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?
Best Answer
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.