Exergy Evaluation of Desalination Processes
Abstract
:1. Introduction
2. Energy, Entropy, and Exergy Relationship
3. Desalination Processes and Operation Principles
4. Energy and Exergy Analysis of Desalination Processes
4.1. First Law of Thermodynamics
4.2. Exergy of Energy Flows
4.3. Exergy Analysis
4.4. Why Exergy Analysis in Desalination Processes
5. Case Studies for Desalination Exergy Performance Analysis
5.1. MSF Desalination
5.2. Multi-Effect Distillation (MED) or Evaporation (MEE) Process
5.2.1. Exergy Evaluation of MED Process
5.2.2. MED-TVC Process
5.3. Reverse Osmosis Membrane Process
5.4. Integrated Membrane Systems
5.5. Solar Stills
5.6. Membrane Distillation
5.6.1. MD Unit without a Heat Exchanger
5.6.2. MD Unit with a Heat Exchanger
5.7. Other Configurations
6. Entropy Generation in Desalination Processes
7. Desalination Exergy Costs (Thermoeconomics)
8. Concluding Remarks
Acknowledgments
Conflicts of Interest
Nomenclature
A | surface area (m2) |
cp | specific heat of ideal gas at constant pressure (kJ kg−1 K−1) |
Δ | temperature difference (K) |
E, ex | exergy (kJ) |
Ė | exergy flow rate (kW) |
h | specific enthalpy (kJ kg−1) |
m | mass flow rate (kg h−1) |
p | pressure (atm.) |
Q | heat energy (kJ) |
total heat transfer rate (kW) | |
s | specific entropy (kJ kg−1 K−1) |
T | absolute temperature or temperature (K) |
To | reference temperature (K) |
U | heat transfer rate (kJh−1m−2) |
W | net work transfer rate (kW) |
w | seawater concentration (kg kg−1) |
Greeks | |
Ψ | exergetic efficiency (%) |
μ | chemical exergy (kJ kg−1) |
η | thermal efficiency (%) |
Subscripts | |
b | brine |
D | destruction |
e | exit, specific exergy |
eff | efficiency |
ex | exergy |
f | freshwater |
hv | heat of vaporization (latent heat) |
i | inlet |
in | input, supply |
o | surroundings |
s | saline water stream, sun |
th | thermal |
v | vapor |
w | withdrawal stream |
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Type of Energy Flow | Specific Energy | Specific Exergy |
---|---|---|
Kinetic | 0.5 V2 | 0.5 V2 |
Potential | gΔz | gΔz |
Heat | q | |
Mechanical | w | w |
Electrical | ItΔV | ItΔV |
Chemical, pure substance | ΔgG | μ − μ0 + RT0ln(c/c0) |
Radiation | I |
Process | Exergy Destruction | Basis for Calculation | |
---|---|---|---|
kWh/ton | J/mol | ||
Reverse osmosis | 0.98 | 63.8 | 35 atm excess pressure at 25 °C |
Electrodialysis | 12.87 | 834 | 0.8 Voltage per membrane pair at 25 °C |
Vapor compression | 2.77 | 179 | Compression unit temp. 60.75 °C; ΔT = 1.5 °C |
Multi-effect evaporation | 2.83 | 183 | Average temperature of 50.75 °C; ΔT = 1.5 °C |
Multi-stage flash desalination | 4.89 | 316.5 | Average temperature of 71.5 °C; ΔT = 3 °C |
Process | Description | Performance | Ref. |
---|---|---|---|
Solid oxide fuel cell–gas turbine (SOFC–GT) hybrid system integrated with a multi stage flash (MSF) desalination unit. | Heuristic optimization method, namely, multi-objective genetic algorithm (MOGA). | Maximum achievable exergy efficiency of 46.7% with optimal design. | [32] |
Desalination capacity—256 m3/day; performance ratio of 8.8 | MSF system exergy efficiency—3.49% | ||
MSF with heat recovery from hot distillate water stages | IPSEpro software was used. Capacity—91,200 m3/day; # of stages 19 (16 heat recovery + 3 heat rejection) with a performance ratio of 8.43 | Overall exergy efficiency—5.8% | [33] |
