Comparative design-point and yearly advanced exergoenvironmental analyses of a solar-biomass organic Rankine cycle power plant

ABSTRACT

This study compared the results of advanced exergoenvironmental analysis at the design point and over a year of operation of a solar-biomass organic Rankine cycle (ORC) power plant based on a real 630 kW ORC plant operational at Ottana (Italy). Although the advanced exergoenvironmental method is now popular in the literature for energy system analysis, it is commonly applied at a single design point. It is, however, not clear yet if such design-point analysis would be acceptably reliable in transient energy systems that depend on varying weather conditions such as solar, a research gap this study attempted to fill. Total impact rates of 7.02 Pts/h were obtained in the hybrid plant at the design point out of which about 4.39 Pts/h is avoidable; 3.50 Pts/h (about 80%) due to the internal operations of each of the individual plant components (endogenous) and 0.89 Pts/h (about 20%) due to interconnections among the system components (exogenous). Also, total yearly impact rates of 91.14 kPts/year were obtained out of which about 69.93 kPts/year is avoidable; 51.13 kPts/year (about 73%) endogenous and 18.80 kPts/year (about 27%) exogenous. While about 63% of the total hybrid plant’s impact rates are obtained avoidable based on the design-point advanced exergoenvironmental analysis, about 76% is obtained in the yearly analysis. These results imply that applying the advanced exergoenvironmental method to a transient energy system over a year of plant operation would offer significantly different insights relative to the analysis at the design point. Going forward, results of the advanced exergoenvironmental method should be interpreted with caution when applied at single design points, especially for energy systems that rely heavily on intermittent weather parameters for their nominal operation.

KEYWORDS:

• Organic Rankine Cycle
• advanced Exergoenvironmental Analysis
• renewable Power Plant
• auxiliary Environmental Indices
• hybrid Solar-biomass Plant

Acknowledgement

The lead author appreciates senior colleagues at the University of Cagliari, ENAS, and other stakeholders in charge of the Ottana Solar Facility for the opportunity to continue collaborative research studies on the experimental ORC plant.

Disclosure statement

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

Nomenclature

Symbols

b	=	specific exergoenvironmental impact (mPts/kWh)
$\dot{B}$I	=	exergoenvironmental impact rate due to irreversibility (mPts/h)
EIE	=	exergoenvironmental impact per unit energy produced (mPts/kJ)
$\dot{E}$	=	rate of exergy (kW)
e	=	exergy
fb	=	exergoenvironmental factor
h	=	enthalpy (kJ/kg)
$\dot{I}$	=	rate of destroyed exergy/irreversibility (kW)
LHV	=	lower heating value
$\text{dotm}$	=	mass flow rate (kg/s)
N	=	plant lifetime (years)
Q	=	thermal energy (kWh)
$\dot{Q}$	=	thermal power (kW)
$\dot{\mathrm{q}}$	=	:heat flux (W/m2)
$\dot{R}$	=	total exergoenvironmental impact rate
rb	=	s: specific exergoenvironmental impact rate difference entropy (kJ/kgK)
T	=	temperature (°C)
$\dot{\mathrm{W}}$	=	electrical power (kW)
$\dot{\mathrm{Y}}$	=	exergoenvironmental impact rate due to component construction/pollutant emission (mPts/h)

Greek letters

$\mathrm{\Delta }\mathrm{T}$	=	temperature change (K)
$\mathrm{\Delta }\mathrm{h}$	=	enthalpy change (kJ/kg)
ρ	=	mass density (kg/m3)
η	=	efficiency
ε	=	exergetic efficiency

Subscripts

A	=	annual
a	=	ambient
biom	=	biomass
c	=	component boundary
cond	=	condenser
eff	=	effective
F	=	fuel
i	=	inlet side
k	=	component identifier
l	=	liquid
o	=	outlet side
P	=	product
pm	=	pump motor
sk	=	sink

Subscripts

av	=	avoidable
en	=	endogenous
ex	=	exogenous
PF	=	pollutant pollution
un	=	unavoidable

Abbreviations

CSP	=	concentrating solar power
HTF	=	heat transfer fluid
LCA	=	life cycle assessment
LFC	=	linear Fresnel collector
ORC	=	organic Rankine cycle
TES	=	thermal energy storage