Solar Powered AC in Camper Van - Advice?

soylentgreen
soylentgreen Solar Expert Posts: 111 ✭✭
in Off Grid Solar & Battery Systems #1
I have a small camper van with pop top, into which I've rigged a 5000 BTU home window air conditioner unit. I've tested this while at an RV park on shore power and it works great. I've also tested the system on the van's built in battery/inverter system, and this also works for short periods of time (the inverter does have enough juice to handle the AC startup surge draw).

I'm planning a desert camping trip to watch the Perseids meteor shower, and the question is: how many days can I run the unit on my van's internal battery bank. Would appreciate help with my calculations.

Data:

Batteries: 2 x Trojan T145s which are rated at 215AH at C/5 rate.

Inverter: Vector "2000w" inverter which claims 87% efficiency.

Solar Panel: 1 x 85W Solar Panel

Charge Controller: MorningStar Sunsaver Duo

Insolation : 8 hours (southwest high desert in mid August, solar panel directly South with about 30 degree slant) This map claims 8 hours ish http://www.nrel.gov/gis/images/map_pv_us_august_dec2008.jpg

AC draw : 500w at startup, but when temperature is reached and duty cycling begins, averages approx 265W (measured with KillaWatt).

Calculations:

By normal calculations, my batteries have 2580WH*50% DOD = 1290WH usable. AC has a 265W draw / 87% inverter efficiency = about 300W.
1290WH/300W = 4.3 hours usable time. So perhaps I could use the AC for one afternoon before I have to go home.

Modifying factors:

* I'm willing to hammer the batteries down to 80% discharged. So that gives me 2064WH usable. I'm up to 6.9 hours now.

* I have a solar panel - assuming clear skies and perfect tilt and perfect 8 hours insolation (it's the high desert, after all), that gives me 85W*8H*0.77 derating = 523WH/ day. So I have 2064WH+523WH=2587WH/300W=8.6 Hours runtime now.

I feel as if this is in the realm of possibility -- clearly I could have about 1 afternoon running the AC. I want to stretch this to 2-3 days if possible.

Decisions to make:

* I could easily mount another rooftop 85W solar panel. Cost $220. This would double my solar input so that I'm adding 170W*8H*.77 derating = 1047WH of solar per day. This would give me 2064WH+1047WH=3111WH/300W = 10.3 hours per day. So if I limited myself to 5 hours AC per day, that'd be almost 2 days. (Of course on day 2 I'd have more solar, so the calculations change: 2064WH+1047WH*2days = 4158WH (over 2 days). 4158WH/300=13.9 hours. So on a 2 day trip, I could run the AC for 7 hours each day.

* I could easily double the battery capacity (using 2 Costco 6V Golf Cart batteries). Cost : $200. Assuming I'm willing to hammer these down to 80% DOD, my total capacity would be 215AH*6V*4*.8DOD = 4128WH. Add in the existing solar (523WH) and I have 4651WH/300W = 15hours.

Questions:

* Am I making any bad assumptions here? Is 8 hours of insolation in the high southwest desert (4000' altitude) in August reasonable?

* The inverter is not a true sine wave. Is 87% efficiency (claimed) going to be true with a 50AMP draw on an an AC compressor reasonable?

* Is the solar derating of 0.77 realistic? I'm assuming that during the heat of the day when I need the AC the most, is when I'll have the strongest solar, so the solar electrons will be going directly to the inverter (rather than panel->battery->inverter).

Advice?

* Adding more batteries vs. more solar panel? About the same cost. Pros & cons to each: More batteries might behave better as the DOD increases (avoiding inverter cutouts due to low voltage). More solar: gives more electricity every day.
· Share on Twitter

Comments

  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #2
    Re: Solar Powered AC in Camper Van - Advice?

    Welcome to the forum.

    Let's start with how the batteries will perform against that air conditioner. Not very well, I'm afraid.
    The T145 is rated as 260 Amp hours @ the 20 hour rate, the standard for solar power. That means at 50% DOD you have roughly 1560 Watt hours DC available. The inverter will consume some (MSW type will be worse), and more will be lost in the conversion to AC. You will be lucky to have a little more than 1kW hour AC from them. Based on the A/C drawing roughly 300 Watts average, that's about 4 hours run time.

    Now if you want to recharge those batteries you'd need about 400 Watts of array. 85 Watts will do nothing for them except maybe stave off the self-discharge. It just doesn't produce enough current to have any serious effect on that much battery, never mind the A/C load. Obviously an 85 Watt panel can not offset a 265 Watt load.

    If you want to extend the A/C run time you'd need enough panel to power it and provide some charging to the batteries so they don't end up being discharged at the end of the day and sit that way over night. If you add more batteries you will only exacerbate the problem. You need more charging power, and a good charge controller as well.

