PV based water pumping system for agricultural irrigation

Abstract

This paper investigates the operation and analysis of the photovoltaic water pumping system in detail. Power electronic controllers were designed and developed for the water pumping system using a boost converter along with an inverter followed by an induction motor pump set. The proposed system could be employed in agricultural irrigation under any operating condition of varying natures of solar irradiances and temperatures. The configuration and implementation of the system were described in detail. Further, the detailed method of analysis and simulation characteristics of such PV water pumping system was also presented. With the concern of shortage of fossil fuel, global warming and energy security, the proposed PV based water pumping system can meet the significant demand of electricity and serve for the agricultural sector.

Keywords

photovoltaic water pumping system / power electronic controller / solar irradiances and temperature

Introduction

Human beings need abundant amount of energy at an increasing rate for their sustenance and good living. Nowadays, this energy is mostly derived from fossil fuel such as coal, oil, natural gas and from nuclear power. But the above resources are not at all stable and reliable. The non-renewable resources like fossil fuel will last only about 50–75 years. The radioactive nuclear waste from the nuclear reactor causes many environmental and personnel hazards. In this era, other alternatives like renewable resources such as solar, tidal, wind etc. have to be considered [ 1– 3].

Of these resources, solar power is the most promising and abundant source of energy available in the universe. Today’s energy crises in the world can be resolved by the effective utilization of solar power. Solar power is the most environmental friendly form of energy available in the world. There are many ways of utilizing the solar energy such as water heating, space heating, space cooling, refrigeration, power generation, distillation, drying and cooking [ 4– 7]. Andria et al. proposed the design and characterization of solar-assisted heating plant for household application [ 6]. Out of these methods, the most efficient way of utilizing the solar power is the use of photovoltaic (PV) cells. PV cells are usually single or multi-crystalline wafers of silicon cells doped with impurity atoms, capable of converting the solar power to electricity. There are many researchers working in the field of solar PV cells. Hence, the price of PV panels is being reduced constantly from year to year [ 8]. This paper focuses on one of the application of solar PV cells, the solar water pumping system. PV water pumping is usually used in remote areas where the line electrification is difficult [ 9]. Solar energy is one of the free form of energy when the initial investments are made.

A solar PV pumping system uses the solar power for driving the pump to pump the water from well to a storage tank at ground for irrigation or drinking. The system consists of an array of solar cells which produces electricity. The generated power is in the form of low voltage which can be increased by using DC-DC converters for utilization. The high voltage output from the converter is used by the inverter to generate the AC power for driving the induction motor [ 10, 11].

Eker [ 12] divided photovoltaic water pumping systems into direct coupled photovoltaic pumping system and battery coupled photovoltaic pumping system. In the direct coupled system, solar energy obtained is directly used for pumping and the water is stored in the tank. This method of pumping is preferred only when the sun light is available. In the battery coupled system, the power obtained from PV is stored in the battery and then it is used for pumping at any time. So, the battery coupled system can be used both at daytime and at night for agricultural irrigation [ 12]. However, the battery coupled PV system is not a preferred option for agricultural sector due to high economic cost, complexity and the maintenance of the batteries. Hence, this paper proposed a simple system for water pumping without battery storage device.

Description of photovoltaic pumping system

The fundamental system consists of a photovoltaic generator, a converter system normally a boost converter, an inverter, a filter and a driving induction motor-pump set, as presented in Fig. 1. The closed loop system contains a controller circuit to improve the performance of the system. The array of solar panels renovates solar energy to electrical energy. The output power obtained from the PV is of a low voltage. The boost converter is employed in the system for boosting up the voltage from the low value to the desired value. The output DC voltage is converted to AC voltage by the sinuscidal pulse width modulation (SPWM) inverter. The output of the inverter obtained contains the DC component, which is filtered out by the LC filter. The capacitor in filter circuit reduces the ripples in the output voltage while the inductor is helped to achieve the sinusoidal current. The value of the inductor in the filter is reduced due to the RL nature of the load which is the induction motor connected to the inverter. Hence, the ripple free the output is supplied to the induction motor for driving the pump [ 11, 13, 14]. So, the performance of the system is enhanced by reducing the harmonic losses.

Photovoltaic generator

The photovoltaic generator consists of an array of PV panels, normally series and parallel connected cells, to obtain the desired voltage and current. The photovoltaic generator can be represented by a current source shunted with a parallel combination of diode and resistor (R_sh), which represents the leakage current and the leakage resistance represented by series resistance (R_se). Fig. 2 is a representation of the PV panel generator [ 15, 16].