Exergy destroyed: | |||
heat recovery stages—55.0% | |||
brine heater—17.0% | |||
heat rejection stages—10.0% | |||
pumps—4.3% | |||
brine streams disposal—14.0% | |||
With heat recovery—14.0% | |||
Recirculating MSF plants in Saudi Arabia, namely, Al-Khobar II, Al-Jubail II, and Shuaibah | Quantitative assessment of MSF desalination plants. | TBT—Exergy efficiency | [34] |
Al-Khobar II: Capacity—194,200 m3/day; # of stages—16 (10 identical units) | 87 °C—4.61% | ||
106 °C—5.21% | |||
115 °C—5.35% | |||
Al-Jubail II: Capacity—940,000 m3/day; # of stages—22 (40 identical units) | 90.6 °C—10.02% | ||
90.8 °C—10.38% | |||
112.8 °C—7.61% | |||
Shuaibah: Capacity—181,818 m3/day; # of stages—19 (10 identical units) | 76.5 °C—3.57% | ||
90.0 °C—1.78% | |||
101.5 °C—1.12% |
Process | Description | Performance | Ref. |
---|---|---|---|
MEE-MVC | Capacity—5000 m3/day; feed temperature—27 °C; compressor—60 °C; heating steam—70 °C # of evaporators—1–6 | Exergy efficiency: | [49] |
1 effect (3.8%); | |||
2 effects (5.8%); | |||
3 effects (6.6%); | |||
4 effects (7.5%); | |||
6 effects (no change) | |||
Capacity—1500 m3/day; feed temperature—27 °C; compressor—60 °C; heating steam—70 °C # of evaporators—1–6 | Exergy efficiency: | ||
1 effect (3.8%); | |||
2 effects (5.8%); | |||
3 effects (6.6%); | |||
4 effects (7.5%); | |||
6 effects (8.4%); | |||
8 effects (no change) | |||
Capacity—3750 m3/day; feed temperature—21 °C; Evaporator 1–65 °C; Evaporator 2–60 °C Evaporator surface area—2670 m2 | Exergy efficiency | ||
With make-up steam—4.34% | |||
Without make-up steam—5.75% | |||
MEE-TVC | Capacity—1200 m3/day; feed temperature and TDS—27 °C and 45,000 mg/L; # of evaporators and surface area—2 and 2160 m2 | Exergy efficiency—2.2% | [50] |
Capacity—5000 m3/day; feed temperature and TDS—27 °C and 45,000 mg/L; # of evaporators and surface area—2 and 8978 m2 | Exergy efficiency—2.1% | ||
MEE-MVC | Capacity—1500 m3/day; feed temperature and TDS—27 °C and 45,000 mg/L; # of evaporators and surface area—2 and 3866 m2 | Exergy efficiency—5.8% | |
Capacity—5000 m3/day; feed temperature and TDS—27 °C and 45,000 mg/L; # of evaporators and surface area—2 and 12848 m2 | Exergy efficiency—5.8% | ||
MED-TVC | Capacity—5000 m3/day; forward feed type, 12 effects combined with TVC at the last effect. Exergy efficiency at different steam extraction pressures: Pa = 4890 kPa, Pb = 2800 kPa, Pc = 1480 kPa, Pd = 700 kPa and Pe = 290 kPa | Exergy efficiencies: | [51] |
Pa—4.9% | |||
Pb—5.0% | |||
Pc—5.1% | |||
Pd—6.0% | |||
Pe—7.1% | |||
Combined CHP-MED-TVC | Capacity—5000 m3/day; forward feed type, 12 effects combined with TVC at the last effect; # of MED units: 0–4. Exergy efficiency at different steam extraction pressures: Pa = 4890 kPa, Pb = 2800 kPa, Pc = 1480 kPa, Pd = 700 kPa and Pe = 290 kPa | Exergy efficiencies: | |
Pa (4-0)—2.5–3.8% | |||
Pb (4-0)—2.6–3.8% | |||
Pc (4-0)—2.8–3.8% | |||
Pd (4-0)—2.9–3.8% | |||
Pe (4-0)—3.0–3.8% | |||
Gas Turbine + MED + RO | Capacity—16874 m3/day; Power production—10 MW. Power—water generation unit consisting of compressor, gas turbine (GT), combustion chamber (CC), Air compressor (AC), high recovery steam generation (HRSG), MED and RO | Exergy Destruction: | [52] |
