The Arab region is characterized by arid and semi-arid conditions with very limited renewable water resources. Most of the surface water comes from transboundary streams and most of the groundwater resources are fossil in nature. Water quality degradation and excessive use of pesticides and herbicides in agriculture pose severe environmental and health risks. The underlying research is a joint effort between Cairo University and the Technical University of Berlin to develop technologies and strategies for sustainable pesticide-free agriculture using saline or brackish water. This project builds on a previously implemented project in Spain by the German research partner that introduced the concept of Watergy, which presents an integrated desalination horticulture solar greenhouse. In this current research, the Watergy greenhouse is further developed to meet more arid climate requirements, reduce construction costs, and increase resource utilization efficiency. Several lab-scale experiments and a 100 m2 prototype were built in Egypt to optimize the process and answer research questions. Lessons learned from this project provided guidelines on the development of the most efficient approach of desalination and water management in the system, devised a cost-effective and efficient heat exchanger using low-cost local material, and established the feasibility of the system in the arid climate together with prospects for wider applications. The proposed greenhouse was estimated to be able to save in irrigation water 40% for cherry tomatoes and cucumbers, and 50% for bell peppers. Maximum crop yield can be achieved at extended upper salinity levels using the proposed greenhouse as follows: from 1,000 to 1,700 mg/L for cherry tomatoes; from 960 to 2,000 mg/L for bell peppers; and from 1,600 to 2,700 mg/L for cucumbers.

INTRODUCTION AND BACKGROUND

The existing water scarcity and the fading of fresh groundwater resources have resulted in significant global shortages in food supply and consequent food security issues. Global phenomena like growing drought and over-exploitation of groundwater stocks provokes a fundamental change of water management including a more efficient use of limited freshwater resources and as increasingly important topic, the use of unconventional water resources like seawater and brackish water.

With a rapidly increasing population and a constant water supply from River Nile water, Egypt is approaching severe water stress which is threatening the country's development. Climate change and anticipated seawater rise add to the stress factors. Water quality degradation and excessive use of pesticides and herbicides in agriculture pose severe environmental and health risks, and limit export potential of the Egyptian produce (Abdel-shafy et al. 2010; UNFCCC 2010).

A vision is needed for Egypt which turns apparent disadvantages into advantages and regard wastes as resources. Egypt is blessed with more than 2,500 km of shorelines which present a potential for development based on seawater desalination. Brackish groundwater resources of salinities in the range of 2,000–10,000 ppm are widely available in the western desert areas and Sinai (Abo Soliman & Halim 2012). The extreme arid climate of Egypt, while limiting water supply from rain to practically zero south of Cairo, can be regarded as a blessing, as Egypt is one of the countries with the highest levels of solar radiation in the world, which could be utilized for the development of conjunctive energy production and desalination (the annual solar radiation energy falling on the High Aswan Dam Lake in Egypt is approximately equivalent to the oil reserves in the Middle East).

The direct irrigation with brackish water poses limitations on the kind of crops to be cultivated, reduces productivity, and ultimately leads to soil salinization and desertification. One approach to tackle the described problems is the cultivation in greenhouses. Horticulture in existing greenhouses enhances water efficiency in relation to produced crops by at least 300% compared to the open field flood irrigation (MWRI 2005). A relatively new approach is the recycling of water and desalination within greenhouses by combined evaporation and condensation. Horticultural production in closed environments will allow the production of more and better food by the practice of CO2 accumulation and pesticide-free plant production while being able to reuse the irrigation water as fresh condensed water and by integrating saline water as an input (Kimball 1982).

This manuscript provides the first cornerstone for the development of intelligent low-tech principles of greenhouse integrated horticultural production and integrated water desalination for arid climate, including condensation of water from humid greenhouse air as a new source of freshwater using the process of combined evaporation and condensation. The main aim of the project is the establishment of a new technology that creates income from increased (quantitative and qualitative) horticultural production due to CO2 supply and pesticide-free production methods in a closed environment, while producing freshwater from brackish water or seawater.

The concept, known in the literature as Watergy (Buchholz 2000; Buchholz & Zaragoza 2004; Buchholz et al. 2005) has been tested in North African and other Mediterranean countries through an EU funded project and showed promising results. The current research advances the development of the Watergy in the following aspects: (1) using brackish water in the greenhouse irrigation system; (2) utilizing very high salinity desiccant in the greenhouse heating/cooling system; (3) enhancing the economic feasibility of the system by researching the use of local materials and innovative low-cost heat exchange units; and (4) developing of cooling methods for closed greenhouses, that will allow the use under climatic conditions of Egypt and the Middle East.

