Most efficient use of a solar collector

[Capt Slog]

Hi.

I'm using al home-made solar collector to heat some water.  The water is taken from and to the same vessel by a circulation pump.

Is it better to have a fast flow through the panel and heat the water by say, 0.1 degree on each pass but have lots of passes of the whole body of water?

OR

Have a low flow through the panel which heats the water by several degrees and then have this hot water 'diluted' when it reaches the main body of water on the return?

My gut instinct tells me that there is no difference, because you can only harvest and move one lot of heat, but I also wonder if one way has a better efficiency and if so, why?


In case you're wondering, the vessel is a 800-900 litre spa-pool which I want to heat by solar to bring down the cost.

[artv]

Hi Capt,....I've been doing testing of this sort for the last 4 days now .There really isn't much difference,by 1:00 in the afternoon my stored water is up to temperature.
1st day ...2min pump every hour on the hour
2nd "....15sec pump  every 15min
3rd  "....45sec pump  every  30 min
So on and so on....the short quick (2nd day) seem to bring the temp up a little quicker, but like I said by 1:00 their all the same.
The 2nd day would see the pump running the least amount of time, but it starts and stops 4x as many times as say the first day,but somebody with better math skills or experience would know better than me which would be more efficent.
Probably the most effcient thing would be a temperature sensor saying when to circulate, but I could only find such a valve almost 2 hours away from me and the guy said it was around 300 dollars........so 30 dollar programable timer is what I'm going to do.
I'm sure there are cheaper sensors' out there but being on dial-up I can't waste the day surfing the net.....good-luck....artv

[GaryGary]

Hi,
From and efficiency point of view, the more flow the better.  More flow keeps the collector absorber cooler which means less heat loss from the absorber out the collector glazing.

But, there is a practical limit to how fast you want to pump water through the collector -- more flow means a bigger pump, more pumping power, and higher pipe velocities.  Most people use around 0.04 gpm per sqft of collector, which seems to be a good compromise.

This page shows the calculated change in efficiency as you change the flow rate through the collector:
http://www.builditsolar.com/References/ColFlowRate.htm

Gary

[prasadbodas2000]

more (or rather optimum) flow would be better from thermal collection efficiency perspective, but it will cost u mich more electrical power at the pump....

[WindriderNM]

I have my collector below my tank and let it thermo-syphon (no pump)