Improvements in permanent magnet materials have resulted in lighter and more efficient permanent magnet generators (PMGs) than wound field generators. The field excitation is provided by permanent magnets that are capable of operation at temperatures up to 260°C. Integrating a high speed PMG with a small gas turbine presents challenges to the designer, such as high speed dynamics and balance, magnet retainment and temperature limitations, choice of cooling system design and evaluation of parasitic losses, maintenance and repair of components, voltage regulation and excitation shut off with internal fault, and frequency conversion to AC power. Overall system DC to AC efficiencies of 95% are possible with small gas turbine-driven PMGs. Power conversion to grid-standard, commercial AC incurs additional losses, due to both inversion and dissipation of heat generation.

An attractive feature of the high speed PMG is the ability to provide a high starting or light-off speed for the turbine, avoiding the need for a separate starting motor and dedicated start fuel injector, thus simplifying the fuel control system. Although high PMG tip speeds may be preferred to reduce rotor length with a stiffer shaft, generator efficiency decreases due to higher windage losses. PMG power capability P is linked to rotor speed and volume by the following relationship:

The terms L and D are shown in Figure 5.3, and the equation has been used to prepared the plot shown for a rotor length to diameter ratio of 4.0. ESS is the electromagnetic shear stress, n is the efficiency, and N is the rotational speed. The rotating shaft is shown crosshatched, and the fixed PMG field is shown immediately outboard of the rotor. The trade-offs that can be made between rotational speed, PMG tip speed, and diameter for the relevant power output range are shown. These three parameters have a major influence upon the aerodynamic and structural design of the complete MT. Generator cooling and heat rejection are major considerations and may incur parasitic power losses equivalent to 5% of the MT output. Cooling approaches using integral fan, air-oil mist, and suction from the compressor inlet have been developed.

Solar Stirling Engine Basics Explained

Solar Stirling Engine Basics Explained

The solar Stirling engine is progressively becoming a viable alternative to solar panels for its higher efficiency. Stirling engines might be the best way to harvest the power provided by the sun. This is an easy-to-understand explanation of how Stirling engines work, the different types, and why they are more efficient than steam engines.

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