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Energy transformation, also known as energy conversion, is the process of changing energy from one form to another.^[2] In physics, energy is a quantity that provides the capacity to perform work (e.g. lifting an object) or provides heat. Energy can be converted to different forms or transferred to a different location or object or living being, but it cannot be created or destroyed.^[3]

According to the first law of thermodynamics, energy can never be created or destroyed from an isolated system, but it can move from one part of the system to another or be converted between different kinds of energy that can do work (free energy) and energy in the form of heat.^[4] However, the second law of thermodynamics also states that the entropy of a system is always unchanging or increasing, never decreasing. Combining the first and second laws, it can be shown that 1) free energy must be spent to generate entropy and therefore 2) energy transfer always decreases (or preserves) total free energy and increases (or preserves) total energy stored as heat. As a result, any time energy is transformed or transferred, the amount of free energy available with which to do work either stays the same or decreases. The conversion efficiency of energy transfer refers to the ratio of total work done divided by total heat transferred, and per the above laws of thermodynamics it is always less than 100%.^[4]

Generally, thermodynamics studies energy transformations through the lens of cycles, or closed paths composed of process steps along a phase space of two thermodynamic variables.^[4] A cycle which uses heat transfer from hot to cold to generate work is called a heat engine or power cycle. Heat engines can be classified based on the type of cycle they implement. Some examples of cycles that can be used as heat engines include the Carnot cycle, the Rankine cycle, the Brayton cycle, and the Stirling cycle. Different cycles have different maximum efficiencies and may be more or less practical to implement for specific applications.^[5] Thermodynamic analysis of a heat engine typically seeks to compute quantities like the work performed at each process step, the heat input, and overall efficiency of the energy transformation. Energy generation for the electrical grid occurs in systems such as steam plants, coal-fired power stations, and gas turbine engines which can all be described as various kinds of heat engines.^[4]

As discussed above, the amount of free energy is always stable or decreasing as free energy is transformed to heat during energy transformations, never gained. A process in which free energy is not lost is called a reversible transformation, while lossy processes are called irreversible transformations.^[4] Due to the close relationship between thermodynamics and information science, reversibility plays an important role in computation, especially in quantum computing where generation of heat is particularly undesirable.^[6] Most energy transformation processes are irreversible, and one hypothetical fate of the universe is that eventually all free energy will be converted to uselessly diffuse, equilibrated heat. This fate is known as “heat death” or “the Big Freeze.”^[7]

In practice, the conversion of free energy to heat can be observed as friction, drag, electrical resistance, direct creation of long-wavelength electromagnetic radiation, or similar processes. Friction and drag are minimal in a vacuum, where for example gravitational potential energy can be converted to and from kinetic energy with very low loss during orbits of celestial bodies in outer space — Kepler’s laws essentially assume no free energy is lost.^[8] Similarly, superconductors have no resistance to direct current and low resistance to alternating currents, so electrical energy can be used to perform reversible computations in superconducting qubits.^[6]

Because transformations between non-thermal forms of energy are constrained only by the conservation of energy, they have a theoretical maximum efficiency of 100%.^[9] By contrast, transformations from thermal energy to other forms of energy are additionally constrained by the second law of thermodynamics and have a theoretical maximum efficiency strictly less than 100% (see Carnot cycle), and typically much lower. In addition, only a difference in the density of thermal/heat energy (temperature) can be used to perform work. This is because thermal energy represents a particularly disordered form of energy; it is spread out randomly among many available states of a collection of microscopic particles constituting the system (these combinations of position and momentum for each of the particles are said to form a phase space). The measure of this disorder or randomness is entropy, and its defining feature is that the entropy of an isolated system never decreases. One cannot take a high-entropy system (like a hot substance, with a certain amount of thermal energy) and convert it into a low entropy state (like a low-temperature substance, with correspondingly lower energy), without that entropy going somewhere else (like the surrounding air). In other words, there is no way to concentrate energy without spreading out energy somewhere else.

