Teardrops n Tiny Travel Trailers

Given restrictions on generator use in a lot of places, there would be some advantages to being able to get air conditioning over an 8 hour night (or so) using just battery power. It seems like it might be practical. Here are some back-of-the-envelope calculations put forward for discussion purposes:

How much heat do we have to remove?- A sleeping person gives off about 250 BTU/hr in heat. So, two people = 500 BTU per hour in cooling is needed just to keep them from heating up the sleeping area.
- Heat gain from the outside: If we assume it is 90 deg F outside at night and we want it to be 70 deg F inside, an R-10 wall (2" of rigid foam insulation) will pass 2 BTU per sq ft per hour. If we imagine a sleeping "capsule" with a queen-size bed and 4' high walls all around, that would be 148 sq ft, or 300 BTU required. But even in Dixie we can expect it to cool off a bit overnight, so let's assume that it will be 80 degrees outside in the morning (8 hours later), and at that point the heat gain will be just 150 BTU/hour through walls, ceiling, and floor.
So, we'll figure an average of 225 BTU/hour gain through the walls for the whole night, plus 500 BTU for the heat produced by the people. We'll round up to 750 BTU/hr on average, total. 8 hours of sleep = 6000 BTU of cooling required for the whole night. Note that there's not much point in going for "superinsulated" walls--the 2" of foam gets us to the point where the majority of the heat we're removing is coming from the two people, it's not heat gain from outside.

Now, that's not a huge amount of energy, and might make generator-free cool sleeping feasible.

A small A/C unit: A small 5000 BTU/hr AC window unit draws about 500 watts when it is running, which is a lot. But we need only 750 BTU/hr of cooling, so the unit will only need to run 9 minutes per hour, and in that time it will draw only 75 watt/hours. For the whole 8 hour night, that amounts to 600 watt/hours. For a 12V battery, that's 50 amp-hours, or about what a fully-charged Group 27 deep cycle battery can yield if drawn down to 50% of capacity. Now, there will be other losses (12VDC to 120VAC conversion, etc), but some of these could be addressed by using a DC air conditioner. Plus, the 5000+ BTU A/C unit can be used to cool the whole trailer during the day using a generator or shore power, the nighttime batteries can be recharged from the generator during "generator hours" each day, from the tow vehicle if on the road, etc. And a major "+" for this approach is that it will constantly be removing moisture from the air, which can make even higher temps much more comfortable.

So, real-world considerations that would need to be overcome:
1) Starting amperage on the AC unit would exceed most commonly-available inverters. Maybe a soft-start kit or some other workaround could help.
2) The heat-gain calculations didn't include the gained heat from ventilation air needed by our two sleepers (i.e. the fresh air brought into the sleeping are needs to be cooled). In addition to lowering the temperature of that air, we also need to remove excess moisture in it, which costs some power.
3) The duty cycle of AC unit as described above is very short during the night (9 minutes per hour), which means either frequent on-and-off or big temperature swings. The ideal air conditioning unit for this job might be something a lot smaller---maybe the "guts" of a household refrigerator could be adapted (and it would probably have a lower start-up current, too).

Apologies for the long, geeky post. Anyone with experience trying an overnight "minimalist" AC installation using just battery power?

Mark

Sounds like fun. I was wondering if maybe the best home built teardrop sized cooling source would be an office watercooler. Not sure the btu/wattage of one but it can't be high. It seem to remember reading that AC a D the like prefer to run all the time to maintain efficiency, the start up is what kills power usage and your service life. Maybe a small water cooler refrigerant system, cooling water which can easily be pumped to a radiator in the cabin would be an ideal solution easily hacked together with computer / automotive parts and an old water cooler. You could even get heat with most coolers!

If someone would make a 2000 BTU AC window unit, that might be possible. If the current draw was below 200 watts with a compressor running (under 20 amps at 12 volts), the idle load was reduced to about 5 amps (or less) for the fan, it might be possible to run one over night with a battery. The problem is, there's no market demand for a unit this size & initial cost of the unit would be high.

It's still a tough question to deal with!

The best place to start would probably be the internals from a 12v compressor based automotive fridge. (ARB, ENGLE, etc..)

They don't have the huge start up current draw that a stand ac or even house fridge would. They run on 12v so no inverter needed. This would reduce conversion loss. And if you use a large size unit it would probably be sufficient to cool a teardrop.

noseoil wrote:If someone would make a 2000 BTU AC window unit, that might be possible. If the current draw was below 200 watts with a compressor running (under 20 amps at 12 volts), the idle load was reduced to about 5 amps (or less) for the fan, it might be possible to run one over night with a battery. The problem is, there's no market demand for a unit this size & initial cost of the unit would be high.

