MPPT - Finding the Sweet Spot

Finding the Sweet Spot
HP Online


All PV cells, modules, and arrays have conditions under which they will perform optimally. Two of the most important of those conditions—amount of sunlight and PV cell temperature—are difficult to control. Ambient temperatures and heat from sun’s rays increase cell temperature, affecting how well a cell converts available light to electrical energy. To date, beyond providing an air gap behind an array, no practical, cost-effective means of cooling cells have been found. Our only recourse is to adjust the module’s electrical condition.

Maximum power point tracking (MPPT) optimizes a module’s power output within the range allowed by its temperature and available sunlight. Every PV module has particular characteristics that are described by I-V curves—a graphed representation of current (I, in amps) versus voltage (V). Power is a product of current and voltage (watts = amps × volts). While the curve changes based on temperature and sunlight, there is only one maximum power point (MPP) on an I-V curve.

Different electronic algorithms can achieve MPPT, but what all methods do is periodically vary electrical conditions and test power output. If the power increases, it is varied a little more until the maximum is attained. If the tested power decreases, then the condition is varied in the other direction to find that maximum power point. That “sweet spot” is maintained until it’s time to again vary conditions and retest. A re-sweep of the I-V curve typically happens every few minutes or when there is a significant power change, though it can vary depending on the tracking method. Since the array is not operating at the MPP during the I-V curve testing, it makes sense to spend as little time as possible during the sweep.

MPPT equipment also varies. Most batteryless grid-tied systems use string inverters, which test and adjust to the MPP for the entire array. Most battery-charging systems rely on a single MPPT charge controller to adjust the entire array. But no two PV modules share the exact characteristics—slight differences between I-V curves, cell temperature, and sunlight (including shading) vary from one place in the array to another. That’s where module-level MPPT comes in.

Microinverters, which are paired to each module, do this automatically. But in a system with a central string inverter, a DC optimizer would be required for each module. A system using optimizers might enjoy an increase of a few percent up to 25% or more in power output, depending on shading and other specifics. A battery-based system would similarly benefit from optimizers. In the case of module-level MPPT, the expense of the equipment should be weighed against the potential gain in energy harvest. Each module will require an optimizer, which can add from a few to several hundred dollars to the system’s overall cost. Optimizer implementation also adds complexity, installation time, and more points of potential failure.

Understanding what MPPT is (and what your equipment options are for various PV system types and scenarios) will help you make an informed choice for your particular situation.

Citation:This article appears in HP online magazine.