Question

Hi,
how can we do an EXERGY budget analysis on a Rankine cycle?
is there a specific formula to use dealing with enthalpy and entropy to do the exergy budget analysis?

Answer 1

Basically, The exergy analysis is employed to evaluate the performance of the system based on the irreversibility that is occurring in every component of the Rankine Cycle system (as non-isentropic expansion and compression)

1. Results and discussions:

The analysis was performed under the following assumptions for every working fluid:

#      The temperature from the solar collector (t8,sol) is 140°C;

#      The ambient temperature is considered to be 20°C;

#      The isentropic efficiency for the turbine was assumed to be 85%.

#         The isentropic efficiency for the pump was assumed to be 60%.

The model described above was used to simulate the behaviour of the ORC system upon variation of the outlet temperature of the solar collector.

Parameters	Units	R123	SES 36	R 365 mfc
Qpreheater	[kW]	10.96	12.50	11.83
Qevaporator	[kW]	11.39	9.49	10.39
Qsuperheater	[kW]	1.054	1.404	1.188
Qcondenser	[kW]	16.94	13.69	15.04
Wturbine	[kW]	3.942	3.701	3.725
Wpump	[kW]	0.155	0.144	0.105
mORC	[kg/s]	0.1007	0.1048	0.0787
WORC	[kW]	3.787	3.555	3.620

Results for t8,sol=140

	Heat exchanger	Condenser	Turbine	Pump
	R123	SES36	R236	R123	SES36	R236	R123	SES36	R236	R123	SES36	R236
ExF[kW]	5.94	5.94	5.94	0.56	0.46	0.48	4.56	4.23	4.23	0.155	0.145	0.105
ExP[kW]	5.35	5.42	5.25	0.42	0.34	0.37	3.94	3.70	3.72	0.005	0.006	0.003
I [kW]	0.59	0.52	0.69	0.14	0.12	0.10	0.62	0.53	0.50	0.151	0.138	0.102

Formula for Enthalpy

ah = (h − hr) − Tr (s − sr)

Formula for Entropy

(∆S = ah/Tr)