Sizing and modelling of photovoltaic water pumping system

International Journal of Sustainable Energy ISSN: 1478-6451 (Print) 1478-646X (Online) Journal homepage: http://www.tandfonline.com/loi/gsol20 Sizing and modelling of photovoltaic water pumping system A. Al-Badi, H. Yousef, T. Al Mahmoudi, M. Al-Shammaki, A. Al-Abri & A. AlHinai To cite this article: A. Al-Badi, H. Yousef, T. Al Mahmoudi, M. Al-Shammaki, A. Al-Abri & A. AlHinai (2017): Sizing and modelling of photovoltaic water pumping system, International Journal of Sustainable Energy, DOI: 10.1080/14786451.2016.1276906 To link to this article: http://dx.doi.org/10.1080/14786451.2016.1276906 Published online: 12 Jan 2017. Submit your article to this journal Article views: 8 View related articles View Crossmark data Full Terms & Conditions of access and use can be found at http://www.tandfonline.com/action/journalInformation?journalCode=gsol20 Download by: [FU Berlin] Date: 14 January 2017, At: 02:28 INTERNATIONAL JOURNAL OF SUSTAINABLE ENERGY, 2017 http://dx.doi.org/10.1080/14786451.2016.1276906 Sizing and modelling of photovoltaic water pumping system A. Al-Badi, H. Yousef, T. Al Mahmoudi, M. Al-Shammaki, A. Al-Abri and A. Al-Hinai Department of Electrical & Computer Engineering, College of Engineering, Sultan Qaboos University, Muscat, Sultanate of Oman ABSTRACT ARTICLE HISTORY With the decline in price of the photovoltaics (PVs) their use as a power source for water pumping is the most attractive solution instead of using diesel generators or electric motors driven by a grid system. In this paper, a method to design a PV pumping system is presented and discussed, which is then used to calculate the required size of the PV for an existing farm. Furthermore, the amount of carbon dioxide emissions saved by the use of PV water pumping system instead of using diesel-fuelled generators or electrical motor connected to the grid network is calculated. In addition, an experimental set-up is developed for the PV water pumping system using both DC and AC motors with batteries. The experimental tests are used to validate the developed MATLAB model. This research work demonstrates that using the PV water pumping system is not only improving the living conditions in rural areas but it is also protecting the environment and can be a cost-effective application in remote locations. Received 27 November 2015 Accepted 15 March 2016 KEYWORDS PV modules; solar radiation; water pumping system; greenhouse gases 1. Introduction Sultanate of Oman is located between latitudes 16°40′ and 26°20′ north and longitudes 51°50′ and 59°40′ east. This geographical location results in a huge potential for solar electricity generation. Recent studies (Al-Badi et al. 2011; Al-Badi 2011a, 2011b, 2013; Charabi and Al-Badi 2015) showed that the desert and northern parts of Oman have the highest solar energy density while the coastal areas in the southern part have the lowest solar energy density and relatively high wind speed. Although the potential of renewable energy in Oman, particularly solar energy, is very promising, the applications of this energy are limited to street lighting, traffic lights, cellular phone towers, telephone lines in remote areas, cathodic protection of pipelines and providing part of energy to some buildings. Diesel pumps have traditionally been used in Oman to pump water for irrigation, livestock and drinking. Today most of these are replaced by electrical pumps that are connected to the grid network. The contribution of the consumer to the extension of the electric distribution network to his/her farm is the minimum. However, the consumer has to pay for the electrical cable required for the extension of the distribution network up to his/her farm. In many areas there are some farms located far away from the distribution system, thus using photovoltaic (PV) for water pumping is an attractive solution in this case. Several reports have demonstrated that solar PV energy is inexpensive compared with diesel energy in standalone remote areas (Is Rooftop Solar Power Cheaper than Diesel/Grid Power? 