John,
examining the air conditioning problem, we would like to remove heat from the interior of the sub both by lowering the temperature of the air and by removing humidity from the air. If we want to reject that heat to the surrounding water we can do that, but if the temperature of the surrounding water is higher than the interior air we need to use a heat pump to move the heat in that direction.
Two common ways of moving heat are the reverse Rankine cycle (vapor compression like in your home window shaker) and thermo-electric coolers. The reverse Rankine cycle is far more efficient than the thermo electric coolers, but the equipment is bulky and the working fluids are not so great for enclosed spaces. I wouldn't think about installing one in the cabin of a
sub. Thermo electric coolers (TECs) are self contained and can be scaled fairly easily.
We used TECs from TE tech http://www.tetech.com/ on a project I did recently. Checking out the website I see they also offer air to liquid coolers, designed to move heat from the liquid to the ambient air. This should be reversable by just reversing the polarity on the input power, making the hot side cold and the cold side hot. I didn't see any specs on the liquid heat exchanger, but we would need to know the pressure specs on it. The TEC option has the added benefit of being able to be used as a heater in cold weather, with an efficiency of greater than 100%.The ice option will work too. Here we aren't rejecting heat to the outside but letting the ice absorb it in a phase change process.
Both the TEC and ice systems are using resources
on the sub. Mass and volume seem to be critical, limited resources on a sub so i think our cooler should be designed to conserve both. The TEC uses power from a lead-acid battery so we need to consider how many joules of heat our TEC can remove from the sub per pound of battery, and compare that to how many joules of heat the ice system can remove per pound.
David Hoffman