Design and Simulation of Photo-voltaic Water Pumping System for Irrigation

In locations where electricity is not available, other means are necessary to pump water for different applications. Using a diesel pump to deliver water causes economic and environmental problems. The higher price of diesel increases operation costs of diesel water pumping system thereby reducing the incomes. In addition, the use of a diesel pump emits a considerable amount of CO2 which cause global warming. A possible solution to these problems is using renewable energy source like solar power, which is environmentally friend and available for free. This paper presents the design and simulation of a photovoltaic water pumping system for irrigation of a potato farm located at a place named Gerenbo in south-west Ethiopia. The system was designed by considering the site’s topography and local meteorological data. The PV system designing was made in such a way that, capable of irrigating a hectare area potato farm with daily water requirement of 7m and total dynamic head of 21m. To run simulations, different tools have been used. Those include CROPWAT, MatLab, and HOMER. Furthermore, after seizing all components and sub-systems, the entire system (from solar radiation to water production) was simulated by using PVsyst. The economic and environmental aspects of PV water pumping systems have been studied. Based on the projected system lifetime production, the equivalent cost of water for PVWPs and DWPs is $0.1187/m and $0.3526/m respectively. Replacement of diesel-powered water pumping system results in an annual reduction of CO2 emissions by 1500kg.


Introduction
When considering the development of the country, energy is one of the key points. Energy can be generated from renewable and non-renewable energy sources. High prices, fast depletion rate and environmental impacts of fossil fuels create problems in electricity generation through conventional energy sources. PV water pumping system have emerged as a promising alternative to conventional diesel and electricity powered irrigation pumps [1]. Solar photovoltaic water pumping has been recognized as suitable for grid-isolated rural locations in developing countries like Ethiopia where there are high levels of solar radiation. The recognizable challenge is the high initial cost of solar water pumping systems; while it demands virtually no maintenance, and requires no fuel [2]. Solar water pumps are most often applied when utility power is not readily available. In Ethiopia, about 73.44% of the population lives without access to electricity [World Bank 2015].
PV power is a more reliable energy source for almost all parts of Ethiopia. This is due to the fact that Ethiopia is situated near the equator and hence solar insolation potential is sufficiently available here. [3][4][5].
The agricultural sector is the leading sector in the Ethiopian economy, comprising 47.7% of the total GDP, as compared to 13.3% from industry and 39% from services [6]. Most of the time, Ethiopian farmers use a diesel-driven generator for irrigation. But diesel pumps have many drawbacks such as high running and maintenance cost, unreliable supply of fuel, and poor availability of spare parts. The site selected for this research is directly facing these problems. Finding the best solution to the problem is very important to increase the profitability of farmers. Using renewable energy is a feasible option to fulfill the energy need without affecting the environment. The objective of this work is to design an efficient and cost-effective photovoltaic water pumping system for irrigation of a potato farm, considering the specific area of Gerenbo to solve drawbacks of a diesel water pump and increase the profitability of the farmers.  The site selected for this research is Gerenbo, which is found in Gurage Zone located 30 km from Welkite and 150 km from the capital city of the country in the southwest direction ( Figure  2). Gerenbo is geographically situated at 8°17′N 37°47′E with an elevation of 1930 meters above mean sea level. Winque is a tributary of the Gibe River. It flows year-round with sufficient capacity. Haram, Gotam, and Wabe are major Feeders. Other minor tributaries also contribute a considerable volume of water.

Crop Water Requirement
The first step in designing a solar-powered water pump system is to determine the overall water requirement for the purpose. The water requirement of crops varies with the type of crop, time and depends on the season and growth of crops [7][8][9]. The type of soil and the climatic parameters are also other factors that affect system design. In this specific research, the crop is potato and the type of soil is categorized under load. Irrigation is the artificial application of water to land for substitute rainfall water. Regional metrological data shows that the amount of rainfall is decreased radically from October (figure 3), therefore, October first is taken as the starting date of irrigation.

