-DC converters and all load current passes through DC-DC converters. In the embodiment1045ofFIG. 29, most power produced by photovoltaic devices1058,1060passes through converter1052, and most power produced by photovoltaic device1068,1070passes through converter1062. Bidirectional upconverters1054,1064are provided to optimize power production by series devices1058,1060, and1068,1070. In this embodiments1045, local controller1056independently adjusts power transfer and voltage gain of each converter1054,1056, to maintain each device1058,1060at their separately-determined maximum power points while developing a block output voltage V1on a local summing bus1055.

Block1048operates similarly to block1046, although at a different voltage level. Local controller1066independently adjusts power transfer and voltage gain of each converter1062,1064independently to maintain each device1068,1070at their separately-determined maximum power points, while developing a block output voltage V2on a local summing bus1065. Any additional blocks in the string, such as block1072, operate similarly.

Similar to theFIG. 27system, certain embodiments of theFIG. 29system can operate to maximize power extracted from their respective photovoltaic devices in applications that constrain either total output voltage or output current. However, in theFIG. 29embodiment, block output voltages (e.g., V1, V2) are directly regulated by the blocks' DC-DC converters (e.g.,1052,1054,1062,1064). Thus, an arbitrary positive total output voltage can be obtained, constrained by the voltage gain characteristics of DC-DC converters, even with DC-DC converters (e.g.,1052,1054,1062,1064) that are not capable of providing a negative output voltage.

Unlike prior systems, such as that ofFIG. 4, having a single block of parallel controllers passing power from photovoltaic devices onto a typically fixed-voltage summing bus, multiple blocks are placed in series to provide an output sum voltage at an array output node1073to drive a load1075; and the voltage gain of each block is independently controlled by a system controller1074. Load1075may, for example, be a battery in a battery-charging photovoltaic system, or a grid-tie inverter in a gird-connected photovoltaic system.

In an alternative embodiment, operable without a system controller1074, each block controller1056,1066, operates to maintain photovoltaic devices of that block at maximum power point while maintaining an output voltage, such as voltage V1or V2, at the highest voltage level possible given output current drawn by the load, while maintaining output voltage at less than a predetermined block-maximum output voltage level. Such operation will tend to apportion output voltages in a string according to power available in each block but may result in overcharging a battery load unless a battery charge controller is also provided.

In embodiments having system controller1074, system controller1074monitors load voltage V3and determines whether and how much power is absorbable by the load1075. When power is absorbable by the load1075, system controller1074also apportions load voltage V3among individual block voltages, such as block1046voltage V1, and block1048voltage V2, such that all blocks operate at or close to the maximum power points of the photovoltaic devices1058,1060,1068,1070within them while providing appropriate current and voltage to the load, and instructs the individual block controllers1056,1066accordingly. When power is not absorbable by the load, such as when the load is a fully charged battery, system controller1074may instruct controllers1056,1066to operate with one or more converters1052,1054shut down, to operate with photovoltaic devices at other than their maximum power points, or alternatively may enable a secondary or dump load1078.

For example, in a system for operation remote from a power grid load1075may be a storage battery and inverter system as known in the art for driving off-grid electrical loads, and secondary load1078may be an electric auxiliary heating system. Similarly, in a system having a primary load of storage batteries for operation of critical systems or for mobile operations, and a grid connection, system controller1074may enable a grid-connected inverter as secondary load1078when primary load1075batteries are fully charged. Such systems may be of use with a motor home or yacht having roof-mounted photovoltaic devices1058,1060, but which spends part of each year in storage. When the motor home or yacht is in storage, system controller1074may enable a grid-connected inverter as secondary load1078to dump excess power produced by the photovoltaic devices into the national power grid through an appropriate metering device. When the motor home or yacht is in mobile operation, primary load1075may include charging storage batteries used to operate such typical mobile electric loads as water pumps, refrigerators, electronic communications, navigation and entertainment devices, automatic sail-trimming and steering devices, and electric lights.