    My advice is to get a small inverter-generator and a battery charger. Run the A/C when you need it and boost/recharge the batteries with the gen. It will be cheaper and easier than trying to outfit your RV with 400+ Watts of panel.
    · Share on Twitter
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #3
    Re: Solar Powered AC in Camper Van - Advice?

    in general, you are doing what nearly all newbies do, over estimate the amount of solar time and hence the power produced during the day and the real capacity due to high drawing currents that logarithmically shorten the time it can deliver the current. you may even be underestimating the draw of the a/c, but i can't confirm this.

    now adding more battery will also up the charging requirements. generally, the batteries should be charged at 5%-13% of the ah rating of the batteries. take the stc rating for the current (imp) and you can derate that by 77% for a good rough charge amount. also between 2 and 6hrs of full sun depending where one is and the time of year. desert sounds like 5hrs with possibly 6hrs summer.
    · Share on Twitter
  • soylentgreen
    soylentgreen Solar Expert Posts: 111 ✭✭
    #4
    Re: Solar Powered AC in Camper Van - Advice?

    Thanks for the replies, let's take them item-by-item, as I want to make sure I understand.

    Niel, you say "over estimate the amount of solar time" -- remember I'm talking about a single three-day trip in a specific place on a specific date. The NREL map shows 8 (ok, maybe 7.5 to 8) hours of insolation for that time/place. Am I misreading that chart somehow? Again, I'm not concerned with year-round averages, shading, bird droppings, etc. This will be full sun and perfect panel orientation.

    Niel and Cariboocoot both express concern that the solar panel is undersized for the battery, and that this problem would worsen with bigger batteries. I'm not sure I understand this : in my experience, the 85W panel does just fine recharging the batteries. Agreed, there's no way that 85W could recharge the batteries while there's a 300W load on the system, but that's not the goal here.

    Niel "you may be underestimating the draw of the AC" - I took actual killawatt measurements to get the 265W average (once the van interior has cooled down) then divided by the 87% inverter efficiency. I can think of a few ways these #s may be low: perhaps in the hot sun the AC won't duty cycle as much as I want (e.g. running full bore at 500W)? I've read that inductive loads such as motors use more juice when running on a non sine wave inverter, but I don't have any numbers for that. On the plus side, my 265W average measurements were not taken with my full insulation panels in (I have reflectix panels to insulate the pop top in really hot weather, so that could help keep the cooling needs lower).

    I appreciate both of your comments, but I have the feeling you are approaching this from the "normal rules" perspective (50% DOD, long-term sustainability, happy battery treatment, etc.) I'm taking a different perspective : If I am happy to drive out of the park with fully discharged (and slightly damaged) batteries, how far can I stretch things?

    I just thought of a third idea: I could also buy a 2nd solar panel that's too big to fit on the roof and just put in on the ground near the van. If I got a 250W panel, then I'd have 250+85=335W which would be getting closer to the average AC load I'm expecting. A major problem with this, however, is that I think none of the 250W panels are built for 12V systems, and I don't have a MPPT controller so that wouldn't work. Perhaps 2 x 135W 12V panels would ?
    · Share on Twitter
  • waynefromnscanada
    waynefromnscanada Solar Expert Posts: 3,009 ✭✭✭✭
    #5
    Re: Solar Powered AC in Camper Van - Advice?

    I also learned a few things the hard way. Niel and Cariboocoot are trying to save you a lot of disappointment. I'll leave it at that.
    · Share on Twitter
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #6
    Re: Solar Powered AC in Camper Van - Advice?

    Over-estimating solar time: unless you continually re-aim the panel directly at the sun you do not really get 8 hours of full insolation. Change the angle, lose power. Even if you do track the sun, the early and late sun will not be as intense as it has more atmosphere to go through. Up here we get 16 hour days, but the equivalent good sun time is only about six of those hours. That's really about the max you can expect anywhere.

    Recharging:
    in my experience, the 85W panel does just fine recharging the batteries.
    No, what you've experienced is that the 85 Watt panel can put back the apparent used Amp hours over time. This is not the same as properly recharging the batteries so that they have a long lifespan. In the old days it was standard formula: big battery bank recharged by small amount of panels over a long period of time. Panels were expensive, batteries were cheap. The batteries didn't last though. Now panels are cheap and batteries are expensive so the recharging standards call for hitting them with enough current to knock the soft sulphation off and sustain high Absorb Voltage long enough to really recharge them. Read through this bit of the Battery FAQ's: http://www.windsun.com/Batteries/Battery_FAQ.htm#Battery%20Charging

    A/C or any thermostatically controlled device is subject to the variables of the weather. We didn't even mention the usual start-up surge sucks the power down so low that the inverter shuts down problem, as you seem to have got around that. I'd really suggest ditching the MSW inverter for a pure sine version for two reasons: they're more efficient and the A/C motor will last longer (induction motors do not run well on MSW).

    The "normal rules" we work from are the laws of physics. Discharge below 50% and you soon won't be able to recharge no matter what, especially if the batteries are left discharge for hours on end. If you're will to kill and toss batteries that's one thing, but it's a pretty expensive way to do it. There is a direct correlation between depth of discharge and cycle life of batteries. Is this a long-term investment or a throw-away experiment?

