The Role of Solid Oxide Fuel Cells in Advanced Hybrid Power Systems of the Future

In pursuing the implementation of highly efficient, emission-free power, the U.S. Department of Energy (DOE) is looking to the development of hybrid power systems that make use of the coupling of an electrochemical device with a heat engine, or more specifically, a solid oxide fuel cell (SOFC) and a gas turbine.1-4 The synergies of coupling these systems in a hybrid configuration provide the potential for reaching the highest possible electric conversion efficiency ever realized.5 As such, advanced hybrid power systems that incorporate a fuel cell and a gas turbine represent fossil or renewable energy production technology that provide the opportunity for a significant improvement in generation efficiency.6 An example of a simplified process diagram of the power cycle in a hybrid fuel cell gas turbine is shown in Fig. 1. While much of the DOE-sponsored research focuses on improving the performance of solid oxide fuel cells, a hardware simulation facility has been built by the Office of Research and Development at the National Energy Technology Laboratory (NETL) to explore both synergies and technical issues associated with integrated hybrid systems. The facility is part of the Hybrid Performance (Hyper) project, and is made available for public research collaboration with universities, industry, and other research institutions. The Hyper facility is capable of simulating high temperature fuel cell systems from 300 kW to 700 kW coupled with a 120 kW turbine. The purpose of the Hyper project is to specifically address this higher risk research by combining the flexibility of numerical simulation with the accuracy of experimental hardware.7 An illustration of the Hyper facility is shown in Fig. 2. The Hyper facility makes use of pressure vessels and piping to simulate the volume and flow impedance of the cathode and a burner controlled by a real-time fuel cell model running on a dSpace hardware-in-the-loop simulation platform to simulate the fuel cell thermal effluent. The hardware used to simulate the fuel cell is integrated with a 120 kW Garrett Series 85 auxiliary power unit (APU) for turbine and compressor system. The APU consists of single shaft, direct coupled turbine operating at a nominal 40,500 rpm, a two-stage radial compressor, and gear driven synchronous generator. The electrical generator is loaded by an isolated, continuously variable 120 kW resistor Fig. 2. Illustration of the Hybrid Performance (Hyper) simulation facility at NETL. Fig. 1. Simplified flow diagram of a representative direct fired, recuperated fuel cell gas turbine hybrid system.