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Potential of the seawater greenhouse in Middle Eastern climates

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  • Seawater Greenhouse Ltd

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The Seawater Greenhouse is a method of cultivation that provides desalination, cooling and humidification in an integrated system. Its purpose is to provide a sustainable means of agriculture in arid coastal areas where the scarcity of freshwater and expense of desalination threaten the viability of agriculture. Because the desalination process is driven mainly by solar energy, sunlight is the weather variable that most influences the performance of the Seawater Greenhouse. Correspondingly, the potential in Middle Eastern climates is excellent. However, other variables such as wind and humidity are also significant and this means that the optimum design and mode of operation may vary across the region. In this paper we describe the Seawater Greenhouse system. We make reference to installations in the Canary Islands and the United Arab Emirates and to the installation now in progress in Oman. The experiments with these developmental versions have led us to create some mathematical models, enabling us to simulate performance in different climates. We analyse historical weather data for some Middle Eastern locations, including the Gulf of Aqaba, and discuss the effects these are likely to have on design considerations and performance.
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International Engineering Conference Mutah 2004
POTENTIAL OF THE SEAWATER GREENHOUSE IN
MIDDLE EASTERN CLIMATES
Philip Davies*, Kim Turner and Charlie Paton
* School of Engineering, University of Warwick, Coventry CV4 7AL, U.K.
philip.davies@warwick.ac.uk
Seawater Greenhouse Ltd, 2A Greenwood Road, London E8 1AB, U.K.
lightworks1@compuserve.com
www.seawatergreenhouse.com
SUMMARY
The Seawater Greenhouse is a method of cultivation that provides desalination, cooling
and humidification in an integrated system. Its purpose is to provide a sustainable means
of agriculture in arid coastal areas where the scarcity of freshwater and expense of
desalination threaten the viability of agriculture.
Because the desalination process is driven mainly by solar energy, sunlight is the weather
variable that most influences the performance of the Seawater Greenhouse.
Correspondingly, the potential in Middle Eastern climates is excellent. However, other
variables such as wind and humidity are also significant and this means that the optimum
design and mode of operation may vary across the region.
In this paper we describe the Seawater Greenhouse system. We make reference to
installations in the Canary Islands and the United Arab Emirates and to the installation
now in progress in Oman. The experiments with these developmental versions have led
us to create some mathematical models, enabling us to simulate performance in different
climates. We analyse historical weather data for some Middle Eastern locations,
including the Gulf of Aqaba, and discuss the effects these are likely to have on design
considerations and performance.
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1. INTRODUCTION
It is hardly necessary to state the dangers posed by the scarcity of fresh water. In recent
years, the seriousness and extent of the problem has prompted a number of detailed
studies into its causes and consequences such as that of reference1.
Nowhere is the problem more acute than in the Middle East. Most states in the region can
be catagorised as suffering from severe water stress, with water consumption greatly
exceeding the available renewable resource. Agriculture accounts for a very large fraction
of water usage, averaging 87% for the MENA region as a whole2,3. As a result, water
shortage has far-reaching consequences in terms of food supplies4 and dependence on
imported food.
Meanwhile, there is a growing opinion that future solutions should involve not only
cheaper and better ways of providing freshwater, but also more economical ways of using
this increasingly precious resource.
In a certain sense, agriculture is very inefficient in its use water. Of all the water used to
irrigate crop, less than 1% can be expected to find its way into the final edible produce.
Even in efficient irrigation systems, a very large fraction of the water is lost through
transpiration.
Plant scientists have studied mechanisms of water loss in great detail. The classic model
for representing water loss from crops is the Penman equation which compares the
process to evaporation from an open pool of water5. In simple terms the equation can be
written as:
Rate of water loss = b R + c D [1]
Where R is the net radiation received by the crop. The term D is the vapour deficit,
meaning the difference between the saturation vapour content of the air and its actual
vapour content. The terms b and c are approximately constant for a given range of
conditions.
The Penman equation suggests two strategies for reducing water requirements.
(i) Reduction of the radiation R by means of shading; or possibly selective
shading to favour photosynthetically active wavelengths of light.
(ii) Reduction of the vapour deficit D through humidification of the air.
Both of these strategies have been employed in the Seawater Greenhouse. In addition, the
Greenhouse addresses the issue of excessive water loss from crops by incorporating them
in a system that recovers some of the water transpired. The Seawater Greenhouse
combines, in a single system, desalination with a water-efficient method of cultivation.
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2. THE SEAWATER GREENHOUSE IN TENERIFE
The first prototype of the Seawater Greenhouse was constructed near Granadilla, in the
south of Tenerife, in 1994. The project was funded by the European Community and
involved a number of European research centres6. This was partly the reason for choosing
Tenerife as a site, since the island is in Spanish territory. The prototype was used to
cultivate a variety of crops including tomato, spinach, dwarf pea, pepper, artichokes,
French beans and lettuces. Some of these, such as lettuces, are quite salt-intolerant but
were nevertheless cultivated very successfully despite being within about 60 m of the sea.
