Direct Conversion of Heat Energy to Electrical Energy (2)
Alkali Metal Thermal Electric Converter (AMTEC)
The AMTEC is an electrochemical device for the direct conversion of heat to
electrical power. It uses a recirculating
alkali metal (Sodium or Potassium) working fluid passing through a solid electrolyte in a closed circuit to produce
an electron flow in an external load.
AMTEC devices depend on the unique properties of some solid ceramic electrolytes such as β" or P" Alumina which, due to their crystal structure, are very good
conductors of ions but poor conductors of electrons.
The working fluid is driven around a closed thermodynamic cycle between a heat source and a heat sink held at different temperatures and, during the vapour phase of the cycle, the available work from the isothermal expansion of the working fluid as it passes through the electrolyte is converted directly into electric power.
AMTEC Operating Principle
The diagram below shows the basic system components.
The thermodynamic cycle works as follows:
- The solid electrolyte BASE which is a conductor of
positive ions but an insulator to electrons is located in the Sodium working fluid circuit and a high temperature difference is maintained across it.
- Heat is added at the anode side increasing the temperature of the Sodium to over 1000°K causing it to vaporise and its pressure to increase to over 20 kiloPascals.
- At the cold side of the device, heat is rejected so that the temperature falls to below 700°K and the pressure correspondingly falls to less than 100 Pascals. Despite it being the "cold" side, the temperature is still relatively high because the Sodium must be kept in liquid form.
- At the anode surface of the BASE the neutral Sodium atoms in the vapour are ionised releasing electrons. (Oxidation - the atom loses an electron) The resulting Sodium ions absorb the latent heat of vaporisation.
- Due to the high pressure difference across
the BASE and its differential conductivity between electrons and ions, the positive Sodium ions are diffused through the BASE to the cathode while the electrodes provide a conduction path for the free electrons to pass instead through the external load doing useful work on their way to the cathode where they are recombined with the Sodium ions to reform neutralised metallic Sodium vapour. (Reduction - the ion gains an electron)
- At the cold side the vapour releases its latent heat of vaporisation and is condensed to liquid Sodium which is transported back to the hot side by an electromagnetic pump, or in small systems, by a simple, passive wick mechanism.
- Back at the hot side the Sodium is vaporised once more in an evaporator and the cycle starts again.
The output voltage generated between the electrodes is between 1.4 and 1.6 Volts DC.
The system has no moving parts and will continue to generate electricity so long as heat is supplied to the system and the temperature differential across the BASE is maintained.
The AMTEC cycle of heating the Sodium vapour to increase its pressure, followed by its expansion and pressure drop through the solid electrolyte, and subsequent cooling can be considered as a heat engine, The maximum theoretical (ideal) energy conversion efficiency, or Carnot efficiency of the cycle is given by (1-Tc/Th)*100% where Th is the temperature at the hot side of the device and Tc is the temperature at the cold side of the device. In the example above, the Carnot efficiency is (1- 700/1000)*100 which is 30% although 40% is theoretically possible with a higher operating temperature.
In practice however the highest conversion efficiencies which have been achieved with AMTEC devices are just over 20% and this compares very favourably with alternative direct energy conversion devices such as semiconductor thermocouple arrays (TEGs) which typically have efficiencies of 5% to 7%. This is particularly important in batteries such as Radioisotope Thermoelectric Generators (RTGs) used in spacecraft applications since the mass of the radioactive heat source required by the AMTEC device to produce a given amount of electrical energy will be only one quarter of the mass needed by an equivalent thermocouple energy converter. This translates into huge savings in system mass, fuel load and cost.
Because the AMTEC device has no moving parts and uses a closed heat cycle its overall conversion efficiency also compares favourably with conventional steam turbine and internal combustion engine (ICE) energy conversion systems which have substantial combustion inefficiencies, frictional losses and in the case of ICEs, pumping losses. While the efficiency of all three systems suffers because of heat loss, steam turbine systems and ICEs must work with much higher temperature ranges to achieve Carnot efficiencies high enough to compensate for the extra losses. See also heat engine efficiency.
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