We often get this question. My car has air conditioning when I’m running down the street so why can’t my boat? First, your car only has air conditioning when the engine is running, which works out fine because we only need air conditioning when the engine is running. Second, the air-conditioning load for the car is a tiny parasitic load compared to the power required to drive the car down the road. It’s actually less than the increased drag caused by driving with the windows rolled down. Thus, it is more or less an engineering after-thought with regards to the energy balance of an automobile.
A boat, on the other hand, unless tied to a dock or running a generator, has only the sun and wind to provide energy, and the area to be cooled is larger. But there are 12-volt air conditioners available that I can run right off the battery. Can I just install bigger batteries and a few more solar panels? Well, yes and no. Air conditioning certainly can run off batteries and solar panels and wind generators, but practicality comes down to the energy balance; how much energy are we generating, how much are we consuming, and how do we store enough to last through the night?
Are you planning to run air-conditioning all day, or only at night so you can sleep more comfortably? The reduction of relative humidity that is so important to human comfort is best achieved from an A/C system that is operating almost continuously. In some circumstances, it may be desirable to provide the required cooling with two separate air conditioning systems, one providing two-thirds of the total capacity. This approach permits operation of one system at times when the exterior temperature does not require the total installed cooling capacity. Of course, air-conditioning all-day is a pipe dream for most sailboats without the addition of a generator.
If you need air-conditioning all day, while out on the water and away from the dock, perhaps sailing isn’t for you. There should be at least some breeze, and what’s the point of being on the water if you aren’t outdoors? Thus, our operating assumption is that you need only air-conditioning for sleeping, about 12 hours through the evening and night.
The Power Draw
In our previous report, Air Conditioning for Sailboats, Practical Sailor June 2018, we discussed the installation of a 12,000 BTU (British thermal units) air conditioner on a cruising catamaran. We will begin our discussion of off-the-grid air conditioning with this example. The unit consumes about 800 watts when operating (12 volts vs. 120 volts actually makes rather little difference-we will come back to this later).
When the outside temperature is 90F, the unit runs an 80- to 100-percent duty cycle through the afternoon until the sun gets low, declining to a 30- to 40-percent duty cycle until dawn, which is a good time to get up. The average afternoon duty cycle is about 60-percent. The overnight draw on the batteries will be about 480 amp hours (Ah).
Neglecting any charging input during the night, lead-acid batteries need to have at least three times that rated capacity in order to avoid a shortened life expectancy due to excessive discharge. Additionally, the discharge will be at a relatively high rate, further reducing battery capacity. That means the battery bank of at least 1440 Ah, or about 12 group 27 lead-acid batteries would be needed just for the air-conditioning load. In the case of our test boat, that would have added 10 percent to the displacement, assuming we could find the space.
Readers have pointed out that this was a rather small air conditioner for this boat. In fact, that was intentional, as we wanted to minimize power consumption so that we could run on 15-amp shore circuits and run on batteries for a short time. Well come back to this decision and how it worked.
12 volts versus 120 volts
Intuitively, air conditioning powered by 12-volt DC system should be more efficient than one plugged into 120-volt AC. In fact, the difference in efficiency between 12 volts and 120 volts is quite small.
A few formulas and rough conversion factors can explain this. Battery capacity varies with temperature and rate of draw down. Voltage varies with the battery state of charge and rate of draw down. Charge controller and inverter efficiency come into play, but these effects are relatively small so we can ignore them for discussion purposes.
Our focus is simplicity. In practice, you can add 30 percent or more battery and charging capacity to make up for approximations, battery aging, and inefficiencies in charging.
Watts (W) = volts (V) x amps (A). AC voltage is about 10 times the DC voltage, but there is about a 5- to 7-percent loss in the inverter. As an approximation, assume the 12V DC draw will be 11 times the advertised 120V AC draw.
Amp-hours (Ah) at 12 volts is related to watt-hours (power required to run the device for an hour) by the formula Ah = Watt-hours/12. So an air conditioner rated at 1000 watt draw will draw about 90 Ah from the batteries when running full time for one hour. It will typically run full time for during the day and for 1-2 hours at night.
For comparison, a solar panel rated at 100 watts will typically produce 100W/12V = 9A during the peak hours. Unfortunately, because of clouds, sun angle, and shading, 5-6 hours should be used in the calculation, even though the days are longer than that.
Here are two examples for comparing energy use at 12V and 120V:
Comfort Cool 6000 BTU, 45 amps at 12V, 540 watts, $3,695
Dometic Turbo 6000 BTU, 4.6 amps at 120V, 528 watts, $1,250
The 120V unit is slightly more efficient, the result of running on higher voltage and more refined engineering. As for the big price difference, the 120-volt units much larger market certainly plays a role.
But what about the power losses, converting battery power to 120V AC? Inverter efficiency ranges from 93 to 97 percent, depending on the unit and the load factor. There is an additional 15 to 25 percent loss in charging hysteresis (the charging voltage is slightly higher than the discharge voltage, even when the amps are same, and some amps are wasted) and wiring resistance. The end result is that a 12-volt unit is about 10 percent more efficient at anchor and the 120-volt AC unit is about 6 percent more efficient at the dock or when powered by a 120-volt AC generator.