2016; Renewable Energy Technologies 2012). Furthermore, the use of PV pumping is suitable for Oman local environment, as there is a natural relationship between the water requirement and the availability of solar radiation. The requirement increases during long-day hot weather periods when the radiation intensity is high and the output of PV is the maximum and vice versa. CONTACT A. Al-Badi albadi@squ.edu.om © 2017 Informa UK Limited, trading as Taylor & Francis Group 2 A. AL-BADI ET AL. The standalone solar PV system has been widely used for power generation in remote areas. In Kazem et al. (2015), the PV water pumping system has been designed and assessed for Sohar city, Oman. The results have shown that the cost of PV energy is 0.309 $US/kWh in comparison with the cost of diesel engine energy which is 0.79 $US/kWh. Bhave (1994) and Kelley et al. (2010) presented that solar water pumping systems are suitable for drinking water and minor irrigation applications in areas where cheaper sources of energy are not readily available. The photovoltaic water pumping (PVWP) systems are shown to be more cost-effective than diesel engine-based pumping systems in Jordan Badia (Al-Smairan 2012). A model to characterise the motor-pump subsystems used in PV pumping installations is discussed in Ould-Amrouche, Rekioua, and Hamidat (2010). The model expresses the water flow output (Q) directly as a function of the electrical power input (P) to the motor pump, for different total heads. Because of the intermittent nature of the solar energy, a storage method is required to meet the demand fluctuation. A pump storage was designed and optimised for a standalone microgrid PV system located in a remote island in Hong Kong (Ma et al. 2014, 2015). It was found that a minimum reservoir capacity should be available to supply the load during the night irrespective of the PV size. A review of solar PVWP system technology for irrigation and community drinking water supplies is presented in Chandel, NagarajuNaik, and Chandel (2015). The authors in Benghanem et al. (2014) studied the effect of pumping head on the performance of PVWP systems using an optimal PV array configuration to drive a direct current (DC) helical pump. The performance of a directly coupled PVWP system is investigated in Mokeddem et al. (2011) and two different DC pumps are compared in Boutelhig, Hadjarab, and Bakelli (2011) with the scope of selecting the optimal direct coupling configuration for providing water to a farm in Algeria. In Senol (2012), the focus was on small and medium-sized mobile PVWP applications for watering purposes in Turkey. Due to the extreme dynamic variability of the parameters affecting the functioning of PVWP systems, mainly solar radiation, the dynamic modelling is an important tool to evaluate their performances as discussed in Ould-Amrouche, Rekioua, and Hamidat (2010). A new approach has been proposed in Campana et al. (2015) to optimise the PVWP system for irrigation with the consideration of groundwater response and economic factors. The optimisation of the number of PV modules for water pumping system and storage capacity is carried out with respect to techno-economic issues (Olcan 2015). Yadav et al. (2015) present the improved performance of using a sine-wave pump controller with maximum power point tracking for solar PVWP system over the most popular controllers using the variable frequency drives. This paper presents and discusses the designing of PV pumping system to be mainly used for farm irrigation in Oman environment. Annual saving in carbon dioxide (CO2) emission per farm is analysed when using a PVWP system instead of diesel generator-based or a grid-connected pumping system. A MATLAB/Simulink model for the overall PVWP system was developed for two types of motor pump, namely the AC motor pump and DC motor pump. The modelling approach of the system was verified by comparing the simulation results and the experimental results. The paper is organised as follows. Sizing of different components of PV pumping system is outlined in Section 2. In Section 3, a small farm is considered as a case study, where all the equations presented in Section 2 are used to calculate the required size of the PV for the farm. For this case, the decrease in CO2 is evaluated when the PVWP system replaces the grid-connected one. Modelling and simulation results are given in Section 4. Experimental results of a laboratory-scale pumping system are presented in Section 5. The paper is concluded in Section 6. 