Introduction

Ztek's Planar Solid Oxide Fuel Cell (SOFC) system has exceptional potential for utility electric power generation because of: simplicity of components construction, capability for low cost manufacturing, efficient recovery of very high quality by-product heat (up to 1000°C), and system integration simplicity. Utility applications of the Solid Oxide Fuel Cell are varied and include distributed generation units (sub-MW to 30MW capacity), repowering existing power plants (i.e. 30MW to 100MW), and multi-megawatt central power plants.

A TVA/EPRI collaboration program involved functional testing of the advanced solid oxide fuel cell stacks and design scale-up for distributed power generation applications. The emphasis is on the engineering design of the utility modules which will be the building blocks for up to megawatt scale power plants. The program has two distinctive subprograms: Verification test on a lkW stack and 25kW module for utility demonstration. A lkW Planar SOFC stack was successfully operated for 15,000 hours as of December, 1995. Ztek began work on a 25kW SOFC Power System for TVA, which plans to install the 25kW SOFC at a host site for demonstration in 1997. The 25kW module is Ztek's intended building block for the commercial use of the Planar SOFC. Systems of up to megawatt capacity can be obtained by packaging the modules in 2-dimensional or 3-dimensional arrays.

The Ztek SOFC Power System Technology offers to TVA the benefits of clean, low cost electricity resulting from low capital cost and ultra high efficiency. In addition to the potential for competitive low cost of electricity, the following additional benefits apply: Maximum Megawatt Capacity per Distributed Site; Maximum Use of Renewable Fuels when Landfill Gas and Biogas are utilized; High Power Quality; Low NOx, SOx and HC Emissions; and Ease of CO2 Capture.

The above leads to significant utility benefits like Customer Service and Retention, Distributed Generation Service, Response to Global Warming Concerns, and Repowering Opportunities.

FUEL CELL/GAS TURBINE BOTTOMING HYBRID SYSTEM

Ztek is proceeding on development of an ultra-high efficiency hybrid system of its Planar SOFC with a gas turbine, realizing shared cost and performance benefits. The gas turbine as the Balance-of-Plant was a logical selection from a fuel cell system perspective because of: 1) the high-power-density energy conversion of gas turbines; 2) the unique compatibility of the Ztek Planar SOFC with gas turbines; and 3) the availability of low-cost commercial gas turbine systems. A Tennessee Valley Authority/Ztek program is ongoing, which addresses operation of the Advanced Planar SOFC system and design scale-up for utility power generation applications.

The advanced SOFC Fuel Cell/Gas Turbine hybrid approach discussed heie4 Figure 1, is potentially capable of reaching electrical efficiencies above 70% (LHV), or heat rate of less than 4800 BTU/kWh (LHV). This alternative has the distinct advantage of being applicable over a wide range of plant capacities, from sub-MW to multi-MW. Ztek's planar SOFC, which operates at 1000°C, has patented features which enhance direct integration with a gas turbine. This approach is based upon applying Ztek's Planar ATI® SOFC as a Combustor And Recuperator Replacement for Advanced Turbine System (CARR-ATS). The SOFC will replace the combustor section, and displace the need for a recuperator for efficiency enhancement Integrating Ztek's patented technology, therefore, can provide increased system efficiency and capacity with reduced NOx emissions.

Assessments revealed the ability to obtain performance with Pressurized SOFC/Gas Turbine Hybrid systems which surpass performance that can be achieved by either a Stand-Alone Gas Turbine or a Stand-Alone Fuel Cell. Figure 2, SOFC/Turbine Hybrid System Combined Efficiency, illustrates the predicted maximum system efficiency (LHV) as a function of gas turbine efficiency, considering the fuel cell efficiency from 50% to 55% (LHV). The maximum combined system efficiency ranged from 64% to 71%, for gas turbine efficiencies of 25% to 35%. This hybrid system approach has the benefit of utilizing gas turbine equipment which has already reached an economic scale consistent with the utility industry. The gas turbine industry can in turn benefit by the improved efficiency and environmental performance of the hybrid system. Ztek, applying its patented technology, is uniquely capable of formulating hybrid systems of its Planar ATI® SOFC with gas turbines which can achieve ultra efficient system performance.

SOFC/GAS TURBINE SYSTEM FOR DISTRIBUTED GENERATION

The incentives and justification for the pressurized operation of the Ztek Advanced SOFC integrated with gas turbine bottoming systems are: the physical and chemical stability of its fuel cell components; structural compatibility with operation at high pressures; compact integration; higher surface power density provision of the fuel cell; and availability of high temperature exhaust gas. For verification of this integrated technology, a development program of the SOFC/Gas Turbine hybrid system, Figure 3, is being initiated, leading to a demonstration of integration with a nominal combined capacity of 250kW in the time frame of 1998-1999. This effort addresses a technology enhancement to the SOFC modules developed for atmospheric operation, which applies to stand-alone or SOFC/Steam Turbine hybrid systems. The complementary components sharing in the SOFC/Turbine integration assures accomplishing the system price target of <$1000/kW.

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