Conference Year
2016
Keywords
energy recovery, heat and mass transfer, dehumidification, membrane
Abstract
To provide a healthy environment inside buildings, there must be some exchange of indoor conditioned air with fresh outdoor air. The outdoor air is then mechanically conditioned to a comfortable temperature and humidity. Research suggests human health and productivity increase with the amount of fresh air available in buildings. This conflicts with the desire to reduce building energy use since the conditioning process is energy intensive, especially in warm and humid climates. While sensible heat recovery using a standard heat exchanger is of some value, much of the energy consumption is due to dehumidification of air. More substantial reductions in energy consumption can be obtained by preconditioning the supply air with the previously conditioned exhaust air using a porous polymer membrane heat and moisture exchanger. As membrane technology improves, the convective heat and mass transfer resistances in the airstream can become the dominant resistance in these devices. This requires potentially new flow passage architecture to optimize heat and mass transfer, while maintaining acceptable pressure loss. Thus, the objective of this study is to develop an analytical model of a counterflow membrane based heat and mass exchanger with different internal flow geometries using the Engineering Equations Solver (EES) platform. The exchanger consists of multiple supply and exhaust air streams flowing in counterflow, separated by a thin membrane layer. The air flow passages are either a bare high aspect rectangular channel, or a high aspect channel with a pin-fin spacer inserted. The model is discretized along the length of the exchanger to more accurately calculate the air and vapor properties along the exchanger. In each segment the model considers the coupled convective heat and mass transfer resistances in each air stream. Heat and mass transfer coefficients are related through the Chilton and Colburn analogy, while representative values for membrane thermal conductivity and effective diffusivity are obtained from the literature. Conservation of energy and mass in each segment provides closure to the model. The model is then used to parametrically evaluate the effect of various exchanger dimensions and operating conditions on the dominant heat and mass transfer resistances, sensible and latent effectiveness, and pressure loss of the exchanger. The primary dimensions considered are hydraulic diameter and geometry (i.e., high aspect versus pin filled channels) of the channels, number of channels, membrane thickness and overall length, width and depth of the exchanger. The effect of operating parameters including air flow rate, water diffusivity of the membrane and allowable pressure drop on system performance and dominant transport mechanisms are also explored.Â