Design

The volt-ampere equation of the photovoltaic system is given by

(1) I=I_{\mathrm{ph}}-I_{\mathrm{0}}\left({\mathrm{e}}^{\frac{V+I{R}_{\mathrm{s}}}{a{V}_{\mathrm{t}}}}-\mathrm{1}\right)-{\displaystyle \frac{V+I{R}_{\mathrm{s}}}{R_{\mathrm{sh}}},}

where V_{\mathrm{t}}={\displaystyle \frac{N_{\mathrm{s}}k{T}_{\mathrm{c}}}{q}}.

Temperature and irradiation correction factors

For standard values of temperature (T_c) and irradiation (S_c), the PV system can be designed as given in subsection 2.1.1. But in the PV system, the temperature and irradiation are not constant. They vary depending upon the climatic and environment condition. The output voltage and current of the PV system vary with the change in temperature and irradiation. The correction factors can be found as follows [ 15, 16].

The change in the output voltage and output current with the variation of temperature can be designated by the temperature coefficients C_TV and C_TI.

C_{\mathrm{TV}}=\mathrm{1}+{\beta }_{\mathrm{T}}(T_{\mathrm{a}}-T_{\mathrm{x}}),

C_{\mathrm{TI}}=\mathrm{1}+{\displaystyle \frac{{\gamma }_{\mathrm{T}}}{S_{\mathrm{c}}}}(T_{\mathrm{a}}-T_{\mathrm{x}}),

{{\displaystyle \beta }}_{\mathrm{T}}=\mathrm{0.004}, {{\displaystyle \gamma }}_{\mathrm{T}}=\mathrm{0.06.}

Any change in the cell irradiation can cause changes in photocurrent and cell temperature which can lead to the change in cell voltage. The change in output voltage and photocurrent with the change in irradiation can be represented by C_SV and C_SI while the change in temperature can be represented by DT_c.

{{\displaystyle C}}_{\mathrm{S}\mathrm{V}}=\mathrm{1}+{{\displaystyle \beta }}_{\mathrm{T}}{{\displaystyle \alpha }}_{\mathrm{S}}({{\displaystyle S}}_{\mathrm{X}}-{{\displaystyle S}}_{\mathrm{c}}),

C_{\mathrm{SI}}={\displaystyle \frac{\mathrm{1}}{S_{\mathrm{c}}}}(S_{\mathrm{x}}-S_{\mathrm{c}}),

\Delta T_{\mathrm{c}}={\alpha }_{\mathrm{S}}(S_{\mathrm{x}}-S_{\mathrm{c}}).

The new values of cell voltage and photocurrent by considering the changes in temperature and solar irradiation can be found by Eqs. (2) and (3).

(2) {{\displaystyle V}}_{\mathrm{C}\mathrm{X}}={{\displaystyle C}}_{\mathrm{S}\mathrm{V}}{{\displaystyle C}}_{\mathrm{T}\mathrm{V}}V,

(3) {{\displaystyle I}}_{\mathrm{p}\mathrm{h}\mathrm{x}}={{\displaystyle C}}_{\mathrm{S}\mathrm{I}}{{\displaystyle C}}_{\mathrm{T}\mathrm{I}}{{\displaystyle I}}_{\mathrm{p}\mathrm{h}}.

Boost converter

The power electronic converter illustrated in Fig. 3 is a DC-DC converter or boost converter, which boosts up the output voltage of the PV array. The boost converter consists of a MOSFET and a diode in the power circuit. The MOSFET is switched at a particular time period determined by the control strategy so that the required voltage is obtained at the output of the converter. The working principle, analysis and operation of the boost converter are described in subsection 2.2.1.

For the analysis of boost converter, the realisation of circuit using a simple switch is demonstrated in Fig. 4. In this paper, the analysis is conducted by considering a small ripple approximation, an inductor volt-second balance and a capacitor current balance [ 17, 18].

It is noted from Fig. 4 that there are two modes of operation in the boost converter. During mode 1, the MOSFET is switched ON. Then, the diode starts to conduct at mode 2 when the MOSFET is OFF. Now, let us consider the mode 1 operation of boost converter. The equivalent circuit of the converter in mode 1 is displayed in Fig. 5.

(4) {{\displaystyle V}}_{\mathrm{L}}={{\displaystyle V}}_{\mathrm{R}},

(5) i_{\mathrm{C}}=-{\displaystyle \frac{v}{R}}.