Short wavelength, such as blue, light is absorbed preferentially by cloud, while longer wavelength, such as infrared, light passes through cloud with much less attenuation. This results in the cloud-shaded (solid circles) and unshaded (open circles) currents produced by a typical three junction stacked cell as illustrated inFIG. 30. It is also apparent that the shorter-wavelength top and middle junctions produce current that somewhat tracks each other when shaded by cloud, while the bottom, longer-wavelength, junction produces current that, although reduced somewhat, becomes relatively much stronger than current produced by the top and middle junctions.

In order to take advantage of the tracking of top1102and middle junction1104current in shaded conditions, while using all power available from the bottom junction1106, a thrifted energy-exchanger circuit like that ofFIG. 31may be used. In this embodiment, filtering capacitors1108provide filtering of voltage transients induced by switching currents. Switching transistors1110and1112, and inductor1114, form the active elements of a DC-DC converter operating under control of control circuitry1116.

In the embodiment ofFIG. 31, in a boost mode operable when bottom junction1106is shaded by cloud and produces significantly greater current than that produced by top1102and middle1104junctions, has inductor1114build current when switching transistor1110conducts, and switching transistor1112acts as a diode; in this mode energy is transferred from bottom junction1106to top1102and middle1104junctions. In a buck mode operable when bottom junction1106is in full sun and produces less current than that produced by top1102and middle1104junctions, inductor1114builds current when switching transistor1112conducts, and switching transistor1110acts as a diode; in this mode energy is transferred from top1102and middle1104junctions to bottom junction1106.

The circuit inFIG. 31can also be used with split-spectrum cells, where junction1106is a separate junction859as shown inFIG. 18D.

In an alternative embodiment having cross section as illustrated inFIG. 32a bottom junction top contact and upper junction bottom contact are brought out separately. WhileFIG. 32illustrates a two junction stack, the separate bottom junction top contact and upper junction bottom contact are applicable to stacks with other numbers of junctions. The device illustrated has a substrate1152, a back contact conductor layer1154, bottom semiconductor absorber1156and junction1158layers, a first transparent conductor layer1160, an upper semiconductor absorber layer1162, an upper junction layer1164, an upper transparent conductor layer1166, a patterned metallic top conductor layer1168, and a passivation or transparent protection layer1172; these layers are fabricated essentially as known in the art of stacked multijunction photovoltaic devices. Additional layers, such as dichroic reflector layers, barrier layers, and antireflection layers, may be incorporated into the photovoltaic device but are not shown here for simplicity.

In order to achieve a low-resistance connection to the first transparent conductor layer1160, and thereby low resistance connection to the top of the lower junction of the stacked device, an additional patterned metallic conductor layer1174and a patterned dielectric layer1176may be added to the multijunction stacked photovoltaic device during fabrication of that device, the conductor1174serving to bring out current from the lower junction, and the dielectric layer1176serving to insulate the top of the lower junction from the bottom transparent contact1178layer of the upper junction. Similarly, a patterned metallic conductor layer1180is provided to provide low resistance connectivity to the bottom transparent contact1178layer of the upper junction.

In some embodiments, inductors illustrated in the schematics are formed of the parasitic inductance of long interconnect wires. In other embodiments, physical inductors are used to implement these circuits.

A module may be divided into sections1202,1204(FIG. 33). In an embodiment each section has series strings of one or more multiple photovoltaic devices, multiple junction photovoltaic devices, or split spectrum devices, equipped with energy exchangers as described herein to maintain maximum power point operation within each section. These sections may in turn be coupled such that each section provides power through a separate section DC-DC converter with the section DC-DC converters coupled in series, as illustrated inFIG. 33, to provide a module or panel output1206. In the embodiment ofFIG. 33, multiple junction or split-spectrum photovoltaic devices1208,1210have high-current-producing junctions1212and lower current producing junctions1214,1216. A simple energy exchanger, here having one bidirectional DC-DC converter1220, is provided to transfer energy from the higher-current-producing junction to the lower-current-producing junction of junctions1214,1216, while a unidirectional energy transfer device, here a unidirectional step-up DC-DC converter1218, transfers energy from high-current-producing junction1212and applies it across the low-current-producing junctions1214,1216. Power from all three junctions is applied to a section converter1222to provide section output. Several section outputs are strung in series to provide module or panel output1206. In the embodiment ofFIG. 33, converter1220,1218and1222of each section cooperate to maintain maximum power production of junctions1212,1214and1216. In an alternate embodiment ofFIG. 33, converters1220,1218and1222of each section cooperate to maintain maximum output power production of each section.

