Impact of shadow on the performances of a domestic photovoltaic pumping system incorporating an MPPT control: A case study in Bejaia, North Algeria

Impact of shadow on the performances of a domestic photovoltaic pumping system incorporating an MPPT control: A case study in Bejaia, North Algeria

Energy Conversion and Management 84 (2014) 20–29 Contents lists available at ScienceDirect Energy Conversion and Management journal homepage: www.el...

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Energy Conversion and Management 84 (2014) 20–29

Contents lists available at ScienceDirect

Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman

Impact of shadow on the performances of a domestic photovoltaic pumping system incorporating an MPPT control: A case study in Bejaia, North Algeria A. Mohammedi, N. Mezzai, D. Rekioua ⇑, T. Rekioua Laboratoire LTII, Universite de Bejaia, 06000 Béjaia, Algeria

a r t i c l e

i n f o

Article history: Received 2 January 2014 Accepted 3 April 2014

Keywords: Pumping system Maximum power point tracking Photovoltaic system Partial shadow

a b s t r a c t Photovoltaic (PV) solar energy is the solution combining economy and efficiency for the supply of stand alone systems. This combination can only be achieved by taking into account the effects of shading which have dramatic consequences on the electrical power delivered. In this paper, we present an experimental study of the effect of shading PV array on a pumping system performance. The experimental bench is installed at the Industrial Technology and Information Laboratory (LTII) in Bejaia (Algeria). In order to test the performances of the proposed system we propose different array configurations which show different behaviors against partial shading conditions. Shadow impacts fundamentally the global PV pumping system production; its influence is difficult to model because it depends on many parameters such as the configuration of the PV array, the relative rate of shadow, and the shaded area of the module. In general, it is better to have a completely shaded string than several partially shaded ones. These are ones of the most important conclusions obtained in this work. Ó 2014 Elsevier Ltd. All rights reserved.

1. Introduction The photovoltaic process is a technology in which radiant energy from the sun is converted to direct current (DC) energy [1]. In photovoltaic (PV) systems, multiple modules are generally interconnected in series and/or parallel to create a system with the desired voltage and loading current capacity [2–13]. Therefore, partially shaded condition is sometimes inevitable because some parts of the module or the PV system may receive less intensity of sunlight due to clouds or shadows from trees, buildings and other neighboring objects [14–15]. It is well-known that partial shading of PV arrays can over proportionally reduce the system’s output power; it has been identified as a major reason for reducing the energy yield of grid connected PV systems. The low output of partially shaded solar cells in a PV module leads to the operation of a bypass diode and results in the reduction of the maximum power in the PV module. The PV pumping has become one of the most promising fields in photovoltaic applications. To achieve the operation’s most reliable and most economical; more attention is paid to their design and their optimal use [16–19]. In the literature, several studies have addressed the photovoltaic pumping systems with different methods and location. ⇑ Corresponding author. E-mail address: [email protected] (D. Rekioua). http://dx.doi.org/10.1016/j.enconman.2014.04.008 0196-8904/Ó 2014 Elsevier Ltd. All rights reserved.

Authors in Ref. [20] have adopted a simulation program to assess the performance of PV pumping systems in the Kuwait climate. They demonstrated that an optimum system parameter search can greatly enhance the performances of a PV pumping system to achieve the required water demand for living in a remote area. The determination of an optimum and an adequate photovoltaic array configuration to supply a DC pump with an optimum energy amount under the outdoor conditions of different sites, has been studied by [8-25]. They concluded that for optimal energy exploitation and an efficient use of the PV pumping system, the design is required and to match the maximum power points of the PV array to the DC pump, an electronic array reconfiguration controller should be included to select the appropriate configuration. In this context, a complete methodology for designing a PV solar system to operate underground water pumping for the selected well-34 was carried out by [26], which showed that using solar energy in the south of Jordan is very practically and theoretically applicable, and is even preferable to other conventional types of energy. Refs. [18,27-29] have been working on the parameters that affect the good operation of photovoltaic pumping systems such as solar radiation, pumping head PV array size, and it has been shown through the results obtained that the variations in one of these parameters leads to a variation in the system performances. Work cited above were done under a uniform radiation, other researchers have introduced the idea of partial shading of the PV

