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 thermodynamical processes involved in an 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 L accumulator, showing that the current estimation techniques for the thermal time constant are not suited for the application studied, predicting higher heat losses 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. The results show that the main heat transfer losses are associated with heat diffusion in the solid parts of the accumulator, making up to 20% of the total heat losses. It is also shown that the heat transfer processes and the thermal time constant are tightly connected to variations in gas mass, in rate of change of volume and compression ratios. Comparison with experimental results validate the CFD model accurately, showing high level of agreement and repeatability between the predicted pressures and temperatures and the experimental measurements.
- Heat transfer
- Thermal time constant model