Thermal energy in equilibrium at a given temperature already represents the maximal evening-out of energy between all possible states^[10] because it is not entirely convertible to a “useful” form, i.e. one that can do more than just affect temperature. The second law of thermodynamics states that the entropy of a closed system can never decrease. For this reason, thermal energy in a system may be converted to other kinds of energy with efficiencies approaching 100% only if the entropie of the universe is increased by other means, to compensate for the decrease in entropy associated with the disappearance of the thermal energy and its entropy content. Otherwise, only a part of that thermal energy may be converted to other kinds of energy (and thus useful work). This is because the remainder of the heat must be reserved to be transferred to a thermal reservoir at a lower temperature. The increase in entropy for this process is greater than the decrease in entropy associated with the transformation of the rest of the heat into other types of energy.

In order to make energy transformation more efficient, it is desirable to avoid thermal conversion. For example, the efficiency of nuclear reactors, where the kinetic energy of the nuclei is first converted to thermal energy and then to electrical energy, lies at around 35%.^[11][12] By direct conversion of kinetic energy to electric energy, effected by eliminating the intermediate thermal energy transformation, the efficiency of the energy transformation process can be dramatically improved.^[13]

The human study and use of different kinds of energy transformations perhaps dates back to prehistory with the first technological uses of fire.^[14] However, formalized and scientific studies of energy transformations are closely tied to the field of thermodynamics. Otto van Guericke invented the first vacuum pump in 1650, which set off a series of studies of pumps and pressurization systems. This line of inquiry eventually enabled development of the first engines which could transform heat into mechanical work. The first engine was invented by Thomas Savery in 1697 and was shortly followed by Thomas Newcomen’s engine in 1712.

These early engines had very low efficiencies, making them impractical for widespread use, but they inspired further study. In 1776, James Watt commercially introduced his improved steam engine,^[14] which achieved higher efficiency by using an external condenser. Soon after, Sadi Carnot established himself as “the father of thermodynamics” with the publication of Reflections on the Motive Power of Fire (1824).^[15] In this book, he developed the idea of the Carnot cycle, one of several theoretical ideal heat engine cycles recognized today.

Today, the study of energy transformations still plays important practical and theoretical roles in modern science. For example, engineers of power plant systems work to minimize losses at different stages of converting energy sources (e.g. fossil fuels, sunlight, or wind) into usable electrical energy.^{[citation needed]}

Energy transformations in the universe over time are usually characterized by various kinds of energy, which have been available since the Big Bang, later being “released” (that is, transformed to more active types of energy such as kinetic or radiant energy) by a triggering mechanism.

A direct transformation of energy occurs when hydrogen produced in the Big Bang collects into structures such as planets, in a process during which part of the gravitational potential is converted directly into heat. In Jupiter, Saturn, and Neptune, for example, such heat from the continued collapse of the planets’ large gas atmospheres continues to drive most of the planets’ weather systems. These systems, consisting of atmospheric bands, winds, and powerful storms, are only partly powered by sunlight. However, on Uranus, little of this process occurs. This is likely because the planet emits very little internal heat compared to other gas giants, possibly due to a past giant impact that released much of its primordial heat or an interior structure that inhibits efficient heat transport.^[16]

On Earth, a significant portion of the heat output from the interior of the planet, estimated at a third to half of the total, is caused by the slow collapse of planetary materials to a smaller size, generating heat.^{[citation needed]}

Familiar examples of other such processes transforming energy from the Big Bang include nuclear decay, which releases energy that was originally “stored” in heavy isotopes, such as uranium and thorium. This energy was stored at the time of the nucleosynthesis of these elements. This process uses the gravitational potential energy released from the collapse of Type II supernovae to create these heavy elements before they are incorporated into star systems such as the Solar System and the Earth. The energy locked into uranium is released spontaneously during most types of radioactive decay, and can be suddenly released in nuclear fission bombs. In both cases, a portion of the energy binding the atomic nuclei together is released as heat.

