# Reverse cycle air con for EV - Efficient climate control



## cts_casemod (Aug 23, 2012)

Hi all,

Some of you are probably know me from the Polo conversion.

So I am opening a new topic for a climate system designed for our cars, as I think this is another big lack on the EV World. I actually have no idea how much power I need (maybe 1KW to mantain and 2KW for quick heating at 0C ambient?) so I would like to start by asking members with an EV how much power you use for heating (Element power and how long is it on) to calculate my required BTU capacity.

My idea is to use the Aircon as a reverse cycle so that I can have either cold or hot air. Reasons are simple. Efficiency. for a 2KW heater I will use about 800-900W from the batteries.

As far as my calculations go the Polo should have a stock 6-9000BTU Air con. I can either use the alternator as a motor or use a 1HP hermetic electric compressor, controlled with an AC Inverter.

I havent found much information about the prius AC system.


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## subcooledheatpump (Mar 5, 2012)

You have an advantage in using an industrial voltage, so you can use inverters to drive standard compressors. Is your original system still intact, and what refrigerant does it use? does it use R-134a (tetrafluroethane) or R1234yf? I'm thinking it used R-134a, and probably used PAG oil. In that case, I would make sure whatever compressor you get either has the same oil type or you change it to whatever was in your system to begin with. Two different types of oil won't take well to each other, and they can plug up your lines. 


Things to consider, "high temperature" compressors made for R-134a have a greater flow (displacement) than other "high temperature" compressors, they do not tolerate high pressures as well though, and might not be the best choice for an automotive application. If you are going for a standard compressor, try getting a "low temperature" rated compressor, or just get a "made for EV" compressor" For a heat pump in a car, you would likely need a larger pressure drop than normal, since you want the evaporator (outside) to have a low temperature, and the condenser (inside) to have a high temperature in heating mode. R-134a is a high temperature refrigerant, meaning it wasn't meant to move heat in very low temperatures, so it will not perform nearly as well for heating unless the outside temperature is above what the normal output temperature of the A/C is. Below 0C the performance of R-134a typically falls greatly, and the output diminishes to nearly nothing. R-134a has a boiling point of -26C, so at low temperatures of say, -6C, the compressor will have to work very hard to maintain a low pressure in the evaporator, and a high one in the condenser. At a relatively higher temperature, like 10 C, the performance will skyrocket as the pressures increase, and the difference in pressures can be relaxed. 

This can be expressed with an equation, COP=QH/(QH-QC)

COP being Coefficient of performance

QH being condenser temperature
QC being evaporator temperature

Just convert temperature from degrees Celcius to degrees kelvin and you'll get an accurite answer

Please let us know more information and keep us informed on your project

Also; I use 5 kW for heating my van, probably not too helpful since your vehicle is much smaller, but it does heat pretty quickly. I only use it when plugged in. More heating power=good. You'll heat up faster and hotter with more heating power, especially when plugged in. When driving, 1kW should maintain and not drain the batteries too heavily


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## Roy Von Rogers (Mar 21, 2009)

Not to mention the metering devices needed to heat and cool.

Riy


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## cts_casemod (Aug 23, 2012)

subcooledheatpump said:


> You have an advantage in using an industrial voltage, so you can use inverters to drive standard compressors. Is your original system still intact, and what refrigerant does it use? does it use R-134a (tetrafluroethane) or R1234yf? I'm thinking it used R-134a, and probably used PAG oil. In that case, I would make sure whatever compressor you get either has the same oil type or you change it to whatever was in your system to begin with. Two different types of oil won't take well to each other, and they can plug up your lines.
> 
> 
> Things to consider, "high temperature" compressors made for R-134a have a greater flow (displacement) than other "high temperature" compressors, they do not tolerate high pressures as well though, and might not be the best choice for an automotive application. If you are going for a standard compressor, try getting a "low temperature" rated compressor, or just get a "made for EV" compressor" For a heat pump in a car, you would likely need a larger pressure drop than normal, since you want the evaporator (outside) to have a low temperature, and the condenser (inside) to have a high temperature in heating mode. R-134a is a high temperature refrigerant, meaning it wasn't meant to move heat in very low temperatures, so it will not perform nearly as well for heating unless the outside temperature is above what the normal output temperature of the A/C is. Below 0C the performance of R-134a typically falls greatly, and the output diminishes to nearly nothing. R-134a has a boiling point of -26C, so at low temperatures of say, -6C, the compressor will have to work very hard to maintain a low pressure in the evaporator, and a high one in the condenser. At a relatively higher temperature, like 10 C, the performance will skyrocket as the pressures increase, and the difference in pressures can be relaxed.
> ...


Thats the beauty of running a >300V DC system. Parts are cheap and readily available for either 230 or 415VAC and unlike PMDC or DC motors I can have full control of torque characteristics with an induction motor.

