Hi Lighting,
"I'd be inclined to go with the flip side. Sure it takes longer to get it hot. But that's a once-only. Then you just have more storage for "sun outages".
Only downsides I can see is the added expense of the bigger tank and the SLIGHTLY higher loses through the larger area (which aren't a real issue if the tank is in the space to be heated). Also, a bigger tank means you don't have to rehack the tank if you add more collector area."
--> The problem with having too much tank in solar space heating applications is the tank spends an awful lot of time at temperatures that are too low to be useful. The 2 gal per sqft is based in the idea that for a typical home and collector area, the collectors will collect and store heat all throgh a sunny day and 2 galons per sqft of collector is enough to store one full sunny day of heat. Then during the night and morning of the next day, typically the house will use all of the stored heat, so the tank is back down to discharged at 80F or so. So, there is no real benefit in having more storage than one days worth of sun UNLESS your house is really really well insulated and has a very low heat loss, or you have a ton of collector area and can generate more heat that your house can use in one day. Living in a warmer climate also reduces heat loss and might make more storage worth it. I've heard from people who put in larger tanks and end up actually running them part full so that they get up to more useable temps more often.
For solar domestic water heating, I think you can make more of a case for a larger tank as long as you have the more collector area to go with it. That is, if you only have enough collector to satisfy you family daily demand on a sunny day, there is no point in having more than one day of storage, but for solar domestic water heating its releatively easy to build in some excess collector area so that on a sunny day, you can make more than one days worth of hot water and have that hot water for a following cloudy day (or 3). This does require more storage because you are saving up as much as a day or two worth of extra hot water. But, its critical that you have enough collector to more than meet your family demand on a sunny day -- else there is no excess to store away.
Anyway, That's my 2 cents
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"I wish I would have researched tempered glass vs polycarbonate glazing... I still haven't found much information, but after I realized that I couldn't measure absorber temperature with my IR gun, I got to thinking.... The gun doesn't measure absorber temperature because the glass blocks IR. I wonder how much more efficient my collector would have been with polycarbonate, which doesn't block IR - or if it makes any difference at all?? This would be a very interesting test to see, and one I would perform if I had both the time and money to do so.
I chose glass for its high durability and temperature resistance - which is great. But if I were to find that polycarbonate gave 5-10% better performance, I may have changed my mind..."
"Blocking (far) infrared is GOOD. It traps the heat in the collector so more of it is harvested. This creates the greenhouse effect.
Virtually all of your solar energy is in the visible, near infrared, and near ultraviolet, which the glass passes. This is absorbed by the collector surface. Then a bunch of it is immediately re-radiated at the temperature of the collector, which corresponds to a thermal distribution centered in the far infrared, which the glass reflects. This gets reabsorbed. Essentially the only way out for the energy is to work its way by conduction through the collector to the heat transfer fluid or through the box to the outside air. Meanwhile the near infrared is a tiny fraction of the incident energy. Switch to something that passes infrared at room-to-boiling-temperature frequency bands and the down-converted energy flies away, to be absorbed by water vapor (or droplets) in the atmosphere.
So the glass plus the black body acts like a diode to capture the energy. Just what you want. But far-infrared transparency gives you ratiative coupling to the upper atmosphere, which is colder than ground-level conditions and takes more heat than it gives back.
I'm not sure what polycarbonate's far-infrared transparency is like. But if you can read the temperature through it, it's letting much of that nice down-converted energy back out."
--> As you say, blocking the IR is good. Polycarbonate has almost exactly the same transimission spectrum as glass, so it blocks IR as well. Not all plastics block IR -- polyethylene for example does not.
I think the only way you can go wrong with glass is to end up with glass that has a lowish transmission. Glass with high iron content has lower transmission, but I don't know any ironclad way to tell for sure -- the iron is supposed to make the glass more green when viewed edge on, but that does not seem like an easy test to apply.
http://www.builditsolar.com/Projects/SpaceHeating/Glazing.htmActually, one way to test some prospective glass you have in hand is to do this kind of test:
http://www.builditsolar.com/Experimental/TreeShade/TreeShade.htmJust put the mystery glass on one of the two collectors and (say) polycarbonate in the other.
I could be wrong about this, but I think that both glass and polycarbonate block the far IR that the absorber radiates by absorbing it. This is not quite a free lunch in that the absorbed IR heats the glazing up some and it looses more heat to the outside air. It would be nice if instead it just reflected it back to the absorber.
But, either way, its a plus.
Jon -- I think you have done a great job of trying some new and well thought out ideas -- there is always a little risk in this, but it certainly helps other people who build the system down the line.
Gary
