Design help needed for off grid air conditioner

stoneunhenged

Here is what I have:

1) A 20' shipping container to use as a shelter.

2) A Sanyo minisplit variable speed air conditioning system (3.0-7.6 amps, 250w-755w)

3) Six Kyocera KD 215 panels

Here's the shipping container:



The shipping container will be super insulated and inhabited only during daylight hours in a sunny pasture in Florida. It will need to be minimally cooled at night or not at all. When the air conditioner is running it will usually be in the middle to low end of its variable range.

Here's the question:

How to design a relatively simple off grid system for powering the air conditioning unit given that I'm locked into the components mentioned above? (I could add more panels if necessary.) A very specific level of detail would be appreciated.

Thanks very much for your help.

Dave Angelini

Re: Design help needed for off grid air conditioner

It will be a challenge as running AC (even a small system) at night pretty much takes the word "simple" out and raises the question what is your budget?

niel

Re: Design help needed for off grid air conditioner

dave is right in that a good charge controller and battery bank is needed along with the psw inverter and associated fuses, electrical boxes, wiring, etc. it really becomes a question of how much power for how long and this dictates the number of pvs and batteries to store it in. this can get to be quite large as the sun isn't always shining during the day and winter hours and solar angles will subtract from the good summertime productions. the batteries should never go below 50% dod and you may want extra capacity for those off times. keep in mind a larger battery bank will need more current to keep a good charge to the batteries and if these batteries are to be put into the interior of the container that you should use agm type batteries as fla types will gas and need vented. add to all of this a battery monitor so you don't go too far and ruin your batteries.

stoneunhenged

Re: Design help needed for off grid air conditioner

I'm willing to spend whatever it takes to build a robust system. For example, if I need to spend a couple of thousand bucks on batteries I'm in. I'd rather err on the side of excess system capacity. I'm not trying to keep it inexpensive, just as simple as possible and still do the job.

I'm very interested in knowing specific recommendations on controllers, inverters, batteries, panels, etc. Reliability and ease of maintenance are more important than saving money.

Thanks.

BB.

Re: Design help needed for off grid air conditioner

OK--I will go through your example--Remember, I am making lots of assumptions and you, more than likely, will need to make adjustments.

First, know your daily loads. Assume 3 amps for 8 hours a night, and 5.5 amps for 10 hours a day at 120 VAC (again--just guessing). The Sanyo mini-split is very off-grid friendly, so finding a 1,000-1,500 watt TSW inverter will be a good start. If you never run it on high, you might even be able to use a smaller inverter (use TSW, no MSW type inverter--again, my guess).

• 3 amps * 8 hours * 120 VAC = 2,880 Watt*Hours at night
• 5.5 amps * 10 hours * 120 VAC = 6,600 Watt*Hours during day
• 9,480 WH per day total

Using PV Watts, Tampa Florida, 1 kW (1,000 watts of panels), fixed array, 0.52 end to end estimated system efficiency (off-grid/battery backed system):

"Station Identification"
"City:","Tampa"
"State:","Florida"
"Lat (deg N):", 27.97
"Long (deg W):", 82.53
"Elev (m): ", 3
"PV System Specifications"
"DC Rating:"," 1.0 kW"
"DC to AC Derate Factor:"," 0.520"
"AC Rating:"," 0.5 kW"
"Array Type: Fixed Tilt"
"Array Tilt:"," 28.0"
"Array Azimuth:","180.0"

"Energy Specifications"
"Cost of Electricity:"," 9.0 cents/kWh"

"Results"
"Month", "Solar Radiation (kWh/m^2/day)", "AC Energy (kWh)", "Energy Value ($)"
1, 4.56, 68, 6.12
2, 5.21, 69, 6.21
3, 5.72, 83, 7.47
4, 6.52, 90, 8.10
5, 5.92, 83, 7.47
6, 5.56, 74, 6.66
7, 5.46, 74, 6.66
8, 5.70, 79, 7.11
9, 5.32, 71, 6.39
10, 5.41, 77, 6.93
11, 4.83, 68, 6.12
12, 4.24, 62, 5.58
"Year", 5.37, 899, 80.91

Let's assume your A/C season is from March thru October... That makes October the lowest productive month. So, you would get around 77 kWH per October or:

