Direct Energy Conversion System

Topics: Thermoelectric effect, Thermodynamics, Thermoelectric cooling Pages: 10 (3252 words) Published: July 10, 2012
Direct energy conversion system
Direct energy conversion systems that could operate in the 100-700°C temperature range with high efficiencies (>30%) provide an attractive compact alternative to internal combustion engines for many military applications. They will also expand the possibilities for waste heat recovery applications. The core of the solution we are proposing is a metal/semiconductor nanocomposite that will allow us to modify four intrinsic material properties in order to fabricate more efficient thermoelectric systems. The concept of a metal/semiconductor nanocomposite as a solid-state thermionic material represents a radical alternative to conventional homogeneous thermoelectric materials. Instead of focusing on semiconducting materials with a highly asymmetric density of states about the Fermi level yielding an optimal doping level of about 10^19/cm^3 at room temperature, the metal/semiconductor nanocomposite concept utilizes the Schottky barrier to filter the electron energy distribution, creating a large difference between the average energy of the conduction electron and the Fermi energy. The high electron density in the metal (>10^22/cm^3) compensates for the negligible contribution to the conductivity from the majority of the electrons in the metal that have energies below the top of the barrier. The high interface density and/or nanoscale embedded nanoparticles in a metal/semiconductor nanocomposite are expected to suppress the transport of mid-long wavelength phonons. This is very important to reduce the lattice contribution to thermal conductivity below the alloy limit. The ability to tune the properties by controlling layer thicknesses and nanoparticles sizes as well as manipulating lattice mismatch and barrier height by alloying offers additional degrees of freedom for materials design. Modeling of the transport properties of metal/semiconductor superlattices suggested that ZT values in excess of 4-5 should be possible if the barrier height is adjusted to be in the range of 4-5 kT. A unique team of researchers experienced in nano-engineered semiconductor materials, physics, electrical and mechanical engineering has been assembled to address fundamental limits to existing materials. This work is closely coordinated with our industrial partner, BSST, who will investigate reliability, manufacturing and scale up production for military systems. BSST is the world’s leading consumer of TE material and is already working with the Department of Defense on the development of several state-of-the-art solid- statethermoelectric systems. “

A thermionic converter is a static device that converts
heat into electricity by boiling electrons from a hot emitter surface (

Thermoelectric effect
The thermoelectric effect is the direct conversion of temperature differences to electric voltage and vice-versa. A thermoelectric device creates a voltage when there is a different temperature on each side. Conversely, when a voltage is applied to it, it creates a temperature difference. At the atomic scale, an applied temperature gradient causes charged carriers in the material to diffuse from the hot side to the cold side, similar to a classical gas that expands when heated; hence inducing a thermal current. This effect can be used to generate electricity, measure temperature or change the temperature of objects. Because the direction of heating and cooling is determined by the polarity of the applied voltage, thermoelectric devices are efficient temperature controllers. The term "thermoelectric effect" encompasses three separately identified effects: the Seebeck effect, Peltier effect and Thomson effect. Textbooks may refer to it as the Peltier–Seebeck effect. This separation derives from the independent discoveries of French physicist Jean Charles Athanase Peltier and Estonian-German physicist Thomas Johann Seebeck. Joule heating, the heat that is generated whenever a voltage is applied across a...
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