Running an AC evaporative air conditioner on solar without batteries

[M7amad]

Hi,

I would like to run an AC evaporative air conditioner during daytime only using solar panels without a battery. These are my system specs:
1. Air conditioner: rated power: 300 W/220V (i assume there is negligible surge here since it's a small unit; a fan and a small pump)
2. Solar array: 2 x 100 W panels (VOC=17 V)--panels connected in parallel
3.Controller: PWM 20 A
4. Inverter: MSW 500 W rated power (surge: 1000 W)--Input: 12V/24V.

My questions:
1. Will this system work? Lets first assume I use the charge controller in parallel. (as shown in the image file attached: Config 1).
2.in the second attached image (Config 2), I removed the charge controller. Is that fine? or is the charge controller necessary?
3. If there is a definitive need for a constant voltage source as a buffer to smoothen voltage input to the inverter, can I simply use a small 5 Ah/12 V lead acid battery?
4. Is it fine if the solar array has a lower power rating than the load for this application? Will this evaporative air conditioner not work at maximum power rating always?
5. When considering not using a battery, will the air conditioner be affected in a negative way?

Any suggestions on how to run this system efficiently without batteries are welcome!

Thanks,

[jflorey2]

Short answer - no.

Slightly longer answer - you need some kind of storage to work with an inverter. And if it's lead acid, it has to be big enough to safely handle the charge and discharge rates that it will see. If you want to get something more exotic ($$$) then you can look into LiFePO4 or ultracaps; these have higher charge and discharge limits but also cost a lot more.

Even longer answer - if you want to do the work to replace the AC pump and fan motors inside the A/C with DC motors, you MAY be able to get a solar direct system working where the power to each motor depends on the power out of the solar array. However it's not an easy thing to do, and usually involves current boosters to reliably start the motors, and some careful balancing to ensure that the lowest impedance motor doesn't get all the power.

[SunEagle]

First off an inverter will not work wired directly to your panels. It requires a constant voltage to start up and run.

Next issue is that a Charge controller is just that. It charges a battery system. It too will not provide what the inverter needs to run.

With 2 x 100 watt panels and a 20A CC you can charge a small 100Ah 12volt battery. That would allow the inverter to work but the battery will only provide about 300 watt hours of usage.

If you only run the AC during the hours of 10am to 2pm, that 200watts might be able to generate about 600 to 800watt hours but some of that will be going to keeping the battery charged.

Unfortunately the only load you can run directly from a solar panel would be some type of DC motor that will accept the voltage from the panels and change speeds as the sun changes intensity.

[M7amad]

aha...some people told me it can be done...but i was always sceptical b/c of the inverter input voltage constraints....i assume: even if i had a PSW inverter the system still wont work...

ok, so lets say I use a (120 Ah/12 V) battery , then it should be easy to run the air conditioner for at least 6 or 7 hours in the daytime right?....but i guess the tricky part is ensuring that battery's capacity is drained alot and remains as close as possible to 100%.

Thanks alot for the feedback...this was relli useful.

[Mike90250]

What they said, gotta use DC motors, maybe a LCB for each motor (Linear Current Booster)

http://realgoods.com/solar-renewable-energy/solar-water-pumps/linear-current-boosters

[sensij]

ok, so lets say I use a (120 Ah/12 V) battery , then it should be easy to run the air conditioner for at least 6 or 7 hours in the daytime right?....but i guess the tricky part is ensuring that battery's capacity is drained alot and remains as close as possible to 100%.

No. 200 W of panels will not provide the energy to run a 300 W air conditioner, if it is drawing 25 A @ 12 V. More details are needed, but figure 800 Wh as the order of magnitude for the amount of usable energy the array can produce with good exposure in the summer. That covers only a couple hours of operation, and the battery capacity would probably limit you to only another hour or two before over-discharge becomes a risk (depending on the type of battery)

If you run the air conditioner until battery failure, you will dramatically shorten the life of the battery. At best, you would be only to run like that only a couple days out of the week, since you'd need a couple days to recharge in between discharges.

[Sunking]

ok, so lets say I use a (120 Ah/12 V) battery , then it should be easy to run the air conditioner for at least 6 or 7 hours in the daytime right?....but i guess the tricky part is ensuring that battery's capacity is drained alot and remains as close as possible to 100%.

There is not an answer to that question. Just like me asking you how far can I drive on 10 gallons of gasoline. You cannot answer that question.

You do not want to discharge your battery more than 20% on any given day, and never ever more than 50%. In other words the battery is sized for 5 day capacity, and that gives you 2.5 to 3 days run time without a recharge.

a 12 volt 120 AH battery has a fixed capacity of 1440 watt hours. If your Swamp cooler uses 280 watts of watts of power, you battery is completely dead in 5 hours. You only got 1 hour run time a day. If you want it to run say 6 hours a day, you would need a 12 volt 720 AH battery to run a Swamp Cooler 6 hours a day.

[SunEagle]

aha...some people told me it can be done...but i was always sceptical b/c of the inverter input voltage constraints....i assume: even if i had a PSW inverter the system still wont work...

ok, so lets say I use a (120 Ah/12 V) battery , then it should be easy to run the air conditioner for at least 6 or 7 hours in the daytime right?....but i guess the tricky part is ensuring that battery's capacity is drained alot and remains as close as possible to 100%.

Thanks alot for the feedback...this was relli useful.

At most you might get a couple of hours but you can easily drain your batteries which would then have to be replaced.

