Technical and Economic Feasibility of Solar Pump Irrigation in the North-Niayes Region in Senegal

Northern Niayes is an area where agriculture is the main activity. Water used for irrigation in this area comes exclusively from groundwater. Farmers do not have access to electricity, thus fuel is the main source of energy used, which is very expensive. The objective of this study was to assess the tech-no-economic feasibility of solar irrigation pumps. Regarding technical feasibility, pump sizing was carried out on the basis of irrigation management, irrigation techniques, and water distribution. Also, the economic feasibility was studied on the basis of the Net Present Value, Benefit-Cost Ratio, Internal Rate of Return and Payback Period. Results showed for cultivated area ranging from 0.05 ha to 1.91 ha, pumps’ flow rate does not vary greatly from one irrigation technique to another. It varies between 2.5 m 3 /h and 31 m 3 /h. However, pressure and power are higher when using drip and sprinkler irrigation techniques, ranging from 27.8 m to 39.9 m for drip and 40.1 m to 58.5 m for sprinkler irrigation. The power varies between 0.05 kW and 1.6 kW The show that the investment cost when the irrigation technique, from when the irrigation and from to 21,403 when sprinkler irrigation technique for pumps. The investment cost is when using In this case, the investment cost between €722 and €6062 for manual irrigation, €1532 and €25,882 for drip irrigation and €900 and €28,000 for sprinkling. However, the total investment cost at the entire lowland scale is higher when farmers use the pumps individually and lower when farmers use the pumps in groups when using manual and drip irrigation techniques. NPV varies between 15,993€ (0.75 ha) and 103,139€ (1.41 ha) and between 13,064€ and 86,139€ when using sprinkler irrigation techniques with PVC pipes and when using the drip irrigation technique, respectively. BCR is estimated to average 2.2, 2, and 2 respectively when using manual, drip, and sprinkler irrigation techniques. In addition, PBP is reached more rapidly when using the manual irrigation technique.


Introduction
Globally, the population is increasing at significant rate resulting in an increase in food insecurity [1]. Demographic growth and food insecurity must be tackled with an expansion of agriculture and irrigation. Production should be increased taking into account environmental concerns. Moreover, the impact on poorer nations facing food scarcity is far more critical because more land and energy resources are required to provide future demands. In fact, water is one of the world's most important resources used in irrigation purposes for food production. Moreover, climate change is exerting negative effects on the earth's water resources which are crucial for sustainable development [2]. Crop diversification as well as yield increase is possible through irrigation [3]. Irrigation is the most extensive water consumption sector. About 70% of the world's freshwater extraction and 90% of the total water use is for irrigation. Additionally, groundwater remains the most important source of freshwater for many agricultural areas when surface water resources are limited. In fact, underground water is a major drought solution for agricultural production [4].
However, irrigation consumes considerable groundwater resources and requires large amounts of energy. The main sources of energy used for irrigation are fuel and electricity which cost high, are not always accessible, and are not sustainable [5]. The cost of this energy source is very high, particularly in Africa, more especially among smallholder farmers, and is associated with a negative environmental impact. The environmental damages mitigation is possible thanks to renewable resources such as solar energy. In fact, solar powered pumping in irrigation provides a green and cost-effective alternative for irrigated agriculture. Solar energy is the world's largest available energy source [3]. Thus, solar power for irrigation water pumping is a promising alternative [5]. The  tion pumps has expanded in both China and India [6]. More lately, it is observed an increase in the use of solar pumps in Africa [6]. However, any study on the technical feasibility of solar irrigation pumps in the Niayes area has not yet been conducted in the Niayes area in Senegal, to the best of our knowledge. In North Niayes, farmers use groundwater for irrigation and do not have access to electricity and, consequently, use fuel as energy source. Therefore, solar irrigation pump may be a good alternative.

Presentation of the Study Area
Nguethiouro is a lowland located in the Gandiolais region, at the southern end of the Senegal Delta. It is an integral part of the Niayes area and occupies its northern extremity. The Niayes area is a natural region located in the northwest of Senegal. Nguethiouro is geographically localized between 16˚28'30'' and 16˚24'05'' in western longitude and 15˚50'20'' and 15˚56'10'' in northern latitude ( Figure 1) with altitudes ranging from 6 m to 13 m a.s.l.
This lowland is composed of 15 plots. The plots vary in size from 0.05 ha to 1.91 ha ( Figure 2).

Technical Feasibility of Solar Irrigation Pumps in Nguethiouro Lowland
The technical feasibility of solar irrigation pumps is studied in Nguethiouro lowland based on three main variants: 1) irrigation management, 2) water distribution, and 3) irrigation techniques ( Figure 3).