The main structure of the greenhouse covered an area of 360 m2 which became
completely planted with crops. Some areas around the Greenhouse were also irrigated
allowing indigenous vegetation to be re-established on this very arid and windswept
coast.
Figure 1: The Seawater Greenhouse in Tenerife, 1994.
The Greenhouse faced into the prevailing wind that it collected through its front wall, this
being a porous structure continuously wetted with seawater. The result of the air coming
into contact with this large moist surface was a substantial humidifying and cooling
effect. Figure 2 compares internal and external temperatures over a period of a week in
the summer of 1995. Corresponding internal and external humidities are shown in
Figure 3.
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The pressure of the wind was sufficient to drive the air through two further elements: a
second wetted wall at the back of the greenhouse and finally through a tube-and-fin type
condenser. This condenser was fed with cold seawater causing freshwater to condense on
its surface. Water production was typically at the rate of 1.5 m3/day.
The water was of excellent quality, generally containing less than 50 ppm total dissolved
solids (TDS). It was used to irrigate the plants via a drip irrigation system and water
usage ranged from 0.6 to 1.2 l/m2/day. This is several times lower than traditional
outdoor cultivation which typically consumes 7 to 8 l/m2/day.
At a number of locations around the world, cold sea water is available where coastlines
plunge deeply into the sea or where cold upwellings occur. Tenerife is an example, and
the condenser of the Greenhouse was suitable for receiving cold water from an offshore
pipe. The design could accommodate up to six such condensers. The economies of scale
of offshore pipes mean that they become viable for large projects. This is because the
flow of seawater that can be collected through a pipe, for a given differential head, rises
sharply as the diameter of the pipe is increased. Thus a pipe of, say 14” diameter can
carry about 4 times as much water as one of 10” diameter, whereas the difference in
diameter does not make a huge difference to the cost, which is more associated with the
laying of the pipe rather than the material itself. For pipes of diameter below about 10”,
warming of the water before it reaches the shore can become an issue.
A finding of the project was that the offshore pipe idea would become viable for projects
covering more than 1 hectare (10 000 m2) in Tenerife and similar locations. This was
beyond the scope of the project, therefore for the purpose of the prototype the cold water
was supplied from a commercial heat pump instead.
Water is an effective absorber of the infrared part of the solar spectrum, which is not
photosynthetically active. The roof of the Tenerife Greenhouse was made of a double
layer of fibreglass sheet thus providing a cavity into which seawater was sprayed. The
system, although effective as optical filter, was troublesome in terms of leaks and had to
be abandoned.
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Figure 2: External ( ) and internal () temperatures measured for the Seawater
Greenhouse in Tenerife, 1995. The day numbers are counted from the beginning of the
year.
Figure 3: External () and internal () humidities measured for the Seawater Greenhouse
in Tenerife, 1995.
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2. THE SEAWATER GREENHOUSE IN THE UAE AND OMAN
Following the Tenerife experiments, the drive became towards a low-cost design that did
not depend on any special oceanographic conditions. It was desirable to be able to
demonstrate the design on a small scale if necessary, as the concept was still somewhat
experimental and large projects might therefore be difficult to fund. These considerations
led us towards the concept of using cold water collected from the front wall of the
Greenhouse to feed into the condenser at the back, in place of cold water piped from the
sea.
The concept was demonstrated in the Seawater Greenhouse constructed in December
2000 at Alaryam in the United Arab Emirates (UAE), as a result of a collaboration with
the Emirates Centre for Strategic Studies and Research (ECSSR). This Greenhouse,
covering 864 m2, is still in operation producing crops year round.
At the start of operation, water yield from the Greenhouse was somewhat lower than
expected. However, a number of improvements were implemented such that water
production approached 1 m3 per day, sufficient to meet the irrigation demand of the
crops. The main change was to add an array of tubes providing solar heating to the
seawater being fed into the second evaporator wall of the Greenhouse.
As in the Tenerife Greenhouse, the product water was of excellent quality with TDS of
9.75 ppm.
Figure 4: Tomato crop growing inside the Seawater Greenhouse in the United Arab
Emirates, June 2001.
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A selective light filter made of commercial polyethelene film was used to shade the
crops, in place of the wetted roof design in Tenerife. However, whereas this film was a
fraction of the cost of a conventional thermal screen, it was found to be less than ideal in
terms of temperature control.
As a result of the work in the UAE, we are now planning a further Greenhouse, to be
constructed as part of a joint project with the Sultan Qaboos University in Oman. The
essential design concept of the Greenhouse is shown in the diagram of Figure 5.
