Sustained Air Conditioning

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dgsloan
dgsloan Registered Users Posts: 22 ✭
in Solar Beginners Corner #1
Ok - So if I had a workshop that I wanted to air condition and I wanted to do it as efficiently as possible without tieing the air conditioner to the grid I would need given a 1000 watt air conditioner - Some backup batteries for when the sun goes behind the clouds - enough to give say 10 amps at 120v for let us say an hour (enough time to either shutdown or cloud goes by). Since in my area the nights tempurature goes below the air conditioned value - I do not need it after sundown. Lets say that I have a pure sine wave inverter to produce the electricity required for the air conditioner, and can charge the battery from the grid. Question - Can I use a set of micro grid tie inverters to supply the required energy for the air conditioner?
The reason that I ask is that I have setup a set of 12 feet by 8 foot tracking solar panels (max 1000watts out) to supplement the power on my trailer when I head south for the winter, but would like to make use of them in the summertime.
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  • drees
    drees Solar Expert Posts: 482 ✭✭✭
    #2
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    Re: Sustained Air Conditioning

    If your workshop is grid tied, you're best off figuring out how much electricity your AC will use, then grid-tie solar panels to offset that usage.

    Charging batteries will increase the amount of PV you'll need by a lot.
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  • Photowhit
    Photowhit Solar Expert Posts: 6,003 ✭✭✭✭✭
    #3
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    Re: Sustained Air Conditioning

    As a point of reference, I have a 1300watt array, and AC a 10x16 cabin with 6" insulation, made in the shade, just enough panels to run a 5300 btu AC (I recall rated at 530 watts and @460 measured with Kil-A-Watt meter) and have a bit extra to charging.

    If you have a large shop with thin walls. I doubt you could even try with out doubling your array..

    I'm with drees...
    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.
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  • System2
    System2 Posts: 6,290 admin
    #4
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    Re: Sustained Air Conditioning

    I'm an A/C guy and not so much of a solar guy. On start-up the compressor in your A/C will pull up to 5 times the FLA or RLA (rated running amps). I don't know if this will effect you or not but I do know for a bit of time the compressor "sees" LRA (locked rotor amps).
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #5
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    Re: Sustained Air Conditioning
    rad1 wrote: »
    I'm an A/C guy and not so much of a solar guy. On start-up the compressor in your A/C will pull up to 5 times the FLA or RLA (rated running amps). I don't know if this will effect you or not but I do know for a bit of time the compressor "sees" LRA (locked rotor amps).

    Yup.
    So your "1000 Watt" AC needs a 5 kW inverter just to start ...
    And a massive battery bank to handle the draw over time (they do not run off the panels; the panels recharge the batteries) ...
    And a lot of panel to recharge that bank.

    Not really a cheap cooling solution.

    Take a look at this thread about AC: http://forum.solar-electric.com/showthread.php?t=5104
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  • RCinFLA
    RCinFLA Solar Expert Posts: 1,484 ✭✭✭✭
    #6
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    Re: Sustained Air Conditioning

    Typically startup surge will exceed LRA rating on compressor. The name plate figure represents LRA at lowest operating voltage, typically 204 vac for a 240 vac branch.

    The wiring gauge used (voltage drop due to surge) effects peak current but if current reduced by wire resistance it just last longer in time to spin up compressor.
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  • System2
    System2 Posts: 6,290 admin
    #7
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    Re: Sustained Air Conditioning

    I'd try to keep any applied voltage within +/- 10% of the data plates' rated voltage.

    LRA is "Locked Rotor Amps", The max amp draw a motor will pull with rated voltage

    applied. I believe for a motor, applying a lower voltage will result in a higher

    amperage.
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  • Photowhit
    Photowhit Solar Expert Posts: 6,003 ✭✭✭✭✭
    #8
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    Re: Sustained Air Conditioning

    The needing a 5000 watt inverter, is really not true as inverters are designed to handle a surge load, typically 2 - 3x the rating for a 1-5 minutes.

    My 1800 watt inverter handles the startup of my wheat grinder just fine it's rated as 10 amp continous or 1200 watt.
    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.
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  • System2
    System2 Posts: 6,290 admin
    #9
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    Re: Sustained Air Conditioning

    As I posted, I'm not a solar pro. (but I wanna be!) The batteries may provide the necessary current during motor start-up?
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,447 admin
    #10
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    Re: Sustained Air Conditioning

    Rad1 and others,

    What is it that you want to do?

    There is conservation/new designed A/C... High efficiency reduces total power use of the A/C Plus lots of Attic insulation, shading against solar heat gain, energy star appliances like Fridge/ TV / smaller laptop vs desk top computers / CFL vs filament lamps / seal against air leaks / used a heat recovery ventilator, etc. reduce heat put into the building.

    Then there are the Sanyo (and others, I am sure) Mini-Splits that use an "inverter" (really a VFD--Variable Frequency Drive) which soft-start the motor and only run the compressor just fast enough for the energy needed (see the thread that Cariboocoot/Marc posted a link too--"Solar_Dave" found that one of these units set on low never used more than ~300 watts, starting/stopping/cycling/etc.--Very off grid / power friendly devices).

    Now that you have reduced your power needs to a minimum... Then look at the power needs.

    The original poster did not say if Grid Tied / Net Metering is available or not... But just as a rough rule of planning... The same kWH for Off-Grid will cost about 4x as much as a Grid Tied system (batteries, extra controllers, replacement batteries every 4-8 years or so, loss in efficiency due to battery cycling, etc.).