2. PV system sizing and components 2.1. Pumping system sizing 2.1.1. The pump Water pumps are classified into three types according to their applications: surface, submersible and floating water pumps. The surface water pump is placed outside the well. This type is efficient when INTERNATIONAL JOURNAL OF SUSTAINABLE ENERGY 3 used to pump water from shallow wells. A submersible pump is submersed in the water well and used to pump from deep wells. A floating water pump is placed on the water surface and pumps water from reservoirs with adjusting height ability. Also, the pumps can be categorised according to their pumping principle • Centrifugal pumps, where water is sucked by the impeller and the centrifugal force directs it to the outlet as the impeller rotates. The water leaves with a higher pressure and velocity than it had when it entered the pump. • Screw pumps, where a screw movement traps the water in the suction side of the pump casing and forces it to the outlet. • Piston pump, water pumping depends on piston movement and two valves. While the piston moves it draws water into a chamber using an inlet valve, and expels it to the outlet using the outlet valve. The selection of an appropriate pump in a solar water pumping system depends on water requirement and water height. The sizing of a PVWP system depends on the expected hydraulic load. The typical head consists of a static and a dynamic components. The static head represents the vertical distance from the water surface level to the point of discharge, as shown in Figure 1 (A + B). During the pumping process, the water level drops (drawdown), which is represented by height C in Figure 1. Furthermore, another component that represents the frictional losses owing to pumping water through the pipe exists. This loss component can be reduced by oversizing the pipe, eliminating bends and reducing the flow rate. Thus, the total dynamic head is the sum of the static head, the drawdown distance and the distance representing the friction losses in the pipe (total dynamic head = A + B + C + friction losses) (Ghoneim 2006). 2.1.2. The motor There are several types of electrical motors that can be utilised to run the pump such as AC, DC, permanent magnet, brushed, brushless, synchronous and asynchronous, variable reluctance and many more. If DC motor is used then the PV array could be directly connected to the motor, however the brushes of the motor needs to be changed regularly. Using AC motor will require the use of an inverter between the PV and the motor. Normally, the motor and pump are built-in together for the submersible and floating pumping systems. In the surface pumping system, it is possible to select the pump and motor separately and evaluate their performance (Meah, Ula, and Barrett 2008). The size of the water pump and the hydraulic system is calculated based on the following equation (Chueco-Fernandez and Bayod-Rujula 2010): P= Figure 1. The typical head of the water pump. rg(h + DH)Q , hb he (1) 4 A. AL-BADI ET AL. where P is the pumping power in W; r is the density of water in kg/m3; g is the acceleration due to gravity (9.81 m/s2); h is the total pumping head in m; DH is the hydraulic losses in m; Q is the water flow rate in m3/s; hb is the efficiency of the pump and he is the efficiency of the motor. The hydraulic energy required per day (kWh) is calculated based on the following equation (Khatib 2010): Eh = rghV = hs E pv , (2) where V is the volume of water required in m3/day, hs is the subsystem (motor, pump and an inverter) efficiency and E pv is the PV energy. 2.2. The PV system 2.2.1. Solar radiation in Oman The global solar radiation values for 25 locations in Oman were discussed in Al-Badi et al. (2011). The average value for global solar radiation in Oman is found to be more than 5.0 kWh/m2/day. Global sunshine duration and solar radiation values for 25 locations covered almost all regions in Oman are presented in Figure 2. 2.2.2. System sizing The schematic diagram for a typical PV pumping system is presented in Figure 3. The PVWP system sizing involves determining the required size of the PV to satisfy the expected load requirements. It is standard to apply a safety factor for the PV size to compensate for the power losses owing to ageing, heat, dust, etc. (New York State, Energy Research and Development Authority 2012). The PV array power (Chueco-Fernandez and Bayod-Rujula 2010) can be calculated based on solar radiation energy as P pv = A pv Gr hr , (3) 2 where P pv is the PV power in W; A pv is the effective area of PV in m ; Gr is the solar radiation at reference temperature (1000 W/m2); and hr is the efficiency of PV at reference temperature (25 C°) Figure 2. Global sunshine duration and solar radiation values for 25 locations. INTERNATIONAL JOURNAL OF SUSTAINABLE ENERGY 5 Figure 3. Schematic diagram for a typical PV pumping system. The daily energy output (Chueco-Fernandez and Bayod-Rujula 2010) of the PV is determined from E pv = A pv GT h pv , (4) where GT is the daily solar radiation on PV surface (kWh/m2) and h pv is the efficiency of the PV under