New air-con tech can use up to 90% less energy

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A new air-conditioner has been developed that uses salt solutions instead of refrigerants, thus eliminating greenhouse gas emission concerns, and a thermally-activated absorption cycle that can slash the electricity needed to run the refrigeration cycle by 50% to 90%.

It can also be powered by natural gas or solar energy. The technology has been patented by the US Department of Energy’s (DOE’s) National Renewable Energy Laboratory (NREL).

Combining the benefits of desiccant and evaporative cooling has long posed a formidable challenge. In a hot and humid region like Dubai, evaporative cooling can reduce the temperature, but not enough to make it pleasant at the height of summer.

Hence refrigeration-based air-con remains the most effective means of keeping cool, but in Dubai it uses up to 70% of the total electricity output in the region. The strain on electricity resources has been seen recently with record temperatures in Kuwait, together with record power consumption levels that could see the introduction of mandatory power cuts in that country.

The DEVap, which stands for desiccant-enhanced evaporative air-conditioner, uses membrane technology to combine the efficiency of evaporative cooling and the drying potential of liquid desiccant salt solutions. Evaporative coolers are a lower-cost alternative to air-con in dry climates that do not get too hot or humid. Water flows over a mesh, and a fan blows air through the wet mesh to create humid, cool air. In humid climes, adding water to the air creates a hot and sticky building environment. Furthermore, the air cannot absorb enough water to become cold.

However, the DEVap is said to solve that particular problem by relying on the desiccants’ capacity to create dry air using heat and evaporative coolers’ capacity to take dry air and make cold air. “The concept of combining the two is by no means novel, but no one has been able to come up with a practical and cost-effective way to do it,” said NREL mechanical engineer Eric Kozubal, co-inventor of the DEVap.

“The idea is to revolutionise cooling, while removing millions of metric tons of carbon from the air. We had been working with membranes, evaporative coolers and desiccants, and saw an opportunity to combine them into a single device for a product with unique capabilities.”

NREL used desiccants consisting of highly-concentrated aqueous salt solutions of lithium chloride or calcium chloride. These have a high affinity for water vapour, and can thus create very dry air. Due to the complexity of desiccant cooling systems, they have traditionally only been used in industrial drying processes. Inventing a device simple enough for easy installation and maintenance has prevented desiccant cooling from entering into commercial and residential cooling markets.

To solve that problem, the NREL device uses thin membranes that simplify the process of integrating airflow, desiccants and evaporative cooling. These result in an air-con system that is said to provide superior comfort and humidity control.

The membranes in the DEVap air-con are hydrophobic, which means water tends to bead up rather than soak through the membranes. That property allows the membranes to control the liquid flows within the cooling core. “It is that property that keeps the water and the desiccant separated from the air stream. We bring the water and liquid desiccant into DEVap’s heat-mass exchanger core. The desiccant and evaporative cooling effect work together to create cold-dry air,” explained Kozubal.

The air is cooled and dried from a hot-humid condition to a cold and dry condition all in one step. This all happens in a fraction of a second as air flows through the DEVap air-con. The result is an air-conditioner that controls both thermal and humidity loads.

Kozubal expects that, over the next couple of years, he will be working on making the device smaller and simpler, and perfecting the heat transfer to make DEVap even more cost-effective. NREL said it will ultimately license the technology to industry.

“We are never going to be in the air-conditioner manufacturing business,” said Ron Judkoff, principle programme manager for Building Energy Research at NREL. “But we would like to work with manufacturers to bring DEVap to market and create a more efficient and environmentally-benign air-conditioning product.”