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Old 04-27-2011, 06:54 PM   #1
Bus Nut
Join Date: Feb 2008
Posts: 362
high efficiency a/c units

So I was doing a lot of research today, as I really don't want to give up air conditioning in my travels. In my research, I rediscovered the ductless mini-split air conditioning systems. Now, I've looked at different models off and on, and keep talking myself out of it due to power requirements. Seems the really efficient ones always want 240V AC input, and the 120V AC ones are inefficient enough you might as well stick with a window shaker.

However, today I dug deeper into the story.

It seems "inverter technology" is the big buzzword in sales these days, and these new super-efficient split systems use this technology. From what I'm reading, using the word "inverter" is very misleading, as it seems they're mostly doing variable DC internally to these systems.

Everything internally seems to be DC, and I think they're using pulse width modulation (PWM) to make things turn only as fast as needed. This goes for all the fans (indoor and outdoor unit fans), as well as the compressor. The compressor is a high efficiency DC compressor. The power needs for a 12,000 BTU indoor unit is 22 watts, and a 9,000 BTU indoor unit is 17 watts. Yes, you read that right. All the real power is in the outdoor unit, which is where the compressor and power supply is located. These things are very quiet, very efficient, and work really awesome (we have a mitsubishi and a sanyo version installed at work).

Now in a tradition A/C unit, the compressor is either ON or OFF. In an "inverter" model, the compressor is variable speed, and can start slow and ramp up, then use just enough power to keep running. With the slow start, ramp up approach, there is no startup surge for the compressor. This means the power rating for an A/C unit is all the power you need to provide to it. Sounds great, right? So just how much power are we talking about?

Well, let's look at a sample configuration. Configuring a 24,000 BTU outdoor unit (1,760 watts), a 12,000 BTU indoor unit (22 watts), and a 9,000 BTU indoor unit (17 watts) gives us a total power usage of 1,799 watts. Did I mention that most of these "inverter" systems are also multi-zone? Also, you can mix and match the indoor and outdoor units (there are charts available to show what configurations will work). This gives you a total of 21,000 BTU of cooling, and runs the system at about 95.5% of capability if fully loaded. That's 11.7 BTU per watt, which is a really impressive number. The seer rating is about 17.

I can see you looking at this and saying "wow, you mean a 2000 watt inverter + 2000 watts of solar + a battery bank big enough will give me 21,000 BTU of A/C?". Well, yes and no. See, these things started in europe, then came to america. In europe, 220V mains are common. When they brought them to america, they basically tweaked them enough to take 60Hz instead of 50Hz, and didn't do much else. So, these things require 220V AC power. This is about 8-9 amps at 220-240V. The problem is that I haven't been able to find a 2000 watt inverter with 240V split phase output. Sure, there are some starting at 3000 watts and up, but none just at 2000 watts. And if you've got the inverter capacity, you better have the battery bank to back it up. And then you better have the means of recharging those batteries (solar, wind, generator, alternator, grid, etc).

Now I got most of this information from this site:

I figure it might be helpful to others.

Is it possible to be air conditioned off grid? I strongly believe the answer is yes. And I believe the key is to make sure you make everything as efficient as possible, then size the equipment appropriately.

An uninsulated bus with 2 8,000 BTU A/C units running 24/7 can't keep it comfortable in 80-90 degree weather. A bus insulated to northern home standards should be easily cooled and heated with much less energy needs. It has been said before, and it bears repeating.... INSULATE, INSULATE, INSULATE. Put as much insulation in everywhere that you can afford (both cost, and space consumption). If you can keep most of the heat from getting in in the first place, it'll be much easier to keep it cool.

I think using a good split-phase inverter, and running one of these really efficient split systems can be made to work, even mobile. It will require a lot of battery bank and a lot of solar capacity. However, what else were you going to do with that roof space? You can even use power from your alternator to augment the solar while driving. Get creative, and rig a wind generator too. Lots of options there.

