CO2 used as a heat pump: why?

[fueliefan]

I'm working on the development of vehicle that will most likely be required to use a CO2 HVAC system. In doing some research, i found quite a few references to the development of automotive CO2-based HVAC systems that can also run as a heat pump. My question is, why is this only being explored with CO2 systems? What is different between CO2 and R134a that makes one better than the other as a heat pump?

Be gentle, i'm not an HVAC guy by trade..

 

[ChrisConley]

CO2 is a 'natural' refrigerant that HVAC and other refrigeration manufacturer's are being pushed to consider. I'm told it is the push from enviromental groups:

Enviromentalist:
CFC - bad, get rid of them
HCFC - almost as bad, get rid of them
HFC - must be bad, get rid of them too

As far as benefits of CO2 over R134a... umm... cheaper refrigerant?

 

[KiwiMace]

According to a quick read of the below (googled) link:
CO2 has a better COP than R134a at low engine speeds, but decreases with speed and is significantly worse by the time you get to 3000 rpm or so.

The equipment for CO2 is 50% heavier.

http://www.sae.org/altrefrigerant/presentations/z-mathur.pdf

CO2 has a much lower global warming potential than R134a.
GWP of CO2 = 1
GWP of 134a = 1300
Vehicle refrigeration systems are inherently more leaky due to vibrations etc, so come under a little more scrutiny than a fixed system.

 

[gerhardl]

I believe CO2 at present to be in opening stage used as coolant, still some technical development to be done before sealings and long-life components are being perfected.

The only reason for use is environmental, as given

 

[fueliefan]

Yeah I understand the push behind its implementation, but what I didn't understand is why all the proposed systems I've been coming across are for loops that can be run either in HVAC mode or reversed and run as a heat pump, whereas I've never seen this functionality in a 134a system before. Any thoughts?

 

[KiwiMace]

R-134a is not particularly good in low ambient operation. plus coil freeze-ups etc.

See this paper for a description of an R-134a vehicle heat pump:

http://www.visteon.com/utils/whitepapers/2004_01_1379.pdf

Suitability of one refrigerant over another can be investigated from a comparison and study of the pressure-enthalpy chart of each fluid.

 

[AbbyNormal]

http://www.danfoss.com/BusinessArea...ng/EducationAndTraining/CO2+Phase+Changes.htm

Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[fueliefan]

those were the answers i was looking for, thanks!