Whatever someone or some YouTube video shows you is more than likely pure bull. Running AC loads on solar and batteries is not financially easy to do. It usually takes a big battery and a lot of panel wattage.

[AzRoute66]

The key is 200W panels powering a 300+W load. Should be a tip-off right there. I also have evaporative cooling and have sorta kinda looked into this. My half horsepower motor is very standard for my area and takes more than 300W, plus your pump is currently taking about 0.9A @ 120VAC (I have no clue if there is a significant power factor on those). I think if you go to a couple of 250ish W panels (24V system) you will be in the ballpark. I also think if you go to a DC fan motor and a DC pump, you probably can get away with getting rid of the battery - but with a lack of a case study I would have a back-up plan. If you do stay with AC, treat the battery as a starting battery/capacitor, not a storage battery, and make sure you turn on and off the load accordingly.

I was looking at the ability to run the fan motor at high speed, perhaps you are content with low speed (which might be where the 300W comes from), I have never looked at my situation from a low speed only perspective. Also, for the DC motor option I had to change the drive pulley/belt to match the desired RPM for the fan - just an additional one time cost...

[jflorey2]

i assume: even if i had a PSW inverter the system still wont work...

Correct.

Ok, so lets say I use a (120 Ah/12 V) battery , then it should be easy to run the air conditioner for at least 6 or 7 hours in the daytime right?....but i guess the tricky part is ensuring that battery's capacity is drained alot and remains as close as possible to 100%.

Yes, if you increase the solar panel power to ~150% of the estimated load AND you shut down as soon as input power gets below 100% of the load (that will protect the battery from overdischarge.) Note, though, that that's usually the hottest part of the day - around 4-5pm.

[SunEagle]

Correct.

Yes, if you increase the solar panel power to ~150% of the estimated load AND you shut down as soon as input power gets below 100% of the load (that will protect the battery from overdischarge.) Note, though, that that's usually the hottest part of the day - around 4-5pm.

I am glad you pointed out that the hottest part of a day is in the late afternoon which is usually after the pv panels are no longer producing anything close to nameplate wattage.

Somehow we need to convince people that powering their AC loads directly from the panels is a complete waste of time and money. Batteries as you know are not the nicest thing to have but are the critical component as the buffer between pv panels and AC loads.

[M7amad]

Correct.

Yes, if you increase the solar panel power to ~150% of the estimated load AND you shut down as soon as input power gets below 100% of the load (that will protect the battery from overdischarge.) Note, though, that that's usually the hottest part of the day - around 4-5pm.

Okay....can we propose this solution: for the same AC 300 W load, use 300 W panels facing North (i am living in the southern hemisphere) and use one 100 W panel facing west...in this way, we can get some extra power in the late afternoon which runs the ac without draining the battery to more than 50%....

one quick relevant question here: whats the best way to measure the exact batter's capacity at any time? a voltmeter and an assumption of a linear relation between V and Ah? or are there battery capacity meters that could be used? i have seen some in Amazon..although i am not sure of their accuracy

[jflorey2]

Okay....can we propose this solution: for the same AC 300 W load, use 300 W panels facing North (i am living in the southern hemisphere) and use one 100 W panel facing west...in this way, we can get some extra power in the late afternoon which runs the ac without draining the battery to more than 50%....

Northwest is probably ideal. If that's not possible, what you mention will work. It won't be ideal (unmatched panel temps) but should work. Having the batteries at 50% almost all the time isn't the greatest, but it will work.

one quick relevant question here: whats the best way to measure the exact batter's capacity at any time? a voltmeter and an assumption of a linear relation between V and Ah? or are there battery capacity meters that could be used? i have seen some in Amazon..although i am not sure of their accuracy

Charge accumulator (like the Flexnet DC by Outback) is the best way to do it. Alternatively let the battery rest for about half an hour and measure open circuit voltage. You can then use a table to look up approximate state-of-charge.

[BackwoodsEE]

As long as the AC or evap cooler is only running when there is enough power coming from the solar panels and you have an MPPT charge controller that adjusts how much power it takes from the PV module(s), even a small battery should be fine. The battery would have little wear and tear from cycling, but its lifetime still would be limited by calendar life.

This might actually be a case where you could get away with a car battery in a solar application. The only time current would be pulled from the battery would be when the compressor or fan motor starts, and it would be a very limited depth of discharge. I could see one of those miniature riding lawnmower batteries serving your purpose just fine.

But you would need to make sure that the AC or evap cooler only runs when you have at least as much power available from the panels as the power consumption of the compressor or motor. You would kill a starting battery in no time if you relied on it for power rather than just buffering the current between charge controller and inverter. You'd probably need to watch the current flow to the battery with an ammeter and be careful about when to switch the AC or cooler on in the morning and off in the late afternoon.

[jflorey2]

As long as the AC or evap cooler is only running when there is enough power coming from the solar panels and you have an MPPT charge controller that adjusts how much power it takes from the PV module(s), even a small battery should be fine.

You still have to heed the charge/discharge limits of the battery. You can't charge a 120ah lead acid battery at 20 amps.

But you would need to make sure that the AC or evap cooler only runs when you have at least as much power available from the panels as the power consumption of the compressor or motor. You would kill a starting battery in no time if you relied on it for power rather than just buffering the current between charge controller and inverter. You'd probably need to watch the current flow to the battery with an ammeter and be careful about when to switch the AC or cooler on in the morning and off in the late afternoon.

That requires a fairly competent controller, so that the AC can be shut off when the battery is being discharged. Most off the shelf controllers will not do that. (Definitely buildable, but not standard.)