Thermal energy is the sum of all the random kinetic energies of the molecules in a substance, that is, the energy in their motions. The higher the temperature, the greater the thermal energy. On the Kelvin temperature scale, thermal energy is directly proportional to temperature.
All matter is composed of molecules or, in some cases, just atoms. In gases and liquids, molecules are relatively free to move around. In a solid they are not so free to move around, but they can vibrate. Although molecules are microscopic, they do have some mass. Combining mass and speed gives them kinetic energy. Depending on the substance, the particles may interact with each other or their surroundings, in which case they would have potential energy along with kinetic energy. Summing the kinetic energy and potential energy of all the molecules gives the total energy of the substance. This energy is called internal energy because it is internal to the confines of the substance.
In a substance where the molecules may move around, the motion is random. No molecule has a definite speed or kinetic energy, but a molecule has a definite average kinetic energy that depends on the temperature. On the Kelvin temperature scale, the average kinetic energy is directly proportional to the Kelvin temperature. The sum of all the random kinetic energies of the molecules is called thermal energy, therefore, thermal energy is directly proportional to the Kelvin temperature.
When one puts a warm hand in contact with cold water, the hand cools and the thermal energy in the hand decreases. The water warms and the thermal energy of the water increases. The exchange of energy stops when both the hand and water come to the same temperature. While in transit, the energy is called heat. When two objects at the same temperature are in contact, no heat flows between either. Accordingly, there is no change in the thermal energy of either, no matter how much thermal energy in either one.
Gasoline-powered engines, diesel engines, steam turbines, and gas turbines are examples of heat engines. All heat engines work on a cyclic principle of extracting thermal energy from some source, converting some of this energy to useful work, and rejecting the remaining energy to something at a lower temperature. In an automobile engine, the ignition of a gasoline vapor-air mixture produces a gas at a temperature several hundred degrees above room temperature. The pressure of the gas forces a piston downward, doing work. The gas cools and is ejected out the exhaust at a temperature significantly lower than at the time of ignition. A heat engine converting thermal energy to work cannot function unless there is a temperature difference between the source and exhaust. The larger the temperature difference, the greater the efficiency of the engine. Usually, the lower temperature is that of the engine's surroundings and the ignition temperature is significantly higher.
If it were it is practical to have a temperature lower than what exists naturally in our environment, a heat engine could be built in which this temperature was the exhaust temperature and the temperature of the environment was the higher temperature. Heat engines extracting thermal energy from the surface water of an ocean, and rejecting thermal energy to the cooler sub-surface water, have been proposed. They would not be very efficient because the temperature difference would be small and they would not be easy to construct. The attraction is related to the huge amount of thermal energy in the oceans, which cover roughly two-thirds of the earth's surface.
Hobson, A. (1995). Physics: Concepts and Connections. Englewood Cliffs, NJ: Prentice-Hall.
Serway, R. A. (1998). Principles of Physics. Fort Worth, TX: Saunders College Publishing.