EML4450L13

EML4450L13 - Sustainable Energy Science and Engineering...

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S ustainable E nergy S cience and E ngineering C enter Direct Energy Conversion: Thermoelectric Conversion References: Direct Energy Conversion by Stanley W. Angrist, Allyn and Beacon, 1982. Direct Energy Conversion by Reiner Decher,Oxford University press, 1997.
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S ustainable E nergy S cience and E ngineering C enter Direct Energy Conversion The best achievable thermal efficiency for electricity generation with conventional methods is about 50%. In general when a process can occur directly rather than passing through several intermediate steps, it is reasonable to expect that it may take place more efficiently. Fuel cells ( a mode of direct energy conversion) have already demonstrated a capability of producing small quantities of electrical energy with considerable efficiency. The ability to convert primary energy directly into the required form at its point of use would be of great use in many systems (distributed power, space exploration, satellites, remote weather stations etc.).
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S ustainable E nergy S cience and E ngineering C enter Thermoelectric Effect One of the ways to directly convert the thermal power to electric power is through thermoelectric generators. In metals and semiconductors, electrons are free to move in the conduction band. They respond to electric fields, which establish a flux of charges or current. These electrons can also respond to a gradient of temperature so as to accommodate a flow of heat. The motion of the electrons transports both their charge and their energy. At junctions of dissimilar materials the electrons flow across a discontinuity in the energy levels of the conduction bands. If the spectrum of electron quantum sates is different in the two materials, the crossing of negatively charged electrons or positively charged holes will not preserve the statistical distribution of electrons around the Fermi level. Where E is the electron energy and k is the Boltzmann’s constant. The maintenance of the current may require addition or removal of heat (thermoelectric generator) or heating the junction will increase or decrease the electric current. f ( E ) = 1 e E μ i kT + 1
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S ustainable E nergy S cience and E ngineering C enter Seebeck Experiment Seebeck Coefficient: When two ends of a conductor of material a are maintained at different temperatures, say Δ T = T 1 -T 2, Seebeck observed in 1822 that a voltage V (varies with Δ T) can be measured when the current is zero. The Seebeck coefficient is then defined as α ab lim Δ T 0 Δ V Δ T V = ab dT T 1 T 2 This equation can be interpreted as that the electrons will acquire an electric potential as they are transported from region of varying thermal energies. Energies are measured by their temperatures. a b b V T 1 T 2
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S ustainable E nergy S cience and E ngineering C enter Seebeck Coefficient
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S ustainable E nergy S cience and E ngineering C enter Peltier Experiment a I b Q Peltier Coefficient: Consider two bars of material a and b that are joined with a current source as shown in the figure below. Peltier observed in 1844 that
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This note was uploaded on 10/22/2011 for the course EML 4450 taught by Professor Greska during the Fall '06 term at FSU.

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EML4450L13 - Sustainable Energy Science and Engineering...

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