Optically-controlled shunt circuit for maximizing photovoltaic panel efficiency

An optically-controlled shunt (OCS) circuit includes a switch and a light sampler. The light sampler is coupled to the switch and is configured to sample light at a photovoltaic (PV) cell corresponding to the OCS circuit and to turn on the switch when the sampled light comprises insufficient light for the PV cell. The light sampler may also be configured to turn off the switch when the sampled light comprises sufficient light for the PV cell. The light sampler may further be configured to partially turn on the switch when the sampled light comprises adequate light for the PV cell and to turn off the switch when the sampled light comprises full light for the PV cell. The switch could include a transistor, and the light sampler could include a photodiode.

TECHNICAL FIELD

This disclosure is generally directed to photovoltaic systems. More specifically, this disclosure is directed to an optically-controlled shunt circuit for maximizing photovoltaic panel efficiency.

BACKGROUND

Solar and wind energy provide renewable, non-polluting energy sources, as opposed to conventional non-renewable, polluting energy sources, such as coal or oil. Because of this, solar and wind energy have become increasingly important as energy sources that may be converted into electricity. For solar energy, photovoltaic panels arranged in an array typically provide the means to convert solar energy into electrical energy.

In operating a photovoltaic array, maximum power point tracking (MPPT) is generally used to automatically determine a voltage or current at which the array should operate to generate a maximum power output for a particular temperature and solar irradiance. Generally, an array includes strings of panels, with the least efficient panel in a string determining the current and efficiency for the entire string.

Shading over a panel in a string introduces resistance in the string. Thus, the shading blocks the flow of current and lowers the power output. One such blockage in the string can lower the available power significantly. Currently available MPPT techniques can observe the available optimum power for each panel and bypass the flow of current, optimizing a cost function to maximize the power flow by “removing” the high-impedance shaded panel from a string of panels. However, while removing a partially-shaded panel increases the efficiency of the string, it also results in the inability to use the energy that is generated by the cells that are not shaded in the panel.

DETAILED DESCRIPTION

FIG. 1illustrates an array100of photovoltaic (PV) panels102in accordance with one embodiment of this disclosure. As described in more detail below, at least one of the panels102comprises bypassable cells104that are capable of being bypassed when shaded.

The PV panels102in the array100are arranged in strings. For the illustrated embodiment, the array100comprises two strings, with each string comprising three panels102. However, it will be understood that the array100may comprise any suitable number of strings of panels102, and each string may comprise any suitable number of panels102. Also for the illustrated embodiment, the panels102in each string are implemented in a series connection.

Each PV panel102is capable of converting solar energy into electrical energy. A DC-AC converter106may be coupled to the array100and is capable of converting the direct current (DC) generated by the panels102into an alternating current (AC) for a load (not shown inFIG. 1), which may be coupled to the DC-AC converter106.

For some embodiments, maximum power point tracking (MPPT) may be implemented for the entire array100and/or for each panel102. MPPT automatically determines a voltage or current at which the array100(or panel102) should operate to generate a maximum power output for a particular temperature and solar irradiance. For example, for a particular embodiment, each of the panels102may be coupled to a corresponding MPPT device (not shown inFIG. 1) that is capable of providing MPPT for that panel102.

For the illustrated embodiment, the panel102acomprises a plurality of bypassable cells104arranged in a string, with each of the bypassable cells104comprising a PV cell108and a corresponding optically-controlled shunt (OCS) circuit110. It will be understood that each of the panels102a-fmay comprise bypassable cells104. In addition, for an alternative embodiment, any one or more of the panels102b-fmay comprise PV cells108without corresponding OCS circuits110instead of bypassable cells104. Also, any of the panels102a-fmay comprise a combination of bypassable cells104and PV cells108.

Each of the PV cells108is capable of generating electrical energy based on solar energy. Each OCS circuit110is capable of sampling the solar energy received at the corresponding PV cell108and bypassing that PV cell108when the sample indicates that the PV cell108is shaded and, therefore, incapable of generating electrical energy in the current lighting conditions.