    The mobile panel solution is one often used by RV'ers. If you can't fit enough panel on the roof (or don't want to park in the sun) you can put some "external" panels out where they'll do you good. The things to watch out for is that panels sitting on the ground are more easily damaged (or stolen) and the wire running back to the RV becomes a bit of a hazard so go for good, heavy outdoor wire that won't break if someone drives over it or trips on it.

    You're right about the high Watt panels being >12 Volt systems can use without the MPPT. They are also large and cumbersome for moving about in a mobile application. The KD 135 or 140 are still 12 Volt (17.7 Vmp) and are about the biggest a person could manage easily on their own. At about 7 Amps each, three of these in parallel on a 25 or 30 Amp PWM controller would make those batteries fairly happy, providing the A/C cycling co-operates as well.
    · Share on Twitter
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #7
    Re: Solar Powered AC in Camper Van - Advice?

    my comments in bold.
    soylentgreen wrote: »
    Thanks for the replies, let's take them item-by-item, as I want to make sure I understand.

    Niel, you say "over estimate the amount of solar time" -- remember I'm talking about a single three-day trip in a specific place on a specific date. The NREL map shows 8 (ok, maybe 7.5 to 8) hours of insolation for that time/place. Am I misreading that chart somehow? Again, I'm not concerned with year-round averages, shading, bird droppings, etc. This will be full sun and perfect panel orientation.

    i have my doubts as i've never heard of it being that high here in the states or anywhere else for that matter. also, the heat will degrade the output some too. then there's the issue of you are moving to other locations.

    Niel and Cariboocoot both express concern that the solar panel is undersized for the battery, and that this problem would worsen with bigger batteries. I'm not sure I understand this : in my experience, the 85W panel does just fine recharging the batteries. Agreed, there's no way that 85W could recharge the batteries while there's a 300W load on the system, but that's not the goal here.

    not sure of what your goal is then.

    Niel "you may be underestimating the draw of the AC" - I took actual killawatt measurements to get the 265W average (once the van interior has cooled down) then divided by the 87% inverter efficiency. I can think of a few ways these #s may be low: perhaps in the hot sun the AC won't duty cycle as much as I want (e.g. running full bore at 500W)? I've read that inductive loads such as motors use more juice when running on a non sine wave inverter, but I don't have any numbers for that. On the plus side, my 265W average measurements were not taken with my full insulation panels in (I have reflectix panels to insulate the pop top in really hot weather, so that could help keep the cooling needs lower).

    ok, in a precooled environment it will cycle, but if pressed into service after it has heated up for quite some time it will tend to run constant or at least far more. if you measured the draw precooled then try it again with it sultry and get the kwh logged as this is more reflective of what you need. also keep a good record of the time it took to record the kwh measured so that a per hour basis can be derived.

    I appreciate both of your comments, but I have the feeling you are approaching this from the "normal rules" perspective (50% DOD, long-term sustainability, happy battery treatment, etc.) I'm taking a different perspective : If I am happy to drive out of the park with fully discharged (and slightly damaged) batteries, how far can I stretch things?

    no not quite as a battery that is stressed with a high draw will not last as long due to the peukert effect. if you look at most quality battery specs they will list the ah with differing times. if you divide it out it shows the draw over that time. example-93ah for 8hrs instead of 100ah for 20hrs. (numbers are made up by me, but are representative of what you will find) now dividing 93ah by 8 gives a draw of 11.625a. the other would be a draw of 5a. now 265w/12v=22a draw and would most likely be at a 1 or 2hr rate. the capacity actually drops under high loads.

    I just thought of a third idea: I could also buy a 2nd solar panel that's too big to fit on the roof and just put in on the ground near the van. If I got a 250W panel, then I'd have 250+85=335W which would be getting closer to the average AC load I'm expecting. A major problem with this, however, is that I think none of the 250W panels are built for 12V systems, and I don't have a MPPT controller so that wouldn't work. Perhaps 2 x 135W 12V panels would ?

    270w x .77eff = 208w. 208w/12v = 17.33a of charge. 17.33a/215ah=8% charge rate, which is good, but your draw is higher than your charge too. maybe add 2 135w pvs (now 140w pvs are available) to the 85w for 355w total. hmm looking back i see the trojan ah is for a c5 rate. that's the discharge rate for those batteries to be dead in 5hrs.:cry:

    just to reiterate here, you definitely don't want to go below 80% dod or you could damage the batteries. high draws will shorten the battery life.
    · Share on Twitter
  • icarus
    icarus Solar Expert Posts: 5,436 ✭✭✭✭
    #8
    Re: Solar Powered AC in Camper Van - Advice?

    One other quick point. Parking in the sun to take advantge of the PV adds considerably to the A/C load. If you park in the shade, use a portable mount that is in the sun, you get closer to cracking the nut. That said, this is a tough nut to crack as designed. If you don't care about cooking a set of batteries, and you are only going to do this for a few days,, then go for it. If are flooding for a real solution, this isn't it.

