New Member and why I'm here with real world question.

[eightbit4055]

Hello All,

I am an electronics technician and I would like to think I know what I'm doing but with no real world experience I know I need to talk to someone who actually knows before I spend roughly $2000 on a solar panel system. So I am going to build a off the grid mobile office in a shipping container. To keep this short I'm going to list what I plan on doing below.

Cargo Container 20'X8'.
Fully insulated and drywalled.
Location: Orlando Fl.
Four 150 Watt Solar panels.
Four 109 Ah batteries.
Battery charge controller: Undecided.
Inverter 1500 Watt: Cobra/Undecided.
Goal: To run a 1000 Watt 120VAC A/C intermittently(non continuous) for 8 hours at night.

My question is will this work in the real world and how long will the four batteries last if this office is used every single night? I need to keep this office cool at night for a full 8 hours. I'm thinking with 436 Ah I should be able to run the A/C for a maximum of 5 hours continuous. Of course the A/C isn't going to be on continuous I'm just trying to figure out if my theory on this is correct. I am hoping that the insulation will be able to seal this thing good enough that at most the A/C will only have to be on for four hours.

My next questions is with my calculations I am assuming in sunny Florida using four 150 watt solar panels I should be able to charge up those four batteries in a little over 8 hours.

Is this a solid system or am I going to have major issues?

Thank You for any advice and Happy New Year!

Edit: I'd like to add that the A/C will not be used at all during the day. So I will have the full day for the batteries to charge.

[SunEagle]

Hello All,

I am an electronics technician and I would like to think I know what I'm doing but with no real world experience I know I need to talk to someone who actually knows before I spend roughly $2000 on a solar panel system. So I am going to build a off the grid mobile office in a shipping container. To keep this short I'm going to list what I plan on doing below.

Cargo Container 20'X8'.
Fully insulated and drywalled.
Location: Orlando Fl.
Four 150 Watt Solar panels.
Four 109 Ah batteries.
Battery charge controller: Undecided.
Inverter 1500 Watt: Cobra/Undecided.
Goal: To run a 1000 Watt 120VAC A/C intermittently(non continuous) for 8 hours at night.

My question is will this work in the real world and how long will the four batteries last if this office is used every single night? I need to keep this office cool at night for a full 8 hours. I'm thinking with 436 Ah I should be able to run the A/C for a maximum of 5 hours continuous. Of course the A/C isn't going to be on continuous I'm just trying to figure out if my theory on this is correct. I am hoping that the insulation will be able to seal this thing good enough that at most the A/C will only have to be on for four hours.

My next questions is with my calculations I am assuming in sunny Florida using four 150 watt solar panels I should be able to charge up those four batteries in a little over 8 hours.

Is this a solid system or am I going to have major issues?

Thank You for any advice and Happy New Year!

Edit: I'd like to add that the A/C will not be used at all during the day. So I will have the full day for the batteries to charge.

Sorry your math is off a wee bit.

If you run that 1000 watt AC unit for 5 hours a day you will need about 1500 watts of panels, a battery system sized for 5 days (we do get clouds down here for a couple of days in a row) which would be about 1050 Ah at 24 volt with a 65 amp MPPT type charge controller or a 525 Ah at 48 volt with a 35 amp CC.

Using batteries to run Air Conditioning , heating or even a coffee pot is very very expensive.

[Sunking]

Is this a solid system or am I going to have major issues?

They major issue you will soon learn is anything you take off grid, will cost you roughly 10 times more than buying it from the POCO for the rest of your life replacing batteries and components.

[eightbit4055]

They major issue you will soon learn is anything you take off grid, will cost you roughly 10 times more than buying it from the POCO for the rest of your life replacing batteries and components.

Yes I agree and this is not a problem. Depending on the number and type of batteries used I will be very happy if they last one year. Which is why I'm coming to more experienced people like yourselves. So one of my questions is and of course I know it depends on the type of deep cycle battery I'm using but are there batteries that would be able to be charged and discharged every day for one year of use?

[Mike90250]

not a chance, as you have it planned.

Batteries = expensive energy storage
Water = cheap energy storage.