Photovoltaic Pumping System Sizing
Sizing of solar irrigation systems is performed through three steps: 1) water requirements assessment, 2) determination of solar pump characteristics (Flow rate, Pressure head, and Power), 3) determination of photovoltaic generator size and number of panels. 1) Water requirement assessment Crop water requirements are determined during the period when water demand is most intense, based on crop types and cropping calendars of each of them. Crop water requirements assessment is realized based on crop evapotranspiration (ETc) and effective rainfall (Peff). Parameters related to ETc and Peff are determined based on [9].
ETc is the product of the reference evapotranspiration and the crop coefficient. It is computed using the formula 1: where ETo is reference evapotranspiration (mm) and Kc is crop coefficient. ETo is calculated using FAO-Penman-Monteith Method with the formula 2: where: ETo is the reference evapotranspiration (mm·day −1 ), Rn is the net radiation (MJ·m −2 ·day −1 ), G is the soil heat flux density (MJ·m −2 ·day −1 ), T is the mean daily air temperature at 2 m height (˚C), Δ is the slope of the saturated vapor pressure curve (kPa·˚C −1 ), γ is the psychometric constant (66 Pa·˚C −1 ), es is the saturated vapor pressure at air temperature (kPa), ea is the actual vapor pressure (kPa), and u2 is the wind speed measured at 2 m height (m·s −1 ).
2) Water requirement assessment Net irrigation requirement refers to the amount of water required to compensate evapotranspiration losses from a crop during a specified period. It is calculated based on formula 3 given by [9]: where: ETc is Crop evapotranspiration (mm) and P eff is Effective rainfall (mm).  After the determination of net irrigation requirement, gross irrigation requirement is determined based on the three different techniques (formula 4).
where: GIR is gross irrigation requirement (mm), NIR is net irrigation requirement (mm), and E a is the irrigation efficiency (0.90 for drip, 0.75 for sprinkler, and 0.85 for manual [10]. 3) Solar irrigation pumping system characteristics determination Solar irrigation pump characteristics are: pump flow rate, pump pressure head, and pump power.  Flow rate o Flow rate for manual irrigation system For the manual irrigation system, the pump flow rate is calculated using the formula 5, given by [11]: where Q is the flow rate (m 3 /h). GIR is the daily irrigation water requirement (m 3 ), and Ho is number of hours of operating time (h).
o Flow rate for drip and sprinkler irrigation pump Flow rate is calculated using the formula 6 [12]: where Q e is the emitters flow rate (L/h), L li is the lateral length (m), E si is the space between emitter or sprinkler (m), L ri is mainline length (m), and L si is the space between laterals (m Technical feasibility of solar irrigation pumps (variants to take into account for solar irrigation pumping system sizing) where Hp is the pressure head, H d is the dynamic high (static height + drawdown) ΔZ is the difference in elevation between the water source and the most unfavorable supply inlet, hf 1 is the head loss from the pump to the most unfavorable supply inlet, hf 2 is head loss from the inlet to the most unfavorable emitter, H em is pressure needed at the emitter, hf 2 and H em are used for drip and sprinkler irrigation techniques.
Drawdown of 10 m is assumed to ensure the availability of well water. Indeed, according to [14] [15], the drawdown in the Niayes area rarely exceeds 10 m.
In addition, specific formulas are used to calculate some parameters related to the pressure head for the different irrigation techniques. Thus, head loss for manual irrigation technique is calculated using the formula 8:

100
J L hf * = (8) where L is pipe length (m) and J represents the head losses (%). It is determined on the basis of water flow, diameter and nature of the pipe. J is determined using the flow rate and diameter curve (%).
where hf is the head loss (m), f is the Darcy-Weisbach resistance coefficient, L: the pipe length (m), D is the inside diameter of the pipe (m), Re is Reynolds number (m), Q is the flow in the pipe (m 3 /s), and g: the gravitational acceleration, 9.81 m/s 2 .
Reynolds number is calculated using the formula 11, given by [16]: where Re is Reynolds number, Q is the Flow rate (L/s), and D is pipe inside diameter.
In addition, the pressure in plot will be calculated by section using the formula 12 [16]: where L is the pipe length (m). J is the head losses (%), is the F: adjustment factor.
Also, head loss less than or equal to 20% compared to the head loss at the previous level is accepted at plot scale (formula 13).  (14) where Eh is the hydraulic power (Wh·day −1 ), Hp is the pressure head (m). Q is the flow rate (m 3 /day), ρ is water density (1000 kg/m 3 ), and g is the acceleration of gravity (9.81 m/s 2 ).
Pump power is calculated using the formula 15 [20]: where Eh is the hydraulic energy (Wh/day −1 ) and ηpum is overall pumping plant efficiency which will be equal to 70% based on [21].

