Tropentag, September 10 - 12, 2025, Bonn "Reconciling land system changes with planetary health"

Enhancing energy efficiency in medicinal and aromatic plants (MAPs) drying through optimised recirculation of exhaust air

¹University of Hohenheim, Inst. of Agricultural Engineering, Tropics and Subtropics Group , Germany
²University of Hohenheim, Inst. of Agricultural Engineering, Tropics and Subtropics Group, Germany

Abstract

Medicinal and aromatic plants (MAPs), valued for their rich bioactive compounds, are widely grown in tropical and subtropical regions. Due to their perishability after harvest, drying represents a critical post-harvest operation for preserving quality and extending shelf life. However, conventional drying systems, especially those reliant on fossil fuels, are typically energy-intensive and suboptimal. Therefore, this study investigates the potential for improving energy efficiency and drying performance of conventional dryers through controlled recirculation of exhaust air. A large-scale flatbed dryer (6.5 × 4.7 × 3.0 m), used for bulk drying of chamomile (Matricaria recutita) in Croatia, was retrofitted with an automated air recirculation system that dynamically regulated the fresh-to-recirculated air ratio based on prevailing air conditions during drying process. Four recirculation strategies were implemented and evaluated. The first strategy, serving as the control, involved continuous intake of ambient fresh air throughout the drying process, reflecting the standard practice. The second and third strategies began with the fresh air intake and switched to full recirculation, without fresh air intake, once the relative humidity of the exhaust air RHout dropped below 40% and 60%, respectively. The fourth strategy employed a dynamic control approach to maintain the relative humidity RHin of drying air at a constant setpoint of 40%. Drying was conducted at 50 °C with an air velocity of 0.15 m·s⁻¹. A total of 1800.3 ± 98.3 kg of fresh chamomile was dried per batch, with a drying capacity of 61.4 ± 2.8 kgH20·h^-1. The RHin = 40% strategy emerged as the most energy-efficient, reducing fuel consumption by 41.9% compared to the control. Although drying time increased marginally by 5.2% due to higher humidity in drying air, higher and stable drying temperatures were observed. This strategy also yielded the lowest specific fuel consumption (7.6 ± 0.8 kg·h^-1) and specific energy utilisation (5.5 ± 0.3 MJ·kg^-1H20), in contrast to 11.1 ± 0.5 kg·h^-1 and 7.6 ± 0.4 MJ·kg^-1H20 under control strategy. Product quality remained unaffected across the evaluated strategies. These findings highlight the effectiveness of controlled exhaust air recirculation in conventional dryers as a practical and scalable solution for improving energy efficiency in MAPs drying systems.

Keywords: Drying performance, energy utilisation, flatbed dryer, fuel consumption, medicinal and aromatic plants

Contact Address: Iris Ramaj, University of Hohenheim, Inst. of Agricultural Engineering, Tropics and Subtropics Group , Garbenstr. 9, 70599 Stuttgart, Germany, e-mail: Ramajuni-hohenheim.de