In the recent edition of Electronic Products there was a fantastic I/V (current / voltage) diagram of a solar panel. It may have originated at Allegro, where, the authors of the article work. It confirmed something I suspected for a long time. The power output of a solar panel falls as it gets hotter. I will put a low-res version of the graph below but you really need to look at the EP article.
This diagram shows how you get less power out of a hot solar cell. Dotted lines are power out, equivalent to the area under the I/V operating point.
This connects with my realization that a solar cell is like any other photodiode. The forward voltage goes down as it gets hotter. But I was not sure what happened in reverse mode I/V. But with what we call a photodiode, you are usually trying to measure light, not draw power from it. So with many photodiode amplifiers, you short the diode into a virtual ground. With no voltage across it, it is not making any power. But the current output is very linear with respect to the light falling on the diode. And note that this current is a reverse current in the diode. You can think of it as a reverse leakage current that gets way worse when light hits the diode. Indeed, the baseline leakage is called dark current.
A photodiode I/V curve.
So here is the diode I/V curve you might see published in a photodiode amplifier book. Note that you can short the diode and its output has to fall on the –I axis. If you put a negative bias on the diode, and still keep it working into a virtual node so there is no voltage generated across it, then it is like the leftmost response. The negative (aka reverse) bias does not materially change the output, but it does greatly lower the diodes capacitance, since a photodiode is also a varactor. If you hook a photodiode, which is pretty much any diode there is, to a resistor, it will make current but the current into the resistor will also make a voltage. That gives you the output of a resistive load in the chart. The value of the resistor sets the slope of that load line. Note that the output is no longer linear. Doubling the light does not double the output current.
Realize the Allegro solar cell curve is showing you the bottom right quadrant of the generalized photodiode curve above. What the solar cell folks do is re-define positive current as what really comes out of the cell, as opposed to having positive current be defined as a forward diode current. So if you can image flipping photodiode curve up around its x-axis, and then tossing out the left side and the whole bottom half as well, you get the Allegro curve. Note that shining light on a solar cell or photodiode will never make forward diode current, but it will affect the operating point if you are putting forward current into the diode.
And note that you can’t get any power out of a cell unless you get both current and voltage at the same time. You short the solar cell and you will get the most current, but no power. If you leave the cell open circuit, you get the most voltage, but with no current flowing you are not getting power. So what you want to do is change the load on the cell until its operating point on the I/V curve has the most area under it.
The red rectangle is smaller since it does not have enough voltage. The blue rectangle is smaller because it does not have enough current. The green rectangle has the maximum area and hence is the MPP (maximum power point) of the solar cell.
So that is what the MPP (maximum power point) or MPPT (maximum power point tracking) concepts are all about. You get no power if you short the cell or leave it unconnected. What you are trying to do is maximize the area under the operation point. That is because power is current times voltage, just like area is X times Y. So the MPP chart I hacked up above shows three different operating points. You can see that the big dot corresponds to the rectangle with the greatest area. If your magnificent Atmel microcontroller multiplies out the voltage and current in real time, it can dither the operating point by changing the operating point of the dc-dc converter that is taking the solar cell power and putting into a battery or onto the ac line. This is the “T” in MPPT. By tracking the maximum power point, you get the most power you can for any particular solar cell, at any particular temperature, at any give illumination.
Now please read that Electronic Products article about measuring current, since you may want to use those Allegro current sensors in your MPPT inverter, or smart meter or other application. Atmel makes the microcontrollers with security and some have integrated power line communications (PLC) modems. We also have parts that integrate the AFE, so you don’t need these external parts in your smart meter. So if you need to measure and log and report and control current, keep Atmel in mind.
Oh and in case somebody hasn’t thought of it yet, it seems obvious to one skilled in the arts that you can combine the shaded evaporative cooling systems that spray water on your roof beneath shutters, with solar panels as the shutters, so now you are cooling both the roof and your panels. Step C, more power.