4) Generator power determination
The photovoltaic generator power, i.e. the power of the solar field is calculated using formula 16 [18].

Q Hp Pgen
Psh npum є * * = * * where Pgen is the generator power (Wh/day), Hp is the Pressure head (m), Q is the Flow rate (m 3 /day), ηpum is overall pumping plant efficiency which will be equal to 70%, Psh is the number of peak sunshine hours per day (kW/m 2 /day), and є: electrical efficiency of the system, 2.73 is calculated using ρa is water density (1000 kg/m 3 ), and g is the acceleration of gravity (9.81 m/s 2 ): The electrical efficiency of the system (є) is determined based on losses in electric cables (3%), in controller (10%), and losses due to temperature (10%) and dust (2%) on the basis of [18] [19].
The number of panels required is calculated using the formula 18: where N is the panels' number, Pgen is the power of the generator (kW), and Ppanel is the power of available panel on the market (kW).

The Net Present Value (NPV)
NPV estimates the total amount of wealth created by the project. It is expressed as: NPV Bo Co = − (19) where: • Bo (€): The discounted present value of benefits, calculated as: • Co (€): The discounted present value of costs, calculated as: where: I is the index of year, NPV is the net present value (€), Bi is the benefit flow of the i year, Ci is the cost flow of the i year, r is the interest rate, and n is the time frame of the project.

The Benefit-Cost Ratio (BCR)
BCR is the ratio between the discounted benefit and cost, and expressed as: The investment is cost-effective when BCR ≥ 1.

The Internal Rate of Return (IRR)
IRR is the average annual rate generated by the project and is the discount rate which makes NPV = 0. It is calculated as: r = IRR when NPV = 0. IRR is expressed in percentage (%).

The Payback Period (PBP)
PBP evaluates the time, it'll take to recover the initial investment made in the project. PBP is expressed in year.

Solar Irrigation Pump and Generator Characteristics for Drip, Sprinkler and Manual Irrigation for Farmers Individually
The pumps' flow rates, pressure heads and power and the generator power are presented in Figure 4.
The flow rate is not much variable considering the three irrigation techniques. It is ranging from 2.5 m 3 /h to 25 m 3 /h; 2.7 m 3 /h to 30.8 m 3 /h and 2.5 m 3 /h to 24 m 3 /h, when using manual, drip and sprinkler irrigation techniques, respectively. Flows rates obtained are relatively low, 86.6% of pumps have a flow rate that is lower than or equal to 15 m 3 /h when using manual irrigation. This is explained by the fact that cultivated plots in this area are relatively small, ranging from 0.05 ha to 1.91 ha. Thus, these pumps are sufficient to meet the daily irrigation water requirements after 1.5 to 5.9 hours, 1.4 to 4.8 hours and 1.7 to 6.9 hours of operation  Niayes area is an environment characterized by its dunes and its depressions or slopes [25]. In addition, groundwater level varies between 3.2 and 8.9 m. The pump power ranges from 0.045 kW to 1.6 kW for manual irrigation, from 0.05 kW to 2.5 kW for drip irrigation and from 0.1 kW to 4.75 kW sprinkler irrigation technique. This increase in pump power is mainly due to the increase in pressure required for drip and sprinkler irrigation system operation. The photovoltaic generators for these pumps are also relatively small and vary between 0.06 and 3.7 kW when using manual irrigation. It ranges from 0.11 to 5.7 kW for drip irrigation. Also, the highest value is evaluated à more than 10 kW when using sprinkler irrigation. The flow rates, pressure head, and power are smaller when using manual irrigation technique with reservoir. It has varied between 2.5 and 21 m 3 /h when reservoirs are used. Thus, smaller pumps can be used by increasing pump operating time, which is a good way to increase pump's efficiency which can be beneficial to farmers. Use of reservoirs is more beneficial for farmers because, they make it possible to obtain water during periods of low irradiance. However, flow rate, pressure head, and power are higher when using reservoir when using drip and sprinkler irrigation technique. The increase in pressure is due to the fact that reservoirs are installed at the highest point in plots in order to provide water to basins by gravity. Figure 5 shows the number of panels connected together in the photovoltaic generator for the various solar pumping systems depending on cultivated areas and for panels of different power.