Equations (4) and (5) give the inductor voltage and capacitor current across the switch during mode 1 of the converter. With small ripple approximation v=V, it leads to

{{\displaystyle V}}_{\mathrm{L}}={{\displaystyle V}}_{\mathrm{g}},

i_{\mathrm{C}}=-{\displaystyle \frac{V}{R}}.

The equivalent circuit of the converter during mode 2 operation is depicted in Fig. 6. It is observed from Fig. 6 that the diode is forward biased so that it is short circuited.

(6) {{\displaystyle V}}_{\mathrm{L}}={{\displaystyle V}}_{\mathrm{g}}-v,

(7) i_{\mathrm{C}}=i_{\mathrm{L}}-{\displaystyle \frac{v}{R}}.

The inductor voltage and capacitor current of the MOSFET in mode 2 of the converter is given by Eqs. (6) and (7). With small ripple approximation,

{{\displaystyle V}}_{\mathrm{L}}={{\displaystyle V}}_{\mathrm{g}}-V,

i_{\mathrm{C}}=I-{\displaystyle \frac{V}{R}}.

The voltage across the inductor and current through the capacitor of the boost converter is exhibited in Figs. 7 and 8. By the inductor volt-second balance, the net volt-seconds are applied to the inductor over one switching period. Equation (8) gives the average value of inductor voltage,

(8) {\displaystyle {\int }_{\mathrm{0}}^{T_{\mathrm{s}}}{V}_{\mathrm{L}}(t)}d\mathrm{t}=V_{\mathrm{g}}D{T}_{\mathrm{s}}+(V_{\mathrm{g}}-V)D^{\text{'}{T}_{\mathrm{s}}}.

The average value of inductor voltage is equal to zero. So Eq. (8) is equal to zero and collecting terms

V_{\mathrm{g}}D{T}_{\mathrm{s}}+(V_{\mathrm{g}}-V)D\text{'}{T}_{\mathrm{s}}=\mathrm{0.}

By solving Eq. (8) there is

V={\displaystyle \frac{V_{\mathrm{g}}}{D\text{'}}\mathrm{.}}

The voltage conversion ratio can be written as expressed in Eq. (9),

M(D)={\displaystyle \frac{V}{V_{\mathrm{g}}}}={\displaystyle \frac{\mathrm{1}}{\mathrm{1}-D}}.

.

Three phase inverter

An inverter is a power electronic circuit which converts the DC at a high voltage to the AC for driving the AC induction motor pump set. For industrial applications, three phase inverters are more common than single phase inverters. The commonly used three phase inverter is a six-step bridge inverter with six switches. Each switch is fired at an interval of 60°. The upper switches named S₁, S₃, and S₅ are fired at an interval of 180°. Similarly, for lower switches named S₄, S₆, and S₂ are also fired at an interval of 180°. Hence, each switch is gated at an interval of 60° in proper sequence to produce the three phase voltage at the output of the inverter [ 19].

Figure 9 shows the schematic diagram of the three phase inverter using MOSFETs. By using the six switches, there are eight combination switching states in the operation. By properly synthesising the switching states, the fundamental component in the output of the inverter can be increased. This process is known as pulse width modulation (PWM). In PWM, the pulses which are given to the inverter are modulated with carrier signal to obtain the output of the inverter.

PWM methods are quite popular in industrial applications of inverters. Different types of pulse width modulation techniques are employed in industry. The classification of PWM techniques are presented in Fig.10.

Normally, sinusoidal pulse width modulation (SPWM) and SVPWM techniques are employed in PV based water pumping system application. The switching frequency has been chosen at 10 kHz for reducing the harmonics in output voltage of the inverter.

SPWM

The most commonly used modulation method is SPWM. In SPWM, the width of the pulses is sinusoidally modulated. For realising SPWM, a high frequency carrier wave is compared with a low frequency sinusoidal wave and gating pulses are produced. The intersection point of these waves provides firing instant or the commutation instant of the devices of the inverter.

In the SPWM technique, the high frequency carrier and low frequency sinusoidal is given to the comparator. Then, the comparator produces output pulses when the sinusoidal wave is greater than the triangular carrier wave. The width of the output pulses so obtained is sinusoidally modulated. The typical waveforms of carrier, reference and gating pulses are shown in Fig. 11.

SVPWM

The SVPWM technique can generate suitable gate pulses for each PWM cycle. This technique can be easily applied to higher power levels and it works efficiently with all high level inverters. The basic idea is the vectorial representation of a three phase system. In this technique, the three phase system can be represented into two dimensional rotating vectors at a particular instant. The revolving mmf in the machines is a well known example of space vector [ 20– 23].

