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I've been reading about DIY solar thermal collectors, and the various ways their performance is influenced. One of the things I learned about solar water heaters for a pool is that it's recommended to run the pump pulling water through the collector at full speed. The reasoning is that the amount of energy that the sun is putting into the collector is constant, and running the water though the pipes faster results in less heat from inside the collector being lost back to the surrounding air (or the glazing, box material, etc.).

If a solar thermal collector for air is pulling air via an intake fan from the house, driving it through pipes in the collector, then pushing it back into the house, does the same concept apply? The fan should be blowing the air as fast as it can, as long as the temperature of the pipes are greater than the intake air temperature? Or am I missing something?

you786
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  • Where did you get the idea that the "amount of energy that the sun is putting into the collector is constant"?? That's clearly NOT true. For instance the amount of energy is high at noon but nearly ZERO at midnight. Hardly a constant. – jwh20 Jul 31 '20 at 02:24
  • The power for pumps or fans is not free (though it can be "almost free" (there is the cost to buy it amortized over time) **and** proportional to the amount of sun if it's driven directly by a solar electric panel with the same exposure as the solar thermal panel. The noise of "fans running full blast" might also put you off having a solar thermal panel unless "full blast" was also "still quiet." – Ecnerwal Jul 31 '20 at 03:02
  • @jwh20 In the context of the question of the effect of fast or slow fluid movement in a collector, yes, the sun's input into the collector is effectively a constant since it's not effected by the fan speed. – you786 Jul 31 '20 at 04:42
  • @ecnerwal, that's a fair point. I think you're getting at the reason why many designs for DIY solar air heaters have small/slow fans: they're cheaper and probably draw less energy . But does the core principle still apply, that faster moving air is actually more efficient at increasing the temperature of the home? – you786 Jul 31 '20 at 04:47
  • @you786 sun’s input is not constant - only need to have clouds go over and the panel energy capture drops. – Solar Mike Jul 31 '20 at 14:52

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In some mythical world where nothing else mattered, pumping your working fluid at maximum speed does limit the temperature rise of the collector surface, which limits the re-radiation of heat from the collector (re-radiated heat is not collected, thus, lost.)

The simplistic theory is that raising 11 pounds or kilos of air or water 0.1 degree is more heat collected than raising 1 pound or kilo of air or water 1 degree. And that's true, in itself.

Like most "purely theoretical and ignoring large parts of reality for the sake of a simple theory" analyses this falls down hard in the face of all that stuff being ignored because it's complicated, in reality.

The 400 mile or kilometer per hour blast out of your "nearly ideal, but perhaps it could be 500 or more" solar air collector will be very unpleasant to live with, deafening, and cost more to run than having no solar collector and just putting that power into a cold climate heat pump. The churning vortex the water version will turn your pool into will probably waste a lot of heat to excess evaporation, and drown swimmers in your pool.

Finding the happy sane point is engineering, and it's not overly simple, because it deals with the real world issues like power to run fans/pumps, efficiency of heat transfer and all that stuff.

Ecnerwal
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You mentioned a solar pool heater in the first paragraph then switched to a solar collector for air in the second paragraph. Are you looking at "eutectic solar collectors" or sensible heat collectors? The beat efficiency is usually obtained when you extract the heat that is being put into the collector at the same rate that it is being collected. Moving too much air or water, will cause an increase in the motor wattage and could "reduce" the heat transfer, which will offset the proposed or estimated cost savings. If you are looking for the least costly solar system, don't forget to look at a passive system which can utilize sensible heat collectors or a eutectic solution collector. A simple system usually saves the most money if that is what you are trying to accomplish. Also don't forget about collector orientation as it relates to the suns rays since that is what heats the collectors.

d.george
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