Experimental Comparison of Control Strategies for Heat Recovery from Compressors Under Variable Operating Conditions

This article presents an analysis and experimental verification of control strategies for a heat recovery system from air compressors operating under varying conditions. The research was conducted on three test benches equipped with compressors with rated powers of 30 kW, 75 kW and 110 kW. Based on experimental data, the dynamic properties of the system were identified using FOPDT models, demonstrating a significant dependence of the model parameters on the operating point. Subsequently, a comparative analysis of P, PI, and PID controllers and various methods for selecting their settings was conducted. Further studies included a PI controller with fixed settings, a gain–scheduling strategy, and adaptive control using online identification of model parameters by the RLS method. It was demonstrated that the variability of the system’s dynamic properties limits the applicability of a PI controller with fixed settings over the entire analyzed operating range. The application of adaptive control enabled continuous adjustment of controller settings to the changing properties of the plant without the need to determine models and settings for multiple operating points in advance, as required by the applied gain–scheduling strategy. In a selected experiment with the adaptive controller, stable water temperature control was achieved with heat exchanger heat balance indicator values exceeding 90%. This indicator defines the ratio of the heat flow rate absorbed by the water to the heat flow rate released by the oil and does not constitute a measure of the overall waste heat recovery efficiency of the compressor. The results confirm the feasibility of applying adaptive control for temperature regulation in heat recovery systems from compressors operating under varying conditions.