In embodiments like that ofFIG. 33, it is anticipated that in some embodiments the DC-DC converters of each section, including converters for transferring energy from higher-current-producing photovoltaic devices to lower-current-producing photovoltaic devices produced by the multiple junction or split-spectrum devices such as converters1218,1222, and the section converter1222, have their controllers and active devices located within a single integrated circuit. Further, in some embodiments, the integrated circuits bearing the active devices of each converter, together with associated passive components such as inductors, are embedded into a module or panel along with the associate photovoltaic devices1208,1210. Similarly, it is expected that active devices associated with the converters associated with each block1046, including converters1052,1054, and their controller1056are located within a single integrated circuit for each block, the integrated circuit being embedded into a module or panel along with associated photovoltaic devices.

In an alternative embodiment resembling that ofFIG. 33, converters1218and1220operate with a fixed, predetermined, voltage gain from input to output, the gain predetermined to place all junctions of the devices approximately at their maximum power points when the devices operate under load.

In embodiments having more than one DC-DC converter, including the embodiments discussed with reference toFIG. 33, it may be desirable to operate the multiple DC-DC converters with transitions on each converter having timing offset, or phased, within a converter cycle from each other. Such phased designs offer benefits of noise reduction by at least partially cancelling ripple currents at their outputs and potentially permit coupled-inductor designs.

In embodiments having more than one DC-DC converter associated with a multiple junction device such as that illustrated inFIG. 33, advantage may result from both properly phasing the converters1246,1248and magnetically coupling inductors1252,1254of the DC-DC converters1246,1248by winding inductors1246,1248on a common core. It may also be desirable to magnetically couple inductor windings of two or more of the DC-DC converters1222provided at section outputs, to extend ripple cancellation to the inductor windings, printed circuit board conductors, and switching devices, thereby promoting higher efficiency and lower current stress.

Various embodiments are designated and have features as follows.

In an embodiment designated by A, an energy transfer device, comprising:

a first port coupled to a first interface unit, the first interface unit being coupled to a capacitor; a second port coupled to a second interface unit, the second interface unit being coupled to the capacitor; wherein the interface units are adapted to operate with the first port having a voltage offset from the second port; wherein the first interface unit is adapted to transfer energy between the first port and the capacitor, and the second interface unit is adapted to transfer energy between the second port and the capacitor; and wherein the first and second interface units are adapted to transfer energy from the first port to the capacitor and from the capacitor to the second port when energy available at the first port is greater than energy available at the second port, and wherein a terminal of the capacitor is coupled to a common return node.

In an embodiment designated by B, a system comprises an energy transfer device, the energy transfer device including: an energy exchange bus; N ports, N being an integer greater than one, each of the N ports being coupled to an energy port of one of N interface units, where each interface unit is coupled to an energy transfer bus and is capable of transferring energy bidirectionally between the energy port and the energy exchange bus; wherein the N interface units are adapted to operate with each of the N ports having a voltage offset relative to each of at least one other of the N ports; and wherein the interface units are adapted to pass energy from a high energy port of the N ports to the energy exchange bus and from the energy exchange bus to a low energy port of the N ports.

In an embodiment designated by C, the energy transfer device designated A or the system designated B, wherein the first and second interface units are adapted to transfer energy from the second port to the capacitor (or energy transfer bus if the parent is designated by B) and from the capacitor or bus to the first port when energy available at the second port is greater than energy available at the first port.

In an embodiment designated by D, the energy transfer device of the embodiment designated by C further comprising a third port coupled to a third interface unit, the third interface unit being coupled to the capacitor (or bus); wherein the third interface unit is adapted to operate with the third port having a voltage offset from the first port and from the second port; and wherein the interface units are adapted to transfer energy from a first selected port having a highest energy provided to the port, the first selected port selected from the group consisting of the first, second, and third ports, the energy being transferred to the capacitor (or bus), at least a portion of the energy being transferred from the capacitor (or bus) to a second selected port having a lowest energy provided to the port.

