1. Field of the Invention
The present invention pertains generally to photovoltaic system charge controllers and, more particularly, to high voltage photovoltaic system charge controllers that employ maximum power point tracking.
2. Brief Discussion of the Related Art:
Photovoltaic (PV) systems that produce electricity from solar energy have established themselves as a successful and reliable option for electrical power generation. Photovoltaic systems have continually been gaining in popularity as the cost of such systems has been reduced, as the cost of utility-supplied power has escalated, and as greater attention has been paid to the need for renewable, alternative energy sources. Basically, a photovoltaic system includes a photovoltaic (PV) array made up of one or more PV panels or modules composed of photovoltaic cells capable of converting solar energy into direct current (DC) electrical energy, a battery bank made up of one or more batteries for storing the electrical energy produced by the photovoltaic array, and a charge controller for controlling the charging of the one or more batteries with the electrical energy produced by the photovoltaic array. The direct current (DC) electrical energy produced by the photovoltaic array and/or stored in the battery bank is available to power a load. In some systems, the load may include an inverter used to convert the direct current (DC) electrical energy into alternating current (AC) electrical energy suitable to power AC loads. Photovoltaic systems are sometimes employed to power loads independently of utility power, such as where electrical power from a public utility grid is unavailable or not feasible, and these photovoltaic systems are commonly referred to as “off-grid” and “stand-alone” photovoltaic systems. In other instances, photovoltaic systems known as “on-grid” and “grid-connected” photovoltaic systems are employed to supply electrical power to the public utility grid as explained further below.
In accordance with programs commonly referred to as “net metering”, many public utilities provide compensation for the net electrical power that is supplied or fed into the utility grid from grid-connected photovoltaic systems. The electrical power produced by grid-connected photovoltaic systems may be used first to operate any connected end load, such as various conventional electrical appliances and devices, and the excess electrical power not consumed by the connected end load would be supplied to the utility grid. If the photovoltaic system fails to produce enough electrical power to operate the connected end load, electricity would be drawn from the utility grid to power the load. Through net metering programs, the owner of the grid-connected photovoltaic system receives compensation for the net outflow of electrical power from the photovoltaic system into the utility grid.
Grid-connected photovoltaic systems utilize inverters, conventionally referred to as “on-grid” or “grid-connected” inverters, that transform the direct current (DC) electrical power produced by the photovoltaic system into alternating current (AC) electrical power suitable for being supplied to the utility grid and for powering any connected AC end load. Grid-connected inverters normally function to ensure that the AC electrical power supplied to the utility grid is in sinusoidal form, synchronized to the frequency of the grid, and limited to a feed voltage, i.e. the output voltage of the inverter, that is no higher than the grid voltage. One way in which the AC electrical power output from an on-grid inverter can be supplied to the utility grid and/or a connected AC end load involves connecting the inverter output to an electrical distribution panel as typically found in residential, commercial, business and/or other types of buildings or structures. The source of DC electrical input to the on-grid inverter may come from various sources including electrical energy stored in the battery bank of the photovoltaic system, flywheels and/or fuel cells, for example.
Photovoltaic systems have been designed with traditional charge controllers that do not employ maximum power point tracking (MPPT), and such controllers may be referred to as non-MPPT charge controllers. Non-MPPT charge controllers connect the PV array directly to the battery bank for charging. Usually there is a mismatch between the output voltage of the PV array and the voltage required to charge the battery bank that results in under-utilization of the maximum power output from the PV array. The reason for the mismatch is that most PV modules are rated to produce a nominal 12V under standard test conditions but, because they are designed for worse than standard test conditions, in actual fact they produce significantly more voltage. On the other hand, a nominal 12V battery for example requires close to an actual 12V (14V typically) depending on battery state of charge. When a non-MPPT charge controller is charging the battery, the PV module is frequently forced to operate at a battery voltage that is less than the optimal operating voltage at which the PV module is capable of producing its maximum power. Hence, non-MPPT charge controllers artificially limit power production to a sub-optimal level by constraining the PV array from operating at maximum output power.
A maximum power point tracking (MPPT) charge controller addresses the aforesaid disadvantage of non-MPPT charge controllers by managing the voltage mismatch between the PV array and the battery bank through the use of power electronics. The primary functions performed by MPPT charge controllers involve measuring the PV module output to find the maximum power voltage (Vmp), i.e. the voltage at which the PV module is able to produce maximum power, and operating the PV module at the maximum power voltage to extract or harvest full power (watts) from the PV array, regardless of the present battery voltage (VB).