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Nomenclature A DC Em E Eref G ht I Iph Ipv Is K KE KT L M MPP MPPT N Np Ns P1,2,3 Phyd

ideality factor of the junction direct current electromotive force (V) insolation in the panel plane (W/m2) reference insolation (1000 W/m2) acceleration due to gravity (980,665 m/s2) total head (m) armature current (A) photocurrent of the PV generator (A) photovoltaic current (A) cell reverse saturation current (A) Boltzman’s constant (J/k) voltage constant torque constant armature coil inductance (H) mass flow rate (l/s) maximum power point maximum power point tracking ideal constant of diode (1–2) number of parallel modules number of series modules constants parameters of the module pump equivalent mechanical output power

generator. Authors in Ref. [30] propose a novel mathematical formulation for the optimal reconfiguration of photovoltaic arrays to minimize partial shading losses. To obtain a technique to configure the modules in the array so as to enhance the generated power from the array under partial shading conditions, authors in [31] use the Su Do Ku arrangement to reduces the effect of shading of modules in any row thereby enhancing the generated PV power. Partial shadowing is a major reason for the energy yield reduction of photovoltaic systems. Its early detection is important, not only due to the immediate power reduction of the PV array, but also to protect the shadowed cells from long-term exposure to increased temperature [32], in this way [33,34] developed a novel procedure for fault diagnosis in PV systems such as fixed object shading with an estimate of distance to the object, small localized dirt, possible hot-spots, module degradation, generalized dirt and cable losses. Few or no studies, taking account the shadow impact on domestic PV water pumping systems performances integrating an MPPT control; have been found in the literature. This lack in literature motivates our study, where the main objective is to observe the behavior of domestic PV water pumping system in several shading scenarios that may arise at any moment and estimating the power and volume losses that can induce this shadow. As illustrated in this work, performances tests with configuration ‘‘A’’ and performances tests with configuration ‘‘B’’ were carried out. In these cases, performances of the system such as output power, flow rate, pumping time, volume losses, power losses, and system efficiencies were compared and analyzed. Obtained experimental results are presented and discussed.

2. Total and partial shadow Unlike thermal panels that can tolerate some shadow, PV modules are very sensitive. The shading of one of the modules constituting a PV array can cause a marked decrease in the production of the entire array. It is therefore important to take into account all the shadows, including those generated by relatively small obstacles.

PMDC PV Ppv q Qv R Rse Rsh S Tem Tj Tjref TL V Vd Vpv

x q npv npump ntot

permanent magnet direct current motor photovoltaic photovoltaic power (W) elementary charge (C) flow rate (m3/h) armature resistance (O) series resistor (O) shunt resistor (O) module area (m2) electromagnetic torque (N m) junction temperature of the modules (°C) reference module temperature (25 °C) load torque (N m) applied voltage (V) effective volumetric displacement of the pump (l/rad) photovoltaic voltage (V) angular motor speed (rad/s) density of water (1000 kg/m3) photovoltaic array efficiency (%) pumping efficiency (%) total system efficiency (%)

– Partial shadow prevents only direct radiation from reaching part of the PV cells (building). Partial shadow effects can also be alleviated through PV array configurations, system architectures, and converter circuit topologies [14]. – Total shadow prevents any radiation from reaching a part of the PV cells or the module (Cover panel).

3. Identity of the site Bejaia city is located 181 km east of the capital Algiers (north Algeria). Situated on the Mediterranean coast with a coastline of over 100 km, it is crossed by the great river Soummam [16], the geographical coordinates of Bejaia are: Longitude 5°040 E, Latitude 36°430 N, Altitude 2 m. The northern part of Algeria has a Mediterranean climate and forms 4% of the country area [13]. Despite its small size, this region is inhabited by more than ninety percent of Algeria’s population, mainly because it is the most fertile region [13]. The average annual rainfall in Bejaia ranges from 800 to 1100 mm [16].

