A seawater greenhouse is a greenhouse structure that enables the growth of crops and the production of fresh water in arid regions which constitute about one third of the earth's land area. This in response to the global water scarcity and peak water and the salt-infecting soil. The system uses seawater and solar energy. It uses a similar structure to the pad-and-fan greenhouse, but with additional evaporators and condensers. The seawater is pumped into the greenhouse to create a cool and humid environment, the optimal conditions for the cultivation of temperate crops. The freshwater is produced in a condensed state created by the solar desalination principle, which removes salt and impurities. Finally, the remaining humidified air is expelled from the greenhouse and used to improve growing conditions for outdoor plants.
Projects
The Seawater Greenhouse Ltd
The seawater greenhouse concept was first researched and developed in 1991 by Charlie Paton's company Light Works Ltd, which is now known as the Seawater Greenhouse Ltd. Charlie Paton and Philip Davies worked on the first pilot project commenced in 1992, on the Canary Island of Tenerife. A prototype seawater greenhouse was assembled in the UK and constructed on the site in Tenerife covering an area of 360 m2. The temperate crops successfully cultivated included tomatoes, spinach, dwarf peas, peppers, artichokes, French beans, and lettuce.
The second pilot design was installed in 2000 on the coast of Al-Aryam Island, Abu Dhabi, United Arab Emirates. The design is a light steel structure, similar to a multi-span polytunnel, which relies purely on solar energy. A pipe array is installed to improve the design of the greenhouse by decreasing the temperature and increasing the freshwater production. The greenhouse has an area of 864 m2 and has a daily water production of 1 m3, which nearly meets the crop's irrigation demand.
The third pilot seawater greenhouse, which is 864 m2, is near Muscat in Oman which produces 0.3 to 0.6 m3 of freshwater per day. This project was created as a collaboration between Sultan Qaboos University. It provides an opportunity to develop a sustainable horticultural sector on the Batinah coast. These projects have enabled the validation of a thermodynamic simulation model which, given appropriate meteorological data, accurately predicts and quantifies how the seawater greenhouse will perform in other parts of the world.
The fourth project is the commercial installation in Port Augusta, Australia, installed in 2010. It is currently a 20 hectare seawater greenhouse owned and run by Sundrop Farms which has developed it further.
The fifth design was constructed in 2017 in Berbera, Somalia. The design was researched to be simplified and inexpensive with advanced greenhouse modeling techniques. This design includes a shading system which retains core evaporative cooling elements.
Sahara Forest Project
The Sahara Forest Project (SFP) combines the seawater greenhouse technology and concentrated solar power and constructed pilot projects in Jordan and Qatar. The seawater greenhouse evaporates 50 m3 of seawater and harvests 5 m3 of fresh water per hectare per day. The solar power production capacity through PV panels produces 39 KW on the 3 hectares area with 1350 m2 growing area. The greenhouses are 15 degrees cooler than the outside temperatures which enables the production up to 130,000 kg vegetables produced per year and up to 20,000 liters of fresh water production per day. Additionally, the project includes revegetation by soil reclamation of nitrogen-fixing and salt-removing desert plants by repurposed waste products from agriculture and saltwater evaporation.
Process
A seawater greenhouse uses the surrounding environment to grow temperate crops and produce freshwater. A conventional greenhouse uses solar heat to create a warmer environment to allow adequate growing temperature, whereas the seawater greenhouse does the opposite by creating a cooler environment. The roof traps infrared heat, while allowing visible light through to promote photosynthesis.
The design for cooling the microclimate primarily consists of humidification and dehumidification (HD) desalination process or multiple-effect humidification. A simple seawater greenhouse consists of two evaporative coolers (evaporators), a condenser, fans, seawater and distilled water pipes and crops in between the two evaporators. This is shown in schematic figures 1 and 2.