In an embodiment designated by E, the energy transfer device of the embodiment designated by D, the third interface unit comprises a bidirectional DC-DC converter including an inductor, the DC-DC converter adapted to operate in a continuous current conduction mode if a magnitude of current transferred by the DC-DC converter is greater than or equal to a threshold value, the DC-DC converter adapted to operate in a discontinuous conduction operating mode if the magnitude of current transferred by the DC-DC converter is less than the threshold value.

In an embodiment designated by F, the energy transfer device of the embodiment designated by C, wherein the energy transfer device further comprises auxiliary circuitry powered by energy drawn from the energy transfer bus.

In an embodiment designated by G, the energy transfer device of the embodiment designated by C, wherein each of the first and second interface units is a bidirectional charge pump converter.

In an embodiment designated by H, the energy transfer device of the embodiment designated by C wherein each of the first and second interface units is a bidirectional Cuk converter.

In an embodiment designated by I, the energy transfer device of the embodiment designated by C, wherein each of the first and second interface units is a bidirectional SEPIC converter.

In an embodiment designated by J, the energy transfer device of the embodiment designated by C, further comprising N energy sources, where each energy source is coupled to a separate port of the N ports of the energy transfer device, and wherein the energy sources are coupled in series.

In an embodiment designated by K, the energy transfer device of the embodiment designated by J, wherein switching devices of at least two interface units operate at constant frequency and duty cycle, and wherein energy flow acts to equalize voltages at ports of the energy transfer device.

In an embodiment designated by L, the energy transfer device of the embodiment designated by C, wherein the energy sources are battery cells and the energy transfer device operates to equalize voltages across the battery cells while the battery cells are charging.

In an embodiment designated by M, the energy transfer device of the embodiment designated by C, wherein the energy transfer device operates to equalize voltages across the energy sources while the energy sources are providing power to an output of the system.

The embodiment designated by M, wherein the energy sources are battery cells.

The embodiment designated by M, wherein the energy sources are fuel cells.

An embodiment designated by N, wherein the embodiment designated by M has N energy sources that are photovoltaic devices of a solar power system.

An embodiment designated by O, wherein the embodiment designated by N further has at least one of the photovoltaic devices further comprising: M photovoltaic cells, the M photovoltaic cells being coupled electrically in series; and a second energy transfer device, including: a second energy exchange bus; M ports, M being an integer greater than one, each of the M ports being coupled to an energy port of one of M interface units, where each interface unit is coupled to a second energy exchange bus and is capable of transferring energy bidirectionally between its energy port and the second energy exchange bus; wherein the M interface units are adapted to pass energy from a high energy port of the M ports to the second energy exchange bus and from the second energy exchange bus to a low energy port of the M ports; wherein each of the M ports of the second energy transfer device is coupled to a separate photovoltaic cell of the M photovoltaic cells.

In an embodiment designated by P, wherein the system of the embodiment designated by O has energy sources that are photovoltaic devices, and wherein the energy transfer device further comprises a controller configured and arranged to adjust a voltage gain of each interface unit based at least partially on a maximum power point of a respective photovoltaic device coupled to the interface unit.

In an embodiment designated by Q, wherein the system of the embodiment designated by P, further comprises an additional photovoltaic device coupled in parallel with one of the N photovoltaic devices.

In an embodiment designated by R, a solar photovoltaic array comprises: a first and a second photovoltaic device each having a positive and a negative terminal, the first photovoltaic device being capable of producing a first electric current at a first voltage when illuminated, and the second photovoltaic device being capable of producing a second electric current at a second voltage when illuminated; wherein the first and the second photovoltaic devices are coupled electrically together in series with the positive terminal of the first photovoltaic device coupled to the negative terminal of the second photovoltaic device; and an energy transfer device having a first terminal coupled to the negative terminal of the first photovoltaic device, a second terminal coupled to the positive terminal of the first photovoltaic device and to the negative terminal of the second photovoltaic device, and a third terminal coupled to the positive terminal of the second photovoltaic device, the energy transfer device being capable of receiving energy from its first and second terminals and providing energy to its second and third terminals if a first parameter selected from the group consisting of the first current and the first voltage is greater than a second parameter selected from the group consisting of the second current and the second voltage, and of receiving energy from its second and third terminals and providing energy to its first and second terminals if the second parameter is greater than the first parameter.

