Performance of solid desiccants for fixed-bed regenerator applications

Abstract

Fixed-bed regenerators (FBRs) are a relatively novel type of energy recovery ventilator (ERV) used to reduce energy consumption in buildings. FBRs operate under cyclical conditions by storing heat and moisture in one recovery period and transferring the accumulated heat and moisture to an air stream in the next period. FBRs are very promising for heat recovery but are yet to be fully adapted for moisture transfer on a commercial scale. In order to enable moisture transfer, adsorptive materials known as desiccants can be coated on FBR plates for energy recovery. The primary goal of this thesis is to apply experimental and numerical methods to select desiccants for FBRs. Desiccants remove moisture by an adsorption process as opposed to absorption where molecules penetrate a bulk media. The sorption properties of the desiccant directly influence the latent effectiveness of FBRs during summer operation. The experimental results in this thesis showed that sorption kinetics is an important parameter for FBRs. A numerical model which includes the sorption kinetics was used to predict the dynamic behaviour of FBRs. The numerical results showed that selecting desiccants with a higher kinetic rate constant value leads to higher moisture transfer rates which in turn will increase the effectiveness of FBRs. The main contribution of this thesis is that the results enable researchers to determine the recovery periods and parameters when sorption kinetics can be neglected, and an equilibrium model is acceptable for FBR applications. The results showed that a 5% relative error in latent effectiveness was achieved in the equilibrium model for a rate constant of at least 0.0014 1/s, thicknesses of up to 25 mm, effective diffusion coefficient of at least 2.9 × 10⁻⁵ m²/s, and cycle periods of at least 60 s.