PV Direct: Crazy or Crazy Good?

Jamie Hall

May I humble suggest that you are overthinking all of this? Just sayin...

As to the plastic pipe -- it will overheat worse if there is no water in it. Your best bet when overheating is a concern (and it can be in the winter time, too) is to keep enough water flowing through the pipe to keep it cool. You may have to figure out a way to dump the excess heat somewhere...

desert_sasquatch

What is the danger in overheating? Is it the breakdown of materials? The expansion of water in a closed loop system? The precipitation of minerals out of the water?

I feel like I don't understand what is driving the "overheating" concern and thus don't know the right questions to ask.

Jamie Hall

Possible concerns -- not all may apply. And there may be others. With plastic pipe, there is a softening temperature and a maximum temperature, and those temperatures for most pipe are below the boiling point of water. Any pressure in there when they are too hot and they may burst -- or at least bulge. If they get really hot they just sort of sag together... very sad. Second, if you have any glycol antifreeze in use, that is seriously affected by overheating and becomes a horrible mess. Third, even if the pipe doesn't deform, the you may get sine pretty high pressures in there.

hot_rod

desert_sasquatch said:

Thanks everyone.

@hot_rod so if I understand that chart correctly (thanks for finding it; I knew I'd seen it but couldn't remember which Idronics it was in) it seems like when delta T is about 60 degrees F and irradiance is about 200 btu.hr.ft^2 that's an inlet fluid parameter of .3, which means most unglazed flat plate collectors won't produce anything. They might produce a little during the peak of the day--11:00 am to 1:00 pm or so--but that's a good deal less than the glazed flat plate which should produce useful energy for maybe 6-7 hours of the day.

Here's the charts I'm referring to, for anyone interested:





I'm close enough to 40 degrees latitude that that should be roughly accurate, I think.

The near 90% efficiency of an unglazed collector is when the fluid going into the collector is near the air temperature.

If air temperature drops, or you drive the collector to a higher operating temperature, performance drops quickly.

Where the yellow and white lines cross is where glazed flat and evac tube perform the same. As ambient drops, the vacuum tubes maintain higher efficiency.

So the glazing or vacuum space certainly matters in winter conditions. That condition is where I suspect that "opaque" covered collector will take a hit.

Its a basic delta T equation and function, based on the laws of thermodynamics hot goes to cold and the rate of transfer is based on the temperature difference ∆T.

Larry Weingarten

Hi @desert_sasquatch , Overheating and freezing are handled in different ways. If poly pipe were put into a standard glazed, insulated box, you can be sure it would be damaged by the heat... unless it could be cooled by running water through it. That's iffy as if the power goes out once, the tubing melts. It should never be exposed to 180F or higher.

Drainback is useful for metal collectors which cannot withstand freezing, but regarding heating, only helps to prevent them making steam on a hot summer day. Also, it avoids the need for glycols, which degrade over time.

It may be that the poly pipe collector just can't perform at low ambient temps, but that's an unknown. It hasn't been tested in a truly cold climate that I know of.


Yours, Larry

jumper

I don't understand these issues with DC. Just couple PV output to resistance heaters in water tanks. You can wire the PV cells for whatever voltage you choose. You can relieve too high temperatures mechanically.

DC switching was around before AC was used. Nowadays solid state devices are available.