Solar DC mini split— thoughts

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peakbagger

Minister of Fire
Jul 11, 2008
7,679
Northern NH
The cost of the batteries would offset the cost of the inverter though. I know some of the units are marketed as not needing batteries, but I don't accept the premise that these heatpumps are going to be effective when shutting down and starting up every time a cloud goes by, not to mention what it would do to compressor/motor longevity.

An APSystems QS1 inverter is $400 and will run 4 panels for a total output of 1200 watts, that cost would buy 2-3 12 volt batteries. I'd still be going grid tied.

I think there are some misconceptions. PV is not on off when a cloud goes over the sun so the minisplit would probably not turn off, it just may not be cooling. A mini split is not like a conventional window shaker or even standard home heat pump. A window unit is generally a single speed motor hooked to a compressor. The compressor is either on or off so there is lots of current inrush everytime the motor compressor starts. It runs at 100% until it overshoots the temperature setting and then the compressor turns off. The fans may have multiple speeds but they do not pull much power. A mini split maintains one temp by running the compressor at variable speed adjusting the compressor speed as needed. Even if it does turn off, its a "soft start" where the motor is ramped up slowly so it will last a lot longer than one that cycled on/off. Thus my guess is if the power from the panels is not high enough the fans may still run but the compressor does not. Conventional old style variable speed drives convert the input AC to DC and then converts it to AC waveform. So having a 48 volt DC input may make the drive simpler without a front end AC stage. Industrially most motors are now VF drives and as long as they are built for VF drive use they are lasting longer and far more efficient as they are soft start and run at lower speeds most of the time.

I think a big selling point of the DC units that do not require battery's is that they are black out and brown out proof, given the issues with the grid in Texas where the legislature is more interested in bailing out the corporations than consumers, black outs and brown outs are likelihood in hot spells. Lot to be said for something that runs for cooling as long as the sun is out. Its not just Texas, CA has issues especially rural areas that get their power shut off to keep from the power lines starting fires. In both areas AC is not optional, people die especially the elderly when the AC goes out.








 

EbS-P

Minister of Fire
Jan 19, 2019
3,511
SE North Carolina
I think there are some misconceptions. PV is not on off when a cloud goes over the sun so the minisplit would probably not turn off, it just may not be cooling. A mini split is not like a conventional window shaker or even standard home heat pump. A window unit is generally a single speed motor hooked to a compressor. The compressor is either on or off so there is lots of current inrush everytime the motor compressor starts. It runs at 100% until it overshoots the temperature setting and then the compressor turns off. The fans may have multiple speeds but they do not pull much power. A mini split maintains one temp by running the compressor at variable speed adjusting the compressor speed as needed. Even if it does turn off, its a "soft start" where the motor is ramped up slowly so it will last a lot longer than one that cycled on/off. Thus my guess is if the power from the panels is not high enough the fans may still run but the compressor does not. Conventional old style variable speed drives convert the input AC to DC and then converts it to AC waveform. So having a 48 volt DC input may make the drive simpler without a front end AC stage. Industrially most motors are now VF drives and as long as they are built for VF drive use they are lasting longer and far more efficient as they are soft start and run at lower speeds most of the time.

I think a big selling point of the DC units that do not require battery's is that they are black out and brown out proof, given the issues with the grid in Texas where the legislature is more interested in bailing out the corporations than consumers, black outs and brown outs are likelihood in hot spells. Lot to be said for something that runs for cooling as long as the sun is out. Its not just Texas, CA has issues especially rural areas that get their power shut off to keep from the power lines starting fires. In both areas AC is not optional, people die especially the elderly when the AC goes out.
I see this system, with a battery and an inverter as equivalent to an expensive stand by generator. Better in that you are getting some use out of it every day. After Florence we saw Generac trucks/vans everywhere for about 12-18 months. Then we didn’t see too many. Now, almost 4 years later they are back in good numbers. I doubt it’s all new installs. But but I could be wrong. Nothing like paying big bucks for an 8-10 Kw system to only need it for a grand total of 13 hours (in my case) in four years. Granted after Florence it was a week for everyone and some people it was 2-3 weeks without power.
 
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ABMax24

Minister of Fire
Sep 18, 2019
1,694
Grande Prairie, Alberta, Canada
I think there are some misconceptions. PV is not on off when a cloud goes over the sun so the minisplit would probably not turn off, it just may not be cooling. A mini split is not like a conventional window shaker or even standard home heat pump. A window unit is generally a single speed motor hooked to a compressor. The compressor is either on or off so there is lots of current inrush everytime the motor compressor starts. It runs at 100% until it overshoots the temperature setting and then the compressor turns off. The fans may have multiple speeds but they do not pull much power. A mini split maintains one temp by running the compressor at variable speed adjusting the compressor speed as needed. Even if it does turn off, its a "soft start" where the motor is ramped up slowly so it will last a lot longer than one that cycled on/off. Thus my guess is if the power from the panels is not high enough the fans may still run but the compressor does not. Conventional old style variable speed drives convert the input AC to DC and then converts it to AC waveform. So having a 48 volt DC input may make the drive simpler without a front end AC stage. Industrially most motors are now VF drives and as long as they are built for VF drive use they are lasting longer and far more efficient as they are soft start and run at lower speeds most of the time.

