Solar Peak Power Trackers
The Microship uses an 8 channel Peak Power Tracker system to maximize solar power transfer into the system battery -- this gives us significant gain over a direct connection. The PPT (Peak Power Tracker) consists of 8 independent DC/DC converters controlled by the PWM outputs of the Motorola DSP56F807. The eight DC/DC converters are connected to eight 60-watt solar panels mounted on the boat. The electricity generated is used to charge the main system battery and run an electric thruster (via another processor that handles power control) to propel the Microship at an estimated 5.5 knots under optimum conditions.
In most photovoltaic applications the solar panels are connected directly to the battery for charging, but this results in certain inefficiencies. Solar panels used in 12-volt applications have very distinct power curves (volts x amps = watts). A graph of voltage vs. current will range from maximum current at zero volts (no power) to maximum volts at zero current (also no power). Somewhere in the middle there will be the Maximum or Peak Power Point (MPP) where the maximum output wattage can be extracted from the solar panel. In solar panels designed for 12 volts systems this MPP is usually between 16 and 17 volts.
The inefficiencies in this system arise when the solar panel is connected directly to the battery. The 12 volt battery fixes the solar panel output voltage to itís own output voltage, usually between 11.9 and 14.4 volts. Since this is not the MPP of the solar panel some power is lost, resulting in lower efficiencies. The solution to this problem is to add a DC/DC converter to step down the voltage of the solar panel at MPP to the 12 volts of the battery. Using this PPT configuration up to 20% more power can be gained from a solar panel array.
The main problem in using DC/DC converters for PPT is figuring out what the MPP for the solar panel is and controlling the DC/DC converter to match that voltage to the battery voltage. This problem is complicated by the fact the MPP for the solar panel continuously changes based on numerous factors including the amount of solar energy falling on the panel, the temperature of the panel, and partial shading. Our solution is to use the Motorola DSP56F807 to sense the voltage and current of each panel and calculate the wattage. An iterative algorithm is then employed to vary the PWM duty cycle of the DC/DC converter until the maximum wattage is found. Since DSP56F807 is constantly searching for the maximum wattage on each channel, all eight solar panels will be operating at maximum efficiency for the current conditions. The DSP56F807 is well suited to this application due to its 8 PWM outputs and 16 A/D inputs.
A photo of the complete power management subsystem is here.
You can find lots of information on the Motorola website about the DSP56F807, with links from there to PDF data sheets, appnotes, and much more.
Finally, you can view Tim's schematic of the Microship's Peak Power Tracking System, as well as a more general block diagram of the whole Microship power system
