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Old 04-23-16, 04:19 PM   #1
Kramer
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Default Mppt inverters for direct coupled solar

I've been thinking about one of these for my pool pump or even supplemental Hvac. It seems cheaper then a charge controller and batteries, even if some solar capacity is wasted. I have a variable speed mini split outdoor unit with a fried circuit board. I am toying with the idea of making my own driver for the compressor using one of these.

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Old 04-25-16, 07:35 AM   #2
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Welcome to the forums! Sounds like a worthy project to me. Please elaborate on what you've got: how much solar panel power/voltage, what model of mini-split unit, what will the unit heat or cool, etc.
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Old 04-25-16, 09:53 AM   #3
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I have a cheap 27k btu multi-unit mini split heat pump that has gone brain dead. I asked the seller to repair/replace the old unit. Their first offer was to send a new unit for $500. Since this allowed me to keep the old unit, which was barely used, I said yes . It is variable speed and has an inverter and runs 410A. I've been wanting to play with solar power for quite a while and this seems like a good opportunity to build solar powered HVAC without the wife having to suffer in the mean time.
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Old 04-25-16, 10:20 AM   #4
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I want to build my own controller for the compressor with an MPPT/VFD drive like the ones being used for solar water pumping. ABB makes one that looks to me to be more then capable (ACS355 Solar pump inverter +N827) , but as a novice, I was hoping someone here might know more. The idea is that as soon as the panels output meets a certain threshold, they power the compressor at the maximum speed possible. This way the unit uses all the solar power available until the panel output is to little to run the compressor. I will need to figure out how to handle the condenser fan and also how to make the best use of all that daytime cooling capacity. I have a few surplus air/refrigerant heat exchangers (both indoor and outdoor.) I have a scheme to use one to make an ice storage tank. If it works it will store all that surplus refrigeration for cooling the house when the sun stops shining.
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Old 04-27-16, 07:43 PM   #5
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The inverter will work if you can figure out the parameters of the compressor you have. Most every motor controller out there has current sensing and soft start capability, and the product you cited has a mppt algorithm built into it, so the easy part of the project would be wiring it all up correctly. The hard part would be paying for everything and tweaking parameters. If you are a PLC programmer, this might not be so difficult. Then automating the operation of the controller to run, say, off a common home thermostat, would not be daunting.

Still don't know anything specific about the guinea pig outdoor unit yet. Unit number, compressor number, brands of each, etc is needed to further investigate. Being overly general, for a two ton unit you will need 2KW of panel power or so to run the compressor at full capacity. It may be less expensive or laborious to try to repair the control board. Not knowing what you have, the price for a repair or replacement board may be here or there, who's to tell?
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Old 04-28-16, 04:03 AM   #6
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I'll be pulling it off the wall soon so I can dissect it. I have the repair manual, and if its feasible to repair without a new control board ($$$) that is what I will do. I have a full basement, and it would be very tempting to use the the unit to run as much as the sun will allow, and pump all the heat into or out of the basement. The vfd drive will run of mains power as well, so I can try a few things out before buying any panels.
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Old 04-28-16, 06:33 AM   #7
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How is the wall mount as far as noise factor goes? I have heard accounts of the units making all manner of hums and buzzes through the wall.
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Old 04-28-16, 07:57 PM   #8
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It made a whistling sound in heat mode, but it was not loud and it was on my bedroom wall.

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