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also an option. [3] Instead of burning fuels, concentrating solar power (CSP) [4] can be used to deliver the heat and power in such systems, enabling a carbon-neutral mCHP system. Furthermore, the use of solid-state technologies, such as Na-TEC, to convert energy from CSP exhibits key advantages over the more mature and economically viable turbomachinery used in current CSP systems. [5] In addition to being carbonneutral, the principle difference between solid-state energy conversion and turbomachinery is the lack of moving parts, which can strongly impact the system lifespan and maintenance requirements, and by extension, the levelized cost of electricity (LCOE). A sodium thermal electrochemical converter (Na-TEC), which is a special variant of an alkali metal thermal-electric converter, [6] can theoretically achieve thermodynamic conversion efficiencies above 45% and rejects heat at a sufficiently high temperature (≈550 K), [7] making it amenable to mCHP applications. [8] Na-TECs convert heat directly into electricity, without moving parts, via the isothermal expansion of Na ions through a beta″-alumina solid-electrolyte (BASE). [6] In the Na-TEC, heat vaporizes Na near ambient pressure in an evaporator. A thermally generated pressure difference drives Na ions through the converter. At the triple phase boundary of Na-anode-BASE, the Na atoms ionize (Na → Na + + e − ) and the electrons simultaneously traverse an external load. A thermally generated pressure difference drives Na ions through the converter. The electrons and ions then recombine on the cathode-BASE interface and the Na atoms return to the vapor phase at lower pressure. This low pressure Na vapor is then cooled and condensed to the liquid phase in a condenser and a wick passively returns the liquid Na to the evaporator. The isothermal ion expansion process, where heat is directly converted to work, is responsible for the high ideal efficiency of this cycle. In one of our separate publications, [7] we reviewed the technical operation of this conventional single-stage Na-TEC and revisited the equations that describe it in more detail. We also reviewed and discussed the key factors and challenges that affect the performance of singlestage Na-TEC. More recently we showed that a multistage Na-TEC can achieve a high practical conversion efficiency by lowering the average device temperature, which in turn reduces A sodium thermal electrochemical converter (Na-TEC) generates electricity directly from heat through isothermal expansion of sodium ions across a beta″-alumina solid-electrolyte. This heat engine has been considered for use with conventional concentrating solar power (CSP) systems before. However, unlike previous single-stage devices, the improved design uses two stages with an interstage reheat, allowing more economical and efficient conversion up to 29% at a hot side temperature of 850 °C. Herein, a cost-performance analysis for this improved design assesses opportunities for distributed-CSP in the context of micro-combined heat and power syste...

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 Sun Shot Initiative.

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Sodium thermal electrochemical converters (Na-TECs) offer a high efficiency advancement for converting thermal energy into electrical energy without moving parts. Since the cell operates using a Na pressure difference between the high temperature evaporator and the lower temperature condenser, a hermetic seal capable of maintaining that pressure difference is essential. This study looked at brazing of the ceramic electrolyte used in these cells, which is a β”-alumina solid-electrolyte referred to as BASE. Since a literature search found no papers pertaining to brazing of BASE, knowledge from ceramic to metal brazing called widegap brazing was used. Specifically, the widegap brazing of α-alumina to nickel-based alloys. Initial brazing trials used a traditional inert atmospheric brazing technique with an Ar-H2 gas mixture. However, the very low pO2 atmosphere resulted in the destruction of the BASE layers due to the diffusion of carbon to the outer surface of the electrolyte during brazing. A new and radically different brazing technique called air brazing was then attempted. This brazing technique proved successful using the brazing alloy Ag-8CuO. Both Ag and Cu are not deleteriously affected by Na corrosion; thus, Ag-8CuO were a good choice for the braze alloy. Leak tests were performed on these cells to establish their hermeticity. This cell structure and brazing technique proved to be successful. Air brazing is an exciting joining operation for these types of cells.