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Water consumption can be reduced by using a greenhouse for agriculture in desert areas. We analyzed the effect of control of ventilation, sprinkler water, and solar radiation shielding on changes of temperature and humidity in a greenhouse under various desert area conditions. We calculated the changes in temperature and humidity in a greenhouse for a whole day in four seasons, and the calculation results of water consumption with and without a greenhouse were compared. When ventilation, shielding, and sprinkler water were controlled under suitable conditions to grow orchids in a desert area, water consumption in July was only 7% of that without a greenhouse.

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. [

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.

The calculation model of temperature and humidity in a greenhouse in a desert area is shown in

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 [_{s1} and absolute humidity X_{s1}), surface of the sand at a place far from the roots (temperature T_{s2} and absolute humidity X_{s2}), air in the greenhouse (average temperature T_{in} and absolute humidity X_{in}), and wall of the greenhouse (temperature T_{gls} and absolute humidity X_{gls}) for the surrounding air (temperature T_{out} and absolute humidity X_{out}), sky temperature T_{ten}, and solar radiation q_{s}.

The equation of heat balance of the node at the surface of the sand (k = 1 and 2) is:

Here, r_{s} is the density of the sand (r_{s} = 1500 kg/m^{3}), C_{s} is the specific heat capacity of the sand (C_{s} = 1100 J/kg K), V_{sk} is the volume of the surface node of the sand ((V_{sk}/A_{sk}) = (h_{s}/2)), A_{sk} is the surface area of the sand for near the roots and a place far from the root (k = 1 and 2), h_{s} is the depth of the sand (h_{s} = 0.15 m), t is the time, q_{s} is the solar radiation heat flux on unit surface area, A_{sya} is the shielding ratio of the roof, a_{s} is the absorptivity of the sand (a_{s} = 1), a_{gls} is the transmissivity of the roof (a_{gls} = 0.7), l_{s} is the thermal conduction coefficient of sand with water (l_{s} = 1.1 W/m K), T_{ave} is the temperature of sand at depth h_{s} = 0.15 m, which is the average temperature of the surrounding air temperature T_{out} for one day, a_{si} is the convection heat transfer coefficient on the surface of the sand (a_{si} = 3 W/m^{2} K), M_{si} is the mass heat transfer coefficient at the surface of the sand (M_{si} = a_{si}/(r_{w}C_{w}) = 7 × 10^{−7} m/s, which is obtained assuming the Lewis number is 1), r_{w} is the density of water, C_{w} is the specific heat capacity of water, L_{w} is the latent heat of water (L_{w} = 2.4 ´ 10^{6} J/kg), and a_{sg} is the equivalent radiation heat transfer coefficient between the surface of the sand and the inside wall of the greenhouse (a_{sg} = 3 W/m^{2} K).

The equation of heat balance of the node of air in the greenhouse is:

Here, a_{gi} is the convection heat transfer coefficient on the inside wall of the greenhouse (a_{gi} = 3 W/m^{2} K), A_{gls} is surface area of the wall of the greenhouse, r_{a} is the density of air, C_{a} is the specific heat capacity of air, and V_{air} is the air ventilation volume (m^{3}/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:

Here, M_{gi} is the mass heat transfer coefficient at the surface of the inside wall of the greenhouse (M_{gi} = 7 × 10^{−7} m/s). The absolute humidity near the surface of the sand and the wall of the greenhouse X_{s} and X_{gls} were obtained from temperature T_{s} and T_{gls} assuming the satisfied condition.

The equation of heat balance of the node at the wall of the greenhouse is:

Here, a_{st} is the equivalent radiation heat transfer coefficient between the sky and the outside wall of the greenhouse (a_{st} = 3 W/m^{2} K), and a_{go} is the convection heat transfer coefficient on the outside wall of the greenhouse (a_{go} = 3 W/m^{2} K). The sky temperature T_{ten} was calculated using an equation proposed by Swinbank [_{out}.

Equations (1)-(4) were calculated by the implicit finite-difference method with a calculation time step of 3 hours.

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.

of shielding ratio of the roof A_{sya} on the temperature T_{in} and absolute humidity X_{in} in the greenhouse for the condition of ventilation V_{air} = 0.2 m^{3}/s. The saturated humidity X_{sat} in relation to temperature T_{in} is also shown in _{in}/X_{sat}. 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 A_{sya} = 0.4 because the absolute humidity X_{in} is almost constant when A_{sya} is less than 0.4, and the absolute humidity X_{in} decreases when A_{sya} is more than 0.4. _{in} in

_{air} on the temperature T_{in}, absolute humidity X_{in} in the greenhouse and saturated humidity X_{sat} in relation to temperature T_{in} for the condition of shielding ratio A_{sya} = 0. The absolute humidity in the greenhouse decreases when ventilation V_{air} is larger than 1 m^{3}/s. _{air} is larger than 1 m^{3}/s. The value of the total water usage in one day with the effect of ventilation V_{air} in _{sya} in

We calculated the change in temperature and absolute humidity every 3 hours under the conditions in Saudi Arabia.

The total water usage in one day in July is 3.8 m^{3}, which is 3 times that in January.

above). The total water usage in one day in July is 0.3 m^{3}, which is only 7% of that without a greenhouse. _{in}, sand near roots of plant T_{s1}, and surrounding air T_{out} in October. The temperature in the greenhouse T_{in} is close to the temperature of surrounding air T_{out} due to the control.

humidity in the greenhouse X_{in} and saturated humidity X_{sat} in accordance with temperature T_{in}. The relative humidity can be calculated with j = X_{in}/X_{sat}. 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.

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, 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 m^{3}/s. The total water usage in one day increased when the ventilation was larger than 1 m^{3}/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.