As summer approaches, I thought it was time to spend some time researching off grid air conditioning. While traditional AC units are generally quite expensive to run off solar, I though that there must be a better way keep cool off the grid, and I wasn’t disappointed.
Five ways to have AC off the grid —
- Passive Solar Design
- Heat Based AC Units — Absorption aka Einstein Coolers
- Earth or Ground Cooling Loops
- Ancient Roman Style Water Cooling
- Solar Powered Wall Air Conditioners and Heat Pumps
- Swamp Cooler
Keeping cool while staying self reliant, most people instinctively gravitate toward using the same AC unit they are accustomed to, just with solar or other renewable power sources. While this can work, thinking outside the box in this case can yield impressive results at a fraction of the cost.
You Can Have AC Off the Grid
The problem that immediately comes up when people plan on putting in an AC unit in their solar power system is much power AC units draw. Even smaller window units typically consume up to 500–1000 W each, and a central systems are typically about 3500 W to run.
While you do have extra solar power available in the summer, running a modest 1000 W system on solar for 8 hours a day would require 8 kWh of electricity. Assuming you started running the AC in May, most areas of the country only produce 4–5 Wh per W of solar capacity during that time, you would need 1.6–2.0 kW of solar panels to run such a system. Which, at going prices of $0.87 per watt, would cost $1,400 – $1,700 in panels alone to run an average size wall AC unit.
- Learn how to calculate your solar needs: How Many Solar Panels? | Sizing your off grid system
So, while off grid solar AC unites are a possibility, I have been looking for smarter, lower cost, and less complex alternatives.
Alternatives to Air Conditioning for Off Grid Cooling
Long before the invention of modern air conditioning units, people have found ways to stay cool that can be low cost and low energy. Both historical and modern developments have provided ways to cool your home without an electrical connection to the grid, and without large amounts of solar panels to power your energy gobbling AC unit.
Passive Solar Design
The best way to get started is to consider “passive solar” design. Which in this context means that you keep the Sun from heating your house up as much as possible. Proper window placement, eave size, and planting appropriate cover vegetation near the house all factor in. Additionally, the use of thermal batteries such as thick earth walls (eg. cob or Earth bag) to help level out the temperature during the night and day may be enough by themselves to keep your home comfortable.
I’ve seen this concept work quite well, even in sweltering hot India. Thick earth walls and good design alone kept the building very cool and comfortable, especially when compared to other buildings near buy that were built with more traditional construction methods.
See below for more information on passive solar
One of the most exciting new developments in the realm of off grid air conditioning is the renewed interest in absorption air conditioners. Unlike traditional Freon and compressor based air conditioning units, absorption coolers can be built from cheap, safe, and readily available materials.
This video show the construction and testing of a simple absorption cooler built from off the shelf parts.
The technology for absorption coolers has a lot of potential for solar applications, because a good part of the energy for cooling actually comes from heating water, which can be done for free with a solar hot water heater. While the demo unit in the video above ended up using about as much electricity as a typical AC unit, the pumps and fans it requires can easily be chosen to run entirely of DC, saving money and energy for a solar installation. And, with a little more development work, these units have the potential to be much more efficient than AC wall units or heat pumps in terms of energy consumption.
See below for more information on absorption coolers
Earth or Ground Cooling (Earth Coupled Heat Exchangers)
My favorite alternative to air conditioning is to use the Earth as a thermal battery. Under ground, the temperature stays near the yearly average temperature in your area, which is between 70 F to 35 F in the continental United States. If you have ever been in a cave in the summer, then you know that Earth temperatures can be quite comfortable.
Earthship buildings have been using this idea for decades to manage the sweltering summer heat in New Mexico, where they originated, and around the world. Earthships are usually built with the north side being a large earth mound with air transfer ducts built in it. Essentially this earth mound functions as giant thermal battery, as well as protection and insulation for the structure itself.
Increasingly modern buildings are being built with or retrofitted with earth cooling, called by their technical name earth coupled heat exchangers. While the name sounds fancy, they are simply a long tube buried a few feet in the ground, that use either water or air to directly transfer heat from the building in to the ground.
Ground based cooling is also great because it helps heat the building in the winter as well. See below for more information on ground cooling
If you have a source of cool water running through your property — such as a stream, spring, or river — you might consider doing like the ancient Romans, and installing a water cooling system in your home.
The more affluent of the ancient Romans were allowed to use water from the famous roman aqueducts, and divert it to run through the walls of their homes for cooling in the summer. The modern homesteader could relatively easily install a water cooling system using either a heat exchangers and fan, like those used by closed loop earth coupled heat exchangers, or install an in floor radiant heating loop, through which you run cool water in the summer.
For those considering installing radiant in floor heating anyway, adapting the existing pluming to switch to a source of cool water seems like a logical choice.
Solar Powered AC Unit
In some cases, people might just prefer to stick with the same old AC unit they are familiar with, or might choose to run AC to supplement other cooling methods on this list. There are a few possibilities to consider when building an off grid air conditioning system.
