Turbine technology developments |
Pressure gain combustion – the next frontier
But it’s about ten years away S C (John) Gülen, ASME Fellow, Bechtel Fellow
The basic gas turbine cycle and construction have not changed in the eighty years since the Jumo- 004 turbojet engine entered wartime service in Germany in 1944. The operation of a gas turbine is described by the ideal “air-standard” Brayton or Joule cycle comprising four basic processes: isentropic compression; constant pressure heat addition; isentropic expansion; and constant pressure heat rejection. The ideal cycle describes a heat engine, which is a poor approximation of the ultimate heat engine operating in a Carnot cycle. (In the latter, heat addition and rejection processes are isothermal.) The actual machine, strictly speaking, is not a heat engine; it is an “energy conversion” device and does not operate in a thermodynamic cycle. This is why a gas turbine power plant is commonly referred to as an “open cycle” – an oxymoron – power plant in the literature. (The exception is the “closed cycle” – a tautology – gas turbine, which has been tried in the past with limited commercial success.) The net shaft power output of a gas turbine is determined by the difference between the power consumed by the compressor and the power generated by the expander (ie, the turbine). In a modern machine, roughly one half of the expansion power is consumed by the compressor. This is the reason why turbine inlet temperature is the key knob to make the ratio more favourable, ie, increase the net power output. The improvement, however, comes at the expense of increased fuel burn in the combustor and adversely affects the thermal efficiency. The balancing act involves an increase in the cycle (ie, compressor) pressure ratio to increase the temperature of the air entering the combustor, which is counter-productive because it increases the power consumption of the compressor. To make a long story short, this balancing act results in a symbiotic relationship between the cycle/compressor ratio (PR) and the turbine inlet temperature (TIT), which resulted in a concomitant rise in both over the last four decades. From the early E class with 1300°C–12:1 (TIT–PR) combo to the F class with 1400°C–15:1 and, finally, to the present-day advanced class gas turbines with close to 1700°C and 25:1, with thermal efficiencies of 43-44% (ISO baseload on a lower heating value (LHV) basis). In a combined cycle configuration, today’s state-of-the-art is around 64% LHV (ISO baseload) with the advanced class gas turbines.
Further increase in thermal efficiency without a change to the underlying Brayton cycle has already become an endeavour of diminishing
returns. Emerging technologies such as additive manufacturing, exotic hot gas path materials, and 3D computational design platforms can (and do) result in incremental improvements but they are unlikely to bring the technology to the next level on their own. (70% net LHV efficiency is an oft-cited target.) One such change, at an ideal cycle level, is a switch from constant pressure (isobaric) heat addition to constant volume (isochoric) heat addition. As can be easily recognised from the ideal gas equation of state, pv = RT, the advantage of this cycle mod is that an increase in working fluid temperature is accompanied by a commensurate increase in pressure. In other words, a portion of cycle pressure rise can be transferred from the power consuming mechanical compression to the heat addition process (ie, thermal compression). In fact, there is an ideal cycle like that in the literature: the Atkinson cycle (frequently referred to as the Humphrey cycle in detonation combustion circles). A graphical comparison of air-standard (ideal) Atkinson and Brayton cycles with the same cycle maximum temperature (ie, T3, the ideal cycle proxy for TIT) is provided in Figure 1. The cycles
are depicted on a temperature–entropy (T–s) diagram, which is the most convenient way to visually describe the cycle thermodynamics. To be precise, there are four ideal cycles shown in Figure 1, namely: Two Carnot cycles, {1–2C–3–4C–1} and {1–2C–3A–4CA–1}, which represent the theoretical maximum efficiency set by the second law of thermodynamics, ie, 1 – T1/T3. The Brayton cycle {1–2–3–4–1}, a poor substitute for the Carnot cycle, whose efficiency is a function of the cycle pressure ratio, PR = P2/P1, only. The Atkinson cycle, another Carnot substitute, where the isochoric heat addition process further increases the cycle pressure ratio. Note that the Atkinson cycle efficiency is a function of the pre-compression ratio, r = v1/v2, and TIT, which sets the thermal pressure ratio rp = P3A/ P2 (= T3A/T2), as mentioned above.
Why the Atkinson cycle is better than the Brayton cycle may not be that obvious to the naked eye. Let us use some numbers for illustration and assume that the Brayton cycle PR = 15:1 (typical F class). The cycle efficiency is 53.9%. For TIT = T3
Figure 1. Ideal cycle comparison of Atkinson (Humphrey) with isochoric heat addition – the ideal proxy for pressure gain combustion – and Brayton with isobaric heat addition – the ideal proxy for deflagration combustion – cycles
30 | July/August 2024|
www.modernpowersystems.com
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