1025 Fuel Cells

Phosphoric acid electrolyte fuel cells (PAFC) are the only commercial fuel cells sold in any quantity to date. The PAFC has been installed in more than 200 locations in the U.S., Europe, and Japan — almost all in CHP mode. PAFCs were the first practical application because, unlike other aqueous acids, PAFCs can be operated above the boiling point of water — typically around 200°C. PAFCs, therefore, can use waste heat from the fuel cell stack to directly reform methane into a hydrogen-rich gas for use as a fuel. They are produced in 200 kW modules that can easily be combined. The heat is used for space heating or hot water, but is not of a high enough quality to be used in other cogeneration applications. The Japanese are particularly advanced in PAFC research and design.

Molten carbonate fuel cells (MCFC) operate at higher temperatures and are more efficient than the PAFC, with estimated efficiencies up to 55% lower heating valve (LHV). The carbonate electrolyte is solid at room temperature but liquid at the operating temperatures of 650 to 800°C. The high exhaust temperature of an MCFC can generate additional electricity in a steam turbine or in a gas turbine combined cycle. The MCFC is expected to target 1 to 20 MW stationary power applications and should be well suited for industrial CHP.

Solid oxide fuel cells (SOFC) are the newest type of fuel cell and are still in laboratory testing for various configurations. Like MCFCs, the high-grade waste heat produced by SOFCs can be used for internal reforming and many other applications, including steam that a steam turbine can use to generate extra electricity in a combined (bottoming) cycle. Even after this process, the heat remaining is sufficient for cogeneration applications. Hybrid systems using gas turbines or microturbines could increase electric efficiencies to 60%.

Proton exchange membrane fuel cells (PEM) operate at relatively low temperatures (80°C) because the polymer membrane melts at higher temperatures. The fuel cell's low-temperature heat is not adequate for traditional cogener-ation,* but does allow a quick startup time.

The type of fuel cell determines the temperature of the heat liberated during the process and its suitability for CHP applications. Low-temperature fuel cells generate a thermal product suitable for low-pressure steam and hot water CHP applications. High-temperature fuel cells produce high-pressure steam that can be used in combined cycles and other CHP process applications. Although some fuel cells can operate at part-load, other designs do not permit on/off cycling and can only operate under continuous base-load conditions.

* Though hydronic heading and desiccant regeneration are possible.

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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