Photovoltaic modules are made up of photovoltaic (PV) cells that have a single operating point where the values of the current (I) and voltage (V) of the cell result in a maximum power output. The maximum power voltage Vmp varies with operating conditions including weather, sunlight intensity, shading, and PV cell temperature. As the maximum power voltage Vmp of the PV module varies, the MPPT charge controller “tracks” the Vmp and adjusts the ratio between the maximum power voltage and the current delivered to the battery in order to match what the battery requires. The MPPT charge controller utilizes a control circuit or logic to search for the maximum power output operating point and employs power electronics to extract the maximum power available from a PV module.
A MPPT charge controller employs power electronics that have a higher input voltage than output voltage, hence Vmp>VB. The power electronics are conventionally designed to include a high frequency DC to DC converter that receives the maximum power voltage from the PV array as converter input and converts the maximum power voltage to battery voltage as converter output. An increase in battery charge current is realized by harvesting PV module power that would be left unharvested using a non-MPPT charge controller. As the maximum power voltage varies, the actual charge current increase that is realized will likewise vary. Generally speaking, the greater the mismatch or disparity between the PV array maximum power voltage and the battery voltage, the greater the charge current increase will be. The charge current increase will ordinarily be greater in cooler temperatures because the available power output and the maximum power voltage of the PV module increase as the photovoltaic cell temperature decreases. In addition, lower battery voltage, as in the case of a highly discharged battery, will result in a greater charge current increase.
Most MPPT charge controllers utilize power electronics designed to include a “buck” converter having topology to “buck” a higher input voltage to a lower output voltage. Buck converters, also known as “step-down” converters, are familiar in the field of power electronics and essentially include an inductor and two complementary switches to achieve unidirectional power flow from input to output. A first of the switches is ordinarily a controlled switch such as a MOSFET or transistor, and the second of the switches is ordinarily an uncontrolled switch such as a diode. The buck converter alternates between connecting the inductor to the input voltage (VA) from the PV array to store energy in the inductor and discharging the inductor into the battery bank. When the first switch is turned “on” for a time duration, the second switch becomes reverse biased and the inductor is connected to the input voltage VA. There is a positive voltage (VL) across the inductor equal to the input voltage VA minus the output voltage (V8), hence VL=VA−VB and there is an increase in the inductor current (IL). In this “on” state, energy is stored in the inductor. When the first switch is turned “off”, inductor current IL continues to flow due to the inductor energy storage, resulting in a negative voltage across the inductor (VL=−VB). The inductor current now flows through the second switch, which is forward biased, and current IL through the inductor decreases. In this “off” state, energy continues to be delivered to the output until the first switch is again turned “on” to begin another on-off cycle.
Use of a buck converter configuration in conventional MPPT charge controllers for photovoltaic systems has various disadvantages including high peak currents and voltages with attendant high power losses, and increasing control problems as the input voltage increases. At the present time, conventionally available photovoltaic system charge controllers that utilize a buck converter configuration to implement maximum power point tracking (MPPT) are limited to an input of 150V, an exception being the MPPT charge controller developed by Australian Energy Research Laboratory (AERL) which is capable of handling an input of 250V. Conventional on-grid inverters, however, operate with high voltage PV arrays up to 600V, such that presently available MPPT charge controllers for photovoltaic systems are generally unsuitable for use in grid-connected photovoltaic systems due to their inability to handle the high voltage.
