Proposal and Thermodynamic Assessment of S-CO2 Brayton Cycle Layout for Improved Heat Recovery
"> Figure 1
<p>S-CO<sub>2</sub> power cycle layouts: (<b>a</b>) simple Brayton cycle, (<b>b</b>) recompression Brayton cycle, (<b>c</b>) recompression Brayton cycle with partial cooling, and (<b>d</b>) recompression Brayton cycle with partial cooling and improved heat recovery. Layouts (a), (b), and (c) are adapted from [<a href="#B28-entropy-22-00305" class="html-bibr">28</a>].</p> "> Figure 2
<p>Temperature – Entropy diagram for different S-CO<sub>2</sub> Brayton cycle layouts with reheat. (<b>a</b>) SBC. (<b>b</b>) RBC. (<b>c</b>) RBC-PC. (<b>d</b>) RBC-PC-IHR. High temperature of the cycle is 500 °C. All state points correspond to layouts in <a href="#entropy-22-00305-f001" class="html-fig">Figure 1</a>.</p> "> Figure 3
<p>(<b>a</b>) Thermal efficiency plots of RBC (no reheat) versus split ratio, with a TIP of 25 MPa, for TIT from 500 °C to 850 °C. (<b>b</b>) Minimum pinch temperature versus split ratio; red dots indicate the minimum pinch temperature of 5 °C.</p> "> Figure 4
<p>(<b>a</b>) Thermal efficiency plots of RBC-PC (no reheat) against TIP, with a fixed split ratio of 0.6, for TIT from 500 °C to 850 °C. (<b>b</b>) Minimum pinch temperature versus TIP; red dots denote the minimum pinch temperature of 5 °C.</p> "> Figure 5
<p>Maximum thermal efficiency plotted versus turbine inlet temperature. Data points are extracted from <a href="#entropy-22-00305-f003" class="html-fig">Figure 3</a>a and <a href="#entropy-22-00305-f004" class="html-fig">Figure 4</a>a corresponding to a minimum pinch temperature of 5 °C in the heat exchangers.</p> "> Figure 6
<p>Cycle thermal efficiency (solid lines) and minimum pinch temperature in the heat exchangers (dashed lines) plotted versus cycle’s minimum pressure for (<b>a</b>) RBC-PC and (<b>b</b>) RBC-PC-IHR. Study conducted with TIP of 17 MPa and 25 MPa for TIT of 500 °C and 850 °C. The red dot indicates the near optimal value of cycle’s minimum pressure (p<sub>7</sub> for RBC-PC and p<sub>8</sub> for RBC-PC-IHR) that satisfies the requirement of minimum pinch temperature of 5 °C. Flow split ratio was set to 0.5 for both configurations.</p> "> Figure 7
<p>Validation of simulation results for SBC, RBC, and RBC-PC with [<a href="#B12-entropy-22-00305" class="html-bibr">12</a>], [<a href="#B28-entropy-22-00305" class="html-bibr">28</a>], [<a href="#B29-entropy-22-00305" class="html-bibr">29</a>]. Input data for the validation are listed in <a href="#entropy-22-00305-t001" class="html-table">Table 1</a>.</p> "> Figure 8
<p>(<b>a</b>,<b>b</b>) Thermal efficiency of RBC-PC-IHR with and without reheat plotted versus TIP, with a fixed split ratio of 0.5, for TIT from 500 °C to 850 °C. (<b>c</b>,<b>d</b>) Minimum pinch temperature versus TIP; red dots denote the minimum pinch temperature of 5 °C.</p> "> Figure 9
<p>Thermal efficiency plots of different S-CO<sub>2</sub> Brayton cycles for reheat and no reheat configurations. Turbine operating pressures (TIP) for each case are the same as specified in <a href="#entropy-22-00305-t002" class="html-table">Table 2</a>.</p> "> Figure 10
<p>Exergy efficiency plots of different S-CO<sub>2</sub> Brayton cycles for reheat and no reheat configurations. Turbine operating pressures (TIP) for each case are the same as specified in <a href="#entropy-22-00305-t002" class="html-table">Table 2</a>.</p> "> Figure 11
<p>Component-wise irreversibility plotted versus turbine inlet temperatures representing (<b>a</b>) net exergy loss in turbines (T1 and T2), (<b>b</b>) net exergy loss in compressors (C1, C2 and C3), (<b>c</b>) exergy loss by CO<sub>2</sub> in Cooler and Intercooler combined, (<b>d</b>) exergy gain by air in Cooler and Intercooler combined, and (<b>e</b>) net exergy loss in the Heater and Reheater together. (<b>f</b>) net exergy loss in heat source.</p> "> Figure 12
<p>Exergy loss versus turbine inlet temperatures in SBC, RBC, RBC-PC and RBC-PC-IHR for (<b>a</b>) HTR, (<b>b</b>) LTR, (<b>c</b>) MTR and (<b>d</b>) all recuperators in the layout combined, plotted for reheat and no reheat configurations.</p> ">
Abstract
:1. Introduction
2. Cycle Configurations
2.1. Simple Brayton Cycle (SBC)
2.2. Recompression Brayton Cycle (RBC)
2.3. Recompression Brayton Cycle with Partial Cooling (RBC-PC)
2.4. Recompression Brayton Cycle with Partial Cooling and Improved Heat Recovery (RBC-PC-IHR)
3. Energy Model
4. Simulation Environment
- The cycle operates under steady-state conditions.