As described in more detail below, for some embodiments, each OCS circuit110may provide a non-variable bypass for its corresponding PV cell108. For these embodiments, the OCS circuit110may be activated when insufficient light exists, thereby bypassing the PV cell108, or deactivated when sufficient light exists, thereby not bypassing the PV cell108. For this case, sufficient light is light that provides enough energy for the PV cell108to operate, while insufficient light is light that fails to provide enough energy for the PV cell108to operate.

For other embodiments, each OCS circuit110may provide a variable bypass for its corresponding PV cell108. For these embodiments, the OCS circuit110may be (i) fully activated when insufficient light exists, thereby completely bypassing the PV cell108, (ii) partially activated when adequate light exists, thereby partially bypassing the PV cell108, or (iii) deactivated when full light exists, thereby not bypassing the PV cell108. For this case, full light is light that provides enough energy for the PV cell108to operate at substantially full capacity. Adequate light is light that provides enough energy for the PV cell108to operate, though not enough to operate at full capacity. Insufficient light is light that fails to provide enough energy for the PV cell108to operate.

As used herein, “completely bypassed” and “bypassed” mean substantially bypassed, “not bypassed” means substantially not bypassed, and “partially bypassed” means less than substantially bypassed and more than substantially not bypassed.

FIG. 2illustrates a bypassable cell104in accordance with one embodiment of this disclosure. For this embodiment, the OCS circuit110comprises a light sampler202coupled to a switch204. The light sampler202may comprise a photodiode or other suitable light-sensitive component. The switch204may comprise a PMOS transistor, an NMOS transistor or any other suitable component capable of being operated as a switch. The light sampler202is capable of turning the switch204off or on (or partially on, depending on the particular embodiment). As described in more detail below, the OCS circuit110may also comprise an optional non-linear amplifier206coupled to the light sampler202and the switch204. For some embodiments, the OCS circuit110may be implemented in the form of a chip that is surface mounted across the terminals of the PV cell108.

The light sampler202, which is in relatively close proximity to the PV cell108, is capable of sampling the light208available at the PV cell108and generating an activation signal210based on the available light208. The switch204is capable of receiving a switch signal212based on the activation signal210and may be capable either of switching on or off or of switching on, partially on, or off based on the switch signal212. For embodiments omitting the non-linear amplifier206, the activation signal210may be the same as the switch signal212.

When the switch204is turned off, the OCS circuit110is deactivated and the PV cell108is not bypassed. When the switch204is turned on, the OCS circuit110is activated and the PV cell108is bypassed. For some embodiments, the OCS circuit110may be either activated or deactivated. For other embodiments, the OCS circuit110may be fully activated, partially activated or deactivated. For these embodiments, the light sampler202is capable of partially turning on the switch204in order to partially activate the OCS circuit110, thereby partially bypassing the PV cell108. In this case, the activation signal210may be capable of indicating the amount of light208available at the PV cell108.

For some embodiments, the relationship between the maximum power output from the array100and the output of a PV cell108is non-linear. Thus, for these embodiments, it may be desirable to accommodate this non-linearity via the signal212applied to the switch204. For embodiments in which the PV cell108may be partially bypassed, therefore, the OCS circuit110may comprise an optional non-linear amplifier206coupled between the light sampler202and the switch204.

For these embodiments, the non-linear amplifier206is capable of receiving the activation signal210generated by the light sampler202that indicates the amount of light208available at the PV cell108. Based upon the amount of available light208relative to full light and no light, the non-linear amplifier206is capable of non-linearly amplifying the activation signal210to generate the switch signal212for the switch204. The gain curve of the non-linear amplifier206may be optimized such that the power output for the array100is maximized.

FIG. 3illustrates details of a bypassable cell104in accordance with one embodiment of this disclosure. For this particular embodiment, the light sampler202comprises a photodiode302and a biasing resistor304, and the switch204comprises a PMOS transistor (the optional non-linear amplifier206is not shown inFIG. 3). The PMOS transistor204is capable of conducting the maximum string current for a string of PV cells108.

