# How Do You Convert Heat Energy To Kinetic Energy?

## Introduction

Heat energy and kinetic energy are two fundamental forms of energy that are often converted between one another. Heat energy, also called thermal energy, is the energy associated with the random motion of atoms and molecules. It is the total internal energy in a substance from the kinetic energy of atoms and molecules. Kinetic energy is energy associated with motion. It is the energy possessed by an object due to its motion. The goal of this article is to discuss the principles and methods of converting heat energy into kinetic energy.

## Laws of Thermodynamics

The laws of thermodynamics govern how heat energy and other forms of energy are converted from one form to another. The first law of thermodynamics states that energy can neither be created nor destroyed – it can only change forms. For example, a stove burner converts electrical energy into heat energy. The total amount of energy before and after remains the same.

The second law of thermodynamics introduces the concept of entropy. Entropy is a measure of randomness or disorder in a system. The second law states that the entropy of an isolated system always increases over time. This means that heat energy tends to dissipate and spread out if not hindered in some way.

An example is a hot cup of coffee left on a table. The heat from the coffee will slowly dissipate into the air and surroundings. The coffee will cool down as the higher concentrated heat energy spreads out and evens out with the cooler room temperature air. This increase in entropy explains why heat engines need a temperature difference to convert heat into work.

## Heat Engines

Heat engines are devices that convert heat energy into mechanical energy or kinetic energy. Some common examples of heat engines include internal combustion engines, external combustion engines, and hot air engines.

Internal combustion engines are perhaps the most ubiquitous heat engines in use today. They operate by burning fuel inside a combustion chamber, which causes the gases to rapidly expand. This buildup of high-pressure gases pushes a piston, which turns a crankshaft to produce rotational motion. In an automobile engine, this rotational motion is then transferred through a transmission and driveshaft to turn the wheels. Other examples of internal combustion engines include small engines used in lawn mowers, generators, and chainsaws.

External combustion engines operate on similar principles as internal combustion engines, except the fuel is burned outside of the engine. The classic example is a steam engine. Here, fuel is burned in a boiler to convert water into high-pressure steam. This steam is then piped into a piston chamber where it expands to drive the piston. Early steam locomotives and steamships used external combustion engines. Modern power plants also generate electricity using high-pressure steam turbines.

Hot air engines provide another example of heat engines. They utilize the expansion of air at high temperatures to move a piston. The most common hot air engines are Stirling engines, originally invented in 1816. These engines contain a gas, like air or hydrogen, inside a closed cycle. Applying heat to one end causes the gas to expand, which moves the piston. Removing heat from the other end makes the gas contract. This cycling of expansion and contraction drives the engine’s pistons without any combustion actually taking place inside the chambers.

## Steam Turbines

Steam turbines are devices that convert the heat energy of pressurized steam into rotational kinetic energy. The high-pressure steam is fed into the turbine and expands across blade rows mounted on a rotor. As the steam expands, it pushes against the curved blades, causing the rotor to spin at high speeds. This spinning rotor is connected to a shaft that can then drive an electrical generator to produce electricity.

Inside the turbine, there are two main sections – the high pressure section and low pressure section. The high pressure steam from the boiler enters the turbine at the first stage blades. These are connected to the rotor and have specially designed aerodynamic profiles that transform the steam’s energy most effectively. As the steam expands through the first stage, its pressure and temperature drop. It then passes through the intermediate pressure stages and finally the low pressure stages. At each subsequent stage, the lower energy steam continues to provide the force to keep the rotor spinning while its own pressure and temperature drop even further. By the exit stage, most of the high pressure steam’s energy has been extracted by the turbine.

The stages within a steam turbine are stacked in succession and enclosed within a metal casing. The fast spinning shaft in the center connects to the electricity generator. Steam turbines provide reliable power output and high efficiencies of over 90% in large power plants. Their ability to extract kinetic energy from high pressure steam makes them an essential technology for generating electricity from heat.

## Stirling Engines

Stirling engines are an external combustion engine that rely on the expansion and contraction of working gases at different temperatures to generate motion. They consist of a sealed working gas (often air, hydrogen or helium) contained within a piston and displacer cylinder. The displacer pushes the gas between a hot heat exchanger (heated externally by combustion or other heat sources) and a cold heat exchanger which removes heat.

As the gas is heated, it expands pushing the piston to generate power. Then as the gas cools, it contracts, and the piston is pushed back by the flywheel or other mechanical linkage. This cycle repeats continuously as long as there is a temperature difference between the hot and cold heat exchangers. The continuous expansion and contraction of the gas turns the crankshaft, converting the heat energy into kinetic energy and usable work.

Stirling engines are known for their high efficiency and ability to utilize small temperature differentials to generate electricity. Their lack of internal combustion makes them very quiet in operation. While more complex than other heat engines, Stirling engines demonstrate a practical method for converting heat gradients into kinetic motion.

## Thermoelectric Generators

Thermoelectric generators are solid-state devices that convert heat flux directly into electrical energy based on the Seebeck effect. They consist of pairs of p- and n-type semiconductors connected thermally in parallel and electrically in series. When one side is heated and the other cooled, charge carriers in the semiconductors diffuse from the hot side to the cold side, generating a voltage.

