Ive read through past posts about air conditioning and running it off solar power. Many people say it cant be done. Some people swear by it. I am confused by it all. Kyocera solar panels is supposed to be the best. There are some other brands that are now coming out with panels that are 230-280 watts (or so they claim). The mini splits are supposed to be the most efficient air. How long do you think until our technology catchs up? There is another guy who says he is an electronics engineer and says he gets far better power out of his solar power by running 6 guage wire instead of the recomended 10 and he has a manual controler and puts it close to his batteries and says his batteries charge much better. There are other people who say this works..... Is it just a pipe dream?
it's all just numbers, exactly what Sean was getting at in the other thread. The AC takes a certain amount of power to run for each hour it's operating. The solar field can create a certain amount of power for it's size and the amount of sunlight it gets. The batteries can store a certain amount of power, and deliver it back over the wiring. It all works by actually pretty simple rules that don't change much. As far as the wiring is concerned, the bigger the wire, the less it takes away when it carries the electricity across it. Yes, 6 gauge wire will take less away than 10 gauge wire. 6 gauge wire is generally considered able to carry 50 amps of current without overheating or causing a lot of voltage loss, while 10 gauge wire is considered able to carry 30 amps. That current capacity is pretty much independent of voltage, but the higher the voltage the more power you get for a given amount of amps, and power is what actually does things, so 48 volts can carry more power across a given wire than say 12 volts can. The goal is simple - minimize losses and be as efficient as possible. Big short wires minimize losses, it's really that simple.
As far as mini split AC units go, there is no real reason for them to be more or less efficient that any other system. They do have an advantage that the condenser unit is outside (or in the bay) and the evaporator unit is inside, but there are other losses that make up for that small advantage. Mini-splits are often intended for home use and so are rated for efficiency, RV units aren't so no one cares.
Bottom line - there's no free lunch, it all matters in the end.
Brian
well, let me try:
To get the 13 amps of 120 v AC to run a big air conditioner, in simple math, you'd need 65 amps of 24 volt DC or 130 amps of 12 volt.
whichever way, it's going to be 1560 watts or so.
And then there will be some losses as you convert the solar DC volts to the AC volts for the air conditioner.
Say 10%?
So, if you can figure out how to get, oh, how about 1750 to 1800 watts of power out of a solar array for all the time you need the AC running, you are in business?
How's that?
happy coaching!
buswarrior
And of course over rate it by a fair % because solar panels only give 100% of their rated output under optimum sunlight conditions and angle. And of course there will be other electrical devices that want/need power.
As an excercise in the math consider about 2000W capacity to power the AC and other smaller items (even a laptop draws 60-90W). For this hypothetical situation suppose you are using 250 watt panels and an inverter with a 10% loss. Factoring inverter loss, you would need 2200 watts. So about nine 250 watt panels. To accomodate less than ideal sunlight, perhaps add 25% making it 12 panels.
Depending on where you want to be able to go, consider that it can be cloudy and still be very hot. Down here in the south we have been cloudy in the afternoons and still have a heat index of 105-110. A couple weeks ago the heat index was 92 at 1am. Out west heat index might not be as much of an issue because it is dryer. But I also know from talking with a friend that in the Vegas area a few weeks ago it was over 100 degrees at 2am.
I am mentioning all that because you also need to think about being able to run your AC and whatever else when the solar array isn't producing peak output or even when it isn't producing any output and in past conversations you have mentioned not wanting a generator. 2200 watts will put about a 185 amp draw on a 12V battery bank or 46A on a 48V bank. So you need to consider that when determining the size of the battery bank. Also remember that to get the best battery life, you shouldn't drain them lower than 50%. With that in mind, hypothetically if you wanted 10 hours capacity at full load with no sun and no generator, at 12 volts you would need a battery bank with approx. 3700Ah rated capacity (about 18 205Ah 12V batteries). That would be 925Ah at 48 volts (still the same number of batteries, just wired together different to get 48V).
