Unified trade-off optimization for general heat devices with nonisothermal processes

Area of Science:

• Thermodynamics
• Heat Transfer
• Energy Systems Engineering

Background:

• General heat engines and refrigerators often operate with nonisothermal processes.
• Understanding performance trade-offs is crucial for optimizing these devices.
• The specific heat of the working medium is a key factor influencing device configuration.

Purpose of the Study:

• To analyze the efficiency and coefficient of performance (COP) for general heat engines and refrigerators with nonisothermal processes.
• To investigate the impact of specific heat on optimal configurations and performance bounds.
• To provide practical insights for designing and operating actual heat engines and refrigerators.

Main Methods:

• Analysis of efficiency and COP under a trade-off criterion.
• Investigation of cycles with constant and nonconstant specific heat.
• Examination of performance bounds as dimensionless contact time approaches infinity and zero.

Main Results:

• For constant specific heat, efficiency and COP bounds match endoreversible Carnot cycles, independent of cycle duration.
• For nonconstant specific heat, alternative upper and lower bounds are proposed under asymmetric limits for infinite dimensionless contact time.
• The endoreversible Carnot model is recovered when dimensionless contact time approaches zero.

Conclusions:

• Specific heat significantly influences the optimal design of heat engines and refrigerators.
• Performance bounds for nonisothermal cycles are established, providing theoretical limits.
• The findings offer practical guidance for improving the design and operation of real-world heat devices.