    I would suggest running your small a/c off a Honda Eu1000 genny. It will run off it, ur a couple of quarts a day, and with the a/c going, wouldn't hear the genny.

    Tony
    · Share on Twitter
  • soylentgreen
    soylentgreen Solar Expert Posts: 111 ✭✭
    #9
    Re: Solar Powered AC in Camper Van - Advice?

    Thanks again to all for the great suggestions, and for playing along with me in my semi-foolish design quest.

    Re: Shade -- agree 100% but where I'm going there are no trees, so full sun it is. The van has a insulated white roof so it's pretty good at keeping the heat out, and I will supplement that with reflectix panels.

    Re: heat effects on solar panels. That's a good point, it's forecast to be frigging hot (like 110F) so I should probably consider that.

    Re: heat effects on the AC needs. Also a good point, it may be that my 265W average is a pipe dream, and that the AC compressor will run 100% duty cycle in 110F temperatures? Definitely need more research there.

    Re: battery discharge rates. I was using the C/5 rates for the trojans, which (if I understand it) is the AH rating (215) that would discharge the battery in 5 hours. The C/20 rate on those is 260AH, but I felt like that was cheating so I used the lower number to be conservative.

    Re: nuking the batteries: I definitely don't want to destroy the batteries, but I'm willing to shorten their life a bit if that's what it takes. I saw one chart that suggests a 80% DOD cuts the lifetime in half, but that's still a few hundred charge/discharge cycles. I don't take the van out that often, so I think that's OK - I think these batteries will die of old age before they die of # cycles.

    Re: Gas generator. Sadly, that's probably the most cost-efficient solution. I'm trying to do this w/o gasoline, on principle, but I agree that a Eu2000i is a tempting choice.

    Other ideas: this is dry desert camping, so another idea I had was a swamp cooler : would have a much lower power draw than an Air Conditioner, but I'm not sure how much water it would take. The van interior is probably 100 square feet or less. Any ideas how much water a swamp cooler would use?
    · Share on Twitter
  • soylentgreen
    soylentgreen Solar Expert Posts: 111 ✭✭
    #10
    Re: Solar Powered AC in Camper Van - Advice?
    Cariboocoot wrote: »
    Over-estimating solar time: unless you continually re-aim the panel directly at the sun you do not really get 8 hours of full insolation. Change the angle, lose power. Even if you do track the sun, the early and late sun will not be as intense as it has more atmosphere to go through. Up here we get 16 hour days, but the equivalent good sun time is only about six of those hours. That's really about the max you can expect anywhere.

    Confused (but open to being proven wrong and learning something). Reading the NREL graphs, it says "Solar Resource, Flat Plate Tilted South at Latitutde" - August" and shows 7.5-8.0 hours at the location I'm at. I'm pretty sure this means that the equivalent sun-hours is 8.0. Can you clarify?

    Cariboocoot wrote: »
    Recharging:
    No, what you've experienced is that the 85 Watt panel can put back the apparent used Amp hours over time. This is not the same as properly recharging the batteries so that they have a long lifespan. In the old days it was standard formula: big battery bank recharged by small amount of panels over a long period of time. Panels were expensive, batteries were cheap. The batteries didn't last though. Now panels are cheap and batteries are expensive so the recharging standards call for hitting them with enough current to knock the soft sulphation off and sustain high Absorb Voltage long enough to really recharge them. Read through this bit of the Battery FAQ's: http://www.windsun.com/Batteries/Battery_FAQ.htm#Battery%20Charging

    I'm not disagreeing here, but the context of my question is "will this work for 3 days" not "what's a good long-term strategy". I think you are answering that latter question.

    Cariboocoot wrote: »
    A/C or any thermostatically controlled device is subject to the variables of the weather. We didn't even mention the usual start-up surge sucks the power down so low that the inverter shuts down problem, as you seem to have got around that.
    I've run only short (1-2 hour tests) so I'm not sure that problem is solved....


    Cariboocoot wrote: »
    I'd really suggest ditching the MSW inverter for a pure sine version for two reasons: they're more efficient and the A/C motor will last longer (induction motors do not run well on MSW).

    I've heard this too -- any idea what % inefficiency hit this adds?

    Cariboocoot wrote: »
    The "normal rules" we work from are the laws of physics. Discharge below 50% and you soon won't be able to recharge no matter what, especially if the batteries are left discharge for hours on end. If you're will to kill and toss batteries that's one thing, but it's a pretty expensive way to do it. There is a direct correlation between depth of discharge and cycle life of batteries. Is this a long-term investment or a throw-away experiment?

    The graphs I've looked at (e.g. http://www.windsun.com/pictures/cyclelife2.gif from this site) shows a 50% discharge giving 1000 cycles, with an 80% discharge dropping it to only 500 cycles. For my application (an RV used perhaps a dozen trips per year, each trip of 3-5 days) I think that's not a difference that I care about. Am I misreading /misunderstanding the graph?