The embodiment designated by R, wherein the energy transfer device comprises at least a first capacitor, and wherein receiving energy from its first and second terminals and providing energy to its second and third terminals is performed by alternately coupling the first capacitor across the first and second terminals, and across the second and third terminals.

The embodiment designated by R, wherein the energy transfer device comprises at least a first inductor, and wherein receiving energy from its first and second terminals is performed by alternately closing and opening a first switching device, the first switching device coupling the first inductor across the first and second terminals.

An embodiment designated by S, wherein the embodiment designated by R, has an energy transfer device that comprises at least a first inductor, and wherein receiving energy from its first and second terminals is performed by alternately closing and opening a first switching device, the first switching device coupling the first inductor across the first and second terminals, and wherein the energy transfer device comprises a bidirectional Cuk converter.

The embodiment designated by S, wherein the Cuk converter is a transformer isolated Cuk converter.

The embodiment designated by R, wherein the energy transfer device comprises at least a first inductor, and wherein receiving energy from its first and second terminals is performed by alternately closing and opening a first switching device, the first switching device coupling the first inductor across the first and second terminals, and wherein the energy transfer device comprises a bidirectional capacitively-isolated Cuk converter.

The embodiment designated by R, wherein the energy transfer device comprises at least a first converter stage having an inductor having a first terminal coupled to a first terminal of the stage, and a second terminal coupled through a first switching device to a second terminal of the stage, and a first capacitor having a first terminal coupled to the second terminal of the inductor and to a first terminal of a second inductor, a second switching device coupled from the first terminal of the second inductor to a third terminal of the stage; and wherein the first and second terminals of the first stage are coupled to terminals of the energy transfer device.

The embodiment designated by R, further comprising a third photovoltaic device having a negative terminal coupled to the positive terminal of the second photovoltaic device, the third photovoltaic device being capable of producing a third current at a third voltage when illuminated; wherein the energy transfer device has a fourth terminal coupled to a positive terminal of the third photovoltaic device, and wherein the energy transfer device is capable of receiving energy from its first and second terminal and providing energy to its third and fourth terminals if the first current is greater than the third current.

The embodiment designated by R, wherein the first and second photovoltaic devices are factory-assembled modules comprising multiple photovoltaic cells and adapted for field assembly into a multiple-module photovoltaic array.

The embodiment designated by R, wherein the photovoltaic array is a factory-assembled module adapted for field assembly into a multiple-module photovoltaic array, and wherein the first and second photovoltaic devices are photovoltaic cells within the module.

An embodiment designated by T, wherein the embodiment designated by R further has the first photovoltaic device comprising at least a first and second photocell electrically connected in series with the second photovoltaic device, and a second energy transfer device is coupled to transfer energy from terminals of a high producing photocell selected from the group consisting of the first photocell and the second photocell of the first photovoltaic device, and to transfer the energy to terminals of a low producing photocell selected from the group consisting of the first photocell and the second photocell of the first photovoltaic device.

In an embodiment designated by U, the embodiment designated by T, wherein the first photovoltaic device is a factory-assembled module incorporating the second energy transfer device and adapted for field assembly into a photovoltaic array.

The embodiment designated by U, further comprising a third photovoltaic device coupled in parallel with the first photovoltaic device.

In an embodiment designated by V, the embodiment designated by T, further comprising a charge controller and a battery.

The embodiment designated by V, further comprising an energy transfer device coupled to balance charge in the battery by transferring energy from a high voltage cell of the battery to a low voltage cell of the battery.

