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ABSTRACT
Exergo-economic analysis of the pinch point temperature difference (PPTD) in both evaporator and condenser of sub-critical organic Rankine cycle system (ORCs) are performed based on the first and second laws of thermodynamics. Taking mixture R13I1/R601a as a working fluid and the annual total cost per net output power Z as exergo-economic performance evaluation criterion, the effects of PPTD in evaporator ΔTe, and the PPTD ratio of condenser to evaporator y, on the exergo-economic performance of ORCs are analyzed. Moreover, how some other parameters influence the optimal PPTD in evaporator ΔTe,opt and the optimal PPTD ratio of condenser to evaporator yopt are also discussed. It has been found that the exergo-economic performance of ORCs is remarkably influenced by ΔTe and y, and there exists ΔTe,opt and yopt. In addition, ΔTe,opt and yopt are affected by heat transfer coefficient ratio of condenser to evaporator ß, the temperature of working fluid at dew point in condenser T1a, and composition of R13I1/R601a: larger ß and T1a lead to lower ΔTe,opt and yopt; by contraries, larger mass fraction of R13I1 makes ΔTe,opt and yopt increase, and yopt increases linearly. The effects of the temperature of working fluid at bubble point in evaporator T3a, mass flow rate of exhaust flue gas mg, and inlet temperature of exhaust flue gas Tgi on ΔTe,opt and yopt are very slight. For comparison, three additional working fluids, namely R601a, R245fa, and 0.32R245fa/0.68R601a, are also taken into account.
Nomenclature
| cT | = | thermal exergy price ($/J) |
| cf | = | flow exergy price ($/J) |
| cp | = | specific heat capacity at constant pressure, J/(kg·K) |
| F | = | heat transfer area (m2) |
| F0 | = | investment unrelated to area cost ($) |
| h | = | specific enthalpy (J/kg) |
| i | = | interest rate |
| IF | = | area-related cost ($/m2) |
| I0 | = | total investment costs ($) |
| K | = | heat transfer coefficient, W/(m2·K) |
| m | = | mass flow rate (kg/s) |
| n | = | expected life of component (in years) |
| Q | = | heat load (W) |
| W | = | work (W) |
| s | = | specific entropy, J/(kg·K) |
| SV | = | salvage value of component at the end of year n ($) |
| T | = | temperature (K) |
| Tgx | = | temperature of flue gas out of evaporating section B (K) |
| TH | = | average temperature of the heat source (K) |
| TL | = | average temperature of the cooling air (K) |
| T0 | = | environmental temperature (K) |
| V | = | average volumetric flow rate (m3/s) |
Greek symbols
| α | = | capital-recovery factor |
| α* | = | present-worth factor |
| ß | = | heat transfer coefficient ratio of condenser to evaporator |
| τ | = | annual operating time (h) |
| θ | = | cost ratio |
| ΔE | = | exergy loss rate (W) |
| ΔTc | = | PPTD in condenser (K) |
| ΔTe | = | PPTD in evaporator (K) |
Subscripts
| a | = | cooling air |
| A | = | single-phase section |
| B | = | two-phase section |
| c | = | condenser |
| e | = | evaporator |
| g | = | exhaust flue gas |
| i | = | inlet value |
| min | = | minimum value |
| o | = | outlet value |
| opt | = | optimal value |
| p | = | pump |
| t | = | turbine |
| w | = | working fluid |
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