HUMIDIFICATION-DEHUMIDIFICATION SYSTEM IN A GREENHOUSE FOR SUSTAINABLE CROP PRODUCTION

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1 Ninth International Water Technology Conference, IWTC9 2005, Sharm El-Sheikh, Egypt 849 HUMIDIFICATION-DEHUMIDIFICATION SYSTEM IN A GREENHOUSE FOR SUSTAINABLE CROP PRODUCTION J.S. Perret 1 ; A.M. Al-Ismaili 1 ; S.S. Sablani 2 1 Department of Soils, Water and Agricultural Engineering, 2 Department of Food Science, College of Agricultural and Marine Sciences, Sultan Qaboos University, P.O. Box 34, Al-Khod 123, Sultanate of Oman; of corresponding author: perret@squ.edu.om ABSTRACT Prolonged aridity in the Sultanate of Oman has resulted in freshwater deficit in some parts of the country. In coastal areas, the aridity coupled with over-pumping of groundwater have often resulted in seawater intrusion. Looking for alternatives to provide freshwater for domestic, industrial and agricultural purposes is an ultimate goal for the government. The use of solar radiation in greenhouses to desalinate saline/brackish water was proposed as an alternative to provide freshwater for irrigation. This paper presents a study aimed at constructing and preliminary testing a humidification-dehumidification system in a Quonset greenhouse for producing freshwater. This greenhouse was modified to work with two humidifiers (i.e., evaporating pads), to increase water vapor inside the greenhouse as much as possible, and two dehumidifiers (i.e., condensers) to condense this water vapor. After evaporation, water leaving the humidifiers was cooler than the incoming water. This cooled water was pumped to the two dehumidifiers and acted as a coolant. Preliminary testing of the performance of the greenhouse showed an increase in the amount of water vapor after the second humidifier. The temperature of the dehumidifiers was always lower than the dew-point temperature of the air passing through them. This meant that there was a potential for condensation. Key Words: Solar Desalination, Greenhouse, Dehumidification, Humidification INTRODUCTION Arid regions rely entirely on irrigation for crop production and as a result, up to 95% of fresh water withdrawn is generally used by the agricultural sector (Al-Mumtaz [1]; Abdel-Rahman & Abdel-Magid [2]; Abdel-Rahman [3]; Al-Ajmi and Abdel Rahman [4]; Norman et al. [5]). Inevitably, such an intensive demand puts considerable pressure on renewable water resources which often leads to groundwater deficit and

2 850 Ninth International Water Technology Conference, IWTC9 2005, Sharm El-Sheikh, Egypt depletion. The economical and social consequences are obvious in many coastal regions of arid countries such Oman where the overuse of groundwater has caused saline intrusion which has reduced the ability to grow crops and resulted in agricultural land being discarded. Long-term solution to this crisis lies in the 1) development and/or implementation of good agricultural/water management practices and water conservation techniques and 2) the augmentation of the fresh water resources through desalination, reuse of treated wastewater, artificial recharge of aquifers, fog collection, etc. (Goosen et al. [6]; [2]). The cost fuel powered desalination techniques (US$ /m 3 of fresh water depending upon the size of the plant and technique employed) is not feasible for arid land agriculture (Goosen et al. [7]). Arid countries may suffer from lack of fresh water but they generally may benefit from a great solar energy potential. This potential combined the development of saline water purification techniques may provide sustainable solution to supply dry regions with fresh water (Chaibi [8]). This study explores the potential of augmentation of the fresh water resource through solar desalination in a greenhouse structure. This concept called Seawater Greenhouse (Paton and Davis [9]), uses the solar desalination principle and works by saturating the air with moisture vaporizing from saline water inside a greenhouse and later dehumidifying, thus, causing freshwater condensation. Recently, Goosen et al. [7] stated that assistance is needed in the design and optimization of the humidification/dehumidification Seawater Greenhouse, in particular in the thermodynamic and economic simulation/ modeling, and construction of greenhouses using local materials. The implementation of the seawater greenhouse in Oman offers great potential, however, it requires, initially, the design and construction of a pilot greenhouse to better understand and optimize the processes of humidification and dehumidification. Humidification of air followed by dehumidification of water vapor to collect freshwater is not a new concept (Delyannis and Belessiotis [10]), but combining these processes in a greenhouse is new (Goosen et al. [7]). In 1992, Lights Work Limited at the UK designed a pilot solar humidification-dehumidification system in a greenhouse type structure for crop cultivation and seawater desalination (Paton and Davis [9]; Goosen et al. [7]; Sablani et al. [11]). This greenhouse was constructed on the Canary Island of Tenerife. Goosen et al. [7] reported that water production by this system was more than crop water requirement of the plant in the greenhouse. Water production is influenced strongly by the temperature of the condenser (Bailey and Raoueche [12]) that should be maintained at levels below the saturation temperature of the moist air in order to form condensation. This system is suitable for arid regions located on the coast (Bailey and Raoueche [12]; Sablani et al. [11]) or inland with shortage of freshwater but access to saline/brackish groundwater.