Estimation of Clear-Sky Radiation on Horizontal
Surface Hottel and Whiller presented a simple model for the estimation of beam radiation in clear sky conditions [10]. The required input data are; the altitude of the location A, day number of the year N and the zenith angle (θz). The clear sky beam radiation on a horizontal surface is: The clear sky diffuse radiation is: The total clear sky radiation on a horizontal surface could be estimated as:

Estimation of Solar Radiation on Inclined Surface
The following formula was given by Liu and Jordan [10], for evaluating the total radiation on a surface of arbitrary orientation from the knowledge of beam and diffuses radiation on the horizontal surface. I 3 I R I 2 R 2 rR 5 I I 2 Where: r=reflectivity of the ground=0.2. Rb, Rd, and Rr are known as conversion factors for the beam, diffuse and reflected components respectively.
The expressions for these are as follows: Sizing of a photovoltaic system is based on the worst operating condition. For December the ratio of water requirement to solar radiations was largest and the recommended average date of the month is the 10 th day.
Therefore, December 10 i. e. N=344 is chosen for PV sizing. The declination angle can be calculated using Hour angle ω and solar time ST in an hour are related as: For the horizontal surface, zenith (θz) can be calculated using: cos θz cos F D cos δ D cos ω sin δ D sin F (9)

Water Storage Design
Size of the water storage tank which depends on water to be pumped per day and daily water consumption [11]. The capacity of the water storage should be sufficient for two up to ten days' worth of water storage in the event of extended cloudy weather or maintenance issues.
Elevated plastic tankers have many drawbacks; in addition to their higher cost, they also increase system head loss, in order to solve these problem design appropriate earthen farm pond for storing water as shown in (figure 4). The storage should be provided with a lining, to minimize water wastage.

System Layout Design
Design layout of the system will be identifying all necessary distances and elevations.

Performance Characteristics of the Piping System
A system head curve indicates the relationship between flow and hydraulic losses in a given piping system. Since hydraulic losses are functions of the rate of flow, size, and length of pipe, and size, number, and type of fittings. The general equation to calculate system head loss due to friction is: Average velocity, For PVC, a b1.5 (10) -6 m, relative roughness is: and Reynolds number Where, V=average fluid velocity D=pipe diameter U=viscosity of water at 10°C

Frictional Head Loss
The Darcy Equation is a theoretical equation that predicts the frictional energy loss in a pipe based on the velocity of the fluid and the resistance due to friction. Normally, it is used to calculate head loss due to friction in a turbulent flow. ).
There are three elbows in the system (figure 6).
The foot valve is required because the pump is located higher than the water level.
Check Valve allows the pump to start easier. It also prevents backflow when changing filter cartridges.

Pump Selection
Selection of the pump depends upon the following factors [12]. The requirement of the water, the requirement of total vertical head, length of the pipe, the temperature of the water, and quality of the water.
Based on design data, the pump chosen here is SCB 10-150-120BL surface solar pump. It is a pressure booster type centrifugal pump equipped with a DC brushless motor and designed for use in standalone water delivery systems, specifically for water delivery in remote locations. Flow rates up to (42LPM) with a total dynamic head of 28.9m. The rated maximum power consumption is 236W. It operates with voltage (75 V DC).

Performance Characteristics of SCB 10-150-120BL
Pump A plot of head and flow rate at a given speed forms the fundamental performance characteristics of a pump. It is useful to plot also the efficiency η against flowrate Q.  In figure 8;

System Configuration
x y z\{e|}WZY ~X•Ve Sizing of PV array is done based on the electrical power requirement of the system.

Module Selection
The selected panel should provide the minimum energy required to run the pump. However, the panels must also have an additional capacity of 25% to account for any potential reduction in power [14].
BP Solar BP 7180 S Saturn technology module, is selected for this purpose because of its best efficiency and better temperature coefficient.

Characteristics of Selected Module
Electrical and mechanical features of BP 7180 S is shown in table 1. Power-Voltage characteristic of the selected module is illustrated in (figure 9).

Economic Analysis of the System
Every project needs to perform an economic evaluation to make sure that it is feasible or not. In this section, the economic evaluation of PVWP system is compared with diesel water pumping. The comparison is conducted in terms of initial investment costs (IIC), life cycle cost (LCC).

Initial Investment Cost (IIC)
The higher component price made PV water pumping system has a higher initial investment cost than the dieselpowered pump but its recurrent cost proved to decline over their current cost. However, in remote areas aspects such as lower operation & maintenance costs, the more reliability, as well as the longer expected useful life of PV systems, could economically justify the higher initial cost of PV systems.