Of course, the problem with solar is that you need to not just power the equipment, but replenish the battery bank from equipment use after the sun went down. Size things appropriately of course. And bear in mind that the 1,799 watts listed above was the FULL ON level. Doing some smart energy savings will reduce that. For example, 75-80 is comfortable in A/C when it's 90+ outside. Turning off one of the indoor units at night will save some energy. Keep an eye on your battery bank, and if it gets too low, make a decision... do without A/C or get power into those batteries another way (ie, via generator).

I don't know if a bus has enough roof to make such a system work 100% of the time just from solar. But I do know you can certainly reduce the fossil fuel requirements with enough solar.

Oh, and for those that haven't done the math yet... 1,799 watts divided by 120V = 14.99A. Hrm... this means a 15A 120V circuit could *almost* power one of these at full load. So, you could augment your solar with a 15A extension cord powering a battery charger for example.

There are systems out there that have lower power requirements (for lower BTU ratings), and there are non-"inverter" systems that aren't as efficient that will run from 120V sources. You have a lot of options to look at when considering air conditioning.

I suspect that air conditioning from PV sources will be extremely costly. However, designing a smart system from the beginning can make it possible, by starting with small capacity and adding it as needed, modularly. Battery banks can be expanded. Some inverters can be connected in parallel, upgrading the output. PV panels can be bought and installed later. A/C capacity can be added later. Wind power could be added at any point.

I don't think we conversion folks need to limit our thinking, and I certainly don't think we need to let the stick and staple RV folks define our options for us. Their basement A/C systems are expensive and proprietary, and can only be sourced via rv parts dealers. The rooftop A/C systems are overpriced, power sucking, inefficient eyesores with plastic shrouds that degrade in UV light. What idiot thought that would be a good idea? Every rooftop unit I've seen over 10 years old has a super brittle cover, or has had it replaced. The rooftop design is hampered by the 14" square hole that all the cold and return air has to go through. It's common for them to condensate inside the rig. Their weight often sags a roof, leading to puddles of condensate (and rain, thanks to the sag) sitting on the roof. Mosquitos just love those free ponds.

Anyway, this is more food for thought.

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Old 04-29-2011, 01:57 PM   #2
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Re: high efficiency a/c units

Hey all,
we have been using split ductless systems off grid for some time now. With a 1800 sq ft home in AZ and summer temps around 100 quite often it was an excelent answer to cooling. Once things cool off the load drops off quite a lot down near 600 watts when the inside cools to 78F. This is quite do able for a mobile unit. main considerations besides plenty of power would be mounting the outside unit and isolating from road vibration, and that could be handled by rubber mounts for a gen set. Make sure air flow to the unit is not restricted. Proper insulation does the trick for reduced heat load also dropping required watts. These things are real efficant and quiet when operating the compresser unit is right outside a window and the noise is very low. Well researched post .
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Old 04-29-2011, 07:21 PM   #3
Bus Nut
Join Date: Feb 2008
Posts: 362
Re: high efficiency a/c units

Thank Chuck for the real-world feedback.

Unfortunately, I can research and research and research, and learn a lot, but it's all theory for me at this point. I haven't *actually* put my ideas into reality yet. Having someone that has been there and done that, and can report the real world results, is invaluable.

We'll be looking at about 360sq feet of space if everything goes according to plans, with minimum R21 in walls and floor, and the ceiling between R28 at the thinnest point and R56 at the thickest point. Windows will be double pane, with R7 shutters. I'm also doing my best to provide thermal breaks between outside and inside, as the steel frame I'm designing would be a great heat sink. I'd prefer to keep the inside climate under my control, not mother nature's control.

600 watts sounds very good. You've got about 5 times the square footage we'll have.

I've got two different mounting locations in mind for the outdoor units, but I'll probably go with the under-body option rather than on a custom shelf on the back of the rig. It'll be less obvious for one thing, and in a shaded location for another. Great suggestion to use generator vibration mounts. I'll have to remember that when it's time to do the install.

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