What a fantastic question! I shall describe a particular topic that, if we find the right materials for, could revolutionize how energy is harvested.

There are many applications that thermal energy is used for, e.g., as a source of heat. Often times though, it is an unwanted form of waste energy due to friction or other inefficient processes. The fact that the car hood gets very hot after you drive is because the combustion reaction that drives the engine is an extremely inefficient chemical process.

But what if we could harvest that waste energy into useful energy?

It turns out we can! With a set of materials known as thermoelectrics.

The basic idea is this: Thermoelectrics convert a temperature difference into electricity. How can this be?

You may have learned in your science classes that matter is made up of many extremely tiny particles known as atoms (side note: not the most accurate depiction of the atom, but it will do here).

A solid is made up of many of these atoms (10²⁶, that's 1 with 26 zeros behind it- read this link If you take a bar of material, heat up one side and cool down the other, you will get a voltage, which can drive electric current. Why is this so?

The particles that are responsible for carrying electric current are the electrons. When you heat one end of the bar, you are giving the electrons on this particular side more thermal energy to move about. This means the electrons are more mobile than the ones on the cold side. The temperature difference across the bar causes the electrons to flow from hot to cold. This results in an electric field

How thermoelectronics work

But here it gets a little complicated. You need the temperature difference to drive the flow of electrons, but the electrons also carry heat with them, which diminishes the temperature difference. So you need a material that is both electrically conductive (i.e., electrons flow easily from one side to another) but is thermally insulating (i.e., heat doesn't travel as easily, so you maintain a temperature difference). That's saying you want the electronic properties of a metal but the thermal properties of a glass. These are two very different, seemingly contradictory criteria!

This is where material scientists and clever engineering come in. It turns out thermal conductivity is also determined by what kind of atoms are in the system, and by tuning which atoms in the periodic table are in the material, we can tune the thermal conductivity, while still maintaining electrical conductivity. The biggest materials challenge in this area is to find a material that is efficient enough at turning waste heat into electricity.

While this currently limits the wide-scale commercialization of thermoelectrics, there are a few technologies with thermoelectrics already in them!

One really cool application that I just learned about from one of my classmates is the use of thermoelectrics in the radioisotope thermoelectric generator RTG An RTG uses nuclear fission to provide the heat source and the coldness of space as the heat sink. If you have read (or watched) The Martian, this might sound familiar to you. An RTG is currently providing the power for Voyager I , which is the first spacecraft ever to enter interstellar space (beyond our own solar system!). It has been running for almost 40 years using the power generated from the RTG, for which one of the key parts is the thermoelectric material.

Heat is difficult to use, because of something called "entropy" (you'll learn more about that in high school chemistry or physics). However, if you have one object with one temperature, and another with a different temperature, you can do work as the heat flows from the hotter object to the colder object. Finally, higher temperature cause chemical reactions to happen faster, so if what you really want to do is cause a chemical reaction, then having extra heat may be needed to cause that reaction.

Thermal energy is used to heat my apartment. It faces southwest, towards the afternoon sun, so I get so much sun, and my apartment gets too hot. It's really cheerful, though, having so much sun, even if it's too hot.

Thermal energy is used to cook my food on my electric stove and in my electric oven. Thermal energy is used to cook my son's food on his gas stove and in his gas oven.

Thermal energy is produced in cars by burning gasoline [chemical energy], and the thermal energy makes pistons move in the engine, which makes the car's wheels move. I don't actually know much about car engines, but I know thermal energy is involved.

Thermal energy makes ice cubes melt when you take them out of the freezer

Thermal energy keeps our bodies warm - our bodies burn food to make thermal energy to keep our bodies at their proper body temperature.

One example of the use of thermal energy is in many electricity production methods which convert thermal energy into electrical energy. Thermal energy in matter is held by vibrations of the particles or atoms in the matter.

In electricity production fuels like natural gas, coal or biomass can be burned to produce heat. The heat is used to boil water or another liquid, which now has a higher thermal energy. The gas phase of the water has thermal energy because the gas particles are vibrating and moving around in space. Thermal energy in this case is alsokinetic energy.

The thermal energy of the gas is transferred into mechanical energy when the gas is used to spin a turbine. Finally, an electric generator can convert the mechanical energy of the turbine into electrical energy.

The most basic way that thermal energy can be used is to heat stuff up. Thermal energy can be thought in many cases as how fast the molecules in a substance are moving on average. This energy can be transferred to molecules that aren’t moving as quickly (a colder substance) when the molecules collide with each other and transfer thermal energy.

One of the most important uses of thermal energy for human use is for heating water. In power plants, water is heated producing steam. This steam then rotates a cylinder which moves a magnet back and forth which “pumps” electrons through a wire making electricity. So in this case, thermal energy is used to make electricity. It’s important to realize that thermal energy is usually considered “low quality” energy in that a lot of processes are considered inefficient if they create a lot of heat. For instance, with a car, you want most of the energy going into moving the car, not heating it. So thermal energy has a lot of uses, but many times it is an undesired side product.