Natural refrigerants For vehicle air conditioning
The more than 100-year history of motorized transportation is characterized by continuous further developments and innovations. In the area of driving comfort, the air conditioning of passenger compartments is a central point. Due to rising customer expectations, air conditioning systems are now part of the standard equipment of almost all vehicles for passenger transport, both on the road and on rail. In all refrigeration systems or heat pumps, refrigerants are used as the working medium. The refrigerant handles the transport of heat in the system. Like in a refrigerator, it absorbs heat on one side through evaporation and releases heat on the other side through liquefaction in a cyclic process. The choice of refrigerant has a significant influence on the overall system of the air conditioning unit or heat pump.
The only question is, which refrigerant is the right one. This question cannot be answered generally, as an ideal refrigerant for all applications does not exist. In addition to the properties required of the medium by the refrigeration system, such as high thermal output with low energy consumption, good miscibility with oil, etc., there is also the environmental aspect. Synthetic refrigerants based on halogenated hydrocarbons were brought to the market in the 1930s. With these substances, refrigerants could be offered for almost all temperature ranges. These substances were designated as safety refrigerants because they exhibit no direct toxicity or flammability. The danger that these refrigerants pose to the environment, as they increase the greenhouse effect and possess an ozone depletion potential, was only proven in the 1980s [Wikipedia]. To improve the environmental compatibility of the products, numerous laws and regulations have been enacted in recent years. This decided the phase-out of the previously used synthetic refrigerants.
Various developments can be seen: on the one hand, the further development of synthetic refrigerants, and on the other hand, the search for alternative refrigerants. In rail transport, for example, a long service life with high annual operating hours is required, which is why long-term, future-proof systems are sought here. In a search for a solution geared towards sustainability, natural refrigerants and specifically propane (R 290) are increasingly moving into focus.
Propane is a natural refrigerant with very good thermodynamic properties. Pressure levels, cooling capacity, and temperature behavior are similar to the synthetic refrigerant R 134a, which is mostly used today. Experience from previous applications can continue to be used, and the new systems also match the dimensions of previous systems with classic technology. This also makes a retrofitting of existing systems conceivable. R 290 is a very efficient and proven refrigerant and, with the appropriate technology, also a safe refrigerant. The refrigerant is available and cost-effective. The disadvantage of R 290 is that it is easily flammable and heavier than air, meaning it can accumulate near the ground.
Refrigeration systems must therefore be designed as closed systems with permanently tight connections. Refrigerant lines may only be routed outside the passenger compartment. They must be protected from heat, spark formation, etc., and the area of the lines must be well ventilated. Forced ventilation can prevent a critical concentration of R 290 from forming, even in the event of a leak in the system.
Although hydrocarbons such as propane are flammable, millions of gas heating systems are operated safely in Europe. Gas burners are also used in motorhomes. These vehicles carry larger quantities of gas with them than is required in vehicle air conditioning systems and have been moving safely across all the world's roads for decades.
From today's perspective, R 290 as a natural refrigerant represents a sensible solution for the air conditioning of vehicles. Even when compared to the refrigerant CO2 (R 744), it shows advantages in the overall assessment. R 744 has an advantage in the assessment of flammability, but on the other hand has disadvantages regarding the required installation space and weight. Propane also scores higher in energy efficiency compared to CO2. Due to the high pressure levels of up to 130 bar, the serviceability and maintainability of CO2 is also to be evaluated negatively in comparison. Component availability and price also favor systems with the refrigerant propane.
In summary, it can be said that when looking at the triad of performance, function, and environment, the use of propane as a refrigerant is recommended when planning refrigeration systems.