Compact cryogen-free modular cooling system for large scale offshore superconducting wind turbines

High temperature superconducting (HTS) generators are expected to pave the way to scale up offshore wind turbines to 10 MW and beyond. Although study of HTS wind generators has been of great interest and several designs have been proposed, the commercialization has not yet come. Despite the demonstrated feasibility and realized technical advantages, cryogenic cooling is still the key barrier to make the use of HTS technology in the field of offshore wind applications more transparent. In order to fulfill the requirements of handling, transportation, maintenance and reliability of long-term offshore operation, we have proposed a compact cryogen-free modular cryogenic cooling system for large scale HTS wind generators. The HTS generator usually contains many identical superconducting coils and each coil is allocated one identical modular cryostat in order to achieve the working temperature of 20-40 K. The modular cryostat enveloping the HTS coil adopts rectangular shape and consists of a vacuum vessel, a thermal shield with multi-layer insulation and corresponding supporting structures. Rods made of titanium alloy are selected as support structures to reduce the heat load. A small scale two-stage Stirling or pulse tube cryocooler driven by oil-free linear compressor will be implemented in each modular cryostat to deliver the required cooling power

Development of a rotary union for Gifford-McMahon cryocoolers utilized in a 10 MW offshore superconducting wind turbine

Superconducting generators (SCG) show the potential to reduce the head mass of large offshore wind turbines. By evaluating the availability and required cooling capacity in the temperatures range around 20 K, a Gifford-McMahon (GM) cryocooler among all the candidates was selected. The cold head of GM cryocooler is supposed to rotate together with the rotating superconducting coil. However, the scroll compressor of the GM cryocooler must stay stationary due to lubricating oil. As a consequence, a rotary helium union (RHU) utilizing Ferrofluidic® sealing technology was successfully developed to transfer helium gas between the rotating cold head and stationary helium compressor at ambient temperatures. It contains a high-pressure and low-pressure helium path with multiple ports, respectively. Besides the helium line, slip rings with optical fiber channels are also integrated into this RHU to transfer current and measurement signals. With promising preliminary test results, the RHU will be installed in a demonstrator of SCG and further performance investigation will be performed.This research is primarily supported by the European Union Seventh Framework Programme (FP7/2007-2013) under grant agreement 308793. Thanks for the technical support from Tom Black and Mark Granoff at Ferrotec

CFD analysis of the regenerator performance of cryocooler under different accelerations

High temperature superconducting (HTS) technology is a promising enabling technology to allow the offshore wind turbines (WT) scaling up to 10 MW and beyond by radical reduction of the head mass. In order to provide adequate magnetic field, the HTS coils are supposed to work at a low temperature of 20-40 K, thus a cryogenic cooling system is required. Comparing with cooling by means of cryogenic liquids, the cryogen-free method which adopts conduction cooling with regenerative cryocoolers leads to certain advantages such as smaller size, simple structure and elimination of safety issues related with cryogens. These advantages are particularly valuable in superconducting WT operating in offshore condition, and thus the cryogen-free cooling system attracts more and more attention in demonstrating HTS WT projects. The cold head of the cryocooler linking to the superconducting filed coil rotates together with the rotor. The resulting centripetal acceleration and oriental variation may introduce streaming inside the regenerator and lead to performance degradation. However, up to now the influence of acceleration on the oscillating flow and heat transfer inside the regenerator has not yet been investigated. As a response to the demands of performance stable cryocooler system, in this paper we will study the impact of the gravity and centripetal acceleration on the regenerator performance. Two dimensional numerical model of the regenerator will be established by Ansys Fluent. The temperature and pressure distribution of the regenerator will be illustrated and the dependence of operating frequency, acceleration and regenerative material will also be discussed

Conceptual design and thermal analysis of a modular cryostat for one single coil of a 10 MW offshore superconducting wind turbine

Superconducting generators show the potential to reduce the head mass of large offshore wind turbines. A 10 MW offshore superconducting wind turbine has been investigated in the SUPRAPOWER project. The superconducting coils based on MgB2 tapes are supposed to work at cryogenic temperature of 20 K. In this paper, a novel modular rotating cryostat was presented for one single coil of the superconducting wind turbine. The modular concept and cryogen-free cooling method were proposed to fulfil the requirements of handling, maintenance, reliability of long term and offshore operations. Two stage Gifford-McMahon cryocoolers were used to provide cooling source. Supporting rods made of titanium alloy were selected as support structures of the cryostat in aim of reducing the heat load. The thermal performance in the modular cryostat was carefully investigated. The heat load applied to the cryocooler second stage was 2.17 W@20 K per coil. The corresponding temperature difference along the superconducting coil was only around 1 K.European Commision's FP

Lightweight MgB2 superconducting 10 MW wind generator

Special Issue in Focus on Superconducting Rotating MachinesThe offshore wind market demands a higher power rate and more reliable turbines in order to optimize capital and operational costs. The state-of-the-art shows that both geared and direct-drive conventional generators are difficult to scale up to 10 MW and beyond due to their huge size and weight. Superconducting direct-drive wind generators are considered a promising solution to achieve lighter weight machines. This work presents an innovative 10 MW 8.1 rpm direct-drive partial superconducting generator using MgB2 wire for the field coils. It has a warm iron rotor configuration with the superconducting coils working at 20 K while the rotor core and the armature are at ambient temperature. A cooling system based on cryocoolers installed in the rotor extracts the heat from the superconducting coils by conduction. The generator's main parameters are compared against a permanent magnet reference machine, showing a significant weight and size reduction. The 10 MW superconducting generator concept will be experimentally validated with a small-scale magnetic machine, which has innovative components such as superconducting coils, modular cryostats and cooling systems, and will have similar size and characteristics as the 10 MW generator.European Union Seventh Framework Programme under grant agreement n° 308793

Experimental Study and Simulation of Quench in MgB2 Coils for Wind Generators

In the frame of the European Union (EU) founded SUPRAPOWER Project, MgB2 conduction-cooled coils are foreseen to be used in the rotor of synchronous wind turbine generators. As a result, a conceptual design of a 10 MW superconducting (SC) generator will be made and a Rotating Magnetic Validator (RMV) will be constructed and tested, with two rotating SC coils. The main dimensions and working conditions will be kept in the RMV and in the 10 MW generator. The RMV has been conceived is such a way that it can be transformed in a generator. Numerical quench simulations have to be performed in order to design a suitable quench protection system. Due to the uncertainty of some material properties and its importance for the machine operation, an experimental validation must be performed in order to resolve discrepancies. Under this premises, a full size MgB2 double pancake coil have been manufactured and instrumented to trigger a controlled quench.European Commission's FP