Off Grid air conditioning

Shadowcatcher · December 2012

This has been discussed here to some extent, but this is trying to be fairly specific in need/target. We cool our teardrop trailer with a 5,000 Btu modified window shaker AC which works very well. The cool and return air flow through 4" hoses and through marine deck plates in the side of the trailer. The problem is that if we are off grid it requires the use of a generator to power it, noisy stinky...
A battery bank sufficient to power this would probably weigh as much as the teardrop and my 340W of solar would not keep up. So at the suggestion of an AC expert I am starting to think in terms of a multifuel absorption chiller approach, those fuels being solar to help, propane and who knows want else, could be coal, wood...
The other much more likely approach will be a small engine driven compressor system using automotive components.

Blackcherry04 · December 2012

Re: Off Grid air conditioning

There a lot of variables that you can do to use what you have, Ill tell you how I do it. It may work for you, may not.

1) Put it on a 24 V thermostat & relay, where the whole unit and fan cut off.

2) Run it a couple hours before you go to bed with the generator and run the temperature down as cold as you can get it. At the same time charge up your batteries as high as you can get them. I start at around 9:00 and go to bed at 11:00. By then the temperature is around 68 deg or so. I then put the thermostat on 75 Deg. Mine will not cycle on till around 1:00 or so. It will only cycle on and off 4-5 times during the night and then it's only for 5 Minutes or so at a time. The A/C has it's own Inverter ( 1,500 watt ) and 4 GC-2 batteries, but I think you could do it on 2 batteries.

It's the start up current draw thats tough to over come. You have a very small space to cool and may have many more cycles and do not have a lot of thermal mass and 2,000 btu would cool what you have. Some type of a chiller system might work for you.

Here is a site that has 12 DC stuff and a forum that you can find some Information. With the right amount of $$$ you can have anything.

http://www.rparts.com/index.php?cPath=13_1_2&osCsid=2lgmhn07kmlm90jjuaim8bugl6
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Eric L · December 2012

Re: Off Grid air conditioning

A battery bank sufficient to power this would probably weigh as much as the teardrop and my 340W of solar would not keep up. So at the suggestion of an AC expert I am starting to think in terms of a multifuel absorption chiller approach, those fuels being solar to help, propane and who knows want else, could be coal, wood...

I can't help with the multifuel chiller idea, but here's an idea on the solar side: consider a set up with a larger panel/battery ratio and run it just during good sun conditions.

In the summer I run two window units (total 15K btu), plus normal house loads on my pv array during the day on a battery-based system with only 17.5KWh of battery capacity. While that's still more battery than you would want, you only need 1/3 the cooling. My charge controller AUX function turns the AC units off before they can pull the batteries down too far (basically enough to run them through about 20-30 minutes of cloud cover, but not longer).

If you were to use AGM batteries, which as I understand it can safely take higher charge and discharge rates, you might be able to have an 5K btu AC running in decent sun conditions with a small battery bank. The basic idea is that the batteries would be there to provide a voltage regulation and enough supporting power to run for short periods (such as passing cloud cover), but no more. Then you'd have a pv array of sufficient size to both run the AC and recharge the batteries. As a guess, maybe something in the range of 1KW of pv for a 550 watt AC unit.

Of course this still wouldn't help you much at night, if you needed it then too. But it's an idea for possibly using your existing array with a modest expansion.

Photowhit · December 2012

Re: Off Grid air conditioning

I am not familiar with motor driven compressors, but it seems like adding a completely new componet.

I have airconditioned a small well insulated cabin (10x16x 13 tall) for 4 hours or so (on thermostat) with a window shaker and 4 - 6volt golfcart batteries, recharging daily off a 1000 watt array in good sun. So perhaps a combination of a bank this size and running a small generator for a couple hours a day...

I had also looked into doing a smaller 'personal' air conditioner and a small 4x4x8 sleeping chamber, problem is there isn't such a small airconditioner and I didn't have the guts to spend the money to try to make one. There is a small danfrost compressor that you can get a kit for a "refridgerator only" cooler or perhaps you can salvage the compressor out of a glass front soda cooler you see in convienience stores. The only other option I've seen is the pet airconditioner which is smaller btu, but uses nearly as much wattage as a standard 5000 btu A/C.

pHredd9mm · December 2012

Re: Off Grid air conditioning

The major problem with most camp trailers is the lack of GOOD sufficient insulation. The first thing I would look at is checking what insulation there is in the roof, walls and floor. A teardrop is not very large (if what I am thinking of is correct). You might be able to throw an insulating blanket over the trailer at night when the sun is not shining to hold in more of the cold and keep more heat out.

