Seawater greenhouse

A seawater greenhouse is a greenhouse structure that enables the growth of crops in arid regions, using seawater and solar energy. The technique involves pumping seawater (or allowing it to gravitate if below sea level) to an arid location and then subjecting it to two processes: first, it is used to humidify and cool the air, and second, it is evaporated by solar heating and distilled to produce fresh water. Finally, the remaining humidified air is expelled from the greenhouse and used to improve growing conditions for outdoor plants. The technology was introduced by British inventor Charlie Paton in the early 1990s and is being developed by his UK company Seawater Greenhouse Ltd. The more concentrated salt water may either be further evaporated for the production of salt and other elements, or discharged back to the sea. The seawater greenhouse is a response to the global water crisis and peak water.

History

The seawater greenhouse concept was first researched and developed in 1991 by Charlie Paton's company Light Works Ltd, now Seawater Greenhouse Ltd. The first pilot project commenced in 1992 with the search for a test site that was eventually found on the Canary Island of Tenerife. A prototype seawater greenhouse was assembled in the UK and constructed on the site in Tenerife. The results from this pilot project validated the concept and demonstrated the potential for other arid regions.^[1]

The original pilot design evolved into a lower cost solution using a lighter steel structure, similar to a multi-span polytunnel. This structure was designed to be cost effective and suitable for local sourcing. The design was first tested and validated through a second seawater greenhouse that was constructed on Al-Aryam Island, Abu Dhabi, United Arab Emirates in 2000. The year 2004 saw the completion of a third pilot seawater greenhouse near Muscat, Oman in collaboration with Sultan Qaboos University, providing 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 predict and quantify how the seawater greenhouse will perform in other parts of the world.^[2]

In 2010, Seawater Greenhouse built a new commercial installation in Australia. The enterprise is now independently operating as Sundrop Farms Pty Ltd .^[3]^[4]

Process

A seawater greenhouse uses the sun, the sea and the atmosphere to produce fresh water and cool air. The process recreates the natural hydrological cycle within a controlled environment. The front wall of the building is a seawater evaporator. It consists of a honeycomb lattice and faces the prevailing wind. Fans control air movement. Seawater trickles down over the lattice, cooling and humidifying the air passing through into the planting area. Sunlight is filtered through a specially constructed roof. The roof traps infrared heat, while allowing visible light through to promote photosynthesis. This creates optimum growing conditions – cool and humid with high light intensity. Seawater that has been heated in the roof passes through a second evaporator creating hot, saturated air which then flows through a condenser. The condenser is cooled by incoming seawater. The temperature difference causes fresh water to condense out of the air stream. 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.

Figure 1. Inside the greenhouse - evaporation: water is pumped from the sea to be evaporated over a cardboard lattice	Figure 2. Inside the greenhouse - condensation: Water vapour is created by an array of hot pipes and successively condenses on cooler pipes

A seawater greenhouse evaporates much more water than it condenses back into freshwater. This humid air is `lost´ due to high rates of ventilation to keep the crops cool and supplied with CO2. The higher humidity exhaust air provides some benefit to the cultivation of more hardy crops downwind of the greenhouse.

Figure 3. The surrounding area from the front of the greenhouse	Figure 4. Two years into operation from the back of the greenhouse

This phenomenon could enable the cultivation of biofuel crops in the area surrounding the seawater greenhouse.

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 and the Qattara Depression, where hydro schemes have been proposed to exploit the hydraulic pressure to generate power, e.g., Red Sea–Dead Sea Canal.^[5]^[6]

Other benefits

If carbon-neutral photovoltaic (PV) panels were available, a seawater greenhouse could be carbon neutral. Unfortunately, current methods of PV production are not perfect. The electrical energy it requires to operate pumps and fans is best produced by solar PV panels as its demand for power is proportional to sunlight.

The use of pesticides is reduced or eliminated as the seawater evaporators have a biocidal effect on the air that passes through them.

Fuels production

A seawater greenhouse produces biological residues. This biomass can be used to help create and enrich the surrounding soil, or alternatively digested to produce bio methane, albeit with less, but still significant quantities of soil nutrients.

Locations

In 2010 a seawater greenhouse was built at Port Augusta, Australia.^[7]

Sundrop Farms, Australia

The seawater greenhouse technology has been adopted by Sundrop Farms in South Australia. This has been a success, growing crops such as tomatoes in desert, with only the use of salt water, and sunlight, which also provides most of their electricity.^[8] An expansion of 20 hectares is planned for 2016.^[9]