DC powered mini-split versus AC, efficiency worth it?

[Swing]

In recent years some DC mini-split airconditioners have come to the market. At least in China and US, here in Europe I would need to import them. And they might not comply to everything needed here, but let's leave that out of the discussion for now.

There are the hybrid types, and there are the ones that can only run on DC (usually 48v).
Focussing on the DC-only models, running in a DC only system (battery/solar etc)

Is it really worth it from an efficiency point of view to use DC mini-splits?
Because the efficiency numbers (SEER and COP) are typlically lower than a good brand mini-split (Mitsubishi / Daikin etc)

So it seems you end up with lower efficiency with the current available DC mini-splits, than if you would use the AC route.
Of course it would save on the need for an inverter.
But it seems like having some loss through the inverter, and then running a very efficient AC powered mini-split comes out more efficient.

Please note that I also look at using it for heating in Western European climates.
So heating when it is -5 or -10 degrees Celcius outside. I am sure they will work, but probably much less efficient than the AC powered minisplits I have been using.

 

[Danke]

SEER ratings, kind of like inverters, seem to be all over the place, and not consistent. So I don’t trust them that much.

But from real world experiences, almost everyone agrees that Mitsubishi is the way to go if you’re in a cold climate. I am currently in Minnesota (cold) and know people who have had good success with Mitsubishi. But they are expensive.

GREE, which is cheaper, also makes one for cold weather, their numbers are almost too good to be believable.

Not sure the cost to import to Europe, but you can get a AC/DC mini split very reasonably from China.

 

[hwy17]

People don't go to DC for efficiency. It's for other benefits; No inverter required, direct solar input, Thomas Edison fandom, etc.

Most people around here will choose and recommend to use a regular heat pump model with a whole home inverter.

 

[timselectric]

I'm using a 48v DC only mini-split. (HotSpot DC4812VRF) and plan to install at least two more.
I chose the DC option because I didn't want to have to increase inverter system size to accommodate the environmental control systems.
And if my inverter system failed (heaven forbid), I wouldn't be left without heating and cooling. While I repaired the system.
I have also included 48v DC lighting, for the same reason.
I like to build redundancy into all of my systems. I don't want a single failure to take everything down.

 

[k490]

I've seen some videos demonstrating couple of panels powering these DC heat pumps I'm guessing it works when it's 85-95F. When it gets to 115F-119F 2 panels won't be enough. I put a tiny 5000BTU window AC unit in a shed last year it only pulled 380 watts at 90F when it got to 119F it pulled 1000 watts of power. Like most heat pumps 22 seer is measured when it's 70F outside they are not working hard the compressor pressures are much lower in cooler weather.

I'm in Arizona our home has a 4-ton central heat pump when it gets between 17-25F in the winter which would be -8 to -4C it hardly works unless the sun is on the condenser coil this is a rooftop packaged unit. I usually wait till 8 30 am sun is melting the frost on the roof before I turn it on. It will blow cold air for 20-30 minutes before start to feel any heat. It's annoying having that blast of cold air on you until it warms up. Then they reverse and go into AC mode to defrost the coil the outside coil. Here in Arizona winter humidity can be in the lower 10-12% range it never really freezes, but if you're in a humid climate you would need those heating elements installed you're just heating the air with direct coils pull tremendous amount of power. If you run them 24/7 it does stay warm enough to avoid that blast of cold air when it goes into defrost.

My neighbors use their 24/7 can hear its reversing valve all night pop open all that refrigerant pressure some of them make a whistling noise at least guy behind me his old heat pump did. Last few years haven't had freezing temps maybe a few days around 0C in January. We use our fireplace, and an electric blanket since I had the windows replaced doesn't' get as cold anymore computers running keep the house warm.

 

[RCinFLA]

Mini-splits use a three-phase inverter to drive a three-phase variable speed compressor motor. The inverter runs on high voltage DC.

The configuration is very similar to a HF hybrid inverter which has a high voltage DC power bus that all power flows through, with exception of AC input to AC output direct pass-through. On HF hybrid inverters, battery power is boosted to the HV DC bus. PV input is boosted to HV DC bus. AC output is generated by PWM chopping the HV DC.

Feeding a mini-split with PV power is similar to PV feeding the HV DC bus in a HF hybrid inverter. The solar powered mini-splits likely use the same PV module HV DC output SCC controller boards used in HF hybrid inverters. It just runs in parallel with HV DC bus powered from AC input to supplement power to HV DC bus from solar HV DC SCC controller.

The process of rectifying AC input to HV DC supply for mini-split inverter is different depending on mini-split size and quality. Cheap, lower btu mini-splits just have simple rectifier-filter capacitor to create the HV DC. This causes the AC load current to be high current pulses just at the AC input sinewave voltage peaks. Battery powered inverters and generators do not like these high peak current pulse loads.

