The Future Pioneers

Hydrostatic transmission for wind turbine application

Driven by climate change and the need to decrease carbon dioxide emissions, the installation of wind turbines for electricity generation has expanded rapidly over the last decade.

The US generates more than 6.3% of its total electricity from wind and has a goal of harnessing 20% of the nation’s energy from wind by 2030 [1].Globally,more than 52GW of clean, emissions-free wind power was added in 2017, bringing total installations to 539 GW. With new records set in Europe, India and in the offshore sector, annual markets will resume rapid growth after 2018.Most utility scale wind turbines (greater than 1 MW) are installed far away from the point of use, increasing transmission cost while incurring about 5% power loss in transmission lines. In contrast, for distributed wind, small (less than 100 kW) and midsize (100 kW to 1 MW)turbines can satisfy local demand and make the electrical grid more reliable and stable. At present, distributed wind turbines are growing slowly because of high installation, operation and maintenance costs[2].

In a conventional turbine (Figure 1), power from the wind is captured by rotor blades. The low speed rotor power is transmitted to high speed generator through a multi-stage fixed ratio gearbox.In a fixed-ratio gearbox turbine, the generator speed changes with wind speed.An expensive power converter is required between the generator and the grid to compensate for the mismatch in voltage and frequency. Studies conducted by the National Renewable Energy Laboratory among others, document failure frequency and downtime for wind turbines [3]. The studies show that gearboxes and generators failure accounts for 95% of the turbine downtime because of the difficulty in replacing them. The failure of gearboxes and generators is due to unsteady wind, causing impact loading which in turn reduces the life of the components. This failure not only decreases the annual energy production of the turbine, but also increases the maintenance cost. So, there is a need for a reliable transmission to replace the existing gearbox.

Proposed Design:

A hydrostatic transmission (HST) is a reliable and continuously variable transmission (CVT). It consists of a hydraulic pump drivinga variable displacement motor (Figure 2). For a continuously variable transmission, at least one unit must have variable displacement.In a wind turbine, the rotordrives the fixed displacement pump creating the hydraulic flow. The pressurized hydraulic fluid is fed to the variable displacement motor driving the generator. Hydraulic pumps and motors have a power density that is ten times higher than electric motors and generators, making the transmission more compact [4].The slight compressibility of the hydraulic fluid in an HST reduces the impact loading on the mechanical components and increases their life. As a CVT, an HST can adjust to varying wind speed.It decouples the generator speed from the rotor speed, rotates the generator at a constant speed and eliminates the expensive power convertor (about 7% of total turbine cost). A hydrostatic transmission has lower transmission efficiency than a mechanical gearbox, but the overall system efficiency is still competitive with a conventional gearbox turbine since there is no need for a power converter.Hydraulics are necessary for high power and high load applications such as construction equipment. Commercial hydraulic components for HSTs in the required power range are readily available at a reasonable cost.

The use of a hydrostatic transmission in a wind turbine creates the possibility of adding energy storage to the turbine using a hydraulic accumulator. When the wind speed is above the rated speed, the excess wind energy can be stored in the accumulator. This excess wind energy is released to the system when the wind speed is below the rated speed.

Power Regenerative HST wind turbine research platform: To demonstrate and validate the performance of the HST, we have successfully designed, constructed and commissioned a unique power regenerative test platform at University of Minnesota (UMN).

This testbed is one of a kind in the world. The testbed can simulate rotor torque generated from real wind profiles. It is a regenerative system consuming less power to operate. The testbed is instrumented with multiple sensors to accommodate and test a variety of hydraulic fluids, components and controls.It consists of two closed loop hydrostatic circuitsas shown in Figure 3. The block in dark gray is the HST under investigation. The other block is the hydrostatic drive (HSD), which is used to simulate the rotor driven by time varying wind. Instead of dissipating the turbine output power, the power is fed back to the HSD. A variable frequency driven electric motor is mounted on the turbine output shaft to make upfor the losses of the HST and HSD. Because of power regeneration, the research platform is capable of generating 105 kW output with only 55 kW of electrical input.

The power regenerative testbed is shown in Figure 4. It has pressure, temperature and flow sensor modules in the hydraulic line and torque sensors and speed encoders in the mechanical line. The testbed is equipped with 27 sensors to monitor the system performance and three analog inputs to control the testbed. It is incorporated with heat exchangers to maintain the temperature of hydraulic fluids. It is designed to operate at maximum pressure of 5000 PSI. It has safety valves to protect the hydrostatic circuit[5,6].

Significance of research:
This research will increase understanding of the HST for wind turbine applications. This will accelerate the development of wind power by increasing reliability, decreasing installation cost up to 12%, reducing maintenance costs and improving the efficiency of distributed wind power generation. The main applications of distributed wind are residences, farms, small industries and institutions. With affordable prices, the distributed wind market will grow and contribute to our wind generation goal. This will also reduce millions of tons of CO2 emission and provide a cleaner environment for our next generation. The outcomes of the project will stimulate industry to develop more efficient hydraulic components, system and control for wind applications and contribute to our green economy.

References:
1. Wind vision report by Department of Energy, https://energy.gov/eere/wind/wind-vision.
2. Distributed wind market report by Department of Energy, https://energy.gov/eere/wind/downloads/2015-distributed-wind-market-report.
3. Sheng, S., Report on wind turbine subsystem reliability- a survey of various databases. National Renewable Energy Laboratory, Golden, CO, Tech. Rep. NREL/PR-5000-59111, 2013.
4. Thul, B., Dutta, R., Stelson, K. A., Hydrostatic transmission for mid-sized wind turbines, 52nd National Conference on Fluid Power, Las Vegas, USA, 2011.
5. Mohanty, B., Wang, F., and Stelson, K. A., “Design of power regenerative hydrostatic wind turbine test platform,” Proceedings of the 10th JFPS International Symposium on Fluid Power Fukuoka, Japan, Oct 2017.
6. Mohanty, B., and Stelson, K. A., “Characterization and calibration of a power regenerative hydrostatic wind turbine test bed using an Advanced Control Valve,” Proceedings of the 11th International Fluid Power Conference, Aachen, Germany, March 2018.

Biswaranjan Mohanty
PhD Candidate
Department of Mechanical Engineering
University of Minnesota, Minneapolis, MN 55455

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