3 Ninth International Water Technology Conference, IWTC9 2005, Sharm El-Sheikh, Egypt 851 MATERIALS AND METHODS This study was carried out in a Quonset greenhouse located at the Agricultural Experimental Station of Sultan Qaboos University, Oman (Figure 1). Originally, this greenhouse had a fan/pad evaporative system consisting of two exhaust fans and a wall of cellulose pads of 6 m wide x 1.5 m high. One pump was used to drive water from an underground sump tank to the top surface of the pads. Water flowed by gravity on the pads and was collected in a gutter located at the bottom of the pads and redirected to the sump tank. In order to introduce the concept of humidification/dehumidification in this greenhouse, several modifications were made. These modifications are briefly discussed below. Figure1. Modified quonset greenhouse at the agricultural experiment station 1. Design and Construction of a Movable Evaporative Cooling Frame Due to the expected increase in air temperature and air capacity as air moves towards the fans, a second wall of evaporative cooling pads was designed and constructed. The pads were mounted on a frame having the same dimensions as the first wall of evaporative cooling pads to maintain the airflow rate through the corrugated cellulose material. The frame was mounted on wheels to test various positions inside the greenhouse. The corrugated cellulose pads were 0.1m thickness with a surface area of 45 m 2 per square meter of pad. Figure 2 shows the movable frame supporting the cooling pads.

4 852 Ninth International Water Technology Conference, IWTC9 2005, Sharm El-Sheikh, Egypt Figure 2. Moving frame mounted on wheels for the second wall of evaporative cooling pads 2. Design and Construction of the Condensers Frame As mentioned earlier, if moist air is cooled below its dew-point temperature, the water vapor in the air starts condensating (Fujii [13]; Sauer & Howell [14]; Moran & Shapiro [15]; Mangold et al. [16]; Huang, [17]). Two 0.9x0.9 m tube-and-fin cross-flow condensers were used (Figure 3) to investigate the possibility of reducing the temperature of the air below its dew-point. These condensers consisted of three rows of brass tubes connected by copper fins. They were mounted on a movable frame to allow different configurations in the greenhouse. Water flowing down the front pads was circulated in the condensers as a coolant since it has been cooled by evaporation. The condensers dimensions were based on availability from a local manufacturer. At this stage, the main purpose of this study was not to optimize the design and size of the condenser but rather to verify if the coolant temperature was below the dew-point of the air entering it and hence validate whether condensation could occur. Figure 3. Schematics of the cross flow condenser

5 Ninth International Water Technology Conference, IWTC9 2005, Sharm El-Sheikh, Egypt Water Circulation System A 3.8 m 3 reservoir was placed outside the greenhouse and oriented south to maximize solar energy input and hence, increase water temperature inside the tank. This relatively warm source of water is circulated on the second wall of evaporative cooling pads to enhance humidification. The water leaving the pads flows by gravity into a sump tank and is then pumped to the condensers. The temperature of the water in the sump tank is expected to be cooler than the water of the outside tank since it has been evaporatively cooled. Water leaving the condensers is then returned to the outside reservoir (Figure 4). Figure 4. Layout of the water circulation system 4. Greenhouse Inner Partitions and Greenhouse Instrumentation To force airflow through the cooling pads and condensers, 3 mm transparent acrylic partitions (baffles) were installed in the arcs of the greenhouse. Transparent material was used to minimize the interference with solar radiation coming in the greenhouse. The greenhouse was equipped with a Delta-T data logger (model DL2E, Delta-T Devices Ltd, UK) which monitored temperature of water and air temperatures and relative humidity in fourteen locations. In addition, solar radiation was recorded outside and inside from the greenhouse. All measurements were recorded with an hourly time interval. Air velocity was recorded with a digital anemometer (model HH31A, Omega Engineering Inc., US) in four cross-sections of the greenhouse (after

