Modeling and experimentation of a novel porous ceramic evaporative cooling heat pipe system

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Kanzari Meryem

Zaghdoudi Mohamed Chaker

1. Contexte scientifique de la thèse

Current Direct Evaporative Cooling (DEC)equipment including evaporative coolers, spray-filled and wetted-surface air washers, spray coil units and humidifiers, evaporate water directly in the air stream. The principle underlying this type of cooler is the conservation of sensible and latent heat. The evaporation process cools fresh outdoor air by adiabatic saturation; in fact, when air is drawn through wet pads its sensible heat evaporates some water and becomes latent heat in vapor in the air within its moisture increased which may affect the human comfort as well as the performance and the cooling capacity of the system.

Indirect Evaporative cooling (IEC) process, on the other hand, involves heat transfer between two separate air ducts: the dry primary air stream and the wet secondary air stream ducts: the dry primary air stream and the wet secondary air stream duct. In existing systems, the primary air is cooled by thin metal or plastic surface separating the two air streams, the opposite sides of which are covered by water film which evaporate into the secondary air stream and yields cooling of the heat exchanger surface. As, the wet air stream involves latent heat while the dry air stream involves sensible heat transfer, no additional moisture is introduces into the cool air of the building.

Several mechanical arrangements and thermal performances of sub-wet bulb temperature Evaporative Cooling systems have been investigated. For instance, Hsu et al. [12] studied the generation of Sub-Wet Bulb Temperature cooling by a counter flow and cross flow using two configurations of closed-loop wet surface heat exchangers. They indicated that for the counter flow closed-loop configuration, the maximum wet bulb effectiveness is 1.3 and is reached at a dry passage number of transfer units (NTU) of 10, while for the cross flow closed-loop configuration; the same maximum effectiveness is reached at NTU of 15.Crum et al. [13] indicated that the Sub-Wet Bulb Temperature Evaporative Cooling is achievable by using multistage system with a cooling tower-heat exchanger combination. They proved that this combination has the greatest thermal potential for air conditioning applications since it can produce lowest temperatures and highest cooling capacities for any value of fraction of inlet air delivered. They also concluded that the coefficient of performance for this equipment can reach 75% in the range of air states seen in air conditioning practice. Boxem et al. presented a model for an Indirect Evaporative Cooler which is a compact counter flow heat exchanger with louver fins on both sides. The authors indicated that their calculations overestimated the cooler performance by 20% for inlet air temperatures below 24C and by 10% for higher inlet temperatures.

Zhao et al. presented a numerical study of a Sub-Wet Bulb Temperature counter flow Indirect Evaporative Cooler. A range of design conditions was suggested to maximize the cooler performance: inlet air velocity 0.3-0.5 m/s, height of air passage 6 mm or below, length-to- height ratio of air passage 200 and working-to-intake air ratio around 0.4. They concluded that the cooler can give wet bulb effectiveness of up to 1.3 under the UK summer design conditions. Anisimov et al. proposed a combined parallel and regenerative-counter flow Indirect Evaporative Cooler. Based on a mathematical analysis, they indicated that such a cooler would have higher efficiency than other types. Riangvilaikul et al. presented experimental results for a sensible evaporative cooling system at different inlet air conditions (temperature, humidity and velocity) covering dry, temperate and humid climates. The results showed that wet bulb effectiveness ranged between 92 and 114%. A continuous operation of the system during a typical day of summer season in a hot and humid climate showed that wet bulb effectiveness was almost constant at about 102%.Hasan [20] proposed four types of cooler configurations to achieve sub-wet bulb temperature: Two-stage counter flow cooler, Two-stage parallel flow cooler, Single-stage counter flow regenerative cooler and Combined parallel-regenerative cooler, their performance isthen compared, a computational model based on mathematical analysis of the heat and mass transfer process inside a cooler is developed. He concluded that with higher number of staged coolers, the ultimate temperature to be reached is the dew point of ambient air.

The proposed Sub-Wet bulb Temperature Evaporative Cooling (SWBT EC) concerns a novel IEC technology that would reduce or eliminate the need of refrigerant based cooling, provide substantial energy savings and avoid the need of duct works and associated air quality problems- this being commensurate with most countries energy policies and targets of reduction of CO2 emissions. Thus the main benefits will be enhanced living conditions in building and lower running costs while providing protection of the environment for the public. The proposed two Stage Sub-Wet Bulb Temperature Evaporative Cooler using porous ceramic and heat pipes as heat exchangers media use a novel and simple cooling technology. A review of the literature on Indirect Sub-Wet Bulb Temperature Evaporative cooling systems based on the combination of heat pipes and ceramic containers reveals no previous studies.

