Rankine cycle

Definition

The Rankine cycle is a thermodynamic cycle that converts heat into work through a series of processes involving a working fluid, typically water or steam. It consists of four main processes: isentropic compression, isobaric heat addition, isentropic expansion, and isobaric heat rejection, making it a foundational concept in the study of heat engines and energy conversion systems.

5 Must Know Facts For Your Next Test

1. The Rankine cycle operates between two heat reservoirs: a high-temperature source where heat is added and a low-temperature sink where heat is rejected.
2. In an ideal Rankine cycle, the working fluid undergoes isentropic processes which ideally conserve entropy during compression and expansion.
3. The efficiency of the Rankine cycle can be improved through modifications like regenerative heating, which reuses some of the exhaust heat to preheat the feedwater.
4. Real-world Rankine cycles often deviate from the ideal due to irreversibilities and non-ideal behaviors of the working fluid, leading to lower thermal efficiencies.
5. The Rankine cycle serves as the basis for many power generation systems, including coal, nuclear, and concentrated solar power plants.

Review Questions

• How does the Rankine cycle demonstrate energy transfer through its four main processes, and what role does the working fluid play in this process?
  • The Rankine cycle illustrates energy transfer through isentropic compression and expansion, along with isobaric heat addition and rejection. The working fluid absorbs heat from a high-temperature source during the isobaric heat addition phase, increasing its internal energy. This energy is then converted to work as the fluid expands isentropically through a turbine. After work extraction, the fluid releases heat to a low-temperature sink in the isobaric heat rejection phase before being compressed back to the initial state. Each process highlights how energy changes forms and moves through the system.
• Evaluate how modifications such as reheat and regenerative cycles improve the performance of traditional Rankine cycles.
  • Modifications like reheat and regenerative cycles enhance traditional Rankine cycles by improving thermal efficiency and reducing turbine blade erosion. The reheat cycle involves reheating steam after it partially expands in the turbine, which increases its energy content before further expansion. This reduces moisture content and prevents damage to turbine components. The regenerative cycle utilizes exhaust heat to preheat feedwater before it enters the boiler, resulting in less fuel consumption for generating steam. Both modifications optimize energy utilization within the system.
• Analyze the impact of irreversibilities on the performance of the Rankine cycle and how these effects can be mitigated in practical applications.
  • Irreversibilities significantly affect the performance of the Rankine cycle by reducing thermal efficiency and increasing entropy generation. Factors such as friction in components, non-ideal behavior of fluids, and temperature differences during heat transfer contribute to these irreversibilities. To mitigate their effects in practical applications, engineers can implement design optimizations like improved insulation to reduce heat loss, better material selection to minimize friction, and advanced control strategies for maintaining optimal operating conditions. These measures help achieve closer approximation to ideal performance and enhance overall system efficiency.

Related terms

Heat engine: A device that converts thermal energy into mechanical work, operating on various thermodynamic cycles including the Rankine cycle.

Thermal efficiency: A measure of how effectively a heat engine converts heat from fuel into work, expressed as the ratio of work output to heat input.

Reheat cycle: An enhancement of the Rankine cycle where steam is reheated after partial expansion to improve efficiency and reduce moisture content in the turbine.