I think a big selling point of the DC units that do not require battery's is that they are black out and brown out proof, given the issues with the grid in Texas where the legislature is more interested in bailing out the corporations than consumers, black outs and brown outs are likelihood in hot spells. Lot to be said for something that runs for cooling as long as the sun is out. Its not just Texas, CA has issues especially rural areas that get their power shut off to keep from the power lines starting fires. In both areas AC is not optional, people die especially the elderly when the AC goes out.

It is quite easy though to have solar output reduced by 80% with cloud cover. Can these DC minisplits have a turndown of 5:1, maybe. To do so will be expensive though, motors, VFDs, and compressors all have limits to their min and max operating range.

Regardless I won't be buying such a unit, to me a house scale battery makes a lot more sense.
 

Poindexter

Minister of Fire
Jun 28, 2014
2,814
Fairbanks, Alaska
When was your house built @Poindexter ? Do you have central AC? In this case, airsealing would REDUCE indoor humidity.
1980 build, state of the art at the time, with vapor barrier under the drywall, but no hats on the connection boxes. I should have bought stock in spray foam products

For Heating I have an oil fired boiler at about 100-110kBTU / hour with hotwater baseboard, and a BK stove at 35k (ish) BTU per hour. Both get run pretty hard in January. Keeping the woodstove raging does lower the duty cycle on the oil fired boiler for the rest of the winter. I do have a spare burner for my new last month boiler, the spare burner is for natural gas. I do have a gas line in the easement at the front of my lot out by the street, but the current price on that stuff is, well the local market is disconnected from the lower 48.

For ventilation I have opable windows and a courtesy fan in each of the two bathrooms.

For AC I got jack doodle. We do have an arc of trees around the house from east to west, but they protect the north side of the house from winter weather.

I didn't really understand vapor barrier until I moved up here (from Chapel Hill, NC) long about 2008. I have never had to deal with vapor barrier in a house that has both heating and AC, but it looks like a challenge.
 
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begreen

Mooderator
Staff member
Nov 18, 2005
96,219
South Puget Sound, WA
The idea of air conditioning in Fairbanks, AK does not compute.
 

ABMax24

Minister of Fire
Sep 18, 2019
1,694
Grande Prairie, Alberta, Canada
For AC I got jack doodle. We do have an arc of trees around the house from east to west, but they protect the north side of the house from winter weather.

Surely those north trees provide some shade to the house on a day like today, with your nearly 22 hours of daylight.

I know that's even an issue for us, with the sun beating on the house for 17+ hours a day, and the nights not cooling off much, it can be difficult to keep the house cool without AC. At this time of year it's not uncommon for our AC to be running consistently until midnight.
 

Poindexter

Minister of Fire
Jun 28, 2014
2,814
Fairbanks, Alaska
Surely those north trees provide some shade to the house on a day like today, with your nearly 22 hours of daylight.

I know that's even an issue for us, with the sun beating on the house for 17+ hours a day, and the nights not cooling off much, it can be difficult to keep the house cool without AC. At this time of year it's not uncommon for our AC to be running consistently until midnight.
The idea of air conditioning in Fairbanks, AK does not compute.

It is all in what you are used to. I flew into Seattle once when they were having the worst snowstorm in 25 years, the place was a madhouse. Because they weren't used to it. It looked like September in Fairbanks to me, but that doesn't help them; or make it normal for them. Christmas-ish 2008 I think it was.

I was talking to my Sarasota, FL area dad last weekend, his heat index was +105-110dF and he was aware it was hot. He gets chilly at +40dF in January down there with like sweaters and pills for his joints and stuff. So 60-70 degree (F) swing, he feels it. I am pretty well acclimated to -20dF for good boots and maybe a, what is that new sweater thing they make out of old Led Zeppelin albums? Fleece, an artificial fleece. They are everywhere now. One of those. But at +70dF, a 90 degree swing, I am miserable and we are looking at +80s all next week.

The trees to the north of the house do moderate nighttime household temps by blocking the sun while the sun is north of west and coming around through north to be north of east overnight. If I had AC and appropriate vapor barrier (I think for cooling I would need a second layer of vapor barrier on the outside surface of the wall, just under the finished surface) I would be shooting for +65dF in the house.

In the +70s I am drinking 6-7 liters of water daily, and sweating like I just got poached or braised or something. Given 'the arctic' is warming twice as fast as the rest of the globe, at least many scientists say so, I probably will be looking an AC system in our next house if we stay up here.