The basic configuration is similar to the UAE. There are two main seawater circuits, one
comprising the first evaporator (at the front wall) and the condenser, the other comprising
the second evaporator (at the back wall) and the tube array used for solar heating. This
tube array is now to be integrated into the greenhouse roof, thereby combining the
functions of providing shade and boosting freshwater production.
A second development in this Greenhouse is the introduction of the all-plastic
Watermaker condenser which is of lower cost and yields more water per kJ of heat
transferred, compared to the previous tube-and-fin condenser made of cupronickel and
aluminium.
Figure 5: Schematic of Greenhouse as developed for Oman. The heavy arrows indicate
flow of seawater.
Crops
Net radiation R
Mass flow of air m
&
h0h1 h2h3h4
Radiation RT
First evaporator Second evaporator
Condenser
Tube array
αm
&
(1-α) m
&
Growing area
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3. MATHEMATICAL MODELLING
A number of theoretical models have been developed and validated alongside the design
and operation of these Greenhouses. The models have become invaluable for predicting
the effects of varying design parameters and are used as a design tool.
The models are useful for drawing some conclusions about the system both generally and
in the case of the Middle East.
3.1 Wind modelling
The Seawater Greenhouse interacts with the wind and the Tenerife prototype
demonstrated that ventilation and cooling could be achieved by wind alone, without using
any fans to move the air.
The Tenerife climate is windier than Middle Eastern climates. In addition, the very high
temperatures in the Middle East make overheating more of a risk in the case of low
windspeeds. For these reasons we use fan ventilation in Middle Eastern applications.
On the other hand, the new Watermaker condenser has a lower air resistance than the
previous design and this introduces the possibility of using wind-driven ventilation at
least some of the time at sites where conditions are suitable. The advantage would be
power saving in the operation of the fans. For reference, the current design uses two fans
each having a nominal rating of 0.75 kW. These are used with a speed controller such
that actual power consumption varies from 0.25 to 1.4 kW in total.
To assess the availability of wind at any particular site, we need to understand the effect
of wind direction on ventilation. The aerodynamics of the greenhouse have been studied
with the help of wind tunnels, computational fluid-dynamic (CFD) analysis and
measurements on the real greenhouse.
Figure 6 shows the flow field around the Tenerife Greenhouse as obtained from 3-
dimensional CFD analysis. The model was used to vary the angle of attack θ of the wind.
The results, in terms of the wind-angle acceptance function f(θ), are shown in Figure 7.
The wind-angle acceptance function is defined as the ratio of the air flow, for a given
angle θ, divided by the airflow resulting from wind approaching the Greenhouse
perpendicularly (θ = 0).
Airflow = constant × ambient wind velocity × f(θ) [2]
It turns from the CFD that f(θ) can be approximated well by a cosine function, as
illustrated by the polar graph of Figure 7. Verification of this against measurements from
the Tenerife Greenhouse is shown in Figure 8.
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Figure 6: Visualisation of the flow field around the Tenerife Seawater Greenhouse,
obtained using the CFD program Flovent.
Figure 7: Polar plot of the wind-angle acceptance function f(θ): from CFD modelling,
cosine model f(θ)=cos(θ).
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0
1
177 178 179 180 181 182 183 184
Day number
Air speed m/s
Figure 8: Verification of the cosine model (–) against measurements of airspeed at the
condenser of the Tenerife Greenhouse().
Based on the cosine model, we can compare the wind availability of different sites in the
Middle East based on time series of wind speed and direction data taken from
meteorological stations7. At each moment in time, we calculate the component of wind in
the prevailing direction for the given site. Only daytime values are used, as the
Greenhouse requires only minimal ventilation at night. This yields a time series of
effective wind speed.
To summarise these values we then determine the median value (see Table 1). Thus, a
median value of 4 m/s would suggest that, if we designed the Greenhouse such that a 4
m/s wind were sufficient to provide wind-driven ventilation, then fans would only be
needed for the remaining half of the time. A 50% power saving would then be achieved.
Table 1: Median effective wind speeds at various Middle Eastern locations.
Tenerife is included for reference. The prevailing direction, measured in degrees
from North, is the angle in which the Greenhouse should be pointed for maximum
ventilation. Where a range is shown, this indicates that the median effective wind
speed is insensitive to the direction within this range.