Europe has regulations on appliance power factor, current crest factor, and load harmonic content. Most of the cheap mini-splits sold in U.S. will not meet European regulations for minimum power factor. The smaller btu, cheap mini-splits have a power factor between 0.5 and 0.65.

Next step at improving AC power factor on mini-splits is to use a heavy, low frequency filter choke between AC rectifiers and filter capacitors. This reduces the filter capacitors peak charging currents to about half the peak current and improves power factor to about 0.8.

Mini-split units like Mitsubishi have electronic power factor correction in their AC to HV DC conversion power supply, similar to circuit used in power factor corrected computer power supplies. Mitsubishi mini splits have an AC load power factor in the 0.97 range with their AC load current being a near perfect sinewave profile.

 

[Plum Crazy Rob]

If strictly AC-power efficiency is your game, SEER2 sure knocked some down. It's all Mitsubishi and Fujitsu at the top (Ruud-Rheem = Fujitsu)

Product Finder — ENERGY STAR Certified Heat Pumps (Mini-Split)

Compare ENERGY STAR Certified Heat Pumps (Mini-Split), find rebates, and learn more.

www.energystar.gov

 

[Partimewages]

@RCinFLA
Will you find this information in the specifications sheet for each unit?
If it's not there can you assume it has a poor power factor?

 

[RCinFLA]

@RCinFLA
Will you find this information in the specifications sheet for each unit?
If it's not there can you assume it has a poor power factor?

Pretty much. If they are spending the extra manufacturing cost for power factor correction, they usually want to advertise it.

Unless it's a Mitsubishi, I have not seen a variable speed compressor air conditioner in U.S. with anything more than a low frequency choke for power factor improvement. They pretty much have to do at least the choke addition on larger btu variable speed units as the peak to rms current is so great without the choke it often pops AC breakers short circuit protection.

You will more often find power factor correction on 240vac units which may be a common SKU sold in E.U. where they have regulations for minimum power factor. Limit on A/C size for 120vac supply is about 15kbtu as they will draw close to rms current of 15 amps of a normal residential AC plug/outlet.

 

[Swing]

Thanks for all the replies. Up until now I have been working with mid to high level split units, just AC powered, from Daikin / Mitsubishi / Panasonic to get a very efficient and quite aircon for heating and a bit of cooling. So I pick the ones that are reasonably efficient for heating.

But I want to do more with DC power, and now there is a project for a holiday home that I want to try and do with DC only. And just the tiniest grid connection there is available for backup.
So yes, the main advantage is not running an inverter all the time.
Efficiency doesn't seem to be a big gain here. Theoretically it could be, but the 48v is quite low. And more importantly, the available DC aircons are of lower quality than a good Mitsubishi or Panasonic or whatever.

@RCinFLA
is 300v a typical DC voltage internally? I saw a 300v DC bus in some technical info of a heat pump once, but I don´t know what is common. It would explain the hybrid AC/DC aircon that I saw which has a solar DC input from 80 to 280v to only be boosted to 300v or something like that.

At some point I would like to dive more into DC systems. There is a whole thing going on tending towards DC bus systems of 350v and 700v. I personally think the 350v level would be great for heatpumps (and thus aircons) and would truly make it more efficient.
For now, powering it with 48v, for it to be boosted to higher DC voltage, doesn´t provide more efficiency it seems.
But it is very easy and safe.

Though not having an inverter running all the time greatly reduces the standby power of the whole system.

I think for now my main worry is if these 48v aircons are any good at heating, and if they are somewhat quite.
I do play with the idea of adding electric underfloor heating, which can also be done at 48v, but I think it becomes to costly.

Water heating could theoretically also be done with a DC heatpump, but for now I´m thinking of just 48v heating and a shower heat recovery system.

The home will be very suitable for holiday use with cheap PV power in the right time (mostly spring and summer), but it still needs to work in darker colder days

 

[RCinFLA]

@RCinFLA
is 300v a typical DC voltage internally? I saw a 300v DC bus in some technical info of a heat pump once, but I don´t know what is common. It would explain the hybrid AC/DC aircon that I saw which has a solar DC input from 80 to 280v to only be boosted to 300v or something like that.

A typical unit with just rectified AC mains has a high voltage DC bus voltage a little less than the AC voltage sinewave peak with some decay ripple from HV DC filter capacitors between full wave rectified peaks. For 240vac which has a sinewave peak of 1.414 x 240vac, that is a little less than 340 vdc. For a 120 vac unit (less than 12 kBTU unit) the DC bus is a little less than 1.414 x 120vac = 170 vdc.

Electronic pre-boost DC-DC power factor correction circuit boost the HV DC a bit greater. For a 240vac AC input, the PF correction module DC output is typically around 360-400 vdc.

If you eliminate the AC to DC power supply, you may find there is some safety control interfaces that you must also defeat to keep the microcontroller from shutting the whole unit down due to lack of AC to DC power supply control connections.

