Increasing efficiency with mirrors
 

Would adding mirrors to a solar heater increase it's efficiency? Mirrors are cheaper per area unit than solar collectors, but how efficient are they at reflecting heat? Would too much focusing too much (how much?) sun on a collector be harmful?

There are other things to worry about too. Stagnation temperatures (temps when you are not cycling water because your tank is already as hot as you want it to get) can be a problem without mirrors on the collectors. Add mirrors and you could possibly run the risk of damaging your collector.

Would mounting mirrors only for the winter months work?

I still believe stagnation temps are a problem even in winter. A lot of light reflects off of snow on the ground. :)

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Solar Water Heating Projects and Plans

In one, they used some sort of pressure-release valve to avoid problems.

If your solar installer does not install a pressure release valve I would not pay them until another installer inspected the system for other safety issues.
I've seen systems boil over in the winter because of the snow reflection when the corner of the panel that had the sensor was shaded for about 3 minutes or less.
So yes, getting more light on to any kind of solar collector will increase your output and void your warranty, you will need more sensors to trigger the pump and higher temp tubing or you will melt some part of it.

I always assumed that the evacuated glass tube collectors were mirrored on the inside on the 'bottom' so that it would be more efficient.

I also thought by rotating the tube, you could 'tune' the system to better collect for a particular setup.

Maybe they are not that way, as looking into now, I do not see any of them mentioning it.

Yes, I've seen evac. tube collectors with parabolic mirrors under them.
I believe only certain types of collectors allow this. The tubes can be rotated a few degrees left or right if the roof isn't exactly facing south. Of course then the parabolic mirrors underneath are more or less worthless (they still may insulate the bottom by reflecting heat which escaped the tube). It would be ideal if the tubes could be rotated automatically during the day, but I haven't seen that.

I spoke to a guy who both researches and installs solar systems and he told me that evacuated glass tubes aren't worth the money. They may be 20% more efficient, but almost twice as expensive, and you'll barely save enough to make up the difference. Well, maybe. But maybe efficiency is more important than money? Which car is better: a cheap SUV or a car that is twice as expensive, but gets twice the milage? It might take long years to make up the difference, but the econocar is still better. At least in my opinion.

Thanks.

Hmmm...I would think in colder climes (or atleast those of us with below freezing winters) that the extra insulation of the vacuum would make a bigger difference.

Of course, I am not sure why you can't have a panel or flat solar hot water collector in a vacuum.

I wonder which one stands up to hail better?

I've never heard of complaints of collector damage from anyone who has them. Hail is not uncommon in my state either.

creating a vacuum with a flat plate of glass is really hard and would take some really thick glass, that is why they use a tube, but the big problem with the evacuated tube collectors is designing a system for them that can handle the more concentrated heat, you have a pice of copper at the top fo them that can reach 280F+ and is the size of a "C" cell battery.

The high temps (and to keep it from freezing as easily) was why they used the new antifreeze/coolant (propylene glycol). Infact, I would have assumed that this substance is what is used down into some kind of heat exchanger that would be in the attic or basement or what have you.

If you could put a bigger tank, that was well insulated, then you could have a heat 'battery' for hot water use into the night.

I also found another misconception I had, that being that the pipes in the evacuated tubes are not flow through, but dead ends. I thought that cold fluid would be pumped up from the bottom, through the tubes, then hot would come out the top. The pump would be controlled by temp difference of the cold vs hot (and obviously minimums and maximums). But, being dead ends, they need to rely on convection to let heat rise to the top. I still think a loop would be more efficient, even if it was just down the 'back' side in the tube.

I also see that many have little solar panels (assume to run the pumps), but if they work well on cloudy days, then would they draw on other electric sources to run the pump if the system started to overheat?

I have asked a company to come out and evaluate my 'site' (roof/front yard), as solar hot water is not as expensive as PV.

Wonder if anyone ever made a Stirling engine run from excess heat produced during the day, when most people are at work? They supposedly run quietly and if you could stick it just below the panel with maybe a automatic valve that once your storage tank is at temperature and maintaining it, then run a stirling engine to produce electricity?

the evacuated tube collectors use heat pipes, they are much much better at transferring heat then a convection of fluid, almost instant try it sometime, take one of those tubes and holding your hand on the hot end put it in the sun, it gets hot in a matter of seconds, about as much time as it takes for the thin copper fins to get warmed by sun light, and because of how a heat pipe works those fins operate at a cooler temp as well so you have fewer losses from radent heat from the collecting fins.
the reason to run the pump with solar is that in an ideal world the panels gets hot when the sun is out, maybe a small battery and thermostat to keep the pump running would be better, but to have two voltages of pumps or to have the pump run off 120v ac would be more wiring to do and more complexity, as the pump is near the thermostat, near the panel.
the hot water storage tank in most solar hot water systems tends to be 80 gallons or more and should be insulated to keep it warm for 2-3 days... those ideas that are being talked about are not just not new but they have been refined and are common on solar hot water systems, when they are good enough to work.

I have used a sheet of foil faced building paper clamped between 2 thin sheets of polycarbonate mounted behind a solar hot water panel to give an added boost in the winter months when the sun is lower on the horizon. The gain in hot water was approx 15%. In the summer it has little effect as the sun is much higher in the sky.

Cheers
Mike