[Greenbuilding] solar heat collection

Gary BIs gary at builditsolar.com
Thu Jul 5 10:40:27 CDT 2012


On 7/4/2012 12:02 PM, Clarke Olsen wrote:
>     I am putting together a system with tilt-adjustable panels and a 1000 gal storage tank, in the hope of collecting
> and storing heat year round. The 1400sqft house has radiant tubing in a 3" concrete floor sitting on 1/2" foam over
> a crawl space. The boost from the tank temp to hot shower is handled by Eemax 6.5kw point-of-use instant water
> heaters. The floor circuit also has an Eemax booster. My plan is to run both the domestic hot water and the floor
> circuit through heat exchangers in the 1000 gal storage tank.
>     My question is: should the storage water run through the collectors directly in a drain-back configuration, or is a
> closed-loop, heat exchanger be preferable? This would make 3 sets of loops in the tank, with the collector circuit
> on the bottom.
> Clarke Olsen
> 373 route 203
> Spencertown, NY 12165
> USA
> 518-392-4640
> colsen at fairpoint.net
>
>
>
>
>
>
Hi,
I like drain backs with the collector water circulated directly from the tank 
through the collectors.  This avoids the cost and inefficiency of another heat 
exchanger.
This is my system: 
http://www.builditsolar.com/Projects/SpaceHeating/DHWplusSpace/Main.htm
As Corwyn says the drain back needs more pump, but when you look at the extra 
pump energy as a fraction of the energy generated by the system, it seems (to 
me) like a small price to pay for the simplicity and reliability of the drain 
back.  If pump power is a big issue, you can put two pumps in series, and run 
both at startup, then turn the 2nd pump off on a time delay -- once the system 
is full of water, you only have the pipe friction to overcome and one pump is fine.

I'd think about whether the adjustable tilt approach is worth it.  For a system 
like this with both winter and summer use, you can optimize tilt for winter when 
you have both DHW and space heating loads, and since you don't have the space 
heating load in the summer, the collectors will provide plenty of heat for DHW 
even at a winter optimized tilt.  The high tilt also tends to eliminate any 
potential overheating problems.

The bit about storing heat in the summer for winter use is a tough goal.  Unless 
you have a house that designed from the ground up for seasonal storage (ie VERY 
low heat loss), the tank sizes to make an real contribution to the winter heat 
load just become prohibitive.  Here is a set of numbers  that may give you some 
idea how tough a problem this is:
http://www.builditsolar.com/Projects/SolarHomes/SandBed/RamlowSandbed.htm  These 
are in the context of sandbed heat storage, but if you take the same sample 2000 
sf very well insulated house I used in the example, it has a 52 million BTU 
winter heat loss.  In your 1000 gallons and using a temperature range of 150F 
down to 90F, you can store (1000 gal)*(8.33 lb/gal)(150F - 90F)(1 BTU/lb-F) = 5 
million BTU -- perhaps 10% of the winter demand.   You can get a rough idea what 
the yearly demand from your house is from: 
http://www.builditsolar.com/References/Calculators/HeatLoss/HeatLoss.htm  and 
compare it to the capacity of the 5 million BTU capacity of your 1000 gallon tank.

For most homes with good to average insulation, 1.5 to 2 gallons of storage per 
sqft of collector works out pretty well.  This is enough storage capacity to 
store a full sunny days worth of solar without overheating the tank -- for most 
homes and unless you have a ton of collector area, this heat will be used by the 
house over a day.
For most homes, have storage over this just adds to the cost and adds to the 
tank heat loss.

Gary







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