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Old 06-01-16, 07:03 AM   #1
MEMPHIS91
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Default Directly solar powered air conditioner

I have been trying to work this out in my head, and do some research. It might be that its so simple that I'm over thinking it. But I have a lot of extra solar power in the summer so I want to divert that power to a small window unit ((500 watt)) and only use it when the sun is shinning. I know I can just wire in my inverter to my battery bank, but I would rather skip that altogether and wire straight from my array. One reason being that I have 800 watts of panels not tied into the batteries yet and the other reason being I want to do this with other appliances in the future.
So here is the question, how would I design a setup to run only when the solar panels are getting enough sun to power the window unit? Seems to me like every time the sun goes behind a cloud I would have to reset everything. Or that the panels would only be making 350 watts and burn something up.
What am I missing here? Do I have to at least have a small battery bank to control everything? Or is there a super awesome device that does all the work for me?

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Old 06-01-16, 07:34 AM   #2
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Your small window unit will take more than even 800 watts to start the compressor, so you'd need at least enough battery to handle starting the compressor. Even most inverters struggle with motor loads like air conditioners even if they have enough voltage on the battery side.

To get an idea of what a startup load is like, listen to this random video of a small 2-stroke generator loading up when it's trying to start the compressor of this air conditioner. You can hear the momentary deep motor startup load before it goes back to a more comfortable 500 watts or so.


I would suggest doing things in this order.
1. Make sure you have an inverter that can handle the startup load of your specific air conditioner.
2. Configure a battery that can handle the startup load without the voltage sagging below the cutoff voltage without solar connected.
3. Connect the solar.

This way you have a system where you've got a battery just large enough to kick over the air conditioner that won't cut out with every cloud that floats over.

A good 800 watt solar panel that's worth it's rating would make 500 watts on the sunniest of days for about 5 hours. Add clouds and you've got less than that. Also keep in mind that you probably won't need the air conditioner at 11am when there is just enough sun to start the air conditioner and when 4 or 5pm hits and you don't have enough sunlight to produce more power than the air conditioner needs, you'll probably want to have some storage to keep it running because that's when things really start getting hot indoors during the air conditioning season.

For what it's worth my air conditioner in my house doesn't even start for the first time of the day until 3 or 4pm usually at the end of May. In hotter weather it runs deeper cycles later in the day, there's some thermal lag involved.
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Old 06-01-16, 08:49 AM   #3
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So you will probably need some kind of battery. I don't know of any way to hook panels directly to an inverter. I don't think the panels would handle the start up surge. But you could use this Relay Driver - Morningstar Corporation it's basically a programable set of relays run by battery voltage. I believe it has times you can set.

So say your batteries go into float for 10 min and you have the relay driver set to kick on the relay to give power to the inverter. But if the in goes away for 10 min you can set the timer on the relay to wait for 20 min before it kicks back off because of low battery voltage.

I don't think that makes sense at all but there is a guy on YouTube that shows how it works. (Sorry I can't find the video) I actually have one but I haven't hooked it up yet. It essentially acts as a smart dump load controller
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Old 06-01-16, 01:53 PM   #4
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Normally panel voltage is too high for an inverter.
My samalex inverter will shut off if it gets to more than 15 volts. It will be damaged by more than 16 volts. Most "12 volt panels" actually produce 18 to 22 volts and usually around 70% of rated amps.

An inverter would under volt and shut off if the demand exceeded the supply.
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Old 06-01-16, 05:06 PM   #5
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Awesome reply guys! Thanks, that is along the lines of what I was thinking.

So next question, would there be a way use grid power to start it and then if the sun is available to use solar power. Then if clouds happen it could jump back to grid? I know your thinking why don't I just feed into the grid and be done with it. I am anti grid, but until I can go full out solar I'm going to make the most of what I have. I would be the first person on my system to grid tie and they pay you about 2 cents less than what they charge for power, people are anti solar around here for sure.
I will start doing so looking into this idea, has anyone seen anything like it?
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Old 06-01-16, 05:46 PM   #6
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Memphis,

I really like your concept of being off grid with solar PV as primary and simply using the grid as a backup (like when a dark cloud goes overhead). Most people do the opposite - use the grid as primary and have PV panels (or wind turbine, generator, etc) as backup.

Think about your comment regarding a dark cloud going overhead. On a hot, humid summer day (what other kind are there in the south!), the PV output drops and the much needed AC output falls to zero - unhappy spouse and family.

But, allow the grid to seamlessly kick in for a bit and all is well . . .

This is exactly what Enphase has with their new AC batteries. The problem is - how long will the "dark cloud" persist. Perhaps nighttime is just an extended "dark cloud". That determines how many of their batteries you need - and they are expensive!

If you can put up with a dark cloud passing overhead, and are willing to suck off the grid for a bit, then what is the difference with the extended dark cloud such as night hours? Isn't it really the same?

I just put in a 10 kW Enphase grid tied system for my cousin (no Enphase battery back up), 40 265 watt panels and 40 M250 inverters. To meet peak demand (summer AC in July & August), he would need about 40 kW, but then much of that would be wasted in other months as the utility takes it and only compensates him for about 1/2 value. So we compromised and figured out just how much would supply most of the power, most of the year.

The next five years will see battery storage decrease in price, much like the exponential cost decreases seen with PV/inverters in the last five years. But for now, I am content to minimize my grid connection. First through conservation, secondly with energy efficient appliances (geothermal heat pumps) and thirdly with electrical generation itself. That triad is a stable three legged stool.

You have done much the same.

The grid connect allows a seamless, instantaneous flow between your generation and what you consumption need requires at very low cost. You might just have to grit your teeth and do it. Right now, it is by far, the cheapest, most reliable and available method to do what you want.

You just have to think of night time as an extended dark cloud . . . . until battery $/kWhr gets a lot cheaper.

Steve
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Last edited by stevehull; 06-01-16 at 05:48 PM.. Reason: clarity
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Old 06-02-16, 10:36 PM   #7
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Grid tie inverters are getting very inexpensive and some of them have limiters so that they will not feed power back into the grid, when they have excess power.
Good for smart meter problems.
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Old 06-03-16, 04:54 PM   #8
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Steve- The problem is with the current setup with my power company I would be getting paid 7 cents for every kwh, and paying 13-16cents. So I would be loosing 50% or more using the grid as a battery. Plus my power company is talking about totally doing away with receiving solar power because no one is doing it and it "cost them more money". REALLY stupid and selfish power company. Hench my whole reason to totally avoid sending them any power, my goal is to use 100% of the power my panels produce, WHILE using the grid to make everything work.

nibs, thanks, I will start looking into some inverters.
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Old 06-03-16, 05:04 PM   #9
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Just found this. 1000W Watt Solar Micro Grid Tie Power Inverter for Solar Panel 10 5 28V AC | eBay
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Old 06-03-16, 07:12 PM   #10
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Memphis,

Yes, you are loosing about 50% of your energy to use the grid as "your battery". But you will loose that and even more so with on site batteries (input/output efficiency). Then the cost of the batteries and the necessary equipment to provide for the seamless switching.

I know, I know - it is hard to accept a 50% loss to use the "grid battery". But it is cheap, existing and very little has to be done to use it as is.

Really take a look at your true cost to put in battery storage. My suggestion is that in five years, on site battery storage will be at a fraction of today's costs. Study, plan, but use what you can NOW.

Recall the phrase, never let the seduction of the perfect, distract you from the incredible utility of the good . . . . .


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