I saw a lot of news articles talking about how the Evil Power Companies were being Meanie McMean by not letting people with solar panels use them when the grid was down. The implication (in many news articles) was that these powerless people with solar panels could use them to power their home while the grid was down, if only the evil power company didn't require that solar not work if the grid was down. The picture painted was one of power company executives, twisting their mustaches, cackling in the glow of their coal fired furnaces, going on about how if they can't deliver power, nobody shall have any power!
That sentiment (and those similar) is somewhere between "showing extreme ignorance of solar" and "actively misleading," depending on the author's knowledge of solar and how it's typically implemented.
So, of course, I'm going to do better. Because I can. And because I'm sick of reading that sort of nonsense on the internet. You will be too, after understanding the issues.
That sentiment (and those similar) is somewhere between "showing extreme ignorance of solar" and "actively misleading," depending on the author's knowledge of solar and how it's typically implemented.
So, of course, I'm going to do better. Because I can. And because I'm sick of reading that sort of nonsense on the internet. You will be too, after understanding the issues.
Solar Panels (Are Weird)
Any detailed understanding of solar power requires an understanding of solar panels - because they're the power supply to the entire system. And, in terms of the available power supplies out there, solar panels are weird. They're substantially different from anything else, and this impacts how you can use them for grid tied and off grid power. The biggest problem is that they're very easy to drive into voltage collapse (and therefore power collapse) if you draw beyond the peak power they can produce at the current temperature and illumination.
This is an example IV (current/voltage) curve out of the datasheet from my panels - it's one I had laying around. The numbers don't matter, because all solar panels work this way - just with different numbers on the scales.
What you're seeing, and what's vital to understand, is that a solar panel will supply a certain current (at any voltage) - up to a certain point. That current is directly affected by the illumination available (the different W/m^2 curves - that's illumination power per square meter of panel area). At a certain voltage, the current starts to drop off, and eventually you hit the open circuit voltage (Voc) - the voltage the panel produces when there's no current draw. The peak power (maximum power point) on the panel comes slightly past the start of the drop in voltage, and the available power drops very rapidly as you go past that point into the voltage collapse. At both the short circuit point (0V, plenty of amps) and the open circuit voltage (0 amps, plenty of volts), the panels are producing zero usable power.
DigiKey has a great diagram that demonstrates how this works for their particular example panel. The red curve is the current, and the blue curve is the power. The dot represents the maximum power point on both curves. Notice that the power curve to the right of the maximum power point is quite steep - it's not a gentle dropoff.
The curves change absolute values somewhat both with illumination and temperature. A colder panel will produce a higher voltage, which a good MPPT controller can extract as extra watts in the winter (when you really want all the watts you can get). Plus, there are curves over the standard 1000W/m^2 illumination you might see in certain conditions that lead to an awful lot of extra power. When might you see that? A vertical panel, with snow on the ground, on a bright, sunny winter day. Also, "cloud edge" effects (the edge of certain cloud formations can focus more light on your chunk of ground than full sun). In those conditions, a panel will produce more than rated current and voltage, and you'd better have designed for that! I've seen north of 11A from 9A panels in the winter reflection condition.
My east facing panels, right now, are producing 1.8A at 58V. In these conditions (afternoon shade, but a partly cloudy day), they'd happily provide 1.8A at 12V, 1.8A at 24V, 1.8A at 40V, 1.8A at 60V... right up until I pass the knee in the curve. Open circuit voltage today is 70V (my little PWM controller can tell me this), so peak power is probably right about 57-60V. And, if I were to try to pull more than 1.8A from them, the voltage would collapse. That's just what they can do right now, aimed as they are.
Swing them around to face the sun, and they're operating at 7.4A at 58V. These panels are connected with a PWM controller (pulse width modulation, or basically a switch that toggles quickly), so they always operate at whatever my battery bank voltage is. That means they're producing more watts when my bank is charging heavily (60V) than empty (48V). But that's the way I hooked them up because it's cheap and plenty good enough for my needs. Since their peak power comes fairly close to my battery bank voltage, the (small) gains of a MPPT controller don't justify the cost on this secondary array. But what's MPPT?
DigiKey has a great diagram that demonstrates how this works for their particular example panel. The red curve is the current, and the blue curve is the power. The dot represents the maximum power point on both curves. Notice that the power curve to the right of the maximum power point is quite steep - it's not a gentle dropoff.
My east facing panels, right now, are producing 1.8A at 58V. In these conditions (afternoon shade, but a partly cloudy day), they'd happily provide 1.8A at 12V, 1.8A at 24V, 1.8A at 40V, 1.8A at 60V... right up until I pass the knee in the curve. Open circuit voltage today is 70V (my little PWM controller can tell me this), so peak power is probably right about 57-60V. And, if I were to try to pull more than 1.8A from them, the voltage would collapse. That's just what they can do right now, aimed as they are.
Swing them around to face the sun, and they're operating at 7.4A at 58V. These panels are connected with a PWM controller (pulse width modulation, or basically a switch that toggles quickly), so they always operate at whatever my battery bank voltage is. That means they're producing more watts when my bank is charging heavily (60V) than empty (48V). But that's the way I hooked them up because it's cheap and plenty good enough for my needs. Since their peak power comes fairly close to my battery bank voltage, the (small) gains of a MPPT controller don't justify the cost on this secondary array. But what's MPPT?
0 Comments