Solar Desalination System Design for Irrigation/Drinking Water and Electricity Generation in Desert or Arid Areas

Climate change and population growth are likely to be future challenges to obtaining water and growing crops in arid areas. Traditional hand pumps are available but give relatively low flows; they are time consuming and are limited by the depth of wells. In low latitudes, solar energy can be the main renewable energy source for water pumping and desalination. In this project, several ways to get irrigation water, drinking water and electricity have been evaluated in the country of Western Sahara. Solar pumps have been proven to be a reliable economic solution for irrigation, but drinkable water is also required in arid areas where the salinity of water wells can be high. There is an obvious synergy when using photovoltaic solar panels for pumping, desalination, and electricity generation, but the feasibility of a project involving all those uses depends on demand and finance. This paper uses resource, technology, and economic assessments to model scenarios that demonstrate the viability of this triple approach. A Net Present Value of €26,887, and an Internal Rate of Return of 10.41% was found where 7 m3 of water was pumped per day.


INTRODUCTION
Around 1/3 of the Earth's land surface is desert, and access to water is one of the main problems for many communities in these areas. Climate change and population growth are putting pressure on resources and the ability to meet Sustainable Development Goal 6 for clean water. In general, water is required for drinking, cooking and washing as well as for livestock and irrigation of crops. Irrigation in dry areas is a challenge because of efficiency requirements and the salinity of water. Some of the main solutions to tackle the salt water issue are farming salt tolerant crops and using desalination systems. Traditionally, the extraction of water in desert areas has been made with manual or diesel pumps. However, the use of solar pumps could be ideal due to high irradiance levels in these areas and the relationship between demand and resource. In addition, diesel fuel supply would not be a limitation. The territories of Western Sahara controlled by SADR (Sahrawi Arab Democratic Republic) are considered particularly interesting for developing solar pumping projects for irrigation. There are many instances of crop farming in desert areas, but in the selected area there is no agricultural tradition. Therefore, there is potential for helping local people to grow food and obtain potable water with a reasonable investment. The selected location is a smallholding close to Meharrize J Sustain Res. 2020;2(2):e200018. https://doi.org/10.20900/jsr20200018 (also known as Mehaires) where a borehole has already been drilled and there are two existing diesel pumps. There is no access to the electricity grid, and water from the well is considered salty, measured at 10,000-12,000 ppm. Drinkable water can be obtained in Meharrize, but when the river is dry it needs to be purchased, which is very expensive. The location of the project is shown in Figure 1.

Aim and Objectives
The aim of this project was to design and to evaluate the feasibility of a system to provide water and electricity in desert areas using solar pumps and PV power.
The objectives of the project were: -To undertake an assessment of water and energy resources in the specific desert area.
-Based on this assessment, to identify suitable technologies for water pumping, desalination and electricity generation at the location.
-To ascertain the water and energy needs in the selected area.
-To design a solar desalination and electricity generation system to provide water and power.
-To perform an economic analysis of the model.
-To perform an economic sensitivity analysis of the model.
The contribution of this project is the design of a medium scale system integrating the most appropriate elements identified by the literature for solar pump irrigation, desalination, and PV solar energy generation in the Western Sahara, an under-researched region of the world. However, the potential impact is through replicating the design in different configurations and locations depending on end-user requirements.

Technical Review
Given the solar resource data and the water available from the well at the site, the technologies for pumping have been investigated. A summary of the comparison of different pumping techniques can be found in The conclusion of the comparison is that taking into account the well depth, cost of fuel, installation requirements, and resources at the project location, a solar pump is the most appropriate technology. Due to the depth of the well, both the pump and motor must be submerged.
As indicated earlier, the salinity of the water is too high for direct irrigation. There are some crops with high saline resistance, but if brackish water is used to irrigate the salt concentration of the soil will increase progressively. Therefore, some desalination system is required to reduce the salinity of the water. Additionally, the demand for potable water in the area, and in desert areas in general, supports further desalination for drinking purposes.  The conclusion of the comparison of different desalination technologies shown in Table 2 is that Reverse Osmosis (RO) is the most appropriate technology for this small/medium scale application. For brackish water of less than 5000 ppm TDS (total dissolved solids), electro-dialysis (ED) could be more appropriate, but the salinity of the raw water in this case (10,000-12,000 ppm) is too high for ED [25]. The salinity range is appropriate for RO. The average power consumption of RO is higher than for some other technologies (using heat) and more maintenance may be needed, but the investment and consequent water production cost for RO is typically lower. The technology is commonly used, and there are commercial portable units on the market. According to [25][26][27][28], RO typically consumes system without batteries (as in [16]) to reduce costs, but it makes the design more complicated and the system less flexible.

Water Requirements
In order to determine the agricultural water needs, the irrigation water requirements (IWR) have been estimated. IWR depends on the cropping pattern and the climate. According to [29], in the selected location the net  -Then the system model was produced, incorporating all the components and devices, the resources available and the requirements captured. Preliminary calculations were made to assess the efficacy of the system.
- precisely specify the elements of the system with the costs assessed as well.
-An economic analysis was performed, including the life cycle costs, payback, and net present value calculations.
-Finally, a sensitivity analysis of the model was investigated to show the impact of changing variables; notably, the discount rate, the inflation rate, the fuel cost and the water cost. The results are now presented.

