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A thermoelectric generator maintains hot and cold junctions of two materials with different Seebeck coefficients. Mobile charge carriers redistribute in the temperature gradient, producing an open-circuit voltage of magnitude $|V|=|S_p-S_n|\Delta T$ for constant coefficients. A load permits electric current; pairs of p-type and n-type legs are commonly connected electrically in series and thermally in parallel. Maintaining the output requires continuous heat transfer from the hot reservoir and rejection of heat to the cold reservoir. The thermoelectric figure of merit $ZT=S^2\sigma T/\kappa$ expresses the favorable combination of large Seebeck coefficient, large electrical conductivity and small thermal conductivity. Contact losses, Joule heating and parasitic heat leaks reduce practical efficiency.

A pyroelectric generator instead exploits a change of spontaneous electric polarization with temperature. If the relevant component of the pyroelectric coefficient is $p=d{P}_s/dT$, an electrode of area $A$ receives a short-circuit electric current approximately $I=pAdT/dt$. A heating stroke and a cooling stroke produce opposite charge changes. Rectification or a synchronized electrical cycle can transfer energy to storage. A spatial temperature difference drives the thermoelectric device; a temporal temperature change drives the pyroelectric device. Uniform, constant temperature gives no continuing current from pyroelectricity even if the material is hot. Reported nanowire generators deliberately use changing temperature, rather than infer continuous generation from a static hot specimen.^[ref]

Both approaches can be compact, silent and free of macroscopic moving parts. The thermoelectric generator gives continuous output under a sustained gradient, but needs two effective thermal contacts and sufficient heat flow; very small $\Delta T$ gives a small voltage and thermal contacts can consume much of the available gradient. Thin films exhibiting pyroelectricity can respond to intermittent heating without a permanently maintained hot/cold separation across their thickness, but require repeated thermal cycling, low electrical leakage and suitable charge extraction. Their thermal time constant limits useful cycling frequency, and sensible heating/cooling can dominate the heat budget. A steady hot pipe with an available cold sink favors a thermoelectric generator; periodic radiative heating can favor a pyroelectric generator. Neither converts equilibrium thermal motion at a single fixed temperature into sustained useful work.

The same current flows through both resistances, $I=V_{\mathrm{oc}}/(R+R_L)$. The load terminal voltage is $I{R}_L$, smaller than the open-circuit voltage. The electrical battery symbol represents the maintained thermoelectric source under this model; heat input is still required.

A thermoelectric generator uses the Seebeck effect: a maintained temperature difference across suitable conductors produces an open-circuit electromotive force. For a pair of legs,when the effective Seebeck coefficient is approximately constant. Repeated p- and n-type legs are usually connected electrically in series and thermally in parallel.

Connecting a load lets part of the heat flowing from the hot side to the cold side become electrical power. Both a heat source and a cold-side heat-removal path are necessary: a uniformly warm device with no maintained temperature difference supplies no sustained output. Internal electrical resistance, heat leakage, contact losses and the finite temperature difference reduce the harvested power; the converter is a heat engine rather than a source of energy from temperature alone.

A thermoelectric generator uses the Seebeck effect and needs a spatial temperature gradient: a couple develops $V\simeq (S_p-S_n)\Delta T$ between hot and cold junctions. It can deliver continuous power under a steady thermal gradient and has no moving parts, but requires sustained heat flow and adequate hot- and cold-side thermal contacts. Thermal conduction, internal electrical resistance and the material's thermoelectric figure of merit limit conversion.

A pyroelectric generator responds to a time-dependent spontaneous electric polarization. Under approximately uniform heating or cooling, $I=pAdT/dt$ when the electrical boundary condition allows charge flow. It can harvest temporal thermal fluctuations without a persistent spatial gradient. However, at a constant uniform temperature its ordinary pyroelectric current stops; practical extraction needs cycling, low-leakage electronics and a suitable load. A large open-circuit voltage is not evidence of a large available average power. Temperature-driven nanowire measurements provide a primary experimental example of this distinction.^[ref]

Alpha decay releases energy in charged particles with short stopping ranges in matter, so much of their kinetic energy becomes local heat. An appropriate radioisotope thermoelectric generator can use a predictable radioactive decay heat source without requiring sunlight or a sustained nuclear chain reaction. Low penetrating photon and neutron output is desirable, but being an alpha emitter does not by itself establish the complete radiation spectrum.

The isotope's half-life must suit the mission. A short half-life gives high initial specific activity but a rapid decline in useful thermal power; a much longer one gives a steadier output but less activity per atom. Selection also depends on recoverable decay energy, specific power, daughter emissions, chemical and thermal stability, compatibility with the power-conversion system, availability and cost. Practical containment and radiological acceptability remain separate system-level requirements.

A thermoelectric generator converts a temperature difference into electrical output through the Seebeck effect; not all decay heat becomes electricity. NASA's operational account describes this heat-to-electricity distinction and the need to match source lifetime to long space missions.^[ref]

For the assumed three-half-life mission, the thermal power falls by a factor of eight. That large decline is the important disadvantage of this particular half-life even though the initial specific thermal power is high.

A thermoelectric generator whose heat supply comes from radioactive decay. The Seebeck effect converts part of a temperature-driven heat flow into electrical output. The source's thermal power and the electrical power delivered by the converter are distinct quantities.