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A completely new refrigeration cycle for air-conditioning with carbon dioxide technology was developed. The main emphasis was placed on the thermodynamic calculation of the cycle and the components according to a revised specification, the development of the components such as compressor, heat exchangers, means for control and expansion, storage vessels and refrigerant hoses, test bench investigations, construction of two vehicles, vehicle tests in wind tunnels and road tests and the evaluation of safety and costs aspects. Following this, a direct comparison to a current serial air-conditioning system under commonly acknowledged conditions became possible.
An automotive air-conditioning system is often operated above the critical temperature of CO(2) at 31.1°C. Therefore a CO(2) system will mostly work in a transcritical cycle mostly. At supercritical conditions (critical pressure: 73.8 bar), pressure and temperature are independent of each other. The conditions in the evaporator remain subcritical. In this transcritical cycle the refrigeration capacity, the compressor work and thereby the cycle efficiency depend on the existing discharge pressure in accordance with the heat rejection temperature. The optimum discharge pressure is a function of the ambient temperature. The refrigeration circuit control should provide sufficient cooling capacity at high efficiency with satisfying passenger comfort, largely independent from the momentary driving and climate conditions.
The vehicle refrigeration circuit consists of a compressor, gas cooler, expansion device, evaporator, accumulator and internal heat exchanger. The packaging shows only slight differences to series vehicles. The small cross-section of the refrigerant pipes makes it easier to find a route through the tight engine compartment. The refrigeration cycle with CO(2) operates at high pressure levels, but this does not represent an significantly increased risk with adapted components. Due to refrigerant properties the new developed components remain nearly comparable in respect of weight and dimensions.
The RACE project has shown that with CO(2) systems undiminished efficiency, capacity and passenger comfort and safety can be reached. At high thermal load, the system performance can be considerably better. This applies to comparisons made with highly developed state of the art systems. The physiological characteristics of CO(2) must be taken into account by careful design and by adjusted production, service and use in the vehicle. This will result in extra components, like sensor and will require a higher level of skills.
The direct effect of refrigerant emissions from HFC-based automobile air-conditioners is a significant part of the TEWI. The leakage scenario of R134a defines the amount of reduction on global warming. The assumed extra weight for the air-conditioning CO(2) system is 3 kg. The system environmental evaluation with a European and North-American user profile and emissions scenario showed a reduction in global warming emissions of 18% to 70%. For a mid sized vehicle, the diminishing of the overall global warming by phasing out HFC-134a refrigerant is between 2% and 8%. The energy consumption for operating CO(2) systems is in the same range as conventional HFC systems. The CO(2) A/C-system allows high passenger comfort with a maximum of ecological compatibility. Serious technical arguments against a vehicle long-term application of the transcritical cycle are not seen during the project. The cost reduction potential of a conventional cycle cannot be reached by the CO(2) system. A successful introduction of the new refrigerants needs a world wide acceptance from the international car industry, only one 'Mobile Air-Conditioning Standard Refrigerant' is suitable in the future.
The proposed research is directed at developing a refrigeration cycle for use in automotive air conditioning systems. The new cycle will use a naturally occurring gas as a refrigerant. Because of the new refrigerants properties and the common working conditions of an automotive a/c cycle it will be necessary to develop a completely new transcritical vapour compression system.
Major tasks are:
(I) Calculation of thermodynamic cycle and of components based on typical car specifications
(II) Development of components - compressor, heat exchangers, expansion device, control device, receiver and hoses -
(III) Bench tests
(IV) Construction of prototypes
(V) Car tests in windtunnel and in-field
(VI) Safety & Acoustics evaluation
Successful completion will provide a long-term solution for an environmentally harmless refrigeration system.
Wissenschaftliches Gebiet
- engineering and technologymechanical engineeringthermodynamic engineering
- natural sciencesphysical sciencesthermodynamics
- engineering and technologymechanical engineeringvehicle engineeringaerospace engineeringaircraft
- natural sciencesphysical sciencesacoustics
- engineering and technologyelectrical engineering, electronic engineering, information engineeringelectronic engineeringsensors
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