Super Simple Solar Energy

The panel cannot be overloaded. It is a constant current source and will survive a short circuit on its output.

I should have said that the constant current is a function of the insolation. If the insolation drops, the current will go down if the panel is attached to a resistor.

You understand the trade offs. I had not seen anything about MPPT'n and it sounds adding one of them would be cost effective. And easy to add at any time.

Thanks,

Dan

Forgot to mention that the other advantage of PV for heating is that the effectiveness of the PV system doesn't drop off as the ambient temperature falls, like a direct heat collection system will. There, the system output temperature is a fairly fixed rise over the ambient temperature so on a really cold day you may be cold too :-). With the PV system the output should actually rise a bit as the panel cools compared to summer peak panel temperatures (I'm sure it will fall again at some really cold temperature but I don't know what that would be or if it matters for this).

I suppose you could blow air through a collector, a simple greenhouse-type box maybe. Is that done?

The energy that powers the fan would just act like a bit of extra resistive heating.

Sunlight looks bright, but it's low density intermittent energy and hard to apply. It's even worse in winter. Better to insulate the basement, probably.

At some illumination level, there is an optimum load current. Anything else wastes power. A solar cell is not a constant-current source.

For a (silly) resistive heater, one could add a big cap and PWM the heater to find the sweet spot.

You will see a dramatic difference between using a fixed load resistance, and some form of MPPT. But I was able to create of form of pseudo MPPT, that's only about 10 to 15% worse than perfect MPPT. To illustrate my scheme I'll use the values I implemented, to charge the Li-ion battery in my bee-hive monitor. My sources are nominal 12V solar panels with various capacities. An important first step is an electrolytic cap charged by the panels. This holds the node voltage during high converter current pulses. I used 470uF 25V to handle up to 20W panels. Next a Schottky diode to handle reverse polarity wiring. Followed by an efficient buck converter setup to create a 4.6-volt charging voltage. Next an important item, an accurate UVLO to shutoff the buck converter whenever the 470uF elec voltage drops below 10 volts. I used a TI TPS54202H, which has an accurate 1.28V cutoff, with two 1% resistors to set the 10V (or maybe 11V). It's the hiccuping of the buck converter that implements MPPT.

Finally there's a charging-controller IC, setting the maximum Li-ion charging current, limiting its voltage to 4.3 volts. The equivalent in your case would be a fixed resistor on the 4.6-volt output. When there's insufficient solar, the buck converter cuts out and the 4.6-volts drops, but the solar panel is always operating with a load from 10 to 13 volts, even with an overcast sky, and it's delivering close to its maximum available power.

It would make more sense for him to install a standard solar system, into the AC line, and go to Walgreens and get a little electric heater for the basement.

Yes, my lazy solution: $100 150W stick on flexible panel + $100 300W micro inverter + $10 heater.

Installation: clean the roof, roll out (17' x 1.5') and stick it on, connect MC4 to micro inverter, plug inverter output into outdoor light or A/C outlet, plug in heater in basement.

The little heaters are usually around a kilowatt or so. 150 watts isn't much heat.

That is very true. In fact, you need a maximum power point tracking (MPPT) load. If you try to draw a fixed amount of power and the solar cell can't deliver that, the resulting power provided goes down a lot more than it sh ould. Draw too much current and the voltage drops disproportionately.

They make MPPT controllers, but they still will only put out some fixed vol tage and for any given lighting condition you won't get optimum power. So in reality you need a MPPT load device instead. Not hard conceptually. It 's basically a switching regulator but the thing being controlled is power to the load rather than voltage or current. Use a low resistance load and a standard buck topology can be used. A small MCU can measure the voltage and current into the load and dither it to find the optimum power point.

Look up the I/V curve for a solar array or cell. It is only constant curre nt up to the knee of the curve where the max voltage is approached and the current drops off dramatically. At that point it is constant voltage.

Even so, in the constant current range the voltage drops off dramatically. Maximum power is at a point on the knee between the two.

Basements are typically not insulated because being below ground they are c lose to the optimum temperature. Only above ground walls are insulated.

John doesn't like solar power so he knocks it. His claim of it being "hard to apply" is directly in contradiction of the OP who is looking for a simp le solution. In this case I think PV solar may be an easy approach. The o nly issue is whether it is cost effective or not.

So far the only issue will be finding a MPPT load device.

A classic illustration of completely not understanding the problem.

Too bad it won't work.

Which part won't work?

Of course it can "work". If he controls the heater carefully enough, he can match its on-average energy use to the solar energy from his panel into the grid. But it may not heat up his basement, very much.

Absolutely, if you want area heat, solar HEATING of water for pumped circulation to the basement is a good idea. It's as effective as a south-facing basement window... except you can turn it off in the summer. Mirror area is cheaper than solar-panel area, and there's black vacuum-insulated glass tubes that are wonderfully effective at heating a water-based fluid (you want some antifreeze).

For my basement, though, i'm more concerned with moisture; it has a resident dehumidifier.

Constant voltage into a constant resistance gives constant current. The insolation is the source. If it is constant, so is the power hence the voltage or current is constant for a given load resistance.

Yes, I know where maximum power occurs on the curve. Where in this thread was maximum power required? I think I said peak power will be reached at a certain point with a resistor load.