I have a small year-round off-grid house at Lat 43.4N Long 75.2W. My usage will be 7kWh per day. The insolation in winter isn't great, maybe 1.8. There would be a 6kW Generac running on propane to make up for shortages. This would be a 120V/240V system. I figure the maximum AC load I would have at one time is 4000W.
The house has a roof that faces East and West - not suitable for an array. Thus, I am planning to build a detached garage with a saltbox style roof with a steep pitch for the solar array, facing due South. There is a good sized clearing for the house, and there is a dropoff to a stream, so I am hoping I will have clear illumination between at least 10am and 2pm on Dec 22 - I will verify this in a few days on a site visit using some measurement devices. The array is not in the shadow of the house, so that is a good start.
Solar panels and batteries
A reasonably sized two car garage roof as described will provide enough space for 16 panels. I chose 275W Mono. 16 x 275 = 4400W. The VOC is 39.4V and the Iss is 9.58A. I did a little research on batteries, and got drawn in by the Aquion AHI "on paper". I liked the safety aspect, not worrying about water, and being able to handle 100% DOD. They aren't cheap, so I settled on under-sizing and planning to use the generator to make up for lack of full capacity. I chose nine 48V stacks, which is 459Ah and 21.6kWh @ 20 hours.
MPPT and Inverter First design
I first had 4 strings of 4 panels, which has a VOC of 158V. The Midnight Solar Classic 150 MPPT can actually handle this (using a proprietary circuit), and puts out 80A max.
I chose the Xantrex XW4548, because it does 120V/240V, 48V batteries, and can handle 4500W to the load. It has an optional controller to start the Generac.
MPPT and Inverter Second design
I was concerned about intermittent shading from trees (that I still need to assess on site), and I noticed the Xantrex MPPT has a "shading algorithm." This was when I became aware of what happens if shading of only one panel occurs. I looked briefly at micro inverters, but decided against them. The Xantrex XW MPPT 80 600 can also put out 80A, and I thought I would lower the amperage (and wire size) by only using 2 strings of 8 panels. The open circuit voltage of 8 is 315V which is no problem.
I liked the idea of using Xantrex for both the MPPT and Inverter, and getting an SCP to control/monitor both. There is also box that interfaces to a LAN available. However, it isn't clear that much benefit exists from these two "working together" for my off grid application.
Comments?
It was just tonight that I discovered the AHI battery hasn't taken off like I assumed. Aquion did give me a quote including shipping, so I assume they are making them and someone is buying. Do you seasoned people have comments on my attempt at a design? I was thinking that worse case I would like the generator running maybe two hours a day in winter. What replacement battery type would you suggest, and what capacity in Ah? Any and all comments are welcome. I had a local installer who is certified that was supposed to help, but he bagged on me, saying "off grid is difficult - all the systems I have done are on grid." The shading issue is bothersome - is there any benefit of four series strings vs two, or the opposite, or are things equally as bad as far as power reduction due to the MPPT if one panel becomes shaded?
I attached a cartoon showing the design as I understand it.
Thanks for any advice,
Don
The house has a roof that faces East and West - not suitable for an array. Thus, I am planning to build a detached garage with a saltbox style roof with a steep pitch for the solar array, facing due South. There is a good sized clearing for the house, and there is a dropoff to a stream, so I am hoping I will have clear illumination between at least 10am and 2pm on Dec 22 - I will verify this in a few days on a site visit using some measurement devices. The array is not in the shadow of the house, so that is a good start.
Solar panels and batteries
A reasonably sized two car garage roof as described will provide enough space for 16 panels. I chose 275W Mono. 16 x 275 = 4400W. The VOC is 39.4V and the Iss is 9.58A. I did a little research on batteries, and got drawn in by the Aquion AHI "on paper". I liked the safety aspect, not worrying about water, and being able to handle 100% DOD. They aren't cheap, so I settled on under-sizing and planning to use the generator to make up for lack of full capacity. I chose nine 48V stacks, which is 459Ah and 21.6kWh @ 20 hours.
MPPT and Inverter First design
I first had 4 strings of 4 panels, which has a VOC of 158V. The Midnight Solar Classic 150 MPPT can actually handle this (using a proprietary circuit), and puts out 80A max.
I chose the Xantrex XW4548, because it does 120V/240V, 48V batteries, and can handle 4500W to the load. It has an optional controller to start the Generac.
MPPT and Inverter Second design
I was concerned about intermittent shading from trees (that I still need to assess on site), and I noticed the Xantrex MPPT has a "shading algorithm." This was when I became aware of what happens if shading of only one panel occurs. I looked briefly at micro inverters, but decided against them. The Xantrex XW MPPT 80 600 can also put out 80A, and I thought I would lower the amperage (and wire size) by only using 2 strings of 8 panels. The open circuit voltage of 8 is 315V which is no problem.
I liked the idea of using Xantrex for both the MPPT and Inverter, and getting an SCP to control/monitor both. There is also box that interfaces to a LAN available. However, it isn't clear that much benefit exists from these two "working together" for my off grid application.
Comments?
It was just tonight that I discovered the AHI battery hasn't taken off like I assumed. Aquion did give me a quote including shipping, so I assume they are making them and someone is buying. Do you seasoned people have comments on my attempt at a design? I was thinking that worse case I would like the generator running maybe two hours a day in winter. What replacement battery type would you suggest, and what capacity in Ah? Any and all comments are welcome. I had a local installer who is certified that was supposed to help, but he bagged on me, saying "off grid is difficult - all the systems I have done are on grid." The shading issue is bothersome - is there any benefit of four series strings vs two, or the opposite, or are things equally as bad as far as power reduction due to the MPPT if one panel becomes shaded?
I attached a cartoon showing the design as I understand it.
Thanks for any advice,
Don
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