Using Staged Compression and Expansion to Enhance the Performance of a Gas Turbine Fuel Cell Hybrid System

It has long been recognized that the heat generated from a solid oxide fuel cell (SOFC) is adequate to drive an external heat engine. The combination of the fuel cell plus the heat engine is called a gas turbine fuel cell hybrid power generation system. In most hybrid systems the heat engine consists of a single compressor and single turbine, arranged in either a Brayton cycle or a recuperated Brayton cycle. One characteristic of hybrid power cycles is that the compression costs are substantial. When this cycle is used in a coal fired hybrid system that is configured with an isolated anode stream to isolate and compress CO₂, the work to compress the cathode air can greatly exceed the work to compress the CO₂. It has also been shown for this same system that using intercooled compression for the cathode air reduces this compression cost. Since there have been no exhaustive studies performed which quantify these effects it is not clear exactly how much reduction in compression cost is possible. In this work we compare three hybrid systems. The first systems has a single compressor and turbine, run at a low pressure ratio as a recuperated Brayton cycle and at high pressure ratio as a simple Brayton cycle (see Figure 1). We then alter the recuperated Brayton cycle using both staged compression and staged expansion. The second system is thus configured with two compressors and two turbines. For this system an intercooler is placed between the compressors and the fuel cell stack is divided into two stacks each followed by a turbine (see Figure 3). Similarly the third system divides the compression and expansion legs of the cycle again into three compressors with intercoolers, and three fuel cell stacks each followed by its own turbine (see Figure 5). As the system configuration is altered by successive divisions of both the compression and expansion legs of the thermal heat engine cycle, the system configuration is transformed from a simple Brayton cycle to a staged approximation to an Ericsson cycle. We show that this new configuration for the gas turbine fuel cell hybrid system not only reduces the high cost of compression, but it makes more heat available for auxiliary system operations. In coal fired systems these auxiliary operations would include pre heating coal for the gasification system, reheating the syngas after cooling or even heating steam for a bottoming cycle.