    Cariboocoot wrote: »
    The mobile panel solution is one often used by RV'ers. If you can't fit enough panel on the roof (or don't want to park in the sun) you can put some "external" panels out where they'll do you good. The things to watch out for is that panels sitting on the ground are more easily damaged (or stolen) and the wire running back to the RV becomes a bit of a hazard so go for good, heavy outdoor wire that won't break if someone drives over it or trips on it.

    You're right about the high Watt panels being >12 Volt systems can use without the MPPT. They are also large and cumbersome for moving about in a mobile application. The KD 135 or 140 are still 12 Volt (17.7 Vmp) and are about the biggest a person could manage easily on their own. At about 7 Amps each, three of these in parallel on a 25 or 30 Amp PWM controller would make those batteries fairly happy, providing the A/C cycling co-operates as well.

    That may be worth a try. This is, as I've tried to emphasize, more of a "mad-scientist - can it be done" type of test rather than a "is this a solid plan for long-term operation". I do have a friend with a KC135, maybe I can borrow one and try to purchase another?
    · Share on Twitter
  • icarus
    icarus Solar Expert Posts: 5,436 ✭✭✭✭
    #11
    Re: Solar Powered AC in Camper Van - Advice?

    To be clear, an Honda EU 1000 will run a small window a/c unit. ( and burn ~1 litre of fuel in ~6 hours I'd memeory serves) it seems sort of penny wise and pound foolish on the "green principle" front to worry about that 1 litre of fuel, when the vehicle itself will burn that my much in a couple of miles.

    Mind you, I am not making a judgement on the merits of green camping or not,,just keeping a bit of perspective. Like I said borrowers, I think the best alternative is the gennie option, given that a good used Eu 1000 can be had for under $500, Weighs less than 30 lbs. One could get rid of the weight of the large battery bank, and save enough fuel driving the rig than the genny burns!

    Tony
    · Share on Twitter
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #12
    Re: Solar Powered AC in Camper Van - Advice?

    Given that battery life is not the primary concern here, the worst problems you're up against are the high current draw to start the compressor and the sheer Amp hours for running the A/C any length of time.

    The first one is best overcome by higher system Voltage, but obviously that's not desirable here. So instead you need sufficient Amp hours that can take the sudden "jolt" and have extra heavy wires to the inverter to minimize Voltage drop under the high current state.

    The second can be at least partially mitigated by some panel, but it would need to be enough to at least slow the A/C draw. 265 Watts of AC could be offset by about 400 Watts of panel. The cycling would allow at least some battery recovery withing that time frame. Otherwise you could simply load up with enough batteries to power the A/C for the whole trip, and worry about recharging them afterward.

    The flaw in the NREL graph is that panels do not put out full power unless the sun is square-on. With the varying angle, the average output within that time frame is less. Add the heat derating factor and you could find your averaged output over the 8 hour time is closer to 70% than 80%. That last number is a typical derating for panels only under "normal" circumstances. The thing is, in the real world there's no such thing as "normal circumstances". :roll: More than anything else, real-world atmospheric conditions will interfere with "ideal" panel output. The #1 mistake people make is expecting their panels to put out the STC rating. Then they're disappointed when they fall far short of that. If you're planning on using panels as a "life extender" for your batteries it is important to realize those panels are going to be putting out much less than STC, no matter how much time they spend in the sun.

    One big advantage you'd have with the portable panels is the ability to re-aim them throughout the day. That would bring your averaged output up over the time frame involved. If you use two Kyocera 135's (relatively inexpensive, easy to handle) you could get slightly more than 1 kW hour AC over the 8 hours with some judicious re-aiming. This might be your best strategy for panel power.

    The MSW inverter is still a bit of a nightmare. They vary a lot in waveform from one to another. Some are really just square wave, but the better ones have many stepped square waves and some are pretty near sine (like noisy utility power with THD around 5%). To make matters worse, different motors will respond differently to different waveforms. It's like a sadistic guessing game: "Will this work? If so, for how long?" 20% decrease in efficiency would not be unrealistic, but again; it all depends.

    If you use those two batteries you've got and supplement them with two 135 Watt panels periodically re-aimed & 20 Amp controller, ditch the MSW for pure sine and connect with big wires you could stretch that A/C time out to the whole 8 hours if the unit will co-operate with cycling time.

    If this seems like a lot of hedging it's because one thing we've learned here over the years is that people who try to run things at the edge often fall over it. :p
    · Share on Twitter
  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #13
    Re: Solar Powered AC in Camper Van - Advice?

    Here are a couple inverter FAQ's from our host (Northern Arizona Wind and Sun):

    All About Inverters
    Choosing an inverter for water pumping

    In theory, an induction motor running on a MSW inverter will use somewhere around 20% more power (while a TSW inverter is usually slightly less efficient than a MSW inverter). The issue is the AC sine wave (voltage and flowing current wave form), is based on the rotating magnetic fields. The RPM of the motor (at 60 Hz) is the fundamental frequency that changes electrical energy to rotational motion. We can look at Square Waves (and modified square waves) as a series of higher frequency sine waves (decreasing in amplitude). All of the energy in the non-fundimental wave forms (i.e, the higher frequencies that make up the "edges/squareness" of a MSW) do not do any good in turning the motor and are, therefore, usually turned into heat energy (warmer motor and wiring).

