Temperature and Humidity Control in Greenhouses in Desert Areas ()
1. Introduction
Irrigation and agriculture in desert areas, such as the Middle East, have been executed successfully. Water consumption can be reduced by using a greenhouse in a desert area because most evaporated water can be confined to the greenhouse. Therefore, agriculture using greenhouses is a good approach in desert areas. However, changes in temperature and humidity in greenhouses are not preferable for growth of plants without control. Therefore, control of temperature and humidity by ventilation, sprinkler water, and solar radiation shielding is indispensable for agriculture using greenhouses in desert areas.
Some studies on the control of temperature and humidity in a greenhouse and agriculture at desert have been reported. Fahmy et al. [1] developed a control technique using an evaporative cooling system to improve the temperature and humidity conditions in a greenhouse and reported a calculation model based on MATLAB. Temperature and humidity in a greenhouse were controlled to fix at the optimal values for growing of herb by adjusting air flow rate of the fan and adding moisture to the air using a PI (proportional integral) control. Ishii et al. [2] measured the effect of circulating fans on air flow and temperature distribution in a greenhouse and reported how airflow and temperature distribution were influenced by the number and position of the fans. Hattori et al. [3] analyzed the temperature distribution in a greenhouse with a circulating fan using a 3-dimensional computational thermo-fluid dynamics code and compared the experimental results. Tanaka and Ishii [4] experimentally studied the effect of a roof absorber that absorbs infrared rays on thermal environment control in a greenhouse. Nagler et al. [5] proposed an evaporation model at a semiarid rangeland. Bruin et al. [6] reported a model of water evaporation and heat transfer at the surface of land. However, there have been few studies on the control of temperature and humidity in a greenhouse in desert areas.
In this work, we analyzed the effect of control of ventilation, sprinkler water and solar radiation shielding on transient changes in temperature and humidity in a greenhouse in a desert area. We calculated the effect of reducing of water consumption by using a greenhouse to grow orchids.
2. Calculation Method
The calculation model of temperature and humidity in a greenhouse in a desert area is shown in Figure 1. Temperature and humidity in the green house were controlled by air ventilation, sprinkler water on the sand, and solar radiation shielding on the roof. Radiation, convection, evaporation, and conduction heat transfer in the calculation model are shown in Figure 1. The greenhouse was 10 m × 30 m × 2.5 m (in height), and the wall and roof were glass. We assumed that the temperature of the sand at a 150 mm depth was constant for all days and water loss by permeation into the sand was negligible. Orchids were planted with a 0.3 m pitch in the greenhouse. Sprinkler water was only present near the roots of the plants. We used the surrounding air temperature, humidity, and solar radiation conditions in Riyadh, Saudi Arabia for sunny days on January 1, April 1, July 1, and October 1 [7] . The conditions to grow orchids in the greenhouse were assumed as follows. Average temperature for one day was approximately 15˚C in January, 25˚C in April, 30˚C in July, and 25˚C in October, and the relative humidity in the air was kept greater than 50% [8] . Dry portions were not allowed to occur in the sand near the roots of plants, and the amount of water that had evaporated from the sand was supplied to the roots of the plants when necessary.
Changes in temperature T (˚C) and absolute humidity X (kg/kg (dry air)) in the greenhouse were calculated using the network method or the lumped model with the heat and mass balance [9] . The nodal points in the calculation were the surface of the sand near the roots of plants (temperature Ts1 and absolute humidity Xs1), surface of the sand at a place far from the roots (temperature Ts2 and absolute humidity Xs2), air in the greenhouse (average temperature Tin and absolute humidity Xin), and wall of the greenhouse (temperature Tgls and absolute humidity Xgls) for the surrounding air (temperature Tout and absolute humidity Xout), sky temperature Tten, and solar radiation qs.
The equation of heat balance of the node at the surface of the sand (k = 1 and 2) is:
(1)
Here, rs is the density of the sand (rs = 1500 kg/m3), Cs is the specific heat capacity of the sand (Cs = 1100 J/kg K), Vsk is the volume of the surface node of the sand ((Vsk/Ask) = (hs/2)), Ask is the surface area of the sand for near the roots and a place far from the root (k = 1 and 2), hs is the depth of the sand (hs = 0.15 m), t is the time, qs is the solar radiation heat flux on unit surface area, Asya is the shielding ratio of the roof, as is the absorptivity of the sand (as = 1), agls is the transmissivity of the roof (agls = 0.7), ls is the thermal conduction coefficient of sand with water (ls = 1.1 W/m K), Tave is the temperature of sand at depth hs = 0.15 m, which is the average temperature of the surrounding air temperature Tout for one day, asi is the convection heat transfer coefficient on the surface of the sand (asi = 3 W/m2 K), Msi is the mass heat transfer coefficient at the surface of the sand (Msi = asi/(rwCw) = 7 × 10−7 m/s, which is obtained assuming the Lewis number is 1), rw is the density of water, Cw is the specific heat capacity of water, Lw is the latent heat of water (Lw = 2.4 ´ 106 J/kg), and asg is the equivalent radiation heat transfer coefficient between the surface of the sand and the inside wall of the greenhouse (asg = 3 W/m2 K).
The equation of heat balance of the node of air in the greenhouse is:
(2)
Here, agi is the convection heat transfer coefficient on the inside wall of the greenhouse (agi = 3 W/m2 K), Agls is surface area of the wall of the greenhouse, ra is the density of air, Ca is the specific heat capacity of air, and Vair is the air ventilation volume (m3/s). Heat capacity of the wall of the greenhouse was neglected in Equation (2).
