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Home Forums General Discussion Air Conditioning and Off-grid : a possible solution?

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    Ouch. I made a mistake in my math. You cannot discharge a lead acid battery 100% – 50% is being optimistic. So actually the cost advantage of storing cold water rather than electricity in batteries for the purpose of A/C is TWENTY TIMES in favor of the water solution.


    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. 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 $1094 worth of batteries from sunelec. Moreover, a quality water storage tank stored inside (so no UV exposure) made of quality plastic and filled with fresh water will last at least 25 years. While a battery bank will need to be purchased about 3 times for that time span. So it’s 10 times cheaper.

    To extract the cold, a small DC pump would circulate the water through a heat exchanger (they make commercial ones 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.

    And that’s how it works.

    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 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.

    This is still a lot of hardware : not the shack we see in the picture for these forums. Probably tens of thousands of dollars of gear for a reasonably sized house. And it’ll never be cheaper than just running off the grid – but it might be MUCH closer in cost than one might think. Just a cheap water storage tank, a small battery bank, and some battery chargers compared to a grid tie system.


    Lead acid can last a lot longer with desulfators;

    Nickel-iron batteries last 25 years;

    Generally, if you need air conditioning, the best thing to do is have a building that is at least partially buried and tends to go toward ground temperature.

    Off grid buildings should not have constant heavy loads from continuous pumps or other devices.

    Check out the Earthship type designs where semi-buried walls and non-solar heated walls in summer help moderate interior temperatures to lower than outside on hot days.


    Air conditioning is bad. I’d rather have a fan and use a spray bottle (spray some water on your face, arms, feet) to keep cool. AC creates heat, hurts the ozone, gives you the illusion of a false temperature. This makes it harder for your body to adjust to the real temperatures outside, and the shock between temperatures from just going outside/inside or vice versa, can make you really really sick!

    People die every year from heat exhaustion, but I find if I have three things, I can always keep cool:

    1: Shade.

    2: Air flow.

    3: Water.

    Hope that helps.


    If you use hydro carbons instead of the refrigerants

    (that hurt the ozone) you can use 30% less electricity

    If you house gets hot and water isn’t a problem – hose it

    down – cooling by evaporation


    Given that water is becoming a scarce resource, perhaps a wiser solution is a more judicious application of evaporation is called for. The phrase hosing down the house evokes images of fire hoses drenching a house. Last summer I tried a humidifier as a cooling device with surprisingly good results.

    Evaporative cooling has been known and used in the Arabian states for several centuries at least from about 1200 AD as far as I can tell. GRIST magazine had an article recently about students in Nevada building a house as a school project that was also cooled in this manner.

    When I worked in the field doing survey work my co workers laughted at me showing up in long sleeved shirts in the heat of the summer. I had the last laugh because I used water from my canteen to soak the collar and cuffs on the shirt so the evaporation kept me cooler than they did wearing only a tee shirt or no shirt and broiling in the sun

    Oh yes I also wore a hat that I soaked. Try it; you may like it.


    I lived in Phoenix and worked outside for 15 years. My first house had a swamp cooler, in which I added up-ducts for added cooling efficiency. Outside I wore a white screen material pith helmet, often white long sleeve shirt and white painter’s pants. Once I even painted my boots white, and my skilsaw white. As the place built up with more heat holding concrete and asphalt, and humidity increase from golf courses, pools, and artificial lakes, the swamp cooler quit working.

    I visited the first Earthship type structure built in AZ, up in Carefree. It was open to the north and fully buried on the roof and most of the sides.

    The underground temperature in Phoenix is 70*F, perfect for no heat or cooling needed underground Earthships (not built in or near washes!). Perfect for solar power, and definitely needing water recycling systems, and low water use appliances with compost toilets.

    The whole place (Phoenix and suburbs), and others like it, don’t do these things (except recycle some of the water, which is full of salts and other impurities), and are just wrong. Monuments to human stupidity, greed, and overpopulation. Unsustainable, and among the first to rapidly lose values, and be depopulated.


    If I had the money I would be tempted to build an earthship as well but the reality is most of us live in existing housing stock and many people simply do not have the financial resources to build anything.

    Dustofer you said the swamp cooler quit working. Was this a failure of one piece of equipment or a failure of that kind of system in that kind of environment?

