Condensation irrigation : a combined system for desalination and irrigation

Sammanfattning: Condensation Irrigation (CI) is a new irrigation method that combines desalination and subsurface irrigation by making use of saline water for supplying clean irrigation. In this system, solar stills are used for evaporating non-potable water, and the formed vapour heats and humidifies the ambient air above the water surface. The warm, humid air is led from the stills into a system of horizontally buried drainage pipes. While flowing through the pipes, the air is cooled by the ground and vapour precipitates as freshwater inside the pipes. The perforations in the pipe wall enable the formed freshwater to percolate into the surrounding soil, and thereby irrigate it. Some of the humid air also infiltrates the soil through the perforations, which further increases irrigation by vapour condensation in the cooler ground. The airflow through the ground supplies soil aeration, which is important for high crop yield.Because the CI system generates freshwater from saline or otherwise contaminated water sources, this system can operate in locations that would normally lack irrigation possibilities. This subsurface irrigation system has also further advantages, such as reduced water losses through surface evaporation and deep percolation, increased soil aeration, and low tech / low cost design. To investigate the potential of the CI system, an implicit transient finite element simulation model, CI2D, was developed in Matlab, that was able to simulate the complex coupled mechanisms of gas, liquid and heat transfer in the soil-pipe system, including water evaporation and condensation. The validated and verified model also included solar radiation, root water extraction, and surface evaporation.The CI2D model was used to simulate a reference example of a theoretical CI facility in Malta. The irrigation rate under steady operation was 3.44 mm d-1 and the root water uptake was 19.8% of the supplied water. By lowering the inlet air temperature, the crop could be placed closer to the pipes without the roots being overheated. The irrigation rate obtained by decreasing the inlet air temperature from 70°C to 50°C, and reducing the pipe spacing from 1.2 m to 0.6 m, was 3.00 mm d-1. The root water uptake was, however, increased to 48% of the irrigation, resulting in a higher root water uptake.The principle behind CI can be used for drinking water production by using pipes without perforations in the ground. The condensed freshwater can then be collected at the pipe endings. This system was simulated under the same reference scenario as the irrigation system. The daily water production rate in a 50 m long pipe was in the example 135 kg d-1, corresponding to 2.26 mm d-1.A small scale laboratory setup where humid air was led through a perforated pipe in a sand box was tested and theoretically simulated. In the experiments, the importance of a free flow path for the gas phase through the soil was visualized. It could therefore be concluded that the CI system should not be implemented in low-permeability soils. From simulations in CI2D, it was evident that soils with high capillarity are unsuitable for CI systems as well, because the water accumulation around the pipe prevents humid air from entering the soil through the perforations.CI is a system with many unexplored possible designs and applications. For example, by leading the cooler saline feed water to the solar stills through the perforated irrigation pipes, the vapour condensation in the pipes would increase. This would also increase the solar still efficiency since the incoming saline water would be preheated by the humid airflow. In future work on this system, this, and other suggested improvements should be explored.