Prospects

S.E. Veyo and W.L. Lundberg Westinghouse Science & Technology Center 1310 Beulah Road Pittsburgh, PA 15235-5098

Presently under fabrication at Westinghouse for EDB/ELSAM, a consortium of Dutch and Danish utilities, is the world's first 100 kWe Solid Oxide Fuel Cell (SOFC) power generation system. This natural gas fueled experimental field unit will be installed near Arnhem, The Netherlands, at an auxiliary district heating plant (Hulp Warmte Centrale) at the Rivierweg in Westervoort, a site provided by NUON, one of the Dutch participants, and will supply ac power to the utility grid and hot water to the district heating system serving the Duiven/Westervoort area. The electrical generation efficiency of this simple cycle atmospheric pressure system will approach 50% [net ac/LHV]. The analysis of conceptual designs for larger capacity systems indicates that the horizon for the efficiency of simple cycle atmospheric pressure units is about 55%. ' i '

An increase in operating pressure to ten atmospheres will yield about a ten percent increase in cell voltage, hence efficiency. At three atmospheres, the voltage increase is about half that at ten atmospheres. Previously reported conceptual design activity (1,2) for pressurized 3 MW and 5 MW SureCELLâ„¢ SOFC/Gas Turbine (PSOFC/GT) power plants operating at a pressure ratio of 8.8 and utilizing a highly efficient two shaft intercooled and recuperated gas turbine yielded combined cycle efficiencies of 61% and nearly 70%, respectively. The performance estimates for these plants have been refined, considering a reduction in SOFC power lead losses that is based upon recent experimental data, and a higher recuperator effectiveness of 93%. In addition, for the 3 MW plant, performance has been estimated for the case in which the reheat combustor at the power turbine inlet is not fired.

The objectives of the analyses reported herein were fourfold. The first was to document the improved performance potential of the two shaft turbine cycle given access to a better recuperator and lower lead losses, and the second objective was to assess the performance of PSOFC/GT combined cycles in the 3 MW plant application that are based on the use of a simple single shaft gas turbine having a design-point turbine inlet temperature that closely matches the temperature of the SOFC exhaust gas, about 850 C. The third objective was to estimate the performance potential of smaller combined cycle power plants employing a single SOFC submodule, and the fourth objective was to evaluate the cogeneration potential of such systems.

As in the previously reported analyses, the basic building block for an SOFC system is the 100 kW (atmospheric pressure) SOFC generator module (1152 cells, each 22 mm diameter by 1500 mm active length), now in manufacture. Using two such building blocks, each with an additional cell row (96 cells), an SOFC submodule was configured with 2496 tubular AES SOFCs in a common canister. This submodule has a nominal capacity of 600 kW dc at nine atmospheres pressure, and pressurized SOFC modules housing one, three, or four such submodules comprise the SOFC systems in the PSOFC/GT power plants that are discussed below.

Two cycles for pressurized SOFC/GT power plants based on a two-shaft gas turbine were described previously (2). In that gas turbine, the first shaft functions as a zero-net-power hot-gas generator, and ac power (1.4 MW) is produced by the power turbine on the second shaft. The gas generator is equipped with two stages of intercooled compression, and the compressor turbine has an inlet temperature requirement (861 C) that is very close to the temperature that is available at the exhaust of an SOFC module. The power turbine design inlet temperature is 863 C, and it is normally achieved by burning natural gas fuel at a reheat combustor. An 88%-effective recuperator in this regenerative gas turbine cycle contributes to its high operating efficiency (43%, LHV).

When a pressurized SOFC module with three SOFC submodules is placed ahead of the high pressure combustor in this cycle, the SOFC operating point can be selected such that no fuel is required by the combustor. This leads to a PSOFC/GT plant rating of approximately 3 MW net ac. Similarly, a second module of identical design can be installed ahead of the low pressure reheat combustor and operated such that the flow of fuel to that combustor is also zero. In that case, a plant of nominal 4.5 MW rating results. Equipping each SOFC module with a fourth submodule results in more power from the plant, in excess of 5 MW, and higher efficiency. Updated performance estimates for the 3 MW and 5 MW power plants are summarized in Table 1; the estimates are based on the reduced lead losses and a 93% effective recuperator.

Table 1. PSOFC/GT (Two-Shaft) Power Plant Performance Estimates

Plant

3 MW-class

I 5 MW-class

i 3 MW-class, j Unfired Power Turbine

Current Density, mA/cm2

0 0

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