6 854 Ninth International Water Technology Conference, IWTC9 2005, Sharm El-Sheikh, Egypt the first and second walls of evaporative cooling pads, and before/after the condensers). 5. Cooling pad and Condensers Configuration As mentioned earlier, the cooling pads and the condensers were mounted on wheels to investigate various configuration scenarios in the greenhouse. For this study, the condensers were installed 2 m away from the exhaust fans located at the back of the greenhouse. The second wall of evaporative cooling pads and the condensers frame were spaced 2 m apart. The fans, pumps and data logger were switched on for a period of one week in March The ability of the cooling pads to increase the relative humidity inside the greenhouse and the capability of the condensers to reduce the temperature of the air below its dew-point was assessed during this period. Estimation of the dew-point of the air entering the condensers was used to determine the temperature of the coolant required to condensate water. The dew-point was calculated using the following equations (ASHRAE [18]): d w 2 3 [ ln(p )] * [ ln(p )] * [ P ] t = * ln(p ) * + w where t d is the dew-point temperature in o C and P w is the water vapor partial pressure in kpa. The water vapor partial pressure P w is obtained by (ASHRAE [18]): w w (1) P w Φ * Pws = (2) 100 T, P and * *10 *T+ T *10 *T *10 *T *lnt P = 1000 * Exp (3) ws where is the relative humidity in %, P ws is the water vapor saturation pressure and P is the total pressure both in kpa and T is the absolute dry-bulb temperature in K. The absolute humidity (amount of water held in air) was also calculated for various points throughout the greenhouse as (ASHRAE [18]): d V = (4) where d v is the absolute humidity in kg/m 3, ω is the humidity ratio (kg of water/ kg of dry air) and v is the specific volume of the moist air mixture in m 3 /kg. The specific volume and the humidity ratio are calculated by (ASHRAE [18]): Plastic film

7 Ninth International Water Technology Conference, IWTC9 2005, Sharm El-Sheikh, Egypt 855 and ( ) T * = (5) p P w = (6) P P w RESULTS AND DISCUSSION 1. Temperature and Relative Humidity through the First Wall of Evaporative Cooling Pads As expected, a reduction in air temperature and an increase in relative humidity was observed as air passed through the first layer of evaporative cooling pads (Figure 5). Temperature, o C Relative Humidity, % m before 1st pad 0.1 m after 1st pad 20 6: :00 6:00 18:00 6:00 18:00 6:00 18:00 6:00 18:00 6:00 18:00 6:00 18:00 6: Time (h) Figure 5. Temperature and relative humidity changes before and after the first wall of evaporative cooling pads

8 856 Ninth International Water Technology Conference, IWTC9 2005, Sharm El-Sheikh, Egypt When the relatively hot and dry outside air is forced to flow through the first layer of wetted pads by the fans, it released part of its sensible heat to the water. Water used this sensible heat and heat from the surrounding (i.e., the water on evaporative cooling pads) to evaporate water from the pads. Thus, this resulted in a reduction in temperature and an increase in the relative humidity. It is interesting to note that maximum cooling occurs at midday when the relative humidity is at its lowest and hence evaporation rate at is highest (due to large vapor deficit). The cooling process is driven by the relative humidity of the incoming air. 2. Temperature and Relative Humidity Gradients through the Greenhouse One would expect the air temperature to increase as it is moving through the greenhouse. This is explained by the fact that the incoming shortwave solar radiations penetrates inside the greenhouse through the covering material, changes wavelength after reflection into longwave infrared radiation. Since the covering material is more transparent to shortwave than longwave the net balance results in trapped radiation that raises the temperature inside the greenhouse. This phenomenon has a positive impact on humidification and dehumidification in the greenhouse as it allows an increase in air capacity. Heat build-up was verified by placing three temperature sensors in the longitudinal axis of the greenhouse. Figure 6 shows the temperature gradient at three sequential locations throughout the greenhouse (0.1 m, 6 m and 12 m from the first layer of pads). At night, most readings were similar to each other due to the absence of solar radiation. On the other hand, the relative humidity of the air decreased as air moved away from the first pad. This decrease in relative humidity was due to the increase in air temperature that caused air to expand and thus, a potential to hold more moisture. This supported the use of a second wall of evaporative cooling pad to bring the air to saturation. Temperature, o C m after 1st pad 6 m from 1st pad 12 m from 1st pad Figure 6. Temperature gradient inside the greenhouse

