DC Air Conditioner (minisplit/rooftop/anything)

[Freqz]

How would you pulse AC? I don't see how it would be possible. It could be "clipped" but the timing would have to be in phase.

PWM DC would be much easier.

you’d turn it on, turn it off— just like DC. It’d be prettier/cleaner if you did it at a zero crossing, sure.

Id argue that pulsed DC has an AC component since it has desired frequency components outside of 0Hz but I’d also argue it’s not really important...what’s important is saving the conversion from DC to 60Hz mains AC and picking up a couple of percent overall efficiency when you already have gobs of available DC anyway.

 

[eXodus]

How would you pulse AC? I don't see how it would be possible. It could be "clipped" but the timing would have to be in phase.

PWM DC would be much easier.

regular DC Motors usually have not as much torque as 3 phase AC Motors. That it something you need with a compressor. The Compressor has to overcome a valve or some sort of metering device.

For Fan - yes there is nothing which is really stopping a fan while ramping up. So a fan motor works perfectly with PWM DC.

Almost all electric cars have 3 phase AC motors for that reason. You can go PWM DC, but usually you need then a gearbox to get the motor into higher RPM for them to have sufficient torque.

I don't know if we are using the correct terminology here - maybe that 3 conductor "AC" motors are truly "pulsed DC" or however it's correctly called.

2 DC conductor go into a controller(inverter) - and 3 conductors go to motor. I don't know what exactly the power looks on those 3.

 

[Supervstech]

AC is a frequency wave of forward and backward energy...

DC is ... direct negative to positive flow energy.

It is pulsed to control RPM.

AC is voltage adjusted, or winding varied to adjust RPM...
POORLY.

 

[Zwy]

regular DC Motors usually have not as much torque as 3 phase AC Motors. That it something you need with a compressor. The Compressor has to overcome a valve or some sort of metering device.

No, it doesn't. Expansion valves meter the refrigerant and will lessen flow if pressure on the low side increases.

I'm also seeing a move towards variable displacement compressors on the motor vehicle side. This allows a reduced load on any motor driven compressor, zero stroke at initial ramp in speed. Easily accomplished with swash plate compressors and from what I'm seeing with the DC air conditioners is they might be using a swash plate compressor and I'll wager it is variable displacement.

2 DC conductor go into a controller(inverter) - and 3 conductors go to motor. I don't know what exactly the power looks on those 3.

Are you sure it's not VVVF ( Variable Voltage Variable Frequency)?

 

[Supervstech]

Many system designs have electronic valve control also. The system can come up to needed rpm, and then actuate the valve slowly to achieve capacity load.

 

[eXodus]

No, it doesn't. Expansion valves meter the refrigerant and will lessen flow if pressure on the low side increases.

I'm also seeing a move towards variable displacement compressors on the motor vehicle side. This allows a reduced load on any motor driven compressor, zero stroke at initial ramp in speed. Easily accomplished with swash plate compressors and from what I'm seeing with the DC air conditioners is they might be using a swash plate compressor and I'll wager it is variable displacement.

Are you sure it's not VVVF ( Variable Voltage Variable Frequency)?

So most A/C diagrams I studied the high pressure side is separated from the low pressure side by a expansion Valve.
Further there is a Valve at the discharge side of the compressor which keeps refrigerant from flowing back. (standard A/C compressor with Scroll technology)
Probably not all make and models are the same. But there is a lot of resistance the compressor to get running.
If you ever tried to turn the A/C compressor of car by hand - you know what I'm talking about - it's very hard to turn.
You need a 2 foot extension to turn that thing.

I know there are newer technologies on the refrigerant side. But very few standard A/C units for Vans or RVs are using them.

 

[Zwy]

So most A/C diagrams I studied the high pressure side is separated from the low pressure side by a expansion Valve.

There is always an expansion device present, it is the only way to change pressure of the refrigerant to allow change of state. Change of state is where heat energy can be transferred.

Further there is a Valve at the discharge side of the compressor which keeps refrigerant from flowing back. (standard A/C compressor with Scroll technology)
Probably not all make and models are the same. But there is a lot of resistance the compressor to get running.
If you ever tried to turn the A/C compressor of car by hand - you know what I'm talking about - it's very hard to turn.

You need a 2 foot extension to turn that thing.

No, I turn the hub by hand daily. I work on mobile HVAC daily. You must be referring to a locked up compressor?

