Summary:
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.
maybe you could specify what you mean by "big concern over CO2". Can you draw on any experience with using R744 in Arizona?
2008-08-27 10:30:04 - jim stack
In Phoenix Arizona and many southern USA States cooloing is the big issue/ You need a system for heating and cooling. Not easy to do and be efficient. The heat pump like used on the past GM EV1 was very efficient for cooling.
Also we have big concern over CO2. IE natural refrigerant carbon dioxide, also called R744
