| Supercritical CO2


prediction and trouble-free operation of sCO2 cycle equipment. Potential problems include


significant shift in the operating point and corrosion. Elimination of these risks requires extensive operation and data collection over the entire operating map of the compressors and the turbine. The STEP test campaign will be a huge step in sCO2


technology advancement in this respect.


Thermodynamics perspective All heat engine cycles are attempts (in many cases, quite feeble) to approach the theoretical ultimate, ie, the Carnot cycle. According to the Kelvin–Planck statement of the second law of thermodynamics, there can be no heat engine operating in a cycle more efficient than a Carnot cycle operating between the same temperature reservoirs (ie, maximum and minimum cycle temperatures, TMAX and TMIN, respectively). In practice, this is an impossible feat to accomplish. Conventional Brayton (gas turbine) and Rankine (steam turbine) cycles are indeed far removed from the Carnot ideal. In simple terms, their mean-effective cycle heat addition and/or heat rejection temperatures (henceforth, METH and METL, respectively) are well short of TMAX and TMIN (in the case of the Brayton cycle, both). Attempts to address these problems, ie, to Carnotise the heat engine cycles, usually involve several thermodynamic ‘tricks’, eg, intercooled compression (to approximate isothermal heat


rejection), recuperation (to increase METH and to reduce METL, simultaneously), and/or reheat (to approximate isothermal heat addition). Supercritical CO2


cycles with recuperation


and intercooled compression (combined with pumping in latter stages) are effectively Carnotised Brayton or Rankine cycles. The temperature–entropy (T–s) diagram of the intercooled-recuperated sCO2


cycle in


Figure 1 is shown (in a generic manner) in Figure 6. As shown in Figure 6, T4 = TMAX, and T1 = TMIN, which is usually assumed to be 15°C (ISO ambient temperature). The ideal cycle performance can be calculated using the air- standard cycle approach, ie, (i) ideal gas equation of state, (ii) constant specific heat, cp gas), and (iii) perfect recuperation, ie, T3 T2


(ie, perfect = T5


= T6 . METH is the logarithmic average of T3


= TMAX, and easy to evaluate. Calculation of METL requires tedious enthalpy/entropy change evaluations for pre-cooling, inter- and after-cooling. (For the AFC, METL ~ 317°C.) Consequently, assuming perfect (isothermal) compression, with METL ~ TMIN = 288 K suffices for a conservative estimate.


and T4


 = 1.18 are shown in Figure 7. Mature technology (plant basis) performance is estimated using a technology factor (TF) of 0.75. This is comparable with the rating performances of modern gas turbine combined cycles with advanced H/J class machines (turbine inlet temperature close to


and


1700°C with cycle PR in the range 24:1 to 25:1). It should be noted that this performance level took close to a half century to achieve after the technology was already at TRL = 9.


The next steps Considerable R&D investment in sCO2 technology


continues. The STEP facility, due to enter the operational phase in late 2023, is expected to make a strong contribution to the advancement of the technology, which is still far from reaching the coveted TRL of 9.


The oxy-combustion variant (notable the Allam–Fetvedt Cycle) with inherent CO2


capture


capability is potentially a worthy competitor to, say, advanced class GTCC with post-combustion capture. The latter is readily capable of 51% net LHV efficiency with mature technology today (eg, capture employing chemical absorption with generic amines).


Even so, recent field experience should make one cautious that even with readily available technologies successful deployment of new applications takes a long time until all the bugs are ironed out and performance potential is fulfilled. As far as CSP and industrial waste heat


recovery applications are concerned, sCO2 is indeed a strong contender relative to, say, organic Rankine or small steam Rankine cycles. Upon the successful completion of the STEP test campaign, the only remaining hurdle will be cost- effectiveness.


Quality solutions for PowerGen


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