The Desert Can Produce Food and Fresh Water with Seawater Greenhouses

February 10, 2009 - 09:59:36 PM

The Process

The 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 entire front wall of the building is a seawater evaporator. It consists of a honeycomb lattice and faces the prevailing wind. Fans assist and 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.

Cool air passes through the planting area and then combines with hot dry air from the roof cavity. The mixture passes through a second sea water 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 replicated in the thermodynamic model and, with appropriate meteorological information, the detailed design and performance of the Seawater Greenhouse can be optimised for every suitable location and environment.

Water Production and Water Savings

The Seawater Greenhouse converts sea water into fresh water, providing a unique local desalination capability. The water is condensed from water vapour in the air, in much the same way as dew. It is pure distilled water, produced without chemical treatment. The quantity produced depends on the climate - the hotter and sunnier, the more water.

The air entering the Greenhouse is both cooled and humidified. High humidity and low temperatures (the Greenhouse operates at approx. 90% relative humidity) reduces plant transpiration substantially, by up to 80%. This reduces irrigation requirements. The irrigation rate in Tenerife averaged 1.2 litre/day/m2 against 8 litres/day/m2 used by local farmers.

The impact of a new source of water on a local area can be highly beneficial. In Tenerife, a barren area 'turned green' as seepage from irrigation reversed saline intrusion and enabled new plant growth.

Of even greater importance is the effect the Seawater Greenhouse can have on reducing demand for mains water and reserves of ground water. Around 8-10 litres per m2 per day can be saved which, on a macro scale, will have an immense impact, freeing existing water supply for other uses.

 Energy Requirements

The Greenhouse is driven by solar and wind energy. Sunlight is separated into visible and infrared light. Visible light passes through the roof to drive photosynthesis. Infrared light is trapped in the roof canopy and is ducted from there to the seawater evaporator. Thus solar energy converts seawater to water vapour.

The structure acts as a 'wind-catcher'. It faces into the prevailing daytime wind to assist ventilation. Fans are required under most conditions although these were unnecessary in Tenerife. The wind-fan combination moves air through the front evaporator and chills the sea water which then provides cooling for the rear condenser and, thus, the production of fresh water.

The electricity requirements are modest and, in the absence of grid power, can be provided by photovoltaic panels without the need for batteries, inverter or standby generator. The Tenerife Greenhouse was built on a wind farm with power supplied by the wind turbines on site. There are thus potential synergies between the Seawater Greenhouse and both wind and solar power.

The overall process is extremely energy efficient. 1kW of electricity expended on pumping will remove 500kW of heat. Water can be produced at low energy costs (<3kWh/m3).

From the Seawater Greenhouse website