Maximum Power Point Tracking

ECE 371Sustainable Energy Systems

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Finding the Maximum Power Point Seems Easy

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Until you actually tryto do it…

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Variation of MPP with Insolation(Single Panel)

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Variation of MPP with Temp(Single Panel)

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Array of Panels Some Panels Shaded

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Array of Panels Some Panels Shaded

Array of Panels Some Panels Different

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Array of Panels Some Panels Different

Credits

Most of the following slides are paraphrased or directly copied from:

“Comparison of Photovoltaic Array Maximum Power Point Tracking Techniques,” TrishanEsram and Patrick L. Chapman, IEEE Transactions on Energy Conversion, Vol. 22, No.2, June 2007.

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MPPT is a Current Research Topic

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Hill Climbing / Perturb and Observe (P&O)

Change the amount of current that we are drawing from the solar.

If the power goes up, keep changing the current in the same direction: Increase Keep Increasing Decrease Keep Decreasing

If the power goes down, change the direction: Increase Decrease Decrease Increase

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Perturb and Observe

If we are to the right of the MPP, then increasing I decreases the power. If we are to the left of the MPP, then increasing I increases the power. If we are to the right of the MPP, then decreasing I increases the power. If we are to the left of the MPP, then decreasing I decreases the power.

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P&O Issues Obviously this simple algorithm can be fooled

by local maxima. It gets much more complicated when the I-V

curve changes with insolation and temperature. Always stepping the current or voltage results in

the algorithm oscillating about the MPP. To reduce oscillations, need a small step.

When far away from the MPP, need a large step to converge quickly.

To fix, use a variable step size.15

Simulink Model

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Diagnostics

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Power Tracker

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Power Tracker

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Control

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Results

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Results

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Incremental Conductance

The incremental conductance method is based on the fact that the slope of the PV array power curve is zero at the MPP, positive on the left of the MPP, and negative on the right, as given by dP/dV = 0, at MPP dP/dV > 0, left of MPP dP/dV < 0, right of MPP.

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Incremental Conductance

Since

Our conditions can be written as dP/dV = 0, ΔI/ Δ V = -I/V at the MPP dP/dV > 0 , ΔI/ Δ V > -I/V to the left of MPP dP/dV < 0 , ΔI/ Δ V < -I/V to the left of MPP.

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𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑

=𝑑𝑑 𝐼𝐼𝑑𝑑𝑑𝑑𝑑𝑑

= 𝐼𝐼 +𝑑𝑑𝐼𝐼𝑑𝑑𝑑𝑑

≅ 𝐼𝐼 + 𝑑𝑑∆𝐼𝐼𝛻𝛻𝑑𝑑

Incremental Conductance

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Incremental Conductance The MPP can tracked by comparing the

instantaneous conductance (I/V) to the incremental conductance (ΔI/ΔV)

Vref is the voltage at which the PV array is forced to operate. At the MPP, Vref = VMPP.

At the MPP, the array is maintained at this point unless a change occurs.

The algorithm decrements or increments Vref to track the new MPP.

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Fractional Open Circuit Voltage There is a near linear relationship between the PV

panel voltage at the MPP and the panel voltage when open circuited. VMPP is linearly related to VOC

Really? Is this true?

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I-V Plot with Varying Insolation

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Collected Data

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Is the Relationship Linear?Varying Insolation

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MPP Variation with Temperature

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MPP Variation with Temperature

Vmpp versus VOC With Varying Temperature

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Fractional Open Circuit Voltage The near linear relationship between VMPP and VOC of

the PV array, under varying irradiance and temperature levels, has given rise to the fractional VOC

method.

𝑘𝑘 is a constant of proportionality and must be computed empirically for the solar panel. (The slope of our plot from the previous slide.)

The power point is checked by periodically shutting down the array and measuring VOC.

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𝑑𝑑𝑀𝑀𝑀𝑀𝑀𝑀 ≈ 𝑘𝑘𝑑𝑑𝑂𝑂𝑂𝑂

Fractional Open Circuit Voltage Disadvantages:

Must periodically shut down the array to measure VOC. Can be avoided by using pilot cells. (Separate cells used as

sensors.

Since the relationship is approximate, the array is never really operated at the MPP

Not good under partial shading. May need to perform I-V sweeps.

Advantages Inexpensive to implement.

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Fractional Short Circuit Current There is a near linear relationship between the PV

panel current at the MPP and the panel current when short circuited. IMPP is linearly related to ISC

Really? Is this true?

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Is the Relationship Linear?Varying Insolation

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Varying Temperature

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Fractional Open Circuit Voltage The near linear relationship between IMPP and ISC of the

PV array, under varying irradiance and temperature levels, has given rise to the fractional ISC method.

𝑘𝑘 is a constant of proportionality and must be computed empirically for the solar panel. (The slope of our plot from the previous slide.)

The power point is checked by periodically shorting out the array and measuring ISC.

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𝐼𝐼𝑀𝑀𝑀𝑀𝑀𝑀 ≈ 𝑘𝑘𝐼𝐼𝑆𝑆𝑂𝑂

Fractional Open Circuit Voltage Disadvantages:

Requires an additional switch to short out the array. (Possibly)

Must periodically shut down the array to measure VOC. Since the relationship is approximate, the array is never

really operated at the MPP Not good under partial shading. Must perform I-V sweeps.

Advantages Inexpensive to implement computationally.

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