Number of Panels Making up the Generator for Different Panel Powers for Farmers Individually
In the case of manual irrigation, the number of panels varied between

Solar Irrigation Pump and Generator Characteristics for Drip, Sprinkler and Manual Irrigation for Farmers in Groups
The The same variation observed when farmers are using pump with reservoir individually is observed when farmer are using pumps with reservoir in groups. Figure 6 shows the number of panels connected together in the photovoltaic generator for the various solar pumping systems depend on cultivated areas and panel powers. Figure 6 shows that the number of panels is very high when the plots are grouped together. Results show that the number of panels varied between 21 (0.93 ha) and 64 (2.29 ha) when using panel with power of 80 W and from 5 (0.93 ha) to 15 (2.29 ha) when using panel with power of 330W, depending on panel power, when using manual irrigation. In the case of drip irrigation, the number of panels varied between 43 (0.79 ha) and 126 (2.29 ha) for 80 W panel's power and between 10 (0.79 ha) to 31 (2.29 ha) for 330 W panel's power. Concerning sprinkler irrigation, the number of panels varied between 79 (0.79 ha) and 254 (2.29 ha) for panels with power equal to 80 W and between 19 (0.05 ha) and 62 (2.29 ha) for 330 panel's power. Moreover, these multiple panels can cover a large area in the plots. This can lead to a reduction in cultivated areas. Thus, the use of larger panels' power may be beneficial in these cases.

Investment Cost Evaluation
The investment cost varies depending on the brand of pump selected. Result of the investment when using manual irrigation show that Lorentz pumps are more expensive (cost varies from 2315 euros (1,518,500 CFA) for 0.05 ha to 4090 euros (2,683,000 CFA) for 0.53 ha), followed by Difful pumps (varying from 1808 euros (1,186,000 CFA) for 0.38 ha to 4798 euros (3,147,500 CFA) for 1.41 ha). In contrast, solar pump, JUQIANG, and SHIMGE pump brands are cheaper (investment cost varies between 669 euros (438,500 CFA) and 1265 euros (825,000 CFA)). However, the investment cost for solar pumping system implementation is higher when using drip irrigation technique compared to manual irrigation technique. This increase in cost is more pronounced when using 900 µ drip lines.
The investment cost varies between 1281 euros and 18,999 euros when using 600 µ drip lines and between 1303 euros and 20,600 euros when using 900 µ drip lines. Indeed, the high investment cost is explained by the high cost of drip irrigation system. In fact, the irrigation system accounts for two-thirds (2/3) and half (1/2) of the pumping system when using the 900 µ and 600 µ dripper lines, respectively.
Concerning sprinkler irrigation, the result show that the investment cost ranges from 819€ to 14,723€ when using PVC pipes, while it ranges from 990€ to 21,403€ when using aluminum pipes. For this purpose, the implementation of a solar pumping system when using the sprinkler irrigation technique with PVC pipes is cheaper compared to the use of the drip irrigation technique. This is because the investment cost of setting up the sprinkler irrigation system with PVC pipes is cheaper. It represents one third (1/3) of the total investment cost. In contrast, the implementation cost of sprinkler irrigation with aluminum pipes represents half (1/2) of the total investment cost.
These investment costs are higher when using a reservoir because of the additional cost of these reservoirs and because of the increase of the pump size when using drip and sprinkler irrigation techniques. In addition, the investment cost is lower when farmers are using pumps in groups in the case of using manual and drip irrigation technique (Table 2).

Part (%) of Solar Irrigation Pumping System Components in the Investment Cost
Results in Figure 7 show that the pump and the PV generator are the most expensive components when considering the pumping system without the irrigation system cost. Thus, the generator can account for up to 54%, 61%, and 65%  of the investment cost when using manual, drip and sprinkler irrigation techniques respectively and the pump can account for up to 43% in manual and drip irrigation and up to 60% in sprinkler irrigation. In fact, pump and photovoltaic generator costs remain higher when using manual irrigation systems, and the irrigation system, in this case, represents at most 29% of the investment cost.
However, the irrigation system cost represents the largest proportion when using drip (77% and 74% of the investment cost when using 900 µ and 600 µ drip lines respectively) and sprinkler (50% and 68% of the investment cost when using PVC and aluminum pipes respectively). These percentages decrease slightly when using reservoir. In fact, the cost of the reservoirs represents the smallest proportion and constitutes 3% to 6% of the investment cost.

Benefit-Cost Analyze
The benefit-cost analysis is conducted based on the NPV, BCR, IRR and PBP The use of the manual irrigation system gives the highest BCR value followed by the use of sprinkler irrigation with PVC pipes and drip irrigation. This is due to the lower investment cost when implementing manual and sprinkler irriga-

Conclusion
The study aims to evaluate the technical and economic feasibility of solar irriga-