Sometimes it might be more convenient to bring the air to the water flow rather than vice versa, but the fan actually uses more power than the heat exchanger loop pump. The current production model of my WS system uses a typical home improvement store attic fan that draws 360 watts. The little pump that I use to send water to the heat exchangers draws 140 watts. Because the water flow rate in the heat exchanger loop is only 2 to 8 gallons per minute, I can use 3/4" PVC to go up on the roof to the heat recovery unit(s). This approach of using a small pump and small diameter tubes is a departure from how solar pool heating is normally done. With a conventional solar panel installation, the entire main pump flow (about 50 gpm) is routed through 1.5" or larger lines onto the roof. Moreover, an automatic power-actuated valve for diverting the main pump flow to roof panels is about twice as expensive as the little auxiliary pump I use for my device. A little flow of water can take-up a lot of heat. I am experimenting with high efficiency fans. One of my prototypes has a blower that moves about the same amount of air as the Home Depot variety fan, but only uses about 190 watts. If I use a brushless DC motor with a high efficiency impeller, I could lower the fan power draw to 100 watts. The air mass flow is one of the limiting factors on the heat output from a WS unit - half the airflow would only give half the heat.
So why not use a PV panel to power a WS system? I think that it would require about 200 Watt to make it work (using DC brushless motors for fans and pumps). Such a PV panel would not fit on top of the 30" by 30" lid of the WS unit. I have not checked prices lately, but a PV panel with this capacity would be considerably more expensive than the rest of the WS system. Another difficulty is that the attic fans will run for hours after sunset to remove the remaining heat within an attic space (lag due to thermal mass). I would love to make a PV-powered WS system, but I feel it is more important to first provide a system that most people can afford.
I have not attempted to establish a thermosiphon for the heat exchanger loop. I would think the water velocity might be lower than optimal for effective heat transfer. Also pool water tends to have air dissolved in the water from waterfalls, splashing, etc. When the water is brought up in temperature in the heat exchangers, some of this air comes out of solution to form bubbles. Having a little bubbling stream coming into my pool is part of what inspired the "W***S***" play-on-words.