Antonio Jarquin Laguna, a researcher in Delft University of Technology, issued a thesis report, exploring a new way of generation, collection and transmission of wind energy inside a wind farm, in which the electrical conversion does not occur during any intermediate conversion step before the energy has reached the offshore central platform.
In particular, Laguna explains that a centralized approach for electricity generation is considered through the use of fluid power technology. In the proposed concept the conventional geared or direct-drive power drivetrain is replaced by a positive displacement pump. In this manner the rotor nacelle assemblies are dedicated to pressurize water into a hydraulic network. The high pressure water is then collected from the wind turbines of the farm and redirected into a central offshore platform, where electricity is generated through a Pelton turbine.
In the report, a numerical model is developed to describe the energy conversion process, as well as the main dynamic behaviour of the proposed hydraulic wind power plant. The model is able to capture the relevant physics from the dynamic interaction between different turbines coupled to a common hydraulic network and controller.
Two case studies are considered in the time-domain simulations for a hypothetical hydraulic wind farm subject to turbulent wind conditions. The performance and operational parameters of individual turbines are compared with those of a reference wind farm with conventional technology turbines, using the same wind farm layout and environmental conditions.
For the presented case study, results indicate that the individual wind turbines are able to operate within the operational limits with the current pressure control concept. Despite the stochastic turbulent wind conditions and wake effects, the hydraulic wind farm is able to produce electricity with reasonable performance in both below and above rated conditions.
Performance Comparison
The performance of both wind farms for the considered conditions is summarized in the following figures:
After including the performances of the main subsystems involved in the conversion and transmission of wind energy in a wind farm, the results show that the overall efficiency of a hydraulic wind farm is lower for a hydraulic concept compared to conventional technology. The total transmission efficiency which includes the complete collection, transmission and generation of electrical power of the wind farm is obtained using the average efficiencies from point A to point C.
For the presented operating conditions, the transmission efficiency of the hydraulic wind farm is between 0.772−0.810 compared to 0.835 from the reference wind farm. The most important losses in the hydraulic concept are attributed to the variable displacement pumps and friction losses in the hydraulic network. Despite having a slower response due to high water inertia, the hydraulic concept also showed higher standard deviations in the generated electrical power, due to pressure transients in the hydraulic network.
Conclusion
The report presented the numerical model of a hydraulic wind power farm, which is used to generate electricity in a centralized manner. The model captures the most relevant physics of a wind farm including transient behaviour of the hydraulic network and Pelton turbine. It was shown that a single turbine is able to operate with a passive control strategy while keeping a stable variable-speed operation. However, when two turbines are incorporated in a farm, the same passive control strategy has strong limitations in terms of performance and stability when these two turbines are coupled through a hydraulic network. An increased stability of the rotor speed for both turbines is possible by operating at a higher tip speed ratio, but the performance of the wind farm is then compromised. For individual turbines with independent lines and nozzles, this is not the case. However, based on the design principles for centralized electricity generation, it is preferred to have a combination of parallel and common pipelines.
From a wind farm perspective, an active control strategy is more suitable to maintain the individual turbines performance. Based on a constant pressure control strategy, the simulation of a hypothetical hydraulic wind farm consisting of five turbines was presented for a stochastic turbulent wind, including wake effects. For the presented case study, the results indicate that the individual wind turbines are able to operate within operational limits with the current active control concept. In terms of performance, the simulations show that the hydraulic collection and transmission has a lower efficiency when compared with a reference wind farm that employs conventional technology. The main energy losses are associated with the variable displacement water pumps and friction losses in the hydraulic network.
Further information may be found by reading the full report:
Source: Delft University
