671Low and High Grade Heat

When small quantities of heat and/or low temperatures typify a fuel cell exhaust, the heat is either rejected or used to produce hot water or low-pressure steam. For example, in a PAFC cycle where the fuel cell operates at approximately 205°C, the highest pressure steam that can be produced is something less than 200 psia. At the other end of the spectrum is the SOFC, which operates at 1000°C and often has a cell exhaust temperature of approximately 815°C after air preheating. Gas temperatures of this level are capable of producing steam temperatures in excess of 540°C, which makes the SOFC more than suited for a steam-bottoming cycle.

Whenever significant quantities of high temperature waste heat are available, high-pressure steam can be generated or a combined-cycle or hybrid heat engine fuel cell approach can be considered. This can be accomplished in many different configurations and can include a large number of technologies (e.g., steam turbine, gas turbine, and Stirling engine) in combination with a fuel cell. In those cycles, heat engines utilize the high grade heat to produce electricity for dramatic increases in overall fuel-to-electricity efficiency.

A bottoming cycle simply adds a heat engine to the fuel cell exhaust for utilizing the heat produced in the fuel cell to produce electricity. This is typically a consideration when the exhaust of the fuel cell is available at low pressure. When a fuel cell operates under pressurized conditions, the high-temperature, high-pressure exhaust could potentially power a gas turbine whose exhaust could be utilized subsequently in a heat recovery steam generator and/or steam turbine.

Many of the cycles described above, which utilize the heat available in a fuel cell system, are called hybrid cycles. These cycles are described in more detail in the following section.

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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