As an added bonus, 120-volt units are also more available, considerably less expensive, and more readily serviced.
In fact, the 1500-watt inverter on our test boat allowed us to run our 12,000 BTU air conditioner for several hours off just three group 27 batteries, without deep discharge, as long as the boat was already cooled down. Not practical for overnight cooling, but darn handy for getting out of a steamy marina. However, the validity of the power system and the concept is proven.
What about the practicality of generating the required power from solar and wind? The test boat had 200 watts of solar panels on the hard top, which we felt was just enough to handle typical cruising loads, including lighting, galley loads, the occasional movie, and allowing for the occasional cloudy day. Assuming were using A/C only at night, we need to push an additional 480 Ah electricity into the batteries.
If we rely on solar, allowing for clouds and sun angle, and shading, conventional wisdom is to use only 5 hours of rated capacity, and so we need an additional 1200 watts of panels, or an additional 125 square feet of panels. Including existing panels, that’s a 10-by-20 foot area on a 34-foot boat assuming zero gaps. It is possible, with considerable inconvenience, on a multihull.
On the test boat, a PDQ 32 catamaran, every inch of the hard top, most of the deck, and a new arch would be required. Access would be impaired and weight and windage would increase. Achieving that much solar capacity is almost impossible on a monohull. If a larger air conditioner is required, there simply isn’t enough square footage on any boat.
What about wind generators? As we found in our wind-generator test, where and when you cruise can have significant impact on wind generation (see Marine Generator Test, July 2007). Some manufacturers quote some big numbers for output. The most we saw in a single day during our test on the Chesapeake was 115 Ah, and the highest 4-day average was 64 Ah.
Also, if you have a good solid 20-knot breeze at anchor, why are you running AC? In reality, if you need air conditioning there probably isn’t enough wind to provide the needed power. Still, if you want to be optimistic, mount a wind generator and figure on 100 amp-hours (Ah) per day. A wind generator can helps meet the energy requirements but is hardly a solution.
A 12,000-BTU unit is a rather small air conditioner for a 32-foot catamaran. This choice was intentional. We wanted to be able to run on 15-amp circuits and to be able to run for a time on batteries. It works because the boat is well insulated, with 1-inches of foam in the cabin roof and -inch in most other areas. The windows are all double glazed and all are fitted with covers to reflect the sun. An awning provides additional cooling for the cabin top.
If we had wanted to cool only the sleeping cabins at night, we would have installed twin 6000-BTU units; one for the salon and galley areas, and one for the aft cabins. This would have dropped our night consumption by 44 percent (not 50 percent, because large units are more efficient). If we had given up early evening cooling, focusing only on sleeping, the overnight load would have dropped to 250 Ah. This would still require a lot of batteries and panels, but not as many as the single unit approach.
A partial solution is switching to lithium iron phosphate batteries (LiP), which allow faster charging, deeper cycling, and reduced weight (see Jumping in on the Lithium Ion Wave, PS May 2011). The down sides are very considerable expense (far more than the air conditioning unit will cost) and the need for more precise control of the charging process.
Any discussion of on-the-hook air conditioning must discuss generators. This could be a small diesel generator or even a portable generator located in a safe place on deck. (The units exhaust must be located down-wind because carbon monoxide is a major concern.)
A high capacity engine alternator could also help replenish lost amps, but do you really run the engine that much? Over the long haul, this type of low-load use can harm the engine.
Hybrid diesel-electric propulsion systems require high capacity energy storage and a powerful generator. This same equipment used to power the drive system can be used to power an air conditioner. Initially embraced by Leopard catamarans and others in the Caribbean charter trade, the systems were extremely popular a few years ago.
However, more recently owners have begun retrofitting conventional diesel engine propulsion systems. Hybrid power works where there are loads surge (buses and passenger cars) or where control is complex (maneuvering systems, such as tugs and cruise ships), but the steady load of a yacht in transit is more efficiently managed by a direct drive diesel engine.
Is it possible to run air-conditioning from renewable energy sources? With enough insulation, a smallish AC unit, the right batteries, and plentiful charging sources, it can be done. In our research we ran across several cruisers who are doing just that. They mounted lots of panels, increased insulation, installed small AC units, and powered them with lithium batteries.
We also learned that they didn’t actually use the AC much, since it required diligent management of battery charging and generally there was a breeze. Feelings seemed to be mixed on whether it was worth the expense, although they certainly like the having lots of power!
We like the idea of designing the power system so that the AC can run off the inverter for short periods to smooth transitions between power sources and while leaving the marina. But the bottom line is that monohulls don’t have enough deck area and even multihulls become overcrowded with panels.
When you are at the dock-and marinas can be unpleasantly windless-simply plug in and run off 120V AC. When at anchor, seek a cooling breeze when hot stifling conditions are expected. Wind scoops help, fans are mandatory, and swimming is fun.
It kind of makes sense that panels and machinery can’t be more efficient at taking the heat away than the sun is at delivering it.