Quantifying the trade-off between water and electricity for tomato production in arid environments

Production of vegetables for fresh consumption in arid regions is usually done in greenhouses fitted with evaporative cooling, which does strain the scarce water resource in those areas so much that [lack of] water frequently becomes the limiting factor. Greenhouses fit with sufficient mechanical cooling capacity (closed/semi-closed greenhouses) not only dispose of the need for evaporative cooling, but also allow for recovery of the water transpired by the crop as condense on the mechanical cooling and dehumidification system. The purpose of this paper is to investigate the trade-off between resources (water and electricity) within a closed greenhouse and evaluate its potential as the ultimate water saving production system. After a series of 8 closed-greenhouse trials in a desert environment, in Riyadh (KSA), benchmark numbers on water and energy use of such a system were established. Thanks to the recollection of more than 80% of the water supplied to the crop, production of 1 kg of fresh tomatoes was achieved at the expense of 4.2 L of water, which is 10 times lower that the lowest reported in evaporatively cooled greenhouses and 40 times lower than commercial practice in similar weather conditions. On the other hand, the required electricity use was about 8 kWh per kg of fresh produced tomatoes. From energy point of view it is shown that using desalinated water for evaporative cooling reduces the electricity consumption of fresh tomato production to about 1 kWh.kg(-1). From economic point of view a closed greenhouse might only result in lower variable costs when 10 liters of desalinated water costs more than 1.75 kWh of electricity.