Researchers have discovered a new way of reducing the power consumption of a home refrigerator by 29 percent while improving its cooling capacity. This can be done by replacing the widely-used and environmentally unfriendly R134a refrigerant with the more energy-efficient R600a refrigerant dosed with multi-walled carbon nanotube (MWCNT) nanoparticles. This test of nanoparticle-dosed refrigerants is a first-of-its-kind and was recently published in Energy Reports, an open-access journal.
Researchers from the University of Johannesburg added that trained technicians could also carry out drop-in refrigerant replacement in the field. They hope that this can help make home refrigeration more accessible for low-income families.
R134a is one of the most widely-used refrigerants in domestic and industrial refrigerators. It is safe for many applications as it is not flammable. However, it has high global warming potential and causes fridges, freezers and air-conditioning equipment to consume a lot of electrical energy. This energy consumption also contributes to climate change.
Meanwhile, a more energy-efficient refrigerant can result in much lower electricity bills and improve energy security for vulnerable households. Improved energy economy and demand-side management can also benefit planners at power utilities, as cooling accounts for about 40 percent of energy demand.
Enhancing power reduction using nanoparticles
Nano eco-friendly refrigerants have been made with water and ethylene glycol. Previous studies showed reduced energy use in nano-refrigeration, where refrigerants were dosed with multi-walled carbon nanotube (MWCNT) nanoparticles. The nanoparticles also resulted in reduced friction and wear on appliance vapour compressors. However, previous studies have not tested the effects of MWCNT on hydro-carbon refrigerants such as R600a.
In a recent study, researchers at the University of Johannesburg tested the drop-in replacement of environmentally-unfriendly refrigerant R134a, in a home refrigerator manufactured to work with 100g R134a.
They replaced R134a with the more energy-efficient refrigerant R600a, dosed with MWCNT nanoparticles.
Reducing electricity use by more than a quarter
The researchers removed the R134a refrigerant and its compressor oil from a household fridge. They used a new refrigerant, R600a, and dosed it with multi-walled carbon nanotubes (MWCNTs) while using mineral oil as a lubricant. The new mix was fed into the fridge and performance tests were conducted. They found that the R600a-MWCNT refrigerant resulted in much better performance and cooling capacity for the fridge.
“The fridge cooled faster and had a much lower evaporation temperature of -11 degrees Celsius after 150 minutes. This was lower than the -8 degrees Celsius for R134a. It also exceeded the ISO 8187 standard, which requires -3 degrees Celsius at 180 minutes,” said Dr. Daniel Madyira from the Department of Mechanical Engineering Science at the University of Johannesburg. “Electricity usage decreased by 29 percent compared to using R134a. This is a significant energy efficiency gain for refrigerator users, especially for low income earners. To gain these advantages, the choice of MWCNT nanoparticles is critical.”
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“The MWCNT’s need to have nanometer-scale particle size, which is extremely small. The particles also need to reduce friction and wear, prevent corrosion and clogging, and exhibit very good thermal conductivity,” he added.
The new refrigerant mix introduces a potential risk though. Unlike R134a, R600a is flammable. On the other hand, it is more energy efficient and can contribute less to global warming. Some refrigerator manufacturers have already adopted production with R600a and these appliances are available in the market.
“To do a safe drop-in replacement, no more than 150g of R600a should be used in a domestic fridge,” said Dr. Madyira. “Before the replacement, the fridge used 100g of R134a gas. We replaced that with 50g to 70g of R600a, to stay within safety parameters.”
He added that only a trained refrigeration technician or technologist should attempt this drop-in replacement.
Advantages of energy-efficient refrigeration
A far more energy-efficient refrigerant, such as the R600a-MWCNT mix, can save consumers money. Vulnerable households in hot climates in developing countries can benefit even more.
Low-income earners in many countries are dependent on home fridges and freezers to safely store bulk food supplies. This greatly reduces the risk of wasting food due to spoilage, or food poisoning due to improperly stored food. Without fridges, people may be forced to buy food daily in small quantities and at much higher prices. As daily buying may not be required anymore, the travel time and cost of buying food can be reduced as well. Medicines that require cooling can also be stored at home.
Grid power still necessary for low-income refrigeration
From a sustainability point of view, it may be preferable to run most home fridges and freezers from solar power. However, solar panels, backup batteries, and direct current (DC) fridges are still too expensive for most low-income families in areas served by power utilities.
Energy-efficient, alternating current (AC) fridges running on grid power may be more affordable for most. The researchers say that further reduction of power consumption with a R600a-MWCNT refrigerant can bring down costs even more.
Refrigeration for all vs demand-side management
As more low-income households and small businesses switch on grid-powered fridges, freezers and air-conditioning, power demand needs to be managed better.
In South Africa where the study was conducted, the state-operated power utility faces huge challenges in meeting demand consistently. Long-lasting rolling blackouts, known as load-shedding, have been implemented as a demand-side power management measure.
The researchers added that shaving off more than a quarter of the power consumption by fridges, freezers and air-conditioning units can free up national power supply for improved energy security in the region.