Heat Engine

Mechanical work done by a shaft submerged in water to the internal energy of the water; this energy may then leave the water as heat. However, any attempt to reverse the process will fail. ==Work can be converted to heat directly and completely, but converting heat to work requires the use of special devices called heat engines.==

Heat engines all follow these principles:

1. They receive heat from a high-temperature source (solar energy, oil furnace, nuclear reactor, etc.)
2. They convert part of this heat to work (usually in the form of a rotating shaft)
3. They reject the remaining waste heat to a low-temperature sink (the atmosphere, rivers, etc.)
4. They operate on a cycle.

Heat engines and other cyclic devices usually involve a fluid to and form which heat is transferred while undergoing a cycle. This is called the working fluid.

Steam Power Plant

A steam power plant is an example of a heat engine. Combustion takes place outside the engine, and the thermal energy released during this process is transferred to the steam as heat. The schematic of a basic power plant is shown.

• $Q_{\text{in}}$ is the amount of heat supplied to the steam in boiler from a high-temperature source.
• $Q_{\text{out}}$ is the amount of heat rejected from steam in condenser to a low-temperature sink
• $W_{\text{out}}$ is the amount of work delivered by steam as it expands in turbine.
• $W_{\text{in}}$ is the amount of work required to compress water to boiler pressure.

The net work output is then:

$$W_{\text{net, out}}=W_{\text{out}}-W_{\text{in}}$$

Recall that for a closed system undergoing a cycle, the change in internal energy $\Delta U$ is zero, and therefore the net work output of the system is also equal to the net heat transfer to the system:

$$W_{\text{net, out}}=Q_{\text{in}}-Q_{\text{out}}$$