4. Proposed system description Fig. 1 shows a synoptic scheme of the PV pumping system used in this work. This solution is more economical and eco-friendly due to the elimination of the electrochemical storage. The electrical energy storage is replaced by water storage in elevated tank which can have many uses. It includes a PV array of 990 W with 55 W for each module connected to each other into three parallel strings; each string consists of six series of modules, the controller which integrated a maximum power point tracking (MPPT) is used to control the pump system and monitoring of the operating states and speed control. The last part is a helical rotor pump (positive displacement pump) which is driven by a permanent magnet DC motor (PMDC).

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Tank Controller With MPPT

flowmetre

11 m

PV array

Well DC motor pump

Fig. 1. The proposed system.

   Iph ¼ P 1 :Es : 1 þ P2 E  Eref þ P3 T j  T jref

ð2Þ

where Iph photocurrent of the PV generator (A), is cell reverse saturation current (A), n ideal constant of diode (1–2), q the elementary charge (C), k Boltzman’s constant (J/K), A ideality factor of the junction, Tj junction temperature of the modules (°K) and Rse, Rsh (O) resistors (series and shunt), E insolation in the panel plane (W/m2), Eref corresponds to the reference insolation of 1000 W/m2 and Tjref to the reference module temperature of 25 °C. P1, P2 and P3 are constants parameters. Table 1 shows the parameter of the PV module SOLARA SM220S/55M used in this study.

Fig. 2. The equivalent circuit of PV cell.

5. Mathematical model 5.1. PV array modeling There are several mathematical models which describe the Ipv–Vpv characteristic [6]. The equivalent circuit (Fig. 2) consists of a single diode for the cell polarization phenomena and two resistors (series and shunt) for the losses. The standard model is the most commonly used. The current source Iph generates a current proportional to the amount of light falling on the cell [12]. Based on this circuit model, the behavior of the PV array with Ns and Np modules model may be described by Eq. (1) [7]:

    q: V pv þ Ipv :Rs V pv þ Rs :Ipv 1  Ipv ¼ Iph  I0 exp AN s kT j Rsh

ð1Þ

The photocurrent Iph is directly depending on both insolation and panel temperature, and may be written in the following form [3]:

5.2. Maximum power point tracking A major challenge in the use of PV is posed by its nonlinear current–voltage (I–V) characteristics, which result in a unique maximum power point (MPP) on its power–voltage (P–V) curve. The

Table 1 Parameter of the PV module SOLARA SM220S/55M. Parameters

Values

Maximum power Current at maximum power point Voltage at maximum power point Short-circuit current Open circuit voltage

55 W 3.1 A 17.6 V 3.4 A 21.3 V

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efficiency, it is imperative to operate the PV source at MPP so that maximum power can be extracted [17]. Implementation of PV system with inappropriate strategy will result in low efficiency. Thus, PV power management strategy including energy conversion system is a very important part in efficiency improvement [22]. However In order to allow the load working at the maximum supplied power of PV generator, a controller for a better matching PV-load around the optimal power is needed [7]. The integrated electronic unit gives the PS1200 system a number of advantages compared to others products. One of these advantages is the built-in microprocessor with MPPT. Thanks to the MPPT function, the pump duty point is continuously optimized according to the input power available [8]. Fig. 4 shows the current/voltage and power/voltage characteristics represented for different irradiance and temperature, using the experimental values obtained with experimental method (Fig. 3) corresponding to SM220S/M55 SOLARA module of 55 Wc. Simulations results unser Matlab/simulink are also presented in Fig. 4. The scheme of MPPT under Matlab/Simulink is given in Fig. 5. We obtain the following results (Fig. 6).

