Steady state thermodynamic performance studies

Various closed-cycle GT heat sources, working fluids and layouts have been studied in literature in order to determine their thermodynamic performances.

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2.5.1.1 Performance comparison with conventional plants

Angelino performed thermodynamic evaluation of four configurations of s-CO2 condensation

cycle and concluded that s-CO2 power cycle has the potential to perform better than reheat steam

cycle on account of efficiency, simplicity and compactness (Angelino, 1968).

Sánchez et al. (2011b) compared the performance of molten carbonate fuel cell (MCFC) hybrid system using s-CO2 closed-cycle GT to a reference hybrid system using air in open cycle

configuration. Results indicated that the MCFC-s-CO2 hybrid system yielded about 10%

efficiency increase with respect to the reference system as a result of improved performance specifications of s-CO2 components (turbine, compressor and heat exchanger). Dekhtiarev (1962)

studied condensing reheated s-CO2 cycles as a good alternative to steam cycle for fossil fuel plant.

Technical-economic analysis of coal-fired s-CO2 Brayton cycle with carbon capture by Le

Moullec (2013) showed promising results with net plant efficiency of 41.3% as well as reduction in levelized cost of electricity and reduction in cost of avoided CO2 compared to superheated

steam power cycle with carbon capture. Hanak and Manovic (2016) proposed s-CO2 cycle instead

of the conventional steam cycle for electricity generation from the high-grade heat of calcium looping process. The calcium looping plant was used to capture 90% of CO2 from the flue gas of

coal-fired power plant. Results of retrofitting the calcium looping process with s-CO2

recompression cycle indicated that a gain in efficiency of about 1-2% point over that of the steam cycle could be obtained. Modelling results of biomass to PCSs based on cascaded s-CO2 cycle

showed a 10% efficiency increase above the convention biomass plant PCS based on ORC or reciprocating internal combustion engines (Manente and Lazzaretto, 2014).

Ishiyama et al. (2008) examined steam Rankine cycle, helium and s-CO2 closed-cycle GT for

nuclear fusion reactor and recommended the s-CO2 cycle based on its reasonable efficiency,

reduced volume and the ease of permeated tritium separation. The coupling of small modular light water reactor to s-CO2 Brayton cycle was investigated by Yoon et al. (2012). Preliminary results

showed comparable efficiency to the conventional steam cycle and potential for further reduction of capital cost of SMR plant due to the small size of s-CO2 cycle components. Santini et al. (2016)

investigated the adoption of s-CO2 cycle for a far lower temperature (about 260 0C) of an existing

PWR. The results indicated that a reheated recompression s-CO2 cycle achieved a net cycle

efficiency of about 34% compared to 33.5% of the existing steam cycle and the plant footprint was 10 times smaller than the steam cycle plant.

In Chacartegui et al. (2011a), s-CO2 cycles were investigated for CSP plants as alternative to the

conventional steam cycle. Performance results showed that s-CO2 cycle has the potential to

compete with the steam cycles based on efficiency and cost. Similarly, Sasol Technology of South Africa benchmarked three s-CO2 cycles layouts and a supercritical steam cycle against a

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superheated steam cycle for CSP plants with molten salt storage system (Cheang et al., 2015). In this instance, results showed that s-CO2 cycles cannot compete with the current steam cycle

technology in term of efficiency and cost. The conflict between the conclusions of the two studies can be attributed to the differences in assumed TITs, gearbox and generator/motor efficiencies, and costs associated with material selection.

2.5.1.2 Studies based on cycle configuration

Herranz et al. (2009) investigated helium direct Brayton cycle with single and three-shaft configurations with emphasis on the effects of intercooling and reheating using the parameters and conditions of PBMR. Thermodynamic and economic assessment indicated that intercooling produces substantial improvement in efficiency, reheating produces no remarkable improvement in performance other than allowing flexibility of operation and use of multi-shaft configuration tends to increase cost of plant without any efficiency improvement.

PhD thesis by Dostal at MIT provided a detailed steady state analysis of s-CO2 cycles for next

generation nuclear reactors based on thermodynamic performance (Dostal, 2004). The study settled on the recompression s-CO2 cycle layout as the preferred option for reactor core outlet

temperature above 500 °C because of its simplicity, compactness, cost and thermal efficiency. Al-Sulaiman and Atif (2015) compared the performance of five different s-CO2 Brayton cycle

configurations (simple, regenerative, recompression, pre-compression and split expansion cycle) for CSP application and the recompression cycle was found to give the best efficiency. Recompression and partial cooling cycles were compared by Neises and Turchi (2014) for CSP, highlighting the potential reduction in cost and improvement of CSP receiver efficiency with the partial cooling cycle.

Recently, Wang et al. (2017) reviewed and compared the main s-CO2 cycle configurations

integrated with molten salt solar power towers having both the main heater and a reheater. Intercooling was introduced into the main compressor of the recompression cycle to further improve the performance. S-CO2 cycles and the various configurations have also been

investigated as bottoming cycles for fuel cell (Bae et al., 2014) and GT system (Kim et al., 2016) as well as an alternative PCS for other waste heat recovery process (Persichilli et al., 2011; Banik et al., 2016) and biomass plants (Manente and Lazzaretto, 2014). Bae et al. (2014) investigated s- CO2 cycle configurations comprising an s-CO2 Brayton-steam Rankine cycle cascade, a

recompression cycle and two simple recuperated cycle (a supercritical and a trans-critical cycle) as bottoming cycles for molten carbonate fuel cell. The different layouts were compared in terms of cycle efficiency, the net electric power output of the hybrid system and physical size.

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Kim et al. (2016) compared the performance of nine s-CO2 cycle layouts together with three newly

developed concept as bottoming cycles for GT plant. It was concluded that although the recompression cycle has a good cycle efficiency, it is not suitable as a bottoming cycle due to its poor heat recovery factor. Pham et al. (2015) carried out the mapping of thermodynamic performance and exergy analysis for different s-CO2 cycle configurations and operating

conditions. The study concluded that the recompression cycle in condensing mode is the most fitting configuration for PWR application due to system simplicity and compactness, and for SFR application due to improved efficiency and optimal IHX inlet temperature. Sakar (2009) performed exergetic analysis of s-CO2 recompression cycle and found the exergetic efficiency

more sensitive to the isentropic efficiency of turbine and the effectiveness of the high temperature recuperator (HTR) than compressor efficiency and low temperature recuperator (LTR) effectiveness respectively.