Shadowcatcher · December 2012

Re: Off Grid air conditioning

I have a large teardrop trailer which is almost an oxymoron, as it is a 6X10 and towed with a Subaru Outback. Because it has an all aluminum frame it is only 1600# and 100# of that is a Lifeline 150AH AGM battery. As far as I know there are very few (I only know of one) that have that much battery capacity. Most tears are 4X8 and a fair number are 5X8.
http://www.tnttt.com/gallery/image_page.php?album_id=803&image_id=96128 Here we are a 10,000 feet last summer. We just barely made it to the camp ground at Saddlebag Lake. So more batteries aint going to happen. We do have 1.5" of EPS insulation and from experience 5,000 Btu (tried one at 2,500) is necessary. As much as anything dehumidification when you have 90+ degrees and 90% humidity is important.
For those that boondock, and it is not just us with very small trailers, who wants to hear some ones generator or your own for that matter humming away. If I understand this correctly an absorption chiller could be powered using a camp fire, probably not real practical and that 90 degrees 90% really puts a damper on fires.

Photowhit · December 2012

Re: Off Grid air conditioning

Well you might look for someone who knows a lot about "absorption chillers", my understanding is making a mistake can cause death, not something I want to entertain. FWIW - doubt you needed the batteries at 10,000 feet, so perhaps something you can reconfigure...

Shadowcatcher · December 2012

Re: Off Grid air conditioning

We only used AC once on our trip to California, in Salt Lake night time temps and lack of humidity were wonderful. But in Ohio and much of the Eastern half of the US the 90 90 is a killer when trying to sleep. When I designed/adapted the current Frigidaire window shaker and Espar diesel heater I had in mind being able to leave them at home when not needed.
If you use the ammonia type chiller yes there is some danger, but as with most 'consumer warnings' they are grossly over stated to prevent stupid people from offing them selves. I have worked around ammonia chillers in industrial situations and as long as you can learn to hold your breath briefly and get out of the area, and are in a well ventilated area there is very little real danger. I envision a stand alone system entirely external to the trailer with only the cooled and return air circulating. Other systems use other use other fluids i.e. polyethylene glycol.
People have made inquiries on this forum concerning air conditioning a number of times and I felt looking at a possibly workable alternative to a compressor based system might be of value.

Photowhit · December 2012

Re: Off Grid air conditioning

You likely know more about the subject than others here at least. I thought the ammonia ran in the coils that the recirculated air flowed over, and I thought your trailer wouldn't be ventilated and you likely sleeping? ...but not my end of the world. If you come up with a workable small system I'm sure others, including me would be interested. I think old camper fridges used them and I would think they'd have to be safe being bounced around.

inetdog · December 2012

Re: Off Grid air conditioning

Shadowcatcher wrote: »

If you use the ammonia type chiller yes there is some danger, but as with most 'consumer warnings' they are grossly over stated to prevent stupid people from offing them selves. I have worked around ammonia chillers in industrial situations and as long as you can learn to hold your breath briefly and get out of the area, and are in a well ventilated area there is very little real danger.

Holding your breath is less of an option if you are asleep when it fails. :-)

Shadowcatcher · December 2012

Re: Off Grid air conditioning

What I have been looking at is a LiBr-H2O (Lithium Bromide water) system no ammonia involved this is the description of an available unit http://www.yazakienergy.com/docs/OP-WFCS-0507.pdf
I am still in the information gathering stage and this is still well above my level of incompetence.
Description of how this works, Note that temps are well with in what can be harvested from a solar collector.

Generator
When the heat medium inlet temperature exceeds 154.4oF (203oF max.) the solution pump forces dilute lithium bromide solution into the generator. The solution boils vigorously under a vacuum and droplets of concentrated solution are carried with refrigerant vapor to the primary separator. After separation, refrigerant vapor flows to the condenser and concentrated solution is precooled in the heat exchanger before flowing to the absorber.
Condenser
In the condenser, refrigerant vapor is condensed on the surface of the cooling coil and latent heat, removed by the cooling water, is rejected to a cooling tower. Refrigerant liquid accumulates in the condenser and then passes through an orifice into the evaporator.
Evaporator
In the evaporator, the refrigerant liquid is exposed to a substantially deeper vacuum than in the condenser due to the influence of the absorber. As refrigerant liquid flows over the surface of the evaporator coil it boils and removes heat, equivalent to the latent heat of the refrigerant, from the chilled water circuit. The recirculating chilled water is cooled to 44.6oF and the refrigerant vapor is attracted to the absorber.
Absorber
A deep vacuum in the absorber is maintained by the affinity of the concentrated solution from the generator with the refrigerant vapor formed in the evaporator. The refrigerant vapor is absorbed by concentrated lithium bromide solution flowing across the surface of the absorber coil. Heat of condensation and dilution are removed by the cooling water and rejected through a cooling tower. The resulting dilute solution is preheated in the heat exchanger before returning to the generator where the cycle is repeated.