the operating condition. The area of the PV can be calculated from Equations (2) and (4) as A pv = rghV . GT hs h pv (5) Thus, the PV array size required in kW can be written as P pv = Eh , GT × F × E (6) where F is the PV mismatch factor (a safety factor for real panel performance) with the typical values range between 0.85 and 0.9 and E is the daily subsystem efficiency which has a typical value range between 0.2 and 0.6 (Chueco-Fernandez and Bayod-Rujula 2010). To compensate for the power losses due to heat, dust and ageing, the selected array size is calculated as 1.2 P pv . The overall efficiency of the PV water pump can be found from the output hydraulic energy and input solar radiation energy as ho = Eh rghV = . Ein GT A pv (7) 3. A case study A farm located near Sohar city (200 km away from Muscat city) is considered as a case study in this paper. The average value for global solar radiation in Sohar (24°34′ N, 56°73′ E) is 5.6 kWh/m2/day (Al-Badi et al. 2011). 3.1 Farm system sizing The size of the farm is 4.5 acres and is irrigated by several methods based on the type of crops (drip, flood and sprinkler). The farm has livestock and provides water supply to three houses. The pump is 6 A. AL-BADI ET AL. of centrifugal type and run by 11 kW induction motor. The motor pump set is grid connected. The well depth is 17 m, the static level is 13.5 m, the drawdown level is 2 m and the flow rate is around 1000 L/min. The existing pumping system is shown in Figure 4. In order to optimise the sizing of the PV, the hydraulic energy required per day should be determined. The parameter of the system is presented in Table 1. Using these parameters in the previous equations, the size of the PV array is found to be 5.8 kW. To compensate for the power losses due to heat, dust and ageing, the selected array size is calculated as 1.2(5.8) = 7 kW. The PV modules are available in different sizes. Using 250 W module with an area of 1.63 m2 gives the required number of modules to be 28 with total area of 46 m2. The specifications of the selected module are presented in Table 2 (Nafath Renewable Energy 2012). If there is surplus power production from the PV, then it can be sent back to the grid (feed-in tariffs) when the farm is connected to the grid system. If the farm is isolated then a battery can be utilised to store the excess power or a water storage tank can be used. 3.2. Evaluation of CO2 reduction The greenhouse gas (GHG) emissions in Oman increases owing to the increase in the total load demand that ranges from 8% to 10% annually due to population growth and economic development. To reduce the CO2 emissions, it is necessary to encourage the use of renewable energy resources especially for water pumping. In this section, the amount of CO2 emission reduction per one farm is calculated when the grid-connected or diesel-connected water pump system is replaced by the PVWP system. Table 3 presents the potential reduction in GHG emissions per MWh of electricity for natural gas and diesel generation facilities in Oman. These values are calculated based on the default emission factors, provided by UN’s Intergovernmental Panel on Climate Change (IPCC 2006), and considering a 10% transmission and distribution (T&D) losses and the efficiencies indicated (Authority for Electricity Regulation – Oman 2008). The annual energy consumed by a diesel generator or a grid-connected electric motor per farm can be calculated as Eg = 365Pt, (8) where P is the power required to pump water per farm which is 7 kW and t is the number of working hours of water pump per day per farm. The average working time of the water pump is assumed to be 7 hours/day (Al Mahmoudi et al. 2014). Figure 4. The pumping existing system. INTERNATIONAL JOURNAL OF SUSTAINABLE ENERGY 7 Table 1. System parameters. Parameter h G g r Q h pv hs V F E Value 18 m 5.6 kWh/m2/day 9.81 m/S2 1000 kg/m3 714 L/Minute 13% 50% 300 m3/day 0.9 0.5 Using the data provided in the last column of Table 3, the required annual energy per farm and the resultant CO2 emission based on the fuel type are calculated and shown in Table 4. Thus using the PVWP system will reduce the annual CO2 emission between 11,804 kg and 13,950 kg for each farm. 4. Modelling of PV pumping system The system presented in Figure 3 was modelled in the MATLAB/Simulink environment. Two different types of water pump are considered in the modelling, namely the DC motor and the AC motor. The model of the DC-powered pumping system with a solar panel directly connected to a DC motor is shown in Figure 5. The parameters of