A seawater greenhouse is a

greenhouse A greenhouse (also called a glasshouse, or, if with sufficient heating, a hothouse) is a structure with walls and roof made chiefly of transparent material, such as glass, in which plants requiring regulated climatic conditions are grown.These ...

structure that enables the growth of crops and the production of fresh water in

arid A region is arid when it severely lacks available water, to the extent of hindering or preventing the growth and development of plant and animal life. Regions with arid climates tend to lack vegetation and are called xeric or desertic. Most ...

regions which constitute about one third of the earth's land area. This in response to the global

water scarcity Water scarcity (closely related to water stress or water crisis) is the lack of fresh water resources to meet the standard water demand. There are two types of water scarcity: physical or economic water scarcity. Physical water scarcity is whe ...

and peak water and the salt-infecting soil. The system uses

seawater Seawater, or salt water, is water from a sea or ocean. On average, seawater in the world's oceans has a salinity of about 3.5% (35 g/L, 35 ppt, 600 mM). This means that every kilogram (roughly one liter by volume) of seawater has appro ...

and

solar energy Solar energy is radiant light and heat from the Sun that is harnessed using a range of technologies such as solar power to generate electricity, solar thermal energy (including solar water heating), and solar architecture. It is an essen ...

. 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 Tenerife (; ; formerly spelled ''Teneriffe'') is the largest and most populous island of the Canary Islands. It is home to 43% of the total population of the Archipelago, archipelago. With a land area of and a population of 978,100 inhabitant ...

. 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 The Sahara Forest Project aims to provide fresh water, food and renewable energy in hot, arid regions as well as re-vegetating areas of uninhabited desert. The founding team was composed of Seawater Greenhouse Ltd, Exploration Architecture, Max Fo ...

(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 Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's activities. Some of this chemical energy is stored in ...

. The design for cooling the microclimate primarily consists of humidification and dehumidification (HD) desalination process or

multiple-effect humidification Multiple-effect humidification (MEH) is a method used for thermal desalination of sea water. It uses multiple evaporation–condensation Condensation is the change of the state of matter from the gas phase into the liquid phase, and is th ...

. 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. The process recreates the natural

hydrological cycle The water cycle, also known as the hydrologic cycle or the hydrological cycle, is a biogeochemical cycle that describes the continuous movement of water on, above and below the surface of the Earth. The mass of water on Earth remains fairly const ...

within a controlled environment of the greenhouse by evaporating water from saline water source and regains it as freshwater by condensation. The first part of the system uses seawater, an evaporator, and a condenser. The front wall of the greenhouse consists of a seawater-wetted evaporator which faces the prevailing wind. These are mostly constituted of corrugated cardboard shown in Figure 3. If the wind is not prevalent enough, fans blow the outside air through the evaporator into the greenhouse. The ambient warm air exchanges the heat with the seawater which cools it down and gets it humidified. The cool and humid air creates an adequate growing environment for the crops. The remaining evaporatively-cooled seawater is collected and pumped to the condenser as a coolant. 1-s2

The second part of the system has another evaporator. The seawater flows from the first evaporator which preheats it and thereafter flows through the

solar thermal collector A solar thermal collector collects heat by absorbing sunlight. The term "solar collector" commonly refers to a device for solar hot water heating, but may refer to large power generating installations such as solar parabolic troughs and s ...

on the roof to heat it up sufficiently before it flows to the second evaporator. The seawater, or coolant, flows through a circuit consisting of the evaporators, solar heating pipe, and condenser with an intake of seawater and an output of fresh water. The fresh water is produced by hot and relatively high humidity air which can produce sufficient distilled water for irrigation. The volume of fresh water is determined by air temperature, relative humidity, solar radiation and the airflow rate. These conditions can be modeled with appropriate meteorological data, enabling the design and process to be optimized for any suitable location.

Applicability

The technique is applicable to sites in arid regions near the sea. The distance and elevation from the sea must be evaluated considering the energy required to pump water to the site. There are numerous suitable locations on the coasts; others are below sea level, such as the

Dead Sea The Dead Sea ( he, יַם הַמֶּלַח, ''Yam hamMelaḥ''; ar, اَلْبَحْرُ الْمَيْتُ, ''Āl-Baḥrū l-Maytū''), also known by other names, is a salt lake bordered by Jordan to the east and Israel and the West Ban ...

and the

Qattara Depression The Qattara Depression ( ar, منخفض القطارة, Munḫafaḍ al-Qaṭṭārah) is a depression in northwestern Egypt, specifically in the Matruh Governorate. The depression is part of the Western Desert of Egypt. The Qattara Depressi ...

, where hydro schemes have been proposed to exploit the hydraulic pressure to generate power, e.g., Red Sea–Dead Sea Canal.

Studies

In 1996, Paton and Davies used the Simulink toolkit under MATLAB to model forced ventilation of the greenhouse in Tenerife, Cape Verde, Namibia, and Oman. The greenhouse is assisted by the prevailing wind, evaporative cooling, transpiration, solar heating, heat transfer through the walls and roof, and condensation which is analyzed in the study. They found that the amount of water required by the plants is reduced by 80% and 2.6-6.4 kWh electrical energy is needed for m3 of fresh water produced. In 2005, Paton and Davis Evaluated design options with thermal modeling using the United Arab Emirates model as a baseline. They studied three options:perforated screen, C-shaped air path, and pipe array, to find a better seawater circuit to cool the environment and produce the most freshwater. The study found that a pipe array gave the best results: an air temperature decrease of 1 °C, a mean radiant temperature decrease of 7.5 °C, and a freshwater production increase of 63%. This can be implemented to improve seawater greenhouses in hot arid regions such as the second pilot design in the United Arab Emirates. In 2018, Paton and Davis researched brine utilization for cooling and salt production in wind-driven seawater greenhouses to design and model it. The brine disposed by the seawater desalination may disturb the ecosystem as the same amount of brine is produced as freshwater. By using the brine valoristation method of wind-driven air flow by cooling the greenhouse with seawater evaporation, salt can be produced as shown in Figure 4. This brine is the by-product of the freshwater production, but can also be the ingredient to make salt, making it into a product that can be merchandised. An additional finding of this research was the importance of the shade-net which is modelled by a thin film in the study shown in Figure 5. It not only provides cooling, but also elongates the cooling plume by containing the cold air plume from the evaporative cooling pad.

References

External links