Exergy Analysis of Fluidized Desiccant Cooling System
Abstract
:1. Introduction
2. Fluidized Desiccant Cooling System
3. Exergy Analysis
3.1. Electric Fans
- The electric fan has an isentropic efficiency of 70% (such efficiency was justified by [19]).
- The increase of the exergy of the air compressed in the electric fan is only due to the pressure rise (the temperature-related exergy is neglected).
3.2. Air Heater/Cooler
- The outer wall of the heat exchanger is adiabatic,
- The heat capacity of the heat exchanger is negligible,
- The water pressure drop in the heat exchanger is negligible,
- Condensation of water vapor on the heat exchange surfaces does not occur,
- In the case of the air cooler, the exergy of water at the outlet is dissipated.
3.3. Regenerative Heat Exchanger
- The outer wall of the heat exchanger is adiabatic,
- The heat capacity of the heat exchanger is negligible,
- Condensation of water vapor on the heat exchange surfaces does not occur.
3.4. Fluidized Beds
3.5. Direct Evaporative Cooler
- The outer walls of the DEC are adiabatic,
- The heat capacity of the DEC is negligible,
- The water stream supplied to the DEC undergoes complete evaporation,
- DEC generates a constant pressure drop of 25 [26],
- The heat of evaporation is exchanged at the temperature level of the outlet air.
4. Results and Discussion
4.1. Air Cooler
4.2. Air Heater
4.3. Regenerative Heat Exchanger
4.4. Direct Evaporative Cooler
4.5. Complete System
Name | Symbol | Unit | Quantity |
---|---|---|---|
Dead state humidity | 0.012 | ||
Dead state pressure | Pa | 101300 | |
Dead state temperature | 30 | ||
Distance between the AC/AH fins | d | m | |
Overall heat transfer coefficient of AC/AH | 50 | ||
Overall heat transfer coefficient of RHX | 25 | ||
Dimension of the RHX channel | m | 0.01 | |
Dimension of the RHX channel | m | 0.003 | |
Number of heat transfer units of AC/AH | − | 6 | |
Number of heat transfer units of RHX | − | 20 | |
Heat capacity ratio of AC/AH | − | 0.2 | |
Air velocity in heat exchangers | U | 3 |
5. Conclusions
- The total exergy destruction in FDC was 4.163 , and the exergy efficiency was found to be 0.577.
- The main sources of exergy losses in FDC were the fluidized beds and the regenerative heat exchanger. These components were responsible for 30% and 20% of the total exergy destruction, respectively. Nevertheless, both components were characterized by relatively high exergy efficiency of 0.58 and 0.77, respectively.
- Optimization of the RHX exergy destruction led also to the decrease of exergy losses in the air heater. For the analyzed case, the optimum was found to be about 20.
- The most inefficient components of FDC were the direct evaporative cooler and the air cooler (featuring an exergy efficiency of 10% and 20%, respectively).
- In order to improve the exergy efficiency, the direct evaporative cooler can be replaced with indirect or indirect/direct evaporative cooling. However, this can result in a significant decrease of the system’s capacity.
- The air cooler efficiency was low due to the dissipation of outlet water exergy. Recovery of this exergy flow would reduce the exergy destruction by 25%.
- Nearly 30% of exergy destruction occurred due to the pressure drop in FDC components. This was caused by a low density and heat capacity of the air. The decrease of the overall pressure drop can significantly improve the exergy efficiency.
Author Contributions
Funding
Conflicts of Interest
Abbreviations
Acronyms | ||
Coefficient of performance | ||
Air cooler | ||
Air heater | ||
Adsorption circuit fan | ||
Desorption circuit fan | ||
Regenerative heat exchanger | ||
Direct evaporative cooler | ||
Greek symbols | ||
Efficiency | ||
Dynamic viscosity | ||
Density | ||
Symbols | ||
A | Heat exchange surface | |
c | Specific heat | |
d | Channel dimension of the RHX | m |
Specific exergy | ||
Specific exergy destruction | ||
E | Parameter of the heat exchanger | - |
Flow of exergy | W | |
Exergy destruction | W | |
F | Correction factor | - |
h | Specific enthalpy | |
Heat of evaporation | ||
k | Overall heat transfer coefficient | |
l | Channel dimension of the RHX | m |
Mass flow | ||
Number of heat transfer units | - | |
p | pressure | Pa |
P | Temperature ratio | - |
Q | Heat flow | W |
R | Individual gas constant | |
Heat capacity ratio | - | |
s | Specific entropy | |
T | Temperature | K |
U | Velocity | |
w | Channel dimension of the air heater/cooler | m |
X | Specific humidity ratio | |
Specific humidity ratio on a molar basis | ||
Subscripts | ||
0 | Dead state | |
a | Air | |
The Carnot cycle | ||
Destruction | ||
Exergy | ||
H | Heat source | |
Inlet | ||
L | Cooling effect | |
M | Heat sink | |
v | Vapor | |
w | Water |
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Name | Symbol | Unit | Quantity |
---|---|---|---|
Desiccant filling height | m | 0.03 | |
Switching time | s | 450 | |
Superficial air velocity | 3 | ||
Desiccant particle diameter | m | 0.001 | |
Fluidised bed height | m | 0.55 | |
Fluidised bed diameter | m | 0.28 | |
Desiccant density | 850 | ||
Inlet temperature of heating water | 70 | ||
Inlet temperature of cooling water | 25 |
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Rogala, Z.; Kolasiński, P. Exergy Analysis of Fluidized Desiccant Cooling System. Entropy 2019, 21, 757. https://doi.org/10.3390/e21080757
Rogala Z, Kolasiński P. Exergy Analysis of Fluidized Desiccant Cooling System. Entropy. 2019; 21(8):757. https://doi.org/10.3390/e21080757
Chicago/Turabian StyleRogala, Zbigniew, and Piotr Kolasiński. 2019. "Exergy Analysis of Fluidized Desiccant Cooling System" Entropy 21, no. 8: 757. https://doi.org/10.3390/e21080757
APA StyleRogala, Z., & Kolasiński, P. (2019). Exergy Analysis of Fluidized Desiccant Cooling System. Entropy, 21(8), 757. https://doi.org/10.3390/e21080757