522 Why Two Turbines

Designing the MT engine to employ two turbines offers several advantages:

• Long engine life — The two-turbine configuration reduces stress by splitting work output between the turbines. Turbine life is extended even further by the relatively low pressure ratio and TITs employed by the engine.

• Direct mechanical output — Any rotating mechanical component can be driven by the power turbine. Thus, the two-shaft MT engine can be used in a wide variety of applications. For example, two-shaft MT systems have been built and tested to directly drive the refrigerant compressor of a vapor-cycle chiller or a screw compressor that would typically be used in ammonia cycles for refrigeration systems.

• Design point flexibility — The two-shaft engine configuration allows the designer greater flexibility in choosing a design point for the power turbine. For example, the power turbine can rotate at significantly lower speeds to better match a particular load requirement. Thus, conventional, reliable, low-cost induction and synchronous generators can readily be connected to the power turbine through a reasonably sized gearbox. Since the power turbine rotates independently, it can accommodate variable-speed applications such as refrigeration systems where the turbine drives a screw compressor.

• Shaft mechanical design — Shaft design issues related to the power turbine and the load component (compressor impeller, generator, etc.) are independent of those associated with the gasifier/turbine shaft. The complexity of each of these simple shafts is, thus, much less than the shaft design required of single-shaft engines. The latter must account for the rotor dynamic, loading, and sealing complexities associated with placing all rotating components (including the generator) on one shaft.

• Component configuration complexity — An independent power turbine allows greater freedom in laying out rotating components. Requiring multiple components to be crowded onto a single shaft often forces poor design compromises. For example, the air flowing into the compressor is often used first to cool the high-speed alternator. Unfortunately, gas turbine engines quickly lose efficiency and power with rising air inlet temperature, and overall performance suffers accordingly.

• Mechanical safety issues — The lower rotating speed of the power components reduces the danger level of catastrophic rotating failure in the system. The higher the rotating speed, the more deadly the "shrapnel."

• Favorable torque/speed characteristics — Since, to the first order, the mass flow through the engine is not affected by power turbine conditions, the power turbine actually delivers more torque as it slows, the opposite characteristic of the turbine in single shaft designs. This improves the engine's ability to handle load changes and maintain operational stability.

Not every advantage lies with the two-shaft approach, of course. Initial capital cost is still an important consideration even when calculating the full life cost of an MT to a facility. In addition, the cost of a design does not necessarily scale with the number of components. However, MT designers must be very careful about cost because of the competitive pressures of the generation market. Unlike single-shaft designs, a two-shaft MT cannot be started by driving the turbine temporarily by the generator (now acting as a motor). Therefore, the designer must build into the system a starting mechanism to bring the gasifier turbine up to an initial operating speed.

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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