A Hot Water Split-Flow Dual-Pressure Strategy to Improve System Performance for Organic Rankine Cycle
Abstract
:1. Introduction
2. System Description
2.1. Basic ORC System
2.2. Conventional Dual-Pressure ORC System
2.3. Split-Flow Dual-Pressure ORC System
3. Mathematical Model
3.1. Assumptions
- The system operates under a steady state.
- In the B-ORC system, the working fluid liquid at the outlet of the working fluid pump is heated to the saturated liquid in the preheater, and then the saturated liquid at the outlet of the preheater is heated to the saturated vapor in the evaporator. In the CD-ORC and SFD-ORC systems, the working fluid liquid at the outlet of the high-pressure working fluid pump or the low-pressure working fluid pump is heated to the saturated liquid state in the preheater of the corresponding loop, and then the saturated liquid at the outlet of the high-pressure preheater and the low-pressure preheater is heated to the saturated vapor in the evaporator of the corresponding loop.
- The working fluid at the outlet of condenser is in a saturated liquid state.
- The inlet temperature of the hot water and the cooling water are constant.
- The changes in kinetic and potential energy are neglected.
- The friction loss and energy loss in the evaporator, condenser and any pipeline are neglected.
- The condensation temperature of the working fluid is constant.
- The cooling water pump and working fluid pump have a constant isentropic efficiency and mechanical efficiency.
- The expander has a constant isentropic efficiency and mechanical efficiency.
3.2. Thermodynamic Model
3.2.1. B-ORC System
- The process from state 1 to 2
- The process from state 2 to 3
- The process from state 3 to 4
- The process from state 4 to 5
- The process from state 5 to 1
3.2.2. CD-ORC System
- The process from state 1′ to 2
- The process from state 1″ to 2
- The process from state 2 to 3
- The process from state 3 to 4′
- The process from state 3 to 4″
- The process from state 4′ to 5′
- The process from state 4″ to 5″
- The process from state 5′ to 1′
- The process from state 5″ to 1″
3.2.3. SFD-ORC System
- The process from state 1′ to 2
- The process from state 1″ to 2
- The process from state 2 to 3
- The process from state 3 to 4′
- The process from state 3 to 4″
- The process from state 4′ to 5′
- The process from state 4″ to 5″
- The process from state 5′ to 1′
- The process from state 5″ to 1″
3.3. Solution
4. Genetic Algorithm Method Schemes
5. Results and Discussion
5.1. Independent Parameters on Net Power Output
5.2. The Effect of Independent Parameters on Net Power Output
5.3. Evaluation of the Genetic Algorithm Method
5.4. Optimal Parameters
5.5. The Maximum Net Output Power
6. Conclusions
- (1)
- The optimal hot water outlet temperature of B-ORC is much higher than that of CD-ORC and SFD-ORC, which indicates less thermal energy could be utilized to convert to power in B-ORC.
- (2)
- The optimal hot water temperature at the outlet of evaporator 1 in SFD-ORC is higher than that in CD-ORC, which means that SFD-ORC could make more efficient use of high-grade thermal energy of hot water.
- (3)
- SFD-ORC has the highest maximum net output power, and B-ORC has the lowest.
- (4)
- With the increase in hot water inlet temperature, the advantage of SFD-ORC becomes increasingly obvious. This indicates that SFD-ORC is more preferable for heat sources of a higher temperature.
- (5)
- SFD-ORC has great advantages in improving net output power.
Author Contributions
Funding
Conflicts of Interest
Nomenclature
A | area (of heat exchanger) (m2) |
cp | specific heat at constant pressure (kJ/(kg∙°C)) |
h | specific enthalpy (kJ/kg) |
K | heat transfer coefficient (W/(m2∙°C)) |
m | mass flow rate (kg/s) |
Q | heat transfer rate (kW) |
t | temperature (°C) |
T | thermodynamic temperature (K) |
W | power (kW) |
∆P | pressure difference (Pa) |
∆T | logarithmic mean temperature difference (K) |
Greek symbols
η | efficiency (%) |
ρ | density (kg/m3) |
Subscripts
AVG | average |
cond | condenser |
cond1 | condenser in high-pressure ORC |
cond2 | condenser in low-pressure ORC |
cw | cooling water |
cwpump | cooling water pump |
cw1 | cooling water exchange heat with high-pressure ORC |
cw2 | cooling water exchange heat with low-pressure ORC |
evap | evaporator |
evap1 | high-pressure evaporator |
evap2 | low-pressure evaporator |
ex | exergetic |
exp | expander |
exp1 | high-pressure expander |
exp2 | low-pressure expander |
hw | hot water |
hw′ | hot water split into high-pressure preheater |
mid | between evaporator and preheater |
net | net |
pin | pinch temperature difference |
pre | preheater |
pump | working fluid pump |
pump1 | high-pressure fluid pump |
pump2 | low-pressure fluid pump |
s | isentropic |
th | thermal |
wf | working fluid |
wf1 | working fluid at high evaporation pressure |
wf2 | working fluid at low evaporation pressure |
0 | environment |
Abbreviations
ORC | basic organic Rankine cycle |
CD-ORC | conventional dual-organic Rankine cycle |
GA | genetic algorithm |
GATBX | genetic algorithm toolbox |
SFD-ORC | split-flow dual-organic Rankine cycle |
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Parameters | Value |
---|---|
Ambient temperature (°C) | 25 |
Ambient pressure (kPa) | 101.3 |
Condensation temperature (°C) | 30 |
Cooling water inlet temperature (°C) | 25 |
Cooling water outlet temperature (°C) | 30 |
Hot water mass flow rate (kg/s) | 324.2 |
Hot water inlet temperature (°C) | 90–130 |
Pinch temperature difference (°C) | 3 |
Working fluid pump isentropic efficiency | 70% |
Working fluid pump equipment efficiency | 75% |
Cooling water pump isentropic efficiency | 80% |
Cooling water pump equipment efficiency | 90% |
Expander isentropic efficiency | 85% |
Expender equipment efficiency | 95% |
Population size | 20 |
---|---|
Maximum number of generations | 50 |
Precision of variables | 20 |
Objective function | Wnet |
Selection method | stochastic universal sampling (SUS) |
Crossover probability | 0.7 |
Mutation probability | 0.01 |
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Wang, S.; Yuan, Z. A Hot Water Split-Flow Dual-Pressure Strategy to Improve System Performance for Organic Rankine Cycle. Energies 2020, 13, 3345. https://doi.org/10.3390/en13133345
Wang S, Yuan Z. A Hot Water Split-Flow Dual-Pressure Strategy to Improve System Performance for Organic Rankine Cycle. Energies. 2020; 13(13):3345. https://doi.org/10.3390/en13133345
Chicago/Turabian StyleWang, Shiqi, and Zhongyuan Yuan. 2020. "A Hot Water Split-Flow Dual-Pressure Strategy to Improve System Performance for Organic Rankine Cycle" Energies 13, no. 13: 3345. https://doi.org/10.3390/en13133345