Automotive Air Conditioning Information Forum (Archives)

4Springs on Sat March 29, 2014 8:14 PM

Year: 1992
Make: Subaru
Model: Brumby
Country of Origin: Australia

Hi guys,
I have converted a Subaru Brumby (Brat) to electric. Nothing in the engine bay now makes much heat, so I need to find another way of heating the cabin. The climate here means that heating is essential and cooling is a nice-to-have.
Some folks replace the heater core with a resistive element run off the traction pack voltage (150VDC), but I'd like to investigate using an automotive airconditioner to provide heat, as this should be more efficient.

I have access to a friendly automotive air-conditioning workshop who are willing to make something up for me if I tell them what I want. So I've been doing some internet research to see if this is even possible in a car. I have no air-conditioning experience myself.

My Brumby didn't come with aircon, but I have an evaporator section which looks like it should fit. I would be able to mount a normal compressor using a shaft from the motor. The motor RPM is up to 5500, so I think this should be fine.

I have two options.
1. Run aircon in reverse as a heater.
2. Run a reversible system which will work as heater or cooler.

According to my research, in theory option #2 needs a reversing valve, two TXVs (with check valves), two receiver/driers (or one bi-directional one) and an accumulator. It also needs a compressor and two coils. There is a good diagram and explanation at this website: Heatpumps

This sounds a bit complicated, so I'd like to go with option 1.
I have my Brumby "evaporator" (indoor coil) which I'd like to use as a condenser. It has a TXV on it, so I think all I'd need to do is to remove that, then it becomes a condenser.
I'll need to buy an evaporator to put in the engine bay. There is no obvious windy spot under there, so I'll need to attach a fan or two to get some airflow. Presumably any evaporator (or even condenser) would do, so I can choose one from the catalogues that meets my size requirements. I'll need a TXV for this evaporator.
So I've read some things that suggest automotive aircon evaporators/TXVs are normally set so that they don't go below freezing. This makes sense in normal use, since we don't want them to ice up. But in my case the outside temperature does go below freezing sometimes, and I will especially want heating when this is the case! So can I set the evaporator/TXV to work at a lower temperature?
If I do this, is it going to ice up straight away? Probably the only way to tell is to try it.
If it does ice up then option 2 would look like a better bet. I could then run it in reverse to heat up the outside coil and melt any ice.

Any thoughts on this? Has anyone done this (e.g for aircooled motors, or for campervans)? There are probably heaps of practical considerations that I don't know about...

mk378 on Sun March 30, 2014 12:26 AM

A dedicated electric motor-compressor makes sense in an EV. Then you can always run the compressor at the best rpm regardless of road speed. This is especially important in city driving since an electric car isn't intended to "idle" when stopped in traffic.

It's a common misconception that a TXV is going to regulate the evaporator to a particular absolute temperature. It will get as cold as it can based on other conditions in the system. There is always another control to prevent ice-up.

4Springs on Sun March 30, 2014 2:51 AM

Quote
Originally posted by: mk378
A dedicated electric motor-compressor makes sense in an EV. Then you can always run the compressor at the best rpm regardless of road speed. This is especially important in city driving since an electric car isn't intended to "idle" when stopped in traffic.Yes, there is an integrated motor/compressor for sale which would (probably) run off my traction pack voltage. It is a very neat little package and I could locate it anywhere I wanted under the bonnet. The first disadvantage is that it costs $1700, whereas a "normal" ac compressor would cost about $300 new. The other thing is a bit more complicated to explain:
I'd like to have the compressor hooked up so that I have three modes selected by a switch on the dash - "off", "on" and "brake". The on mode would be "full time" mode, it would run whenever the motor was spinning (and as you say it is not spinning when you are stopped, but I don't drive much in the city so I'm not too worried about that). The brake mode would mean that the compressor would be enabled when the foot was taken completely off the accelerator. This would give a little bit of engine braking, and would basically give me some free heating when going down hills. The car is a manual, and has no re-generative braking. This system (with factory aircon) has been implemented by another electric vehicle builder who dubbed it "poor man's regen".

