PV Inverter

Work in progress! PIC based 50Hz square wave generator. DIL switches set the duty cycle - here set to 50% (0x80).

12 volt DC input current is 1A, with AC load of 12W - so it's 100% efficient!

This was quick; and useful; and gave an insight of the wrong approach!

Another way

Maximum Power Point Tracker

The panel need to be loaded at the Maximum Power Point. See this page. (Image size is a bit big!)

http://www.redrok.com/neat.htm#mppt

A summary of Redrok's ruminations here:

Pulse Width Modulation [PWM] Solar Power Regulators.
Maximum Power Point Controllers [MPPC]
Maximum Power Point Trackers [MPPT]

The Maximum Power Point Tracker is a special form of buck/boost, sometimes buck only, power converter designed to deliver the maximum possible power to a load, or storage battery, from limited input power sources. These work similar to a normal Pulse Width Modulating, PWM, voltage regulator except that the pulse width control is designed to track the maximum power available from the PV panel. Of course, when the battery voltage is high enough then the converter is shut down effecting charge regulation. 

PV panel output voltage, and consequently the maximum power point, varies in many ways. The maximum power point varies with temperature, light influx, cloud cover, dirt, and panel age. The important thing is that this maximum power point voltage rarely matches the battery voltage. 

These converters are sort of reverse voltage regulators that instead of controlling the output voltage, which is fixed by the battery, controls the input voltage. 

There are several ways to accomplish the control of this power conversion: 

Fixed Ratio. The simplest way is to just manually set the pulse width of the power converter. While this is not technically a controller because the pulse width is fixed it does produce some power gain over a shunt controller. 

Open loop power tracker. A better way is to first characterize the panel voltage vs. the input light vs. output power. A photo sensor circuit controls the input voltage that the panel runs at. While these work reasonably well and more efficiently than the simple regulators they are not the optimum solution. In my opinion they are no cheaper to build than the better tracking circuit described next. 

Closed loop power tracker. More complicated and more accurate power tracking controllers use wattage sampling techniques to continuously find the optimal panel operating voltage. The way they work is to periodically introduce a small change in the controlled panel input voltage, measure the current, then calculate the input wattage. If the wattage has increased over the last sample then the next change in voltage should be in the same direction as the last change. However, if the wattage is less than the last sample then the next voltage change should be in the opposite direction to what the present change was. 

The panel voltage is continuously being adjusted in a dynamic way to see if the output power can be increased. While the panel voltage is technically never at the true peak point the error is negligibly small. The result of this voltage dance is the ability of the controller to track the maximum power point no mater what the input PV panel conditions. 

Can the circuit be made simpler? YES! Closed loop current tracker. It turns out that there is a major simplification in the control of maximum power point battery charger controllers. Delivered power is the multiplication of delivered current times battery voltage. Since battery voltage is relatively constant one needs to only use the delivered current to represent the approximate delivered power. No complex multiplication needs to be performed. 

In the above example where power was used to determine the direction of the change of the input voltage The simplified controller uses the change of the delivered current sample as the clue for determining the direction of the change of the input voltage.