• 77,000 WH per Oct / 31 days per Oct = 2,484 WH per day per 1kW of panels

And your system "needs" 9,480 WH per day--So solar panel wise, your estimated panel / array requirement would be:

• 9,480 WH per day / 2,484 WH per day per 1,000 watt panels = 3,816 Watts of solar panels

You are using 215 watt panels:

• 3,816 watt array / 215 watt per panel = 17.8 = ~18x 215 watt solar panels

Battery wise, I would normally suggest that you have 2 days of no sun and 50% maximum discharge--You may get away with 1 day of no-sun if you do not use the system during cloudy weather and/or night--but for now:

• 9,480 WH * 1/0.85 inverter eff * 1/48 volt battery bank * 2 days * 1/0.50 max discharge = 929 AH @ 48 volt battery bank

Anyway, before we go any further--What do you think of the number so far?

-Bill

stoneunhenged

Re: Design help needed for off grid air conditioner

Let me start by saying that I am very grateful for that detailed and sophisticated analysis.

A few observations:

This will be used as a farm office each day of the work week and can be completely shut down at night and on weekends. So, figure about nine hours of use five days a week.

I live in North Florida. Closest major city is Tallahassee. The heat really begins in late May and is over by the end of September.

I'm going to mount the PV panels on the roof of the container on an elevated and angled south-oriented tube steel structure. The maximum feasible number of panels that can be mounted would be 12.

I am willing to add a generator to the system.

Thoughts?

Dave Angelini

Re: Design help needed for off grid air conditioner

As long as you are honest with yourself about shutting it down at night this can be done. Bill has good data as we expect... You will have to be careful at both extremes of the day when the array is off axis. You also should read the whole thread about using a min-split as you cannot just turn it on in a hot room. The temperature differential programming of it is critical. The amp readings you listed from the specs are cycling averages and they can easily be exceeded if the pump is not cycling. Full on is somewhere north of 1200 watts plus your inversion loss and battery loss when the array is off axis or out of sun.

If this is not a forever structure and is temporary, a generator and a big propane tank may be a better option.

I just keep thinking of the first people with electric light offgrid. They probably said "we will use candles most of the time and not use the electricity." It is hard to go back to the days without AC. In Florida, when the heat is on, there is no going back to sweating. Good Luck

Seven

Re: Design help needed for off grid air conditioner

You state that you already have the mini split. If that container is to be insulated as you say, a window unit would be more than enough to cool it. It looks to be a 20x10 unit so I don't thhinkyou will be able to get 12 of those panels on it. I would put as many panels as possible to help keep the sun off of the roof.

Solar Guppy

Re: Design help needed for off grid air conditioner

A shameless plug ... :roll:

I have the inverter, batteries and extra panels that could make this possible and I'm only 4 hours away

http://forum.solar-electric.com/showthread.php?t=13357

stoneunhenged

Re: Design help needed for off grid air conditioner

That's not shameless. Helpful maybe.

Shadowcatcher

Re: Design help needed for off grid air conditioner

One other possibility is a self contained engine powered unit i.e. Thermo King truck unit.

Tricksailing

Re: Design help needed for off grid air conditioner

I have a similar sized "container" (boat) which spends a lot of time in Mazatlan, Mexico. It's pretty well insulated and sealed but has a few windows. I use a 1/2 ton (8000 BTU) window unit to keep it cool. It has an ~50% duty cycle during the day (around 90 in the summer) which puts it at an average power use of ~400W - around 10KWh per day

You might look for a heat loss/gain calculator to predict your insulation and power needs for AC. I googled Heat load calculator and found http://www.shophmac.com/info-center/hvac-calculators/heat-load-calculator.php. It says the uninsulated box would need ~2500 W to cool it 20 degrees and a well insulated box ~300 W. That's still 7 kWh per day. What about doors and windows?

I also have a Honda 20i (H 2000 equivalent) which easily runs the AC. It has an automatic throttle control, so idles when power isn't needed. It consumes less than 1 gallon of gasoline per day. You might consider this as an emergency power source. They are very quiet too. Might get you going relatively inexpensively while you figure out the solar.

Will you need additional power for lights and office equipment?

BB.

