Utilization of sustainable energy sources, as alternatives to petroleum sources, has become an increasingly important objective. Solar cells transform energy from an essentially unlimited source into useable electricity. The level of energy from the sun that is available at the solar cell location is variable in accordance with changing shade conditions and atmospheric effects. The optimum power point at which the solar cells can operate varies with these changing conditions. Direct connection of solar cells to batteries or inverters in grid-tie systems rarely allows optimum power transfer. The need thus exists for a maximum power point tracker that can facilitate load transformation of power from a solar source at its optimum power point operation.
A typical one hundred twenty watt solar panel contains forty eight photovoltaic cells connected in series, and bypass diodes connected in parallel with each group of twenty four cells. With uniform isolation and evenly distributed sunlight, a power-voltage curve can be obtained such as shown in FIG. 1a. The curve is derived by applying a voltage to the solar panel that is varied from zero (or short circuit condition) to a maximum (or open circuit condition) and detecting the power, as a function of current drawn over the voltage range. Maximum power is obtained at a clearly defined voltage level. Under partial shading conditions, however, there can exist multiple local maxima on the power-voltage or power-current curve of a solar panel. FIG. 1b is a power-voltage curve for the solar panel under weak partial shading. FIG. 1c is a power-voltage curve for the solar panel under strong partial shading. As all cells in the series chain must pass the same current, local maxima are created at each cell's optimum current level. As current increases, shaded cells are bypassed, cutting their power output, while power from the remaining cells increases.
Typical schemes for solar panel operation have ignored the problem of multiple global maxima, deeming such detection too difficult to solve without the use of expensive, complex elements such as analog to digital converters and microprocessors. One such approach would be to operate the solar panel at a set percentage of maximum voltage, based on an assumption that such voltage level approximates the point of maximum power output. However, with inevitable variability of sunlight conditions, operation will often be at less than maximum available power output.
The need exists for efficient and inexpensive tracking of a characteristic that is variable over an operating range and identifying a point in the range at which the characteristic is a maximum, or minimum. A particular need exists for a maximum power point tracker that can determine a global maximum power point and can avoid large space consuming hardware and costly complex components.