An Ambient Pressure Absorption Heat Pump Using Microporous Membranes: Design, Modeling, and Experimental Investigation

Woods, Jason David

Graduate Thesis Or Dissertation

An Ambient Pressure Absorption Heat Pump Using Microporous Membranes: Design, Modeling, and Experimental Investigation Public Deposited

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Citeable URL: https://scholar.colorado.edu/concern/graduate_thesis_or_dissertations/fj236248s

Abstract

A membrane absorption heat pump uses absorbent (a salt solution) and refrigerant (water) flows separated by a membrane to create a temperature difference, or temperature lift, used for heating or cooling. Compared to conventional absorption heat pumps, an ambient-pressure membrane heat pump is built from simpler, more compact, and potentially less expensive components. Storing the absorbent in an unpressurized tank offers unique options for thermal energy storage for solar heating and cooling of buildings and potential applications in long-distance thermal energy transport.

The contributions of this thesis can be summarized as: (1) design characterization of this novel process, focusing on controlling the heat and mass transfer in a membrane device for energy storage and transport applications, (2) modeling the process, including detailed analyses of the transport phenomena and a generalized analysis of membrane pore-size distribution, which is applicable to a wide range of membrane processes, and (3) experimental characterization of this process, with validation of the model.

Results from a first-principles numerical model shows that using a 1-mm air gap between two membranes gives temperature lifts four times higher than using a single membrane with no air gap. Predicted temperature lifts for the air-gap design range from 5-25^ᵒC, with higher inlet temperatures giving higher temperature lifts. Experimentally measured temperature lifts over a range of flow rates, salt mass fractions, and temperatures match the modeling within 15% with an R² of 0.91. The maximum temperature lift achieved was 9^ᵒC, but temperature lifts up to 20^ᵒC are anticipated with a future design using more porous hollow fibers.

The detailed analyses of the transport phenomena led to the following conclusions. First, natural convection in the air gap is negligible for the geometries considered here. Second, the membrane's porosity, tortuosity factor, and pore size are adequate to predict membrane mass transfer coefficients, with pore-size distribution having a minimal effect. Third, an accurate estimate of the membrane's effective thermal conductivity is unimportant for modeling a membrane heat pump. Fourth, most of the complex phenomena occurring in the boundary layers are unimportant for predicting the Nusselt and Sherwood numbers for the flows. Experiments on the three prototypes reinforce these conclusions.

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