the used solar panel are: open-circuit voltage = 12 V and short-circuit current = 1.2 A. A 12-V DC motor with armature resistance and inductance of 2 Ω and 12 mH, respectively, and back emf constant of 0.8 × 10− 3 V/rpm is used in the simulation. Simulation results of the motor voltage and flow rate are shown in Figure 6 (a,b), respectively. At steady state, the motor input voltage (solar panel output) is 12 V with a 45% overshoot. The pump flow rate is 2.6 L/min. Simulink model of the AC-powered pumping system is shown in Figure 7. A singlephase inverter is included in the model to provide AC input voltage to the motor from the solar panel. In this model, we use the same PV panel of the DC system. The AC motor used is a singlephase, split-phase induction motor with the following rated values: power = 2 W, input voltage = 12 V (rms) and frequency = 50 Hz. The auxiliary winding of the motor is assumed to cut out of the circuit at 75% of the synchronous speed. The universal bridge of Simpower systems toolbox of the Simulink is used as a single-phase inverter. The motor voltage and flow rate are presented in Figure 8(a,b), respectively. It is clear that the PV panel is capable to supply the AC motor with 12 V rms and the steady flow rate reaches 0.9 L/min. Simulation results of both the DC- and AC-powered solar pumping systems are compared in terms of the maximum overshoot and settling time of the voltage and flow rate. The maximum overshoot is defined as the maximum deviation of the signal from its steady-state value. The settling time Table 2. Specifications of the PV. Parameter Type of modules Dimensions (m) Nominal peak power Maximum power voltage Maximum power current Short-circuit current Open-circuit voltage Optimised cell efficiency Operating temperature Maximum system voltage Power temperature coefficient Value Polycrystalline silicon (c-Si) 1.64 × 0.992 × 0.04 250W+3% 30.00 V 8.25 A 8.98 37.6 17.2% −40°C to +90°C 1000 V DC −(0.5 ± 0.05)%/K 8 A. AL-BADI ET AL. Table 3. Assumed properties for CO2 calculation. Fuel type Natural gas Diesel tCO2/MWh Efficiency tCO2/MWhElectricity 0.20196 0.26676 0.34 0.38 0.66 0.78 is defined as the time at which the signal reaches 98% of its steady-state value. The comparison is given in Table 5. 5. Experimental set-up and validation A laboratory-scale PVWP system is implemented experimentally to validate the simulated results. The physical layout of the proposed PVWP system is shown in Figure 9 and the experimental set-up is given in Figure 10. The functions of the main components of the proposed system are discussed in the following sections. 5.1. The PV arrays The PV panels are the main source of electric energy of the proposed system. Different types of PV panels are available with variety of specifications. Selection of solar panels rating is based on the rated voltage and current of the motor/pump set. Practically, solar panels are connected in series and/or parallel to form an array suitable to supply the required voltage and current. 5.2. The regulator The function of the regulator is twofold. It is used to protect the battery against overcharging and to prevent reverse current flow to the PV panel. A manual switch is used along with the regulator to have the options of feeding the system directly from PV panels or from the battery. 5.3. The controller The system is equipped with two sensors, namely water-level sensor and soil-moisture sensor. The water-level sensor is used to detect the water level in the reservoir and the soil-moisture sensor is used to detect the soil humidity. Based on the sensors’ reading, a program is developed to control the operation of the motor/pump set. The program is embedded in an ATmega32 microcontroller. 5.4. Motor/pump set In the proposed system, two different types of motor/pump sets are used. One is a DC motor/pump set and the other type is an AC motor/pump with inverter. 5.5. Inverter A single-phase inverter is used to convert the DC output voltage of the PV panels into AC voltage level suitable to run the AC motor. Table 4. CO2 emission calculation and annual energy consumption specifications of the PV. Fuel type Annual energy (kWh) Emission of CO2 (kg/year) Natural gas Diesel 17,885 17,885 11,804 13,950 INTERNATIONAL JOURNAL OF SUSTAINABLE ENERGY 9 Figure 5. Simulink model of DC-powered pumping system. Figure 6. Simulation results of DC water pumping system: (a) motor voltage and (b) flow rate. 5.6. Battery A battery is used for the purpose of storing the energy and as an alternative source of power in case of the cloudy weather. 