The transient response of rotary desiccant wheels through experimentation and numerical analysis

Rotary desiccant wheels are commonly used for industrial dehumidification systems and significant research is being done to increase their application in other HVAC systems. The transient response is of concern because it can affect overall system performance and previous research efforts have indicated the transient response to be of substantial duration.; This research experimentally and analytically investigated the transient response of rotary desiccant wheels.; A model was developed to predict the transient response of rotary desiccant wheels with significant improvements to previous versions. The basic model uses fundamental principles of heat and mass transfer with the finite difference method. The parabolic concentration profile was used to approximate moisture gradients within the desiccant particle. This concept is fundamentally more correct than previous lumped capacitance models and avoids the computational difficulties of more rigorous models which include an extra second order differential equation. The NTU terms from the finite difference equations were also calculated at each point rather than as constants for a stream or element since they are significantly temperature and moisture dependent. The numerical solution techniques improved the speed and flexibility of the model by using a tridiagonally banded matrix solver for the difference equations and the bisection method was used for the parabolic concentration profile.; Experimental work was performed at the National Renewable Energy Laboratory (NREL) in Golden, CO on a commercially available rotary desiccant wheel made by NovelAire. Step changes to regeneration temperature, wheel speed, and flowrate were performed.; The model was validated by comparing the experimental data to model predictions using statistical and visual analysis of the outlet condition (temperature, humidity ratio) response curves. An alternative validation technique was also performed by statistically comparing thermographic images of the desiccant wheel surface with numerical calculations.; A parametric analysis was completed to observe the impact of different parameters on the transient response. Operational factors, ambient conditions, and wheel construction can have a substantial impact on the transient response time. Ambient temperature, for example, can increase transient time by up to 97% from a baseline of 16 minutes.; A control strategy comparison was done that determined cycling of rotary desiccant wheels could be an effective and efficient strategy to meet moisture removal requirements.