Abstract

Purpose: This study aims to evaluate the energy efficiency and operational optimization of a variable-speed, low-temperature heat pump drying system designed for sewage sludge treatment. The research addresses the need for experimental validation of inverter-controlled heat pump systems that can reduce Specific Energy Consumption (SEC) and emissions compared to fossil-fuel-based dryers. Research Design & Data: A pilot-scale closed-loop belt dryer equipped with an inverter-driven scroll compressor (30–200 Hz) was fabricated and tested at compressor frequencies of 130, 160, and 180 Hz. Experimental data were collected for power use, temperature, pressure, and moisture content, supplemented by external verification from the Korea Testing & Inspection Institute (KTI). Research Results: The SEC decreased from 0.580 to 0.564 kWh kg⁻¹-H₂O as frequency increased, while final moisture contents remained ≤ 10 wt %. Average energy savings reached 31.5 % compared with conventional dryers. The system maintained stable condensation (17.6–20 bar) and evaporation (5.3–6.7 bar) pressures, confirming robust thermodynamic performance. Conclusion: The results demonstrate that optimal operation occurs near 170–180 Hz, balancing energy efficiency and moisture removal. The inverter-controlled heat pump dryer offers a technically stable, zero-emission alternative for wastewater facilities, aligning with carbon-neutral and circular-economy goals, and providing a scalable foundation for future hybrid sludge–biomass drying systems.