5.7. Experimental results The data of the different components of the experimental set-up are given in Table 6. Experimental results of the DC pumping systems are shown in Figure 11(a,b). It is obvious that the designed prototype solar water pumping system is capable to pump water at a steady-state flow rate of 2.3 L/min with motor voltage of approximately 12 V. Comparing the results obtained from the simulation Table 5. Comparison between simulation results of DC- and AC-powered solar systems. System type Voltage maximum overshoot (%) Voltage settling time (s) Flow rate settling time (s) DC powered AC powered 45 18 0.08 0.15 0.2 0.4 10 A. AL-BADI ET AL. Figure 7. Simulink model of AC-powered pumping system. Figure 8. Simulation results of AC water pumping system: (a) motor voltage and (b) flow rate. model (Figure 6 (a,b)) and the experimental results we observe that the experimental flow rate is somewhat less than the simulated value because of the frictional loss due to bending of the pipe. The motor voltage reaches the steady-state value of 11.8 V with an approximate overshoot of 50%. Figure 9. Physical layout of PV water pumping system. INTERNATIONAL JOURNAL OF SUSTAINABLE ENERGY 11 Figure 10. Experimental set-up. Table 6. Data of different components of the PV pumping system. DC motor AC motor Battery Inverter PV panel Transformer 12 V–500mA, 1600 rpm, 3.8 L/min Mini DC submersible water pump 12V–200mA, 2.5 L/min Li-ion, 1.3Ah and 12 volts. 150 W, 10–15 V DC/110 V AC, 50 Hz 6 W, 500 mA, 12 V Frame size: 349 × 290 × 17 mm 110 V/12 V AC Experimental results of the AC pumping systems are shown in Figure 12 (a,b). From these results, we note that the steady flow rate is approximately 0.88 L/min and the rms voltage is 11.8 V, which agrees with the simulation results. A comparison between the maximum overshot and settling time of the experimental results for both systems is shown in Table 7. From the comparison given in Table 5 (simulation results) and Table 7 (experimental results), it is clear that the maximum overshot for the DC-powered solar system is much higher than that of the AC-powered solar system. The larger the maximum overshoot, the higher the voltage stress on the insulation between motor turns. This will affect the motor lifetime. On the other hand, the flow rate of the AC system reaches steady state slower than the DC system. Figure 11. Experimental results of DC water pumping system: (a) motor voltage and (b) flow rate. 12 A. AL-BADI ET AL. Figure 12. Experimental results of AC water pumping system: (a) motor voltage and (b) flow rate. Table 7. Comparison between experimental results of DC- and AC-powered solar systems. System type Voltage maximum overshoot (%) Voltage settling time (s) Flow rate settling time (s) DC powered AC powered 50 25 0.1 0.25 0.4 0.5 As far as the construction, the DC motor has the disadvantage of sparks that occur due to commutator and brush assembly. This phenomenon dictates regular maintenance of the DC motor. The AC motor, however, is a maintenance-free motor. In addition to these performance differences between the DC and AC systems, it is needless to say that the AC system requires inverter and transformer as additional components. 6. Conclusions Oman has high level of solar radiation with annual average value of more than 5 kWh/m2/day. Using PV water pumping for irrigation, livestock and drinking in remote areas becomes an attractive solution with the decline in price of the PV in the international market. A method used to size a PV pumping system for irrigation was presented and discussed. The system consists of PV modules, inverter, AC motor, well and tank. A case study for a small farm near Sohar city was selected and size of the PV panels required to run the water pump was calculated which depends on the amount of the hydraulic load. The decrease in the GHG emission, owing to the use of PV pumping system, was calculated for two possible cases. The first case is a diesel generator-connected pumping system used mainly in remote areas and the second case is a grid-connected pumping system. It was found that each farm in Oman can reduce the annual CO2 emission by at least 13,950 kg. This means that the PVWP system will not only improve the living conditions but it also protects the environment. A MATLAB models for the AC and DC pumping systems have been developed. The modelling approaches were validated by comparing the simulation results of both models with experimental results of DC- and AC-powered pumping systems. 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