- Pressure and heat losses in all pipelines and equipment are considered zero.
- The turbine and compressor adiabatic efficiencies are 93% and 89%, respectively.
- The heat exchanger effectiveness is 95% with a minimum pinch point temperature of 5 °C for all heat exchangers.
- The cycle maximum pressure is 25 MPa.
- Compressor inlet temperature and pressure are maintained at 32 °C and 7.5 MPa corresponding to state ‘6’ for SBC, state ‘7’ for RBC, state ‘9’ for RBC-PC, and state ‘10’ for RBC-PC-IHR.
- The turbine inlet temperature varies from 500 °C to 850 °C.
- Turbine (T1) pressure ratio (p1/p2) is set to 1.0 and 2.0, respectively, for no reheat and reheat configuration.
- Reference temperature used in the exergy analysis is 25 °C.
- Temperature of Heat Source is fixed at 900 °C.
5. Parameter Adjustments
6. Simulation Validation
7. Energy Analysis of Proposed Layout (RBC-PC-IHR)
8. Exergy Model
9. Exergy Analysis
9.1. Overall Exergy Performance
9.2. Component-Wise Exergy Performance
9.2.1. Turbines and Compressors
9.2.2. Coolers
9.2.3. Heat Source
9.2.4. Recuperators
10. Conclusions
- Simple Brayton cycle (SBC) has the lowest thermal efficiency which offers at least, on average, 6.5% and 6.0% (in percentage points) less thermal efficiency, respectively, for reheat and no reheat layouts, in comparison to other configurations incorporating heat recovery units. However, it is still considered an attractive option due to its simplicity and compactness.
- Exergy analysis showed that the maximum overall exergy losses occurred in SBC layout (with and without reheat), where the losses in Cooler and HTR were significantly high. This was due to ineffective recovery of exergy in HTR and associated pinch point problem.
- All configurations showed monotonic increase in the thermal efficiency with the turbine inlet temperature.
- Proposed layout, RBC-PC-IHR, had the maximum thermal efficiency and noticeably higher than RBC and RBC-PC layouts for both reheat and no reheat configurations. In comparison to RBC and RBC-PC, the average improvement of thermal efficiency from RBC-PC-IHR in percentage points was RBC (reheat) 1.2%, RBC (no reheat) 1.3%, RBC-PC (reheat) 1.1%, and RBC-PC (no reheat) 1.4%.
- Like thermal efficiency, the overall exergy efficiency of the RBC-PC-IHR layout was significantly higher. The average increase of the overall exergy efficiency in percentage points was RBC (reheat) 1.6%, RBC (no reheat) 1.4%, RBC-PC (reheat) 1.4%, and RBC-PC (no reheat) 1.8%. Component-wise exergy analysis showed that this improvement was due to considerably less exergy loss in the Cooler and Intercooler and Heat Source. Thus, it may be concluded that the introduction of the third recuperator in the proposed layout decreased the overall external irreversibilities.
- Results showed that the exergy losses occurring in the heat exchangers and heat source were significantly greater compared to that of turbo-machineries.
- For low to medium heat source temperatures, S-CO2 Brayton cycle is an attractive alternative to conventional steam Rankine cycles due to its higher energetic and exergetic performance and compactness.