    Sine wave     - Wikipedia, the free encyclopedia
    In 1822, Joseph Fourier, a French mathematician, discovered that sinusoidal waves can be used as simple building blocks to describe and approximate any periodic waveform including square waves. Fourier used it as an analytical tool in the study of waves and heat flow. It is frequently used in signal processing and the statistical analysis of time series.

    You might not see this using a Kill-a-Watt meter... They are not really designed to measure/calculate True RMS (root mean square) power measurements (unlike the, not cheap, Fluke and other brands that have True RMS function for current/etc.).

    If you have an inverter (or access to both types of inverters), measuring the DC current input required by the inverters will (pretty much) be your "gold standard" measuring system performance.

    Where are you planning on camping? Using PV Watts for a fixed array in Las Vegas, tilted to latitude gives:
    Month Solar Radiation (kWh/m 2/day)
    1 5.19
    2 5.98
    3 6.57
    4 7.32
    5 7.42
    6 7.35
    7 7.37
    8 7.29
    9 7.31
    10 6.56
    11 5.75
    12 5.05
    Year 6.60

    So, in the desert southwest, you can get average of 7+ hours per day during summer with a fixed array.

    But move away and get into places that have "weather" even during summer, it does start to drop. Here is Flagstaff Az:
    Month Solar Radiation
    (kWh/m 2/day)
    1 5.19
    2 5.92
    3 6.27
    4 6.44
    5 6.56
    6 6.61
    7 5.95
    8 5.54
    9 6.59
    10 6.19
    11 5.47
    12 5.07
    Year 5.98

    For an RV, I can understand the 80% discharge cycle... Even if the battery bank only lasts ~500 cycles, the battery is probably going to "age out" before it cycles out. You are not cycling the battery bank every day for the next decade.

    There are so many losses with solar--That the "average" solar PV system only has around 52% end to end efficiency (solar panel rating * hours of sun * derating) if you charge during the day and discharge at night while using an AC inverter for powering loads...

    If you are running the AC only during sunlight (for the most part), then you can get better efficiencies (something like 0.77 panel+charge controller * 0.85 inverter eff = 0.65 efficiency cutting out the battery cycle).

    The problem is that solar is best used when you are using most of the system's power for something like 9+ months of the year.

    If you are using a solar PV system for several months of the year, the costs of not using the power while in storage make's the $$$/kWH costs higher (lost use of energy while parking). So, many times, for RV's and 1-2 season cabins, using propane powered refrigerators, hot water, etc. is more cost effective--and use the Generator for heavy loads (A/C, power tools, etc.). And use a small battery bank (plus solar and inverter if you wish) for small loads (quiet time at evenings/night).

    In the end, energy use is highly personal. That you want to run your A/C from a battery bank + solar array, if it has value for your, we are more than happy to help. Having a backup genset (Honda eu1000i/eu2000i/or equivalent) will allow you to cut corners a bit as you will have backup power if things don't work out as planned (use power power than estimated, less sunny weather, etc.).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
    · Share on Twitter
  • techntrek
    techntrek Solar Expert Posts: 1,372 ✭✭✭
    #14
    Re: Solar Powered AC in Camper Van - Advice?

    I've worked out the numbers to run a mini-split A/C in my small camper for 8 hours a night, 2 nights. No solar input. Set to low this is only a 260 watt load, with no surge (the benefits of the mini-split). Factoring in the Peukert effect I would need 6 deep cycle batteries to prevent an excessive DOD. Thought I would throw this out there for the OP to chew on.

    Partly to show that his one battery with a much higher load isn't going to work for 3 nights, even with his existing PV. Partly to introduce him to the idea of a mini-split - but I'll warn him that they are very expensive and not a drop-in solution.

    Bottom line, go with the inverter genset option others have mentioned, it will be your easiest and cheapest option.
    4.5 kw APC UPS powered by a Prius, 12 kw Generac, Honda EU3000is
    · Share on Twitter
  • solar_dave
    solar_dave Solar Expert Posts: 2,397 ✭✭✭✭
    #15
    Re: Solar Powered AC in Camper Van - Advice?

    Perhaps you could reconsider and use a small portable swamp cooler for the high desert. The draw would be much less than a AC unit.
    · Share on Twitter
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #16
    Re: Solar Powered AC in Camper Van - Advice?
    solar_dave wrote: »
    Perhaps you could reconsider and use a small portable swamp cooler for the high desert. The draw would be much less than a AC unit.