When insufficient light208is available for the entire panel102that comprises the illustrated PV cell108, such as at night or when that panel102is completely shaded, no photo current is generated by the panel102. Thus, no current is flowing and no power is being generated.

However, when light208is available for at least a portion of the panel102, including for the illustrated PV cell108, the photodiode302essentially samples the light208at the PV cell108by being exposed to that light208, which results in the photodiode302being turned on. In this case, the gate-to-source voltage (VGS) of the PMOS transistor204is held low by the conducting photodiode302. The PMOS transistor204is thus held in an off state, which deactivates the OCS circuit110, allowing the PV cell108to generate power in a normal manner.

When light208is available for at least a portion of the panel102, but is not available for the illustrated PV cell108, the photodiode302samples that unavailable light208at the PV cell108, which results in the photodiode302being turned off. In this case, the gate-to-source voltage (VGS) of the PMOS transistor204, which is biased by the voltage divider defined by the photodiode302and the biasing resistor304, increases. Thus, the PMOS transistor204is in an on state, which either partially or fully activates the OCS circuit110depending on the particular embodiment, and the PV cell108is at least partially bypassed.

FIG. 4is a graph400illustrating an example of voltage variation with light for the optically-controlled shunt (OCS) circuit110ofFIG. 3in accordance with one embodiment of this disclosure. For this embodiment, a non-variable bypass is provided for the PV cell108by the OCS circuit110. Thus, the OCS circuit110is either activated or deactivated.

When light208is fully available for the PV cell108, the gate-to-source voltage (VGS) of the PMOS transistor204is low, and the PMOS transistor204is off. In this case, the OCS circuit110is deactivated and the PV cell108is not bypassed. However, as the light208decreases, the impedance of the photodiode302increases and VGSbegins to rise.

For the illustrated embodiment, the PMOS transistor204may be substantially off when the light208is above a sufficient light threshold (LSuff) that corresponds to a voltage threshold (Vth) for VGSof the PMOS transistor204. When the light208drops below LSuff, raising VGSabove Vth, the PMOS transistor204may be substantially on. In this case, the OCS circuit110is activated and the PV cell108is bypassed.

FIG. 5illustrates a method500for bypassing a cell108in a PV panel102using the OCS circuit110in accordance with one embodiment of this disclosure. Initially, the light sampler202samples the light208received at the PV cell108(step502). For example, the photodiode302may be exposed to the light208available at the PV cell108.

If the PV cell108is receiving sufficient light208for operation (step504), the light sampler202deactivates the OCS circuit110by turning off the switch204(step506). For example, the photodiode302may be turned on by the available light208, causing the gate-to-source voltage of the PMOS transistor204to be held low. This turns off the PMOS transistor204, deactivating the OCS circuit110. As a result, the PV cell108may function normally.

However, if the PV cell108is receiving insufficient light208for operation (step504), the light sampler202activates the OCS circuit110by turning on the switch204(step508). For example, the photodiode302may be turned off by the lack of available light208, causing the gate-to-source voltage of the PMOS transistor204to increase. This turns on the PMOS transistor204, activating the OCS circuit110. As a result, the PV cell108is bypassed.

The light sampler202continues to sample the light208at the PV cell108(step502) in order to make adjustments to the OCS circuit110based on changing light208conditions. In this way, a non-variable bypass of the PV cell108may be provided. As a result, when shaded, the PV cell108does not represent a blockage to the overall flow of power, resulting in the power delivery of the panel102being maximized.

FIG. 6is a graph600illustrating an example of voltage variation with light for the optically-controlled shunt (OCS) circuit110ofFIG. 3in accordance with another embodiment of this disclosure. For this embodiment, a variable bypass is provided for the PV cell108by the OCS circuit110, i.e., as the light208decreases or increases, the PMOS transistor204may be gradually switched between an off state, a variable partially on state, and a fully on state. Thus, the OCS circuit110is either deactivated, partially activated or fully activated.

