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Most of the world’s energy can go back to our sun. Every day we are heated by its electromagnetic rays, and plants use the sun’s energy to make sugars and ultimately proteins and other good things to eat. Fossil fuels were also once made from these plant and other organisms that relied on the sun’s energy millions of years ago. Today, humans can convert the sun’s energy directly into electricity, through solar thermal and solar photovoltaic systems.

SOLAR THERMAL OR PHOTOVOLTAIC?

Solar panels, also called solar thermal, convert sunlight to heat and then heat to electricity. Photovoltaic cells, or solar cells, convert sunlight directly into electric current by way of carefully-engineered semiconductor materials.

Though solar photovoltaics are more efficient converters of sunlight, they are also more expensive.

As of May 2011, the world’s largest solar power plant is a concentrating solar thermal power plant in the Mohave desert in California. Solar Energy Generating Systems has a capacity of 310 megawatts and uses parabola-shaped reflective troughs to concentrate electromagnetic radiation.

The world’s largest solar photovoltaic plant is probably the Sarnia Solar Project in Ontario, Canada. It has a capacity of roughly 80 megawatts.

HOW SOLAR THERMAL WORKS

Sunlight heats a design element (water, air, chemical fluids), and that thermal energy is transmitted for other applications, such as heating water, heating space, or generating electricity. In solar thermal power plants, sunlight heats a specialized fluid, which in turn heats water into steam, which can run turbines and produce electricity.

Solar thermal power plants use concentrators that bounce the sunlight off elliptical mirrors to a central tube, in which the specialized fluid lies.

HOW PHOTOVOLTAICS WORK

Photovoltaic cells are made of specialized diodes. Electrons (natural components of atoms) in the photovoltaic cells absorb light, which excites them to a state where they can be conducted as electrical current. This difference in energy, between the valence band (the state of a normal electron staying around its home atom) to the conduction band (electron free to move between atoms) is called the band gap.

Well-engineered photovoltaics have a band gap that coincides with the energies of as broad a spectrum of light as possible, to convert the maximum amount of the sunlight into electricity.

As sunlight energy pops electrons into the conduction band and away from their home atoms, an electric field is produced. The negatively-charged electrons separate from the positively-charged “holes” they leave behind, so that when electrons are freed into the conduction band, they move as electric current in the electric field, electricity.

PHOTOVOLTAICS ARE MADE OF SPECIALIZED MATERIALS

An ever-expanding variety of semiconductor materials can be used to make solar cells; universities and companies worldwide are researching these options, from special bio-plastics to semiconductor nanocrystals. Nonetheless, the photovoltaic cells available today require precise manufacturing conditions and are therefore far more expensive to produce than solar panels.

Silicon has to be processed under clean room conditions — carefully regulated atmospheres — to remove impurities and prevent introducing contaminants, both of which can change the band gap. Thin film-based photovoltaics require special production methods, like chemical vapor deposition. Semiconductor processing also uses strong acids and often dangerous chemicals for etching.

Today, commercially-sold cells are made from purified silicon or other crystalline semiconductors like cadmium telluride or copper indium gallium selenide.

WHERE DO WE GET THE STARTING MATERIALS?

Silicon is plentiful in the Earth’s crust. Cadmium is a readily available but highly toxic heavy metal, as is arsenic, another chemical used in some cells. As tellurium demand is only recently rising in response to solar demand, it’s unknown what the global supply is for this unusual element but it may be quite abundant. Photovoltaics are a lively area of research, and the future production and environmental costs of starter materials, production, and pollution are difficult to predict.

California, Massachusetts, Ohio, and Michigan produced the most photovoltaics in 2009. However, that year, 58 percent of photovoltaics were imports, primarily from Asian countries like China, Japan, and the Philippines.

LED TECHNOLOGY: MORE THAN HEADLAMPS

Photovoltaic cells work in the opposite direction of light-emitting diodes, or LEDs. LEDs are used interchangeably with other lighting, like light bulbs. However, LED’s work in a completely different manner, far closer to the way photovoltaics work.

