Optimizing Viscosity for Maximum Power in a Hydrostatic Transmission Wind Turbine

In basic hydraulic theory, efficiency is dependent on volumetric and mechanical losses. Components dominated by mechanical losses favor low viscosity fluids, while those dominated by volumetric losses favor high viscosity fluids. For a hydrostatic transmission (HST), efficiency of both the pump and motor must be considered. The goal of this work is to model the HST viscosity-efficiency curve for finding the optimal viscosity and to validate the results with experiments. This paper reports on preliminary results with only partial success. This study uses a mathematical efficiency model that is dependent on non-dimensional viscosity, two non-dimensional length ratios, and fractional swash plate displacement. Rather than using physical dimensions for the non-dimensional length ratios, these parameters were determined for the pump and motor through constant viscosity experiments. The predicted results of the model were compared to validation experiments where viscosity was varied by changing temperature. Results showed high accuracy for the pump model at all tested conditions, but the accuracy of the motor model was lacking, deviating significantly from experiments at low viscosities. For future studies, it is recommended to increase the number of data points at low dimensionless viscosity to create a more accurate model fit. Alternative models that provide more realistic results will be considered.