Ionic liquid, co-fluid, absorption, non-volatile, carbon dioxide
Carbon dioxide is undergoing a renaissance as an alternative to synthetic refrigerants due to its environmental advantages in addition to a high density and excellent transport properties. A weakness of carbon dioxide is having a critical point which occurs at a lower temperature and higher pressure than most other fluids used as refrigerants. This combination leads to high operating pressures, especially on the heat rejection side of the thermodynamic cycle. Ionic liquids, which are salts which remain in their liquid phase at room temperatures, have been shown to strongly absorb carbon dioxide. Due to recent advances in ionic liquids, the cation and anion groups are able to be formulated to tailor a variety of fluid properties including liquid-vapor equilibrium characteristics. By selecting appropriate ionic liquids, it is possible to reduce the operating pressure of an air-conditioning system utilizing carbon dioxide to be in the range of conventional refrigerants. Not only are ionic liquids able to physically absorb volatile refrigerants as in other co-fluid cycles, but ionic liquids also offer the possibility of chemical absorption thereby giving the opportunity for greater enthalpy changes. Conceptually, the ionic co-fluid cycle is similar to a traditional vapor compression cycle. In the high pressure heat exchanger, heat is rejected to lower the enthalpy and to absorb carbon dioxide into the ionic liquid. The enthalpy is further reduced in an internal heat exchanger before the high pressure liquid is passed through a valve to decrease the pressure which causes the fluid mixture to cool. Heat is absorbed by the mixture from the environment, thus boiling additional carbon dioxide. After passing through an internal heat exchanger, the fluid is mechanically compressed and the cycle is repeated. System modeling work was utilized to identify important thermodynamic characteristics for achieving good performance. These characteristics included heats of mixing, solubility, entropy of mixing, and viscosity. Using experimentally and numerically determined IL-CO2 mixture properties, system models were able to predicatively select anion and cation pairs for optimizing performance. The ionic liquids selected from the modeling exercises were subsequently synthesized for demonstration in a laboratory. An air conditioning system was built from components designed for use with conventional refrigerants. The system was installed in a facility which was instrumented to measure air and refrigerant pressures and temperatures. Air flow rate and temperature information allowed the cooling capacity to be measured. The power consumption of the pump and compressor used to circulate the working fluids was measured so that COP could be determined. Modeling results were validated with experimental findings. The emphasis of modeling and experiments was to determine the effect of operational parameters on system performance. The loading of ionic liquid and carbon dioxide, along with valve opening and compressor speed, was found to dramatically alter the operating pressures. The difference and ratio between high and low side pressures directly affected the specific cooling capacity and COP, respectively. While the model had strong agreement with the experimental results, non-idealities to be incorporated in more sophisticated models are identified.