I know there are newer technologies on the refrigerant side. But very few standard A/C units for Vans or RVs are using them.

Scroll compressors for residential HVAC were designed to get the SEER rating much higher, minimal resistance. The scroll compressor works by rotation of closely fitted scrolls where on is offset and runs in an elliptical to the other. In layman's terms, the scroll walks 2 pieces against each other and forces the refrigerant to move. I work on some occasionally, I usually find the internals ground to a fine powder due to the friction. The force required to turn on initial startup and running are similar as the friction of the 2 scrolls rubbing against each other is the major drag other than initial motor draw when turned on.

In the automotive side, recently the move has been towards variable displacement compressors with an adjustable swashplate to adjust the angle of the swash plate shown as number 5 in your above diagram. The reason you find the compressor initially harder to turn on a fixed displacement compressor is the angle of the swashplate is not at a low degree which would allow for lower initial force to start rotation. On a variable displacement, the angle can be close to zero which only requires turning the shaft and swashplate. Once those are spinning, the swashplate angle can be changed to allow the pistons to move. As the angle increases, the stroke becomes longer and greater displacement occurs.

Each as pros and cons. As high side pressure increases, both will require more energy to work against the higher pressure. Several advances in condenser design have lead to greater efficiencies of the energy needed to compress against the higher pressures and allow high heat transfer in a small space. Parallel flow multipass condensers are the norm now. There are some advances in subcooling also that can help lower high side head pressure and increase cooling capacity while using less refrigerant which leads to less friction on the liquid portion of the system.

 

[curiouscarbon]

grateful for the in depth discussion and delineation between traditional and scroll type.

@Zwy do you know of any DC scroll compressors?

i’m getting to the point of just simply learning how to safely braze safely pressure test safely charge safely purge a system and buying a DC scroll compressor due to their advantages vs compressor one might find in a cheap mini fridge.

it would be very tedious and involved

ideally i want a DC scroll compressor based water chiller with a simple analog input for power output. with precharged coolant lines to a decent sized condenser outside part. then chill water and cool air and stuff with that. ensuring to not hit dew point on cooled water side.

of course whatever firmware controlling the compressor should be programmed with appropriate safe slew rates and minimum activity as per the data sheet, if that’s still a thing with scroll type.

what i’m saying is, i want to chill water with a scroll compressor and pump it through tubes in my chair to directly cool me down ?

edit:something like this? https://www.guchen-eac.com/what/low-voltage-electric-compressor/ld20a24ab-24v-ac-compressor.html



only coefficient of performance of 2.2?

 

[Supervstech]

Lemme know if you guys want me to cut open any residential HVAC equipment, or compressors.
Happy to tear into parts, or explain issues.

 

[Zwy]

grateful for the in depth discussion and delineation between traditional and scroll type.

@Zwy do you know of any DC scroll compressors?

i’m getting to the point of just simply learning how to safely braze safely pressure test safely charge safely purge a system and buying a DC scroll compressor due to their advantages vs compressor one might find in a cheap mini fridge.

I agree, something designed for road vibration and works at various angles is what is needed.

it would be very tedious and involved

ideally i want a DC scroll compressor based water chiller with a simple analog input for power output. with precharged coolant lines to a decent sized condenser outside part. then chill water and cool air and stuff with that. ensuring to not hit dew point on cooled water side.

All you need is a temp sensor and the right programming.

of course whatever firmware controlling the compressor should be programmed with appropriate safe slew rates and minimum activity as per the data sheet, if that’s still a thing with scroll type.

As long as rpm can be changed, the overall displacement can be adjusted. A worn out scroll will displace enough refrigerant to allow cooling at high rpm but will fail cooling at low rpm.

what i’m saying is, i want to chill water with a scroll compressor and pump it through tubes in my chair to directly cool me down ?

edit:something like this? https://www.guchen-eac.com/what/low-voltage-electric-compressor/ld20a24ab-24v-ac-compressor.html

Yes, that one fits the need. DC brushless induction motor. Variable speed. Controller is installed right on the compressor making for a compact package. I like it. Only question is what inputs they have designed it for.

If this is a mobile application, you would most likely want to utilize hose. Hard lines will fatigue over time without some type of vibration isolation. Fittings on the compressor are peanut type and easily found.

You're making me want to quick buy one and start building myself, but I just don't have the time to do so.

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only coefficient of performance of 2.2?

