Solar cells and power, Part 2 – power extraction

Part 1 looked at the solar cells themselves; Part 2 looks at how a cell or panel is managed for maximum performance.

Q: How can the useful output of a solar cell or panel be maximized?

A: While an active, in-use load or a power-generating facility could be connected directly to the output of a cell or panel, this is feasible only in very unusual applications where power needs are low and non-critical. To understand the reality of solar-cell output, begin with the simplified equivalent circuit, Figure 1, where R_s is a resistance in series with the diode and R_sh is a resistance in parallel with the diode. The objective is to match the source and the load such that maximum power is transferred.

As the amount of impinging sunlight changes, the load characteristic that results in the largest amount of power transfer efficiency changes. Therefore, the efficiency of the system is optimized when the load characteristic changes to keep the power transfer at the highest efficiency. This load characteristic is called the maximum power point, and the challenge is to find this point and keep the load characteristic there.

This situation is analogous to how an RF source (the power amplifier or PA) must be matched to its load (the antenna) for maximum power transfer and minimum VSWR (voltage standing wave ratio) via a specially designed matching circuit. The main difference between RF and solar is that the RF situation is usually static, as the source and load impedances are fixed. Therefore, once the matching circuit is defined and fabricated, the work is done. This is unlike the case with solar cells, where the matching function must track operational changes.

Q: What’s the matching situation for a solar cell?

A: For the solar cell, the parameters of the source change with impinging light intensity, temperature, and many other factors. PV cells and panels have a nonlinear voltage-current characteristic curve, where the I-V relationship spans the short-circuit current (I_SC) at zero volts, all the way to zero current at the open circuit voltage (V_OC), Figure 2.

There is a point on this curve (called the “knee”) where the panel produces its maximum electrical power. To produce maximum DC output power, the power-management interface circuitry must constantly adjust the load, trying to meet the specific point on the I-V curve that produces maximum DC power.

Q: How is this adjustment done?

A: The general name for the dynamic matching technique is called maximum power point tracking, or MPPT, Figure 3. There are many ways to implement MPPT, ranging from all-hardware circuitry, a combination of circuitry and software, or an approach based almost entirely on software and various algorithms.

Q: What are some of the commonly used MPPT approaches?

A: There are several in wide use, each offering differences in implementation difficulty, consistency of results, scalability, and cost.

In the “perturb and observe” approach, the controller “dithers” the voltage from the array by a small amount and measures the power. If the power increases, the controller makes further adjustments in that direction until power no longer increases (this method is also called “hill climbing”). This is easy to implement but can result in oscillations of power output.

In the “incremental-conductance” method, the controller measures incremental changes in PV array current and voltage and then tries to predict the effect of a voltage change. The incremental conductance method compares the incremental conductance (I_Δ / V_Δ) to the array conductance (I / V). When these two are the same, then the output voltage is the MPP voltage. It requires more numerical processing but is faster at tracking changing conditions than perturb and observe. It, too, can produce oscillations in power output.

The “constant voltage” or “open voltage” MPPT approach method momentarily interrupts the power delivered to the load is momentarily interrupted and measures the open-circuit voltage at zero current is measured. It then resumes operation with the voltage controlled at a fixed ratio of the open-circuit voltage, which is usually a value which has been determined to be the maximum power point, either based on test data or simulation and modeling, for expected operating conditions.

Q: Where is the MPPT function located in a solar-array system?

A: This is another “it depends” question. Since each solar cell or small panel has different characteristics than others in the array, ideally there would be an MPPT installation for each panel. In larger solar arrays, this is costly, so an MPPT unit may be placed to support a group. At the other extreme, a single MPPT function can be used for the entire array, but there will be a loss in the overall efficiency which can be achieved. This is a tradeoff that must be made on the basis of how much inefficiency can be tolerated, and at what is the cost for improvement (and how much improvement will that yield).

References