Location Prevailing direction
deg. from N
Median effective
speed m/s
Tenerife 90 6.3
Sharm-el-Sheikh 20 4.8
Aqaba 15 4.0
Jeddah 290 – 320 4.6
Salalah 190 3.6
Qarn Alam 180 2
Muscat 40 3.0
Abu Dhabi 330 4.4
Kuwait 310 – 360 3.1
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From Table 2 it can be seen that, whereas all Middle Eastern locations have less wind
than Tenerife, the potential for wind ventilation at certain locations such as Aqaba,
Sharm-el-Sheikh, Abu Dhabi and Jeddah is significant. Of course, these data are taken
from meteorological stations, mainly at airports. Local wind conditions vary due to the
influence of topographical features.
The Seawater Greenhouse requires a supply of seawater and discharges seawater into the
sea. It is therefore intended for use near the coast. On the other hand, oil wells often
dispose of large quantities of production water, which can be seawater piped in from the
coast. This introduces the possibility of building Greenhouses inland. The Greenhouse
could make use of the production water either before or after it has been used in the oil
well. As an example of inland operation, we have analysed wind data available for Qarn
Alam (Oman). It can be seen from Table 2 that the potential for wind ventilation is rather
low in this case. This is because, unlike at the other locations, which are coastal, the wind
lacks directionality. Ventilation by fans alone would probably be preferred in this case.
3.2 Simplified thermodynamic modelling
The large number of processes occurring simultaneously in the Seawater Greenhouse
means that simple analytical models are not usually accurate and that computer models
have to be used. On the other hand, simple models are useful in exposing the relations
among the main parameters. They can also give some indications of performance under a
set of idealised assumptions, enabling the real performance to be compared against the
ideal. Areas for improvement can then be identified.
The basic method of analysis is heat and mass balance. Under approximately steady-state
conditions, it is possible to write a heat balance equation for each stage of the process
shown in Figure 5. The symbols in the following equations are defined in Table 2.
Regarding the two seawater circuits shown in Figure 5, some bleed off and top up is
needed otherwise the concentration of the seawater would keep on increasing. However,
these flows are much smaller than the circulating flows and are therefore ignored in the
heat balance calculations. The first evaporator is in the same circuit as the condenser, so
the heat added at this stage equals that removed by the condenser:
Qhhm &
&=)( 01 [3]
Between the inlet and outlet of the growing area of the Greenhouse, the heat added
corresponds to the net radiation received over the plan area, multiplied by the fraction of
light transmitted through the tube array:
RAThhm =)( 12
& [4]
The first evaporator must be capable of producing sufficient water at the wet-bulb
temperature to feed the condenser. For this to be the case, there must more air flowing
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through the evaporator than through the condenser. This is achieved by allowing a
fraction α (typically 0.5>α>0.3) of the air that leaving the growing area to bypass the
condenser. Therefore, the mass flow rate is reduced to the fraction (1-α) at the second
evaporator. The heat addition corresponds to the radiation captured by the tube array:
)1()()1( 23 TRAhhm =&
α
[5]
The rate of heat removal in the condenser is:
Qhhm &
&=)()1( 43
α
[6]
Let us introduce the expression for the effectiveness of the condenser, based on the
enthalpy drop on the air side:
03
43 hh hh
=
ε
[7]
The maximum possible effectiveness of unity would correspond to the situation in which
the air is cooled to the temperature of the water entering the condenser i.e. the ambient
wet-bulb temperature. The enthalpy of the air, which is saturated at this point, would then
be the same as the ambient enthalpy h0. Note that it is more usual to define effectiveness
in terms of the corresponding temperatures, but the definition in terms of enthalpy is
convenient here and for small temperature ranges there is little difference between the
two definitions.
Two simplifying assumptions are now introduced:
1. On leaving each of the evaporators, the air is saturated with water vapour.
Although in practice 100% humidity is never reached, humidities above 95%
can occur with a correctly designed evaporator. This is seen in Figure 4.
2. The amount of water contained in the saturated air is approximated by a linear
function of enthalpy for the range of interest.
We wish to determine the conditions for the Greenhouse to be self-watering. The
estimation of water use by plants using the Penman equation is complicated by the need
to determine the constants in equation [1]. On the other hand, a simple approach is to say
that the amount of water evaporated in the growing area cannot be greater than that
required to result in the exit air (position 2) being saturated. This should give a
conservative estimate of the amount of watering required.
Now, on the basis of assumption 2 above, the amounts of fresh water condensed in the
condenser and evaporated in the planting area can be written purely in terms of mass
flows multiplied by enthalpy changes. This leads to the following condition for the
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amount condensed to be greater than the amount evaporated, i.e. for self-sufficiency in
water to be achieved:
)())(1( 1243 hhmhhm
&&
α
[8]
The enthalpy terms can be eliminated from the above system of equations. The inequality
[8] then leads to the following result for the required effectiveness of the condenser:
α
ε
2)/1(1
1
+
T [9]
The inequality shows that, as the fraction T of transmitted light is increased, a more
effective condenser will be needed to produce enough water for the plants. This can be
explained by the facts that (i) the greater radiation falling on the plants will lead to greater
transpiration and (ii) the corresponding decrease in the amount of solar heat collected by
the tube array will lower the rate of water production.