When full light (LFullor more) is available for the PV cell108, the gate-to-source voltage (VGS) of the PMOS transistor204is low (VFullor lower), and the PMOS transistor204is turned off. In this case, the OCS circuit110is deactivated, and the PV cell108is not bypassed.

However, as the light208decreases, the impedance of the photodiode302increases and VGSbegins to rise. When the light208drops below full light but remains higher than adequate light (LAd), VGSincreases to more than VFulland less than VAd. In this case, the PMOS transistor204is partially turned on, which partially activates the OCS circuit110such that the PV cell108is partially bypassed. The amount that the PMOS transistor204is partially turned on is a function of the available light208. For example, the PMOS transistor204is mostly turned off when the light208is near LFulland mostly turned on when the light208is near LAd. For some embodiments, the optional non-linear amplifier206may be used to provide a non-linear reaction in the PMOS transistor204to the decreasing or increasing available light208.

As the light208continues to decrease, the impedance of the photodiode302continues to increase and VGScontinues to rise. When the light208drops below LAd, VGSincreases above VAdand the PMOS transistor204is turned on. In this case, the OCS circuit110is fully activated, and the PV cell108is completely bypassed.

Similarly, as the available light208increases, the PV cell108may change from being completely bypassed to partially bypassed when the light208increases above LAdand from being partially bypassed to not bypassed when the light208increases above LFull.

FIG. 7illustrates a method700for bypassing a cell108in a PV panel102using the OCS circuit110in accordance with another embodiment of this disclosure. Initially, the light sampler202samples the light208received at the PV cell108(step702). For example, the photodiode302may be exposed to the light208available at the PV cell108.

If the PV cell108is receiving full light (step704), the light sampler202deactivates the OCS circuit110by turning off the switch204(step706). For example, the photodiode302may be turned on by the available light208, causing the gate-to-source voltage of the PMOS transistor204to be held low. This turns off the PMOS transistor204, deactivating the OCS circuit110. As a result, the PV cell108may function normally.

If the PV cell108is not receiving full light (step704) but is receiving adequate light for operation (step708), the light sampler202partially activates the OCS circuit110by partially turning on the switch204(step710). For example, the photodiode302may be partially turned on by the available light208, causing the gate-to-source voltage of the PMOS transistor204to partially increase. This partially turns on the PMOS transistor204, which partially activates the OCS circuit110. As a result, the PV cell108may be partially bypassed.

For some embodiments, this partial activation of the PMOS transistor204may be provided based solely on the partial activation of the photodiode302. For other embodiments, the optional non-linear amplifier206may non-linearly amplify the activation signal210from the partially turned-on photodiode302to generate the switch signal212for the PMOS transistor204.

If the PV cell108is receiving insufficient light for operation (step708), the light sampler202fully activates the OCS circuit110by fully turning on the switch204(step712). For example, the photodiode302may be turned off by the lack of available light208, causing the gate-to-source voltage of the PMOS transistor204to increase. This fully turns on the PMOS transistor204, fully activating the OCS circuit110. As a result, the PV cell108is completely bypassed.

The light sampler202continues to sample the light208at the PV cell108(step702) in order to make adjustments to the OCS circuit110based on changing light208conditions. In this way, a variable bypass of the PV cell108may be provided. As a result, when shaded, the PV cell108does not represent a blockage to the overall flow of power, resulting in the power delivery of the panel102being maximized.

AlthoughFIGS. 5 and 7illustrate examples of methods500and700for bypassing a cell108in a PV panel102, various changes may be made to these methods500and/or700. For example, while the methods500and700were partially described with reference to the OCS circuit110ofFIG. 3, the methods500and/or700may be implemented using any other suitable implementations of the OCS circuit110. Also, while shown as a series of steps, the steps in the methods500and/or700may overlap, occur in parallel, occur multiple times, or occur in a different order.