RESULTS AND DISCUSSION
A system capable of providing electricity and water for both drinking and irrigation has been designed. The only resources needed to operate the system are the sun and brackish water from a well in a desert area.
Financially, there is a moderate investment that can be recovered.
Additionally, different water flow rates and electricity production for one or several dwellings have been modelled.

Resource Assessment
For the solar irradiance, the most appropriate data identified was from [31]. According to the water analysis carried out, the salinity was around 11 g/L (between 10,000 and 12,000 ppm or 17 dS/m) (an electrical conductivity EC = 17 dS/m has been measured. A conversion 1 dS/m→640 mg/L, from EC to TDS (total dissolved salts) has been applied), which means that the water was brackish water (BW). This value has been used for the basic design of the desalination unit, but once the plant designed in this study is operational samples would have to be tested regularly. The condition of aquifers and soil quality have been checked from several sources such as [33]. According to the consulted sources, the aquifer in the selected location seems to be between sedimentary and basement fracture, and the productivity is low. The risk of water

System Diagram
The main equipment and elements required for the system are shown in Figure 2.   and Case 6. Case 0 is not relevant in this situation because the salinity of the water is too high for irrigation, and desalination is needed. Drinking water must be considered, so Case 1 is only illustrative. Cases 4 and 7 (with 4 dwellings) have not been considered for a first phase of implementation because the cost would be is higher, and the dwellings have not yet been constructed. Only one dwelling has been considered (in Cases 3 and 6) for the people taking care of the crops.

Water and Solar Calculations
The existing borehole is 50 m deep. However, water is already present at 20 m, according to the local contact. But it has been estimated based on experience that the water flow can be higher if the well is drilled deeper.
For hydraulic calculations, a depth of 75 m has been assumed, but a total head H of 100 m has been used to include the pressure drops of the system and to allow for some margin.
For the basic design of the RO unit, two stages have been considered. In the 1st stage, the salt concentration is reduced from 11 g/L to 1 g/L (1000 ppm). This water is used for irrigation. The inlet of the 2nd stage is not the brine but the partially desalinated water stream. In the 2nd stage the concentration is reduced from 1 g/L to 0. The monthly averaged insolation incident on an equator-pointed tilted surface has been obtained from [16], as shown in Table 4. During the winter months, the solar irradiance is much lower, but the water consumption will be also considerably lower because no irrigation is required. With a fixed tilt of 24.6° (optimum angle), the annual average insolation will be above 5.94 kWh/m 2 /day, which is the figure for a tilt of 26° (same as latitude) and this value has been used for the calculations.
Solar irradiance could be optimized with a solar tracking system, to adjust the angle as required. The irradiance is similar to that found in a study in Mexico that also highlighted the use of PV for brackish water desalination [17].  The size of the PV array can be calculated from the following Equations [19]. The efficiency of the pump can be included in the subsystem efficiency . In this case, already has been considered, and is the efficiency of the subsystem from the PV array to the pump and consumers, including the batteries and inverter for AC (alternating current). A subsystem efficiency of 0.80 has been assumed, so the daily energy demand is 20.79 kWh (from Equation (3)).
is the effective area of the PV array in m 2 ; is the irradiance of reference ( = 1000 W/ 2 )(the solar irradiance G in W/m 2 is the total radiative power density incident on a collector. The solar irradiation H in J Sustain Res. 2020;2(2):e200018. https://doi.org/10.20900/jsr20200018 kWh/m 2 is the total radiative energy received by a collector in a specified time); = is the daily solar radiation on the PV array surface ( = 5.94 kWh/m 2 ), or daily irradiation (some references, such as [11], use GT instead of HT in Equation (6) From Equations (4) and (5), PR is the quality indicator Performance Ratio (the performance ratio is the difference between the electric generated by the PV modules and that delivered by the system taking into account system losses), calculated from and (alternative notation is also commonly used: (operating efficiency) instead of and (standard conditions efficiency) instead of or ).
If PR is 0.85 (typically 0.80-0.90), then the PV array power P is 4.12 kWp (from Equation (8)). From Equation (4), if the conversion efficiency is 0.14, then the estimated area of the modules is 25 m 2 .
In contrast to Equations (6) and (8), some authors use the PSSH (peak sunshine hours) instead of the irradiation (the figure of "peak sunshine hours" PSSH (or peak sun hours) is numerically identical to the average daily solar insolation (or irradiation). A peak sun hour is an hour when the intensity of the sunshine reaches 1 kW/m 2 . It can be visually explained as converting the area under the curve "solar radiation vs time" in a square 1 kW/m 2 × PSSH (h)), the mismatch or thermal factor ℎ and the efficiency of the main elements in the system.
If PSSH is 5.94 h, ℎ is 0.83, the battery efficiency is 0.90 and the inverter efficiency is 0.90, then the PV array power P is 4.16 kWp (from Equation (9)).
Similar calculations have been performed with PVSyst for all cases, getting comparable results, which have been summarized in Table 5.
J Sustain Res. 2020;2(2):e200018. https://doi.org/10.20900/jsr20200018 As indicated above, only cases 2, 3 and 6 have been considered relevant for further analysis. Case 2 is the base example for irrigation and drinking water. In Case 3, the system produces the same amount of water, but electricity is provided to the site dwelling. Case 6 keeps the variable for drinking water and electricity the same but increases the water for irrigation.

