So single glazing would work ok, and avoid high temps? It would also save me some money. an automated vent system is possible I have access from the barn to get to the back of the panel. What would you recommend for pumps? Is there a self priming pump I could use for the collector?I want to avoid all all holes in liner. My transfer lines would be 1" (?) 100' into sequential sidearms. Thanks, My name is also Gary
Hi Gary,
I think the combination of steep tilt and single glazing can get the stagnation temps down to OK levels, but I think that a collector tilted at (say) 45 degrees with single glazing is still going to get to hot when stagnated in good sun. You need both the steep tilt and the single glazing to get temps down to what pex can take when stagnated.
I know you are not wanting to hear the copper word, but it does have a lot of advantages for collectors -- just costs more than pex.
Single glazing will perform well even in cold climates -- my shop heating collector is single glazed and it still puts out a lot of heat down at 0F in good sun.
For the collector pump, for such a large collector, you probably want one of the Grundfos or Taco HVAC pumps -- there is a rundown on some of the comone ones here:
http://www.builditsolar.com/References/Pumps/PumpCurves.htm
These HVAC pumps are good reliable pumps and even though they are not paragons of efficiency, they are not bad and pumping energy will be a very small fraction of the heat energy collected. If you do want to look into a more efficient pump, one to look at is the Grundfos Alpha series.
Basically for a drain back, you need a pump with a startup head (zero flow) that is about 1.2 times the vertical distance from the tank water level to the top of the collector. This is to get the flow started. Once the flow is started and the full circuit is full, you need a pump that will deliver the about 0.04 gpm per sf of collector with the pipe friction you have -- 16 gpm in your case. It is important to get a pump that meets this startup requirement in that if the pump is not able to pump the water to the top of the collector at sartup, the system will not work at all. A Taco 008 might be a little on the small side, but might be OK -- it won't quite make the 0.04 gpm/sf, but maybe close enough -- this is a good pump -- I use it on my Solar Shed collectors. Or, a Grundfos 15-58 three speed pump on high speed might do it -- again, it won't quite make the 0.04 gpm/sf, but probably close enough. I've got three of the 15-58's -- very nice pumps.
If you want to go through the whole pump sizing routine, there is a page here on it:
http://www.builditsolar.com/Projects/SpaceHeating/PipeSizing/PipeSizing.htm
The pump to the house is a tradeoff: if you go for a small pipe diameter, it keeps the pipe cost down and reduces pipe heat loss, but increases the pipe friction, which makes for a larger pump. A larger diameter pipe reduces pipe friction and allows a lower head, lower power, less expensive pump to be used.
I ran numbers for several pipe sizes for my Solar Shed setup, which has over 100 ft of tank to house pipe. This is a 240 sf collector. I ended up with 3/4 inch pipe as the compromise. The pump I use to pump the water from the tank to the house and then through the radiant floor loops is just one Grundfos 15-58 running on high speed. This works OK in that while the flow rate is on the low side, it is able to transfer all the heat from the fully heated 400 gallon tank to the house after a sunny day well before the start of the next collection period.
Since your collector is larger, a 1 inch pipe might be better for yours.
For 250 ft of 1 inch pex at 5 gpm, the pressure drop is 3.95 psi, or 9.1 ft of head.
For 250 ft of 3/4 inch pex at 5 gpm, the pressure drop is 13.15 psi, or 30.4 ft of head -- quite a bit of head loss.
There would be some additional pressure drop for fittings etc.
5 gpm with a 10F temperature drop will transfer about 25,000 BTU per hour.
Your collector might do 320000 BTU on a sunny day, so the pump would need to run about 13 hours to move it all to the house.
All rough numbers.
Self priming pumps that work well in solar service are hard to come by, but you can avoid the tank wall penetrations by using a U-tube -- as on this system:
http://www.builditsolar.com/Projects/SpaceHeating/DHWplusSpace/Plumbing.htm
The water stays in the U-tube up to the water level inside the tank, so the pump does not lose its prime. Its important to locate the pump as far down as possible so that it has a little positive head at its inlet. You do have to protect the U-tube from freezing if the tank is in a cold area.
Gary