5.3. Permanent magnet DC motor modeling Fig. 3. Experimental bench for electrical characteristics with PS1200 system.

Many PV water pumping systems employ DC motors because they could be directly coupled with PV arrays and make a system very simple. Among different types of DC motors, a PMDC motor is preferred in PV systems because it can provide higher starting torque [9]. A schematic diagram of the DC motor used is shown in Fig. 7.

3

50

2,5

40

Power (W)

2 1,5 1

30 20 10

0,5 0

0 0

5

10

15

20

25

0

30

10

30

20

Voltage (V)

Voltage (V)

(a) Experimental results 50

3

45

E=708W/m2, T=21.1°C E=493W/m2, T=16.3°C E=292W/m2, T=14.6°C

2.5

E=703 W/m2, T=21.1°C E=496 W/m2, T=16.3°C E=292 W/m2, T=14.6°C

40 35

2

Power (W)

Current (A)

Current (A)

matter is further complicated due to the dependence of these characteristics on solar insolation and temperature. As these parameters vary continuously, MPP also varies. Considering the high initial capital cost of a PV source and its low energy conversion

1.5 1

30 25 20 15 10

0.5

5 0

0

5

10

15

20

25

30

0

0

5

10

15

Voltage (V)

Voltage (V)

(b) Simulation results Fig. 4. Ipv(Vpv), Ppv(Vpv) electrical characteristics.

20

25

30

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P

Es

703

puiss

Constant3 -C-

Tj

V

IG

Constant1 VG

i

PV model

XY Graph

Vpv

1/c3 il

Vdc Idc

Ipv MPPT t Clock

To Workspace

Fig. 5. Bloc diagram of PV model with MPPT.

The equations that describe the motor electrical components are as follows [21,24]:

V ¼ Em þ I:R þ L

dI dt

efficiency under low power conditions [3]. The major disadvantage associated with helical rotor pumps in solar applications is stiction which leads to increased starting torques and later start up times. The equivalent mechanical output power of the pump is calculated as [10]:

ð3Þ

where V is the applied voltage, Em is the motor e.m.f., I is the armature current R is the armature resistance, L is the armature coil inductance.

E m ¼ k E :x

Phyd ¼ m:g:ht ¼ nt :Pmot ¼ nt :x:T L

where m is the mass flow rate (l/s), ht is the total head (m), nt is the overall efficiency of the pump. The load torque of the pump is now given by:

ð4Þ

where kE is the voltage constant and x is angular motor speed. The electromagnetic torque is shown below:

T em ¼ kT :I

ð6Þ

TL ¼ ð5Þ

m:g:ht V d :g:ht ¼ nt :x nt

ð7Þ

where Vd is the effective volumetric displacement of the pump (1/rad).

where kT is the torque constant. 5.4. Helical rotor pump modeling

6. System efficiencies These pumps have the ability to operate efficiently over a wide speed range, whereas the efficiency of centrifugal pumps deteriorates away from the rated peed [10]. As compared to the centrifugal pump the positive displacement pump presents a better

The PV array efficiency npv is given as a ratio between the operating electrical power and the incident power radiation received on the surface of the PV array:

3

50 45

2

E=708W/m2, T=21.1°C E=496W/m2,Y=16.3°C E=292W/m2,T=14.6°C

40

E=703W/m2, T=21.1°C E=496W/m2, T=16.3°C E=392 W/m2, T=14.6°C

35

Power (W)

Current (A)

2.5

1.5 1

30 25 20 15 10

0.5

5

0

0

5

10

15

Voltage (V)

20

25

30

0

0

5

10

15

Voltage (V)

Fig. 6. Electrical characteristics Ipv(Vpv), Ppv(Vpv) under simulation with MPPT.

20

25

30

A. Mohammedi et al. / Energy Conversion and Management 84 (2014) 20–29

I

Table 2 Description of different cases-first scenario.

