Small power sources such as photovoltaic cells, and in particular solar cells, are being used to power portable devices or to charge a battery pack. The well-known boost chopper power converter circuit has been used in conjunction with such power sources in portable applications. This type of power converter takes input current at a low voltage, e.g., a single silicon solar cell has an output of about 0.5 volts, and produces an output current at a higher voltage that can be used to operate handheld electronics or charge a battery pack.
When it is desired to use the power of multiple power sources the outputs of the individual sources have to be combined. Traditionally, the power sources such as like-type silicon solar cells have been connected in some series-parallel combination to provide the voltage and current outputs required for the particular application. For example, one type of solar cell battery charger connects the solar cells in series to approximate the battery voltage. The output is then fed into the battery through a diode. However, this architecture is inefficient and the rechargeable batteries quickly wear out. Another approach simulates the AC outlet that users can plug into with a solar cell powered equivalent. This architecture is inherently inefficient since the battery chargers incorporated in most consumer electronics are not designed for efficient operation with limited sources of power.
A photovoltaic cell connected to an electrical load such as a resistor acts as a power-limited current source. Its current output depends on the rate of incident radiation and the temperature of the cell. A typical plot of cell current versus cell voltage for a photovoltaic cell is shown in FIG. 1.for five different incident radiations. Curve A corresponds to an incident radiation of 1000 W/m2, curve B—800 W/m2, curve C—600 W/m2, curve D—400 W/m2 and curve E—200 W/m2. These curves clearly show that cell current depends on the degree of incident radiation. The cell output current collapses if the load attempts to draw too much power from the cell. A typical plot of output power versus cell voltage for a photovoltaic cell is shown in FIG. 2 for the same five different incident radiations used in FIG. 1. Curve A corresponds to an incident radiation of 1000 W/m2, curve B—800 W/m2, curve C—600 W/m2, curve D—400 W/m2 and curve E—200 W/m2. There is a specific operating point, the maximum power point, MPP, with a specific voltage, VMPP, and current, IMPP, at which maximum power output is produced from the cell. As apparent from FIG. 2, the MPP is a function of the incident radiation. The MPP is also a function of the cell temperature.
It is most efficient, and therefore highly desirable to use Maximum Power Point Tracking (MPPT), i.e., to operate a photovoltaic cell at its MPP. This means finding and tracking the VMPP and IMPP. It is even more challenging to accomplish MPPT for a photovoltaic array made up of many solar cells that operate under slightly different temperature and irradiance conditions. Two sensors are usually required to measure photovoltaic cell voltage and photovoltaic cell current. Current sensors are typically more expensive and bulky than voltage sensors.
To obtain maximum power from a photovoltaic array of solar cells, one has to perform MPPT on every solar cell. The number of sensors and hardware complexity required to run MPPT on every solar cell in the array is prohibitively expensive. Instead, MPPT is usually performed on one solar cell or on one group of connected solar cells in the array and the results applied to the whole array.
Recent solar cell architectures use multiple cells that efficiently absorb different portions of the solar spectrum and thereby achieve higher overall conversion efficiency than traditional solar cells using a single type of cell. Since the cells and their characteristics are not identical, to obtain maximum power from such a solar cell architecture MPPT must be carried out for each cell.
Thus there is a continuing need for a method for MPPT a photovoltaic cell, an array of photovoltaic cells and an architecture of different type photovoltaic cells.