Click here see a bar chart comparing how much energy is used by various light sources.

LEDs absorb energy in the form of electricity, exciting electrons into the conduction band. When the electrons in the semiconductor material drop back into the valence band from the conduction band, they emit energy in the form of photons, or electromagnetic radiation.

It’s a highly efficient process because energy isn’t wasted on producing heat, which happens with standard tungsten filament bulbs. LEDs also last a much longer time as they do not have filaments to burn out, and because they are very small and several units are used to replace one large traditional lamp, they do not all burn out at once. That makes LEDs a good choice for stoplights or other safety critical applications.

 

Thermodynamics is the study of how energy moves and changes form, usually by way of heat, as suggested by the components of its name: thermo-dynamics. Its laws and equations help to predict what could happen in various situations, based on the temperature, pressure, materials, and shape of a system.

Thermodynamics tells us how to calculate the ultimate temperature of a refrigerator or how much energy we can get out of a steam engine. Thermodynamics can also be applied to chemistry and the world on an atomic level, predicting which compounds are stable at specific temperatures and pressures. Thermodynamics explains why diamonds form naturally and spontaneously from carbon-based compounds deep inside the Earth, but they cannot form spontaneously here on the surface.

Thermodynamics relies on the idea that energy is conserved, even if it is transferred from or to a system to its surroundings through heat, changes in momentum, or other forms of energy.

 

TEMPERATURE AND HEAT

Heat and thermal energy are directly related to temperature. We can’t see individual atoms vibrating in solids, liquids, and gases, but we can feel their kinetic energies as temperature. Atoms in solids, liquids, and gases do vibrate. If they didn’t, they would be at absolute zero, a theoretical state of zero thermal energy at ­-459.67 Fahrenheit.

When there’s a difference between the temperature of the environment and a system within it, thermal energy is transferred between them as heat. Something doesn’t have heat. Instead, as an object or system gains or loses heat, it increases or decreases its thermal energy.

Adjacent objects that exhibit different temperatures will spontaneously transfer heat to try to reach the same temperature as each other, or equilibrium. However, how much energy it takes to change the temperature of an object is based on what its made of, a property called heat capacity or thermal capacity.

Water has a higher heat capacity than steel, for example. An empty pot on the stove takes almost no time to get to 212 degrees Fahrenheit, or the boiling temperature of water. A pot with some water in it will take far much longer to reach the same temperature, because water needs to absorb more energy — per weight, per degree — to gain the same number of degrees as metal. (Even though the vaporization temperature of metal is far, far higher than the water’s).

 

THERMAL ENERGY STORAGE: A SOURCE OF POWER

Thermal energy storage exploits the difference in temperature between a system and the environment. In the late 1800s, Americans used thermal energy storage by cutting blocks of lake ice during the winter and storing them underground packed in insulating wood shavings. When the summer rolled around, they retrieved that stored ice to make food cold, exploiting the difference in temperature to force thermal energy out of the food and into the ice.

Thermal energy storage can also happen in the other direction. Electricity or other forms of energy can be used to heat various materials, which are stored in insulated containers. Later, when the energy is needed, the hot materials can heat water into steam, and that steam can push turbines, which in turn produce electricity.

Solar panels use thermal energy storage. The panels absorb the heat of sunlight and store that energy so it can be transformed into electricity with turbines. There are several kinds of solar panels, but all rely on heat for energy, unlike photovoltaic cells.

Energy comes in two basic forms: potential and kinetic.

Potential Energy is any type of stored energy; it isn’t shown through movement. Potential energy can be chemical, nuclear, gravitational, or mechanical.

Kinetic Energy is the energy of movements: the motion of objects (from people to planets), the vibrations of atoms by sound waves or in thermal energy (heat), the electromagnetic energy of the movements of light waves, and the motion of electrons in electricity.