The SVPWM produces different switching states. By the proper implementation of these switching states, pulses with modulated width are obtained. Then, the pulses are used to trigger the inverter switches. SVPWM provides unique switching time calculation for each switching states.

In the inverter, there are six switches so that eight possible combinations can be achieved. The combinations are represented as the switching states of the inverter. For the sake of simplicity, the ON states of upper switches are represented by 1 and the ON states of lower switches are represented by 0. The combinations are 000, 001, 010, 011, 100, 101, 110 and 111. Among these, the vectors 000 and 111 are represented by zero vectors (V₀). The remaining vectors are known as active vectors represented by V₁–V₆.

The representation of these 8 possible combinations (8 vectors) in the ab-plane is exhibited in Fig. 12. The zero vector is represented in the horizontal axis and the active vectors are represented with 60° phase difference between the adjacent vectors. This is due to the fact that each switch in the inverter is fired at a phase difference of 60°. The switching pulses obtained in SVPWM technique are shown in Fig. 13.

Results and discussion

Simulation of photovoltaic generator

The simulation is performed on a SOLKAR 36 W photovoltaic module characterized with the Matlab platform. The electrical characteristics and details of the SOLKAR 36 W PV module are listed in Table 1 [ 24, 25].

The simulation results obtained with a constant irradiation and varying temperatures are given in Figs. 14 and 15. Figure 14 shows the V-I characteristics of the module with a constant irradiation at different temperatures. The test is conducted with a constant irradiation of 1000 W/m² at 25°C, 50°C and 75°C. It is noted from Fig. 14 that the short circuit current of the module increases and the open circuit voltage of the module decreases when temperature rises.

Figure 15 shows the P-V characteristics of the module with a constant irradiation at different temperatures. The test is conducted with a constant irradiation of 1000 W/m² and at varying temperatures of 25°C, 50°C and 75°C. As the operating voltage increases, the power reaches the maximum and then starts to decrease. As temperature increases, the maximum power obtained reduces as well as the voltage at which the maximum power obtained also shifts from the right to the left side of the plot.

The results obtained using varying irradiation and constant temperatures are given in Figs. 16 and 17. The V-I characteristics of the module with a constant temperature and varying irradiation conditions are shown in Fig. 16. The test is conducted at a constant temperature of 25°C and varying irradiations of 1000 W/m², 600 W/m² and 200 W/m². As the irradiation increases, both the short circuit current and open circuit voltage increase.

Figure 17 shows the P-V characteristics of the module at a constant temperature and varying irradiations. The test is conducted at a constant temperature of 25 °C and varying irradiations of 1000 W/m², 600 W/m² and 200 W/m². It is noted from the characteristics that the power increases, as the voltage increases and reaches a maximum at a particular voltage and then it starts to decrease.

The results with varying temperatures and irradiations are shown in Figs. 18 and 19. For the analysis, three irradiations of 400 W/m², 1000 W/m², 600 W/m² and three temperatures of 40°C, 60°C, 50°C are taken into consideration. Figure 18 shows the three V-I characteristics obtained with 400 W/m² and 40°C, 1000 W/m² and 60°C, 600 W/m² and 50 °C. Figure 19 shows the three P-V characteristics obtained with 400 W/m² and 40°C, 1000 W/m² and 60°C, 600 W/m² and 50°C. It is evident from the characteristics that the PV module delivers a maximum output when the irradiation is maximum and temperature is minimum. Figure 20 shows the PV cell voltage and current along with corresponding output power of the PV panel.

Simulation of power electronic converters

The boost converter is connected to the PV array to boost up the input voltage. The simulation of the boost converter is performed in a Matlab environment and the results are presented in Fig. 21. The test is performed with an input voltage of 12 V and a duty ratio of 0.6. It is noted from Fig. 21 that the voltage is increased to 30 V.

The inverter is a power electronic circuit which converts DC to AC. The simulation of the proposed system along with a three phase inverter fed induction motor drive is performed in a Matlab environment. The three phase output current and output voltage of the inverter is obtained and presented in Fig. 22.

Conclusions

A detailed investigation of a PV water pumping system was conducted and presented in this paper. The detailed working and analysis of the various components of the proposed PV water pumping system was described. The simulation of the PV water pumping system was performed using the Matlab simulation tool and their results are furnished. The results confirm the satisfactory working of the system under varying solar irradiances and temperatures. The present paper provides a significant solution for farmers in remote or rural areas where electrification is not available. Photovoltaic water pumping is also one of the best solutions for future energy crisis in the agriculture sector. The analysis and development of robust control strategies for the system gives the future scope of this paper.

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