Integration of Solid-Oxide Fuel Cells and Compressed Air Energy Storage for Peaking Power with Zero Carbon Emissions

In recent years, sustainable production of environmentally friendly electricity has become a major topic of discussion and research worldwide. One promising technology is the solid-oxide fuel cell (SOFC) power generation system [1]-[4]. An SOFC utilizes a carbonaceous fuel gas (synthesis gas, for example) and an oxidant (typically air) to efficiently produce electrical power through an electrochemical reaction across a solid oxide barrier. There are key advantages to using SOFCs for power production. For example, the anode exhaust is mainly H₂O and CO₂, which are easily separated if the anode and cathode streams are not mixed downstream, and the cathode exhaust is mostly N₂, which can be used for additional power generation or heat recovery and then safely vented. However, a significant disadvantage of the SOFC system is that there are currently cost-prohibitive operational challenges associated with its dynamic use, which limits the usefulness of an SOFC system for following a typical diurnal demand profile for electrical power.

This work investigates a novel system that integrates a natural gas fuelled SOFC system for base load power production (Adams and Barton (2010)) with a compressed air energy storage (CAES) plant for load-following capabilities [1]. This new system takes advantage of the already hot and compressed cathode exhaust by temporarily storing it underground with a relatively low parasitic energy penalty. The SOFC/CAES plant may be switched from storage mode (where the CAES system consumes power to store compressed cathode exhaust) to expansion mode (where the CAES system generates power by releasing stored compressed exhaust through a turbine). The plant operates in storage or expansion mode depending on whether the current power demand is lower or higher than the base power output provided by the SOFC system, respectively. The cathode and anode exhaust streams of the SOFC system are not mixed, and thus 100% CO₂ capture from the anode exhaust can be maintained at all times. Simulations of the combined system under a variety of charging and discharging conditions are performed in Aspen-Plus and dynamic simulations of the load-following scenarios and dynamic mass balances on the storage cavern are executed in MATLAB.

Simulation results based on real scaled market demand and pricing data show promising results for the SOFC/CAES system with regard to both its ability to provide peaking power as well as improve gross revenues due to hourly variations in electricity pricing. Moreover, the addition of CAES turbomachinery allows for effective load-following capabilities to be added to the SOFC system with very minor reductions in overall plant efficiency (~1% HHV). Furthermore, unlike other standalone CAES plants, the SOFC/CAES hybrid plant does not require any additional air or fuel in order to operate the CAES section; all of the required electricity, heat, and compressed air can be obtained from the SOFC and its waste streams. Therefore, the SOFC/CAES system provides load-following power generation from fossil fuels with essentially zero CO₂ emissions at high efficiencies. A techno-economic analysis of the combined SOFC/CAES system is also performed, and its profitability is discussed and compared to other systems.

References

[1]        Adams T.A. II, Barton, P.I. High-efficiency power production from natural gas with carbon capture. Journal of Power Sources. 2010, 195, 1971.

[2]        Adams T.A. II, Barton P.I. High-efficiency power production from coal with carbon capture. AIChE J. 2010, 56, 3120.

[3]        Duan, L.; Yang, Y.; He, B.; Xu, G. Study on a novel solid oxide fuel cell/gas turbine hybrid cycle system with CO₂ capture. Int. Journ. of Energy Res. 2012, 36, 139-152.

[4]        Becker, W.L.; Braun, R.J.; Penev, M.; Melaina, M. Design and technoeconomic performance analysis of a 1 MW solid oxide fuel cell polygeneration system for combined production of heat, hydrogen, and power. Journal of Power Sources. 2012, 200, 34-44.