In the world of thermodynamics, 100 percent efficiency is not possible. For example, a car cannot produce energy out of thin air. The efficiency of a machine depends on its parts. These parts must be frictionless.
Some examples of high efficiency are a superconductor or an electric resistance heater. A heat engine can convert up to 40% of its heat into workable energy, although this is nowhere near the theoretical maximum.
An ideal heat engine would be able to convert all its heat into work. This is not an impossible goal. However, it is not practical.
In practice, most heat engines operate at 30 to 50% efficiency. It is also difficult to get an engine to achieve 100%. This is because the heat must be cooled down before it can be extracted. There is also friction, which prevents conversion of heat.
However, there are some notable exceptions. Superconductors are close to 100% efficient. Similarly, heat pumps move thermal energy.
The efficiency of a system is inversely proportional to its entropy, or the tendency of a closed system to reduce order. As parts of a system become out of sync, more and more energy is lost.
One example of a hypothetical example is the Carnot engine. The Carnot engine is a hypothetical example of the most efficient heat engine.
Although the Carnot engine is the most impressive of the lot, the true test of its efficiency is whether or not it is actually capable of producing all of its claimed output.