Project B4: Phase changes in liquid metals for direct energy conversion

PI: Wolfgang Hering (KIT)
Partners: KIT-CN, KIT-CS

1. Scientific case for the project

1.1 Background

In power conversion systems, a gap exists between the range of Rankine cycle up to 550°C (limited by increased pressure) and the Brayton cycle, which has very low efficiency below 700°C. Above 550°C and especially at high system pressures affordable materials are very scarce. Reducing the pressure to ambient levels allows higher temperature and less material consumption. A very efficient direct energy conversion is known for the case of phase changes, i.e. processes involving condensation and evaporation. The project considers the Alkali Metal Thermo-Electric Converters (AMTEC) which runs on ambient pressure level.

Fig. 9: Schematics of an AMTEC cell.	Fig. 10: Central component of an AMTEC device built at KIT.

AMTEC is one of the most promising technologies for direct conversion of thermal energy to electricity with a high efficiency close to the Carnot value. The major component (see Fig. 9) is the solid electrolyte, which allows the alkali metal sodium to penetrate as ion. The sodium ions are driven by thermodynamic potential caused by a temperature difference across the electrolyte. The electrolyte, ß-Aluminate Solid Electrolyte (BASE), is an electrical isolator, so the electrons cannot penetrate. This leads to an electrical potential across the BASE, which can supply power to an external load as indicated in Fig. 9. As investigated some years ago on a small experiment (see Fig. 10), the critical issue are the electrodes on both surfaces of the BASE, because competing processes limit the density of the electrodes and hence the achievable current density.
At the hot side, the receiver temperature is above saturation so that sodium steam is produced. At the cold side the sodium is cooled by cold sodium or another liquid metal at temperatures of appr. 350°C. The excess energy can be stored in a high temperature thermal storage device as shown in Fig. 11. The sodium is circulated back to the hot side by a pump, which is the only active part of the system.
Fig. 11: Scheme of a general Power Conversion System including AMTEC.

Based on previous experiences [1,2] and research ongoing in USA [3] and China [4] several challenges were identified, thereof the most stringent two are encountered by the time-dependent power degradation of the BASE and is stability as well as the relatively low efficiency compared to its theoretical value. However, compared to other direct thermal to electric converters (DTECs), such as the thermoelectric converter (TEC), thermionic converter, and thermo-photovoltaic converter, the efficiency is significantly higher.

1.2 Most important goals of the planned work

First goal is to recover the technology and to solve technical problems associated with the BASE and the electrodes. This will also provide education capabilities on liquid metal phase change technology within the Alliance. Final objective is an operating two phase AMTEC device, which allows optimizing thermal hydraulics and flow/power control, in particular for power and thermal transients.
Since AMTEC is considered as a part of a PCS (Power Conversion System, Fig. 11), the implementation is optimized, including a high-temperature stratified storage system. In a next step, an optimal coupling strategy of the AMTEC cold side to the storage will be elaborated. The activities are supported by analytical investigations which will simulate the AMTEC and the thermal storage pool and, thus, allows to simulate the most significant parts of the PCS.
AMTEC is considered as a favorable topping system in order to enhance the efficiency of concentrated solar power CSP systems. On the cold side of AMTEC a temperature level of 520°C is sufficiently high to feed conventional Rankine cycles for high efficient conventional power production.
With such a combined system, the efficiency can be significantly higher than at existing CSP systems. For base load operation, a thermal storage is integrated to decouple the receiver from the PCS allowing stable and permanent operation. The permanent operation allows taking benefit from highly optimized steam turbines and opens the market to base load electricity production. The compact layout of CSP systems allows a safety oriented design, based on long term experience with sodium at KIT. In AMTEC, the principle is similar to the sodium-sulfur battery; sodium ions travel through a solid electrolyte and produce a voltage between the surfaces, since the electrolyte is a ceramic isolator.
The DC Voltage can be coupled via DC/AC converter directly to the grid, or the voltages can be added to feed a high voltage DC transmission line, so that the conversion losses are minimized. Since only sodium is circulating, it can quickly react to changing needs in the grid.

2. Existing competencies and infrastructure

Already between 1990 and 2000 investigations of AMTEC were focused on its application to energy systems. The activities took credit from ongoing developments of the sodium sulfur battery (NaS-battery), especially the solid electrolyte based on aluminum oxide. However, that promising project was stopped due to financial difficulties and technical problems associated with the limited stability problems of the available electrolyte. Today, such electrolytes are available and used in industry (Japan) for NaS Batteries to buffer wind power and to level electric power consumption in Japanese cities.
The activities will be performed in the Karlsruhe Sodium Laboratory (KASOLA), which is currently erected at the Institute for Neutron Physics and Reactor technology (INR) of KIT.

3. Resource Planning and Budget Justification

Although the AMTEC principle is not new, the high temperature and the material compositions (metal/ceramic) as well as the fluid dynamic optimization require a wide range of expertise as available within the LIMTECH alliance. Starting with the available experiences and technology and extending it to two phase liquid metal applications is a challenge, which requires at least an additional scientist, responsible for planning the experiments and the devices, data acquisition and reduction. The PhD at KIT-CS will focus on the optimization of the device, the fluid dynamics, and the surface structures to stabilize the evaporation process.

Links: There are close links to the projects A5, B2 and B5.

References

[1] Steinbrueck, M., Heinzel, V., Huber, F., Peppler, W., Will, H., Voss, M., 1993, Investigations of beta-alumina solid electrolyte for application in AMTEC cells. Proc.of the 28th Intersociety Energy Conversion Engineering Conf. (IECEC-93), Atlanta, August 8-13, Vol. 1, 1.799-1.807.
[2] Heinzel, V., Huber, F., Peppler, W., Steinbrueck, M., Voss, M., Will, H., 1993, AMTEC-Zellen. Ergebnisse des Kernforschungszentrums Karlsruhe. Sonnenenergie, Nr.2, 14-16.
[3] M.A.K. Lodhi, A. Daloglu, 2000, Performance parameters of material studies for AMTEC cell, Journal of Power Sources 85, 203–211.
[4] Shuang-Ying Wu, 2009, A review on advances in alkali metal thermal to electric converters (AMTECs) Int. J. Energy Res. 33, 868–892.
[5] W. Hering, R. Stieglitz, 2011, Qualification requirements for innovative instrumentation in advanced nuclear systems, PAMIR-8, Int. Conf. on Fundamental and Applied MHD, 529-533.