Engineering Applications and Energy Conversion Characteristics of Power and Refrigeration Cycles

Combining practical industrial systems, this article analyzes the operational characteristics of power cycles and refrigeration cycles in energy conversion and reverse heat transfer.
Represented by large-capacity coal-fired or gas turbine power plants, the core task of power cycles is to convert high-grade thermal energy released by fuel combustion into mechanical work, ultimately outputting electrical energy. In practical industrial systems, this process typically relies on the Rankine cycle or Brayton cycle. The working fluid absorbs heat from a high-temperature heat source in a boiler or combustion chamber and subsequently expands to perform work in a steam or gas turbine.

However, the operational characteristics of actual power cycles are severely constrained by irreversible losses. Due to fluid friction resistance, heat transfer temperature differences, and mechanical efficiency limitations of expansion and compression components, the actual expansion process is not an ideal isentropic process. This results in the effective work output of the working fluid being lower than the theoretical maximum. To approach thermodynamic limits, industrial systems widely adopt regeneration, reheat, and combined cycle technologies. For instance, in ultra-supercritical power plants, multi-stage steam extraction for feedwater heating not only increases the average heat absorption temperature but also effectively reduces the heat transfer temperature difference between the boiler and the working fluid, thereby macroscopically enhancing the energy conversion efficiency of the entire power cycle.

Characteristics of Reverse Heat Transfer in Refrigeration Cycles in Chemical and Cold Chain Industries

In contrast to power cycles that convert heat into work, refrigeration cycles and heat pump systems represent a typical class of reverse heat transfer processes. In industrial scenarios such as petrochemicals, large-scale cold storage, and air separation, refrigeration cycles utilize externally input work (such as electricity-driven compressors) to forcibly transfer heat from a low-temperature environment to a high-temperature environment. Their basic working principle relies on the phase-change properties of the working fluid, achieving low-temperature heat absorption through throttling and depressurization, followed by high-temperature heat rejection through compression and temperature rise.

The operational characteristics of practical industrial refrigeration systems are centrally reflected in the pursuit of the "Coefficient of Performance" (COP) and the strict selection of thermophysical properties of working fluids. In cryogenic engineering, single-stage compression often struggles to meet large temperature difference heat transfer demands, hence multi-stage compression and cascade refrigeration cycles are widely applied in practice. Meanwhile, since the reverse heat transfer process is likewise accompanied by compressor volumetric losses, mechanical friction, and finite temperature difference heat transfer in heat exchangers, the actual power consumption of the system is significantly higher than that of the theoretical Carnot cycle. Engineering technicians must finely adjust the expansion valve opening and compressor speed to match load fluctuations under variable operating conditions, ensuring continuous and stable system operation at the optimal energy efficiency ratio.

Coupled and Integrated Operational Characteristics of Power and Refrigeration Systems

In highly integrated modern industrial systems, power cycles and refrigeration cycles often do not exist in isolation, but are closely coupled through the cascade utilization of energy. For example, in natural gas liquefaction plants or distributed energy systems, high-temperature exhaust gas discharged from gas turbines can drive absorption refrigerators via waste heat boilers, or recover low-grade waste heat to generate electricity through Organic Rankine Cycles (ORC).

The characteristic of such coupled operation lies in achieving matched optimization of energy quality (exergy) and quantity. Power cycles focus on the efficient work conversion of high-grade thermal energy, whereas refrigeration cycles focus on the creation and maintenance of low-grade cooling capacity. Through engineering integration and optimization, the system not only reduces thermal pollution caused by the direct discharge of waste heat into the environment, but also significantly improves the comprehensive primary energy utilization rate of the entire industrial installation. Understanding and mastering the operational characteristics of these two cycles in practical systems is an important theoretical and technical foundation for achieving industrial energy conservation, consumption reduction, and low-carbon transition.