An Investigation of Reversing Axial Turbomachinery for Thermal Energy Storage Application

This paper investigates the feasibility of reversing axial turbomachinery for a thermal energy storage application. Under the proposed concept, the direction of flow and spool rotation is reversed when changing from an electrical generation to heat pump mode, and the axial turbomachinery components function as either a compressor or turbine depending on the energy storage system’s mode of operation. Such reversing pump-turbines are widely used in pumped hydro installations but have not been employed for compressible flow applications to our knowledge to date. A genetic algorithm based optimization has been developed to determine the coupled turbomachinery design parameters that meet performance objectives (efficiency, power output, etc.) in both modes. The aerodynamic design approach will be described in detail, followed by the CFD-based turbomachinery performance projections shown for the case of a 50MW thermal storage plant. The requirement to achieve high efficiency in both modes leads to an unconventional design with a large number of stages for a relatively low pressure ratio. Variable guide vanes at each of the two turbomachines are also critical for flow matching between modes. Finally, counter-rotating stages are also employed principally to further reduce the size and cost of the machine. It is concluded from the CFD results that the reversing axial turbomachinery achieves sufficiently high efficiency in both modes of operation to merit further virtual and physical development and testing.