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Everything R744


Heating And Cooling An Electric Vehicle

27 July 2008

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Global warming and climate change is forcing humanity to take action. The electric vehicle has during the past year shown an increasing potential, implementing new Li-Ion batteries. These provide a performance similar to today’s fossil fueled vehicles.

The drawback of highly efficient cars is however the lack of waste heat, used today for cabin heating. An interesting alternative to the presently used oil burners is the vapor compression cycle, used for air conditioning cars. The future of refrigerant choice for the automobile sector is today uncertain due to the demand for substitution of R134a.

This MSc thesis discusses the possibility of implementing a transcritical vapor compression cycle based on the natural refrigerant carbon dioxide, also called R744, in future electric vehicles. It would be used for both cooling and heating.
Although R744 was one of the first refrigerants to be used, very little is known about its transcritical performance. This study describes the special characteristics of this cycle. An improved cycle is proposed and compared with the “standard” transcritical cycle using R744 and a subcritical R134a cycle. The improved cycle requires an 8 port 2 way valve, enabling alterations in the refrigerant flow and an expander recovering work. A flash-gas by pass concept, decreasing the pressure drop in the evaporator and improving heat transfer, is also introduced. Optimal correlations and equations of state provided by EES are used in the simulation providing ideal and thus comparative results.

The improved automobile R744 TVCC HP/AC cycle showed an average of 23 % improvement in COP compared to the standard R744 cycle when simulated in heating mode. The nature of the transcritical cycle implies a need to keep the passengers safe from the high-pressure and temperature of R744 entering the gas cooler. At an ambient temperature of -2 °C the refrigerant was estimated to have a pressure of 120 bar and corresponding gas cooler inlet temperature of 120 °C in order to keep the Cabin warm.

R134a can however be proven to outperform both analyzed R744 cycles. An average of 20 % in reduced power consumption can be expected if operating the standard cycle with R134a compared to the improved R744 cycle. In spite of this, the R134a refrigerant can not be recommended due to its facing out, starting 2011. Therefore it is recommended to continue developing improved R744 alternatives, as well as studying other refrigerants in direct and indirect systems.

Finally this thesis describes further improvements on the proposed cycle. By integrating a second micro-channel tube bundle evaporator in the EV-battery core, it is possible to recover heat generated in the battery. The design presents an additional 12- port- 5 way valve enabling constant refrigerant flow direction during the cycles multiple operational modes. This should improve the COP even more and reduce the temperature drop in the cabin during de-frosting. The battery can further on be temperature regulated, heated and cooled from the inside, leading to a more energy efficient use of the EV as well as possibly prolonging the lifespan of the battery.

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Marc Chasserot

Please have a look at my entry on the Blog. thanks marc
added 2008-07-29 14:14:24

Anonymous User

Being that I\'VE worked in the trade for 33 years and FYI R134 is also explosive. Your statements on CO2 are quite understandably self serving. Diesel injector pumps are in steel small dia. lines and you are talking rubber lines connecting CO2 components. I doubt very much that we will see 500 g charges in cars my friend, and draught beer systems I\'ve worked on since before you were born and vapour pressuriziation of less than 15 psi (1bar) is irresponsible to be considered in the same sentence.
From my information HFO1234yf i only flammable under particular guys in Europe seem to love the propane fridges so what\'s the problem with this?
It always amazes me how we can step back 50 years and then claim a forward progress. My suggestion is keep looking...1234yf is not acceptable, neither is CO2....for efficiency and service safety reasons. But what the hell do I know.
added 2009-02-18 14:32:25

Anonymous User


It is good to show you have solid experience, and you are right in saying that high pressure for CO2 is a challenge. May I ask, out of your 33 years of experience in the sector, how many have actually been on CO2?

It would be no surprise that a CO2 system would bust if you use rubber lines to connect CO2 components. That\'s why all suppliers are working with aluminium components, you simply cannot use rubber! And that is also one of the reasons why CO2 implies a higher investment cost at the beginning. But that is not say that CO2 cannot work. It can work, but it requires training and a different way of designing the system, as the properties of the refrigerant are quite different.

For some hints, I suggest an article about Behr testing of a complete fleet over 100+ vehicles with CO2. You can find it on, by simply doing a search about Behr.
added 2009-02-18 15:22:16

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Publication Info

27 Jul. 2008 - PDF
Year: 2008
Type: Report / Study
Source: Research / Science
Language: English
File Size: 2 MB
Number of Pages: 77
R. Arboix
Royal Institute of Technology Sweden KTH
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