You also have to be aware of any circuits that might use the AC mains for a subcircuit supply, like a small 240vac to 24vac control transformer to provide power to house thermostat, or internal power relay coils, or heat/cooling compressor reversing valve control coil.

The devil is in the details. I would not mess with an expensive Mitsubishi mini-split without a full troubleshooting manual including a complete schematic to be sure you know what you are dealing with.

 

[Swing]

@RCinFLA

For now the goal is to use "off the shelf" 48v DC mini-splits. I say "off the shelf" because I need to import them, I haven't seen them in Europe.

The 350v route is something that needs more of my attention, studying new NPR 9090 standard, that describes how to setup that up. What wiring to use and what kind of protections. Basically, everything to setup a DC micro-grid.

When I go that route, I would take the time to modify an air-to-water heat pump to input the 300 or 350v DC, most likely via a DC-DC converter anyway. And I would like to do that with some technical support from the manufacturer.

For now, going the 48v route, the voltage probably gets boosted to the 300 to 400v region.
So, the efficiency of the DC voltage boosting might actually be lower than the AC rectifiers (as you have showed them above)
Do you have any effiency numbers on an AC rectifier part setup for a high power factor? (higher than 0.9)
It seems that these typically are very efficient with numbers above 95%?

And as mentioned, the 48v DC minisplit models are of lower quality/efficiency than a good AC powered mini-split.

 

[Swing]

I'm using a 48v DC only mini-split. (HotSpot DC4812VRF) and plan to install at least two more.

For that one at least there is some data
Indoor noise is 26db at the lowest. Which isn't very quite compared to 21db of Panasonic/Mitsubishi etc
It says very little about heating. It mentions quite a low COP and no working temperatures are mentioned.

I chose the DC option because I didn't want to have to increase inverter system size to accommodate the environmental control systems.
And if my inverter system failed (heaven forbid), I wouldn't be left without heating and cooling. While I repaired the system.
I have also included 48v DC lighting, for the same reason.
I like to build redundancy into all of my systems. I don't want a single failure to take everything down.

That is kind of the school of thought for this project as well, hopefully completely skipping the inverter.
That actually depends most on the kitchen equipment.
It is a small holiday home with all the technology in one side, which is convenient for 48v cabling.

Do you have some links to what you have been doing with 48v lightning?
It is not so hard, but I would like to try and find some options for E27 bulbs that give of 2700K or lower.

 

[timselectric]

For that one at least there is some data
Indoor noise is 26db at the lowest. Which isn't very quite compared to 21db of Panasonic/Mitsubishi etc
It says very little about heating. It mentions quite a low COP and no working temperatures are mentioned.

It's very quiet. I don't know if it is running unless I walk over to it and feel the air coming from it. I do slightly hear it in turbo mode. But I only use that when I first come in and want to warn up or cool myself quickly.
So far it has cooled and warmed my space without any problems. We have only seen a couple of days below freezing, so far this year. So, I can say how well the heating works at very cold temperatures, yet. It definitely had no problem cooling in above 100 F temperatures, last summer.

 

[timselectric]

Do you have some links to what you have been doing with 48v lightning?

Nothing in particular.
All led , and I prefer 5000k for a bright light. (I'm old)
I have both 48vdc and 12vdc available. Which gives me more decorative options.

 

[Swing]

Nothing in particular.
All led , and I prefer 5000k for a bright light. (I'm old)
I have both 48vdc and 12vdc available. Which gives me more decorative options.

Yes it makes sense to have a 12 or 24v circuit for lighting, to have more options.

But, these retro transparent filament LED bulbs use these LED sticks which are all 50v as far as I know.
I think it would be great to have these bulbs but instead of 4 of these 50v sticks in series (for 230v AC) just parallel?
I haven't seen something like that for sale though.

 

[timselectric]

Yes it makes sense to have a 12 or 24v circuit for lighting, to have more options.

But, these retro transparent filament LED bulbs use these LED sticks which are all 50v as far as I know.
I think it would be great to have these bulbs but instead of 4 of these 50v sticks in series (for 230v AC) just parallel?
I haven't seen something like that for sale though.

Most of what I am looking at is RV lighting.
There are some 48v options. But much more available for 12v.

 

[Plum Crazy Rob]

For that one at least there is some data
Indoor noise is 26db at the lowest. Which isn't very quite compared to 21db of Panasonic/Mitsubishi etc

Mini Splits are very quiet, but those numbers are unquestionably deflated and/or at lowest operating output. I'd expect 40-50db are more realistic as an owner of several...

 

[Swing]

I have experience with mini splits and they can be quiet (inside unit) when at lower speeds.

And there are big differences. 40 or 50db is very loud and unacceptable for longer periods.

Values of 20 to 28db are quite common for lower speeds / quiet mode.

For the Hotspot it says 26db for other Chinese 48v DC minisplits online I see 32 db which is quite a difference

 

[timselectric]

I don't hear mine.
I only know that it's working because the room temperature is where it should be. And I sit 15' from it.