Thermoelectric generators have no moving parts, are compact, have high reliability, and are environmentally friendly. They can be used to convert waste heat into electricity from sources like vehicle exhaust systems, industrial processes, and appliance flue gases. NASA has used radioisotope thermoelectric generators powered by the heat of decaying plutonium to provide long-term power for space missions.

While low conversion efficiency has limited applications in the past, nanotechnology and new materials research are improving performance. Segmented and cascaded designs have increased efficiency to over 15% in the lab. With further advances, thermoelectric generators have potential for small-scale distributed power generation.

## Solar Thermal Energy

Solar thermal systems convert the sun’s energy into usable heat for power generation and domestic or industrial heating applications. There are two primary ways this is achieved:

Concentrated solar power (CSP) systems use mirrors or lenses to concentrate sunlight onto a receiver containing a heat transfer fluid. The concentrated sunlight heats the fluid to high temperatures, which is then used to boil water into steam to drive a conventional steam turbine that generates electricity. CSP technologies include parabolic troughs, linear Fresnel reflectors, power towers, and parabolic dish/engines.

Solar hot water and space heating systems are the most common solar thermal designs. They use rooftop collectors containing tubes or panels with a heat transfer liquid inside that absorbs the sun’s energy and transfers it to a storage tank to heat water. This hot water can then be used for tasks like washing, heating buildings via radiators, or low-temperature industrial processes. These simple systems convert heat into kinetic energy via convection and conduction.

## Geothermal Power

Geothermal power plants harness the natural heat from the Earth’s core to generate electricity. They tap into reservoirs of hot water or steam located deep underground. Wells are drilled into these geothermal reservoirs to bring the hot water or steam to the surface. The steam can then be directed to turn turbines which power generators that produce electricity.

The geothermal reservoirs require three key elements: heat, fluid, and permeability. Heat comes from the Earth’s molten core and mantle. The fluid is usually naturally occurring water that has percolated down and been heated up. Permeability means there are cracks and porous spaces in the rock that allow the heated fluid to flow through it. Fractured crystalline rocks or permeable sedimentary rocks work best for geothermal reservoirs.

There are three main types of geothermal power plants: dry steam, flash steam, and binary cycle. In dry steam plants, the geothermal steam from the reservoir is piped directly to the turbine generator unit. For flash steam plants, high pressure hot water is brought up from the reservoir and is converted to steam by reducing the pressure. This “flashed” steam then spins the turbine. Binary cycle plants pass the hot geothermal water through one side of a heat exchanger while a separate “working” fluid with a much lower boiling point, like isobutane or isopentane, passes through the other side. The working fluid vaporizes and powers the turbine. The water and working fluid are kept in separate closed loops.

Geothermal plants have low emissions and provide reliable baseload power not subject to fluctuations in weather or climate. However, suitable geothermal reservoirs are located in specific regions mainly along tectonic plate boundaries. There can also be challenges with emissions of hydrogen sulfide or disposal of depleted geothermal brines. Overall though, geothermal is a promising source of renewable electricity generation.

## Examples in Nature

There are numerous examples in nature where heat energy is converted into kinetic energy through natural processes and cycles.

One of the most notable examples is the global convection cycle. The sun heats the surface of the Earth unevenly, causing air and water near the equator to be warmer than at the poles. This temperature difference creates convection currents in the atmosphere and oceans, as warm air and water rises, cools, then sinks back down towards the poles. These massive convection currents drive winds and ocean currents, converting heat energy into the kinetic energy of moving air and water.

Another example is sea breeze and land breeze cycles near large bodies of water. During the day, land heats up faster than water, causing the air above it to warm and rise. This creates a breeze from the cooler water towards the warmer land. At night, the land cools faster, reversing the cycle and creating a breeze from land towards the cooler water.

Metabolic processes within living organisms also frequently convert heat to motion. For example, ATP synthase enzymes use a proton gradient to drive a tiny molecular motor and synthesize ATP energy molecules. On a larger scale, muscles convert chemical energy from food into kinetic energy through heat-releasing metabolic reactions.

These and many other examples in nature demonstrate the continual process of converting heat energy into motion, from microscopic scales up to the global climate system.

## Conclusion

There are several methods for converting heat energy into kinetic energy, which involves putting heat energy to work. This content examined some of the most common man-made and natural processes that perform this conversion.

Among the man-made methods are steam turbines, Stirling engines, and thermoelectric generators. Steam turbines use high-pressure steam from heated water to spin turbine blades and generate electricity. Stirling engines use an external heat source to expand gas inside a sealed engine and move pistons to produce kinetic energy. Thermoelectric generators rely on the Seebeck effect, where temperature differences between two metals produce an electric current.

Solar thermal energy uses mirrors or lenses to concentrate sunlight into heat, which can produce steam to drive turbines. Geothermal power taps into natural underground heat to bring steam or hot water to the surface for electricity production. Examples in nature include wind formation, which uses uneven heating of the atmosphere, and hydrothermal vents on the ocean floor that eject superheated water.

While the methods vary greatly, they all exemplify the thermodynamic principle of converting heat into mechanical motion or electricity. This ability to harness thermal energy as kinetic energy plays an essential role in power generation and enables utilization of diverse heat sources.