672Hybrid Fuel Cell Heat Engine

Hybrid fuel cell systems have been designed to obtain the highest possible fuel-to-electricity efficiency by using the heat produced by the electrochemical oxidation of fuels within a fuel cell to produce electricity. A hybrid system recovers the thermal energy in the fuel cell exhaust and converts it into additional electrical energy through a heat engine. Several heat engines have been considered for this type of system including gas turbines, steam turbines, and reciprocating engines. The only conversion device which has been tested in this role to date is a microgas turbine (or micro-turbine generator, MTG).

A microgas turbine fulfills this role with some particular synergistic benefits:

• MTGs require relatively low turbine inlet temperature which can be supplied by the exhaust of a high temperature fuel cell.

• MTGs operate at relatively low pressure ratios amenable to hybrid fuel cell applications.

• The fuel cell can be operated under pressurized conditions, improving its output and efficiency.

• Sufficient thermal energy is contained in the fuel cell exhaust to power the compressor (for fuel cell pressurization) and an electric generator (to produce additional electricity).

• The power density of the system can be increased.

The hybrid fuel cell-gas turbine concept integrates a high-temperature MCFC or SOFC with a gas turbine, air compressor, combustor, heat exchangers, and several balance-of-plant items to produce a hybrid system. Synergis-tic effects of the combined fuel cell-gas turbine lead to electrical conversion efficiencies of 72 to 74% lower heating value (LHV) for systems under 10 MW. Larger hybrid systems are being considered which may be able to achieve fuel-to-electricity efficiencies greater than 75%.

Figure 6.6 presents a schematic system diagram for a generic hybrid fuel cell-gas turbine system to illustrate the concept of hybrid fuel cell systems. Compressed air and fuel pass through a gas-to-gas heat exchanger (recuperator) which is typically used to recover heat from the combustion product gases leaving the gas turbine. The heated fuel and air streams pass into the anode and cathode fuel cell compartments of the fuel cell, respectively, where the electrochemical reactions take place. Fuel cell exhaust gases that already contain thermal energy from the electrochemical reactions are subsequently mixed and burned, raising the turbine inlet temperature. The thermal energy contained in this stream replaces that typically delivered by the conventional combustion section of the gas turbine engine. Expansion of the fuel cell exhaust gases through the gas turbine provides an inexpensive means for recovery of the fuel cell waste heat.

High-pressure hybrid systems or topping arrangements are the most likely of the hybrids to be commercialized in the near future. The simplest of these systems is a topping-cycle SOFC integrated with a recuperated gas turbine, as shown in Figure 6.6. This particular arrangement operates with an SOFC system that can capture and recirculate steam-laden anode exhaust gases to an internally integrated fuel reformer in order to produce hydrogen and


Generic hybrid solid oxide fuel cell gas turbine cycle schematic.


Generic hybrid solid oxide fuel cell gas turbine cycle schematic.

carbon monoxide, as shown in Figure 6.6. The potential benefits are obvious. High electrical conversion efficiencies are possible. The combination of fuel cells and heat engines provides a cost-effective new system with greater flexibility to meet the needs of the distributed power generation market.

Hybrid cycles are myriad. Typical fuel cell gas turbine configurations include topping cycles (where the fuel cell replaces a combustor and generator, and the gas turbine is the balance-of-plant) and bottoming cycles (where the fuel cell uses the gas turbine exhaust as an air supply and the gas turbine is balance-of-plant). In general, topping cycles lead to the highest efficiency systems with high oxygen concentration at the cathode, fewer cells required in the fuel cell stack as compared to low pressure systems, and higher power density. Bottoming cycles perform well depending on fuel cell type and are simple to integrate, easy to start, and simple to control. To achieve high efficiencies, most of the electricity of a hybrid system is produced in the fuel cell (typically between 70 and 80%).

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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