A high voltage maximum power point tracking bidirectional charge controller for photovoltaic systems that have a high voltage photovoltaic array, a battery bank and a high voltage DC load comprises a bidirectional isolated DC to DC converter electrically connectable to the high voltage photovoltaic array, the battery bank, and the high voltage DC load. The bidirectional isolated DC to DC converter receives DC input from the photovoltaic array and operates in a first direction to step down the voltage of the DC input received from the photovoltaic array to obtain a stepped-down DC output of appropriate voltage to charge the battery bank. The converter receives DC input from the battery bank and operates in a second direction to step up the voltage of the DC input received from the battery bank to obtain a stepped-up DC output of the appropriate voltage for the high voltage DC load. The converter performs maximum power point tracking of the high voltage PV array while simultaneously feeding the high voltage DC load and providing interface to the battery bank. The high voltage DC load can be a grid-connected inverter for transforming DC electricity received from the charge controller into AC electricity appropriate for being supplied to a public utility grid and to a connected AC load. The bidirectional isolated DC to DC converter may include a full-bridge bidirectional isolated DC to DC converter configuration, a dual active full-bridge bidirectional isolated DC to DC converter configuration, or a parallel resonant bidirectional isolated DC to DC converter configuration. The bidirectional isolated DC to DC converter may include a first bridge, a second bridge, and a switch located along the electrical path between the photovoltaic array and the battery bank to prevent current from flowing from the battery bank back to the photovoltaic array when the voltage of the DC input from the photovoltaic array is less than the voltage of the DC output from the converter to the battery bank. When the maximum output power of the photovoltaic array is higher than the power required by the high voltage DC load, the bidirectional isolated DC to DC converter delivers the power difference to the battery bank. When the power required by the high voltage DC load is higher than the maximum power output of the PV array, the converter takes the power difference from the battery bank and delivers it to the high voltage DC load. The high voltage maximum power point tracking bidirectional charge controller tracks the maximum power operating point of the photovoltaic array, which corresponds to average power, by adjusting battery input power to match the difference between the maximum power operating point of the PV array and the input power required by the high voltage DC load. The converter operates in a back-up mode when the public utility grid is down, such that the converter supplies the stepped-up DC output from the battery bank to the inverter to power any connected AC load when the connected AC load requires more power than is available from the PV array, and the converter supplies the stepped-down DC output from the PV array to charge the battery bank, if needed, when the AC load requires less power than is available from the PV array. The high voltage maximum power point tracking bidirectional charge controller can be used in photovoltaic systems having a high voltage photovoltaic array of up to 600V.
A method of charge control for grid-connected photovoltaic systems having a high voltage photovoltaic array, a battery bank and a grid-connected inverter comprising the steps of delivering DC electricity produced by the high voltage photovoltaic array as DC input to a bidirectional DC to DC converter of a maximum power point tracking charge controller; stepping-down the voltage of the DC input in a first direction through the DC to DC converter to obtain a stepped-down DC voltage; delivering the stepped-down DC voltage from the maximum power point tracking charge controller to the battery bank; delivering DC electricity from the battery bank as DC input to the DC to DC converter of the maximum power point tracking charge controller; stepping-up the voltage of the DC input from the battery bank in a second direction through the DC to DC converter to obtain a stepped-up DC voltage; and delivering the stepped-up DC voltage from the maximum power point tracking charge controller to the grid-connected inverter.
Various objects, advantages and benefits of the invention will become apparent from the following detailed description of the invention taken in conjunction with the accompanying drawings.
A high voltage maximum power point tracking (MPPT) bidirectional charge controller 10 is illustrated diagrammatically in
The PV modules of the PV array 14 are composed of photovoltaic (PV) cells capable of converting solar energy into direct current (DC) electrical energy. The battery bank 16 is capable of storing the DC electrical energy produced by the PV array 14, and the MPPT bidirectional charge controller 10 controls charging of the battery bank 16 with the electrical energy produced by the PV array 14. The MPPT bidirectional charge controller 10 receives input voltage from the PV array 14, and output voltage from the MPPT charge controller 10 is supplied to the battery bank 16. The electrical energy produced by the PV array 14 and stored in the battery bank 16 is available to power the load 18, which supplies AC electrical output to the AC load 21 and/or the utility grid 19. The MPPT bidirectional charge controller 10 also controls the transmission of DC electrical energy from the battery bank 16 to the load 18 as explained further below. Accordingly, the MPPT charge controller 10 may be referred to as “bidirectional” since it operates in one direction to deliver DC electrical energy to the battery bank 16 from the PV array 14 and operates in the opposite direction to deliver DC electrical energy from the battery bank 16 to the load 18.