The embodiment designated by T, further comprising a grid-tie inverter, energy from the photovoltaic devices being coupled to power the grid-tie inverter, the grid-tie inverter capable of synchronously feeding energy into an alternating-current power distribution system.

In the embodiment designated by R, wherein the energy transfer device is a transformer-coupled bidirectional converter.

In the embodiment designated by R, wherein the photovoltaic array is capable under standard conditions of producing power substantially in excess of a power rating of the energy transfer device.

An embodiment of a solar photovoltaic array designated by W and comprising: a first, a second, and a third photovoltaic device each having a power output port having positive and negative terminals, the first photovoltaic device being capable of producing a first electric current at a first voltage when illuminated, the second photovoltaic device being capable of producing a second electric current at a second voltage when illuminated, and the third photovoltaic device being capable of producing a third electric current at a third voltage when illuminated; wherein the power output ports of the first, the second, and the third photovoltaic devices are coupled electrically together in series; an energy transfer device having a first port coupled to the power output port of the first photovoltaic device, a second port coupled to the power output port of the second photovoltaic device, and a third port coupled to the power output port of the third photovoltaic device, the energy transfer device being capable of receiving energy from its first port and providing energy to its second port if a first parameter selected from the group consisting of the first current and the first voltage is greater than a second parameter selected from the group consisting of the second current and the second voltage, and of receiving energy from its second port and providing energy to its first port if the second parameter is greater than the first parameter; and wherein the energy transfer device is capable of receiving energy from the first port and providing energy to its third port if the first parameter is greater than a third parameter selected from the group consisting of the third current and the third voltage, and of receiving energy from its third port and providing energy to its first port if the third parameter is greater than the first parameter.

The photovoltaic array designated by W, wherein the energy transfer device comprises at least a first and a second converter stage, where each converter stage comprises: an inductor having a first terminal coupled to a first terminal of a first port of the converter stage; a switching device for coupling a second terminal of the inductor to a second terminal of the first port of the converter stage; an isolation capacitor having a first terminal coupled to the second terminal of the inductor and a second terminal coupled to transfer power between the first port of the converter stage and a second port of the converter stage; wherein the first port of the first converter stage is coupled to the first port of the energy transfer device; the first port of the second converter stage is coupled to the second port of the energy transfer device; and the second port of the first and second converter stages are coupled together and to a common energy storage device.

An embodiment of a system designated by X comprising a first energy transfer device for transferring energy from a high-producing device to a low-producing device, the first energy transfer device, comprising: a first port for coupling to a first photovoltaic device; a second port for coupling to a second photovoltaic device; a controller for determining a port attached to a low producing device selected from the group consisting of the first port and the second port, and for determining a port attached to a high producing device selected from the group consisting of the first port and the second port; at least a first inductor, and a first switching device coupled in series with the first inductor, the first inductor being coupled to the first port; wherein the energy transfer device is operable with the first and the second ports coupled together in series; and wherein energy transfer from the first port is performed by a method comprising alternately closing and opening the first switching device at a high frequency, and wherein opening the first switching device disconnects at least one terminal of the inductor from the first port.

The system designated by X, wherein the energy transfer device is incorporated into a module for assembly into a photovoltaic array, the module further comprising at least a first photovoltaic device coupled to the first port and a second photovoltaic device coupled to the second port, wherein each of the first and the second photovoltaic devices comprises a photovoltaic cell.

An embodiment designated by Y of the system designated by X, further comprising at least one photovoltaic module coupled to the first port of the first energy transfer device and at least one photovoltaic module coupled to the second port of the first energy transfer device, wherein each photovoltaic module comprises at least a plurality of series-connected photovoltaic cells assembled to a common module substrate.

An embodiment designated by Z of the embodiment of the system designated by Y wherein at least one photovoltaic module further comprises a second energy transfer device capable of transferring energy from at least one high-producing photovoltaic cell of the module to a lower-producing photovoltaic cell of the module.

The embodiment designated by Z, wherein at least one port of the first energy transfer device is coupled to at least two photovoltaic modules coupled together in parallel.

The embodiment designated by X, wherein the first inductor is part of a CUK converter.

