Investigation of Chemical Looping for High Efficiency Heat Pumping
The demand for heat pumping technologies is expected to see tremendous growth over the next century. Traditional vapor compression cycles are approaching practical limits of efficiency and running out of possibilities for environmentally friendly and safe refrigerants. As a result, there is an increasing interest in pursuing non-vapor compression technologies that can achieve higher efficiencies with alternative working fluids. The chemical looping heat pump (CLHP) investigated here utilizes a chemical reaction to alternate a working fluid between more and less volatiles states. This allows the main compression to take place in the liquid phase and enables the utilization of a range of different working fluids that would not be appropriate for vapor compression technology.
Thermodynamic models were developed to assess the potential performance of a chemical looping heat pump driven by electrochemical cells. A number of potential working fluids were identified and used to model the system. The thermodynamic models indicated that the chemical looping heat pump has the potential to provide 20% higher COPs than conventional vapor compression systems.
An experimental test stand was developed to investigate the efficiency with which the electrochemical reactions could be performed. The working fluids selected were isopropanol and acetone for reasons of performance and availability. The test stand was designed to measure not only the power consumed to perform the conversion reaction but also the concentration of products formed after the reaction. The experimental tests showed that it was possible to perform the reactions at the voltages required for an efficient chemical looping heat pump. However, the tests also showed that the reactions proceed much slower than expected. To increase the rates of the reactions, an optimization effort on the membrane and catalyst selections was performed.
Traditional catalyst materials used by solid polymer electrochemical cells, like those used in the testing, perform best in hydrated environments. The fluids isopropanol and acetone tend to displace water in the membranes, reducing the system conductivity. Multiple membrane types were explored for anhydrous operation. Reinforced sPEEK membranes were found to be the most suitable choice for compatibility with the CLHP working fluids. Multiple catalyst mixtures were also tested in the experimental setup. Density functional theory was used to develop a computational framework to develop activity maps which could predict the performance of catalyst materials based on calculated parameters.
A detailed model of the CLHP electrochemical cell was developed. Built on open-source tools, the model was designed to determine the charge, mass, and heat transfers within the cell. The conversion of reactants along the channel of the cell as well as overall power consumption are predicted by the model. The model was validated against measurements and used to determine parameters for a CLHP cell that would have improved conversion performance and energy efficiency compared with the tested cell.
The cell model was integrated into an overall system model which incorporates the effect of concentration changes throughout the entire cycle. Compared to the early-stage thermodynamic modeling, consideration of incomplete reactions provided more accurate predictions of the potential performance of CLHP systems. Different cell and system architectures were investigated to boost system performance. The model predictions demonstrated that the CLHP has the potential to provide high heat pumping efficiencies, but more work is still needed to improve the energy density of the system.