Some AC systems are getting efficient enough that solar
powered AC/heat pump units on a boat are practical. The Mitsubishi Mr. Slim split system line has a 26 SEER 9,000 BTU unit for example and there are 25 SEER 12,000 BTU units. Those numbers equate to approximately 350 and 500 watts peak usage respectively. This is doable with several 200 Watt to 300 Watt solar panels
as a power source. It is not as many BTUs as normally recommended for most boats, but at the very least you can keep the aft cabin cool at night for a good nights sleep, and have slightly dryer cooler air during the day. Unless you want a huge battery
bank, you will need to budget
amp hours carefully anyway.
For boat use some modifications will increase efficiency. The indoor units run on DC, so there is no point converting to AC and then back. You may want (need?) to put the solar panels
in series to raise the voltage. In fact I would change the electronics
so the entire boat was at higher voltage 24v to 48V, to decrease powers losses in the wiring
, but DC to DC converters work too. If you do convert to higher voltage you can still run a small DC to DC converter (such as Vycor) for your 12 volt equipment
The overall unit gains much of its improved efficiency by using an inverter
that supplies power as needed to the compressor by modifying the electric
frequency/compressor speed as needed for steady low power operation. That would mean bypassing the rectifier here as well and using a DC to DC converter if necessary to raise the voltage to the necessary input voltage. I wish I had a circuit diagram. The compressor fan is also DC, so it should be run directly too if you stick with air cooling. But who wants that ugly thing on deck.
Here you can look at water cooling. This would mean replacing the condenser with tubing and possibly fins that can tolerate seawater. For best efficiency you would want to passively circulate water if possible rather than waste energy pumping it, but I hate the idea of having it outside the hull
. A pump probably uses more energy than the fan you are trying to replace, but you might recover that energy with increased efficiency of a water cooled condenser. If you can draw deeper cooler water even better.
Between water cooling and eliminating the rectification-DC conversion step in the original design it should be possible to get an overall SEER rating better than 26 SEER. I think this would be a good project for an engineering student. As with any non-marine grade electronics
a liberal coating of marine
grade electronics grease should be applied to all circuit boards and bare connections.
Solar Panels have the added benefit of reflecting light energy including infrared (IR) that makes up nearly half the energy. Some well placed panels can help with cooling, but it is a good idea to have an air insulation
layer between them and the boat. Think about reflective window treatments that are designed to reflect IR and it may be ugly but aluminized Mylar is a great light/IR reflector. The less heat you let in in the first place the less AC you need.