Abstract
Production of vegetables for fresh consumption in arid regions usually takes place in greenhouses with evaporative cooling during the warm months of the year. In this period, water use for cooling easily exceeds irrigation water use. The purpose of this paper is to investigate how far water use for evaporative cooling can be lowered by adapting design elements of the greenhouse and the evaporative cooling system. In a greenhouse trial in a desert environment, in Riyadh (KSA), two different greenhouses: one traditional and one modified, both equipped with a pad and fan system, have been compared in terms of productivity and water use with main focus on water use for cooling. The modified greenhouse design resulted in about 14% higher fresh weight production and more than 40% water saving was achieved on evaporative cooling. The climate and water use data recorded during the trial were used to validate a greenhouse climate simulation model including pad and fan cooling. Then, we used the validated model in a scenario study and we quantified the effect of each one of three design elements on use of cooling water. It was shown that the extracted air temperature has a major influence on water use for cooling. Increasing the extracted air temperature with 4 K resulted in about 27% saving in water use for cooling. The latter was done by repositioning of the exhaust fans in order to take advantage of the vertical air temperature gradient in the greenhouse. Additionally, a 5% higher cooling efficiency of the pad wall could yield an extra 12% water saving for evaporative cooling. Finally, the greenhouse cover-to-ground area ratio was also found to affect the water use for cooling. In summary, we have shown that there is much scope for saving on water use for evaporative cooling by improving design of greenhouses and of the cooling system, and that a good greenhouse climate model can be a useful tool in this process. The findings of the current research provide clear guidelines for the construction of more water efficient evaporatively cooled greenhouses.
Highlights
Fresh vegetable production needs to be increased in the upcoming years to ensure a healthy diet for everybody (Krishna Bahadur et al, 2018)
The water used for irrigation in the M greenhouse (1246 L m− 2) was about 27% more than the corre sponding amount of water used in the L greenhouse (980 L m− 2), as a result of both higher production and higher transpiration, both initiated by the higher amount of light in the M greenhouse
Evaporative cooling is by far the largest water use in the greenhouses where it is applied
Summary
Fresh vegetable production needs to be increased in the upcoming years to ensure a healthy diet for everybody (Krishna Bahadur et al, 2018). Plentiful solar radiation would seem to provide semi-arid and arid regions with great potential for agricultural opera tions, it is the high temperature which does not, as it typically rises above optimal values This is the case for most of Saudi Arabia, with an average yearly sun radiation sum of 7.6 GJ m-2 and mean monthly maximum temperatures between 39 ◦C and 46 ◦C for at least 5 months each year. When such values occur, relative humidity does not exceed 20%, being less than 10% during the day, in the central region of the country (Al-Helal et al, 2004). Evaporative cooling has been shown to deliver conditions more favorable than natural (or even forced) ventilation in less dry conditions than deserts, for instance the is land of Cyprus (Nikolaou et al, 2019)
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