2. Problématiqueetobjectifs du travail

This thesis aims to investigate a novel technology of Evaporative Cooling attending the Sub-Wet Bulb Temperature which uses heat pipes embedded into ceramic containers as heat exchanger tool. Theoretical and experimental studies are expected in order to evaluate the performance of such a system.The scope of integrating porous evaporators with heat pipes to provide passive cooling in buildings presents design and thermal performance optimization challenges that will be thoroughly investigated. The proposal will have the added benefit of extending the knowledge of porous material and heat pipes characterization, heat transfer in porous material and heat pipes and provide results for novel cooling designs in other applications. The architectural and engineering implication of integrating such passive cooling system will also be thoroughly investigated as part of this proposal.

3. Approche méthodologique

Part 1: Theoretical modeling and selection of porous materials
A full literature review of available publications and research knowledge on porous ceramic materials and their application for cooling is carried out. A Selection of the suitable ceramics for the proposed application was effected. Then, a Mathematical model is carried out to establish the potential for maximizing heat transfer rates into and out of the porous ceramic panel compartment by providing an understanding of heat transfer mechanisms in the unit that would help optimize the design. The sub-wet bulb temperature evaporative cooler is modelled using common energy and mass conservation laws. In the model the dry and wet channel were divided into small elements (finite volumes) to which the energy and mass transfer equations were applied. Then, differential equations are discretized and applied to each finite volume element along the dry and wet channel length.

Part 2 : Theoretical study of cooling systems using heat pipes and ceramic porous material

Several studies are carried out to assess cooling effectiveness using heat pipes. A literature review of available publications and research knowledge on heat pipes heat exchanger and their application for cooling is also carried out. The combination of heat pipes and ceramic porous containers and the integration of both systems in the sub wet bulb evaporative cooling still not proposed by manufacturers and researchers.

Part 3: Design, construction and laboratory testing of a small scale system

The aim is to generate a design that would prove the concept of using porous ceramic materials and heat pipes heat exchanger for Indirect Evaporative Cooling to reduce energy consumption for air conditioning in buildings. Based on the outcome of the previous parts, a prototype unit incorporating selected porous ceramic heat transfer interface elements, heat pipes, a fan propeller and air ducts will be achieved.

A purpose built laboratory test rig should be constructed to test the thermal cooling performance of the prototype system. The system will be equipped with three heat pipes embedded into ceramic containers, controlled fan speed to deliver required air flow rates to the system, controlled supply air temperature to evaluate heat transfer at various temperatures and humidity, data acquisition system and measurement instruments such as anemometer, thermocouples, and a manometer to monitor performance parameters such as air flow velocities, air and inlet and outlet temperatures, air humidity, and fan power.

Part 4: System integration into buildings: economic and environmental analysis

The integration of the proposed system into buildings will be examined, for a range of building types and for different climatic conditions. Indices describing energy performance and economic attractiveness will be assessed. The most suitable type of buildings for application of the proposed technology will be identified.

Part 5: Modelling, Design, construction and laboratory testing of the full scale system

Based on the outcome of the previous parts, the aim is to generate a design that would prove the concept of using several modules of ceramic-heat pipe, heat exchanger systems, in order to improve the performance of the proposed cooler by enhancing its cooling capacity.The full specification of the system will be implemented with a view of making the design versatile to allow improvement to be made to the system at various stages of construction and testing process and ultimately to be viable for further development.

Part 6: Field trails and performance monitoring

The aim is to install the system in an occupied building and monitor its performance over a period where demand for air conditioning is highest (June, July and August) under realistic operating conditions. This will take the form of continuous monitoring of system parameters. Data on temperature, air velocity, air distribution, relative humidity, evaporation rate, heat transfer and thermal comfort will be recorded and analyzed. Indices describing energy performance and economic attractiveness will be then assessed based on the result of the modeling, laboratory testing and field monitoring. Economic viability of the proposed system will be assessed taking into account investment costs, life cycle cost and energy consumption prices.

4. Moyens utilisés ou nécessaires

As mentioned in the different phases of the thesis project, the performance of the proposed cooler will be evaluated using simulation programs in which the heat transfer using porous ceramic and heat pipes will be modelled. The simulation programs will be performed using MATLAB software. The Economic and Environmental performances of the system will be evaluated using “RETScreen4” software which is an Excel-based clean energy project analysis software tool that helps decision makers quickly and inexpensively determine the technical and financial viability of potential renewable energy, energy efficiency and cogeneration projects. The experimental part is essentially the design, construction and laboratory testing of the full scale system which is mainly composed by:

  • Heat pipes heat sink: the full scale system will be composed by 10 heat pipes adjusted in two parallel rows. As presented in the figure below, the evaporator sections will be the main component of the dry channel and the condenser sections will be embedded into ceramic containers and integrated into the wet channel of the cooler.

Figure . A schematic of the sub-wet bulb temperature evaporative cooler

  • Ceramic containers: the system will use cylindrical shaped porous ceramic panels embedded with the condenser part of the heat pipe, adjusted in two parallel rows into the wet channel of the cooler.

