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AMTEC

The Alkali-Metal Thermal to Electric Converter (AMTEC) is a device for direct transformation of heat into electric energy. Most of them developed up to date, use sodium. Their key component is a beta”-alumina solid electrolyte (BASE). As a special feature of this ceramic, sodium ions can hop through its crystal structure so that it is permeable for sodium ions, but it is an isolator for electrons and neutral sodium atoms.

In an AMTEC cell, the BASE separates two regions, both filled with sodium. One of them is heated – in our case by solar power – to a temperature of up to 1000 °C, the pressure being about 0.2 MPa. In the other region, temperature may be below 400 °C and pressure as low as about 10 Pa. In the high temperature region, sodium ions may form at the metallic coating of the BASE, the anode. The electrons are collected at the anode and redirected to the cathode on the cold side of the BASE via an external load in order to produce electricity. At the cathode, the electrons recombine with sodium ions to form sodium vapour. It is condensed and transported back to the hot side so that the cycle can restart. As a thermodynamic cycle, it corresponds nearly to an isothermal expansion of sodium between the two pressures on either side of the BASE. The theoretical value for conversion efficiency is about 40 %; presently 20–25 % has been reached.

AMTEC has several advantages, such as a larger efficiency comparative to other direct converters, a large power-to-weight ratio, and no moving components are necessary. Even for pumping, a MHD pump or a wick can be used to avoid moving components. For this reason, it is very silent in operation and quasi maintenance free. In addition, its response time is short. Furthermore, for commercial applications, the conversion facility can be erected in a modular form.

Though the principle of this converter exists since the 1960s, several issues are still open. The related experimental research in our institute aims at the development of improved designs for AMTEC cells to assess its usage in a concentrating solar power (CSP) plant as a topping system, increasing therefore the global efficiency of the plant. It will address issues as material investigations, improvements, and development, studies of different metal-ceramic interfaces, a characterization of the BASE stability and improvement for long term operation, extension of the operational life time, as well as the increase of the efficiency of the AMTEC conversion process.

The studies will benefit from our in-house experimental experience of about 10 years, a period in which several AMTEC prototypes were tested in our institute. They will be performed in the frame of the HGF LIMTECH (LIquid MEtal TECHnology) and HGF HEMCP (Helmholtz Energy Materials Characterization Platform) projects. In the frame of the HEMCP project, the investigations will be performed in close collaboration with other institutes within KIT.