The equation of water mass balance of the node of air in the greenhouse is:
(3)
Here, Mgi is the mass heat transfer coefficient at the surface of the inside wall of the greenhouse (Mgi = 7 × 10−7 m/s). The absolute humidity near the surface of the sand and the wall of the greenhouse Xs and Xgls were obtained from temperature Ts and Tgls assuming the satisfied condition.
The equation of heat balance of the node at the wall of the greenhouse is:
(4)
Here, ast is the equivalent radiation heat transfer coefficient between the sky and the outside wall of the greenhouse (ast = 3 W/m2 K), and ago is the convection heat transfer coefficient on the outside wall of the greenhouse (ago = 3 W/m2 K). The sky temperature Tten was calculated using an equation proposed by Swinbank [10] , which is a function of the surrounding air temperature Tout.
Equations (1)-(4) were calculated by the implicit finite-difference method with a calculation time step of 3 hours.
3. Calculation Results of Effect of Parameters
We calculated the effect of control of ventilation and shielding on temperature and humidity in a greenhouse under typical conditions at noon on October 1. Figure 2 and Figure 3 show the calculation results of the effect
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Figure 2. Effect of shielding ratio on temperature in greenhouse.
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Figure 3. Effect of shielding ratio on absolute humidity in greenhouse.
of shielding ratio of the roof Asya on the temperature Tin and absolute humidity Xin in the greenhouse for the condition of ventilation Vair = 0.2 m3/s. The saturated humidity Xsat in relation to temperature Tin is also shown in Figure 3. The relative humidity j is defined as j = Xin/Xsat. The temperature in the greenhouse decreases linearly in according with the shielding ratio of the roof. The relative humidity in the greenhouse is maximum when Asya = 0.4 because the absolute humidity Xin is almost constant when Asya is less than 0.4, and the absolute humidity Xin decreases when Asya is more than 0.4. Figure 4 shows the effect of shielding ratio on total water usage in one day. The curve of total water usage in one day is similar to that of the absolute humidity Xin in Figure 3.
Figure 5 and Figure 6 show the calculation results of the effect of ventilation Vair on the temperature Tin, absolute humidity Xin in the greenhouse and saturated humidity Xsat in relation to temperature Tin for the condition of shielding ratio Asya = 0. The absolute humidity in the greenhouse decreases when ventilation Vair is larger than 1 m3/s. Figure 7 shows the effect of ventilation on total water usage in one day. The total water usage in one day increases when ventilation Vair is larger than 1 m3/s. The value of the total water usage in one day with the effect of ventilation Vair in Figure 7 is about 10 times that with the effect of shielding ratio Asya in Figure 4.
4. Calculation Results of Effect of Greenhouse to Grow Orchids in Desert
We calculated the change in temperature and absolute humidity every 3 hours under the conditions in Saudi Arabia. Figure 8 shows the total water usage in one day without a greenhouse to grow orchids in the desert on October 1, January 1, April 1, and July 1. Dry portions were not allowed to occur in the sand near the roots of the plants at any time, and the amount of evaporated water from the sand was supplied to the roots of the plants.
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Figure 4. Effect of shielding ratio on total water usage in one day.
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Figure 5. Effect of ventilation on temperature in greenhouse.
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Figure 6. Effect of ventilation on absolute humidity in greenhouse.
The total water usage in one day in July is 3.8 m3, which is 3 times that in January.
Figure 9 shows the total water usage in one day with a greenhouse to grow orchids in the desert on October 1, January 1, April 1, and July 1. Ventilation, solar radiation shielding and sprinkler water were controlled so that the average temperature for one day and relative humidity were near the specified conditions (as explained
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Figure 7. Effect of ventilation on total water usage in one day.
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Figure 8. Total water usage in one day without greenhouse.
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Figure 9. Total water usage in one day with greenhouse.
above). The total water usage in one day in July is 0.3 m3, which is only 7% of that without a greenhouse. Figure 10 shows the change in ventilation and shielding ratio used in the calculation for October. Figure 11 shows the change of water supply rate. Figure 12 shows the calculation results of the change in temperature in the greenhouse Tin, sand near roots of plant Ts1, and surrounding air Tout in October. The temperature in the greenhouse Tin is close to the temperature of surrounding air Tout due to the control. Figure 13 shows the change of absolute
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Figure 10. Change of ventilation and shielding ratio in October.
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Figure 11. Change of water supply rate in October.
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Figure 12. Change of temperature in greenhouse and sand air in October.
humidity in the greenhouse Xin and saturated humidity Xsat in accordance with temperature Tin. The relative humidity can be calculated with j = Xin/Xsat. The absolute humidity in the greenhouse is almost constant.
Therefore water consumption can be reduced very much by using greenhouses for agriculture in desert areas with an optimum control of ventilation, sprinkler water, and shielding solar radiation according to this work.
5. Summary
We analyzed the effect of control of ventilation, sprinkler water, and solar radiation shielding on transient
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Figure 13. Change of absolute humidity in greenhouse in October.
changes in temperature and humidity in a greenhouse in a desert area, and the following results were obtained.
1) The temperature in the greenhouse decreased in accordance with the shielding ratio of solar radiation. The relative humidity in the greenhouse was maximum when the shielding ratio was 0.4.
2) The relative humidity in the greenhouse decreased when the ventilation was larger than 1 m3/s. The total water usage in one day increased when the ventilation was larger than 1 m3/s.
3) When ventilation, shielding, and sprinkler water were controlled under suitable conditions to grow orchids, water consumption in July was only 7% of that without a greenhouse.
NOTES
*Corresponding author.