Thermodynamic Modeling of a Membrane Dehumidification System
In warm and humid climates, a primary source of building energy consumption is dehumidification of conditioned air supplied to the building spaces. The proposed system utilizes a selective membrane to remove water vapor from ambient air as opposed to a vapor compression cycle or a desiccant. This work provides an analysis of the membrane dehumidification system with a focus on the energy performance of the system. A system performance goal was set at the beginning for a given inlet and outlet ambient air condition and a total cooling load of one ton. The target COP of the combined sensible and latent cooling is 3.58 with a target value for only the latent system of 3.34.
Two different simulations were developed including an initial simulation which uses a basic mass transfer model and a simpler condenser model. The initial model was used to develop the system, analyze operating parameters and provide initial performance results. The initial simulations indicate that the system requires two optimizations to meet the target performance: condenser pressure optimization and the use of multiple membrane segments operating at different pressures. The latent only COP including the optimizations was a maximum of 4.23. A second model was then developed which uses a more detailed mass transfer model and a more detailed condenser model based on the operating conditions. This simulation yielded a maximum latent only COP of 4.37 including the optimizations.
The work also analyzes two different combined systems capable of providing both sensible and latent cooling. The first utilizes a conventional vapor compression cycle for sensible cooling and has a maximum COP of 3.93. The second uses multiple evaporative coolers in between multiple membrane dehumidification steps and was found to have a maximum COP of 3.73.
Second law analysis of the systems was also conducted and found that the greatest reduction in latent system exergy loss can be obtained by improving the selectivity of the membrane. Apart from improving the membrane selectivity, the results show the greatest improvement can be found in improving the operation of the gas compression devices.