The inequality [9] is represented in Figure 9. It can be seen that, for a value of T of 0.5,
the condenser effectiveness ε should be at least 0.45.
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0.4 0.5 0.6 0.7 0.8 0.9 1
Fraction of radiation transmitted T
Condenser effectiveness ε
Figure 9: Required minimum effectiveness ε of the condenser to achieve self-watering for
a given value of transmittance T of radiation of the tube array. Based on the simplified
thermodynamic model, relation [9]. The value of α is fixed at 0.4.
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Whereas increasing the size of the condenser would increase water production further, it
is important to note that there could be a substantial cost penalty. For example, a
condenser of ε=0.7 will tend to be about twice the physical size compared to one of
ε=0.45.
Table 2: Symbols used in the simplified thermodynamic model, equations [3] to
[9].
Symbol Units Meaning
A m2 plan area of Greenhouse
h0 kJ/kg specific enthalpy of ambient air
h1 kJ/kg specific enthalpy of air leaving first evaporator
h2 kJ/kg specific enthalpy of air leaving growing area
h3 kJ/kg specific enthalpy of air leaving second evaporator
h4 kJ/kg specific enthalpy of air leaving condenser
m
& kg/s mass flow of air entering Greenhouse
Q
& kW rate of heat removal by condenser
R kW/m2 net solar radiation flux
T fraction of radiation transmitted to growing area
α fraction of air bypassing the condenser
ε effectiveness of the condenser
3.3 Detailed thermodynamic modelling
The simplified model provides the basis for the more detailed model that has been
developed into a bespoke program called Waterworks, based on the MATLAB
programming environment8.
The detailed model includes a number of experimental corrections and correlations
omitted from the simplified model. For example:
Real performance of the evaporators, without assuming 100% saturation.
Detailed model of the condenser, based on its size, with scaling from laboratory
experiments with small-scale models.
Heat loss or gain through the walls of the greenhouse.
Simulation over a series of time steps, with varying inputs of ambient
temperature, humidity, sunshine and wind.
The interface to the Waterworks program presents the user with a world map, allowing
him or her to zoom into a region of interest and then select a potential Greenhouse site.
Figure 10 shows the interface with the Middle East region selected.
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Figure 10: Interface to Waterworks program.
The eight sites shown were selected to reflect a certain geographic spread and according
to the data available, which was generated from references 7 and 9. Year-round
simulations were performed to give values of annual water production for a Greenhouse
of nominal area A=500 m2, between the first and the second evaporator. The results are
shown in Table 3 together with average ambient humidities. A variation in water
production of about 25% is predicted among the sites. This correlates somewhat with the
relative humidities. In Oman, a slightly larger water production can thus be expected at
the inland site of Qarn Alam where conditions tend to be less humid than in Muscat (see
comments in section 3.1 about the use of oil-well production water at inland sites). The
relatively high water production at Kuwait and Sharm-el-Sheikh is due to drier offshore
winds compared to the humid onshore winds at some of the other sites.
A lower relative humidity results in a lower wet-bulb temperature. This means that colder
water is fed into the condenser. Meanwhile, the air is also entering the Greenhouse at a
cooler temperature. This favours the absorption of heat through the walls of the
Greenhouse. Further, it is possible to choose a slightly lower ventilation rate without
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encountering overheating in the greenhouse. Consequently, a larger temperature
differential tends to occur at the condenser which favours heat transfer and fresh water
output.
Table 3: Annual water productions from a nominal Seawater Greenhouse (A=500 m2),
based on simulations.
Location Water production
m3/year
Average ambient
relative humidity %
Sharm-el-Sheikh 325 41
Aqaba 275 59
Jeddah 265 63
Salalah 230 66
Qarn Alam 310 45
Muscat 260 59
Abu Dhabi 270 58
Kuwait 330 40
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4. CONCLUSIONS
The principle of operation of the Seawater Greenhouse can be understood from the
standard theory of crop transpiration based on the Penman equation. Effectively, the
Greenhouse creates an environment in which watering requirements are substantially
reduced compared to cultivation in the ambient environment. As a consequence, the value
of the freshwater produced by the Greenhouse is enhanced, such that one cubic metre of
product water substitutes for several cubic metres of water as used in more conventional
cultivation.
The Seawater Greenhouse has evolved through the constructions in Tenerife and the
United Arab Emirates. Aspects of these designs are being incorporated into a new project
underway in Oman. The practical developments have been paralleled by mathematical
models, which give projections of performance under various design options and site
conditions.
The Middle East is an area of prime interest, due to the suitable climate conditions and
the need for innovative solutions to the problems of water and agriculture.
