A workable idea for an off-grid Airconditioner?

Right now, air conditioning is difficult to do with the power budget an off grid home usually has. The biggest obstacle is the cost of batteries. Not only are the batteries expensive, but they die like flies...lead acid batteries last what, 8-10 years at most? And they lose capacity the entire life cycle?

So I have been trying to figure out on paper a solution. And I thought of one that is both simple, elegant, and the numbers work very well on paper.

First, while the cost of batteries has stayed fairly constant over the last few years, the cost of the solar panels have not. Sunelec.com will sell you panels in bulk for 98 cents a watt. Also, while batteries last perhaps 8 years, the panels last at least 25 with acceptable performance at the end of lifespan (meaning they don't really need to be replaced at 25, you could just add more panels to the array every 10 years to make up for the capacity loss of aging panels). With ultra-cheap panels, generating the energy needed for an air conditioning system is actually plausible, especially since the times of year that the A/C is needed the most happen to be the same timespans that the solar systems produce the most electricity.

Anyways, the solution is simple. Purchase a geothermal airconditioning/heat system called "water to water", where the output of the geothermal system is heated or cooled water. A 3 ton system draws about 2500-3000 watts electric. Have at least that wattage in panels, and a small battery bank to act as a buffer. The water-water system will send the cold water to an uninsulated 550 gallon storage tank in a closet in the center of your home. By my math, this tank will cost about $300 and store 46 killowatt-hours of potential energy if the water is chilled to 45 degrees and the target temperature in your home is 80 degrees farenheit. With a coefficient of performance of 3.5 for the geothermal system, that's the same as $2200 worth of lead-acid batteries from sunelec (assuming you discharge the batteries to 50%). Moreover, a quality water storage tank stored inside (so no UV exposure) made of quality plastic and filled with fresh water will probably last at least 25 years. While a battery bank will need to be purchased about 3 times for that time span. So it's 20 times cheaper for water tanks. 550 gallons might or might not be enough storage, it depends on the thermal load of the house.

To extract the cold, a small DC pump would circulate the water through a heat exchanger (they make commercial air handlers that use chilled water or you could use a car radiator) and you have a fan blow air over it and a duct system. Install an energy recovery ventilator with the intake vent sucking air from the closet the tank is in (to remove condensation...need a drain in that closet as well and to make the walls out of a material that can withstand moisture) and have the air coming from outside for the ERV go to the air intake for your heat exchanger and A/C system.

The tank doesn't need to be insulated - any cold that escapes the tank into your house just means that the air handler fans and pumps need to run less - this actually boosts efficiency. You could alternatively use more expensive tanks that can take hot water as WELL as cold and use the system for heating in the winter the exact same way. (alas, the insulated tanks that can take hot water are very expensive - $3600 for a 500 gallon one.)


You'd use an Arduino board to control the system : if the A/C is armed, then when the incoming power from your solar array exceeds a certain level (the arduino board would have a data link to your inverters) and the charge in your battery bank is above a certan threshold, you turn on the A/C and run it until either the water in your storage tank is at ~45 farenheit or the charge in your battery bank drops below a certain setpoint. (because clouds obscured the sun or other power draws in the house drained your battery or whatever)

One final state : at a certain point on some days the water in your system will be pretty warm, and running a heat exchanger won't make sense any more. You'd have your system switch modes : the water-air heat exchanger would shut down and you'd use the cold water from your tank as the water source for a swamp cooler.

Note that the water in the tanks would not be pressurized, it would basically be a cistern. Glycol and anti-fungal additives might be needed, although they might reduce cooling performance.