9 Ninth International Water Technology Conference, IWTC9 2005, Sharm El-Sheikh, Egypt Temperature and Relative Humidity through the Second Wall of Cooling Pads Increase in air temperature and decrease in relative humidity as air was moving through the greenhouse offered a great potential for air to hold more water vapor before the condensers justifying the construction of a second evaporating frame. Like the first layer of pads, there was a decrease in air temperature and an increase in the relative humidity as air was passing through the second layer of wetted pads. Temperature and relative humidity were measured 0.1 m before and after the second pad. The relative humidity after the second pad was usually greater than that measured after the first pad (Figure 7). Again, this strongly justifies the use of a second wall of evaporative cooling pads. However, the air after the second layer of pads was not always saturated. For instance, the relative humidity was always lower than 100% on the interval from 9:00 am to 6:00 pm. Ideally, the relative humidity should reach saturation at all times. This can be achieved by increasing water distribution efficiency on the pads. Saturation could also be achieved by changing air flow rate through the pads until the optimal flow rate (i.e., that gives the highest relative humidity) is obtained. In addition, high relative humidity could be enhanced by covering with reflective material the area of the greenhouse after the second layer of cooling pads. This would prevent temperature increase due to solar radiation and hence reduction in relative humidity m before 2nd pad 0.1 m after 2nd pad Relative Humidity, % Figure 7. Relative humidity increase between air entering and leaving the second wall of evaporative cooling pads 4. Absolute Humidity As mentioned earlier, the absolute humidity is an indicator of the amount of water held in that air. It can be observed that the first layer of evaporating pads was adding moisture to the coming air (Figure 8). Similarly, the absolute humidity after the second layer of pads was further increased. Figure 9 shows the difference between absolute humidity of the air entering and leaving the greenhouse expressed in terms of

10 858 Ninth International Water Technology Conference, IWTC9 2005, Sharm El-Sheikh, Egypt evaporation rate. By integrating the curve over time, one can obtain the volume of water evaporated from the two evaporative cooling walls for a given period. For instance, it was found out that on average roughly 2.85 m 3 of water evaporated from the cooling pads each day Absolute humidity, kg/m Air entering greenhouse 0.1 m after 1st pad 0.1m after 2nd pad Figure 8. Absolute humidity gradient of air traveling through the greenhouse Evaporation rate from pads, kg/h Figure 9. Rate of water loss due to evaporation 5. Temperature of Water Flowing on the First and Second Pads The temperature of water leaving the pads was always lower than the temperature of the water arriving on top of the pads by up to 2 o C (Figure 10). This was due to the cooling effect of evaporation (i.e., evaporative cooling). Water flowing from both frames of the evaporating pads was collected in the sump tank. Water temperature in

11 Ninth International Water Technology Conference, IWTC9 2005, Sharm El-Sheikh, Egypt 859 the sump tank was relatively cooler than that in the outside tank (Figure 11). Difference in water temperature between the outside and sump tanks reached 4.8 o C and the average difference was 3.1 o C. 28 Top of 1st pad Bottom of 1st pad 26 Temperature, oc Figure 10. Water temperature changes between top and bottom of first wall of evaporative cooling pads Outside tank Sump tank Temperature, oc Figure 11. Temperature differences between the outside tank and sump-tank (feeding the condenser) 6. Condensation Potential The dew-point of the air entering the condenser was calculated using Equation 1. Below this temperature, water vapor in the air condensate (ASHRAE [18]). Figure 12 shows the air dew-point temperature and the temperature of coolant flowing through the two cross flow condensers. It can be observed that condensers temperature was always lower than the dew-point temperature of the air with a maximum difference of