Re: Design help needed for off grid air conditioner

I cannot argue with Dave Sparks--he has much more experience than I in the off grid living/power stuff... However, I had understood that Sanyo Mini-Split was pretty good about power usage--very little surge current. If you set it to low--it would draw ~300 watts with little to no surge... But, I could be wrong.

I believe, for long term use, the Sanyo is much better than your "typical" window shaker A/C system... More efficient and less surge requirements.

Regarding your overall power use... Besides the A/C, do you have a computer and lighting too? Need to recharge two way radios/cells phones, etc.?

Choosing energy efficient lighting and tools for the office (i.e., laptop instead of a desktop workstation, etc.) will help a lot too. Plus energy efficient appliances put less hot air into the conditioned space.

Depending on your time line--I would probably suggest purchasing or renting an eu2000i Honda genset (or its big brother the eu3000i if needed; or any of the Yamaha inverter/generator family would be good too).

First, you can run the genset and get a load baseline (use a kill-a-watt meter or equivalent meter to measure your daily power loads). Plus, you probably will need a genset for backup power anyway.

While you can get a bigger and/or cheaper genset--The Honda eu family are very fuel efficient and quiet... You will get around 4-10 hours on 1.1 gallons of fuel for the eu2000i--which should be enough for you to run your full office+A/C (assuming you don't go overboard using power and put a refrigerator in there, etc.).

And, if a eu2000i cannot power your loads--You probably need to rethink the solar pv system--The one we are talking about may not be large enough.

-Bill

Fullpower

Design help needed for off grid air conditioner

Hate to sound Luddite...
But why not dig a pit, and bury the box?
Build a short staircase. No A/C needed.

BB.

Re: Design help needed for off grid air conditioner

In Florida? They may have problems with high water table and still have high humidity...

-Bill "my guesses" B.

Seven

Re: Design help needed for off grid air conditioner

Fullpower wrote: »

Hate to sound Luddite...
But why not dig a pit, and bury the box?
Build a short staircase. No A/C needed.

That box cannot handle the side load pressures of being buried. It is not designed that way. It is more cost effective to start from scratch than to reinforce a shipping container for burial. Plus it would be a pain in the butt to climb a 8' set of stairs and you would lose floor space where the staircase would be.

techntrek

Re: Design help needed for off grid air conditioner

Seven wrote: »

That box cannot handle the side load pressures of being buried. It is not designed that way. It is more cost effective to start from scratch than to reinforce a shipping container for burial. Plus it would be a pain in the butt to climb a 8' set of stairs and you would lose floor space where the staircase would be.

I disagree, they are designed for a top load of a dozen fully-loaded containers bolted above, part of that is achieved by using corrugated steel walls. Corrugated material can handle a large side load.

No need to loose any floor space, build the stairwell entirely outside of the container. Or forget it altogether - dig a ramp and dig the pit from the side instead of from above, then leave the ramp.

Seven

Re: Design help needed for off grid air conditioner

techntrek wrote: »

I disagree, they are designed for a top load of a dozen fully-loaded containers bolted above, part of that is achieved by using corrugated steel walls. Corrugated material can handle a large side load.

p.

We agree to disagree then. Stacking is possible because the strength of the structure is in the frame. If you walk up to that container and push on the wall with your hand, it will move.

stoneunhenged

Re: Design help needed for off grid air conditioner

Seven wrote: »

You state that you already have the mini split. If that container is to be insulated as you say, a window unit would be more than enough to cool it. It looks to be a 20x10 unit so I don't thhinkyou will be able to get 12 of those panels on it. I would put as many panels as possible to help keep the sun off of the roof.

I already have the mini-split so I'm committed to trying it. One of the selling points was that it required merely a 3" hole in the wall of the container. I'm a fan or reducing the size of the holes I'll have to cut.

Yes, my plan is to put 12 panels on the roof at a stationary 20° south facing angle. At my latitude this should put the array dead on in mid-August; about the time I will have the biggest air conditioning load. Seems like a good compromise. The 12 panels should be about 10' x 20' in aggregate area. The container roof is 8' x 20'.

I am also painting the container in a light green color and using the Insul-Add thermal reflective additive. Any comments on this would be welcome.

BB.

Re: Design help needed for off grid air conditioner

The thermal paint additive is worthless as insulation.

