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Old 08-01-18, 09:56 AM   #11
jeff5may
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On the opposite side of your question, the actual variable speed compressor in nearly all of the "novelty" outdoor unit is going to operate like a super size computer fan. Permanent magnet rotor and multiple phase stator. Depending on how the manufacturer designed the compressor, it may or may not have a 3 phase rotor and stator. This is very important, because a generic 3 phase motor drive might not work. More phases equates to more precise control and maybe higher efficiency, and these smallish capacity variable speed units are uncommon by nature. As such, the designs are all over the place.

Above the 2 ton rating, most of the units are built with 3 phase drives, and as capacity increases the permanent magnet design becomes much less implemented. At a certain point the novelty goes away.

The smaller DC units are pretty much all designed for use with a battery bank in mind, so the drives use multiples of 12 volts. Controls are built for mobile ac or refrigerating duties. The PV residential twist exists, but in general all of these smallish DC units are super uncommon (residential heat pump or mobile commercial refrigeration plus DC power equals single digits parts per million products offered). Not converting source power from DC to AC back to DC has a definite efficiency advantage, but the 120/220 VAC controls are way more common in general.

As to the nature of the units you're looking into, read this article:
https://coolautomation.com/wiki/vrv-or-vrf/
There are more topics in the blog section that will give you lots of quick tidbits to wrap your head around.

Recently the manufacturers have been making 2 stage compressors for the top efficiency outdoor units with variable capacity. They're designed for Sub-Zero temps outside and btu maintenance inside and/or heavenly performance ratings during less demanding conditions. Nearly all of the units are able to run or bypass the second stage of compression on the fly. They can be reduced to 10 percent of the rated output and increased to around 150 percent, so they're very versatile. Naturally these units are the opposite of cheap.


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Old 08-01-18, 04:05 PM   #12
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Quote:
Originally Posted by jeff5may View Post
I'm not sure I can agree with the full load efficiency listed for variable speed compressors.

..sure, a 30+ SEER 9k or 12k unit might be at 18 EER(or a little lower depending on this unit) under high load, but this really isn't bad at all because you are still kicking the crud out of 13 SEER 11 EER units.

In some cases the lower efficiency sub-20 SEER variable speed units, especially larger units do seem to be as bad at high load at cheaper 13 SEER units but that's only at peak loads which isn't an all the time scenario and isn't worse than a common old fashioned single stage split unit.

Moral of the story but a high EER rated inverter mini-split if you are planning to run at high load and that's not an issue and you get the best efficiency in any condition. It seems that the outdoor condensor coils on the smallest units often have the same coil on them between the 9k and 12k sizes and sometimes even have the same coil as up to an 18k size, in which case getting the 18k size might be a compromise but the efficiency becomes clear looking at the EER number. ..of course aiming for a high SEER will give you the best information for cooler conditions and lower load but I wouldn't oversize a unit based on the chart above if buying a good top efficiency unit.

I'm still not sure how you'd arrange getting the compressor to ramp load with the excess solar generation though, that's the trickiest part.
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Old 08-01-18, 07:54 PM   #13
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Jeff, i've never seen anything but VS (inverter) or 2 stage/step in a <5 ton standard split system. the only application i'm readily familiar with digital scrolls in is CRACs. I don't see those as a good fit for purely running on solar - they modulate the scroll on/off full load, so if it's not actually grid-tied, it might be a problem. otherwise average draw should work for % modulation.
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Old 08-02-18, 11:23 PM   #14
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I'm just giving the op a survey of what's out there as far as variable capacity pots. I agree with you guys, most of the sub-heavenly priced stuff in the mini and whole home residential range is going to be brushless permanent magnet stuff.

I'm still kinda intrigued about the whole supply matching strategy here. As of now, the proposal seems like a leveraged heat dump to me. The thing that baffles me is how much extra heat can be dumped into the floor? At a certain point, enough should be enough. Then what?
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Old 08-03-18, 02:35 PM   #15
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what MIGHT work and be simple (haha):
get one of those Bosch BOVA 3T or 5T ODUs, then put a coax/brazed plate exchanger indoors. they modulate from 25-110% based on load calculated by the ODU. all you'd have to do is control the water flow rate through the indoor hx to get the ODU to modulate up/down.

i've seen the 3T (0.75-3.6 ton range) available online for under $2k USD.
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Old 08-18-18, 12:42 AM   #16
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Hi All, Been reading through this over the years and figured I should join. I have seen recently a few products offered which may be of interest. There is a solar powered minisplit made by hot spot energy and there is a solar powered hydronic water to water heat pump system available as well. Scott Hunt has several videos of these systems in operation from his practical preppers youtube channel and of course on his website he sells the systems.
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Old 01-14-19, 09:20 AM   #17
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It must be so helpful to follow up. These techniques would have worked with the heat pump.

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