11-02-2013, 12:17 PM
New Trends in the Field of Automobile Air Conditioning
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Abstract
This article presents new trends in the area of automobile air conditioning, which is fast
becoming standard equipment. Attention is focused on the refrigerant and ventilation circuit
of the air conditioning equipment, and on the control system. An analysis is presented here of
a commonly used contemporary refrigeration system working with the refrigerant R134a and
a promising trans critical refrigeration system using CO2 as a refrigerant. The article also deals
with the configuration of some components of a trans critical refrigeration system. Additionally, attention is paid to physiologically controlled air conditioning and multi zone air conditioning systems which make it possible for every individual in the automobile cabin to have an optimum microclimate.
Introduction
Automobile air conditioning has become more and more commonplace. The air conditioning
system ensures a thermal comfort for passengers, but also contributes to the defogging of
windows and thus increases the active safety factor. Automobile air conditioning equipment is
markedly different from the air conditioning facilities found in buildings since, in contrast to
the stable conditions of a building, conditions inside an automobile vary markedly in both
time and space. Among the most significant new trends in automobile air conditioning is the use of alternative refrigerants to replace the more usual R134a, the use of physiologically controlled air conditioning, and multi zone air conditioning systems. Because of its contribution to the
greenhouse effect, the long-term prospects for R134a, which has been used to the present time, are not very good. The best candidate to replace R134a as a refrigerant in automobile air conditioning is CO2. The refrigeration cycle functioning under CO2 is trans critical and, due to
the working pressures in the refrigeration system, some components require special configuration. At the present time, the majority of fully automatic air conditioning equipment used in automobiles regulates themselves based upon temperature measurements in the area near the driver. Evolution is toward so-called physiologically controlled air conditioning, in which an
optimum thermal microclimate is reached by means of several sets of sensors which measure
the thermal state in the automobile cabin.
The Refrigerant Circuit in the Automobile Air Conditioning
The basic components of the refrigerant circuit are the compressor, the condenser, the
refrigerant cartridge with safety device and drying pad, the expansion valve and the
evaporator (Fig. 1). The compressor must ensure that vapor of the refrigerant is removed from the evaporator in such a way as to ensure that both pressure and temperature are maintained at required levels. At the same time, circulation of the refrigerant must be maintained in the cooling circuit. Given that the compressor is run by the engine of the vehicle, it must operate with wide
variation in rotating speeds, which will impact the refrigerant equipment capacity. Modern
compressors with variable capacity allow transport quantities to vary between 0 and 100%
and thereby lessen fuel demands.
Air Conditioning Control Systems
Control system of the air conditioning equipment ensures the desired temperature of the inner
cabin air, and controls the amount of air intake as well as its distribution. In manually
controlled systems, the desired air temperature, distribution and air intake level are set using
the appropriate manual controls. In temperature controlled systems, the chosen temperature is
automatically maintained, while air distribution and air intake are controlled manually.
Fully automatic control with programmable selection independently ensures the desired
internal air temperature, air intake amount and air distribution. The desired temperature is
maintained using either an air régime or water régime (Fig. 3). Fresh air drawn in by the fan
(1) is either cooled in the evaporator (2) or warmed in the heater (4) to the desired 4
temperature and then, depending upon the position of the valves, is moved into individual
areas within the internal environment of the automobile (b, c, f). The electronic control unit
(8) registers the temperatures as indicated by the sensors (3, 5, 7) and the temperature to
which the selector (6) is set. The desired temperature is compared to the actual temperature
and the difference is converted in the control unit into regulation variables for heating (4, 11)
and cooling (2, 10) capacity, managing the amount of air (1), and regulating air distribution
(by the position of valves b, c, d, e, f), depending upon the program choices made by passengers.
a -fresh air ,b- windshield outlet, c- upper outlet, d- internal circulation, e- bypass, f- lower outlet.
Analysis of Air Conditioning Systems Operating with CO2 and R134a
Automobile air conditioning systems presently use the refrigerant R134a. Because of its
contribution to the greenhouse effect, which is 1300 times that of CO2 , its long-term prospects are not good. Intensive research and development is underway into refrigeration systems using alternative refrigerants (Joudi et al 2003). CO2 is among the alternative refrigerants used for automobile air conditioning. CO2 refrigeration system functions in air conditioning equipments right at the critical pressure of 7.38 MPa, or sometimes above it (Kim et al 2004). Heat transfer therefore takes place in many cases at supercritical temperatures and the cycle is trans critical, i.e., it has a subcritical low-pressure side and a supercritical high-pressure side. At supercritical
pressures, saturation conditions do not exist and pressure is independent of temperature. For a given evaporation temperature and minimum heat rejection temperature on exit from the cooler, a trans critical cycle has greater thermodynamic loss than a condensation cycle (Fig. 4).
Draft for a CO2 component system
In CO2 systems, the compressor works under high median effective pressure and the pressure
ratio (discharge pressure to suction pressure) is relatively low. The pressure ratio to deliver
identical refrigerating capacity is 3.1 using CO2 and 5 using R134a. The compressor becomes
more efficient as the pressure ratio tends lower. For the trans critical cycle, the compressor
requires thicker walls, but it is still smaller than a compressor which delivers the same
capacity using R134a as a refrigerant. Piston and rotary compressors (rotary vane, rolling
pistons, scroll) both single- and two-stage, are being developed for CO2 refrigeration systems.
The use of two-stage compressors improves COP by up to 20%. A CO2 gas cooler has better heat transfer as a result of greater convection heat transfer in the vicinity of the critical point
and as a result of the high pressure, which allows for higher velocities of refrigerant flow. Because of the high pressures, a cooler with flat micro channel tubes is used, fitted with folded louvered fins as we can see in Fig. 5 (Pettersen et al 1998). The internal heat exchanger, in which the refrigerant exiting the cooler is sub cooled by vapors exiting the evaporator, can increase the efficiency of the cooling cycle working under CO2 by up to 25%. An example of micro channel
configurations for the internal heat exchanger is given in Fig. 6 (Kim et al 2004). These configurations, in comparison with the conventional concentric tube designs, reduce demands
on the material by 50% and increasing effectiveness by 10%.
Conclusion
The analysis offered here of automobile air conditioning systems working with the refrigerant R134a and more promising trans critical systems using CO2 demonstrates that CO2 technology brings with it many advantages. As an example, when refrigeration systems work with CO2 and R134a and with heat exchangers of identical proportions, system using CO2 will have greater refrigerating capacity and allow lower cabin temperatures to be achieved in the automobile at the same time it cuts fuel demands by 25 to 30%. Some components of refrigeration systems working under CO2, however, require fresh configuration. Modern physiologically controlled air conditioning makes possible the automatic control not only of temperature in the automobile cabin, but also humidity and velocity of air, and permits automatic switching between external and internal air circulation. With multi zone automatic air conditioning systems, it becomes possible to create an optimal microclimate for each individual in the passenger compartment of the automobile.