The Greenhouse in Tenerife was ventilated and cooled by the wind without the use of any
fans. Although fans are likely to be used in Middle Eastern projects, providing ventilation
for at least some of the time, this study shows that certain sites in the Middle East have
significant wind potential. The median effective wind speed at Sharm-el-Sheikh, for
example, is 4.8 m/s compared to 6.3 m/s at Tenerife
A simplified thermodynamic analysis emphasises the importance of shading in the
Greenhouse and correct sizing of the condenser that has been an expensive element of the
Greenhouse. A low-cost condenser is currently under development.
The more detailed thermodynamic analysis, carried out by computer simulation, shows
that a variation of about 25% may be expected among the sites studied. Larger rates of
freshwater production tend to occur at sites with lower relative humidities. The use of the
Seawater Greenhouse at inlands sites, where humidities are low and seawater may be
available in the form of oil-well production water, may be an interesting possibility in
spite of the fact that the Greenhouse is intended primarily for use on arid coastlines.
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CT-920508. Prepared by Light Works Ltd.
7 NOAA: Integrated Surface Hourly Observations, 2002.
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Renewable Energy Sources for Water Production, Santorini 1996, pages 163-166.
9 Meteonorm v4.0, Meteotest, Bern, Switzerland.
... Furthermore, shading [10][11] or selective shading mechanisms, where only photosynthetically active radiations can penetrate inside the greenhouse, can effectively reduce the water requirement of the plants [12]. Currently, the amount of water allocated for food production is nearly 11.8 cubic meter per day (m 3 /day) per person which can decrease by 80% using greenhouses [13]. ...
... The humidified air is then condensed to produce freshwater. Ref. [12,[51][52] have reported four of such greenhouses, built in Spain, UAE, Oman, and Australia. The first greenhouse, built in Spain [12], has a WPR of 0.0042 m 3 /day/m 2 which only uses the wind energy for the cooling system. ...
... Ref. [12,[51][52] have reported four of such greenhouses, built in Spain, UAE, Oman, and Australia. The first greenhouse, built in Spain [12], has a WPR of 0.0042 m 3 /day/m 2 which only uses the wind energy for the cooling system. Its roof is made up of two glass layers, so the shading mechanism is achieved by spraying the SFW between the two glass layers of the roof, but it can cause leakage problems [12,52]. ...
Article
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This paper is motivated by the crisis of freshwater in remote areas around the world and responds to the growing need for sustainable food production in arid lands. It focuses on utilizing solar energy to yield freshwater from the sea or brackish water with less environmental impacts, for greenhouses, which can produce sustainable food all over the year. The integration of various solar‐driven desalinations such as solar still, humidification‐dehumidification, reverse osmosis, electrodialysis, and multieffect and multistage flash with greenhouses are evaluated, for better sustainability towards greenization. The paper first discusses the specifications of solar‐driven desalinations and compares their advantages and limitations. Then, different types of greenhouses are introduced, and their total water requirement is discussed based on their locations, crop type, greenhouse technology, irrigation type, and environmental conditions, as well as their cooling and heating strategies. Later, the existing integration of solar‐driven desalinations with greenhouses are reviewed, and their advantages and limitations are deliberated. Finally, the paper discusses the criteria to be considered when selecting solar‐driven desalinations for greenhouses and presents a detailed comparison between the water production rate and cost as well as the energy consumption of these systems. In the end, the most appropriate combinations of solar‐driven desalinations with greenhouses are recommended based on their water requirement and production cost.
... The SWGH is a promising technology for agricultural production in places experiencing two major constraints, namely, extremely high air temperatures and a chronic shortage in freshwater resources (Stanger, 1985;Norman et al., 1997;Norman et al., 1998;Al-Ajmi and Rahman, 2001). From an agricultural viewpoint, the SWGH technology offers a sustainable solution to overcome both constraints provided that seawater, brackish groundwater, or oil-field water by-product is available (Bailey and Raoueche, 1998;Davies et al., 2004;Bourouni et al., 2011). ...
... Seawater, pumped from a well located 50 m from the sea, is used to moisten the evaporative cooling systems of the SWGH. Because the amount of condensation is directly influenced by the temperature difference between the condenser and cross-flowing moist air (Davies et al., 2004;Eslamimanesh and Hatamipour, 2009;Alkhalidi et al., 2010;Mahmoudi et al., 2010;Alkhalidi et al., 2013), the second evaporator is connected to a solar water heater to heat up and saturate the air stream before reaching the condenser. Additionally, the temperature difference is further amplified using the evaporatively cooled seawater flowing from the first evaporator as a coolant in the condenser (Al-Ismaili and Jayasuriya, 2016). ...
