The key benefit of aquaponic systems, as contrasted to conventional agriculture, is reduced water usage. Water is only lost from absorption and transpiration by plants, evaporation from fish ponds, and biomass removal. Plant absorption and biomass removal are usually desired; evaporation and plant transpiration however, are water losses which are unproductive – and potentially avoidable. The capstone project for my internship with Nourish The Planet concerns the design of a greenhouse which captures the water lost from evaporation and plant transpiration, and reuses it in the system’s water cycle.
In 2003, the European Union commissioned a greenhouse design from Watergy that would do just that. Over the course of three years, two prototypes were tested; the system built in Spain consisted of an airtight greenhouse combined with a secondary solar collector and cooling tower (concept shown in figure below). It was able to recycle evaporation and plant transpiration, desalinate seawater, and passively cool the air during the day and heat the air at night – all by virtue of its design and water coolant circulation.
During the daytime, sunlight heats the greenhouse’s moist air, causing it to rise up the outside of the cooling tower. The tower is colored black to absorb as much heat as possible, thus encouraging warm, moist air to gather from the entire lower section of the greenhouse. Cool water is circulated from a reserve tank outside the greenhouse and into a water-to-air exchanger built into the inner tube of the tower. This encourages the moist air near the top of the tower to condense droplets onto the exchanger. These droplets stream down the inside of the tower to the bottom, where it is collected and reused. The dry air resulting from this process continues to the lower section of the tower and out into the greenhouse, thus cooling the plants.
During the condensation process the heat from the air in the top of the tower is transferred into the circulating exchanger-water. This causes the water in the reserve tank to gradually heat up throughout the daytime. The water from this reserve tank continues circulating in the same direction all throughout the day and night. At night, this causes the air in the greenhouse to reverse direction, as the warm air-to-water exchanger encourages the air near the plants to rise up the inner tube of the tower. This air then falls down the outside of the inner tube, heating the greenhouse. By morning, the water in the reserve tank has returned to its originally-cool temperature, and is ready to begin the cycle anew. 
I’ll be posting updates and results for the application of this concept to a small-scale aquaponics system in the next few weeks. Stay tuned!
Written by: Alex Gan, Water Quality Intern
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