Economic Analysis
For Case 2, the estimated costs (although information from suppliers and several sources has been consulted for the costs, assumptions have

Sensitivity Analysis
A sensitivity analysis has been performed for Case 2, plotting NPV when changing the discount rate i, inflation, the fuel cost and the water cost. A similar approach was taken by [13]. Decreasing the discount rate from 0.1 to 0 transforms the project from an unprofitable one into a very profitable one, as shown in Figures 5 and 6. Inflation does not affect the investment, but it affects all prices from the first year. Thus, the impact of inflation on the NPV is significant, especially when it is above 5%. Without inflation NPV is close to 0. The study also showed that the drinking water cost is much more important than the fuel cost in the economic analysis, as shown by comparing the slopes of the curves in Figures 7 and 8.

Water Flow (Case 6)
In Case 3, the electric power consumption of one dwelling has been included. Because the designed system is able to provide AC electricity and to store solar energy in batteries, the size of the PV array can be increased to be a realistic one because, according to the local contact, PV panels are normally used in new installations). However, the cost of this additional electricity will be definitely lower than for a new installation.
Another variation has been analysed in Case 6 where the irrigation volume was increased from 4 to 9 m 3 in order to substantially extend the crop area. The total energy required is 41.28 kWh per day with P rated at 10.22 kWp, and 62 m 2 . The NPV for this case is still positive but is much lower than in the base case because the added value of irrigation is not considered in the analysis (the economic value added to the irrigation could be calculated based on the produced food and the generated salt, but this calculation is beyond the scope of this work.).
The choice between Case 2 (base case) and Case 3 depends on the funding and subsidies for the initial investment. However, in principle, it seems reasonable to consider the consumption of one dwelling, taking into account the costs saved compared to building a separate installation for the house. Regarding Case 6, the available sizes and prices of the RO should be double checked in order to make a decision, provided that the funding allows it. There are some other potential improvements related to the control system such as tracking PV modules or moisture sensors/probes and automatic valves at the outlet of the irrigation tank [34,35].
Furthermore, utilising IoT monitoring and management could improve resource use and reduce costs, for example as demonstrated for irrigation in [36].

CONCLUSIONS
This project shows the details and feasibility of a system capable of providing water for both irrigation and drinking, as well as electricity, in desert areas for many years with a reasonable investment to meet sustainable development goals. The study has met the objectives set out in the introduction. The water resource assessment was considered sufficient for the design, but it should be verified in terms of quality and quantity. The salinity was assessed at 11 g/L. Based on the energy resource assessment and review of suitable technologies, PV solar energy has been J Sustain Res. 2020;2(2):e200018. https://doi.org/10.20900/jsr20200018 determined to be a suitable technology for water pumping, desalination, and additional electricity generation [11,16,19]. The main water and power requirements in the project area have been identified. The need to be flexible in the design led to several scenarios being considered in the study. A model, PV-BWRO, to provide water and electricity, based on solar pumps and PV power" has been designed and evaluated. After analysis of the different scenarios, one case with 4 m 3 of water for irrigation and 2 m 3 for drinking water was proposed for implementation. 1 m 3 of brine was also produced. The economic analysis of the model shows the savings of the selected case compared to a scenario utilising diesel pumps. Finally, a sensitivity analysis has been performed to evaluate the impact of changing certain variables with four of the most significant presented in Additionally, two variations of the design have been presented: One with electricity generation for one dwelling and the other with higher flow rates for irrigation (9 m 3 ). The decision to adopt these alternative systems would mainly depend on funding. The size of the PV array can be increased to provide electricity for more consumers, but the system would need to be analysed to verify that all elements are suitable for doing so.
There are also some possible improvements and alternatives for further research, such as tracking panels, automatic valves and the use of IoT [36].
This project is reproducible and scalable, but a proper analysis of water and well characteristics wold be required in advance. The use of the salts from brine also should be defined. If the system were grid-connected, then excess electricity could potentially provide a revenue stream, depending on tariffs. [37]. Further areas for research could include using grey water from the drinking supply for irrigation and the potential to grow crops between rows of PV modules where larger solar irrigation systems are deployed [38][39][40].

DATA AVAILABILITY
Data generated from the study is available in the supplementary file.

SUPPLEMENTARY MATERIAL
The supplementary material is available online at https://doi.org/10.20900/jsr20200018.

AUTHOR CONTRIBUTIONS
JI and RB designed the study. JI developed the model and analyzed the data with input from RB. JI and RB wrote the paper.

CONFLICTS OF INTEREST
The authors declare that there is no conflict of interest.