What is defined as the 'thermal cycle' in power plants?

The 'thermal cycle' refers to the series of processes that convert heat into work and then back into heat within a power plant. This is a fundamental concept in thermodynamics, particularly relevant to power generation, where heat energy is transformed into mechanical work, typically through a working fluid within a heat engine.

In a typical thermal cycle, such as the Rankine or Brayton cycle, the working fluid absorbs heat from a heat source (like combustion gases or steam), expands to perform work (such as driving a turbine), and then releases heat to a cooler environment before returning to its initial state to repeat the cycle. This cyclic process is essential for efficient power generation, as it allows the continual conversion of thermal energy into mechanical work, which can then be turned into electrical energy.

The other options do not accurately capture the comprehensive definition of a thermal cycle. Cooling down the system is just one part of the process but doesn't reflect the complete cycle of heat work conversion. The maintenance schedule pertains to equipment upkeep rather than thermodynamic processes. Fuel selection contributes to efficiency but is not described as a thermal cycle itself. Thus, option B stands out as the correct description of a thermal cycle in power plants.