Thermo-economic Optimization of a Solid Oxide Fuel Cell, Gas Turbine Hybrid System

Large scale power production benefits from the high efficiency of gas-steam combined cycles. In the lower power range, fuel cells are a good candidate to combine with gas turbines. Such systems can achieve efficiencies exceeding 60%. High temperature Solid Oxide Fuel Cells (SOFC) offer good opportunities for this coupling. In this paper, a systematic method to select a design according to user specifications is presented. The most attractive configurations of this technology coupling are identified using a thermo-economic multi-objective optimization approach. The SOFC model includes detailed computation of losses of the electrodes and thermal management. The system is integrated using pinch based methods. A thermoeconomic approach is then used to compute the integrated system performances, size and cost. This allows to perform the optimization of the system with regard to two objectives: minimize the specific cost and maximize the efficiency. Optimization results prove the existence of designs with costs from 2400 $/kW for a 44% efficiency to 6700 $/kW for a 70% efficiency. Several design options are analysed regarding, among others, fuel processing, pressure ratio or turbine inlet temperature. The model of a pressurized SOFC-μGT hybrid cycle combines a state-of-the-art planar SOFC with a high speed micro gas turbine sustained by air bearings. ⇤Address all correspondence to this author. INTRODUCTION For decentralized electricity production, the solid oxide fuel cells (SOFC) have emerged in the last years as an ideal candidate to be combined with gas turbines. This kind of hybrid system takes advantage of the SOFC high operation temperatures to valorize the fuel energy. Many studies have assessed the feasability and operating conditions of such systems, proposing a variety of design alternatives. The U.S. Department of Energy high efficiency fossil power plant program has demonstrated [1] the feasability of a low cost SOFC-GT system of 220 kWe, integrating a Mercury 50 gas turbine and a Siemens Westinghouse SOFC. Such systems achieve efficiencies of 60% for an expected installation cost of 1170 $/kW . Massardo and Magistri [2] analyzed pressurized and atmospheric systems with efficiencies varying from 65 to 75%. The latter work also presented a thermo-economic analysis of the system components. At the present time, few studies have approached the design of hybrid SOFC-GT systems as an optimization problem. Yi et al. [3] have optimized an internal reforming solid oxide fuel cell and intercooled gas turbine hybrid cycle using analysis tools based on a design of experiments (DOEx) approach. For plants of around 600 MW, efficiencies higher than 75% based on LHV are reached. Marechal et al. [4] have demonstrated a method to optimize PEM fuel cell systems that integrate a micro-gas turbine. The same approach has been applied by Palazzi et al. [5] on SOFC systems and will be applied here for the design of hybrid SOFC microGT. 1 Copyright c 2005 by ASME Acell Single cell area m2 Ccell Cell cost $ Cct Compressor and turbine cost $ CDriveSpec Drive specific cost $.kW 1 Ch,spec Stack Housing specific cost $.cm 2 CFC,spec Cell surface specific cost $.cm 2 CFCstack Fuel cell cost $ CmicroGT Complete μGT cost $ CVP Volume purchase cost $ LHV Lower heating value kJ.kg 1 Nstack Number of stacks Psha f t Available mechanical power kW Pelec Net electrical power kW PC,out Compressor outlet pressure Pa R Gas constant J.kmol 1 Tfuelcell Fuel cell temperature K Texhaust Exhaust gas temperature K Tr Reformer temperature K ci Polynomial coefficients -