Hydraulic pitch systems provide robust and reliable control of power and speed of modern wind turbines. During emergency stops, where the pitch of the blades has to be taken to a full stop position to avoid over speed situations, hydraulic accumulators play a crucial role. Their efficiency and capability of providing enough energy to rotate the blades is affected by thermal processes due to the compression and decompression of the gas chamber. This paper presents an in depth study of the thermodynamic processes involved in a hydraulic accumulator during operation, and how they affect the energy efficiency of the component. An initial evaluation of the popular thermal time constant model is made and compared with experimental results for a 6-liter accumulator, showing that the current estimation techniques for the thermal time constant are not suited for the application studied, predicting higher heat loses in the gas and resulting in lower pressure buildup. Furthermore, it is shown that the assumption of a constant value for the thermal time constant can provide extremely accurate results, provided that the compression ratios of the process are known in advance. For varying compression ratios, dynamical effects play an important role and the accuracy of the model decreases. To study the thermal processes, a simplified axisymmetric CFD model of the accumulator is developed, using COMSOL Multiphysics for meshing and STAR-CCM+ as a solver. The model provided an interesting close up view to the gas movement and temperature distributions during operation, which describe a particularly nonlinear behavior of the heat losses and the thermal time constant. The model was successfully validated with experimental data, and provides a repeatable and accurate prediction of the gas states, regardless of the operational conditions, with maximum prediction errors of 10%. Finally, a practical approach on how to improve the thermal efficiency of the accumulators by introducing foams on the gas side is shown, effectively decreasing the heat losses in the accumulator, and improving the efficiency of the compression-expansion cycles.
Keywords: CFD, Hydraulic Accumulators, Heat Transfer, Thermal Time Constant