A system designated by AA and comprising a first energy transfer device for transferring energy from a high-current-producing junction of a first stacked multijunction photovoltaic device to a low-current-producing junction of the photovoltaic device, the first energy transfer device comprising: a first port for coupling to the high-producing junction of the photovoltaic device; a second port for coupling to the low-producing junction of the photovoltaic device; at least a first inductor coupled to at least one port selected from the group consisting of the first and second port, and a first switching device coupled in series with the first inductor; and a controller for monitoring voltages at the high-current-producing and low-current-producing junctions and for determining switching of the first switching device to maintain at least an approximate maximum power point for each junction of the multijunction photovoltaic device.

A system designated by AB according to the system designated by AA further comprising a second energy transfer device for transferring energy from a high-current-producing junction of a second stacked multijunction photovoltaic device to a low-current-producing junction of the second photovoltaic device, the second energy transfer device, comprising: a third port for coupling to the high-producing junction of the second photovoltaic device; a fourth port for coupling to the low-producing junction of the second photovoltaic device; and at least a second inductor coupled to at least one port selected from the group consisting of the third and fourth port, and a second switching device coupled in series with the second inductor.

A system according to the system designated by AB further comprising a third energy transfer device for transferring energy from the first stacked multijunction photovoltaic device to the second stacked multijunction photovoltaic device.

An embodiment designated by AC comprises a system for driving a load comprising: a first DC-DC converter coupled to transfer power from a first energy source to a capacitor; and a second DC-DC converter coupled to transfer power from a second energy source to the capacitor; wherein the first and second energy sources, and the capacitor, are coupled electrically in series to drive the load; and wherein the energy sources are selected from the group consisting of photovoltaic devices, batteries, and fuel cells.

An embodiment of the system designated by AC wherein the energy sources are batteries

An embodiment of the system designated by AC wherein the energy sources are photovoltaic devices.

An embodiment designated by AD comprises a system for driving an electrical load comprising: a plurality of blocks, the blocks having outputs electrically coupled together in series, each block further comprising a first photovoltaic device coupled to power a first controllable DC-DC converter, the first controllable DC-DC converter coupled to power the output of the block; a second photovoltaic device coupled to a second controllable DC-DC converter, the second DC-DC converter coupled to power the output of the block, and a controller; wherein the controller of each block comprises apparatus for determining a maximum power point for the first and for the second photovoltaic device, and apparatus for controlling the first and the second DC-DC converter to operate the DC-DC converters at the maximum power point while maintaining the output of each block at less than a predetermined maximum voltage.

The embodiment designated by AD further comprising a controller for monitoring a load voltage and for apportioning desired output voltages among the plurality of blocks.

An embodiment designated by AE comprises a subsystem comprising: at least one multiple junction stacked photovoltaic device; at least a first DC-DC converter coupled to transfer energy from a high-current-producing junction of the at least one stacked multijunction photovoltaic device to across at least a low-current-producing junction of the photovoltaic device, the high-current-producing junction electrically coupled in series with the low-current-producing junction; a controller for monitoring voltages at the high-current-producing and low-current-producing junctions and configured to determine switching of at least one switching device of the first DC-DC converter.

The embodiment designated by AE wherein the controller optimizes power output from the multijunction photovoltaic device by determining switching of the at least one switching device of the at least one DC-DC converter to maintain both the high-current-producing and low-current-producing junctions at approximately their respective maximum power points when the system is coupled to provide power to a load.

An embodiment designated by AF comprises the embodiment designated by AE wherein the multiple junction stacked photovoltaic device comprises at least three junctions, and further comprising a second DC-DC converter coupled to transfer energy from a second high-current-producing junction of the at least one stacked multijunction photovoltaic device to across at least one low-current-producing junction of the photovoltaic device.

The embodiment designated by AF wherein the controller optimizes power output from the multijunction photovoltaic device by determining switching of the at least one switching device of the first DC-DC converter and switching of at least one switching device of the second DC-DC converter to maintain both the second high-current-producing and the at least one low-current-producing junctions at approximately their respective maximum power points when the system is coupled to provide power to a load.

