Abstract
Growing size of wind turbines, especially off-shore wind turbines, has strong advantages of increased wind speed with less turbulent at higher altitudes and the larger rotor sizes that can capture more energy. A hybrid hydraulic-electric wind turbine concept is proposed by introducing a hydraulic motor in the nacelle to convert rotor shaft work into hydraulic power that can be transmitted to the electric generator at ground/sea level. This hybrid design can greatly reduce the nacelle/drive components, help simplify or even eliminate the gearbox to reduce maintenance, and greatly reduce the head weight as much as 50%. To quantify the benefits of a hybrid hydraulic-electric power generation and of possible energy storage, the National Renewable Energy Laboratory (NREL) off-shore 5-MW baseline wind turbine was employed as a baseline. Based on analysis, the tower mass can be reduced by 33% to 50%, maintaining the tower natural frequencies. So the capital costs of wind turbine manufacture and assembly can be reduced.
Integrating Compressed Air Energy Storage (CAES) to a power supply system with a variable and unsteady energy source can also bring a number of advantages. The hydraulic-electric generator concept is compatible with a novel approach to compressed air energy storage that allows high storage efficiency. This concept involves a liquid piston utilized as a compressor and expander with water spray to promote heat transfer to allow near isothermal processes. For the 5 MW reference off-shore wind turbine which integrates a spray-based compressed air energy storage with a 35 MPa accumulator, the overall compression is proposed in three stages with pressure ratios of 10:1, 7:1, and 5:1, all operated at 1 Hz to maintain moderate liquid surface acceleration. Based on a simple and fundamental description of the system, compression efficiency is found to be strongly dependent on droplet surface area, which can be achieved through either high mass loading or small drop sizes. The one-dimensional simulations show that direct injection spray can increase overall three-stage compression efficiency to as high as 89%, substantially better than the 27% associated with a conventional adiabatic compression at the same pressure ratio. In addition, this liquid piston concept is examined with detailed two-dimensional flow. Multi-phase computational fluid dynamics is implemented in an axisymmetric domain to investigate compression in a cylinder for first-stage and second-stage compression, using a spray discharge within the cylinder at various mass loadings. The spray was generally achieved by operating a single pressure-swirl nozzle directed along the centerline and operating at the maximum liquid mass flux capable that still permitted a small average droplet diameter. The results indicating that a single pressure-swirl nozzle injection resulted in an injected mass loading of 1.6 and yielded efficiency as high as 93% for a first-stage compression cycle. To satisfy both high mass loading and uniform distribution for second-stage operation, a multi-nozzle configuration should be considered. Pre-mixed approach may serve to increase the degree of mixing and the mass of drops than can be injected.
