Introduction to Evaporative Cooling Technology
Evaporative cooling technology has emerged as a promising alternative to traditional vapor compression cooling systems, offering the potential to provide thermal comfort with minimal power consumption and without the use of environmentally harmful refrigerants. Unlike vapor compression systems that rely on energy-intensive mechanical compressors and synthetic refrigerants, evaporative cooling employs the natural process of water evaporation to extract heat from the air and cool the surrounding environment.
The two main evaporative cooling approaches are direct evaporative cooling and indirect evaporative cooling. In direct evaporative cooling, hot ambient air is drawn through a wetted cooling pad, where the heat of the air is absorbed by the evaporation of water, resulting in cooler and more humid air being supplied to the conditioned space. This process can theoretically lower the indoor air temperature to the outdoor wet-bulb temperature. However, the introduction of additional moisture to the cooled air can lead to higher indoor humidity levels, which may compromise human comfort.
Indirect evaporative cooling, on the other hand, utilizes a heat exchanger to separate the air to be cooled from the evaporative cooling process, avoiding the direct introduction of moisture to the conditioned space. While indirect systems can maintain lower indoor humidity levels, they are generally more complex in design and may have lower cooling efficiencies compared to direct evaporative cooling systems.
Combining Direct Evaporative Cooling and Dehumidification
To address the challenge of increased indoor humidity associated with direct evaporative cooling, researchers have explored the possibility of integrating a dehumidification process into the system. By incorporating a solid desiccant dehumidifying pad downstream of the evaporative cooling section, the moisture content of the cooled air can be effectively reduced before it is supplied to the conditioned space.
In this study, the performance of a modified direct evaporative cooling system that combines a cooling pad and a removable dehumidifying pad was experimentally evaluated for space cooling applications. The cooling pad was constructed using a mixed media of luffa fiber and charcoal, while the dehumidifying pad was made from activated carbon derived from tamarind seed.
Experimental Setup and Methodology
The modified direct evaporative cooling system was designed and constructed at the Department of Mechanical Engineering, Ahmadu Bello University, Zaria. The system consisted of a double-pad configuration, with the cooling pad at the evaporative cooling section and the dehumidifying pad placed downstream to remove the moisture from the cooled air before it entered the conditioned space.
Cooling Pad Design and Fabrication
The cooling pad was a mixed media pad, made of luffa fiber lagged with charcoal. Luffa, a fibrous skeleton of the luffa aegyptiaca and luffa acutangula fruits, was chosen for its porous nature and ability to retain water. Dry pieces of charcoal were embedded into the luffa fiber matrix to provide a large adsorptive surface area.
Dehumidifying Pad Design and Fabrication
The dehumidifying pad was fabricated using tamarind seed activated carbon granules. The activated carbon was prepared by chemical activation, following a one-step pyrolysis method. The tamarind seed was impregnated with calcium carbonate (CaCO3) in a 1:2 ratio and carbonized at 600°C for 30 minutes. The activated carbon granules were then wrapped in plastic nets and packed into a wire mesh frame to create the dehumidifying pad.
Experimental Procedure and Performance Evaluation
The modified direct evaporative cooling system was used to cool a test room with a peak cooling load of 4.53 kW, as determined through TRNSYS simulations. Two experimental days were conducted, one with the dehumidifying pad removed (Day 1) and the other with the dehumidifying pad in place (Day 2).
During the experiments, measurements were made of ambient temperature, indoor dry-bulb temperature, outdoor wet-bulb temperature, outdoor relative humidity, indoor relative humidity, and water consumption. The following performance parameters were calculated:
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Cooling Capacity: The amount of heat removed from the conditioned space, calculated as the product of the mass flow rate of air, the specific heat of air, and the temperature difference between the inlet and outlet air streams.
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Cooling Efficiency: The ratio of the actual cooling capacity to the theoretical maximum cooling capacity, which is the product of the mass flow rate of air and the enthalpy difference between the inlet and outlet air streams.
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Coefficient of Performance (COP): The ratio of the cooling capacity to the total power consumption of the system.
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Water Consumption: The difference in the volume of water in the storage tank between two successive measurement times.
Uncertainty analysis was conducted using the quadratic power method to determine the relative uncertainties of the cooling efficiency and cooling capacity.
Results and Discussion
Experimental Day 1 (without Dehumidifying Pad)
On the first experimental day, the dehumidifying pad was removed from the system, and the modified direct evaporative cooling system operated with only the cooling pad.
Temperature and Humidity Profiles
The results showed that a minimum room temperature of 24°C was achieved, resulting in a maximum temperature drop of 11°C from the ambient temperature. However, the indoor relative humidity increased significantly, reaching a maximum of 84%, while the outdoor relative humidity was only 30%.
Cooling Performance
The system achieved a maximum cooling capacity of 3.84 kW, a maximum cooling efficiency of 84.6%, and a maximum COP of 16.1. The total water consumption for the day was 8.93 L, with an average water consumption of 0.99 L/h.
The high cooling performance during the mid-day hours was attributed to the availability of higher heat of vaporization due to the lower outdoor relative humidity, which enhanced the evaporation of water and resulted in better cooling capacity and efficiency.
Experimental Day 2 (with Dehumidifying Pad)
On the second experimental day, the dehumidifying pad was incorporated into the system, allowing the cooled moist air to pass through the desiccant material before entering the conditioned space.
Temperature and Humidity Profiles
The results showed that a minimum room temperature of 26.5°C was achieved, resulting in a maximum temperature drop of 10°C from the ambient temperature. However, the maximum indoor relative humidity recorded was 49%, while the outdoor relative humidity was 34%, indicating that the dehumidifying pad was able to effectively absorb the moisture from the cooled air.
Cooling Performance
The system achieved a maximum cooling capacity of 3.2 kW, a maximum cooling efficiency of 71.4%, and a maximum COP of 13.4. The total water consumption for the day was 9.73 L, with an average water consumption of 1.08 L/h.
Compared to the system operation without the dehumidifying pad, the incorporation of the desiccant material resulted in slightly higher indoor dry-bulb temperatures due to the heat of adsorption released during the dehumidification process. However, the system was able to maintain the indoor relative humidity within the recommended range for human comfort.
Conclusion
The experimental evaluation of the modified direct evaporative cooling system, which combined a cooling pad and a dehumidifying pad, demonstrated the potential to achieve effective space cooling while addressing the issue of increased indoor humidity typically associated with direct evaporative cooling systems.
The key findings of the study are:
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Sensible Cooling Capacity: The system was able to achieve a maximum temperature drop of 11°C from the ambient temperature, indicating its effectiveness in providing sensible cooling.
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Humidity Control: The addition of the dehumidifying pad successfully reduced the indoor relative humidity from a maximum of 84% (without the pad) to a maximum of 49%, maintaining the indoor environment within the recommended comfort range.
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Cooling Performance: The system achieved high cooling efficiencies of up to 84.6% and COPs up to 16.1 when operating without the dehumidifying pad, and 71.4% and 13.4, respectively, when the dehumidifying pad was incorporated.
The integration of a dehumidification process into a direct evaporative cooling system, as demonstrated in this study, offers a promising approach to addressing the moisture-related challenges while leveraging the inherent energy-efficiency and environmental benefits of evaporative cooling technology. Further research and optimization of the system design and materials could lead to even greater performance and wider applicability in various building and industrial cooling applications.
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