12 860 Ninth International Water Technology Conference, IWTC9 2005, Sharm El-Sheikh, Egypt almost 4 o C. Even when including an uncertainty of ±2% in thermocouple readings in the calculations of dew-point to take into consideration the instrumental error, the temperature of the condenser remained lower than the dew-point temperature air. Thus, theoretically, water vapor condensation is possible. However, the tipping bucket gauge installed to record condensation rate did not registered outflow from the condensers. Formation of water droplets on the fins and tubes of the condensers were observed but only a small fraction of the droplets made their way to the gutter collecting the condensate. This is probably due to 1) high air speed through the condensers and 2) surface tension created between the water droplets and fins of the condenser. Air velocity was measure and averaged from nine points 0.25m after the first and second wall of evaporative cooling pads and the condensers. The airflow velocity through the two walls of cooling pads was 1.5 m/s. This flow velocity could be reduced as evaporative cooling through 0.1 m cellulose pads is optimal around 1.25 m/s (Wiersma et al. [19]). Air flow velocity through the two condensers was relatively high due to their small cross-sectional area (0.9 x 0.9 m). Although, the condensers were cold enough to condensate air moisture, this high airflow velocity may have left enough time to bring below dew-point only a fraction of the water molecules in vapor phase. In other words, air speed did not allow sufficient time for surface contact between the moist air and the condensers to reduce air temperature below saturation. Air velocity regulation trough the condensers should be investigated. It can be achieved by increasing the area of contact of the condensers and/or with varying fan speed. The fins design should also be looked at critically to avoid droplet entrapment by surface tension. 28 Air dew-point Coolant Temperature, o C Figure 12. Difference between dew-point temperature of air entering the condensers and temperature of the coolant in the condensers CONCLUSIONS A humidification-dehumidification system in a greenhouse at the Agricultural Experiment Station of Sultan Qaboos University was constructed. It was based on a Quonset design modified in order to include two humidifiers to increase water vapor

13 Ninth International Water Technology Conference, IWTC9 2005, Sharm El-Sheikh, Egypt 861 inside the greenhouse. Two dehumidifiers (i.e., condensers) were constructed in the greenhouse after a second layer of evaporative cooling pads. Performance of the humidification and dehumidification processes was tested for a period of seven days. According to this preliminary test, several conclusions could be drawn. First, the second set of pads has met its construction objective by increasing the relative humidity before the condensers up to saturation most of the time. Evaporation from the two pads resulted in a reduction in water temperature up to 3 o C. Relatively cold water flowing out from the cooling pads was pumped through the condensers and used as a coolant. The temperature of the condensers was always lower than the dew-point temperature of air passing through them. Thus, condensation of water vapor on the condensers is possible. However, with the existing condenser design, condensation of water vapor was insufficient to result in measurable quantities. Low condensation can be attributed to the high air flow velocities through the condensers which did not allow enough time for surface contact between the air and the condensers and thus, obtaining the required reduction in air temperature to condense water vapor. Other factors, such as water droplets entrapment between condensers fins due to surface tension have also contributed to low condensation rate. Near condenser design and airflow velocity appears to be the bottle neck of the dehumification process. These aspects of the humidification-dehumidification system will be investigated in the near future. ACKNOWLEDGMENT This study was supported by Sultan Qaboos University project IG/AGR/BIOR/01/01. REFERENCES 1. Al-Mumtaz M. (2000). Water desalination in the Arabian Gulf region. In M. F. A. Goosen and W. H. Shayya (ed.), pp Water management, purification & conservation in arid climates. Volume 2: Water purification. Technomic Publishing Company, Inc., Pennsylvania 2. Abdel Rahman H. A.; Abdel Magid I. M. (1993). Water conservation in Oman. Water International, 18(2): Abdel-Rahman, H. A. (1996). Traditional systems application of modern irrigation techniques in the Sultanate of Oman. Agircultural Mechanization in Asia, Africa and Latin America 27(1): Al-Ajmi H. A.; Abdel-Rahman A. H. (2001). Water management intricacies in the Sultanate of Oman: The augmentation-conservation conundrum. Water International, 26(1): Norman W. R.; Shayya W. H.; Al-Ghafry A. S. (1998). Irrigation water costs and management practices among farms in Northern Oman, Agricultural Sciences, 3: 1-8.

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