Author Contributions
Funding
Conflicts of Interest
Nomenclature
CO2 | carbon dioxide | heat input to Heater (kW) | |
LNG | liquefied natural gas | heat input to Reheater (kW) | |
specific entropy (kJ/kg K) | ER | Expansion ratio | |
reference temperature (°C, K) | Subscripts | ||
source temperature (°C, K) | j | state point | |
p | pressure (kPa) | in | inlet |
LTR | low temperature recuperator | out | outlet |
MTR | medium temperature recuperator | T1 | turbine 1 |
HTR | high temperature recuperator | T2 | turbine 2 |
TIP | turbine inlet pressure (kPa) | C1 | compressor 1 |
SR | split ratio | C2 | compressor 2 |
PR | pressure ratio | C3 | compressor 3 |
T | temperature (°C, K) | HI | inlet of hot stream to heat exchanger |
TIT | turbine inlet temperature (°C, K) | HO | outlet of hot stream to heat exchanger |
power (kW) | CI | inlet of cold stream to heat exchanger | |
Irreversibility | CO | outlet of cold stream to heat exchanger | |
exergy (kJ/kg) | input | input | |
thermal efficiency | source | source | |
exergy efficiency | loss by CO2 | ||
net power output from turbines (kW) | gain by air | ||
net input power to compressors (kW) | t | total | |
heat exchanger effectiveness | |||
h | enthalpy (kJ/kg) | ||
mass flow rate (kg/s) |
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Turbine Efficiency | 93% a |
Compressor Efficiency | 89% |
Turbine Inlet Temperature (TIT) | 500–850 °C |
Heat Exchanger Effectiveness ( | 95% |
Cycle High Pressure | 25 MPa |
Compressor Inlet Temperature | 32 °C |
Minimum Pinch Temperature ( | 5 °C |
Turbine (T1) Pressure Ratio (p1/p2) | 1.0 for no reheat and 2.0 for reheat |
Split Ratio | Adjusted for each TIT to maintain minimum pinch temperature of 5 °C in heat exchangers |
TIT | 500 °C | 550 °C | 600 °C | 650 °C | 700 °C | 750 °C | 800 °C | 850 °C | Average |
---|---|---|---|---|---|---|---|---|---|
Thermal efficiency (%)-No reheat | |||||||||
RBC | 4.9 | 5.5 | 5.7 | 6.2 | 6.4 | 6.5 | 6.6 | 6.7 | 6.1 |
TIP (MPa) | 18.9 | 19.6 | 21.1 | 21.9 | 22.7 | 23.4 | 24.9 | 24.9 | |
RBC-PC | 4.5 | 4.8 | 5.5 | 5.9 | 6.2 | 6.7 | 7.1 | 7.5 | 6.0 |
TIP (MPa) | 12.1 | 12.5 | 13.6 | 14.3 | 15.1 | 16.2 | 17.4 | 18.5 | |
RBC-PC-IHR | 5.7 | 6.3 | 6.8 | 7.3 | 7.8 | 8.0 | 8.4 | 8.8 | 7.4 |
TIP (MPa) | 17.2 | 17.9 | 18.7 | 19.3 | 20 | 20.8 | 21.3 | 22.2 | |
Thermal efficiency (%)-Reheat | |||||||||
RBC | 4.9 | 5.3 | 5.9 | 6.4 | 6.9 | 7.1 | 7.5 | 8.0 | 6.5 |
TIP (MPa) | 21.9 | 21.9 | 22.7 | 23.4 | 24.9 | 24.9 | 24.9 | 24.9 | |
RBC-PC | 5.0 | 5.7 | 6.2 | 6.7 | 7.0 | 7.2 | 7.3 | 7.4 | 6.6 |
TIP (MPa) | 20.4 | 21.9 | 23.4 | 24.9 | 24.9 | 24.9 | 24.9 | 24.9 | |
RBC-PC-IHR | 5.6 | 6.3 | 6.9 | 7.5 | 8.1 | 8.5 | 9.0 | 9.4 | 7.7 |
TIP (MPa) | 18.3 | 19.2 | 19.9 | 20.8 | 21.4 | 22.2 | 22.9 | 23.8 |
TIT | 500 °C | 550 °C | 600 °C | 650 °C | 700 °C | 750 °C | 800 °C | 850 °C | Average |
---|---|---|---|---|---|---|---|---|---|
Overall Exergy efficiency (%)-No reheat | |||||||||
RBC | 6.6 | 7.1 | 7.6 | 8.2 | 8.7 | 9.3 | 9.8 | 10.0 | 8.4 |
RBC-PC | 5.9 | 6.3 | 7.2 | 7.8 | 8.4 | 8.9 | 9.4 | 9.9 | 8.0 |
RBC-PC-IHR | 7.8 | 8.4 | 9.0 | 9.7 | 10.3 | 10.6 | 11.1 | 11.6 | 9.8 |
Overall Exergy efficiency (%) – Reheat | |||||||||
RBC | 7.9 | 8.4 | 8.9 | 9.4 | 10.0 | 9.8 | 10.2 | 10.4 | 9.4 |
RBC-PC | 8.2 | 8.7 | 9.3 | 9.8 | 10.2 | 10.0 | 10.2 | 10.1 | 9.6 |
RBC-PC-IHR | 9.1 | 9.6 | 10.3 | 10.9 | 11.5 | 11.7 | 12.2 | 12.5 | 11.0 |
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Siddiqui, M.E.; Almitani, K.H. Proposal and Thermodynamic Assessment of S-CO2 Brayton Cycle Layout for Improved Heat Recovery. Entropy 2020, 22, 305. https://doi.org/10.3390/e22030305
Siddiqui ME, Almitani KH. Proposal and Thermodynamic Assessment of S-CO2 Brayton Cycle Layout for Improved Heat Recovery. Entropy. 2020; 22(3):305. https://doi.org/10.3390/e22030305
Chicago/Turabian StyleSiddiqui, Muhammad Ehtisham, and Khalid H. Almitani. 2020. "Proposal and Thermodynamic Assessment of S-CO2 Brayton Cycle Layout for Improved Heat Recovery" Entropy 22, no. 3: 305. https://doi.org/10.3390/e22030305