If you had a 2,000 w PV array, you could use a 1,000w AC to chill water in the daytime. Barrels of it, line one side of the container with 55 gallon drums. fill with water, daisy chain siphons in them and circulate chilled water. This will pre-chill the container, and if you can keep the humidity down, temps in the 75F range are fine. When you get all that water cooled down, it will keep the container cool for a long time. To kick start it, you may need to dump some block ice into the barrels, in an 5 hour solar day, you may not be able to chill the water down.

Batteries, cheapest would be 6V golf cart, 200ah style. wire 8 in series for a 48V, 200ah bank. Deeply cycling them, you may get a year out of them. BatteryServiceLife.jpg

[eightbit4055]

not a chance, as you have it planned.

Batteries = expensive energy storage
Water = cheap energy storage.

If you had a 2,000 w PV array, you could use a 1,000w AC to chill water in the daytime. Barrels of it, line one side of the container with 55 gallon drums. fill with water, daisy chain siphons in them and circulate chilled water. This will pre-chill the container, and if you can keep the humidity down, temps in the 75F range are fine. When you get all that water cooled down, it will keep the container cool for a long time. To kick start it, you may need to dump some block ice into the barrels, in an 5 hour solar day, you may not be able to chill the water down.

Batteries, cheapest would be 6V golf cart, 200ah style. wire 8 in series for a 48V, 200ah bank. Deeply cycling them, you may get a year out of them. [ATTACH=CONFIG]1450[/ATTACH]

Mike90250, I actually like the idea. I just have a few questions.
1. Do you know of any other resources I could go to learn more?
2. Basically your saying put 55 gallon drums inside the container. Run a pump and lines in front of the a/c to chill the water and circulate it. Then let the chilled 55 gallon drums keep the container cool at night?
3. I can't think of it right now since i'm on my way to work but isn't there a more natural system for this? Where you run a pump and water line underground since 20 feet underground is already 50 degrees year round? That way I wouldn't need solar panels or batteries.

I like this a lot. I'm glad I came here. I'm getting a lot of good ideas.

[russ]

Where you run a pump and water line underground since 20 feet underground is already 50 degrees year round?

Nope - not even close - that is an old wives tale - the underground temperature varies from area to area but Orlando is nowhere 50°F.

Some cooling systems do just what you mentioned. In the northerly climates a ground coupled heat pump may use only ground temp for cooling

[inetdog]

Nope - not even close - that is an old wives tale - the underground temperature varies from area to area but Orlando is nowhere 50°F.

Some cooling systems do just what you mentioned. In the northerly climates a ground coupled heat pump may use only ground temp for cooling

And even if the ground temperature is a "constant" 50°F, the heat from the pipe will soon raise the temperature of the soil surrounding the pipe.
A ground source heat pump, for example, may need hundreds of feet of pipe spaced over a wide area to be able to take advantage of the earth temperature.

I assume you were talking about 20 feet below the surface, which is almost certainly below the water table. That reduces the length of pipe you would need.

[eightbit4055]

Sorry your math is off a wee bit.

If you run that 1000 watt AC unit for 5 hours a day you will need about 1500 watts of panels, a battery system sized for 5 days (we do get clouds down here for a couple of days in a row) which would be about 1050 Ah at 24 volt with a 65 amp MPPT type charge controller or a 525 Ah at 48 volt with a 35 amp CC.

Using batteries to run Air Conditioning , heating or even a coffee pot is very very expensive.

Four 150 Watt solar panels = 600 Watt's. Charging four 109 Ah batteries which equals 435 Ah, 12 volts*435 Ah = 5220 Watt's. 5220 Watt's / 600 Watt's = Batteries should be charged in 8.7 hours.

If the four batteries can hold up to 5220 Watt's of energy it should run a 1000 Watt A/C unit for 5.22 hours.

The only math I'm a wee bit off that I know of is the energy loss from the wires, charge controller and my inverter. Of course all this is great as long as I have 100% sunlight perpendicular to my solar panels for a full 8.7 hours each day and everything in the electronic world is perfect.

Keep in mind I'm not running the A/C during the day. Also on bad days with no sun or cloudy weeks my back up plan is a generator which is what I've been using.