The maximum power voltage (Vmp) of the PV array 14 is the voltage where the product of current and voltage (amps×volts) is greatest, and it varies with operating conditions including weather, sunlight intensity, shading, and photovoltaic cell temperature. The MPPT bidirectional charge controller 10 employs maximum power point tracking (MPPT) to manage the disparity between the output voltage of the PV array 14 and the voltage required to charge the battery bank 16. The MPPT bidirectional charge controller 10 operates a maximum power point tracking algorithm to identify and track the maximum power voltage Vmp of the PV array 14, even as the maximum power voltage Vmp changes with operating conditions, and utilizes power electronics that have a higher input voltage VA than output voltage VB to adjust the ratio between the maximum power voltage Vmp and the current delivered to the battery bank 16 in order to match what the battery bank requires. The maximum power point tracking algorithm, which is fully automatic, tracks the maximum power voltage Vmp as it varies and ensures that maximum power is harvested from the PV array 14 throughout the course of each day. Any appropriate MPPT algorithm may be used in the MPPT bidirectional charge controller 10 to effectuate maximum power point tracking of the PV array, including conventional MPPT algorithms. The power electronics used in the MPPT bidirectional charge controller 10 receives the Vmp from the PV array 14 as input VA and converts the Vmp to battery voltage VB as output. In addition, the power electronics used in the MPPT bidirectional charge controller 10 controls the transmission of DC electrical energy from the battery bank 16 to the load 18 by converting the DC electrical energy stored in the battery bank 16 to DC electrical energy of the appropriate voltage for the load 18. Where the load 18 includes a conventional on-grid inverter, the charge controller 10 converts DC electricity from the battery bank 16 into DC electricity of sufficiently high voltage for the on-grid inverter.
In order to lay the groundwork for understanding the approach taken in the MPPT bidirectional charge controller 10, it is helpful to consider prior DC to DC converter configurations for charge controllers used in PV systems.
When the switch Sw1 is turned on, the inductor L is connected to the input voltage VA(IN) and the switch Sw2 becomes reverse biased or turned off, resulting in a positive voltage VL across the inductor equal to VA(IN)−VB(OUT) and an increase in the inductor current IL. Furthermore, when the switch Sw1 is on, the input current IA is equal to the inductor current IL (IA=IL), and the current Isw2 across switch Sw2 is equal to zero. In this “on” state, energy is stored in the inductor L. When the switch Sw1 is turned off, inductor current IL continues to flow due to the inductor energy storage, resulting in a negative voltage VL across the inductor equal to −VB(OUT). The inductor current now flows through the switch Sw2, which is forward biased or turned on, and current IL through the inductor decreases. The input current IA is now equal to zero and the current Isw2 across switch Sw2 is equal to the inductor current IL. In this “off” state, electrical energy continues to be delivered as output until the switch Sw1 is again turned on to begin another on-off switching cycle.
If a high voltage PV array is used to supply VIN to the DC to DC converter 20 depicted in
where η is the converter's efficiency and the input voltage VIN is the high voltage input received from the high voltage PV array.
The foregoing principle is further understood with reference to
In addition to voltage stepping-down applications as described above, DC to DC converters have been used in the past to “boost” or “step-up” a lower input voltage to a higher output voltage, and these types of DC to DC converters may be referred to as step-up or boost converters.
In some voltage stepping-up applications, and stepping-down applications depending on transformer ratio, a unidirectional transformer-isolated DC to DC converter configuration may be employed in DC to DC converters instead of a standard boost converter or buck converter configuration.
where η is the converter's efficiency.
The diagram of
where η is the converter's efficiency. As a result, power losses and cost for the isolated DC to DC converter 120 of
In the high voltage MPPT bidirectional charge controller 10 of the present invention, the isolated DC to DC converter 220 is implemented to include a bidirectional isolated DC to DC converter configuration 228 as shown in
The high voltage DC load 18 may be an on-grid inverter 30, which may be a single-phase or three-phase on-grid inverter as depicted in
switch S at location B closed,switch S′ at location D open=Vin−Vout;
switch S at location B open,switch S′ at location D closed=Vin.
The foregoing arrangement is advantageous in order to extend the low input voltage operating range by the amount of Vout.
Inasmuch as the present invention is subject to many variations, modifications and changes in detail, it is intended that all subject matter discussed above or shown in the accompanying drawings be interpreted as illustrative only and not be taken in a limiting sense.
This application claims priority from provisional patent application Ser. No. 61/253,538, filed Oct. 21, 2009, the entire disclosure of which is incorporated herein by reference.
Number | Date | Country | |
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61253538 | Oct 2009 | US |