The motor normally runs quite quickly, it is most efficient to run it at its maximum rpm. So I'd say normally running above 4000 rpm.
Quote
Originally posted by: mk378
It's a common misconception that a TXV is going to regulate the evaporator to a particular absolute temperature. It will get as cold as it can based on other conditions in the system. There is always another control to prevent ice-up.Ah, good. I can see a small capillary on my evaporator - I assume this is the "too cold" control. It goes into a switch with a small screw on it. I'm assuming that this would normally be connected to electrically disable the compressor if the evaporator got too cold. The screw on the switch may be a temperature adjustment.
So any tips on how to avoid ice-up? I need to buy an evaporator - perhaps if I get a particularly big one? Would I be right in thinking that a larger evaporator would get less cold, and so may be less likely to ice up?

Leggie on Mon March 31, 2014 1:05 AM

You will need to let it run into freezing if you want heat on a cold day. If you're going to run the coil just shy of freezing, the use is very limited.

In a house heat pump, the compressor and motor are together and much of the motor's heat is recovered. If you use an open-drive, you won't have use of motor heat. Heat pumps don't provide toasty hot air out the vent. If you reduce the flow to increase the air temperature, you reduce the efficiency as well as capacity. Increasing the temperature difference between the two points only reduces efficiency.

Car A/Cs are very inefficient and COP is often below 1. I wouldn't bother with a heat pump idea unless you're certain you can get COP during heat mode to be well over 2.0 and don't expect hot air out the vents. It will be 10-15F hotter than interior temperature.

Edited: Mon March 31, 2014 at 1:10 AM by Leggie

4Springs on Mon March 31, 2014 5:21 AM

Quote
Originally posted by: Leggie
Car A/Cs are very inefficient and COP is often below 1. I wouldn't bother with a heat pump idea unless you're certain you can get COP during heat mode to be well over 2.0 and don't expect hot air out the vents. It will be 10-15F hotter than interior temperature.Are the inefficiencies to do with the compressor or other parts of the system? If it is mostly the compressor then this would sway me more towards the expensive electric drive version.

mk378 on Mon March 31, 2014 11:01 AM

Auto systems have to use tiny heat exchangers because of size limitations. In a conventional cool only system, the condenser is always the big limitation. Look at the size of the condenser in a car vs. the one on a 24000 btu central unit.

The fundamental laws of thermodynamics dicatate that heat pump efficiency drops rapidly when pushing to higher hot side temperatures. Thus in a home, there is slightly warm air at high flow.

Leggie has a good point that heat recovery from the compressor motor is essential to ensure that COP stays above 1.0, otherwise resistance heating is better.

Edited: Mon March 31, 2014 at 11:04 AM by mk378

Leggie on Wed April 09, 2014 2:39 PM

The low COP is primarily due to very large temperature differential and limited surface area of heat exchangers. It is the kW of thermal absorption vs kW of shaft input.

One thing about resistive heating element is that the output is not related to temperature. You can get the air out the temp as hot as 150C if you slow the airflow enough or only 1C above ambient if there airflow is very high.

4Springs on Thu April 10, 2014 3:42 AM

Thanks people for your musings on this topic.
If I am reading you correctly then I should be able to get this to work, but it may not be as efficient as resistive heating, depending on the ambient conditions and the efficiency of the system as a whole.
To give it the best chance of working I could do the following:

1 Use the electric compressor here: Link to Compressor I could site this quite close to the condenser (compressor in engine bay with a very short line to the condenser inside cabin).
2 Choose a fairly large evaporator
3 Give the evaporator lots of airflow using a good 12V fan
4 Direct airflow from the (air-cooled) car motor onto the evaporator. Once the car motor warms up a bit that should help (motor gets to 70 degrees C (hot to the touch) on a warm day, probably 40 degrees C (blood temperature) on a cold day).
From what was said above, I'm thinking that using a standard automotive compressor is probably not going to be efficient enough. Plus it would have to be situated at the front of the engine bay, quite a distance from the cabin. So I would have to lag the tubes quite well to stop heat loss.
There compressor efficiency tables in the linked document here: Link for Specifications. If I am reading these correctly I think that this compressor should be good enough? I would use the 150V version. It looks like the attached controller can be told to run the motor at different speeds. So I could either just always run it at the fastest speed, or I could design some electronics to vary the speed based on condenser/evaporator temperatures.
My ambient temperatures will range from -2 to 15 degreees C (28 - 60 F).
Can anyone see any problems with the points I've made above?

wptski on Thu April 10, 2014 7:46 AM

A big project here! I assume that you are able to braze and have a inert gas source also? I was wondering on a project like this, how is the refrigerant charge calculated?