[eightbit4055]

And even if the ground temperature is a "constant" 50°F, the heat from the pipe will soon raise the temperature of the soil surrounding the pipe.
A ground source heat pump, for example, may need hundreds of feet of pipe spaced over a wide area to be able to take advantage of the earth temperature.

I assume you were talking about 20 feet below the surface, which is almost certainly below the water table. That reduces the length of pipe you would need.

Yes you're correct. The ground around the pipe would actually slowly start to heat up but that's why you need 100 to 400 feet of this depending where and what you're trying to cool. I really like this idea and there are a lot of resources(waterfurnace, geothermal cooling) for this but this cuts out the whole mobile thing that I'm looking for. Mike has a nice idea with the 55 gallon drums the only issue with that is it looks like I'd have to develop my own system because I can't find anything on it except for the reverse which is storing heated water but not cold water.

[Sunking]

Four 150 Watt solar panels = 600 Watt's.

Correct and with 4 or 5 Sun Hours in peak of summer and taking into account 66% efficiency of a battery system only generates 600 watts x 5 Hours x .66 = 2 Kwh of usable energy. You next major flaw is you have forgotten about Peuket Law applied to batteries at a 5 hour discharge rate changes 400 AH battery into 270 AH.

[eightbit4055]

Correct and with 4 or 5 Sun Hours in peak of summer and taking into account 66% efficiency of a battery system only generates 600 watts x 5 Hours x .66 = 2 Kwh of usable energy. You next major flaw is you have forgotten about Peuket Law applied to batteries at a 5 hour discharge rate changes 400 AH battery into 270 AH.

That's totally epic, Thanks! I calculated 287.6 Ah but I was kind of logically guessing on the Peukert constant. I will have to do some research on that one. Well I got to rework my numbers a bit and thanks for the help.

Edit: I'd like to know why you say that there will only be a 66% efficiency of the battery system?

[inetdog]

That's totally epic, Thanks! I calculated 287.6 Ah but I was kind of logically guessing on the Peukert constant. I will have to do some research on that one. Well I got to rework my numbers a bit and thanks for the help.

Edit: I'd like to know why you say that there will only be a 66% efficiency of the battery system?

There are at least three contributors to the overall energy cycle efficiency:
1. Losses in panels, wiring and CC.
2. Losses in inverter and its supply wiring.
3. What is called Coulombic efficiency, which measures the difference between the charging current you put in and the load current you can draw to get the battery back to the exact same state. This varies with battery chemistry and design but is typically more than 95% for FLA. And a lot lower for NiFe.
There is also the fact that the internal resistance of the battery and chemical factors will require you to put the current back in at a higher voltage than you take it back out again. This will either be built into 1 and 2 or calculated separately depending on your procedures.

Where that total comes out to 66% is, I think, a matter of long experience.

[SunEagle]

Four 150 Watt solar panels = 600 Watt's. Charging four 109 Ah batteries which equals 435 Ah, 12 volts*435 Ah = 5220 Watt's. 5220 Watt's / 600 Watt's = Batteries should be charged in 8.7 hours. First off you will never get more than 6.5 hours of useful sunlight during the year and maybe only 4.25 hours during the Winter. You also forgot to derate the charging efficiency to about 60%. Less sunlight hours and a lower charging efficiency requires you to use much more than the 600 watts.

If the four batteries can hold up to 5220 Watt's of energy it should run a 1000 Watt A/C unit for 5.22 hours. If you discharge you battery by 100% it will quickly die. A battery should be sized for only a 20% daily discharge. Also getting 4 x 12 volt batteries wired in parallel to charge equally will be an issue due to wiring resistances

The only math I'm a wee bit off that I know of is the energy loss from the wires, charge controller and my inverter. Of course all this is great as long as I have 100% sunlight perpendicular to my solar panels for a full 8.7 hours each day and everything in the electronic world is perfect. Again we get an average of 5.4 hours a day of useful sunlight and never get close to even 7 hours. If your calculations require more than one day of charging your batteries will never get back to 100%

Keep in mind I'm not running the A/C during the day. Also on bad days with no sun or cloudy weeks my back up plan is a generator which is what I've been using.

See my answers in red. You will need more batteries and more solar panels to run your AC 5 hours a day.