One would think given it is a DC inverter controlled brushless motor it would have a COP greater than 2.2. However, what test conditions were used? Europe for example runs a different standard. Other things that play into it are diameter of piping, dry bulb temp, condensing capacity and expansion device setting. In this case, the only thing that would be constant is compressor displacement/per watt. I'm not sure how that could be easily computed for COP.

Air conditioners typically run a lower COP than a heat pump, many which are using thermally conductive heat transfer such as ground loops instead of air to air. Typical COP of air conditioning is 2 to 4. Improvements can be made to improve COP and considering this is just a compressor without the rest of the system, I'm not certain the COP given would be a true indication of efficiency.

 

[curiouscarbon]

Thank you @Zwy for your thoughts on this! ?

 

[eXodus]

Air conditioners typically run a lower COP than a heat pump, many which are using thermally conductive heat transfer such as ground loops instead of air to air. Typical COP of air conditioning is 2 to 4. Improvements can be made to improve COP and considering this is just a compressor without the rest of the system, I'm not certain the COP given would be a true indication of efficiency.

A regular one stage on-off A/C unit may have a good COP, but it takes a while after restarting for the pressures in the system to get up to operating conditions. And how often does normal A/C starts in an hour? 3-6 times?

Lot's of lost efficiency there. Not even talking about the inrush current.

I know that the systems I built - I could downsize the system size, when switching to a inverter unit. (went from 3 ton to a 2 ton in house)
So here is my thesis on this - human comfort is a weird thing - we feel cooler when air is moving - even when the temperature is higher - a regular A/C unit - is primary not running. (total cooling time of the year / run time) While a inverter unit is almost always running.
Further the inverter units are much better in dehumidifying the air - since they can adjust the coil temperatures. (Fan speed up, or down)

Most Systems are sized on the maximum load - which hardly ever happens (one month in the summer) - at this load it would run most efficient. That means the remainder of the year it's not running at it's fullest potential.

In a vehicle we are energy and roof space constraint.

Energy (inputs)
1. Reduce heat gain in the RV (ceramic tinting, awnings)
2. Get the most battery you can afford
3. Get the most efficient solar panel per sqft

Energy (outputs)
1. Use the stored energy most efficient - A/C unit which makes the most BTU out of the least amount of watts.

Thus, we should be avoiding conversion losses and use the most efficient refrigeration cycle available. Further reducing components and so making the system more reliable

My current setup:
DC Solar
DC Battery
AC Inverter
AC Window A/C unit (which internal - does:
AC (power supply)
DC (controller) - 3 phase AC (compressor) or however you want to call that pulsed DC


Moving to:
DC Solar
DC Battery
DC Controller - 3 phase AC (Compressor)

Looks like the simpler system setup

 

[eXodus]

12V 24V DC Powered Air Conditioner for Truck, Van, Special Vehicle

As a liable DC Powered A/C manufacturer, Guchen produce the DC powered A/C system for sales over 20 years, we also offer DC powered A/C parts and compressor for customers.

www.guchen.com

Found something more

Ecooler Electric Air Conditioner for Truck Cab-Saving More Fuel

Ecooler ac unit for truck cab is electric DC powered 12 or 24v, roof top mounted ac unit, apply on truck cab, tractor, excavator

www.guchen.com

 

[eXodus]

When you start searching for "24V Truck A/C" a bunch of stuff comes up.

Truck Sleeper Cab Air Conditioning-Roof Top Mounted|King Clima

E-Clima series truck rooftop air conditioning units are battery powered, 12v 24v, 1KW to 4KW cooling ac for all kinds of truck cabin solutions.

www.kingclima.com

Australia and Europe seem to have implemented far reaching no-idle laws and so the truckers are pushing for those DC units.

 

[Zwy]

When you start searching for "24V Truck A/C" a bunch of stuff comes up.

Truck Sleeper Cab Air Conditioning-Roof Top Mounted|King Clima

E-Clima series truck rooftop air conditioning units are battery powered, 12v 24v, 1KW to 4KW cooling ac for all kinds of truck cabin solutions.

www.kingclima.com

Australia and Europe seem to have implemented far reaching no-idle laws and so the truckers are pushing for those DC units.

I mentioned this is another thread and Europe is way ahead of the US implementing 24V ac systems. I'll make my move to one in the future, just how long that will be remains to be seen. I'm waiting for 2 things, one, the units become more widespread here in the US, however 24V trucks are not as common here, most are 12V. As adoption becomes more widespread, we will see prices start to possibly come down. Two, I want to see real live test results and how efficient these units really are. And how much efficiency improves over time.