If you would rarely need that capability you could derate that some by planning to drain the batteries below 50%. And of course under all but the worst conditions, the AC compressor would cycle on and off so it wouldn't be drawing the heavy load 100% of the time. However, with one AC unit cooling the whole bus, in hot conditions it is going to run a high percentage of the time I would bet.
On the wire size issue, if you are looking at a 48volt system capable of delivering 2200W, the DC wires need 50A capacity and 6awg technically will do that. But for lower voltage drop, I would consider going up a notch in size to 4awg wire.
In your situation I would still strongly suggest having a generator to back it all up because stuff happens and extreme heat exposure can kill.
But.. the day will come where these things will be possible, and this thread will give the young folks such a chuckle!!!
I do know of an MC9 with the designed capability to put up to 16 golf cart batteries in the spare tire compartment, and will run the bedroom AC overnight on battery power with only 8 of them in there.
Assuming the temp already pulled down, yadda yadda yadda
If you hate noise enough, you will insulate and isolate sufficiently to make your dreams come true with current technology!
happy coaching!
buswarrior
Teresa; Are you only interested in A/C, or would this work?
http://www.turbokool.com/ (http://www.turbokool.com/)
Looks like it would work with a much smaller Solar system...Cable
Quote from: happycamperbrat on August 13, 2010, 02:52:56 PM
Ive read through past posts about air conditioning and running it off solar power. ... Some people swear by it. ... There are other people who say this works.
If these people to whom you are referring are claiming to effectively air-condition an RV, steady-state, with RV-mounted solar panels, I will tell you without equivocating they are blowing smoke. I challenge anyone to demonstrate this live. At this writing, with today's technology, it is technically impossible. (I am talking here about stuff you can buy over-the-counter, not someone's research project.)
As I wrote in the other thread, you can run air conditioners from batteries. We do it all the time, and can run eight hours between charges, enough to sleep overnight. But the energy has to get into the batteries somehow, and you simply can not put enough energy into them with 300 square feet of today's panels.
With today's technology, you can potentially get about 3,600 watts of panels into that space (more realistically it will be less, because panels are not made in sizes to perfectly cover the roof). However, the rating of a panel in watts is the peak output under ideal conditions. In the continental US, the average insolation of panels yields three watt-hours per day for each rated watt of panel. You might get that figure up to four watt-hours or even a bit more in, say, Arizona with careful placement and elevation of the panels. But, by contrast, in the more northern latitudes it will be as little as two watt-hours.
3,600 rated watts times three yields an average of 10.8 kWh of energy delivered by the panels. To use this effectively it will need to be stored in batteries, and the act of doing so extracts about a 10% penalty, leaving you with 9.7 kWh of usable energy.
The air conditioner you linked in the other thread has the advantage of being able to use this directly, without the roughly 5-7% inverter penalty that a 120-vac unit would require. It draws 1,850 watts according to its manufacturer. That figure divided into 9.7 kWh yields a total of 5.25 hours of run time.
So if you completely covered the roof of a 40' bus, and you used no other electricity at all (no water pumps, no lights, no fans, no appliances, etc.), you could get a little over five hours of air conditioning a day, if the bus was parked in full sunlight. And at that, it would only be 18kBTU/h, not the 25-40 kBTU/h required to cool a typical 40' bus in full sunlight on a hot day.
In order to store the 9.7 kWh of power, you'd need eight size 8D batteries weighing 1,200 lbs. The solar panels will cost you at minimum $8,000-$11,000 at today's prices, the batteries will cost another $2,000-$4,000 depending on technology, and the charge controller another $500.
To put that in perspective, a portable generator capable of running a 15kBTU/h roof air conditioner would cost less than $2,000, and the roughly $10,000 or more you would save in capital costs will run that generator for over 13,000 hours on $3 per gallon gasoline. 13,000 hours is a year and a half if you ran it 24/7; at 5.25 hours per day (what the $12-$15k in solar would get you, as described above) it could go for nearly seven years of daily use. And if you don't need air conditioning every day of the year, it could take decades before you'd pay off the solar equipment; in many cases, it will never pay back.
