Design and experimental characterization of a membrane-based absorption heat pump

A membrane absorption heat pump uses absorbent and refrigerant (solvent) flows separated by a membrane to create temperature gradients (aka temperature lifts) used for heating or cooling. Compared to vacuum absorption heat pumps, an atmospheric-pressure membrane heat pump provides more compact designs, potentially enabling applications such as energy-efficient cooling for electronics. In addition, storing concentrated absorbent offers unique options for energy storage for solar heating and cooling of buildings. A new membrane heat pump module was built using two sets of rows of hollow fibers with stagnant air between the fibers to reduce conductive heat transfer. Transport coefficients for the complex air-gap geometry were estimated with a three-dimensional finite-volume heat transfer analysis of the air gap region with results fitted to a modified conduction shape factor. A two-dimensional finite-difference model of the entire process shows good agreement with experiments performed over different air-gap widths, flow rates, inlet temperatures, and absorbent concentrations. Temperature lifts up to 9 °C were achieved with 39% (mass) LiCl (aq) feed solution and 35 °C inlet temperatures. Extensions of the modeling to higher-porosity, larger-pore-size membranes suggest that temperature lifts of 14 °C at ambient inlet temperatures are achievable in our module geometry.

Research highlights▶ Experiments performed on new design for a membrane absorption heat pump. ▶ Design uses alternating layers of paired rows of water and salt solution. ▶ Space between rows filled with air (‘air gap’) reduces conductive heat losses. ▶ Air gap geometry modeled with relations based on conduction shape factors. ▶ Experiment and finite-difference model match well (for temperature lift, R2 = 0.96).