Thermoelectric generation using molten salts

Thermoelectric generation of electrical energy from heat energy has been reviewed, the histor.f of the subject has been reassessed, and the materials used for thermoelectric generation have been examined. The suitability of a material for thermoelectric generation is determined by the product of the figure of merit, Z, and the 2.bsolutetemperature, T. The figure of merit of a material depends upon the thermoelectric potential, eX. , and the ratio of the electrical conductivity, 0-, to the thermal conductivity, K : Z T = 0< 2 T a-/ K The most suitable semiconductor materials have (dimensionless) ZT values between 0.5 and 1, and the most suitable molten salts developed in this work have ZT values of about 0.5. The advantages of molten salt cells over semiconductol' thermoelements are the absence of conta.ct junction, doping and thermal expansion problems, and the low cost of the materials. The major disadvantage is the mass transfer between the electrodes, and this caused the failure of many of the initial generation thermocells de signed by the author. Pure molten salts are found to have ZT values ranging up to 0.3 and mixtures of salts have been examined in an attempt to increase the ZT value. The .A{,-cI - AgI system has been studied as an example of a. common-cation The thermoelectric potential has been measured up to a temperature of a.bout 12000C and. the thermal conductivity has been measured using a transient hot wire method which was adapted for measurements on molten salts at high temperatures. The maximum ZT value is 0.4 for the AgCl - AgI eutectic mixture. The AgCl - CuCl - LiOl 2 system has been studied as an example of a common anion mixture. A theory to the thermoelectric potential of common anion mixtures has been developed and partially substantiated by experiment. Some salt mixtures are expected to havo ZT values in excess of unity, yielding actual conversion efficiencies of 7 - 10%. The thermoelectric effect in a molten salt thermocell is due to the flow of ions between the electrodes, and this flow of ions causes an electrochemical mass transfer. In addition there is a mass transfer between the electrodes caused by natural convection currents in the salt. A cell has been developed in which the mass transfer problem has been overcome by arranging for the mechanisms just described to transfer metal from a molten metal alloy electrode to another electrode situated above it, and allowing gravity to return the metal to the lower electrode. Convection currents have been decreased by inserting a fibrous material in the salt. to the thermoelectric potential of a thermocell with alloy (rather than pure metal) electrodes has been developed and confirmed. In order to demonstrate the feasibility of thermocell generation a smaIl generator consisting of up to 44 of these cells has been constructed and has given an open circuit potential of 13 volts and a short circuit current of 0.6 amps. This is believed to be the first thermoelectric generator using a liquid thermoelectric material and also the first utilising the thermoelectric effect of the flow of ions