Eric L · December 2012

Re: Off Grid air conditioning

What I have been looking at is a LiBr-H2O (Lithium Bromide water) system no ammonia involved this is the description of an available unit

It looks like those start at 10 tons, which would certainly keep your teardrop cool but would likely be huge.

There was a Spanish company that made small absorption chiller-based air conditioners for homes designed to be used with solar, but I think they went out of business. Sopogy in HI makes "smaller" solar-thermal collectors (although they're still pretty big) for absorption chillers, generators, and so on:

http://sopogy.com/

I think it will be a real challenge to make a solar absorption chiller as small as you want. Remember you'd still need electricity for the pumps. Although it's almost certainly possible to do, it's also almost certainly easier and cheaper to do it with a pv + battery + inverter + compressor A/C unit. That said, if you manage rig something up with solar-thermal and an absorption chiller you will have to post many pictures and details, since it would be super-cool. 8)

techntrek · December 2012

Re: Off Grid air conditioning

Photowhit wrote: »

... I think old camper fridges used them and I would think they'd have to be safe being bounced around.

Modern RV fridges still use the ammonia absorption system. But considering how much bulk is needed just to keep a few square feet of highly insulated and enclosed space cool, I wouldn't want to consider one for an entire trailer. Even a well insulated and enclosed teardrop.

Blackcherry04 · December 2012

Re: Off Grid air conditioning

I was thinking about the Air Conditioning in the Chevy Volt and The Nissan Leaf. Looks like they both use the pack voltage in some way. I didn't think the voltage was as high as stated below, I thought the battery packs were around 200 V DC, but what do I know. The Compressor alone was over $700, so it wouldn't be doable.

Attachment not found.

" The A/C compressor is a "scroll" type pump that receives 360V DC which is internally inverted to 3-phase AC to drive the variable speed motor. So as in-car heat load varies the speed of the pump is controlled as opposed to clutch cycling or variable displacement schemes. "

Ken Marsh · December 2012

Re: Off Grid air conditioning

One thing you might want to consider is that the absorption cycle is not a so called "heat engine".
Heat engines take energy at one temperature and pump it up to another temperature.
The theoretical energy required to move a given amount of energy to a different temperature is the difference
of temperature between the two divided by the the temp of the high in absolute (Carnot's equation).

The COP of the absorption cycle in practice is always less than one.
The COP of compression/expansion (Rankine cycle) air conditioners, such as your window shaker, is typically 3 or 4.

So much for the thermodynamics. What does this mean in your case?
An absorption cycle system is going to take a lot more energy to operate.

inetdog · December 2012

Re: Off Grid air conditioning

Ken Marsh wrote: »

So much for the thermodynamics. What does this mean in your case?
An absorption cycle system is going to take a lot more energy to operate.

Thanks for the reminder of that, Ken!

And that balances against the greater efficiency and lower cost (no batteries!) of a solar thermal array to run the absorption cycle against the lower efficiency and higher cost of a solar PV array to run the compressor.
Not sure where the final balance point will be, but the difference in COP between the two systems will at least reduce the thermal system's advantage if not actually favor the PV.

Ken Marsh · December 2012

Re: Off Grid air conditioning

Right Interdog,

Absorption cooling was big in the days of low cost energy.
It seems like every one of them I have had to work with we ended up replacing with vapor compression air conditioning.
One of them I remember in particular, a 600 ton unit, took more electricity to run the auxiliaries than we could do the whole thing with regular cooling.

Last year we replaced the absorption refrigerator in our cabin with a Sunfrost.
The absorption unit was drawing an amp and a half electrical @ 12 V just to run the controls.
It had a down converter providing 12 v from the 24 v battery.
I did not keep records but the solar system doesn't seem to know the difference and we are using a third the propane.

techntrek · December 2012

Re: Off Grid air conditioning

Blackcherry04 wrote: »

I was thinking about the Air Conditioning in the Chevy Volt and The Nissan Leaf. Looks like they both use the pack voltage in some way. I didn't think the voltage was as high as stated below, I thought the battery packs were around 200 V DC, but what do I know. ..." The A/C compressor is a "scroll" type pump that receives 360V DC which is internally inverted to 3-phase AC to drive the variable speed motor. So as in-car heat load varies the speed of the pump is controlled as opposed to clutch cycling or variable displacement schemes. "

You are thinking of the Prius with a ~200 volt traction battery. For A/C and heat it does the same thing - the traction battery drives a dedicated inverter which outputs 3-phase AC to drive the heat pump at variable speed. Buying a Volt/Leaf/Prius that has been totalled in an accident, but has an undamaged front-end, might be a way to salvage the parts cheaply. If you are willing to part out the whole thing to make your money back.

Off Grid air conditioning

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