It's still a tough question to deal with!

Actually, there is a 2000 BTU a/c unit, but because the demand is so small, it's really expensive.

Dumb thought! Could propane refrigeration equipment be hacked to cool the interior ?

I never really understood building a fire to cool anything anyway but they must work !

Here's a 2500 Btu unit: http://www.climaterightair.com/products/climateright-2500-btu-mini-ac-heater.html for $399.

So, a little more digging around has turned up some potentially useful information:
- The guy at this site ( http://www.beananimal.com/articles/dorm-fridge-aquarium-chiller.aspx ) has some interesting feedback on the use of parts from a small refrigerator to do constant cooling (in his case--of a salt-water aquarium). He says a small dorm-style fridge has a cooling capacity of about 150 BTU/hr, while bigger "neck high" units can move 500 BTU/Hr. Per the calcs in my OP, I'd need about 750 BTU/HR, so it would take something around the size of a typical "regular" home refrigerator to do the job--running full-time when the weather is hot

BUT: These refrigerators are not designed for high duty-cycle operations (i.e. running full time). They are designed to cycle on for 10-20 minutes, then stay off for quite awhile. He says the motor windings will burn up, and the way the refrigerant/compressor is set up (esp the designed "suction side" pressure) is not optimum for steady use where the difference between the input and output pressure across the compressor is high.

So, maybe refrigerator/water cooler type mechanicals won't work for this application.

Now--some good news: "Hard-Start Kits". It appears these may allow us to get by with a smaller inverter and still get a typical 5K BTU window AC unit started. It's just a large capacitor and a relay, and it allows electricity to be stored up (in the capacitor) and used to get the compressor motor going with less peak current draw from the generator. Here's a link to info on one (no endorsement implied) http://www.amazon.com/gp/product/B002JP3MEK/ref=as_li_qf_sp_asin_il_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=B002JP3MEK&linkCode=as2&tag=lo02f-20 It appears they are commonly used by those in the RV world running 13K BTU rooftop AC units from 2000 W generators. These generators often can't provide the peak current to get the AC started, but these "hard start" kits allow them to start te compressor--after that, the generator can supply enough power to run the AC without a problem. So, with one of these installed in our 5K BTU window AC unit, maybe a "regular" DC>AC inverter would have enough amperage to do the job using our battery(ies).

Another idea: To reduce the need for the unit to cycle on and off (with concomitant starting loads and temperature swings in the sleeping compartment), maybe what we need is some thermal mass. If our AC unit is cooling down a chunk of heavy "something" when it runs, and if the fan stays on to keep the air blowing over that "something", then that block of "something" could keep the air cool while it slowly heats up and it would be longer before the AC needed to come on again. Nobody wants to carry around extra weight, but if we could easily use our stored drinking water (5 gals = about 40 lbs) for this job, it could work out pretty well. The challenge would be doing this in a practical way (light, reliable, inexpensive, everything easily cleanable to keep the potable water in good shape). Putting a few cases of bottled beer in front of the AC output would help, and be pretty simple!

aggie79 wrote:Here's a 2500 Btu unit: http://www.climaterightair.com/products/climateright-2500-btu-mini-ac-heater.html for $399.

Pretty neat. About four times the cost per BTU of a typical $175 5K BTU window unit, but everything is in one box with simple "in" and "out" ducting fittings already provided.
I'm surprised that it takes 480 watts to run it, that seems high compared to the regular "window shakers" which move about 10 BTU per watt.

vigilant1 wrote:

aggie79 wrote:Here's a 2500 Btu unit: http://www.climaterightair.com/products/climateright-2500-btu-mini-ac-heater.html for $399.

Pretty neat. About four times the cost per BTU of a typical $175 5K BTU window unit, but everything is in one box with simple "in" and "out" ducting fittings already provided.
I'm surprised that it takes 480 watts to run it, that seems high compared to the regular "window shakers" which move about 10 BTU per watt.

I had the predecessor to this unit. Others had different experiences but it cooled my teardrop well. The greatest test was a June camping trip - full sunlight, 100 degrees daytime temp, 60% humidity. It held to about 80 degrees but cooled further down as the sun set and temperatures dropped. After about 5-6 years of service it recently quit running and I'm looking for a replacement. I am now preparing to replace it with a 5K Btu unit.