    He did ask about that, but so far no one has offered up any figures on their power consumption.
    · Share on Twitter
  • solar_dave
    solar_dave Solar Expert Posts: 2,397 ✭✭✭✭
    #17
    Re: Solar Powered AC in Camper Van - Advice?

    here is one that claims 60 watts off 120V
    http://www.compactappliance.com/Sunpentown-Portable-Air-Cooler-with-Ionizer-and-LCD-Controls/SF-613,default,pd.html?cgid=Air_Quality-Portable_Air_Coolers-Residential_Portable_Air_Coolers
    · Share on Twitter
  • soylentgreen
    soylentgreen Solar Expert Posts: 111 ✭✭
    #18
    Re: Solar Powered AC in Camper Van - Advice?

    techntrek replies, and with DATA :-) (thank you!)

    techntrek wrote: »
    I've worked out the numbers to run a mini-split A/C in my small camper for 8 hours a night, 2 nights. No solar input. Set to low this is only a 260 watt load, with no surge (the benefits of the mini-split).

    My first estimate of my cooling load was 265W, interesting that we are so close (I suspect just a coincidence, but..)
    techntrek wrote: »
    Factoring in the Peukert effect I would need 6 deep cycle batteries to prevent an excessive DOD. Thought I would throw this out there for the OP to chew on.

    I need to read up on this effect, methinks. :-)

    techntrek wrote: »
    Partly to show that his one battery with a much higher load isn't going to work for 3 nights, even with his existing PV. Partly to introduce him to the idea of a mini-split - but I'll warn him that they are very expensive and not a drop-in solution.

    My existing system consists of two Trojan T145s (6V@200ish AH), decently beefy batteries. Not sure how that compares to yours, what batteries do you have?

    techntrek wrote: »
    Bottom line, go with the inverter genset option others have mentioned, it will be your easiest and cheapest option.

    Agreed, but let's just take it as given that if "easy" and "cheap" were my M.O., I wouldn't be driving out to the middle of nowhere in 110F temperatures...

    Can you say more about installing a mini split in a camper? Was it a 12V unit or do you run it off an inverter?
    · Share on Twitter
  • soylentgreen
    soylentgreen Solar Expert Posts: 111 ✭✭
    #19
    Re: Solar Powered AC in Camper Van - Advice?

    Re: Swamp coolers : it does kinda feel like cheating, but this may be the way to go. I just looked and the historical humidity where I'm going is under 10% during the heat of the day. So a swamp cooler would work wonderfully.

    The van already has a maxxAir maxxfan deluxe, which can either pull or push air (and uses <30W on a reasonable setting). Perhaps I can build some sort of attachment that holds some swamp coooler filter media, with a water source?
    · Share on Twitter
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #20
    Re: Solar Powered AC in Camper Van - Advice?

    there are 2 headaches with using swamp coolers and they are,

    1> carrying enough water and
    2> cleaning it out of pollutants and mineral buildups.
    · Share on Twitter
  • Jim45D
    Jim45D Solar Expert Posts: 102 ✭✭
    #21
    Re: Solar Powered AC in Camper Van - Advice?

    OK, you ask so I'll throw it out. Here it is. www.Turbokool.com. They are pricy. I have used one for going on three years in a 33' RV. However, when the humidity is above 15% forget it. It is designed to operate off a 75 to 80 watt panel. I'm in the desert also if that's any consolation. Check it out. They have everything you need from water supply tanks, to bigger fans. It's no big deal to clean. To solve the problem of lime/hard water buildup......with each refill of the water reservoir, simply add a half cap/lid of Downey fabric softener.

    Jim
    · Share on Twitter
  • techntrek
    techntrek Solar Expert Posts: 1,372 ✭✭✭
    #22
    Re: Solar Powered AC in Camper Van - Advice?
    soylentgreen wrote: »
    I need to read up on this effect, methinks.
    The short description is the slower you draw the amps, the more total amps you will get from a battery(s). This applies to a single battery - note the difference between the 20 hour rate and other hour rates for any given battery - or multiple batteries in series or parallel.
    My existing system consists of two Trojan T145s (6V@200ish AH), decently beefy batteries. Not sure how that compares to yours, what batteries do you have?

    The ones in my home system are Wal-Mart marine deep-cycle. I haven't built the system for the camper.
    Can you say more about installing a mini split in a camper? Was it a 12V unit or do you run it off an inverter?

    They come in two parts, one part needs to be installed on a wall, the other on a roof or the tongue of a camper. They are intended for homes so it will require some work. There are some that run on 120 volts, but most (including the most efficient models @ 26 SEER) require 240 volts.
    4.5 kw APC UPS powered by a Prius, 12 kw Generac, Honda EU3000is
    · Share on Twitter
  • Photowhit
    Photowhit Solar Expert Posts: 6,006 ✭✭✭✭✭
    #23
    Re: Solar Powered AC in Camper Van - Advice?

    Gosh I thought this thread was dead after 6 or so post, one query a couple very good responses.

    Forget solar, w/900watts of array I was close to an even draw vs production in much more idea situation.