Dougflas on Fri April 11, 2014 1:32 PM

Here is somethig else to think about. During the heat mode, your system must have enough space for the refrigerant to be in the evaporator as this is now the "condenser" of a cooling system. If not emough space, then you will hve to adjust the charge from cooling mode to heat mode. This is exactly why there are so many improperly charged AC systems out in the HVAC field. My feelings?? Let the engineers design the systems and let us techs repair them.

4Springs on Fri April 11, 2014 6:42 PM

Quote
Originally posted by: Dougflas
Here is somethig else to think about. During the heat mode, your system must have enough space for the refrigerant to be in the evaporator as this is now the "condenser" of a cooling system. If not emough space, then you will hve to adjust the charge from cooling mode to heat mode. This is exactly why there are so many improperly charged AC systems out in the HVAC field. My feelings?? Let the engineers design the systems and let us techs repair them.
This system will only be in "heat mode". I will not be using this system for cooling, I think that would be too complicated for several reasons including the one that you point out.

Quote
Originally posted by: wptski
A big project here! I assume that you are able to braze and have a inert gas source also? I was wondering on a project like this, how is the refrigerant charge calculated?
I can't do anything! There is a local automotive A/C shop whose owner is willing to put together parts for me - he can do the brazing, charging etc. You have to have a license to work on A/C systems here. He said that he can install a system for me to my specifications.
As to the refrigerant charge - does this need to be calculated? I was assuming that refrigerant was put in until it was "full" in some way.

wptski on Fri April 11, 2014 9:57 PM

Quote
Originally posted by: 4Springs


I can't do anything! There is a local automotive A/C shop whose owner is willing to put together parts for me - he can do the brazing, charging etc. You have to have a license to work on A/C systems here. He said that he can install a system for me to my specifications.

As to the refrigerant charge - does this need to be calculated? I was assuming that refrigerant was put in until it was "full" in some way.
From your links, it's full of oil only.

Leggie on Wed April 16, 2014 2:43 AM

Remember the traditional engine coolant runs pretty close to 100C.

It's not a good idea. If it's 0C outside and say you get 50C at interior coil and you get dry 40C air out the vents. The air is dry as absolute humidity relates to 100% RH of outside air.

Dry air at 40C will feel very chilly to occupants.

4Springs on Thu April 24, 2014 6:48 AM

Quote
Originally posted by: Leggie
Remember the traditional engine coolant runs pretty close to 100C. Ah, I hadn't thought about that. The cabin is small (it's a ute), and so the heater used to warm it up in no time flat once the engine was warm.
Quote
Originally posted by: Leggie
If it's 0C outside and say you get 50C at interior coil and you get dry 40C air out the vents. The air is dry as absolute humidity relates to 100% RH of outside air.
Dry air at 40C will feel very chilly to occupants.
It would be dry if it came from outside, but in that scenario I'd be most likely re-circulating the air from inside the cabin. So (perhaps) 10C air being heated up with a 50C interior coil sounds a bit better. Still not as good as the original though.
It's not 0C very often, more likely 5 or 10. I've been experimenting with the air that comes from the (air-cooled) traction motor and it is noticeably warm even if the motor itself is only relatively cool (about 40C). There is plenty of room to put an evaporator in this airstream.

northernbeach on Fri April 25, 2014 2:02 AM

Thanks for such a nice article it really helps me to differentiate between which type of air conditioner to be used for better performance.

-------------------------
[url=http://www.northernbeachesair.com.au]gas heating[/url]
[url=http://www.northernbeachesair.com.au]ducted heaters[/url]