 

[Zwy]

A regular one stage on-off A/C unit may have a good COP, but it takes a while after restarting for the pressures in the system to get up to operating conditions. And how often does normal A/C starts in an hour? 3-6 times?

Lot's of lost efficiency there. Not even talking about the inrush current.

I know that the systems I built - I could downsize the system size, when switching to a inverter unit. (went from 3 ton to a 2 ton in house)
So here is my thesis on this - human comfort is a weird thing - we feel cooler when air is moving - even when the temperature is higher - a regular A/C unit - is primary not running. (total cooling time of the year / run time) While a inverter unit is almost always running.
Further the inverter units are much better in dehumidifying the air - since they can adjust the coil temperatures. (Fan speed up, or down)

Most Systems are sized on the maximum load - which hardly ever happens (one month in the summer) - at this load it would run most efficient. That means the remainder of the year it's not running at it's fullest potential.

In a vehicle we are energy and roof space constraint.

Energy (inputs)
1. Reduce heat gain in the RV (ceramic tinting, awnings)
2. Get the most battery you can afford
3. Get the most efficient solar panel per sqft

Energy (outputs)
1. Use the stored energy most efficient - A/C unit which makes the most BTU out of the least amount of watts.

Thus, we should be avoiding conversion losses and use the most efficient refrigeration cycle available. Further reducing components and so making the system more reliable

My current setup:
DC Solar
DC Battery
AC Inverter
AC Window A/C unit (which internal - does:
AC (power supply)
DC (controller) - 3 phase AC (compressor) or however you want to call that pulsed DC


Moving to:
DC Solar
DC Battery
DC Controller - 3 phase AC (Compressor)

Looks like the simpler system setup

Around here, humidity is the biggest factor when it comes to getting comfortable. I used to make it to the desert mid summer now and then, then return back to Iowa and when I stepped back into the humidity here, you just about melt.

If you can remove the humidity, a 90F day is easy in the shade. I'm a firm believer in undersizing a system so that it runs longer in order to remove the humidity.

 

[eXodus]

Around here, humidity is the biggest factor when it comes to getting comfortable. I used to make it to the desert mid summer now and then, then return back to Iowa and when I stepped back into the humidity here, you just about melt.

If you can remove the humidity, a 90F day is easy in the shade. I'm a firm believer in undersizing a system so that it runs longer in order to remove the humidity.

another interesting challenging of norms.
All the sizing charts / calculations of A/C units had been created in a time when single stage was the norm. Are those charts outdated? Is oversizing or under sizing something different with variable speed? Sure there is some underlying physics - you need to move the heat from the inside to the outside.

Maybe the HVAC guys can chime in on that.

 

[eXodus]

I mentioned this is another thread and Europe is way ahead of the US implementing 24V ac systems. I'll make my move to one in the future, just how long that will be remains to be seen. I'm waiting for 2 things, one, the units become more widespread here in the US, however 24V trucks are not as common here, most are 12V. As adoption becomes more widespread, we will see prices start to possibly come down. Two, I want to see real live test results and how efficient these units really are. And how much efficiency improves over time.

really US-Trucks run on 12V ? I learned something new today.
I got told in Driving School in Germany that "all Trucks" run on 24V a few decades ago. Apparently that's all "Euro-Trucks"

For my purpose - I got an RV and I'm currently deciding on the battery system I'm going to use. Was debating between 24v and 48V

So far I found a bunch of 24V appliances (Microwave, Fridge), including a many different varieties of A/C units. Further the Inverters for 24V are cheaper per watt. But when I get only 12V/24V appliances, I don't even need a inverter anymore

 

[Q-Dog]

These sleeper cab A/Cs are likely too small for an RV, unless it is one of those tiny canned hams, and that might still be a problem. These units would likely struggle to cool down a hot truck cab, but their use case is to run once the truck parks so they don't have to idle the engine to keep the cab cool. In most cases the cab is already cooled down after driving for several hours.

 

[Partimewages]

From an electrical standpoint a 24 or 48 volt system would be much easier to implement in an existing RV. There is likely no way to use existing wire with the amps you would see in a 12 volt system. I suspect that even with 24 volts the microwave and a few others would need new wire. If your building/remodeling it's probably not a problem otherwise it's going to be a pain.