The embodiment designated by AF wherein the first and second DC-DC converter cooperate to optimize power output of the multiple junction stacked device.

The embodiment designated by AF wherein the first and second DC-DC converter cooperate to optimize power output of the subsystem.

An embodiment designated by AG of the embodiment designated by AF wherein at least the first DC-DC converter is a bidirectional converter.

The embodiment designated by AG wherein the second DC-DC converter is a unidirectional converter.

An embodiment designated by AH of a subsystem comprising: at least a first photovoltaic device selected from the group consisting of a multiple junction stacked photovoltaic device having a plurality of photovoltaic junctions coupled electrically in series, and a split-spectrum photovoltaic device having a plurality of photovoltaic junctions coupled electrically in series; at least a first bidirectional DC-DC converter coupled to transfer energy between an output of the first photovoltaic device and at least one specific junctions of the at least a first photovoltaic device; a controller configured to determine switching of at least one switching device of the first DC-DC converter to optimize power output from the first photovoltaic device.

The embodiment designated by AH wherein the controller monitors voltages at junctions of the first photovoltaic device to control switching of the at least one switching device.

The embodiment designated by AH wherein the subsystem provides power through a second DC-DC converter, and further comprising at least a second photovoltaic device selected from the group consisting of a multiple junction stacked photovoltaic device having a plurality of photovoltaic junctions coupled electrically in series, and a split-spectrum photovoltaic device having a plurality of photovoltaic junctions coupled electrically in series; at least a third bidirectional DC-DC converter coupled to transfer energy between an output of the second photovoltaic device and at least one specific junctions of the at least the second photovoltaic device; a fourth DC-DC converter coupled to receive power from the second photovoltaic device; and wherein outputs of the second and fourth DC-DC converters are electrically coupled in series.

An embodiment designated by AJ of the embodiment designated by AH further comprising a second photovoltaic device electrically coupled in series with the first photovoltaic device, and a second bidirectional DC-DC converter coupled to transfer energy between the first photovoltaic device and the second photovoltaic device to equalize current, thereby optimizing output power of the first and second photovoltaic devices.

The embodiment designated by AJ wherein the photovoltaic devices are stacked multiple-junction devices, and wherein the first and second DC-DC converter cooperate to optimize power from at least one junction of the photovoltaic devices.

The embodiment designated by AJ wherein the photovoltaic devices are stacked multiple junction devices, and wherein the first and second DC-DC converter cooperate to optimize power from the subsystem.

An embodiment designated by AK comprises a subsystem comprising at least a first and a second photovoltaic junction coupled to an energy transfer device adapted to equalizing current produced by the first and a second photovoltaic junction, the first photovoltaic junction capable of producing more current than the second photovoltaic junction, the energy transfer device comprising a DC-DC converter coupled to transfer energy from the first to at least the second photovoltaic junction.

An embodiment designated by AL of the embodiment designated by AK further comprising a third photovoltaic junction and a second DC-DC converter, the third photovoltaic junction capable of producing less current than the first and second photovoltaic junctions, the second DC-DC converter coupled to transfer energy from at least the first to at least the third photovoltaic junction.

The embodiment designated by AL wherein the first and second DC-DC converters have an output coupled across the second and third photovoltaic junctions, and the second DC-DC converter has an input coupled across the first and second photovoltaic junctions.

The embodiment designated by AL wherein the first and second DC-DC converters operate with a fixed, predetermined, gain.

The embodiment designated by AL wherein the first and second DC-DC converters operate with switching transitions phase-offset between converters.

The embodiment designated by AL wherein inductors of the first and second DC-DC converters are magnetically coupled.

Certain changes may be made in the above methods and systems without departing from the scope hereof, and the matter contained in the above description or shown in the accompanying drawings should be interpreted as illustrative and not in a limiting sense. Those skilled in the art should appreciate that items as shown in the embodiments may be constructed, connected, arranged, and/or combined in other formats without departing from the scope of the invention. The following claims cover all generic and specific features of the invention described herein, and all statements of the scope of the invention which, as a matter of language, might be said to fall therebetween.