# Comparison of the Trilateral Flash Cycle and Rankine Cycle with Organic Fluid Using the Pinch Point Temperature

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## Abstract

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## 1. Introduction

## 2. Theoretical Modeling

## 3. Results

## 4. Discussion

## 5. Conclusions

- (1)
- The evaporation temperature influenced the net power and thermal efficiency of the TFC system. When the temperature increases, each working fluid can achieve relatively large power generation and thermal efficiency. The pinch temperature relatively limits the operating maximum temperature of the ORC system. The excessive evaporation temperature reduces the working fluid mass flow rate and system performance.
- (2)
- Under fixed environmental conditions, the increase in mass flow rate has a limit, and the mass flow rate decreases with the evaporation temperature. When the mass flow rate is higher than the optimum value, the net power generation of the system decreases. Additionally, increasing the working fluid mass flow rate only increases the net power at the minimum permissible pinch-point temperature difference and does not increase thermal efficiency. Thermal efficiency only changes with the evaporating temperature adjustment. At optimal operating conditions, the ORC system requires a lower working fluid mass flow rate than the TFC system.
- (3)
- Net power and heat transfer are negatively correlated with the pinch temperature. When the pinch point is close to the minimum temperature, more heat transfer is intercepted from the system, which can generate larger net power. The heat exchange efficiency of the ORC system is limited by the pinch point generated from the phase change of the working fluid. By contrast, the efficiency of the TFC system is relatively high because the TFC is non-isothermal and phase-unchanged, which gives it a more favorable heat extraction rate for the heat source.
- (4)
- From the simulation results, under the same heat source condition of 80 °C, the thermal efficiency of each working fluid in the ORC was compared with that in the TFC, and the TFC system exhibited a more favorable heat extraction rate and higher pump consumption, and the net power output was approximately 30% higher than the ORC system.

## Author Contributions

## Funding

## Acknowledgments

## Conflicts of Interest

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**Figure 1.**A photograph of the 200-kW organic Rankine cycle system. Source: Industrial Technology Research Institute (ITRI).

**Figure 2.**(

**a**) T–s diagram for organic Rankine cycle (ORC); (

**b**) T–s diagram for trilateral flash cycle (TFC).

**Figure 5.**Effect of evaporation temperature on net power generation and efficiency in the TFC system.

**Figure 6.**Finding the most favorable power generation by fine-tuning the mass flow rate and evaporation temperature in the TFC system.

**Figure 7.**Net power and efficiency of the TFC system for different mass flow rates (80 °C hot water).

**Figure 8.**Effect of evaporation temperature on net power generation and efficiency in the ORC system.

**Figure 9.**Finding the most favorable power generation by fine-tuning the mass rate and evaporation temperature in the ORC system.

**Figure 12.**(

**a**) The maximum net power generated by the TFC and ORC (ORC); (

**b**) the maximum heat exchange by the TFC and ORC.

Parameter | Value | |
---|---|---|

Heat source | 80 °C Water | 4.16 kg/s |

Heat sink | 30 °C Water | 26.6 kg/s |

Cycle | Type | TFC, ORC |

Working Fluid | R245fa, R134a, R236fa, R1233zd | |

Condensation temperature | 37 °C | |

System Componet | Pump efficiency | 0.7 |

Expander efficiency | 0.75 | |

Heat exchanger | Pinch temperature 5 °C | |

Condenser | Pinch temperature 1 °C | |

Pressure drop | No pressure drop |

Working fluid | R245fa | R134a | R236fa | R1233zd |
---|---|---|---|---|

Mass flow rate(kg/sec) | 12.46 | 10.82 | 13.18 | 13.58 |

Evaporation(°C) | 75 | 75 | 75 | 75 |

Evaporator pressure(kPa) | 695 | 2364 | 1108 | 581 |

Heat exchange capacity(kW) | 692 | 675 | 687 | 692 |

Gross power(kW) | 25.08 | 33.95 | 28.37 | 24.42 |

Working fluid pump(kW) | 6.35 | 18.88 | 10.08 | 6.03 |

Net power(kW) | 18.73 | 15.06 | 18.28 | 18.38 |

Net Cycle Eff(%) | 2.71 | 2.23 | 2.66 | 2.65 |

Power generation per unit mass | 2.01 | 3.13 | 2.15 | 1.79 |

Working fluid | R245fa | R134a | R236fa | R1233zd |
---|---|---|---|---|

Mass flow rate(kg/sec) | 1.61 | 1.77 | 2.06 | 1.54 |

Evaporation(°C) | 55 | 55 | 55 | 56 |

Evaporator pressure(kPa) | 400 | 1491 | 668 | 349 |

Heat exchange capacity(kW) | 348 | 348 | 347 | 330 |

Gross power(kW) | 12.67 | 13.32 | 12.46 | 12.78 |

Working fluid pump(kW) | 0.29 | 1.15 | 0.57 | 0.26 |

Net power(kW) | 12.37 | 12.17 | 11.89 | 12.51 |

Net Cycle Eff(%) | 3.55% | 3.49% | 3.42% | 3.78% |

Power generation per unit mass | 7.86 | 7.52 | 6.04 | 8.29 |

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**MDPI and ACS Style**

Lai, K.-Y.; Lee, Y.-T.; Chen, M.-R.; Liu, Y.-H. Comparison of the Trilateral Flash Cycle and Rankine Cycle with Organic Fluid Using the Pinch Point Temperature. *Entropy* **2019**, *21*, 1197.
https://doi.org/10.3390/e21121197

**AMA Style**

Lai K-Y, Lee Y-T, Chen M-R, Liu Y-H. Comparison of the Trilateral Flash Cycle and Rankine Cycle with Organic Fluid Using the Pinch Point Temperature. *Entropy*. 2019; 21(12):1197.
https://doi.org/10.3390/e21121197

**Chicago/Turabian Style**

Lai, Kai-Yuan, Yu-Tang Lee, Miao-Ru Chen, and Yao-Hsien Liu. 2019. "Comparison of the Trilateral Flash Cycle and Rankine Cycle with Organic Fluid Using the Pinch Point Temperature" *Entropy* 21, no. 12: 1197.
https://doi.org/10.3390/e21121197