Being able to run something like this system PRN in the summer and then maybe use the solar panel output for something else the rest of the year is something to consider pretty seriously.
 

sloeffle

Minister of Fire
Mar 1, 2012
1,070
Central Ohio
The idea of air conditioning in Fairbanks, AK does not compute.
I'm glad our hotel had it when we visited Denali a few years ago. ;)

A few day earlier we were bundled up in coats ( mid 50's ), and the folks from Alaska were walking around in shorts. We chatted with a local at a pizza place in Fairbanks and he knew right away we weren't from Alaska.
 
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tlc1976

Minister of Fire
Oct 7, 2012
1,139
Northwest Lower Michigan
I see this system, with a battery and an inverter as equivalent to an expensive stand by generator. Better in that you are getting some use out of it every day. After Florence we saw Generac trucks/vans everywhere for about 12-18 months. Then we didn’t see too many. Now, almost 4 years later they are back in good numbers. I doubt it’s all new installs. But but I could be wrong. Nothing like paying big bucks for an 8-10 Kw system to only need it for a grand total of 13 hours (in my case) in four years. Granted after Florence it was a week for everyone and some people it was 2-3 weeks without power.
Yes and in addition to upfront costs, that standby generator costs a lot in fuel when you do need it. I have an 8kw Generac.

Solar mini split- one time purchase, no fuel required, self contained, and cools more as the sun beats down harder. Yes it’s not perfect (won’t work at night and probably not as good when it’s cloudy). But if I lived in an area that was hot and sunny most of the time, it’d be a no brainer, I’d be getting one. Especially if the place had a history of grid problems. For a long time I’ve wondered why they don’t use absorption fridge technology to do something similar, or maybe they have.
 

peakbagger

Minister of Fire
Jul 11, 2008
7,679
Northern NH
the Coefficient of performance for a Mini Split is around 3 (I watt of power in for 3 watts of cooling out). Absorption chillers are far lower, for a single effect hot water unit the COP is less than 1 (usually .8). Of course the heat is "free" but you need a lot of it. There was a Swiss individual once that posted that they did sell house hold absorption chillers in Switzerland. The smallest I have seen in the US is a Yazaki and I think the smallest one is 5 tons.

I have a 85 ton unit one of my projects. It dwarfs the 500 ton centrifugal sitting next to it.
 

Brian26

Minister of Fire
Sep 20, 2013
682
Branford, CT
the Coefficient of performance for a Mini Split is around 3 (I watt of power in for 3 watts of cooling out). Absorption chillers are far lower, for a single effect hot water unit the COP is less than 1 (usually .8). Of course the heat is "free" but you need a lot of it. There was a Swiss individual once that posted that they did sell house hold absorption chillers in Switzerland. The smallest I have seen in the US is a Yazaki and I think the smallest one is 5 tons.

I have a 85 ton unit one of my projects. It dwarfs the 500 ton centrifugal sitting next to it.
I don't think you and others realize how efficient a properly sized modern high seer 38-42.5 seer AHRI rated mini split heat pumps is. A COP of 3 for cooling is what a good Carrier central air unit was like 30 years ago. My mini split at its minimum compressor speed is rated at a cop of 10.62 cooling at 82 degrees using only 80 watts. I verified all these numbers with my Emporia Vue Energy monitor which is measuring voltage and adjusting for power factor. My observations were perfectly matched up with the below chart.

I have been on net metered solar for 8 years here in CT that covers close to 90% of my yearly electricity usage. I ran some numbers and even if I had to purchase the electricity these high seer mini splits use so little electricity that the savings over a central unit are massive. Most central air units are are around 16 seer.

Here is my yearly monthly electricity load on just both my mini splits that cover 90%+ of my yearly heating and cooling load. . As you can see my downstairs unit carries most of the heating load and my upstairs does most of the cooling. I haven't even used 100 kwhs this month for cooling so far which if I purchased the electricity would cost me around $26.

Screenshot_20220625-060423_Chrome.jpg Screenshot_20220625-090352_Monitor.jpg
 
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Piney

Member
Nov 29, 2015
77
Frozen North
Slightly tangential: as far as ac goes, we are off grid. We run ac when needed. We find that when it is so hot we need ac during the day there is enough sun on the panels to let us. We set the ac to max, continuous run and drive the house a little cooler than we like so that when the early evening comes and the ac is off we are ahead of the game. We don’t need ac at night no matter how hot the day was because our area has the greatest diurnal temperature swings in North America. It’s always cold at night. We open a window. I realize this isn’t useful information for those in the humid south but it might give ideas to those at higher elevations. It’s amazing what a 7 or 9000 btu wall (why waste a window? We cut a hole) machine can do running full tilt, non-stop, on ‘free’ power for hours on end.