Direct FuelCell^® Turbine

Fuel Cell Energy Inc. has developed a hybrid concept combining the Direct FuelCell (DFC®) and an unfired gas turbine. This hybrid system uses a network of heat exchangers to transfer waste heat from the DFC system to the turbine, which converts a portion of the waste heat to mechanical energy and then electricity. The system adds 10 to 15 percentage points to the efficiency of the DFC. For large systems in the long term, unsurpassed net electric efficiency of close to 80% is possible. In the nearer term, it is believed that cost effective small MW class hybrid systems can be configured with efficiencies of 70% or better. Although power plants utilizing this system are not yet available, the design is showing promise in its development stage. Under a Department of Energy-supported Vision21 program, activities are underway to operate a subscale system and to develop an ultra-high-efficiency 40MW powerplant design.

What distinguishes this hybrid concept from many other proposed hybrid systems is the novel approach of integrating the turbine with an atmospheric pressure fuel cell and recovery of the waste heat in a Brayton cycle. The fuel cell does not need to operate at the turbine pressure, instead it operates at the preferred ambient pressure and is independent of gas turbine cycle pressure ratio. The system works efficiently with a wide range of turbine compression ratios (3 to 15). This means that in principle the concept can be applied from the multi-MW scale (with industrial size turbines operating at 9 to 16 pressure ratios), to smaller systems using microturbines at a lower pressure ratio. Key features of the system include:

• Fuel Economy—The DFC/Turbine hybrid has the potential for achieving an electric efficiency of more than 70% in the power plants as small as the single MW size range, reducing fuel consumption by a factor of two for power plants in this size range.

• Unmanned Remote Operation—The design approach preserves the ambient pressure operation of fuel cell, avoiding the need for a high-pressure boiler. The power plants can be sited almost anywhere, including unattended remote locations, without the need for an operator and without complications from local safety codes for high-pressure boiler operation.


• Simplicity in Design—The system retains much of the simplicity of our direct fuel cell technology which eliminates fuel processing equipment including external reformer and shift reactors. This results in the most efficient and reliable fuel cell power plant configuration.

• Low Pressure Fuel—The required fuel supply pressure is at the natural gas line pressure which is typically available at about 15 psig on most commercial sites. A fuel compressor is not required.

• Load Following and Reliability—The turbine section can be used to load follow utilizing stored kinetic energy, while the fuel cell is efficiently operated at constant power. This is only possible because of the decoupled nature of the fuel cell and turbine sections of the plant. This feature, not realizable in other pressurized fuel cell systems, imparts attractive load following characteristics in grid-independent applications. If an air blower is incorporated in the system, it will be possible to operate the fuel cell while the turbine is being maintained, which could be a major reliability benefit.

• Environmental Benefits—The Brayton cycle in the hybrid system is an unfired system, indirectly heated with fuel cell waste heat. This results in low NOx generation and yields the highest efficiency, since all primary fuel consumption is done in the fuel cell - which is the more efficient portion of the system. The hybrid system has near lower emissions per kWh than our simple cycle power plants, due to the higher efficiency.