The field of thermodynamics has long been a cornerstone of engineering, underpinning the principles that govern energy conversion and efficiency. Among the various cycles that have emerged, the Brayton cycle stands as one of the most influential, particularly in the realms of aerospace and power generation. Yet, as technology advances and we combine ever more conscious of the ‘energy return’ we get, for ‘energy invested’, there are exciting lines of enquiry that can see further gains in the future of thermodynamics.

The Brayton Cycle: A Brief Overview

The Brayton cycle, also known as the Joule cycle, is the fundamental process behind gas turbine engines, which are widely used in aircraft propulsion and power plants. The cycle operates on the principles of thermodynamics, involving the compression of air, its heating at constant pressure, expansion through a turbine, and subsequent release as exhaust.

In essence, the Brayton cycle is an idealised model of a gas turbine engine, capturing the essence of energy conversion from chemical to mechanical energy. Its efficiency, while significant, is primarily governed by the temperature difference between the combustion chamber and the turbine, and the compression ratio of the air entering the cycle.

Advancements and Limitations

Despite its widespread use, the Brayton cycle is not without limitations. The efficiency of gas turbines, though improved over the years, is inherently capped by the maximum temperature the materials can withstand. Furthermore, the environmental impact of burning fossil fuels in these turbines has become a growing concern, as the world seeks cleaner and more sustainable energy solutions.

To address these challenges, researchers and engineers have been exploring modifications and enhancements to the Brayton cycle. One such innovation is the regenerative Brayton cycle, which reuses the heat from the exhaust to preheat the compressed air, thereby improving efficiency. Another approach is the combined cycle, which couples the Brayton cycle with the Rankine cycle, using the waste heat from the gas turbine to produce steam and generate additional power.

Beyond Brayton: The Future of Thermodynamic Cycles

As we look beyond the traditional Brayton cycle, several alternative thermodynamic cycles are gaining traction, each with the potential to revolutionise energy conversion and efficiency.

1. The Supercritical CO2 Cycle: This cycle utilises carbon dioxide in its supercritical state, where it behaves as both a gas and a liquid. The supercritical CO2 cycle promises higher efficiency than traditional Brayton cycles, with lower environmental impact due to its closed-loop system and reduced greenhouse gas emissions. Its compact size and potential for integration with renewable energy sources make it a promising candidate for future power generation.
2. The Stirling Cycle: The Stirling engine, though an older technology, is experiencing a renaissance due to its high efficiency and ability to operate on various heat sources, including solar energy. The Stirling cycle involves the compression and expansion of a working fluid, typically a gas, in a closed system. Unlike the Brayton cycle, the Stirling cycle is isothermal, meaning it operates at a constant temperature, which can lead to higher efficiencies under the right conditions.
3. The Organic Rankine Cycle (ORC): The ORC is a variant of the traditional Rankine cycle, using organic fluids with lower boiling points than water. This makes the ORC particularly suitable for low-temperature heat sources, such as geothermal energy, waste heat recovery, and solar thermal power. The ORC's flexibility and efficiency in utilising low-grade heat make it a key player in the future of renewable energy.
4. The Kalina Cycle: A lesser-known but highly efficient thermodynamic cycle, the Kalina cycle uses a mixture of water and ammonia as the working fluid. This mixture allows for variable boiling points, which can be tailored to optimise heat absorption and power generation. The Kalina cycle is particularly effective in waste heat recovery and geothermal power, offering efficiencies that surpass those of the Rankine cycle.

Principled Pathways

As we advance into the future, the principles of thermodynamics will continue to guide the development of energy systems. The Brayton cycle has served us well, providing the foundation for modern gas turbines and jet engines. However, the challenges of climate change and the need for sustainable energy require us to explore beyond Brayton.

The supercritical CO2 cycle, Stirling cycle, Organic Rankine cycle, and Kalina cycle represent the next frontier in thermodynamics, each offering unique advantages in efficiency, sustainability, and adaptability. As these technologies mature, they will play a critical role in shaping the energy landscape of tomorrow. ARIA’s continued efforts to be at the vanguard of a ‘just’ energy transition, mandate that our research focusses on ‘next-gen’ which can remedy some of the bottlenecks which are already appearing in the energy transition.