Momentum and Energy
What does energy efficiency mean?
In most cases thermal energy is not a useful form of energy for a system. You want the energy content in the gasoline in your auto to give it kinetic energy, not to make it hotter. The cooling system uses a water-antifreeze mixture to cool the engine and warm the radiator, where air flowing through it is heated, thus cooling the fluid. The thermal energy in the heated air is often called rejected or waste energy. An auto is about 20% efficient. That is, only 1/5 of the energy in the gasoline is converted into the kinetic energy of the auto. In addition to the hot air from the engine, tires get warm from flexing, and the brakes get hot when they are applied. All this thermal energy is rejected or waste energy.
Your home furnace converts the chemical energy in oil or gas or electrical energy into thermal energy, either of air or water, depending on whether you have forced-air heat or hot water radiators. But not all the energy goes into heating the house; some leaves through the chimney as rejected or waste energy. Heating systems used to be about 60% efficient. Newer systems can be as much as 95% efficient.
Means of increasing the efficiency of auto and home appliances is an active area of research as nations try to conserve as much of the produced energy as they can.
Your body also uses only a fraction, again about 20%, of the food energy to move your limbs when you walk or run. Your body is cooled by contact with the air, or by evaporating liquid—either perspiration or the humid air expelled by your lungs.