METHODOLOGY

The main aim of the project is the establishment of a new technology, that creates income from increased and pesticide-free horticultural production due to CO2 supply in a closed environment and that allows the integration of saline water either in irrigation or in the cooling process depending on the salinity of the given sources.

The project work started by a planning and prototype design phase. Research aspects included design options, material selection, crop varieties, and operational and cost-reduction aspects. This was followed by a prototype construction phase. Blueprints were developed with the partnership of Cairo University and Technical University of Berlin (TU-Berlin). Teams from the two universities participated in the start-up and commissioning of the prototype greenhouse (100 m2 greenhouse constructed in the premises of Cairo University). Upon successful commissioning of the prototype, an empirical experimentation phase is underway. During this phase, the design of heat exchangers and desalination processes are being optimized, in addition to testing the horticulture of specific crops in the greenhouses. Results are being interpreted to establish the economic feasibility of the technology, the range of brackish water to be used, and the possible irrigation water-saving potential.

GREENHOUSE CONSTRUCTION AND OPERATION

The main idea behind the project is to use the energy conservation theory to preserve the temperature and humidity within the greenhouse within the adequate limits for crop growth even during the summer time (Zaragoza et al. 2004; Buchholz et al. 2006).

A key component of a closed greenhouse is the cooling mechanism. This manuscript proposes a cooling technique using evaporative cooling using desiccant solution. Commercial desiccants are expensive, making the technology infeasible. However, bittern solutions that remain after evaporation and crystallization of sodium chloride (table salt) from seawater can have high hygroscopic capacity similar to commercial desiccants (Davies & Knowles 2006; Lychnos et al. 2010).

A thorough search for a proper desiccant in the by-products of main industrial facilities within Egypt has resulted in finding a residue of the salt production industry in El-Max, Alexandria, Egypt. This residue (bittern solution) consists mainly of MgCl2 and MgSO4. However, MgCl2 is the main constituent that provides the required hygroscopic properties (Lychnos et al. 2010) Testing the water quality of this industrial by-product has revealed that a sufficient concentration of MgCL2 (>70%) with other salts (MgSO4, NaCl, and minor impurities). This composition was proven in the literature to act as pure MgCl2 in terms of its hygroscopic capacity (Lychnos et al. 2010). MgCl2 solution of mass concentration 31% would result in an equilibrium relative humidity (ERH) of 50%, ERH curve for MgCl2 (Davies & Knowles 2006) was used to design the absorption column in this project. Thus, this industrial by-product (in liquid form) is a suitable desiccant that can absorb greenhouse humidity during day time (in a wet absorption column), and hence promote evaporative cooling in the closed greenhouse.

Figures 13 depict a three-dimensional (3D) view, top view, and side view of the greenhouse, respectively, where (1) desiccant collection sump, (2) desiccant storage, (3) absorber, and (4) phase change material (PCM) storage and heat exchanger.

Figure 1

A 3D diagram of the greenhouse design.

Figure 1

A 3D diagram of the greenhouse design.

Figure 2

Top view of the greenhouse ground including the design measurements.

Figure 2

Top view of the greenhouse ground including the design measurements.

Figure 3

Front view of the greenhouse showing the desiccant and air cycles.

Figure 3

Front view of the greenhouse showing the desiccant and air cycles.

Greenhouse operation relies on two distinct cycles: the day and night cycles of the desiccant and the greenhouse internal air. Figure 4 shows a simple diagram of the desiccant flow and the air flow including the stages though which each will pass. The operation modes during the day and night cycles are described herein.

Figure 4

A schematic of the desiccant and air cycles.

Figure 4

A schematic of the desiccant and air cycles.

Greenhouse day cycle

  1. As the desiccant comes in contact with the humid air in the absorber tower, the humidity is absorbed by the desiccant, raising its temperature and decreasing its concentration, while the air loses humidity and gets cooled by the originally cool desiccant, and therefore decreases the temperature of the surrounding greenhouse environment.

  2. This now dilute warm desiccant goes through a heat storage area containing a PCM which absorbs the heat from the desiccant. The outlet desiccant enters the absorber once more to the first stage (lower part of the tower).