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I am confused by it all. Kyocera solar panels is supposed to be the best. There are some other brands that are now coming out with panels that are 230-280 watts (or so they claim).
PV panels are a commodity item. I would not lend much credence to the idea that Kyocera is significantly different from Siemens, SunPower, Sharp, BP, or anyone else. All of these firms are constantly leapfrogging one another. You can buy a Kyocera panel that has been sitting in someone's warehouse for two years and be eclipsed by a SunPower that was made last month. SunPower, BTW, is making the highest-efficiency commercial panel today, perhaps twice as efficient as my example above, but they are pricey and delicate.
A 230-watt panel is no better (in fact it is arguably worse) than a 115-watt panel if it is also twice as large. What you are looking for, in order to get maximum power from the available real estate, is watts per square foot. Bear in mind that for a coach application, smaller panels that can be arranged to cover more roof are often better than larger panels, even more efficient ones, with less coverage. Working out your mounting points, brackets, and anything on the roof that needs to be avoided, such as vents, in order to get the greatest coverage can deliver more energy than simply buying the largest panels available.
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The mini splits are supposed to be the most efficient air.
This is also a bit of mythology. The most efficient unit is highly application-dependent; high-efficiency package roof units will generally deliver the best performance in an RV (although there are exceptions). There is nothing magic about split systems that make them inherently any more efficient than package units. Age of the unit, compressor quality, and proper refrigerant charge will all have a more measurable effect, and placement is paramount. Almost anything with a basement condenser is going to be less efficient than something with a rooftop condenser in a coach application.
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How long do you think until our technology catchs up?
If past history is any indication, a decade or two. Now, a revolutionary breakthrough in PV technology could happen at any time, and that might change things. But it helps to understand the physics -- today's PV panels are deriving nearly 12 watts per square foot, and that's the number I used in my calculations above. The average peak (high noon on a clear day) insolation of the Earth's surface is about 100 watts per square foot. So if a breakthrough happened tomorrow that made PV panels 100% efficient, it would be a factor of eight -- not even an order of magnitude.
More realistically, efficiency of commercially available PV panels might double every decade. If it doubled twice, that would be enough to air condition an RV with RV-attached PV panels. But my crystal ball is cloudy; perhaps we could see 40% efficient PV panels within the decade. Efficiency improvements in air conditioners come at a somewhat slower rate, but they do come, and at some point, yes, it will meet in the middle. But not in the short term.
QuoteThere is another guy who says he is an electronics engineer and says he gets far better power out of his solar power by running 6 guage wire instead of the recomended 10 and he has a manual controler and puts it close to his batteries and says his batteries charge much better.
Any time you are running high DC current for any distance, bigger is always better. 10-gauge is only "recommended" for very small installations. You need to do the math, and select a gauge that brings the voltage drop down below 3% or so. The right gauge for 200 watts is going to be way too small for 1,200 watts (at a given voltage).
I don't know what he means by "manual controller"; the state of the art in PV controllers today is microprocessor controlled and will yield the maximum power flowing from the panels to the batteries. The controller should always be located close to the batteries.
HTH,
-Sean
http://OurOdyssey.BlogSpot.com (http://ourodyssey.blogspot.com)
Thanks everyone for all the great info! I got a lot to digest here. You guys really put a lot into your very thorough answers!
Cable, yes I am also considering a swamp cooler. I already have one roof top but I think I will be needing something to supplement with. Swamp coolers work great where I am right now (BONE DRY HOT DESERT) and I even have one in my house, but I also have an air conditioner in my house. The air conditioner is absolutely a power guzzler compared to the swamp cooler, but it is also dramatically more effective at cooling on certain days....... though the air "feels" more artificial.
BTW, Cable I had to make sure that sight you linked to wasn't inappropriate and against the new TOS otherwise I would have to get my attorney involved lol ;D
It's OK Teresa. I ran it by my legal dept. before I posted it... ;) Cable