A sodium thermal electrochemical converter (Na-TEC) converts heat directly into electricity without moving parts by isothermal expansion of ions through beta"-alumina solid-electrolyte (BASE). These generators are most similar to thermoelectric generators; however, they are considerably more efficient than the best performing thermoelectric materials. While these heat engines have been considered for CSP applications, literature review found that the efficiency of single-stage Na-TEC could readily achieve 20% even though ideal cycle efficiencies predict above 45% efficiency at elevated temperatures. Thermal parasitic loss has been identified to be responsible for the largest drop in the efficiency. Our recent study shows that staging helps to improve thermal management of the Na-TEC, due to the lower average temperature of the device, which can reduce the thermal parasitic loss. We demonstrate that dual-stage device can improve the efficiency by up to 8% over the best performing single-stage device. We are currently designing and developing a modular dual-stage Na-TEC power block with target efficiency of 33%. We emphasize modularity because this power block can be potentially deployed for both small-scale dish solar, which is appropriate for distributed residential scale (2-3 kWe), and large-scale heliostats and parabolic trough CSP, which is appropriate for centralized industrial scale. A fundamental cost-scaling relationship for this technology was developed based on this design. System variables and component manufacturing methods with material selection for processes were established. The current off-the-shelf component costs indicated an overnight capital cost of $2,044/kWe. The costs of BASE, manufacturing, and electrode preparation have driven the overall price of the module. The paper demonstrates $/W design optimization and cost scaling analysis to reduce the system capital $/W metric below $1,500/kWe, with the goal being to achieve the cost target of <900/kWe set by Department of Energy's SunShot Initiative.
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