  3. More concentrated desiccant from the bottom of the underground storage tank is fed to the top of the absorber with a low flow rate to increase the concentration of the circulating desiccant, and the overflow from the collection tank is recycled to the top of the storage tank.

Greenhouse night cycle

  1. The desiccant (now diluted from absorbing humidity during the day cycle) is made to contact the air again, but this time increasing its humidity to the saturation point.

  2. The saturated air cools down at the top of the greenhouse, making the water to condense on the greenhouse cover and collected at the bottom of the sheet to the condensate water tank.

  3. The desiccant goes to the PCM storage to be heated and then is returned to the absorber to complete the cycle.

  4. Diluted desiccant goes to the top of the absorber (the reverse of the day cycle) to give more humidity to the air.

To properly monitor the greenhouse, irrigation water flow rate, condensed water volume, desiccant daily usage are being monitored and stored in a computer log sheet. Real-time data acquisition is being undertaken for the following parameters: temperature probes in a grid setup inside the greenhouse, relative humidity probes, and two CO2 sensors.

RESULTS AND DISCUSSION

The main focus in this paper relates to the desalination capacity of the proposed greenhouse. Other aspects including greenhouse cooling, horticulture diversity, and material selection are described elsewhere.

The greenhouse configuration has been designed according to an energy balance for a period of 24 hours for a typical Egyptian hot day (20th July) based on climatic data of Cairo, Egypt. For the daytime period, a total solar radiative input of 600 kWh was given with a peak radiation of 900 W/m². Outside temperatures were in the range of 27 and 40 ° with a maximum greenhouse temperature of 45 °. The heat input was led to different heat sinks and is distributed out of the system during the 24 hours via heat transfer through the greenhouse cover, heat transfer in and out of the soil and heat transfer through the desiccant into the storage. The latter transfer is mainly characterized as a latent heat flow, with (1) evaporation of the crops, (2) absorption of water vapor into the desiccant, (3) storage of heat in the desiccant, (4) evaporation of water out of the desiccant during the night (desorption), and finally (5) condensation of water on the inner surface of the greenhouse with heat release out of the greenhouse. For the related energy and mass transfer, an air circulation of 3,000 m³/h and a desiccant flow rate of 3 m³/h have been used for the extreme midday situation.

Day-to-day temperature and relative humidity changes are being monitored and recorded in the greenhouse and the surrounding ambient air. Information about solar radiation is extracted from a nearby Ministry of Environment weather station.

For the water production system, the closed greenhouse concept is aimed at establishing an increased rate of relative humidity in the closed system with either (1) lower daily evaporation rates in the range of 2–3 L/m² or (2) equal evaporation rates and related higher water recuperation rates (5 L/d and 4 L condensed water produced). In the first case, the salinity of the irrigation water can be higher due to the lower total daily uptake into the biomass. In the second case, a higher amount of condensed water is available and a part of it can be used for dilution of the irrigation brackish water with consequently much lower salinity.

Initial results from the constructed greenhouse have demonstrated that the condensed water within the greenhouse amounts to 2.5–3.8 L/m2/d. Proper piping for condensate water collection along with the steep slope of the greenhouse bow construction has established the potential for collecting nearly 80% of this condensed water (Figure 3). Thus, the collected freshwater condensate within the greenhouse (salinity ∼= 0) amounted to 2–3 L/m2/d.

In Egypt and the Middle East, vegetable crops like tomatoes, cucumbers, green peppers, peas, and other field vegetables are the main greenhouse crops. However, in recent days, higher value crops like broccoli and lettuce started to be cultivated in controlled greenhouses to achieve high-quality pesticide free organic crops. High brackish water salinity prohibits the cultivation of such crops due to their relative salinity intolerance. A typical new land agricultural field size in Egypt is 50 feddans (∼ 20 hectares (ha)).

To examine the potential applications of the proposed system the following scenarios were modeled if being cultivated in the proposed closed greenhouse.

  • Scenario 1: cultivating cherry tomatoes in a 20 greenhouse setup.

  • Scenario 2: cultivating cucumbers in a 20 greenhouse setup.

  • Scenario 3: cultivating bell peppers in a 20 greenhouse setup.