Each form of energy can be transformed into any of the other forms, but energy isn’t destroyed or created. Losses of energy can always be accounted for by small transformations to other types of energy, like sound and heat. Power plants convert potential energy or kinetic energy into electricity, a type of kinetic energy, and electricity in turn can be converted back into other forms of energy, like heat in an oven or light from a lamp.

 

Forms of Potential Energy

CHEMICAL

Chemical energy is stored in the bonds between atoms. (See here for more about atoms.) This stored energy is released and absorbed when bonds are broken and new bonds are formed –  chemical reactions. Chemical reactions change the way atoms are arranged. Like letters of the alphabet that can be rearranged to form new words with very different meanings, atoms go through chemical reactions to be reorganized to form new compounds  with vastly different properties. Each compound has its own chemical energy associated with the bonds between the atoms it contains.

When we burn sugar (a compound made of hydrogen, oxygen, and carbon) during exercise, it’s components are reorganized into water (H2O) and carbon dioxide (CO2). These reactions both absorb and release energy, but the net reaction releases energy.

Chemical reactions that produce net energy are called exothermic. When gasoline is burned, the reactions taking place are exothermic and thermal energy is released, which can be used to power an engine. Meanwhile, chemical reactions that absorb net energy are called endothermic.

 

NUCLEAR

Nuclear energy is the stored potential of the nucleus, or center, of an individual atom. Most atoms are stable on Earth; they retain their identities as particular elements, like hydrogen, helium, iron, and carbon, as identified in the Periodic Table of Elements. Nuclear reactions change the fundamental identity of elements.

Unlike everyday chemical reactions that change how atoms are stuck together (rearranging the letters of a word), nuclear reactions change the name of the atoms themselves. (Sort of as if the letter “m” was split into the letters “r” and “n,” or the letters “l” and “o” combined to make the letter “b”).  In nuclear reactions, atoms split apart or join together to form new kinds of atoms, called fission and fusion, respectively.

When atoms split apart or fuse together, they release stored nuclear energy, sometimes in huge quantities.

Today’s nuclear power plants are fueled by fission, a breaking apart of uranium or plutonium atoms that releases lots of energy. Hydrogen atoms in the sun experience nuclear fusion, combining to form helium and subsequently releasing large amounts of kinetic energy in the form of electromagnetic radiation and heat.

 

ELASTIC

Elastic energy can be stored mechanically in a compressed gas or liquid, a coiled spring, or a stretched elastic band. On an atomic scale, the basis for the energy is a reversible strain placed on the bonds between atoms, meaning there’s no permanent change to the material.

These bonds absorb energy as they are stressed, and release that energy as they are relaxed.

 

GRAVITATIONAL

Systems can build up gravitational energy as mass moves away from the center of Earth or other objects that are large enough to generate significant gravity (the sun, other planets and stars).

For example, the farther you lift an anvil away from the ground, the more potential energy it gains. The energy used to lift the anvil is called work, and the more work performed, the more potential energy the anvil gains. If the anvil is dropped, that potential energy becomes kinetic energy as the anvil moves faster and faster toward Earth.

 

 

Forms of Kinetic Energy

MOTION

A moving object has kinetic energy. A basketball passed between players shows translational energy in the motion that gets the ball from player A to player B. That kinetic energy is proportional to the ball’s mass and the square of its velocity. To throw the same ball twice as fast, a player uses four times the energy.

If a player shoots a basketball with backspin or topspin, the basketball will also have rotational energy as it spins through the air. Rotational energy is proportional to how quickly the ball spins, as well as the ball’s mass, and the size and shape of the ball. A hollow ball needs more energy than a solid ball of equal mass to spin at the same rate. The hollow ball requires more energy because it’s mass is farther from its center.

In shooting a basketball, players often try to add rotational energy as backspin, because it results in the greatest slowdown in speed when the basketball hits the rim or the backboard, increasing the chance that the ball stays near the basket. The opposite direction of spin, a topspin, can be used in games like tennis, because it will help speed up a ball after impact and lowers the angle it travels after the bounce.