Your results make sense, on a van. Thats a quite large area. I would never consider a resistor element to heat such a large area even if the draw is smaller when driving, but you could use a water heater that you can pre-heat at home. Even if you have to go to several places on the same day it would keep some of the water warm for a few hours after it was plugged of.

My car uses R134a HFC. R134a like you say is not the best to heat in winter because I need the condenser to be at -20 or below. The current TXV valve is set to 0C, so I will need to replace it with suitable ones on both the evaporator and condenser.

I am planing to use R22 or equivalent, so I can go down to -40. I will need to use a different compressor. I may be able to use an R410 compressor with a VFD (increased speed, reduced voltage). So say a 12.000BTU R410 compressor could be run at 100Hz 230V for peak power (12KBTU) or 50Hz 110V for 6000BTU and I could go as low as 30Hz at 80V for the low speed setting.

For those that cant see the logic, R410 is double the pressure of R22 so I would run the compressor at double speed for 100% load or half voltage for 50% load, making it universal.

If I use a R134a compressor from a car I will have problems because they are variable displacement so they keep the pressure constant regardless of the motor speed. This would give me excessive head temperatures. Same applies for fixed speed compressors running from the mains.

A few years ago I did tests using R22 blended with R134 on older R12 systems. The R22 would carry the mineral oil so no modifications were required. It would also help to cool down as R134 is not as efficient as R12 on the smaller condensers.

I did tests using only a few grams of 100% R22. There was an increased load on the compressor over 30C Ambient due to high head pressures, on the other hand the evaporator temperatures were -20C, really cold. On winter the low evaporator temperatures defrosted the windows in seconds. The prototype run for 2 years until I scrapped the car without any issues. The condenser was however improved, which helped increase efficiency and reduce head pressures.

Cost was essentially zero as I could fill a car AC with 100-150g of refrigerant, but any leaks would be a huge problem (I had none)


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## subcooledheatpump (Mar 5, 2012)

I like the idea of using R-410a, but only with an inverter. Like you said, with fixed displacement just variable compressor speed. That could help simplify the system and allow pressure and flow control. 

Assuming you had a properly rated R410a compressor with PAG oil, then in this case the only thing I would be concerned with is the pressures that the existing condenser and evaporator would be subject to, even without the compressor running. Like on a hot day the R410a pressure can increase by quite a bit. Also maybe freezing of the evaporator, that could be defrosted by reversing the cycle


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## cts_casemod (Aug 23, 2012)

subcooledheatpump said:


> I like the idea of using R-410a, but only with an inverter. Like you said, with fixed displacement just variable compressor speed. That could help simplify the system and allow pressure and flow control.
> 
> Assuming you had a properly rated R410a compressor with PAG oil, then in this case the only thing I would be concerned with is the pressures that the existing condenser and evaporator would be subject to, even without the compressor running. Like on a hot day the R410a pressure can increase by quite a bit. Also maybe freezing of the evaporator, that could be defrosted by reversing the cycle


 
I only use the R410 compressor, not the R410 Freon. The rubber o-rings that seal parts in the air con would not hold the pressure (Can go to 100Bar)


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## subcooledheatpump (Mar 5, 2012)

Darn, I was afraid of that. I was thinking of using it myself for a similar project for a heatpump in my van. I'll have to check back on your results and see if I want to pursue the same


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## cts_casemod (Aug 23, 2012)

subcooledheatpump said:


> Darn, I was afraid of that. I was thinking of using it myself for a similar project for a heatpump in my van. I'll have to check back on your results and see if I want to pursue the same


 
To start I am going to use a standard portable AirCon I got on ebay for £30. It comes much cheaper that getting a compressor. I believe is actually R404. I will place a duct to for the cold air to escape.

Most of this units dont support condensing temperatures much lower than 0C so I will have to see. Its hard to plan without nothing on hand. Most use a resistive element for heating as standard to save on a few components.

I also want to use the original condenser, the one on the car is quite small/inefficient and the connections are alluminium. I rather use the  copper pipping I can actually weld something to.


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## cts_casemod (Aug 23, 2012)

I am getting really **** right now. Customs clearance complete on monday (called them gave them all they need) parcel force now is holding the package and I have no car yet... 

Well back to the air con issue...

I got the unit - 1.5KW/12BTU.
Capilary tube expansion device, single phase compressor (Cant change speed and have to wait 3 minutes for a hot restart)

Done some tests with the unit, but the lowest I could get was 12C, inside the house. Outside the temperature was 5C.

Anyway... 
Both the evaporator and condenser were being feed with air at 12C during the test.

After 5 minutes operation 
Evaporator outlet temperature was 5C
Condenser outlet temperature was 35C

The output temperature was over 20C higher than the inlet. Obviously if i was using recirculation, instead of feeding the condenser with 12C air the temperature would rize much faster on the small area of a car.

Now I wonder how would the system operate at -5C.
I am going to play with it this weekend just to see how quick it can heat the car. I think its a bit oversized, given that I am not able to change the speed of the compressor.

Power consumption was 1100Watts, including both Fans.


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