scientific breakthrough carnot efficiency heat engines

Rethinking Carnot: Scientists Break Power-Efficiency Limits in Thermodynamics

Revolutionizing Thermodynamic Principles

Overcoming the Limitations of the Carnot Engine

Revolutionizing long-standing principles of thermodynamics, a recent study in Physical Review Letters suggests that a heat engine could theoretically be engineered to reach maximum power output while nearing Carnot efficiency.

The Carnot Heat Engine: A Theoretical Foundation

The Carnot heat engine is a theoretical thermodynamic system that harness heat energy from two temperature reservoirs—one hot and one cold—to perform mechanical work.

The engine operates by extracting heat from the hot reservoir, converting a portion into mechanical work and expelling the remaining heat to the cold reservoir. This process follow the thermodynamic principles of the Carnot cycle.

In an ideal scenario, the Carnot engine would achieve perfect reversibility and operate at maximum efficiency. However, real-world heat engines are inherently irreversible and dissipate energy as heat.

According to the second law of thermodynamics, no real heat engine can achieve the efficiency of a Carnot engine functioning between the same thermal reservoirs.

The fundamental limitation of constructing a heat engine with near-Carnot efficiency is that the process would take infinitely long while delivering minimal power.

The Fundamental Challenge of Heat Engines

The Carnot Efficiency Trade-Off

To tackle this challenge, the researchers developed a biochemical heat engine. In an interview with publishing website, study co-author Assistant Prof. Yu-Han Ma (Beijing Normal University) and Dr. B. Shiling Liang (Center for System Biology Dresden) shared their insights.

Prof. Ma explained, "This collaboration began through a discussion between Shiling and me in late 2022, during which Shiling, in his initial research observed that degeneracy could enhance the efficiency at maximum power (EMP) in heat engines."

The Role of Degeneracy in Enhancing Efficiency

Dr. Liang explained, "Using a polymer folding model from my earlier work, i designed a minimal heat engine that much to my surprise, showed the possibility of surpassing conventional efficiency limits at maximum power. This unexpected finding led me to approach Yu-Han, which marked the beginning of our joint research."

Energy-Efficiency Equilibrium and the 1/2-Universality Principle

The Balance Between Efficiency and Power

Heat engines, which transform thermal energy into valuable work have played a crucial role in advancing human civilization since the Industrial Revolution.

However, heat engines have long encountered a significant trade-off in achieving Carnot efficiency: they can either operate at maximum efficiency by moving at an exceedingly slow pace (producing minimal power) or generate useful power at the cost of efficiency.

Understanding the 1/2-Universality Principle

The "1/2-universality principle" is particularly significant. It asserts that heat engines operating in the linear response regime (with small temperature differences) can only achieve half of the Carnot efficiency at maximum power.

According to Prof. Ma, "The universality of this trade-off relation has been confirmed in several scenarios, especially in low-dissipation heat engines, where the maximum power efficiency has a clear upper bound and there is a substantial difference between this and the Carnot efficiency."

The solution was found in the form of a system with degenerate energy levels where each energy level is associated with multiple microscopic states or configurations, all of which correspond to the same energy level.

Phase Transition and the Role of Degeneracy

High-Energy States and Molecular Configurations

The model includes two states, one low-energy and one high-energy, with the higher-energy state able to support a significantly greater number of molecular configurations indicating a higher degeneracy.

There are two distinct reaction pathways for transitions between energy levels: one governed by ATP hydrolysis at low temperatures and the other a spontaneous transition occurring at high temperatures.

First-Order Phase Transitions and Energy Loss

At high temperatures, the system naturally favors the high-energy state, as it can access the numerous configurations available there. This increases the likelihood of a spontaneous transition which does not require ATP.

At lower temperatures the reaction driven by ATP hydrolysis is more likely to occur, leading the system from the low-energy state to the high-energy state.

As the system size grows the high-energy state can accommodate increasingly more configurations compared to the low-energy state, making the transitions sharper and more abrupt. These transitions are classified as first-order phase transitions occurring with minimal energy loss.

"Through the creation of a minimal model that includes this feature we were able to illustrate how it breaks conventional thermodynamic boundaries and identify the physical mechanism behind the collective advantage," said Dr. Liang.

Exceeding Conventional Thermodynamic Boundaries

How the Biochemical Engine Surpasses Carnot Efficiency

The researchers showed that their biochemical engine can reach Carnot efficiency while sustaining maximum power output as the system size increases indefinitely. The power scales linearly with the system size, while efficiency nears the Carnot limit.

The Power of High-Degeneracy Systems

Dr. Liang discussed the design principle of their model, stating, "By engineering systems with high-degeneracy states, the performance of heat engines can be greatly improved."

Much like the idealized Carnot engine has influenced the development of practical heat engines over centuries, the concept of increasing degeneracy can now be seen as a valuable strategy for enhancing heat engine efficiency, even if perfect degeneracy remains unattainable.

Breaking Established Universality and Re-thinking Thermodynamic Constraints

Violation of Well-Established Universality

The study also reveals a breach of a well-established universality.

"Our research indicates that this universality breaks down in the limit of large degeneracy." explained Prof. Ma.

The Impact of Large Degeneracy on Thermodynamics

"The sequence in which the limits of Carnot efficiency and degeneracy are taken influences the proportionality coefficient of the EMP relative to the Carnot efficiency. This suggests that when the system exhibits a diverging intrinsic quantity certain traditional thermodynamic constraints may need to be reconsidered."

Future Directions: Biochemical Engines and Practical Applications

Relevance for Biological Systems

Furthermore, the system operates as a biochemical engine that synthesizes ATP, thereby making it relevant for biological systems. The next challenge lies in identifying practical heat engines with these attributes. Dr. Liang highlighted biopolymers as strong candidates, given their natural possession of highly degenerate unfolded states.

Source

"Discover how groundbreaking research is reshaping our understanding of heat engines and efficiency! Scientists are challenging long-established thermodynamic limits with innovative biochemical heat engines and the implications for energy systems could be profound.

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