Thermoeconomic Optimization of Scroll-Based Organic Rankine Cycles with Various Working Fluids

Abstract

In this study, thermoeconomic optimization method is applied to an organic Rankine cycle system to evaluate the cost of the system and net generated revenue as a function of exergy efficiency to be able to generate most economical electricity, depending on market conditions and fuel prices. Thermoeconomic optimization can serve as one of the most important stages in the design procedure which, for defined boundary conditions, makes it possible to find the optimal values of independent variables. The values that minimize or maximize the chosen optimization criteria are considered to be optimal in this case. It may be the annual net profit, time of return of investment, or any other economic profitability criterion. Here, the cost per unit of net electric power generated by the cycle is chosen as the primary measure of the performance of the system. It is considered to be the most universal optimization criterion since the other two mentioned are dependent on price at which electricity can be sold. This price may vary depending on the country and the type of application. It is found that best possible solutions can be obtained for the exergy efficiency of approximately 55 % in terms of cost of the system and net revenue.

Nomenclature

Ex:

Exergy, J

$\dot{E}x$ :

Exergy rate, J/s

h:

Specific enthalpy, J/kg

P:

Pressure, Pa

Q:

Heat, J

$\dot{Q}$ :

Heat rate, J/s

s:

Specific entropy, J/kg K

T:

Temperature, °C

V_ee :

Expander expansion chamber volume, m³

V_ei :

Expander intake chamber volume, m³

W:

Work, J

$\dot{W}$ :

Work rate, J/s

η:

Efficiency

θ:

Orbiting angle, rad

ρ:

Density, kg/m³

φ:

Involute angle, rad

φ_e :

Rolling angle (involute ending angle), rad

φ_i,s :

Starting angle of the inner involute, rad

φ_i0 :

Initial angle of the inner involute, rad

φ_o,s :

Starting angle of the outer involute, rad

φ_o0 :

Initial angle of the outer involute, rad

CB:

Cost of boiler

CC:

Cost of condenser

CE:

Cost per unit of electricity produced

CF:

Cost of fuel

CFUEL_f :

Fuel cost of producing hot fluid

cp:

Compressor

CT0T_f :

Cost of producing the hot fluid by the low pressure furnace

CV:

Control volume

ex:

Exergy

exp:

Expander

FC_f :

Fixed charges for the low pressure furnace

fix:

Fixed

MPE:

Market price of electricity

N_obj :

Number of objectives

N_param :

Number of parameters

NRG:

Net revenue generated by the sale of the generated electricity