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SPECIAL SECTION ON THE 2ND EUROPEAN FUEL CELL TECHNOLOGY AND APPLICATIONS CONFERENCE

# A New Concept for High Temperature Fuel Cell Hybrid Systems Using Supercritical Carbon Dioxide

[+] Author and Article Information
D. Sánchez1

Escuela Técnica Superior de Ingenieros de Sevilla, Camino de los Descubrimientos s/n, 41092 Sevilla, Spaindavidsanchez@esi.us.es

R. Chacartegui

Escuela Técnica Superior de Ingenieros de Sevilla, Camino de los Descubrimientos s/n, 41092 Sevilla, Spainricardo@esi.us.es

Escuela Técnica Superior de Ingenieros de Sevilla, Camino de los Descubrimientos s/n, 41092 Sevilla, Spainfcojjea@us.es

T. Sánchez

Escuela Técnica Superior de Ingenieros de Sevilla, Camino de los Descubrimientos s/n, 41092 Sevilla, Spaintmsl@esi.us.es

1

Corresponding author.

J. Fuel Cell Sci. Technol 6(2), 021306 (Mar 04, 2009) (11 pages) doi:10.1115/1.3080550 History: Received January 29, 2008; Revised July 21, 2008; Published March 04, 2009

## Abstract

Hybrid power systems based on high temperature fuel cells are a promising technology for the forthcoming distributed power generation market. For the most extended configuration, these systems comprise a fuel cell and a conventional recuperative gas turbine engine bottoming cycle, which recovers waste heat from the cell exhaust and converts it into useful work. The ability of these gas turbines to produce useful work relies strongly on a high fuel cell operating temperature. Thus, if molten carbonate fuel cells or the new generation intermediate temperature solid oxide fuel cells are used, the efficiency and power capacity of the hybrid system decrease dramatically. In this work, carbon dioxide is proposed as the working fluid for a closed supercritical bottoming cycle, which is expected to perform better for intermediate temperature heat recovery applications than the air cycle. Elementary fuel cell lumped-volume models for both solid oxide and molten carbonate are used in conjunction with a Brayton cycle thermodynamic simulator capable of working with open/closed and air/carbon dioxide systems. This paper shows that, even though the new cycle is coupled with an atmospheric fuel cell, it is still able to achieve the same overall system efficiency and rated power than the best conventional cycles being currently considered. Furthermore, under certain operating conditions, the performance of the new hybrid systems beats that of existing pressurized fuel cell hybrid systems with conventional gas turbines. From the results, it is concluded that the supercritical carbon dioxide bottoming cycle holds a very high potential as an efficient power generator for hybrid systems. However, costs and balance of plant analysis will have to be carried out in the future to check its feasibility.

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## Figures

Figure 1

Recuperative gas turbine engine and Brayton cycle

Figure 2

Influence of turbine inlet temperature and pressure ratio on useful work

Figure 3

Influence of turbine inlet temperature and pressure ratio on efficiency

Figure 4

Influence of recuperator effectiveness and pressure ratio on gas turbine efficiency (TIT=950°C)

Figure 5

h-s diagram and layout of supercritical CO2 cycle

Figure 6

h-s diagram and layout of open air cycle

Figure 7

Hybrid systems integration schemes

Figure 8

Global efficiency versus current density at 1800 A m−2 (SOFC)

Figure 9

Specific power versus current density at 1800 A m−2 (SOFC)

Figure 10

Specific power versus temperature at 0.65 V (SOFC)

Figure 11

Global efficiency versus temperature at 1200 K (SOFC)

Figure 12

Specific power versus temperature at 1200 K (SOFC)

Figure 13

Specific power versus temperature at 1800 A m−2 (MCFC)

Figure 14

Global efficiency versus temperature at 1800 A m−2 (MCFC)

Figure 15

Specific power versus temperature at 0.8 V (MCFC)

Figure 16

Specific power versus temperature at 900 K (MCFC)

Figure 17

Global efficiency versus temperature at 900 K (MCFC)

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