-Bill

Seven

Re: Design help needed for off grid air conditioner

stoneunhenged wrote: »

I already have the mini-split so I'm committed to trying it. One of the selling points was that it required merely a 3" hole in the wall of the container. I'm a fan or reducing the size of the holes I'll have to cut.

Yes, my plan is to put 12 panels on the roof at a stationary 20° south facing angle. At my latitude this should put the array dead on in mid-August; about the time I will have the biggest air conditioning load. Seems like a good compromise. The 12 panels should be about 10' x 20' in aggregate area. The container roof is 8' x 20'.

I am also painting the container in a light green color and using the Insul-Add thermal reflective additive. Any comments on this would be welcome.

Good call on the three inch hole.

The panels you have are a bit more than three feet wide, so you are only going to get 6 of them, side by side, on the roof, in one row. You would have to have quite a bit of overhang in order to have two rows, without any shading, with your tilt angle.

Easiest way to help keep it cool is to keep the sun off of it in the first place. If you could safely attach an awning, that will help on the sides. Your panels should keep the sun off of the roof. A fence around it will help a good bit if you build it close. Cost may be a factor with the fence. Framing the inside and using normal insulation would cost you floor space, but it is effective. I would suggest the foil backed foam board against the walls and roof no matter what way you go.. It has about an R5 value and is only 3/4" thick. Again, anything you put on the outside that keeps the sun off of the metal will help.

stoneunhenged

Re: Design help needed for off grid air conditioner

Seven, agreed on all counts. I'll post pics as the plan progresses. The interior framing is designed with 2x4 studs and will be filled with closed cell foam insulation. The idea is to make the container into one giant Igloo cooler.

Next month we should be pouring the concrete foundation to support the container and an ancillary 8'x8' shed to the rear (north) of the long container wall.

Seven

Re: Design help needed for off grid air conditioner

What kind of flooring is in that unit?

Vic

Re: Design help needed for off grid air conditioner

Regarding the strong and weak parts of the cargo containers;

The weight of containers above is designed to be carried by the "pockets" at each end. There is a heavy frame at each end designed to do this. None of the weight of containers stacked on the one below is DIRECTLY carried by the longitudinal structure or the walls.

The "racking" strength is created by the walls, and ends. It is very difficult to cause the walls to deflect very much at all by the average human pushing on them.

The floors of non-refer containers is often Luan -- mahogany plywood. Some I've seen have plastic floors, probably PVC.

As these cargo containers are fairly narrow, building the frame outside, and insulating that will allow a bit more useable width ( only abut 7 inches). A skin can be placed on this outside frame. This can make a fairly large difference is the way the space can be used. Realize that the internal frame can be useful for consealing wiring, attaching shelves etc.

Have framed, insulated and am just now having the exterior of one container, here, Stuccoed. The exterior frame would allow the use of greater R-value insulation (obviously). This may sound bizarro, but this particular container is the power room for a new cabin, and the rest of the structure is built onto this box, and the 30 degree roof is mostly comitted to the PV array.

YMMV. Nice project, stoneunhenged, Vic

techntrek

Re: Design help needed for off grid air conditioner

Vic wrote: »

...The "racking" strength is created by the walls, and ends. It is very difficult to cause the walls to deflect very much at all by the average human pushing on them.

...As these cargo containers are fairly narrow, building the frame outside, and insulating that will allow a bit more useable width ( only abut 7 inches). A skin can be placed on this outside frame.

Agree. I was in the overseas shipping biz for about 5 years so I know they are built to be thrown around and take a beating in the process.

If the closed-cell foam board insulation is used (pink or blue), nothing would need to be done to protect it from rain so it could go on the outside. You might want to cover it for aesthetic reasons and to protect it from UV damage. Saving 7 inches on the inside width of a shipping container is a lot.

stoneunhenged

Re: Design help needed for off grid air conditioner

Seven wrote: »

What kind of flooring is in that unit?

3/4" white pine over the composite plywood flooring that came with the container.

stoneunhenged

Re: Design help needed for off grid air conditioner

My plan is to use 2x4 stud framing on the interior, sheath it in finished drywall, and insulate the voids with the spray closed cell foam insulation. While the interior framing narrows the available interior space, exterior sheathing strikes me as significantly more expensive and complicated. Also, I want to conceal the wiring in the wall in the conventional manner and mount electrical outlets between the studs.