- Daytime only, no battery systems
- Direct DC solar AC units
Batteries are one of the major costs of off grid solar systems. Since AC is most needed when the sun is out and bright, one cost saving measure is to run your air conditioner directly from solar panels (with inverter if an AC unit). This will limit the time when AC is able to run, but saves money in the long run.
An additional option to consider is to buy a direct DC air conditioning unit, which is able to run from solar power without the need for an inverter. Inverters not only cost money, but loose some percent of your solar power in the conversion processes, usually %5 – %20 depending on operating conditions.
See below for more information on solar air conditioning
One last consideration is low cost swamp coolers. For those unfamiliar, swamp coolers use a source of water to lower room temperature through evaporation. Swamp coolers are cheap, but they end up increasing the humidity of the room. While probably not the best option for a living space, swamp coolers are potentially useful for cooling livestock or other agricultural spaces.
See below for more information on swamp coolers
Passive Solar Design
Passive solar design leverages simple concepts such as shading and cooling masses to stay cool and save money through systems that don’t use any power at all. If you are planning to build or remodel, then you absolutely need to consider passive solar design, which can reduce heating and cooling requirements by %75 in some climates.
One key element of passive solar design is managing how the sun enters your home. Most homes are built without any consideration for sun position and climate, which finds windows facing any old direction, and eves that do nothing to protect from the sun in the summer, or maximize solar heating in the winter.
Good solar design commonly dictates that windows are mostly south or east facing (ie in the direction of the sun). This allows for maximum solar heating in the winter, optimum insulation on the cold norther side of the building, and protection from hot afternoon sun from the west.
To protect the home in the summer, eaves should be the right length for your latitude to shade the entire window when the sun is high in the sky, but allow lower winter rays to enter and heat the home.
Even existing constructions with poorly designed windows can benefit from plants as a living shade. Fasting growing vines planted along the sides of the house exposed to the Sun make a tangible difference the cost and energy requirements for cooling a home in the summer.
For more permanent fixtures, deciduous trees planted on the west or southwest side of the home offer up superior protection from the sun.
Living shades have the benefit that they either die back or drop their leaves in the winter, providing shade and cooling in the summer but direct access to the sun warming in the winter.
Because it is generally cooler out at night than during the day, using a “thermal battery” to even out the internal temperature throughout the day can reduce cooling costs or eliminate the need for AC altogether.
I’ve seen this idea work handily in very hot climates, like India, where the cooling mass of earth walls was able to keep the house cool all day without air conditioning. Earth construction such as Cob and earth bag construction inherently provide the benefit of thermal masses that keep your home cool the same way as a cave.
For homes using traditional stick frame construction, passive solar designs typically leverage the use of massive floors such as earth floors as a means of storing cool and heat.
Other fast and low cost methods of adding thermal mass to your home includes —
- Building a rocket mass or masonry stove
- Barrels of water
- Trombe walls
Passive solar always works best with optimum insulation value. Anything you can do to provide more insulation for the building supports passive solar design. Common ways to improve your home’s insulation value are —
- Better weather stripping
- Insulated doors
- Double or triple paned windows
- Blow in attic or in wall insulation
- Additional high R value foam insulation (pink foam)
Absorption Based Air Conditioners
Absorption based air conditioners are a completely different technology than Freon and compressor based air conditioners or heat pumps typically found in residential AC units. The biggest benefit to absorption air conditioners is that they can be built at home, DIY, and don’t require any special equipment to service. Currently, absorption based AC are not necessarily more efficient that modern commercial units, but they have the potential to be much more efficient that current designs, and some leading-edge firms are developing absorption designs that are far more efficient than any existing AC design.
As side benefit of absorption based AC unites is that they inherently dehumidify the room air. For typical AC unites, the majority of energy expenditure is actually just to remove water from the air, since moist air feels warmer than dryer air. Nice absorption units actually function by removing water, you may be able to keep the room warmer while still feeling just as comfortable.
How Absorption Air Conditioners Work
Absorption based air conditioners rely on the principal that the processes of water being absorbed into another substance requires heat, and thus cools the air in the process. The material that does the absorbing is called a “desiccant”, and while there are many options for desiccants, one of the most popular is liquid calcium chloride in water.
The reference design shown in the video above operates in three phases.
Phase One. The first phase sprays cool calcium chloride solution through the room air, pulling out moisture and cooling the air. This leaves the calcium chloride solution wet and warm.
Phase Two. In order to get the calcium chloride ready for the next passage around the loop, it needs to be dried out. This is accomplished by first heating it, then spraying it in a column of out door air, to allow excess water to evaporate out of the desiccant. This stage is where much of the electrical efficiency comes in, because the desiccant can be heated for free with a direct solar water heater or cheaply with a gas flame.
Phase Three. Lastly, the desiccant is pre-chilled, first through a radiator to bring it down to ambient temperature, then it is passed through water chilled through evaporation, just like a swamp cooler, except the excess moisture is kept outside the room.
