Optimization of a MCFC/Turbine Hybrid System for Cogeneration

Abstract

High temperature fuel cells can be integrated in a hybrid cycle with a gas turbine and achieve lower heating value (LHV) efficiencies of about 70%. A hybrid cycle designed for cogeneration applications could lead to even higher LHV efficiencies such as 78% to 80% without post combustion and 85%–90% with post combustion. The purpose of the present paper is to optimize the integration of a high temperature fuel cell in a cogeneration cycle. We used Gatecycle™ heat balance software by GE Enter Software, LLC, to design a 20–80 MW high efficiency cogeneration plant. Since Gatecycle™ does not have an icon for the fuel cell, we calculated the heat balance for the fuel cell stack in Microsoft® Excel and we imported the results into Gatecycle™. We considered a 8.5 MW, a 17 MW and a 34 MW fuel cell by scaling up of the commercially available 3MW molten carbonate fuel cell (MCFC). Our goal was to evaluate the optimum ratio between the fuel cell size and gas turbine size using a family of curves we developed showing LHV “electric” efficiency versus power for different ratios of “fuel cell–to–gas turbines size”. Similar curves showing LHV “cogeneration” efficiency are also presented. In addition configurations with a back pressure steam turbine and with a condensing steam turbine are evaluated. The influence of steam generation pressure in the overall system efficiency is discussed, as well as the performance of the hybrid system for different temperatures (0°F–80°F) and elevations (0 ft–3000 ft). Our conclusion is that high temperature fuel cells in a hybrid configuration with gas turbines could be successfully integrated into a cogeneration plant to achieve very high efficiencies.