There are many equivalent statements of the 2nd Law of Thermodynamics. For our purposes, an appropriate (albeit simplified) statement of the 2nd Law is that no process in a system can proceed without increasing the level of “disorder” in the system + surroundings combination. The level of disorder is determined by a system property called entropy. Entropy (disorder) can be increased by heat transfer into a system, friction, or other losses. As a consequence of this law, not all of the energy input from an energy source can be used to generate work output (η0 < 1). Instead, some of that energy input is lost to internal friction, heat transfer to low temperature thermal sinks, or other losses. In fact, for work output produced by an engine drawing energy from a high temperature energy source (temperature TH), the 2nd Law requires that some energy be lost to a low-temperature thermal sink (temperature TL). It can be shown that the maximum possible thermal efficiency for this process is η0 = 1 - TL / TH, known as the Carnot efficiency. The Carnot efficiency is for an idealized process, however, and real systems have thermal efficiencies less than this. Nevertheless, the thermal efficiency of real systems increases as TL / TH decreases, similar to the Carnot efficiency.