 

THERMAL ENERGY AND TEMPERATURE

Heat and thermal energy are directly related to temperature. We can’t see individual atoms vibrating, but we can feel their kinetic energies as temperature, which is a reflection of the energy with which atoms vibrate. When there’s a difference between the temperature of the environment and a system within it, thermal energy is transferred between them as heat.

A hot cup of tea in a cool room loses some of its thermal energy as heat flows from the tea to the room. The atoms in the hot tea slow their vibrating as the tea loses heat, and over a few hours the tea cools to the same temperature as the room. At the same time, the room gains the lost thermal energy from the tea, but because the room is much larger than the tea, the temperature of the room increases by so little a person wouldn’t notice it.

Adjacent objects that are different temperatures will spontaneously transfer heat to try to come to the same temperature. However, how much energy it takes to change the temperature of an object is based on what its made of, a principle called heat capacity or thermal capacity. Water has a higher heat capacity than steel, for example. An empty pot on the stove takes almost no time to get to 212 degrees Fahrenheit (the boiling temperature of water). A pot half-full of water will take much longer to reach the same temperature, because water needs to absorb more energy — per weight, per degree — to get as hot as metal.

 

SOUND

Sound waves are made through the transmitted vibration of atoms in bulk — though atoms can also vibrate through heat — and sound can travel by the motion of atoms regardless of whether they are in liquid, solid, or gaseous states. Sound cannot travel in a vacuum because a vacuum has no atoms to transmit the vibration.

Solids, liquids, and gases transmit sounds as waves, but the atoms that pass along the sound don’t travel (unlike the photons in light). The sound wave travels between atoms, like people passing along a “wave” in a sports stadium. Sounds have different frequencies and wavelengths (related to pitch) and different magnitudes (related to how loud).

Even though radio waves can transmit information about sound, they are a completely different kind of energy, called electromagnetic.

 

ELECTROMAGNETIC RADIATION

Electromagnetic energy is the same as radiation or light energy. This type of kinetic energy can take the form of visible light waves, like the light from a candle or a light bulb, or invisible waves, like radio waves, microwaves, x-rays and gamma rays. Radiation — whether it’s coming from a candle or nuclear fission of uranium — can travel in a vacuum, and physicists like to think of electromagnetic radiation as divided into tiny energy packets called photons. Each photon has a characteristic frequency, wavelength, and energy, but all photons travel at the same speed, the speed of light, or nearly 1 billion feet per second.

Electromagnetic energy can be converted to stored chemical energy by plants during photosynthesis, the process by which plants, algae, and some other small organisms use the sun’s electromagnetic radiation to turn carbon dioxide gas into sugar and carbohydrates.

 

ELECTRIC

Electric energy is to the kinetic energy of moving electrons, the negatively-charged particles in atoms. For more information about electricity, see Basics of Electricity.

 

 

 

The world depends on fossil fuels for its energy, and the United States is no exception. The vast majority of U.S. energy — more than 80 percent in 2009 — comes from burning fossil fuels. (more…)

As is the case with so many scientific fields, the history of nuclear physics and energy development has always been wrapped up with the history of modern warfare.

An unprecedented level of research went into the American bomb program, applying a rapidly evolving understanding of nuclear physics immediately to building a weapon. That investment spurred the rest of the world to pursue nuclear fission, often using the energy as an excuse for the weapons development. Rather than isolate nuclear energy from its less peaceable counterpart, the timeline incorporates all types of nuclear history.

Recent nuclear media coverage:

Germany begins shutting down old reactors and considers swearing off nuclear power entirely. Germany Dims Nuclear Plants, but Hopes to Keep Lights On.

New evidence that Japan’s troubled reactors were destined to malfunction, tsunami or not, in The Explosive Truth Behind Fukushima’s Meltdown.

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