... Many researchers have emphasized that the overall effectiveness of the SWGH greatly depends on the effectiveness of the condenser (Davies et al., 2004;Dawoud et al., 2006;Zurigat et al., 2008;Alkhalidi et al., 2010;Ghaffour et al., 2015). ...
Article
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Seawater greenhouses (SWGH) utilize seawater or saline/brackish groundwater for cooling the microclimate and providing freshwater for irrigation through a humidification–dehumidification desalination process. The overall effectiveness of the SWGH greatly depends on the effectiveness of its condenser. The present study provides a good review on the available simulation models of the SWGH condenser and proposes a multiple linear regression model to predict the dehumidification rate of the condenser in the Oman SWGH. Four climatic and operational input variables were considered, including solar irradiance, inlet moist air temperature, inlet humidity ratio, and inlet air mass flow rate. The results showed that the model accurately predicts the dehumidification rate when compared against experimental values [a mean predictive error (PE) = −0.127 kg/h and root mean square error (RMSE) = 4.691 kg/h]. The model also outperformed some other model in several accuracy indicators such as PE, mean absolute predictive error, RMSE, R², index of agreement and fractional variance.
... The numerical simulation technique has not received proper attention by researchers. In one study, CFD approach was used to simulate the wind flow surrounding the Tenerife SWGH and to find the effect of wind direction on freshwater production (Davies et al., 2004b). From this study, the best orientation for the SWGH was identified. ...
Article
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In arid climates, extremely high temperatures in the summer and the chronic water scarcity put a firm barrier against agricultural development and sustainability. The SWGH technology is an engineering phenomenon that came to overcome both the constraints particularly in areas where seawater is accessible and/or brackish groundwater is available. It is a greenhouse used to cultivate crops and at the same time produce its own freshwater need. This study aimed to highlight the models that were carried out to simulate the SWGH as a whole or only the dehumidification rate of the SWGH condenser. Four types of simulation models were identified, namely, analytical, numerical, empirical and artificial neural network simulations. The factors affecting the dehumidification rate were also discussed taking into consideration the results from the simulation models.
... The values of each parameter in the decomposition table indicates areas where a certain stills can be improved to make it competitive and to reach the efficiency frontier. This integrated fuzzy AHP DEA approach can be used in other desalination system to find the relative efficiency of desalination processes like multi-effect flash distillation (Baig et al., 2011;Choi, 2016;Elzahaby et al., 2016), membrane distillation (Nakoa et al., 2015;Orfi et al., 2016;Wang, 2011;Zhang et al., 2015), FO & RO(forward and reverse osmosis) (Altaee and Hilal, 2015;Delgado-Torres and García-Rodríguez, 2010;Khanzada et al., 2017;Mokheimer et al., 2013;Mudgal and Davies, 2016;Qasim et al., 2015), ion exchange (AlMarzooqi et al., 2014;Hilal et al., 2015aHilal et al., , 2015b, seawater greenhouse techniques (Davies et al., 2004Davies and Paton, 2005;Yetilmezsoy and Abdul-Wahab, 2014), etc. by selecting the techno-economic input/ output parameters. This integrated approach can also be employed in other applications like renewable energy sectors (solar, wind, tidal, biomass, etc.) and power generation sectors (conventional and non-conventional power plants). ...
Article
Desalination using solar stills is an ancient economic method for water desalination. Over the years, research and development in the area of solar still has resulted in increased distillate yield by means of integration of PCM (phase change material), photo-voltaic thermal (PVT), etc with the still. Nano-PCM is an upcoming technology which modifies the thermal performance of PCM. The aim of this research is to analyze the efficiency of 20 solar stills including nano-PCM based solar stills considering various input and output criteria using integrated fuzzy analytical hierarchy process (AHP) and data envelopment analysis (DEA). The efficiency derived here is relative with regard to the parameters and stills considered in this study. The result infers that, even though the productivity of stepped solar still with sun tracking system was high, but when techno-economic aspects were considered it is not among the top solar stills. The analysis indicated pyramid type solar still, single slope solar still with PVT, solar still with NPCM (paraffin + copper oxide), solar still with NPCM (paraffin + titanium dioxide) and solar still with PCM (paraffin) occupies the top five positions with relative efficiency of 100, 100, 88.47, 88.46 and 76.93% respectively.
... As the seawater becomes more concentrated, giving increased concentration of calcium and alkalinity, the S & DSI will lower to about 6.8 suggesting a risk of calcium carbonate precipitation in the cooling pads. Nonetheless, earlier Seawater Greenhouses have been operated [69] leading to the practical observation that precipitation tends to occur mainly in the sump tanks of the evaporative cooling pads, rather than on the pads themselves. This may be attributable to the relatively stagnant conditions within the tanks relative to the continuously flowing conditions in the evaporator pads, as indeed the S & DSI prediction applies to equilibrium conditions and certain time lag in the precipitation of scaling species can be expected. ...
