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need bb
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Posts:3
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| 06/02/2009 8:22 PM |
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| Six months ago I had a solar thermal DHW system installed. The systems consist of three flat plate Scuhco slim V panels and a Marathon 105 gallon tank for preheating. The system works well with two people, but it was designed for four people and I don’t think I will see the expected performance. Yesterday which was a 70F clear day in Boston, MA with brilliant sunshine, the system heated the 105 gallon tank 50F, from 80F to 130F. I calculated that the system then produced about 50K BTU’s and I was hoping to see more like 70K BTU’s. The outlet temperature of the panels reaches around 170-180F by noon regardless on the tanks temperature. Is my assessment correct that the heat exchanger is undersized for the system, since the outlet delta between the tank should be 20-30F and not 50-60F? For a system with 75 Sq feet of panels am I missing some performance?
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Dana1
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Posts:648
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| 06/03/2009 10:27 AM |
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50F x 105gallons x 8.34lbs/gallon=43785BTUs, substantially less than 50K.
Did you use any hot water that day? (Gotta count those BTUs as well.)
Outlet temps of 170-180F may be OK (but not great) when the tank is already 130F (a delta-T less than 50F), but is significantly cutting into realizable collector efficiency if it's running that hot when the tank is under 100F. Flat panels are ~40% efficient when running 100F above ambient with more reasonable loop delta-Ts, compared to ~60% when running 50F above ambient. So if your tank is up (from 80F) to 90F by noon, and the outdoor temp is 70F but your panel output temps are 170F you're giving up at least a third of what you could/should be collecting during peak midday hours with low tank temps.
The heat exchanger is one possibility, but not as likely as other issues. Anything that reduces flow has an impact, such as a faulty or undersized pump, or a crimp/scale/restriction of some sort etc.
I assume you've verified the glycol loop has the correct pressure and has been properly purged of air(?). If you don't know how to do it, have the installer check that stuff (I'd assume they would warranty their work for at least a year.) It's an annual-maintenance sort of thing. If you don't have sufficient pressure corrosion goes up over time, and any air infiltration/vapor locks can wreak havoc on net flow as well. I'd look there first. An undersized heat exchanger isn't very likely, and it's a bit early to have scaling issues on the heat exchanger in all but the very worst of hard-water conditions. Assuming the pumps and heat exchangers were sized correctly in the beginning (unlikely but possible, I s'pose), air in the glycol loop would be my first guess (even if the system is properly pressurized.)
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need bb
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New Member
Posts:3
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| 06/04/2009 8:39 AM |
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Well I suspect I have air in the line since the pressure during the day is at 32psi, but drops to around 20psi during the night as the gylcol cools. The air should have purged itself out of the system since it has been running in this state for multiple months.
For the size of the heat exchanger and pump, how do I know if it is properly sized? I am using the pursist pumping station here:
http://www.puristenergy.com/
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