Earth cooling leverages the natural heat storage capacity of the earth to provide low cost heating and cooling for your home. All you need to do this is a length of underground pipe to exchange your room’s heat with the ground. Functional designs exist where room air is blown directly through the underground pipe (direct air heat exchangers). Or, where liquid, usually water, flows through the underground pipe and through a radiator which cools the room air.
How Much Underground Pipe Do You Need For Earth Cooling
The geothermal heat exchange design manual recommends about 150 to 200 feet per ton of cooling for vertical loops, and 30% to 50% more for horizontal loops. One ton of cooling is the same as 12,000 BTU/h or 3.5 kW of cooling.
Does Earth Cooling Work?
Absolutely! I have personally worked in larger buildings that utilized ground cooling, and can attest that it is very effective. Earth cooling has been utilized all of over the United States and Europe, and often boast impressive efficiency compared to typical AC units.
Currently, earth cooling is less common in residential applications, primarily because it is an unfamiliar technology for home builders and home owners. But, for off grid applications it as absolutely ideal. Earthship style construction has used earth cooling for many decades with much success.
One of the biggest benefits for the DIY off gridder is that it is a simple, DIY friendly technology that is easy to understand and easy to service.
AC Wall Air Conditioners or Heat Pumps
So, you want to run AC off the grid. If you haven’t read the above options, especially passive solar design and ground heat exchangers, then you may want to check those out before you commit to off grid AC.
While off grid AC is completely possible and not particularly difficult to accomplish, in my opinion it is just a matter of throwing money at the problem. But there are times when traditional AC units may be the only option, or may be used as a supplement for other less costly design concepts.
Installing an AC air conditioner on a solar system
Running an off the shelf AC (alternating current) air conditioner or heat pump is no different than running any other appliance off the grid, although there are some improvements that you might be able to make since most of the time AC will be run during the day.
The first step is to consider power consumption of your AC appliance. Wall units are typically 500 W – 1200 W in size, while central air systems are typically in the neighbor hood of 3500 W. This figure is the maximum energy consumption of the unit. Since AC units turn their compressors on and off based on the need, there is no way to be certain exactly how much it will use on a particular day. If you built your solar system based on the maximum figure, you will almost certainly have enough power, but you may end up over sizing your system, and thus paying for than necessary for solar power.
For detailed stops on sizing your solar system, check out my article below which walks you through the process.
Direct DC Solar air conditioners
One easy improvement to running a typical AC air conditioner is to find a DC models made to be run from solar panels and batteries. Direct DC unites are much more efficient than AC units, since you do not need to convert the electrical power form DC to AC using an inverter.
There are not many DC units on the market, but one that I have heard recommended is the Securus Air direct DC air conditioners.
It is also possible to convert a commercial refrigerator or air conditioner with an external motor to run off DC by changing out the AC motor to a equivalently sized DC motor. Commercial unites are the best option, because many of them have electric motors that are external to the compressor, and can be easily changed out.
Unfortunately, typical residential air conditioning units are not possible to convert to DC since they usually seal the motor inside with the compressor. Any attempt at modification would end up releasing the refrigerant, which requires special tools and a license to replace.
Off Grid AC with Microhydro Power
One thing worth mentioning about off grid air conditioning is the use of micro-hydro power as an energy source. Because hydro power is relatively consistent throughout the day, and can produced AC power directly, it is a good match for powering off grid air conditioners and heat pumps.
More on micro-hydro power systems here:
Battery-less Solar for Air Conditioning
Another option to consider when designing an off grid air conditioning system is to remove the batteries from the system. Because most of the time air conditioning is most needed during the heat of the day, the extra expense of batteries and battery controllers can be avoided if you are willing to only run your AC unit during the day.
A battery-less design could be accomplished by using a separate solar array just for the AC unit (that could be re-purposed in the winter for other uses) which would be connected directly to a DC air conditioning unit, or directly to an inverter powering your AC air conditioning unit. No charge controller would be necessary, although you may want one to function as a DC to DC converter if your array voltage is higher than your inverter or DC air conditioner can support.
Swamp coolers are probably the simplest air conditioning units possible, they simple run water through a pad, that allows it to cool the air via evaporation. Direct evaporate cooling like this works best in dry climates, and work poorly in very humid climates.
Another option is to spray a fine mist of water in to the air, cooling it directly.
Swamp coolers require a source of water to operate, but take very little power if any to operate, and are worth considering if you live in a dry climate where a little extra humidification would be appreciated.
Because of their low cost, swamp coolers are very commonly used in agricultural applications. Mist sprays are a good way to cool a patio, parts of a garden, or out door spaces where people spend their time in the summer, in order to encourage a healthier outdoor lifestyle.
How do you run the AC off the grid?
AC is fully compatible with off grid solar or other power, but can be expensive due to high power consumption. Low energy alternatives such as passive solar, ground heat exchangers, absorption coolers, and swamp coolers offer potentially lower cost alternatives.
Can you run air conditioner off solar panels?
Yes, with enough solar panels you are able to run an AC unit. For a typical 1000 W wall unit, you would need about 1.6 kW to 2.0 kW of solar capacity to run the AC 8 hours a day. Central heat pumps would need about 3–4 times more power.