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