In a similar chain of transformations beginning at the dawn of the universe, nuclear fusion of hydrogen in the Sun releases another store of potential energy which was created at the time of the Big Bang. At that time, according to one theory^[which?], space expanded and the universe cooled too rapidly for hydrogen to completely fuse into heavier elements. This resulted in hydrogen representing a store of potential energy which can be released by nuclear fusion. Such a fusion process is triggered by heat and pressure generated from the gravitational collapse of hydrogen clouds when they produce stars, and some of the fusion energy is then transformed into starlight. Considering the solar system, starlight, overwhelmingly from the Sun, may again be stored as gravitational potential energy after it strikes the Earth. This occurs in the case of avalanches, or when water evaporates from oceans and is deposited as precipitation high above sea level (where, after being released at a hydroelectric dam, it can be used to drive turbine/generators to produce electricity).

Sunlight also drives many weather phenomena on Earth. One example is a hurricane, which occurs when large unstable areas of warm ocean, heated over months, give up some of their thermal energy suddenly to power a few days of violent air movement. Sunlight is also captured by plants as a chemical potential energy via photosynthesis, when carbon dioxide and water are converted into a combustible combination of carbohydrates, lipids, and oxygen. The release of this energy as heat and light may be triggered suddenly by a spark, in a forest fire; or it may be available more slowly for animal or human metabolism when these molecules are ingested, and catabolism is triggered by enzyme action.

Through all of these transformation chains, the potential energy stored at the time of the Big Bang is later released by intermediate events, sometimes being stored in several different ways for long periods between releases, as more active energy. All of these events involve the conversion of one kind of energy into others, including heat.

A coal-fired power plant involves these energy transformations:^[17][18]

1. Chemical energy in the coal is converted into thermal energy by burning coal in a boiler to produce exhaust gases.
2. Thermal energy of the exhaust gases in the boiler is converted into thermal energy and kinetic energy of high-pressured steam through heat exchange between the gases and the boiler water.
3. Kinetic energy of steam is converted to mechanical energy as the steam flows into a turbine.
4. Mechanical energy of the turbine is converted to electrical energy by spinning the generator. This is the ultimate output.

In such a system, the first and fourth steps are highly efficient, but the second and third steps are less efficient.^{[citation needed]} Other types of power plants can be more efficient. Fore example, natural gas plants in the US in 2019 overall converted 45% of the energy stored in the fuel into usable electrical energy.^[19] In comparison, coal plants converted only 33% of stored energy into electrical energy.

In a conventional automobile, the following energy transformations occur:

1. Chemical energy in the fuel is converted into kinetic energy of expanding gas via combustion
2. Kinetic energy of expanding gas converted to the linear piston movement
3. Linear piston movement converted to rotary crankshaft movement
4. Rotary crankshaft movement passed into transmission assembly
5. Rotary movement passed out of transmission assembly
6. Rotary movement passed through a differential
7. Rotary movement passed out of differential to drive wheels
8. Rotary movement of drive wheels converted to linear motion of the vehicle

There are many different systems, both natural and engineered, which act as transducers that convert one energy form into another.^[20] A short list of examples follows:

• ATP hydrolysis (chemical energy in adenosine triphosphate → mechanical energy)
• Battery (electricity) (chemical energy → electrical energy)
• Electric generator (kinetic energy or mechanical work → electrical energy)
• Electric heater (electric energy → heat)
• Fire (chemical energy → heat and light)
• Friction (kinetic energy → heat)
• Fuel cell (chemical energy → electrical energy)
• Geothermal power (heat→ electrical energy)
• Heat engines, such as the internal combustion engine used in cars, or the steam engine (heat → mechanical energy)
• Hydroelectric dam (gravitational potential energy → electrical energy)
• Electric lamp (electrical energy → heat and light)
• Microphone (sound → electrical energy)
• Ocean thermal power (heat → electrical energy)
• Photosynthesis (electromagnetic radiation → chemical energy)
• Piezoelectrics (strain → electrical energy)
• Radio transmitters (electrical energy → electromagnetic wave energy)
• Thermoelectric (heat → electrical energy)
• Wave power (mechanical energy → electrical energy)
• Windmill (wind energy → electrical energy or mechanical energy)

• “Energy—Volume 3: Nuclear energy and energy policies”. Applied Energy. 5 (4): 321. October 1979. doi:10.1016/0306-2619(79)90027-8.
• Energy Transfer and Transformation | Core knowledge science