R

V

L

Em

Ppv Es :S:Ns :Np

ð8Þ

The pumping subsystem efficiency npump is defined as the ratio between the hydraulic power and the operating electrical power of the subsystem. The hydraulic power depends on the water flow rate and the total head. The equation of pumping subsystem efficiency npump is given as follows:

npump ¼

q:g:Q v :h 3600:Ppv

Description

Case Case Case Case Case Case Case

Without shading Shading of two panels in the first string Shading of four panels in the first string Shading of all the first string Shading of all the first string and two panels in the second one Shading of all the first string and four panels in the second one Shading of the two strings

(0) (1) (2) (3) (4) (5) (6)

Cases

Description

Case (a) Case (b) Case (c)

Shading of one panel in each string Shading of two panels in each string Shading of three panels in the first string and two panels in the others Shading of three panels in the first and second string and two panels in the third one

Case (d)

due to gravity, 3600 is the number of second per hour and Qv is the water flow rate (m3/h).

ð9Þ

The total efficiency of the PV pumping system is defined as the product of the efficiencies of the PV array and the pumping subsystem:

ntot ¼ npv :npump

Cases

Table 3 Description of different cases-second scenario.

Fig. 7. Schematic diagram of the permanent magnet DC motor.

npv ¼

25

ð10Þ

where Ppv is the operating electric power of the system (W), S is the area of one module (m2), q is the density of water, g is the acceleration

7. Methodology of the tests Shadow problems are absolutely crucial in regard to the nonlinear nature of the relationship between the shadow and production losses. For the same percentage of shading on a PV array, the impact can vary from 0% to 100% depending on where the shadow takes form and the topology of the module circuit in the PV array. For this purpose two configurations applied on an 18 modules

Fig. 8. Case (1) of configuration ‘‘A’’.

Fig. 9. Case (a) of configuration ‘‘B’’.

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Table 4 Overall system performances with configuration ‘‘A’’ E = 650 W/m2, T = 26.4 °C. Config. (A)

Vpv (V)

Ppv (W)

Ppump (W)

Qv (l/min)

npv (%)

npump (%)

ntot (%)

Case Case Case Case Case Case Case

113 110.4 110.2 109.3 93.3 92.9

316.4 306.9 305.2 296.2 237.9 232.2 –

312.4 297.2 293.6 287.0 227.7 224.7 –

43.66 42.55 42.44 42.33 36.33 36.15 –

5.82 5.65 5.62 5.45 4.38 4.27 –

22.29 22.39 22.45 23.08 24.66 25.14 –

1.299 1.265 1.262 1.259 1.080 1.075 –

(0) (1) (2) (3) (4) (5) (6)

Table 5 Overall system performances with configuration ‘‘B’’ E = 650 W/m2, T = 26.7 °C. Config. (B)

Vpv (V)

Ppv (W)

Ppump (W)

Qv (l/min)

npv (%)

npump (%)

ntot (%)

Case Case Case Case Case

113 95.5 74.6 72.8

316.4 238.7 171.6 163.8 –

312.4 228.3 161.9 152.9 –

43.66 36.49 28.57 27.90 –

5.82 4.39 3.16 3.01 –

22.29 24.69 26.89 27.51 –

1.299 1.085 0.849 0.830 –

(0) (a) (b) (c) (d)

(b) Water flow

(a) Pumping power

(d) Photovoltaic efficiency

(c) Pumping efficiency

(e) Total efficiency Fig. 10. Pumping power, flow and efficiencies under different cases with configuration ‘‘A’’ E = 650 W/m2, T = 26.4 °C.

A. Mohammedi et al. / Energy Conversion and Management 84 (2014) 20–29

array with three strings connected in parallel, each string consists of six modules connected in series. 7.1. Configuration ‘‘A’’ In this configuration six cases of shadow were considered, Table 2 and Fig. 8 shows the scheme adopted for configuration A. All other modules of the array are at maximum illumination. 7.2. Configuration ‘‘B’’ Fig. 9 shows the second scenario with uniform shading across the three strings. Four shading situations were considered (Table 3). All other modules of the array are at maximum illumination. 8. Experimental results and discussions In order to test the performance of the proposed PV pumping system under a partial shadow, this section was considered to practically analyze and compare the different results of PV array configurations. The total head was fixed at 11 m and the motorpump subsystem was supplied by the PV generator configurations