    Look at a homemade swamp cooler, mostly a fabric that draws water, like a chamis(mispelt) cut in strips run an effiecent fan, Like an O2Cool and you should be able to work this out. A fellow can from AZ to Missouri and said the heat index was in the 90's even with the 110 temps due to low humidity, same as it was here in Missouri(Misery) There are also mister's that make a fine spray in front of fans. Worth looking into.

    ...but if it's a 1 time thing, and you want to be comfortable... rent a quality genny and a be comfortable, ...enjoy.
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former backup...lol), 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
    - Assorted other systems, pieces and to many panels in the closet to not do more projects.
    · Share on Twitter
  • dano
    dano Registered Users Posts: 3
    #24
    Re: Solar Powered AC in Camper Van - Advice?

    Recommend MaxxFan Deluxe Roof Vent....SuperCool your RV at Nite....1000cfm-3.0-4.0Ah.....works even in the Desert...Check em Out www.maxxair.com
    · Share on Twitter
  • plongson
    plongson Solar Expert Posts: 115 ✭✭
    #25
    Re: Solar Powered AC in Camper Van - Advice?

    Or get use to the heat and save your money. Seriously, the more AC one uses, the lower the temp becomes to be comfortable. Modify your habits, do more at night and less in the middle of the day etc. Obviously old, very young or sick need the cool without a doubt but for camping trip...deal with it, or change your itinerary.
    3500w solar, 800AH with Rolls Surrette, Magnum inverter, Midnite charge controller, Kubota 21kW diesel genset...private well...and just recently connected to city power for additional options...nice to have options 
    · Share on Twitter
  • Ken Marsh
    Ken Marsh Solar Expert Posts: 114 ✭✭✭✭
    #26
    Re: Solar Powered AC in Camper Van - Advice?

    Soylentg, First, congrats on your analysis. You understand it well.
    Its just that there are some non-obvious gotchas.
    Your biggest prob is going to be what CCoot brings out.
    When you get those bats a little discharged they are not going to supply the surge needed to start that air condx.

    I disagree a little with the common consensus of needing a TSW for motors.
    Induction motors run fine on MSW even though the TSW are better theoretically at least.

    The mini-split principle is the way to go.
    They use a poly-phase compressor motor and an phase changing inverter (MSW) to supply the poly phase directly.
    No problem generating the rotating phase needed to start the motor.
    As a result they start with no inrush and run at higher efficiency.
    Probably in five to ten years all new aircondx units will be that way. But we are not there yet.

    What you need for your planned expedition is to get your solar production up to approx your use level.
    300 watts of panel isn't that big. It seems like you could get that on your roof.
    This way your bats would stay charged and could supply the starting inrush.
    · Share on Twitter
  • soylentgreen
    soylentgreen Solar Expert Posts: 111 ✭✭
    #27
    Re: Solar Powered AC in Camper Van - Advice?

    Update:

    Thanks to all for playing along with this experiment, at times perhaps more of a "thought" experiment than a reality-based one.

    After thinking this over, something hit me: I'm on vacation, in beautiful country. I don't want to be cooped up in a metal box, even a nicely cooled metal box.

    Thinking "outside the box" (ha!) led to this epiphany : goal of being on vacation in summer is to be outside.

    Long story short, my companion and i made a shade structure out of heavy duty tarp, poles, and tie-down straps. Purchased a Misty Mate cool camper http://www.mistymate.com/cool-camper-6.html and brought an extra 40 gallons of water. Bought some really comfy lounge chairs from REI.

    Voila! an outdoor seating area with misting system which kept us very comfy in 100F temperatures.

    About the only downside is that we both got terribly sunburned - still trying to figure out how that happened : either the "sunblocker" tarp we used wasn't, or perhaps we were getting UV reflected off the underside of the tarp?
    · Share on Twitter
  • techntrek
    techntrek Solar Expert Posts: 1,372 ✭✭✭
    #28
    Re: Solar Powered AC in Camper Van - Advice?

    Mister! Excellent idea for outside I've thought of getting one for our back porch. However, my biggest issue is getting to sleep when its hot and humid (east coast problem) and for that A/C is a must.
    4.5 kw APC UPS powered by a Prius, 12 kw Generac, Honda EU3000is
    · Share on Twitter
  • OverTheTopCargoTrailer
    OverTheTopCargoTrailer Registered Users Posts: 3
    #29
    Re: Solar Powered AC in Camper Van - Advice?

    Ha Ha ......

    This is a funny thread... I have this exact set up, so here is what I am currently using.
    4 x 210 panels = 840 watts, A 5,000 Btu window ac converted into a roof top model.
    a 3" blower to help move the ac air through the ac unit.
    a Magnum MSW 1512 inverter - runs at 50 amps with the ac on. Approx .670 Kw on the charge controller
    This provides power while the sun is up until 10 am- 4 pm on flat panels , longer if tilted.
    I have 400 amp L16 Surrette Batteries - they would go empty pretty fast.
    Also I have a 2000 CFM under floor fan ...that also works great

    P.S. what is the point of having colored fonts if you come and edit them ??


    http://overthetopcargotrailer.blogspot.com/
    · Share on Twitter

Categories