Modeling and thermodynamic analysis of gas-supercritical carbon dioxide combined cycle system

ABSTRACT

Gas turbine power generation has the advantages of flexibility, efficiency, and lower carbon emissions compared with coal-fired power generation. Under the situation of energy structure transformation and upgrading, gas turbine power generation will have bright prospects. In this paper, the modeling and thermodynamic analysis of various gas-supercritical carbon dioxide(sCO₂) combined cycle systems are carried out based on the calculation method of total physical properties, and six typical combined cycle layouts are selected for detailed research. In order to obtain the optimal state of the combined cycle system, the parameters of the sCO₂ bottom cycle are optimized within a certain range of the gas turbine pressure ratio. It is found that the combined cycle system has high thermal efficiency when the top cycle adopts the air preheating cycle, and a higher top cycle regenerative degree does not represent a better performance. In fact, the optimal regenerative degree is related to the bottom cycle performance. The study also found that the air preheating-dual heated cascade combined cycle system has the highest thermal efficiency of the layouts studied, reaching 65.3%, but it comes at the cost of a complex operating system. The heat recovery performance of the sCO₂ partial heating bottom cycle is relatively poor, and its mass flow rate is the highest, so it is not suitable for the bottom cycle of the combined cycle. The sCO₂ dual heated cascade cycle has the highest output work and the smallest CO₂ flow, so this paper considers this bottom cycle as the most suitable bottom cycle for the combined cycle system. This research can provide a certain basis for the design of gas turbine combined cycle system.

KEYWORDS:

• Gas turbine
• waste heat recovery
• supercritical carbon dioxide cycle
• thermodynamic analysis

Nomenclature

$W$	=	Power,$kW$
$m$	=	Mass flow rate,$kg/s$
h	=	Enthalpy,$kJ/kg$
$\mathrm{\Delta }{H}_r$	=	Chemical reaction enthalpy,$kJ/kg$
P	=	Pressure,Mpa
$\mathrm{\Delta }p$	=	Pressure loss,%
t	=	Temperature,°C
Rp	=	Pressure ratio
$Q_{net}$	=	Low calorific value of fuel, $kJ/kg$
$Q$	=	Heat exchange,$kW$
out	=	The mass fraction of cool air in air,%
X	=	Fuel-air ratio
Greek Symbols	=
$\eta$	=	Isentropic efficiency,%
$\phi$	=	Total pressure recovery factor
$\epsilon$	=	Recuperator effectiveness
$\kappa$	=	Mass fraction of water generated by combustion reaction
Subscripts	=
$c$	=	Compressor
t	=	Turbine
a	=	Air
g	=	Gas
f	=	Fuel
$C{O}_2$	=	Carbon dioxide
s	=	Isentropic process
in	=	Parts inlet
out	=	Parts outlet
cool	=	Recuperator cool side
hot	=	Recuperator hot side
air	=	gas mixed with air

Abbreviations

sCO2	=	Supercritical carbon dioxide
HTR	=	High temperature recuperator
LTR	=	Low temperature recuperator
K	=	Cycle efficiency,%

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This work was supported by Science Center for Gas Turbine Project [P2021-AB-I-003-001], National Science and Technology Major Project (HT-J2019-II-0010-0030), National Science and Technology Major Project [HT-Y2019-I-0002-0003], and Shanghai Science and Technology Committee with Grant No. [22010503100].