(20a) Thermodynamic Analysis of Thermally-Integrated Stationary High Temperature Fuel Cell Systems with Absorption Chillers

Thermodynamic Analysis of Thermally-Integrated Stationary High Temperature Fuel Cell Systems with Absorption Chillers

Whitney G. Colella, Ph.D., M.B.A.

Principal Research Engineer, Gaia Energy Research Institute, Arlington, Virginia, 22203
Tel: +1-650-283-2701, Email: wgc@gaia-energy-research-institute.com, web:http://www.linkedin.com/in/wgcgaia

This research discusses thermodynamic analyses of thermally-integrated stationary high temperature fuel cell systems (FCS) with both absorption chillers and with more traditional electric chillers. This research describes the detailed thermodynamics of novel, stationary combined cooling, heating, and electric power (CCHP) FCSs using chemical engineering process plant models. Unrecovered heat from a high temperature FCS can be converted to cooling power for space cooling, refrigeration, and freezing via different thermodynamic cycles. Models developed here describe FCSs thermally coupled to absorption chillers to produce chilled water for space cooling and refrigeration. Unrecovered heat from the FCS provides heat to the absorption chiller’s thermally-driven generator(s). An absorption chiller uses heat to generate a refrigerant vapor in a boiler/generator, condenses the refrigerant by transferring heat to a medium temperature environment, and then evaporates the refrigerant at reduced pressure to absorb heat from the low temperature cooling load. Finally, a concentrated absorbent solution absorbs the refrigerant where heat is removed to a medium temperature environment before the now dilute solution is pumped back to the generator to repeat the cycle. Single, double, and triple-effect lithium bromide (LiBr) absorption chillers are evaluated. Models encapsulate the effect of absorption chiller pressure, coefficient of performance (COP), internal heat exchange, generator temperature, LiBr/water solution concentration, and LiBr/water solution crystallization temperature on CCHP FCS performance. Models also describe the thermodynamics of a solid oxide fuel cell (SOFC) system, including the behavior of cell voltage as a function of operating temperature, pressure, and current density. Model results support the conclusion that compared with conventional energy generators, CCHP FCS can be expected to reduce fuel consumption and greenhouse gas emissions by about 46%. For a CCHP FCS using a double effect LiBr chiller with a COP of ~1, model results indicate that the overall system efficiency (the summation of the electrical, heat recovery, and cooling efficiencies) can be as high as 99%. For such a system, model results indicate that the net electrical efficiency can be expected to range from 22% to 38%, and the net heat recovery efficiency can be expected to range from 28% and 45%. For such a chiller that recovers heat in the generator from a constant-temperature slip stream, the cooling recovery efficiency can be expected to be fairly constant at 35% of the higher heating value (HHV) of the fuel input. The performance of these CCHP FCSs is also compared with CCHP FCSs coupled with electric chillers.