It is probably a situation of diminishing returns electrically. The blower on the predecessor had the same CFM as my window unit, so even though the compressor demands scaled down, the power to run the blower is about the same.

BTW, I mentioned the direct sunlight condition above. I believe the radiation heat gain would be in addition to any convection heat gain.

Vigilant1 - love the idea of using the capacitor to help balance the starting load. I admit that in our little trailer we have a 3.3 ft3 Haier dorm refrigerator that during travel we run from a 2000 watt Centech inverter, with tow vehicle charging the trailer battery. It has always cranked up and run fine, in fact really, really well. We have to turn it way down to keep it from freezing everything in the fridge.

I've been working on building the DIY petcool. I was using an old 5k btu window unit I got from an RV scrap yard for $10. It does work, but it is just about rusted all to heck, so I really did not want to use it. Tah Dah - last week we had a multi-family garage sale and my neighbor was selling a 5k btu window shaker that looked like it just came out of the box. I got it for $15. So now the nice looking one will be used for the DIY cooling unit to tie into my duct system. This thread has now got me really thinking about being able to run the unit with the inverter. Actually, the AC uses less power than the fridge. I guess I need to go out and see if it will crank up on its own, but I think I'm going for the capacitor simply to help it out.

dave

. . .

vigilant1 wrote:

daveesl77 wrote: I guess I need to go out and see if it will crank up on its own, but I think I'm going for the capacitor simply to help it out.

Dave,
It would be interesting to see if it will crank, and the current draw (DC amps) when it is running. While the compressor will draw a a lot of juice to start, it is for such a short time that I don't think it will significantly impact the total available running time on battery (unless it is cycling on and off every 5 minutes). BTW, if the "hard start kit" capacitor provides lot of current as quickly as the motor calls for it, it probably helps reduce wear/heating of the compressor motor (vs waiting for a genset to come up to speed)

aggie79 wrote:It is probably a situation of diminishing returns electrically. The blower on the predecessor had the same CFM as my window unit, so even though the compressor demands scaled down, the power to run the blower is about the same.

Yes. I'd guess that scaling down the compressor results in less and less efficiency (e.g. volume reduces faster than the friction-inducing swept area of the pistons/vanes/whatever).

aggie79 wrote:BTW, I mentioned the direct sunlight condition above. I believe the radiation heat gain would be in addition to any convection heat gain.

Agreed. Of course, not a significant issue at night (unless parked over a hot, black parking lot, etc), but in the daytime getting into the shade (or putting a light colored awning over the rig) would make a big difference.
Tom,--did you ever try to run the small AC unit from a battery?

Mark

I just ordered the spp6 from Amazon for $11. What the heck, worth a try. I did find a youtube video where another teardrop guy put this same thing in a Haier 5k and ran it from his Honda 1000 without any problems. We shall see.
Oh, I did just try doing a direct run from my inverter, but it tripped the inverter circuit breaker. I then realized the battery had not been charged in quite some time, so now charging it and will try again tomorrow. In addition, the battery I presently have is a 6+ year old mopar that a friend gave me a few years ago. Darned thing still holding a charge (well up to now, so we'll see how it is now) really well and has worked for us in both the old truck camper and now the trailer. Yes, it is way past its "stale" date, but what the heck, if it still works, I'm happy. If not, I've gotten a few years and many thousands of miles of use from it in the two campers.

dave

daveesl77 wrote:I just ordered the spp6 from Amazon for $11. What the heck, worth a try. I did find a youtube video where another teardrop guy put this same thing in a Haier 5k and ran it from his Honda 1000 without any problems. We shall see.

Super. Hopefully it will work.

daveesl77 wrote:Oh, I did just try doing a direct run from my inverter, but it tripped the inverter circuit breaker. I then realized the battery had not been charged in quite some time, so now charging it and will try again tomorrow.

I'd guess the circuit breaker tripping was all about a flow of current that exceeded the unit's capacity, and not due to a weak battery. When you get the spp6 installed maybe it will start okay. If you get a chance: what's the circuit breaker rating on the inverter, and what is the amperage/VAC or wattage rating of the AC unit (as shown on the dataplate)? Sometimes they show starting current, but often it's just the running current. For example, here (http://products.geappliances.com/appliance/gea-specs/AEM05LV ) is a low-price GE EnergyStar 5200 BTU unit with a running current of 3.7A at 115VAC, or about 420 watts (= 35 Amps at 12 VDC, plus any conversion losses). I'm guessing the starting amperage is probably double that, if no "hard-start" kit is used.


Thanks,

Mark