Cool CO2

Author: John Monczunski

istock photo

Notre Dame engineering researchers have come up with a new, green take on an old air-conditioning technology that has the potential to save money and benefit the environment. Ironically, the new eco-friendly cooling system employs carbon dioxide, one of the primary greenhouse gases implicated in global warming.

In recent years there has been strong interest in CO2 as a refrigerant because its global warming potential is 1,000 times less than those currently used. However, a problem exists: Carbon dioxide requires extremely high pressure to work as a coolant, and the equipment needed to withstand those pressures is expensive.

Now a team of researchers led by Notre Dame chemical engineering Professor William Schneider may have a solution. Until now, researchers could never find an ideal co-fluid to work at lower pressure with CO2 in a cooling system. However, while working with certain compounds known as ionic liquids on research designed to remove carbon dioxide from power plant emissions, Schneider recognized these organic salts might be ideal for a CO2 based refrigeration system.

Since ionic liquids remain fluid over a broad temperature range and have an affinity for CO2, they should be excellent candidates for use in low pressure/low energy vapor-compression systems like those common in household, commercial and transportation cooling.

Using powerful computers, Schneider and his colleagues have designed ionic liquids with just the proper strength chemical bond — strong enough to bind with CO2 but not so strong as to take too much energy to break the bond. Such fine-tuning allows them to work efficiently with CO2 at lower pressure and hence lower cost.

In the refrigeration cycle, a cool, low-pressure stream of CO2 dissolved in ionic liquid takes in heat, breaking the CO2-ionic liquid bonds. This warm gas-liquid mixture is then compressed, driving up its pressure and temperature. The hot mixture ejects heat to the surroundings, allowing the CO2 to recombine with the ionic liquid. The mixture then flows through an expansion valve, decreasing the temperature further, and the mixture begins the cycle all over.

The ND researchers hope to have an ionic liquid/CO2 air conditioner up and running by the end of the year.