Experimental validation of thickness and moisture effects on the energy efficiency of intermittent vs. continuous drying

Intermittent drying (ID) is often reported to be more energy-efficient than continuous drying (CD), although previous studies frequently lacked consistent comparison conditions. A recent meta-analysis suggested a probable correlation between material thickness, initial water content, and the specific energy savings of ID. The present study experimentally validates this hypothesis under controlled and comparable conditions. Drying experiments were conducted using a porous sponge model with thicknesses ranging from 12 to 48 mm and initial water contents from 30% to 70% (wet basis), while maintaining identical operating conditions and equal total drying time for both ID and CD. The results demonstrate that ID is not universally more energy-efficient than CD, but instead exhibits condition-dependent behavior governed by internal transport limitations and moisture redistribution during tempering periods. For thin or low-moisture samples, the energy ratio remained close to unity (∼0.97–1.00), whereas for thicker (≳36 mm) and wetter (≳60%) samples, values reached ∼1.05, corresponding to energy savings of up to 4–5%. A transport-based interpretation using characteristic diffusion time scales supports the observed dependence on thickness and moisture content. This study provides the first controlled experimental isolation of material thickness and moisture effects under identical drying conditions and equal total drying time, revealing a transition-like regime governing the effectiveness of intermittency. The findings demonstrate that the energy advantage of intermittent drying is not universal but emerges primarily under conditions where internal transport limitations become significant.