Geisenberg & Stewart (1986) have demonstrated that tomatoes, peppers, and cucumbers have the following average annual consumptive use under typical growing conditions: 50 m3/ha/d; 40 m3/ha/d; and 50 m3/ha/d, respectively. Table 1 summarizes the results of cultivating cherry tomatoes, bell peppers, and cucumbers in the proposed greenhouse as compared to other typical greenhouses.

Table 1

Salinity tolerance, water saving, and crop yield enhancement in the proposed greenhouse

Cropping pattern (20 ha) Cherry tomatoes Bell peppers Cucumbers 
Water consumption per cropping season (m350 m3/ha/d*20 ha*180 days = 180,000 40 m3/ha/d*20 ha*210 days = 168,000 50 m3/ha/d*20 ha*90 days = 90,000 
Condensed water recovery per cropping season (m32 L/m2/d*20 ha*180 days = 72,000 2 L/m2/d*20 ha*210 days = 84,000 2 L/m2/d*20 ha*90 days = 36,000 
aTarget irrigation water salinity for optimum yield (mg/L) <1,000 <960 <1,600 
bMaximum salinity for brackish water supply – proposed greenhouse system (mg/L) 1,700 2,000 2,700 
cProductivity – proposed greenhouse system (% of maximum yield) 100 100 100 
aProductivity – other typical greenhouses (% of maximum yield) 90 77 77 
% increase in productivity 10 23 23 
% water saving accounting for recovered condensate water 40 50 40 
Cropping pattern (20 ha) Cherry tomatoes Bell peppers Cucumbers 
Water consumption per cropping season (m350 m3/ha/d*20 ha*180 days = 180,000 40 m3/ha/d*20 ha*210 days = 168,000 50 m3/ha/d*20 ha*90 days = 90,000 
Condensed water recovery per cropping season (m32 L/m2/d*20 ha*180 days = 72,000 2 L/m2/d*20 ha*210 days = 84,000 2 L/m2/d*20 ha*90 days = 36,000 
aTarget irrigation water salinity for optimum yield (mg/L) <1,000 <960 <1,600 
bMaximum salinity for brackish water supply – proposed greenhouse system (mg/L) 1,700 2,000 2,700 
cProductivity – proposed greenhouse system (% of maximum yield) 100 100 100 
aProductivity – other typical greenhouses (% of maximum yield) 90 77 77 
% increase in productivity 10 23 23 
% water saving accounting for recovered condensate water 40 50 40 

aValue extracted from Mass (1993).

bValue calculated using ‘mass balance calculations’ of brackish water supply (unknown salinity but known flow rate) and condensate water (known salinity and flow rate) to reach the target optimum yield salinity (known from Mass (1993)).

cThis is because the crops are irrigated with the brackish water mixed with condensate water and thus the salinity levels do not exceed the maximum level required for maximum yield.

The contribution of groundwater to total water supply in Egypt has been very moderate over the years; however, it remains to be an important freshwater resource. It is the sole source of water for people living in the Egyptian deserts. The main groundwater systems in Egypt are Nile aquifer system, Nubian Sandstone aquifer system, the fissured carbonate aquifer, the coastal aquifer, the Moghra aquifer, and the Hardrock aquifer system.

Groundwater also exists in the non-renewable deep aquifers in the Western Desert region. In all other Middle Eastern countries, groundwater is mostly the major, if not the only, source of irrigation. Salinity levels in most of these aquifers exceed 2,000 mg/L and spans up to 10,000 mg/L (Abo Soliman & Halim 2012). Under possible climate change scenarios, sea-level rise will likely increase the salinity of groundwater (UNFCCC 2010). Over-mining of groundwater also increases groundwater salinity and thus it is expected that most of the groundwater resources in the Middle East will increase in salinity levels to exceed 3,000 mg/L in most cases (UNFCCC 2010; Abo Soliman & Halim 2012). Under such circumstances, the proposed desalination greenhouse can provide a cost-effective technology for cultivating crops that are difficult to grow otherwise with increased crop yield and maximum crop per drop. Table 2 presents the calculated benefits of cultivating in the proposed desalination greenhouse in terms of water productivity and crop yield for tomatoes, cucumbers, and bell peppers.

