Many international organisations have begun to discover the advantages of using hydrocarbons such as propane and propene as they are climate-neutral refrigerants, apart from being energy efficient. Eurammon, the European initiative for natural refrigerants, presents case studies to support this view
According to the International Institute of Refrigeration1 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. This implies that reducing this energy demand would make an important contribution towards 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 climateneutral. Expert opinion recognises ammonia as the most efficient refrigerant. But hydrocarbons such as propane, propene and isobutane also have outstanding thermodynamic properties. Refrigerating and air conditioning systems that run on these refrigerants are particularly energy-efficient. This fact has been recognised by numerous international companies, including Ben & Jerry, Pepsi and Unilever. They use hydrocarbons for refrigeration in both their chilling units and freezers. 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 company is one of the world’s largest manufacturers of room air conditioners, with a production output of more than 70 million units a year.
The Chinese use HCFC R22 as a standard refrigerant, but these refrigerants contribute considerably to global warming as well as destroying the ozone layer. Altogether, China’s air-conditioning systems generate annual HCFC emissions amounting to 260 million T of carbon dioxide equivalent, thus constituting one of China’s largest sources of emissions. This is why in late 2009, Gree, assisted by the implementing agency GTZ Proklima, started pilot production of room air conditioning systems based on propane. The quantity of refrigerant ranges from 200 to 350g for rated cooling capacities of 2 to 4 kW, depending upon the model. Significantly, the air conditioners have a higher efficiency than both R22 and R410A models, whilst requiring a smaller mass of system materials. In addition to the reduced charge size, GTZ Proklima with UK-based consultant Daniel Colbourne, assisted with the safe design of the air conditioners. A production line will turn out 180,000 systems per year.
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 translates into benefits in terms of lower electricity bills.
The best project practice, which is intended to have a role-model effect for China’s entire air conditioning industry and beyond, is being funded by the German Federal Ministry for the Environment, Nature Conservation and Nuclear Safety, within the framework of the International Climate Initiative based on a decision of the German Federal Parliament.
FRUIT STORAGE – DELAYING THE RIPENING
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 completed in 2008 with an output of 1,150 kW. Five air-cooled factory-assembled packages charged with altogether 90 kg propene provide refrigerating energy for the secondary circuit at a temperature of -9°C.
The special safe design of these chillers and a detailed safety analysis was provided by Re-phridge. 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. The secondary refrigerant also cools the preparation and loading areas.
The evaporators in the warehouses are defrosted by off-cycle defrost. 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 output is 12 kW at a secondary refrigerant outlet temperature of 0°C, and 6.5 kW at -40°C. 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, commissioned in 2006, 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. 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 the refrigerant must be clearly marked as such, so that service technicians are informed accordingly and can take corresponding precautions. Good initial and advanced staff training plays a crucial role, as faulty maintenance is one of the greatest risks when operating refrigeration systems with hydrocarbons.”
Having said that, Witt rests her case when she points out: “But even if the flammability of hydrocarbons pose a challenge, these substances can still be handled safely, as demonstrated every day in thousands of refuelling stations all over the world.”
Presentation: “Improving Energy Efficiency in Refrigeration”, Jean- Luc Dupont, Head of the Scientific and Technical Information Department, International Institute of Refrigeration, Asgabat, March 1, 2007.