    I once saw air conditioning of an outdoor space done at a country fair. a large tent was set up and green house misting sprayers set up high above people. Every 15 seconds water was sprayed in a very fine mist and as it evaporated cooler air resulted. cooler air falls while hot air rises. The air under this awning tent was 15 degrees cooler that just outside the tent.

    admittedly it was not the bone chilling fast freeze you get with modern air conditioning but it was good enough to comfort the people who stopped in for a refreshing break. Being a typical curious engineer I studied this system. It consisted of a triple filter to remove any particulates that might clog the nozzxles. the presure pump was from a pressure washer. A timer relay cycled the whole thing and SS tubing distributed the high pressure water. The nozzles came from a commercial green house equipment supplier who had actually assembled the entire system. these nozzles are very special they are synthetic ruby drilled by lazers to around 5 mill size. This is far smaller than the smallest number drill you can buy.

    The pressure gauge showed around 800 PSI. I had to climb up and place my hand within a couple of feet in order to feel the mist from the nozzle.

    Later I found out this is standard equipment for large commercial green houses such as was found in that agricultural community. I also learned boats and large yachts use something similar for air conditioning outdoor cockpits and deck areas in coastal climates. Somebody had claimed this kind iof system would never work in any but a desert climate.

    For off grid use the principal challenge will be to find a way to create the high pressure supplied by the electric pump.

    There may be a solution somewhere between the straight evaporative effect of water on a cloth mesh and the ultra fine mist of the sprayer I saw.

    Oh how I wish I had the funding to do some experiments!


    Originally, when Phoenix was much smaller, in the outskirts the temperature would go down to 60*F like out in the desert at night. The humidity was around 5% most of the time and swamp coolers’ evaporative cooling worked well. Then as Phoenix swelled with overpopulation, it would only go down to 90*F at night in the 6 month summer. Humidity went up gradually to average 35% from golf courses, artificial lakes, and pools. The increased humidity made it so the evap coolers no longer could cool much, and in fact the increased interior humidity made it feel even hotter than outside. Some started using pre-coolers for the water to help, at an extra energy cost. Many converted to energy hog air conditioners as more huge lines of wires went up with steel and copper stretched from Hoover Dam and coal fired plants south of the Grand Canyon. In a place with 70* year around underground temps and over 300 days per year of sun—perfect for solar power.

    As you move up in latitude and/or altitude, the underground temperature cools even more, so you don’t have to totally bury your Earthship and must have it pointed South to SSE, for thermal gain in winter. Backup heating is needed for cloudy days and codes.

    People ignorantly chose to live in above ground structures and subdivisions which required long commutes. Cities became monstropolises, ever more unsustainable with energy, food, and water. Overpopulation first hit with more demand for jobs and thus lower wages, and more demand for food, water, energy and products—-increasing their prices and pollution with depletion. Fewer people could afford to go green, and with the dumbing down of the average, fewer were taught basic ecology and building trades in High Schools. A vicious cycle.


    Look into Geo-heating and cooling you’d just need a small amount of power to run the pump.

    Basicly they drill several holes into the ground and circulate 50/50 glycol thru pipes and help bring the inside of the cabin to ground tempiture, in most places in the USA is around +48-+52 F.


    Heating and Cooling Contractor

    Repair Expert

    & Installation Serviceman

    At Afterglow


    What is “tempiture”?? From the post above the “repair expert”, ” with the dumbing down of the average, fewer were taught basic ecology and building trades in High Schools. A vicious cycle.” Har, har….

    Ground temp in Phx is 70, no need for cooling or heating, while up at the Canadian border or up at 10,0000′ it is 48 or less, so the need for heat.


    Living at 42 degrees lat, and in Canada I can tell you that the Canadian Sheild ( rock ) maintains a temperature of 42 degress, with air temps ranging from -40 to 100 f. It’s not the temperature that matters it the costs or more importantly how the temperature transfer takes place. You would only use glycol if there was a danger of freezing, water is much better at transfering btu’s than glycol, and cheaper and easier to get and handle.


    GMonroe: did I miss it or did you say where you live? Whats the climate there? rainfall? summer heat levels? humidity levels? Average sunshine? Wind? How much money do you have? do you have cash or will the money have to be borrowed from a bank for construction? I assume that a/c in your area is far more important than heat throughout the year?

    Insulation and thermal barrier keep the cold in and the heat out. Ground will cool obviously down to ground temperatures. Thermal mass will keep the inside at a more constant temperature. If cost is the concern then what should also be a concern is the volume of space that needs to be cooled in cubic feet.

    One almost needs layers and zones with differing comfort levels. Your main living zone will be the most well designed and probably most expensive such as a den(living room) and possibly a master bedroom. For example if its just you and your wife stay in the cool rooms and let the others warm up. If you have guest then turn up the a/c in those other rooms and spend a little more until they are gone. You can design all you want and still its going to be difficult to get away from taking what you would have otherwise given to a utility month by month and investing that in construction instead up front.

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