Solar Air Heaters

Re: Solar Air Heaters

If my limited memory of classes I took decades ago is still there,,, The most efficient is to pick up the fewest number of btu's per pass. In other words in a water system, circucate the water faster with a smaller Delta T gets more net btus than higher Delta with lower volume.

I would assume that would be true for air as well. Having said that, with air you want to avoid drafts at room temps. A warm air heater blowing at 90f will feel drafty, as opposed to one blowing at 200f feel warm. So the balance of efficiency has to be off set by the practical. Picking up 1f @150cfmmight be the most efficient, but it might feel cold, as opposed to picking up 15f @ 15 cfm for example.

Try rigging up either a variable speed fan, (try 24vdc fan on 12vdc for example,, should move ~1/2 as much air) or use a series of fans since this is a beta test.

Tony

Re: Solar Air Heaters

Neil,

Don't take my opinion to the bank,, but if memory serves,, what I said is correct.

Lets consider this example. Let's say that your garden hose is 50' long and holds 2 gallons of water. If you turn on the hose,lying in the sun, with it flowing at say 5gpm. At the end lets say that it picks up .05degree f at the outlet. (You are going to make me think here!) 5 gallons X 11lbs/gallon=66 lbs X .05 degrees=3.3 btu If you run that for 6 hours you will get 3.3 X 60 X 6 =1188 btus into the water.

The made up numbers here are the Delta T (temp rise) all the other numbers are real numbers,,, unless I err which is more than possible.

Now,, lets pose a different scenario.

If you reduce the flow rate to 1/10,, or .5 gpm I don't think that the temp rise would rise ten fold,,, or 5f. Of course the only way you can test this is with know flow rates and know Delta T's.

Now,, as the memory comes back,, I believe that one of the reasons for the more efficiency is that less heat is lost out of the collector. Picture this,,,Energy efficiency in houses is measured with R-values. R-values are an inverse relationship to a U-value. The R-value of a component is the resistance of that material to heat flow. The U value is the calculation of heat loss (in btus) per degree of Delta T.

Intuitively we know that our house (and it's components) loses more heat on a colder day than a milder one. That said, a solar collector should loose more heat as it's Delta T relative to the outside air is greater. I think that is the key. As I said,, I learned some of this stuff more than 30 years ago and don't really use it on any regular basis.

Does this make any sense, or am I getting it clear as mud?

Tony

Ps I'm as likely to be full of crap as anyone,, probably more so!

Re: Solar Air Heaters

I think my example stunk quite frankly. I started on one train of thought and ended on another with no real connection between the two,,not so uncommon

I THINK, that my contention that it is more efficient to collect heat in many passes through a collector, gaining a small number of btus each pass than fewer passes with greater gain per pass. I BELIEVE that this is true because the hotter the collector is, the greater the temp difference between the collector and the outside air. The greater that temp difference the more heat is lost to the air from the collector.

I'm not sure this make it any clearer however.

Tony

A quick google search reveals:http://www.directdriveservice.com/Ensysce_brochure.pdf

Re: Solar Air Heaters

Back in the '70's the Mother Earth News published articles on solar air heaters and movers. They used no moving parts. They found that convection worked well enough.

They were basically a trough with 2X4 sides, plywood bottom and a glazed top.

The ends were open and inside was a sheet of corrigated metal roofing painted black.

They were installed, slanted down at about a 45* angle on the South side of a building with the high end stuck through a window.

I always thought that several of them installed on a pitched roof could be vented to suck hot air out of an attic in the summer and pump hot air into the attic in the winter.

Re: Solar Air Heaters

http://www.fieldlines.com/story/2009/2/24/24929/4256

Re: Solar Air Heaters

lefty,
in it's simplest form that works just fine for either air or water. water has freezing worries of course. the completely open area filling with air or water will create eddy currents and may not be the best way to heat it as some of it may escape before being heated up very well. baffles or channels for the air would do slightly better for it imho.

Re: Solar Air Heaters

If you want to buy, here's another source - good guy from some of the solar newsgroups building solar air heating panels.

http://www.iedu.com/DeSoto/index.html

However, solar air heating panels/collectors are one of the easiest and most cost effective do it yourself projects out there - no leaks or at least no potential mess as with hot water panels, cheap, simple. I think Gary's site (the build it solar link previously posted - another newsgroup guy) covers the current state of the art well, though I have not read through it in it's new, expanded version - the best air collectors are typically built with screen or furnace filter material inclined at an angle so that cool air filters from the front/bottom/outside through the black screen/filter and into the back/top/inside hot side.

The collector material in these designs does not need to conduct heat well, as it is directly exposed to the air, and transfers heat to the air without needing to conduct it anywhere. They are far more effective than old flat plate air heaters where a hot surface is next to the glazing, and heat must be conducted through it, and then transferred into an air stream passing behind it. Enough more effective that I have 4 of the old type I scavenged which I'll be changing to "the new system" before I deploy them.

Tony's recall is good - more BTU's are extracted at minimal temperature rise, due mostly to less loss from less elevated collector temperature - however, in practice this needs to be traded off with fan power consumption and fan noise (if using fans), temperature required to make thermosyphon work (if not using fans) and comfort issues.

Making sure that the collectors do not "back thermosyphon" at night is also important - things as simple as a "check valve" made from plastic wrap can do that, but not doing that can actually increase your fuel use. Fan-driven collectors can be designed so that they cannot back thermosypon, without needing any check valve.