According to the International Institute of Refrigeration, 20% of the global warming potential of refrigerating and air-conditioning systems come from leaks (direct emissions), while 80% results from their energy consumption (indirect emissions). Today's refrigeration systems consume around 15% of the world's available electrical energy, so that reducing their energy demand would make an important contribution to reducing the threat to global warming.

In this context, special importance is attributed to natural refrigerants such as ammonia, carbon dioxide and hydrocarbons which offer high energy efficiency as well as being climate-neutral. But hydrocarbons such as propane, propene and iso-butane also have outstanding thermodynamic properties; refrigerating and air-conditioning systems that run on these refrigerants are particularly energy-efficient. Various tests in the field have confirmed energy savings between 10 and 30% compared to HFC systems.

In addition, certain hydrocarbons can also be used as a “drop-in solution” for synthetic refrigerants. For example, propane (R290) and propene (R1270) have similar thermodynamic behaviour to the HCFC R22. They use the same technology which means that many of the existing installed components are compatible. For higher ambient temperatures or higher humidity levels, propane and propene are more efficient than R22.

Making air-conditioning systems environment-friendly

The Chinese air-conditioning system manufacturer Gree Electric Appliances is one of the companies using propane to replace R22 and R410A in new systems. The Chinese use HCFC R22 as a standard refrigerant, but these refrigerants make a considerable contribution to global warming as well as destroying the ozone layer.

Significantly, the air-conditioners have a higher efficiency than both R22 and R410A models, whilst requiring a smaller mass of system materials. The change-over in refrigerant will save 560,000 t of carbon dioxide equivalents in direct emissions over the entire service life of the air-conditioning systems. To this should be added a further 320,000 t of carbon dioxide equivalents in indirect emissions saved by the improved energy efficiency of the systems. For the final consumer, this benefits in terms of lower electricity bills.

Fruit Storage – delaying the ripening of fruit

Companies in other industries are also opting for hydrocarbons, such as the British fruit grower Mansfields. The family company stores apples and cherries in a controlled atmosphere so that they will be available in top quality all year round regardless of when they were picked. State-of-the-art measuring, control and refrigerating systems monitor temperature, humidity, oxygen and carbon dioxide levels, keeping them at the required level to delay the ripening of fruit and vegetables.

Mansfields wanted an efficient, HFC-free refrigeration system for the warehouse in Chartham near Canterbury. The refrigeration experts International Controlled Atmosphere Storage and SRS Frigadon designed a propane secondary refrigerant system with an output of 1,150 kW.

A brine mixture of water and salt is used as the secondary refrigerant. The circuit is filled with 30,000 l and works at an operating pressure of only 1.5 bar to cool the heat transfer fluid down to -3°C. The brine is pumped to the 36 controlled atmosphere cold storage rooms which are kept at a constant air temperature of -0.5°C and 1.5°C. This entails interrupting the refrigeration process so that the brine absorbs heat from the ambient air which is used for defrosting. This method prevents the products being cooled from absorbing unnecessary heat and saves energy. The system design minimises the quantity of refrigerant and guarantees an ESEER (European Seasonal Energy Efficiency Ratio) of more than 4.2, although based on local conditions the real seasonal cooling COP (coefficient of performance) is around 6.

Supporting biotechnological research

One important process in the research and production of biotechnological products is freezing and defrosting substances for transport and storage. At the pharmaceuticals company Roche, this takes place in tanks with a volume of 300 l which are cooled down to -40°C in a clean room atmosphere. Here the company wanted an efficient refrigeration system to cope with fast changes in temperature between -50 and 130°C with an accuracy of +/- 1 K, with automatic drainage and refilling of the tank's cooling jacket.

Furthermore, compliance with the Roche Environment Protection Guidelines restricted the choice of refrigerants to substances that protect the ozone layer and the climate. To meet these requirements, Peter Huber Kältemaschinenbau developed a chiller that works with a small charge of 1.8 kg propene. The core element of the system is a two-stage semi-hermetic reciprocating Bitzer compressor, which is designed for use with propene. After being brought down to a temperature of -60 to -30°C, the propane then cools the silicone oil circulating in the cooling jacket.

The safety concept of the system comprises separate refrigeration circuits into several sections so that in the event of a burst pipe, any refrigerant leak is limited to the affected section rather than the complete charge. Additional components in the refrigeration system include a plate heat exchanger acting as evaporator, a water-cooled coaxial condenser and a Modbus-based control unit.

As safe as refuelling stations

"The case studies show that non-halogenated hydrocarbons are suitable for reliable refrigeration in many different branches", says Monika Witt, Chairwoman of eurammon, the European initiative for natural refrigerants. "However, certain requirements have to be met when using these substances. Potential sources of ignition have to be identified and eliminated early during the planning phase. The systems have to be designed so as to avoid leaks: this includes reducing the number of joints and applying permanent corrosion protection,” Monika Witt says. “As far as possible, the refrigeration system should be installed on the roof or equipped with a gas detection and ventilation system so that the gas can be exhausted in the event of a leak. Components containing refrigerant must be clearly marked as such so that service technicians are informed accordingly and can take corresponding precautions. But even if the flammability of hydrocarbons pose the greatest challenge, these substances can still be handled safely, as demonstrated every day in thousands of refuelling stations all over the world."

Source: eurammon

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