12 Volt from solar panel converted to 120VAC without battery

Question

I have solar panels that will supply about 200 watts. (They are rated for 400 watts.)

I have a load that needs about 150 watts, 120 Volt AC, off grid, and only when the sun shines.

The 12 volt solar panels have an open circuit voltage about 17 Volt.

All the 12V inverters have an input range 10 to 15 volt, and 17 volt is an overvoltage.

I don't want to use a battery because I don't need to store the power. I don't want to invest in one, batteries don't last long.

How can I overcome the mismatch of the panel output voltage and inverter input requirement?

How can I do it inexpensively and without losses? (Like voltage regulator, or DC to DC converter.)

Grid tie inverters have larger input voltage range, but I don't want to connect to a grid.

Answer 1

Your inverter can't handle more than 15V, but the panel may put out a higher voltage in bright sunlight. Therefore you must find a way to drop the voltage. You want a cheap solution, but does it have to be lossless?

Since you are using the power directly from the solar panel with no storage, it doesn't matter if dropping the voltage results in a power loss (since you won't be using that extra power anyway). What does matter is that the regulator doesn't introduce extra loss when the panel is producing the minimum voltage required to power the inverter.

A series or shunt linear regulator that simply wastes the excess power produced by the panel could still have 100% efficiency when it is really needed. The obvious solution is to simply put a high power 15V Zener diode across the panel, but how much power will it have to handle and can it be done 'inexpensively'?

Here's the IV curves for a typical '12V' 130W solar panel. First thing to note is that it puts out 22V open circuit (at 25°c) and about 17V at the maximum power point.

Your panel might actually put out 12V at maximum power and 17V open circuit, though that would be an unusual spec for a '12V' panel. But let's assume it is correct and your panel produces an IV curve similar to the green line on the graph.

Your panel can produce 400W in full sunlight, but your inverter is only drawing about 190W (150W at 80% efficiency). So the Zener will have to absorb about half the power produced by the panel, ie. ~200W. That's going need several large transistors on big heatsinks.

The other alternative is a series regulator. This drops the excess voltage at the current being drawn by the inverter (~190W/15V = 13A). A good low resistance FET could pass this current with virtually no loss at lower panel voltage, and only has to dissipate about 26W in full sunlight (assuming the panel puts out 17V at 13A). This circuit should be considerably cheaper to make than the 200W shunt regulator.

Or you could just use an off-the-shelf switching regulator designed to deliver 12V at 16A or higher. This may have a slightly higher minimum voltage drop, but will the difference be significant?

The panel may only have to produce a fraction of a volt more to compensate for voltage drop in the regulator. But light intensity and temperature variations have a much greater effect on panel output. The difference between the panel producing just enough power to run the inverter and not enough power is so small that it will hardly be noticed, so 99% of the time the extra loss in the regulator will be nothing to worry about.

Answer 2

You need to check how much current needs to be drawn to bring the voltage down from 17V to the 15V that the inverter requires.

If it's very small, then simply sticking a 15V zener diode across each panel would be sufficient. If the current would be too large, then a zener diode switching a power transistor could have the same effect.

The advantage of this is that it would only waste power when you aren't using it anyway. As soon as the inverter fires up and starts feeding the load the voltage will drop anyway.

I suspect your biggest problem is the inverter constantly cycling on and off on a cloudy day, when there's not enough power to drive your load.

Answer 3

Easiest way to manage, regulate and get max performance is with a charge controller, Morningstar being a good brand. Charge controllers will expect a battery-like thing to be present. That doesn't have to be a battery and certainly doesn't have to be a lead-acid.

MPPT charge controllers will dynamically adjust the panel load to maximize Watts (volts * amps = watts).

Ask the off-grid people about care of large battery packs, but lead-acid battery life radically depends on several factors including how deeply you discharge it. Drain it to near-dead every night, you'll be lucky to get 30 cycles. If you keep it above 80% (i.e. rarely draw more than 20% of its capacity) it should last for 5+ years unless it's cheap junk. So if you use a real battery, add a circuit that shuts off your load below X volts (that being 80%).

You could also use a stack of 10 Ni-Cad batteries (they are far more resilient to being drained to zero daily) or just burn through cheap $20 lawnmower batteries if you wanna. Or use a stack of ultracaps or just a big fat electrolytic. As long as there's a semi-capacitive load to make the charge controller happy. So here's your arrangement

Solar panel -> charge controller -> battery or cap -> load shed circuit -> inverter -> appliance

Answer 4

Throw on a 300W buck converter to step down from 17VDC to 14VDC. going to run about 15$. Max current for those is 20A, 15A suggested continuous. Putting out the 14VDC at 15A is 210W. They're about 95% efficient