5. Résultats attendus ou obtenus

- Modeling of the proposed cooler using only ceramic panels as heat exchangers: The computer modeling was performed using MATLAB software. The discretized differential equations wereapplied to each finite volume element with initial condition criteria.

Calculation of the air flow operating parameters along the ducts length was performed until satisfactory convergence conditions. The results of the converged solution includes temperature profiles of air along the dry and wet channel, air moisture content along the wet channel and temperature profile of the water film on the ceramic panel surface. The evolution of ducts temperature was calculated for an initial inlet air temperature of 30oC and relative humidity of 35% (i.e., wet bulb temperature of 18.8oC and dew point of 12.5oC).

The temperature of the air in the dry channel decreases from 30oC to 14.9oC. It reaches a value below the wet bulb temperature value (Twb,i=18.8oC), showing that the evaporative cooler would perform adequately in such climatic conditions.

The water film temperature increases from 14.4oC to 16.5oC along the duct as the balance between heat gain from the air in the dry channel and heat loss by evaporation to the air in the wet channel is positive. The heat loss through evaporation of water to the air in the wet channel is due to the increase in the airflow temperature and water content profiles along the channel.

The working air temperature at the turning point of the airflow is higher than that of the water film (Ta,i=14.94oC >Tw,i=14.4 oC) , they intersect at Ta=Tw= 14.7oC and then the working air temperature drops to 14.6oC which is below that of the water film before resuming the normal trend and then rejected at a temperature of 15.4oC.

In the wet channel, the evaporative fluid takes heat and stores it in the working fluid as increased latent heat which appears as moisture content increase from 0.0069 kg/kg to 0.00726 kg/kg.

Finally, the effectiveness of the evaporative cooler was also evaluated using two different methods: the wet bulb and dew point effectiveness. For the design inlet air conditions of 30oC and 35% relative humidity the web bulb effectiveness is =1.23 and dew point effectiveness is = 0.779 . This shows that the sub wet bulb temperature evaporator cooler has wet bulb effectiveness higher than unity, a thermal performance that can compare favourably with more mechanical vapour compression systems and can contribute to reducing overall energy consumption for air condition in buildings.

Environmental and economic analysis: The proposed Sub-Wet Bulb Temperature Evaporative cooling project using innovative technology was investigated in order to evaluate its environmental and economic impact in a Middle East country in which traditional air conditioning systems are used. The economic study based on the financial, cost, and risk analysis, using important indicators of financial viability, has assessed to evaluate the project based on their own criteria. It was proofed the proposed technology is cost-effective with low initial cost and fuel consumption and remain affordable during the project life. Otherwise, the environmental impact was evaluated using energy and emission analysis based on the GreenHouse Gas Emission and reduction. The resulting indictor shows an important energy savings and corresponding GreenHouse Gasemission which can play an important part in thechallenge of meeting global CO2 and other greenhouse gas emission reductions targets.

6. Valeur ajoutée attendue ou obtenue

The proposedresearch is of prime interest of many sectors of the industry including refrigeration, air conditioning, manufacturing, energy and buildings. In hot and humid countries demand for a low cost and energy efficient system to provide comfort cooling is becoming increasingly important. The proposed system is suitable for development as a commercial product appropriate for application in both commercial and domestic buildings. It has the potential to find a large market in hot climate countries, and so could bring economic benefits to the industry.

7. Publications parues, à paraître ou soumis

Kanzari, M., Boukhanouf R., Zaghdoudi, M.C., Ibrahim H.G., “Mathematical modelling of a sub-wet bulb temperature evaporative cooling using porous ceramic materials”, International Conference on Sustainable Technologies, Dubai, 2013.

Kanzari, M., Boukhanouf R., Zaghdoudi, M.C., Ibrahim H.G., “Mathematical modelling of a sub-wet bulb temperature evaporative cooling using porous ceramic materials”, International Journal of Chemical, Materials Science and Engineering Vol:7 No:12, 2013.

M. Kanzari, R. Boukhanouf, M.C. Zaghdoudi and H.G. Ibrahim, “Investigation of a novel porous ceramic evaporative cooling heat pipes system”, International conference on Mechanics and Energy, ICME’2014, Tunisia, 2014.

Biographie du doctorant

Ms Meryem Kanzari was born in Tunisia, in 1983. She received his Degree in Instrumentation and Industrial Maintenance from Institut National des Sciences Appliqueeset de Technologie (INSAT), TUNISIA in 2008. In 2009, she obtained his Master of Science (Msc) Degree in Mechanics, Energetic, Genie civil and Acoustics (specialized in Acoustic and Vibration), from Institut National des Sciences Appliquees de Lyon (INSA Lyon), FRANCE.c:\users\tosh\desktop\screenshot_2014-06-29-16-55-35.png

Currently, sheis a PhDstudent in GenieEnergetique at Ecole Doctorale Sciences et Techniques de l'Ingénieurs and a member of the laboratoire Matériaux, Mesures et Applications (MMA), Institut National des Sciences Appliquées et de Technologie (INSAT).
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