Giving 20th-Century Absorption Refrigeration Systems the Cold Shoulder

Posted on | February 22, 2012 | No Comments

Ionic Liquids for Absorption Refrigeration

Joan F. Brennecke

  • Joan F. Brennecke
    Keating-Crawford Professor
  • Chemical and Biomolecular Engineering
  • Director
    Notre Dame Energy Center

Ideal for carbon dioxide capture, ionic liquids (ILs) — organic salts that are liquid at room temperature — are proving equally as attractive in a variety of energy related applications, including absorption refrigeration. In fact, faculty in the College of Engineering have successfully designed several ILs with specific thermodynamic properties that promise to increase the efficiency of absorption refrigeration systems, providing more cooling for less energy.

Absorption refrigeration, typically used for industrial climate control, is not a new process. Invented in 1858, it replaces the large amounts of electrical energy needed for compressors in normal vapor compression refrigeration units with an energy-stingy pump and low-grade waste heat. Most of today’s systems use an ammonia-water or a water-lithium bromide solution for the refrigerant-absorbent. Both types of systems present challenges.

Ionic liquids

Notre Dame researchers are designing ionic luquids with specific thermodynamic properties to address a variety of energy needs.

Much more popular before freons were introduced, ammonia is a toxic compound with a pungent odor. The water-lithium bromide system has other problems: using water as the refrigerant limits the temperature range. If it falls below 32 degrees Fahrenheit, it could form ice in the system, bursting the pipes. Excessive concentrations of lithium bromide, which is a corrosive, can also solidify in the pipes of a system, sending maintenance costs soaring.

Neither method is ideal.

Joan F. Brennecke, the Keating-Crawford Professor of Chemical and Biomolecular Engineering and Director of the Notre Dame Energy Center, has been leading a multidisciplinary faculty team in search of a more energyefficient absorption refrigeration system. It is one of the many projects focused on energy efficiency being conducted in the energy center.

“We know that ionic liquids (ILs) are incredible solvents. They are salts, so they don’t evaporate to cause air pollution. But, more important in cases like this, we have had a great deal of success designing ILs with unique properties, so that they function the way we want them to in specific processes. That was one of the major reasons we began investigating using water and ILs as the refrigerant absorbent combination.”

Coefficients (COP) of numerous ionic liquid-water systems

Tested over a wide range of generator temperatures, the coefficients (COP) of numerous ionic liquid-water systems are higher than the standard commercial lithium bromide-water system at typical evaporator, condenser, and absorber conditions.

Funded by the Department of Energy, the researchers have been measuring the thermodynamics, including vapor-liquid equilibrium, enthalpy of mixing, and heat capacities, involved in the absorption refrigeration process to calculate the coefficient of performance (COP) for several of the new ILs they have designed. And, they have been very pleased with the results.

The COP for a refrigeration cycle is the cooling that you get divided by the work (power) needed to obtain it. For refrigeration, it is the electrical work needed to run the pump plus the heat you need to add to the absorbent to desorb the refrigerant. A COP of 1.2 or greater is desired for heating efficiency, while 0.7 or greater is the target for cooling.

The next step for Notre Dame will be to build a benchtop model and demonstrate the physical behaviors promised in the simulations. “We’re excited to move to the demonstrationphase for stationary systems [making more efficient buildings],” says Brennecke, “but we are equally encouraged about what this work means for mobile cooling systems such as cars and recreational vehicles.”

Suggested Reading

Simoni, Luke D.; Ficke, Lindsay E.; Lambert, Caitlin A.; Stadtherr, Mark A.; and Brennecke, Joan F., “Measurement and Prediction of Vapor- Liquid Equilibrium of Aqueous 1-Ethyl-3- methylimidazolium-based Ionic Liquid Systems,” submitted to Industrial & Engineering Chemistry Research, 2010, 49 (8), 3893-3901.

Simoni, Luke D.; Chapeaux, Alexandre; Brennecke, Joan F.; and Stadtherr, Mark A., “Asymmetric Framework for Predicting Liquid — Liquid Equilibrium of Ionic Liquid — Mixed-Solvent Systems. 2. Prediction of Ternary Systems,” Industrial & Engineering Chemistry Research, 2009, 48, 7257-7265.

Simoni, Luke D.; Brennecke, Joan F.; and Stadtherr, Mark A., “Asymmetric Framework for Predicting Liquid — Liquid Equilibrium of Ionic Liquid — Mixed- Solvent Systems. 1. Theory, Phase Stability Analysis, and Parameter Estimation,” Industrial & Engineering Chemistry Research, 2009, 48, 7246-7256.

Ficke, Lindsay, E.; Rodriguez, Hector; and Brennecke, Joan F., “Heat Capacities and Excess Enthalpies of 1- Ethyl-3-methylimidazolium-based Ionic Liquids and Water,” Journal of Chemical Engineering Data, 2008, 53 (9), 2112-2119.

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