Improving Hybrid Efficiency and Flexibility by Integrating Thermal Energy Storage into the Fuel Cell System

D. Tucker, N.F. Harun, V. Zaccaria, L. Shadle
National Energy Technology Laboratory,
United States

Keywords: fuel cell, energy, efficiency

Summary:

Power plants must be flexible in operation to meet changing load requirements throughout the day while ensuring economic feasibility. As renewable energy systems penetrate the market producing inconsistent power, the primary concern of power plants today is being able to load follow. Thermal energy storage (TES) can be added to the system to increase its flexibility. Hybrid systems allow multiple technologies to be integrated and better system flexibility. This reduces dependency on fossil fuels. Solid oxide fuel cells (SOFC) operate between 600 and 1000 ˚C at high efficiencies. It has been shown that TES can be integrated into Combined Heat and Power (CHP) systems with gas turbines to better match power production and electricity demand. TES, when combined with renewable energy, takes care of intermittent power production [1–3]. SOFCs can be used as storage devices when operated in reverse as an electrolysis cell [4, 5]. Another means of storage is to integrate the SOFC with metal hydride tanks for hydrogen storage were also studied [6–8]. However, with NETL’s concept there is no separate energy storage device; minimizing additional system costs. SOFC have high heat capacities, mainly due to their large interconnect mass. The interconnect material, typically stainless steel, has a larger specific heat than the cathode or anode material. For a standalone SOFC system, the interconnect material is typically minimized to reduce the cost of the fuel cell [9]. However, to optimize system performance, SOFCs have a potential to be used directly as TES for active power control and to increase the flexibility of a hybrid system. NETL’s concept is to incorporate TES directly into the fuel cell design [10]. This concept was developed using the hybrid performance project facility [11]. By increasing the mass and specific heat of the interconnects within the fuel cell, the overall heat capacity and thermal energy storage can be increased. For example, a 400 kW SOFC has ~2GJ of stored energy. This could be easily doubled substantially improving the thermal energy storage capacity. During transient operation heat could be stored or removed without adversely affecting the temperature gradient within the fuel cell. This dramatically increases system flexibility. Even at steady state the temperature profile is more uniform (Figure 1). Another benefit of increasing interconnect size is the resulting increase in stack efficiency (Figure 2). The reason is that the increased cross section area in the interconnects reduces ohmic losses and the more uniform temperature profile reduces activation losses. System efficiency is also improved by the reduced need for cooling airflow. Adding storage to the SOFC with the interconnect material reduces the resistance to heat conduction and ionic flow. Efficiency of the fuel cell is increased by an increased voltage due to increased cross-sectional area and reduced polarizations. In a hybrid system, flexibility is increased dramatically by being able to extract thermal energy and convert that thermal energy to electricity in a recuperated turbine cycle.