AC—2.0% | |||
CC—14.0% | |||
GT—1.5% | |||
HRSG—5.9% | |||
MED—3.9% | |||
RO—0.5% |
System 1 | System 2 | System 3 | |
---|---|---|---|
Process description | Direct RO | NF pretreatment + RO | MF + NF pretreatment + RO |
Brine flow rate, m3/h | 629.9 | 504.5 | 531.9 |
Brine concentration, g/L | 57.6 | 71.9 | 68.0 |
Fresh water flow rate, m3/h | 421.2 | 547.0 | 517.6 |
Fresh water concentration, g/L | 0.34 | 0.27 | 0.27 |
Fresh water recovery, % | 40.1 | 52.0 | 49.2 |
Process | Capacity, m3/day | Exergy Efficiency, % | Reference |
---|---|---|---|
RO | 7250 | 4.3 | [53] |
RO | 2850 | 0.7 | [57] |
SWRO | 7586 | 5.8 | [58] |
MF-NF-RO | 12,408 | 30.9 | [59] |
Desalination Device | Description | Performance | Ref. |
---|---|---|---|
Passive solar still | Exergy analysis of individual compounds | Collector—12.9% | [60] |
Brine—6.0% | |||
Solar still—5.0% | |||
Passive solar still | Single and Double slope solar still | Thermal Efficiency | [70] |
Single—22.6–31.3% | |||
Double—25.4–34.3% | |||
Exergy Efficiency | |||
Single—0.18–1.25% | |||
Double—0.13–1.16% | |||
Passive solar still | Wind and insulation effects | Max. exergy efficiency—9.48% | [71] |
Daily Avg. exergy efficiency—4.93% | |||
Exergy saved by Insulation—7.71% | |||
Active solar still | Evaporator maintained under vacuum with an air-cooled condenser | A 12% increase in basin absorptivity increased distillate by 27%, energy utilization by 25%, and exergy efficiency by 39%. | [64] |
Effect of absorptivity of basin and heat loss reduction from basin walls | A 75% reduction in basin heat losses increased distillate production by 87% and exergy performance by 152%. | ||
Solar still with energy storage | Phase change material thermal storage | Daytime exergy efficiency < 5% | [72] |
Nighttime exergy efficiency > 80% | |||
Pyramid-shaped solar still | Comparison of summer and winter conditions, effect of water depth | No significant difference. Higher exergy efficiency at a lower water depth (4–8 cm) | [73] |
Single-effect horizontal basin-type passive solar stills | Thermodynamic model development | Ultimate energy efficiency 80.0% | [74] |
Optimum exergy efficiency 21.1% | |||
Weir type cascade solar still | Computer simulation package, effect of brine flow rate | Inlet brine flow rate of 0.065 kg/min—10.5% | [75] |
Inlet brine flow rate of 0.2 kg/min—3.14% | |||
High brine inlet flow rate—3.8–7.34% | |||
Thermoelectric assisted solar still | Dynamic thermodynamic modeling study | Energy efficiency—19.8% | [76] |
Exergy efficiency—0.95% | |||
Exergy destruction in thermo-electric module—63.4% | |||
Active solar still | Solar still integrated with solar pond | Energy efficiency—38.6% | [77] |
Exergy efficiency—2.7% |
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Gude, V.G. Exergy Evaluation of Desalination Processes. ChemEngineering 2018, 2, 28. https://doi.org/10.3390/chemengineering2020028
Gude VG. Exergy Evaluation of Desalination Processes. ChemEngineering. 2018; 2(2):28. https://doi.org/10.3390/chemengineering2020028
Chicago/Turabian StyleGude, Veera Gnaneswar. 2018. "Exergy Evaluation of Desalination Processes" ChemEngineering 2, no. 2: 28. https://doi.org/10.3390/chemengineering2020028