Performance analysis of an air-source heat pump for low-temperature sludge drying in a water treatment plant

Addressing the issues of high energy consumption and carbon emissions in municipal sludge drying, Air-Source Heat Pump (ASHP) technology demonstrates significant potential as a high-efficiency, low-carbon solution. However, its dynamic thermodynamic behavior under variable operating conditions and the primary factors controlling energy efficiency require further analysis. This study investigates a full-scale ASHP low-temperature sludge drying system in a municipal wastewater treatment plant (WWTP). Based on continuous operational data collected over 118 days, a mechanistic coupled model incorporating the main controller, heat pump cycle, and drying chamber heat and mass transfer kinetics was established using MATLAB/SIMULINK. Validated against experimental data, the model achieved a mean relative error of only 1.35% for COP prediction, demonstrating high physical consistency. The results indicate that in practical engineering operations, the system stably raises air temperature to 48–55°C, effectively reducing sludge moisture content from 57%–66% to 20%–40%. The system achieved an average Specific Energy Consumption (SEC) for water evaporation of 0.40 kWh/kg and a Specific Moisture Extraction Rate (SMER) of 2.5 kg/kWh, significantly outperforming traditional thermal drying processes. Furthermore, a Random Forest algorithm was introduced to perform a multi-factor nonlinear coupling analysis of operational parameters. The feature importance ranking revealed that “sludge feed rate” is the dominant factor influencing specific energy consumption, rather than the traditional drying temperature. The study identified an optimal feed load range of 9–12 tons/batch, providing a theoretical basis and data support for the engineering application and refined low-carbon operation of ASHP sludge drying technology. • Development and validation of a dynamic MATLAB/Simulink model for an air-source heat pump (ASHP) sludge drying system, demonstrating high accuracy (MAE ±0.66°C, COP error <1.35%). • Simulation study conducted for a full-scale wastewater treatment plant demonstrated that over a 118-day simulated operation period, the system performed stably in the actual operating environment and effectively reduced the sludge moisture content from 57~66% to 20~40%. • Identification of sludge feed quantity as the dominant factor influencing specific energy consumption (SEC), with an optimal operating range of 9~12 tons, revealed through random forest algorithm analysis. • Significant energy savings demonstrated by an average SEC of 0.4 kWh/kg, substantially lower than conventional hot-air drying systems (0.8~1.2 kWh/kg).