Abstract:
The growth of the world’s population and the consequent food shortage,
requires the expansion of agriculture into arid zones which constitute about 60
% of the earth’s land area and are characterized by high levels of solar radiation
and shortage of freshwater. The objective of the seawater greenhouse for arid
lands was to develop and demonstrate a cost effective means of producing both
crops and pure water in hot, arid coastal regions. The project exploited both
the high solar radiation and prevailing wind to drive most of the energy
exchange processes in the greenhouse. In addition to a crop grown inside the
greenhouse, a shade tent provided shelter for nursery plants and an outdoor
planting scheme was maintained with the supply of freshwater as well as
protected by the structure of the greenhouse itself.
The greenhouse is cooled with an evaporative cooling pad (Celdek™) through
which an air flow is promoted by the prevailing wind. The water vapour
transpired by the plants combines with the cooled and humidified ventilation air
stream to generate a high relative humidity in the exhaust air. A second
evaporation pad (Celdek™) is used to further humidify the exhaust air which
passes through a condenser cooled by seawater to produce fresh water. The
wind also promotes an air flow through a roof cavity in the greenhouse where
more seawater is evaporated and the humid air passes through the condenser.
A computer program was modified to describe the greenhouse, which at first
was triangular in plan, and models were written to describe the evaporative
cooling systems, the selectively absorbing roof, the humidification of the air in the roof and the seawater condenser. These models were incorporated into the
existing greenhouse model.
Analytical techniques were used to create the hourly values of solar radiation,
air temperature, humidity, cloud cover and wind speed required by the
simulation model.
This simulation model was used to predict system performance and
determine the sensitivity of the greenhouse environment and fresh water
output to air and water flowrates and the climatic conditions. The sensitivity
analysis showed that adding the second (rear pad) evaporative cooling pad
increased the fresh water production compared to using only the front
evaporative cooling pad. The use of cooling water at the wet bulb temperature
gave a lower condenser output than using surface seawater. The results showed
that an effective desalination system would consist of an evaporating cooling
pad coupled directly to the seawater condenser.
Data were recorded in the greenhouse in Tenerife in order to determine the heat
and mass transfer coefficients of the second Celdek™; these established the
program of the system and provided data for validating the model.
The experiments performed on a prototype Celdek™ heat exchanger in the
laboratory were used to design the Celdek™ condenser.
The validity of the seawater greenhouse simulation model was checked using
experimental data recorded on the prototype seawater greenhouse in Tenerife,
in December 1994 and June 1995. The model was revised as a result of the
validation exercise.