Life Cycle Cost Analysis (LCCA)
The life cycle costs analysis has been computed using Homer and MS Excel; it takes into account all costs of acquiring, owning, and disposing expenses incurred over the life period of the systems. The life-cycle cost of both alternatives can be calculated using the formula: Where, CC= Capital cost of a project MC=Maintenance cost EC=Energy cost RC=Replacement cost SC=Salvage value The Estimation of solar resources can be alternatively performed with the software Cropwat. The result indicates that the site is blessed with good solar radiation levels, varying from a daily average of 3.92kWh/m 2 in the least irradiated month (nonirrigating months) to 5.82kWh/m 2 in those months with best solar irradiance.  Table 3. Head losses as a function of Q. Performance characteristics of the piping system (Head losses as a function of Q) can be illustrated as follows. Figure 11. Performance characteristics of the piping system.

Working Point
A working point is the TDH-flow pair corresponding to the pump function, taking into account the head loss in the pipe system. When the pump operates, a particular working point is reached, corresponding to the balance between flow and TDH.  Figure 12 shows, the pump flow rate at the working point is close to the design flow rate. This indicates that the selected pump is working in its best operating condition. Figure 13. The monthly average PV output.

The Monthly Average PV Output
The amount of monthly average PV output is reduced starting from Jun up to September but there is no need for irrigation water at this period.

Monthly Average Water Production
Now the simulation process operates on the monthly, daily, or hourly basis, visualizing them as "Graphs" and "Tables". At 5.5 kWh/m 2 daily solar irradiation the system can be pumped more than 7 m 3 of water, which satisfies daily crop water requirement. Figure 15 represents the overall annual system loss diagram for the system. The amount of annual pumped water is 1512m 3 . It is greater than the user's water need. as a result, the designed system satisfies irrigating a hector potato farm. Figure 12 presents the variation of LCCs for PVWP and DWP systems over 20 years (equation 21). Despite the lower operation and maintenance cost and no fuel costs, the high PV modules and the low diesel prices made the PVWP system unaffordable compared to the DWP system in the first year (i. e. PVWPs has higher initial investment cost than DWPs). The PVWPs could still not compete with the DWPs in the fifth year, but gaps became smaller. PVWP has started to be more competitive after the fifth year. Lately, the gap between the two alternatives was further increased.

Environmental Advantages
The main cause of global warming is CO 2 emission released from different sources of energy. The amount of CO 2 emitted varies depends on energy produced and power technology used for conversion. When comparing with diesel powered system; the operation of PV technology is CO 2 emissions free. The calculation of CO 2 emissions was done considering of DWPs used to pump water for irrigation purpose. The CO 2 emission from DWPs was obtained per year and it is equivalent to 1,500 kg of CO 2 emissions. In addition to CO 2 , using of diesel water pump has a cause of other pollutants (Table 4).

Conclusion
The study has accomplished its objectives. From the entire discussions good results could be achieved; based on the results of an investigation, the following conclusions were drawn: 1. Assessment of crop water requirements and available solar resources are the two basic design data essential to the design of PVWP system.
2. Solar PV power is more reliable for small scale irrigation for almost all parts of Ethiopia. This is due to the fact that solar energy is in phase with water demand to irrigate crops, secondly; Ethiopia has an abundant supply of solar energy throughout the year. Especially in irrigation periods i. e. from October to May, the horizontal daily solar radiation of the site is about 5.2 Kwh/m 2 which is very promising for solar PV utilization.
3. The designed system of solar PV water pumping system was capable of irrigating a hector area of potato, within 6.5 hrs with a daily water requirement of 7m 3 and total dynamic head of 21m. 4. The results of the study indicate that irrigating crops using PV systems is beneficial and suitable for long-term investments as compared to diesel-powered engines, as life cycle cost of PV system considering the life span of 20 years was found to be ` $6,183.65 while the life-cycle cost of the diesel engine was ` $18,365.98.
5. Based on the projected system lifetime production, the equivalent cost of water for PVWPs and DWPs is $0.1187/m 3 and $0.3526/m 3 respectively.
6. Application of PVWPs instead of DWPs results in an annual reduction of CO 2 emission by 1,500 kg. 7. Production of electricity from PVWPs is not only at irrigation seasons. Therefore, further researches are required for using this energy to other purposes especially, at the nonirrigating season.