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Power generation

Main article: Thermoelectric generator

Approximately 90% of the world’s electricity is generated by heat energy, typically operating at 30–40% efficiency, losing roughly 15 terawatts of power in the form of heat to the environment. Thermoelectric devices could convert some of this waste heat into useful electricity. Thermoelectric efficiency depends on the figure of merit, ZT. There is no theoretical upper limit to ZT, and as ZT approaches infinity, the thermoelectric efficiency approaches the Carnot limit. However, no known thermoelectrics have a ZT>3. As of 2010, thermoelectric generators serve application niches where efficiency and cost are less important than reliability, light weight, and small size.
Internal combustion engines capture 20–25% of the energy released during fuel combustion.Increasing the conversion rate can increase mileage and provide more electricity for on-board controls and creature comforts (stability controls, telematics, navigation systems, electronic braking, etc.)^[5] It may be possible to shift energy draw from the engine (in certain cases) to the electrical load in the car, e.g. electrical power steering or electrical coolant pump operation.
Cogeneration power plants use the heat produced during electricity generation for alternative purposes. Thermoelectrics may find applications in such systems or in solar thermal energy generation.

 

Refrigeration

Main article: Thermoelectric cooling

Thermoelectric materials can be used as refrigerators, called "thermoelectric coolers", or "Peltier coolers" after the Peltier effect that controls their operation. As a refrigeration technology, Peltier cooling is far less common than vapor-compression refrigeration. The main advantages of a Peltier cooler (compared to a vapor-compression refrigerator) are its lack of moving parts or circulating fluid, and its small size and flexible shape (form factor). Another advantage is that Peltier coolers do not require refrigerant fluids, such as chlorofluorocarbons (CFCs) and related chemicals, which can have harmful environmental effects.


The main disadvantage of Peltier coolers is that they cannot simultaneously have low cost and high power efficiency. Advances in thermoelectric materials may allow the creation of Peltier coolers that are both cheap and efficient. It is estimated that materials with ZT>3 (about 20–30% Carnot efficiency) are required to replace traditional coolers in most applications. Today, Peltier coolers are only used in niche applications.

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Thermoelectric materials

Thermoelectric materials show the thermoelectric effect in a strong and/or convenient form. The thermoelectric effect refers to phenomena by which either a temperature difference creates an electric potential or an electric potential creates a temperature difference. These phenomena are known more specifically as the Seebeck effect (converting temperature to current), Peltier effect (converting current to temperature), and Thomson effect (conductor heating/cooling). While all materials have a nonzero thermoelectric effect, in most materials it is too small to be useful. However, low-cost materials that have a sufficiently strong thermoelectric effect (and other required properties) could be used in applications including power generation and refrigeration.
A commonly used thermoelectric material in such applications is bismuth telluride (Bi₂Te₃).

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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 charge carriers in the material to diffuse from the hot side to the cold side.
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 balt-German physicist Thomas Johann Seebeck. Joule heating, the heat that is generated whenever a voltage is applied across a resistive material, is related though it is not generally termed a thermoelectric effect. The Peltier–Seebeck and Thomson effects are thermodynamically reversible, whereas Joule heating is not.

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The Photovoltaic Effect

The photovoltaic effect was discovered more than 150 years. The direct conversion of sunlight into electricity for the first time succeeded Alexandre Edmond Becquerel, French physicist in 1839. For many years this effect has been exploited only in the photograph at the exposure. The most important application today is the photovoltaic power generation with solar cells. The solar cells consist of two superposed layers consisting mostly of crystalline silicon. On the outside there are contacts that are made of metal. Since the two silicon layers treated with different materials such as boron and phosphorus were, they have different electrical properties. In the so-called boundary layer, which is only about one thousandth of a millimeter is strong there is an electric field with positive and negative poles. Now, if light falls on the boundary layer, the electrons gain energy from the photons of light, loose binding of the crystal and move in positive direction. The flowing current can now be removed by the consumer. Light (Greek: Photo) thereby causing an electrical voltage. Since one is measuring the voltage in volts it up the term photovoltaic (PV too).
The photovoltaic effect is observed only in semiconductors, which can absorb photons at all on the one hand, on the other hand, can simultaneously produce charged particles which have enough energy to move through the material to leave the semiconductor and thus to produce electricity.
By absorption of a photon of appropriate energy, an electron goes from the ground state to an excited state, whereby a free electron-hole pair is created. Is there a sufficiently strong electric field as it exists for example in a pn junction of a semiconductor, the two charge carrier drift in the opposite direction. The electrons migrate to the n-layer and the holes in the p-layer. By the charge transfer results in a photovoltage which causes a corresponding photocurrent, if the two layers are connected via an external circuit, where electrical energy from light is produced (semiconductor photoelectric effect).

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photovoltaic II

 

Nellis Solar Power Plant at Nellis Air Force Base in the USA. These panels track the sun in one axis.

 

Photovoltaic SUDI shade is an autonomous and mobile station in France that replenishes energy for electric vehicles using solar energy.

 

Solar panels on the International Space Station

Photovoltaics (PV) is a method of generating electrical power by converting solar radiation into direct current electricity using semiconductors that exhibit the photovoltaic effect. Photovoltaic power generation employs solar panels composed of a number of solar cells containing a photovoltaic material. Materials presently used for photovoltaics include monocrystalline silicon, polycrystalline silicon, amorphous silicon, cadmium telluride, and copper indium gallium selenide/sulfide. Due to the growing demand for renewable energy sources, the manufacturing of solar cells and photovoltaic arrays has advanced considerably in recent years.
Solar photovoltaics have long been argued to be a sustainable energy source. By the end of 2011, a total of 67.4 GW had been installed, sufficient to generate 85 TWh/year. Solar photovoltaics is now, after hydro and wind power, the third most important renewable energy source in terms of globally installed capacity. More than 100 countries use solar PV. Installations may be ground-mounted (and sometimes integrated with farming and grazing) or built into the roof or walls of a building (either building-integrated photovoltaics or simply rooftop).
Driven by advances in technology and increases in manufacturing scale and sophistication, the cost of photovoltaics has declined steadily since the first solar cells were manufactured and the levelised cost of electricity (LCOE) from PV is competitive with conventional electricity sources in an expanding list of geographic regions. Net metering and financial incentives, such as preferential feed-in tariffs for solar-generated electricity, have supported solar PV installations in many countries. With current technology, photovoltaics recoup the energy needed to manufacture them in 1 to 4 years.

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http://www.pbs.org/wgbh/nova/tech/saved-by-the-sun.html