What is exergy?
Exergy is the maximum useful work a system can produce relative to the environment it sits in. The first law of thermodynamics tracks the quantity of energy and states that it can be neither created nor destroyed. The second law adds that energy also has a quality: one kilojoule at 1000 K and one kilojoule at 300 K are the same amount of energy, but they cannot produce the same amount of work. Exergy is the quantity that measures this quality, and unlike energy it is not conserved — every real process destroys some of it irreversibly. That loss is called exergy destruction, and it is a direct measure of the irreversibilities inside the system.
Why energy analysis alone is not enough
When the energy efficiency of a gas turbine is calculated, part of the fuel’s chemical energy is seen to leave the system with the exhaust gases and as heat loss. Energy analysis, however, lumps all of that into a single figure and concludes only that a loss exists. In reality some of that loss is unavoidable under the laws of thermodynamics, while some could be recovered through better design. Exergy analysis separates the two and shows which component the loss concentrates in. In gas turbines the largest exergy destruction typically appears in the combustion chamber, because converting the chemical energy of a fuel into heat through an uncontrolled temperature rise is a highly irreversible process. This is a conclusion an engineer looking only at energy would miss: on an energy basis the combustor appears extremely efficient.
Advanced exergy analysis: whose fault is the loss?
Conventional exergy analysis reports the destruction in each component but leaves one question open: does the loss in a component stem from that component’s own inefficiency, or from the operating conditions the rest of the system imposes on it? Advanced exergy analysis answers this by splitting the destruction into endogenous and exogenous parts, and the same approach also separates avoidable from unavoidable losses. The practical consequence is important: a large loss in a component does not by itself mean that investing in that component will pay off — if the loss is exogenous and unavoidable, the improvement has to be sought elsewhere. Prof. Dr. Yasin Şöhret and his colleagues have published this decomposition for the main components of an experimental turbojet engine.
Exergy efficiency is not a single number
An engine’s exergetic performance cannot be treated as a fixed coefficient; it varies with altitude, flight speed and power setting. A meaningful assessment therefore has to examine the engine across the whole flight envelope rather than at a single design point. Şöhret’s work includes an exergy analysis of a three-spool turboprop engine over the flight of a cargo aircraft, and an exergy mapping of an unmanned aerial vehicle through a reconnaissance mission envelope. Studies of this kind make visible why behaviour under real mission conditions diverges from ground test results.
How alternative fuels change the picture
Switching fuel changes not only emission figures but the thermodynamic behaviour of the engine. Hydrogen differs from kerosene in lower heating value, combustion temperature and flame speed, which directly affects the irreversibility in the combustion chamber and therefore the exergy destruction. Şöhret’s research in this area includes the effect of hydrogen use on the exergetic performance of a turbojet engine, an exergy-based evaluation of a conceptual hydrogen-burning ramjet, and the effect of ignition timing on the energy and exergy balance of a hydrogen-fuelled spark ignition engine.