    So, if there is no way that Grid Tied is available (government/power company say no)--then you really have to look at see if $1-$2 per kWH is really worth installing an off-grid system for (not really "micro system"--this will be a fair sized system with goodly power capabilities).

    However, if Grid Tied is allowed and practical--there is a Hybrid variant available. That is a system that has batteries and such--but the inverter can be run Grid Tied when the utility is functioning (batteries just "float" more or less unused) and when the power fails, the inverter switches over to off-grid mode and runs from the battery bank.

    Very roughly, the cost of power from a hybrid system will be roughly 2x grid power... Not cheap, but can be nice if emergency power is needed (long outages from ice storms, hurricanes, etc.). Since batteries are not "cycling" most of the time, the system is more efficient and the batteries can last longer.

    One of the big issues with an Off-Grid system for a dedicated use (like air conditioning)--is you only need A/C a few months of the year and, realistically, a solar powered system should be used 9+ months of the year to generate/use most of the power generated (keeps $$$/kWH prices a bit lower). Of course, if you get an A/C system with heat pump capabilities--you can probably extend its useful time into cooler times of the year.

    There are even some interesting ways of connecting a Hybrid Inverter/Charger system... Some folks have A/C utility power at the edge of their property--but have a 0.5+ mile run from the drop to their home. Either they have to pay a lot of money to run a high voltage/high power line on their property (on poles or buried)--Or they can bring a smaller line in (say 10 amps at 240 VAC) to the hybrid inverter... The system will pass the power through for use, but use the batteries to manage power surges (will assist the low current line, or even a small generator) to power heavy loads. After the loads are removed, the system will recharge the battery bank from the utility/small generator power.

    So--depending on what the needs are--there are several ways of addressing the issues.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • dwh
    dwh Solar Expert Posts: 1,341 ✭✭✭
    #11
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    Re: Sustained Air Conditioning
    dgsloan wrote: »
    Question - Can I use a set of micro grid tie inverters to supply the required energy for the air conditioner?

    Not without trying them to the grid.

    Grid-tie inverters require a grid source to sync their oscillation to. If they don't see it, they shut down.

    Grid-tie is grid-tie. You'd need to get the permits and inspections and whatever else is required to do a grid-tie. If you did that, then you would be in the situation that drees referred to - generating enough power to off-set the power used by the air conditioner - but the a/c would still be running from the grid.
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  • System2
    System2 Posts: 6,290 admin
    #12
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    Re: Sustained Air Conditioning

    Thanks BB!! Good info indeed!! I checked the mini that was linked to in your post,

    seems like a nice unit!! I am a newby who wants solar but has a misses who likes

    to use POWER.... What to do is what I'm after! Thanks for the post! My Apologies to

    dgsloan for the thread hi-jack. I was curious to know if the high starting amps of an

    A/C motor would be detrimental to a PV system.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,447 admin
    #13
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    Re: Sustained Air Conditioning

    From a solar system point of view:
    1. The solar panels are a "current" source. Their output is proportional to the sunlight on them--no surge capabilities there.
    2. Inverters--Pretty straight forward... They typically can surge 2-3x their rated output--assuming the DC power (battery) and wiring is capable. Note that inverters have virtually no energy storage internal to them--they draw all needed power from the battery bank.
    3. Wiring--One of the "weak points" for supplying energy. Battery systems are low voltage (12-48 volts typically) and so have up to 10x or more the current of the AC load (12 volt to 120 volts, 10 amp 120 VAC becomes around 135 amps from the battery). For a 12 volt system, the battery may be drawn down to 11.5 volts from loading and the inverter will cut out at 10.5 volts--so you only have 1.0 of voltage drop to play with--Higher voltage battery banks are recommended for high load/surge installations).
    4. And battery bank... Sometimes I am amazed that they work as well as they do... Typically we try to size a battery for 20 Hour Rate for "average loads", 8 Hour Rate for heavy sustained loads, and no less than 2.5 hour rate for surge power for standard flooded cell battery banks (we also try not to use more than 50% of battery capacity for longer life). AGM batteries are better battery for sustaining high loads (and much cleaner because the acid is more or less sealed inside)--but they are also 2x as expensive.
    So--We try to size the battery bank to the expected average/heavy/surge loads--then size the rest of the system (wiring, inverter, chargers, solar panels, genset) around the battery to give it the best chance at a long life and have you happy with your system's operation.

    If you want to talk about your own system (current or future plans)--please feel free to start your own thread--We try to keep every one to their "own thread" when discussing systems so that we don't get even more confused who is typing about which system. ;)

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • dgsloan
    dgsloan Registered Users Posts: 22 ✭
    #14
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    Re: Sustained Air Conditioning

    Thanks for all the replies - Of coarse I should have added a few more details - The reason that I wanted to add the air conditioner to a battery bank was because of all the other motor loads in the workshop - so I would not blow any breakers. I have tested a similar 6000 BTU air conditioner with the inverter and it does handle the surge - I am using some LiFePO4 batteries that I picked up at a low cost by bidding on ebay - use some for my Ebike but got carried away. The workshop is insulated and 14 by 16 so the BTUs should be close. My main concern was not having to add additional wiring to the shop - I have not blown any breakers yet but my watt meter is telling me that I am close to the max amps for the saws /shop vac and air cleaners that operate at the same time. The other alternative is to run an additional feed but I was trying to work with what I had available. Grid tie would work if I tried to add 240v but the inverters that I am using would be hard to split and then the inspection and configuration would be more complicated. The breakers and fuses are all marine grade but may not be grid grade. I definitely have covered the ground / lightning issues. Thanks again for the help.
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