Article
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Brine disposal is a major challenge facing the desalination industry. Discharged brines pollute the oceans and aquifers. Here is it proposed to reduce the volume of brines by means of evaporative coolers in seawater greenhouses, thus enabling the cultivation of high-value crops and production of sea salt. Unlike in typical greenhouses, only natural wind is used for ventilation, without electric fans. We present a model to predict the water evaporation, salt production, internal temperature and humidity according to ambient conditions. Predictions are presented for three case studies: (a) the Horn of Africa (Berbera) where a seawater desalination plant will be coupled to salt production; (b) Iran (Ahwaz) for management of hypersaline water from the Gotvand dam; (c) Gujarat (Ahmedabad) where natural seawater is fed to the cooling process, enhancing salt production in solar salt works. Water evaporation per face area of evaporator pad is predicted in the range 33 to 83 m 3 /m 2 ·yr, and salt production up to 5.8 tonnes/m 2 ·yr. Temperature is lowest close to the evaporator pad, increasing downwind, such that the cooling effect mostly dissipates within 15 m of the cooling pad. Depending on location, peak temperatures reduce by 8-16 °C at the hottest time of year.
... The SWGH is similar to any ordinary greenhouse used for crop cultivation but solar desalination mechanisms are integrated in the same structure for the purpose of providing freshwater necessary for irrigation. To date, four SWGHs have been constructed; first was built in Tenerife, Spain in 1994 (Goosen et al., 2000), second was built in Abu Dhabi, UAE in 2000 (Davies et al., 2004), third was built in Muscat, Oman in 2004 (Al-Ismaili, 2009) and fourth was built in South Australia in 2012 (Aljazeera English, 2012). The SWGH in Oman is shown in Figure 1. ...
... The SWGH is similar to any ordinary greenhouse used for crop cultivation but solar desalination mechanisms are integrated in the same structure for the purpose of providing freshwater necessary for irrigation. To date, four SWGHs have been constructed; first was built in Tenerife, Spain in 1994 (Goosen et al., 2000), second was built in Abu Dhabi, UAE in 2000 (Davies et al., 2004), third was built in Muscat, Oman in 2004 (Al-Ismaili, 2009) and fourth was built in South Australia in 2012 (Aljazeera English, 2012). The SWGH in Oman is shown in Figure 1. ...
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Considering the drive to develop innovative and sustainable food production systems, this study analyzes a novel renewable energy powered self-sustainable greenhouse. The system is designed based on the principles of decentralization within food production systems and sustainability to improve the food security of a region. The greenhouse unit utilizes the humidification-dehumidification phenomena using saline groundwater to provide optimum growing conditions to the plants throughout the year thus making self-sustaining agriculture possible in arid climates. The subsystems integrated in the proposed system include a greenhouse unit, parabolic trough collector, organic Rankine cycle, absorption cooling system, and thermal energy storage. A detailed thermodynamic model is developed using the mass, energy, entropy, and exergy balance equations for all the components of the system. A comprehensive parametric study is performed to determine the performance and relationship between different inputs on the outputs of the system. Results illustrate that the proposed system is capable of providing year-round essential requirements for the sustainable greenhouse in an efficient and environmentally friendly manner. The outputs of the system include 17.5–27.3 m³/day produced freshwater, 4.3 MW cooling, 1.03 MW electricity and gained output ratio of about 2.10–3.3 while maintaining optimum temperature and humidity level inside the greenhouse.
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The prediction of freshwater production from the condenser of an agricultural seawater greenhouse is important for designing the greenhouse process. Two models, namely, Artificial Neural Network and multilinear regression (denoted as ANN and RA, respectively), were developed and tested to predict the freshwater production rate considering ambient solar intensity, condenser inlet moist-air temperature, humidity ratio and mass flowrate, and inlet coolant temperature. Statistical analysis indicated that all parameters significantly affected the prediction (p < 0.05). The accuracy of the ANN and RA models was then compared to two models previously developed by Yetilmezsoy and Abdul-Wahab and Al-Ismaili et al. (denoted as Yetilmezsoy model and Al-Ismaili model, respectively). The ANN model showed the best prediction when seven statistical criteria were considered. The Pearson correlations for ANN, RA, Yetilmezsoy, and Al-Ismaili models were observed as 1.00, 0.98, 0.88, and 0.96, respectively, while mean absolute percentage errors (MAPE) were 17.84, 79.72, 63.24, and 80.50%, respectively. Hence it could be recommended to use ANN model for the prediction of freshwater production rate, however other three simple models could also be used with lower accuracy in the cases of unavailability of the ANN model.
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