How much 'gain' would we get if we sunk a 55 gallon barrel into the relatively cool ground here in Florida, filled it with calcium free water, and sprayed the condenser in a closed loop cycle when it is running?
Any evaporated water would be made up with condensate.
I can imagine two advantages. Evaporative cooling (how humid is your area? The more humid, the less benefit you'll get). The other advantage is that some of the heat will be lost to the cool earth.
There must be some reason it isn't yet common practice. Maybe that reason is not much benefit. And maybe the reason is the equipment and install costs.
Please make a flow-through design, rather than a deisign where water is collected and stays. If ther is no drain (just evaporation) then eventually the minerals and solids will concentrate.
Window AC are designed to spray the condensate water on the condensor coils. Howver, the wet condensors tend to attract dust. And so, window AC need cleaning every couple years. Wet coils for your outdoor unit will likely also need cleaning more often.
Save the effort of burying the barrel... The btus are found in the phase change of the water... Roughly 970 btu/lb of water. The "relatively cool" water from a buried barrel will only give you 1btu/dF of the temp difference. 65dF water to 212 dF will yield 147 btu/lb, relatively low compared to that phase change... it won't involve a shovel.
I'd route the gutters to it & collect the rainwater, filling with tap water adds too many variables. You could raise the barrel above the unit & let gravity power the water flow rather than a pump, although a spray might require a few extra feet of elevation. You'd also make monitoring the water quality easier with an above ground barrel.
The idea of spraying coils w/water has been around for years, though most I've run into are water coils w/prop fans. I'm sure someone makes a refrigerant type system...
Since most outdoor units are built to handle rain & the like, I don't see why it would hurt if common sense is also used... I would be very careful to avoid a heavy spray that would bypass the coil & blow straight on the motor.
It might be more advantages to have a couple "drip tubes" rather than a spray on the coil, especially with the elevated barrel. Then there's always the question of what your going to do to drain the base pan.
All in all, I don't think the efficiency increase from such a project would justify it's cost & maintenance issues. It'd probably be easier to put the efforts & $$$ in insulation, Programmable stats, window treatments, etc... But if you give it a try, we'd all like to hear the results.
A new Texas upstart company around 1998 [called AC2] tried to market a residential evaporative condenser that used water and air. Problem was, citys and municipalities would not approve because of the excessive water usage. [We're in a water drought in most western states.] The problem came due to the 'flushing' of the TDS. The required flush cycle washed out the TDS as the unit ran. But the water flush was so often and so high [to prevent TDS build up and copper coil caking] that it became prohibitive in most areas.
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