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described above. We pump a volume of 500 L for each configuration for irradiance of 650 W/m2. Currents, voltages, power, flow rate, pumping time well as irradiance and temperature were measured and summarized respectively to each configuration in Tables 4 and 5. The tests were held for 33 days in June and July 2013. Figs. 10 and 11 show some results of experiments performed on the system described with configuration ‘‘A’’ and ‘‘B’’ respectively. A significant shadow on a PV array can reduce 50% of the production of PV modules; this effect can be illustrated in Figs. 10 and 11, the presence of two inflection points makes the MPP search difficult. Regardless of the point that it will identify, it will not correspond to the real point of the unshaded module, which inevitably decrease the global production system. Regarding the cases (6) and (d), the pump was unable to start because the generated voltage 70.7 V (Table 4) and 65.1 V (Table 5) respectively, is low compared to the range of operating voltage of the system (72–96 V). We remark that in the case of modules arranged in series and in parallel; if the shadow completely obscures some strings of the PV array, the impact will vary with the number of affected modules and also makes difficult to find the MPP. Otherwise, if the shadow partially obscures all strings of the PV array, the impact on power is more important which is demonstrated by the power losses of our

(b) Water flow

(a) Pumping power

(d) Photovoltaic efficiency

(c) Pumping efficiency

(e) Total efficiency Fig. 11. Pumping power, flow and efficiencies under different cases with configuration ‘‘B’’ E = 650 W/m22, T = 26.7 °C.

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Fig. 12. Influence of the number of shaded panel on the water flow.

Fig. 15. Comparison between case (3) and case (b).

PV performance and flow rate. To pump 500 l at a height of 11 m, there is a difference of 5 min and 51 s between the pumping time of the two cases (3) and (b), this value represents a loss in the volume of water about 247.5 l equivalent to 49.5% if the configuration (b) was used to supply the system. The positive displacement pump presents a better efficiency under low power conditions this feature is validated by the subsystem efficiencies in the case (b), which is higher than the obtained one in the case (3). In general, it is better to have a string completely shaded than several partially shaded strings.

Fig. 13. PV power losses in the two configurations.

system shown in Fig. 13. These losses affect directly and proportionately the flow rate and all system efficiencies in the two configurations (Figs. 12 and 14). Fig. 15 shows a comparison between the different performances of the system with two different configurations and the same number of shaded modules. We see that in case (3) the generator produces more than in case (b) This can be explained by the presence of bypass diodes that protect cells, by becoming conductive the diode blocks the use of the series of shaded cells and the current will be transferred to the next series allowing other modules of the string to continue to operate. This difference in power results in a similar gap on the

9. Conclusion To increase the economic viability of PV pumping systems and for a better integration of this system, the problem of shadow on photovoltaic generators should be taken into consideration. Most modules are now equipped with bypass diodes to minimize the effect of shadow (and to protect cells) but these effects are significant. In this work a complete system consisting of PV array with controller integrated an MPPT, PMDC motor and helical rotor pump was presented and tested. We have presented the mathematical models of the various components of our system. In order to test the performance of the proposed PV pumping system we proposed different array configurations which show different behavior against partial shading condition and a comparative study was carried out. Shading impacts fundamentally the global PV pumping system production, its influence is difficult to model because it depends on many parameters such as the configuration of the PV array, the relative rate of shading, and the shaded area of the module. It will be necessary to choose a suitable location for the installation of photovoltaic system while minimizing the presence of shading. If obstacles around a photovoltaic system cannot be avoided, the design of photovoltaic generator and the introduction of an MPPT control will help mitigate the impact of these obstacles on the performance of the photovoltaic system especially domestic photovoltaic pumping systems. An electronic controller can be used to reconfigure the generator according to the shaded part.

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Fig. 14. System efficiencies in the two different configurations.

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