Table 2

Salinity tolerance, water saving, and crop yield enhancement in the proposed greenhouse for a hypothetical 3,000 mg/L irrigation water salinity

Cropping pattern (20 ha) Cherry tomatoes Bell peppers Cucumbers 
Water consumption per cropping season (m350 m3/ha/d*20 ha*180 days = 180,000 40 m3/ha/d*20 ha*210 days = 168,000 50 m3/ha/d*20 ha*90 days = 90,000 
Condensed water recovery per cropping season (m32 L/m2/d*20 ha*180 days = 72,000 2 L/m2/d*20 ha*210 days = 84,000 2 L/m2/d*20 ha*90 days = 36,000 
aTarget irrigation water salinity for optimum yield (mg/L) <1,000 <960 <1,600 
Hypothetical source water irrigation water salinity (mg/L) 3,000 3,000 3,000 
bProposed greenhouse modified salinity (mg/L) 1,800 1,500 1,800 
aProductivity – proposed greenhouse system (% of maximum yield) 92 88 96 
aProductivity – other typical greenhouses (% of maximum yield) 72 55 71 
% increase in productivity 20 33 25 
% water saving 40 50 40 
Cropping pattern (20 ha) Cherry tomatoes Bell peppers Cucumbers 
Water consumption per cropping season (m350 m3/ha/d*20 ha*180 days = 180,000 40 m3/ha/d*20 ha*210 days = 168,000 50 m3/ha/d*20 ha*90 days = 90,000 
Condensed water recovery per cropping season (m32 L/m2/d*20 ha*180 days = 72,000 2 L/m2/d*20 ha*210 days = 84,000 2 L/m2/d*20 ha*90 days = 36,000 
aTarget irrigation water salinity for optimum yield (mg/L) <1,000 <960 <1,600 
Hypothetical source water irrigation water salinity (mg/L) 3,000 3,000 3,000 
bProposed greenhouse modified salinity (mg/L) 1,800 1,500 1,800 
aProductivity – proposed greenhouse system (% of maximum yield) 92 88 96 
aProductivity – other typical greenhouses (% of maximum yield) 72 55 71 
% increase in productivity 20 33 25 
% water saving 40 50 40 

aValue extracted from Mass (1993).

bValue calculated using ‘mass balance calculations’ of brackish water supply (unknown salinity but known flow rate) and condensate water (known salinity and flow rate) to reach the target optimum yield salinity (known from Mass (1993)).

CONCLUSIONS AND FINAL REMARKS

The underlying research is a joint effort between Cairo University and the Technical University of Berlin to develop technologies and strategies for sustainable pesticide-free agriculture using saline or brackish water in an integrated desalination horticulture solar greenhouse. Several lab-scale experiments and a 100 m2 prototype were built in Egypt to optimize the process. Initial results in terms of salinity management have demonstrated the following:

  • Condensed water within the greenhouse is found to be 2.5–3.8 L/m2/d. Proper piping for condensate water collection along with the steep slope of the greenhouse bow construction has established the potential for collecting nearly 80% of this condensed water. Thus, the collected freshwater condensate within the greenhouse (salinity ∼= 0) amounted to 2–3 L/m2/d.

  • Tomatoes, bell peppers, and cucumbers are the crops widely grown in greenhouses in Egypt and the Middle East. The proposed greenhouse was estimated to be able to save in irrigation water 40% for cherry tomatoes and cucumbers and 50% for bell peppers.

  • Maximum crop yield can be achieved at extended upper salinity levels using the proposed greenhouse as follows: from 1,000 to 1,700 mg/L for cherry tomatoes, from 960 to 2,000 mg/L for bell peppers, and from 1,600 to 2,700 mg/L for cucumbers.

  • Under a possible scenario of increasing groundwater salinity to 3,000 mg/L from excessive mining and possible sea-level rise, the proposed greenhouse can serve increasing crop yield by 20%, 33%, and 25% for cherry tomatoes, bell peppers, and cucumbers, respectively. This significant increase in crop yield is also coupled with irrigation water saving of 40% for cherry tomatoes and cucumbers and 50% for bell peppers.

  • The proposed technology not only provides water saving and increase in crop yield under brackish water conditions, but also provide organic pesticide-free produce.

  • Closed greenhouse setups like the one proposed can even increase the crop yield by boosting the CO2 level in the greenhouse. Hydroponic irrigation can save on water even more, making this system an attractive cultivation alternative for arid regions.

ACKNOWLEDGEMENTS

This work is financially supported under the Egyptian German Research Fund (GERF II). Financial support from the Egyptian Science and Technology Development Fund and the German Federal Ministry of Education and Research (BMBF) is highly acknowledged.

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