0D[LPXP 3RZHU3RLQW7UDFNLQJXVLQJ +\EULG 3HUWXUE 2EVHUYHDQG,QFUHPHQWDO &RQGXFWDQFH 7HFKQLTXHV Bilal Masood, M. Shahzad Siddique, Rao M. Asif, M. Zia-ul-Haq Department of Electrical Engineering, Superior University, 15-KM Raiwind Road Lahore Email:
[email protected],
[email protected],
[email protected],
[email protected] m Abstract- The development of renewable energy has been an increasingly critical topic in the 21st century with the growing problem of global warming and other environmental issues. With greater research, alternative renewable sources such as wind, water, geothermal and solar energy have become increasingly important for electric power generation. Although photovoltaic cells are nothing new, their use has become more common, practical, and useful for people worldwide. Although each cell output is relatively low voltage, if many are connected in series, a solar photovoltaic (PV) module is formed. Although the price for such cells is decreasing, making use of a solar cell module still requires substantial financial investment. At a given temperature and insolation level, PV cells supply maximum power at one particular operation point called the maximum power point tracking (MPPT). However, the MPP locus varies over a wide range, depending on PV array temperature and insolation intensity. Instantaneous shading conditions and aging of PV cells also affect the MPP locus. In this research paper two reformed algorithms have been proposed for MPPT, the most widely used ones are the ‘Perturb and Observe’ (P&O) and the Incremental Conductance algorithms. The duty cycle of the converter will be controlled, so that the source will send maximum power to the load. The research paper presents a new reformed technique of harvesting MPP from PV cells after evaluating the previous ones. All the previous techniques were thoroughl y studied before making any reform in the traditional MPP techniques. Both techniques have their own significance at different situations of weather. In this paper new technique is introduced by using both of above P&O and Incremental Conductance. Keywords – Perturb and Observe (P&O), Maximum Power Point Tracking (MPPT), Photo Voltaic Cell (PV cell)
,
Unlike other sources solar cell need a specific method to extract maximum power from it. The PV systems when connected directly to the load result in overall poor efficiency where as such MPPT should have been introduced in PV systems that increase the efficiency of the system. Solar radiations, load impedance and module temperature are the three factors which affect the maximum power extraction from solar PV module. Unlike conventional energy sources, it is desirable to operate PV systems at its MPP. Thus, to make a PV module useful, it is necessary to extract as much energy as possible from such a system. To achieve operation at the MPP, a time varying matching network is required that interfaces the varying source and possibly the varying load. The role of this matching network, called the MPPT network, is to ensure operation of the PV array at its MPP, regardless of atmospheric conditions and load variations. I-V curve of PV module is a function of insolation and temperature which affects output current and voltage. The increased temperature decreases the open circuit voltage (Voc) while increased intensity of solar radiations increases short circuit current (Isc). The concept of MPPT is to monitor the terminal voltage and current continuously and update the control signal accordingly to achieve maximum power point (MPP). A DC/DC convertor with MPPT algorithm is used between PV module and load to extract maximum available power. An example of MPPT with a typical cell of voltage and current is shown in figure below. It shows where the maximum power point lies for that typical cell is.
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Solar cell when invented by a French scientist in 19th century than at that time its efficiency was not good. That time solar cell was only used in different type of lab experiments. Anyhow when further research was carried out on solar cell than its efficiency became 6% in 1954. At that time solar was 1st used for space applications. Later in 1980’s researcher show their huge interest in solar cell efficiency improvement. Now a day’s solar cell efficiency is up to 25%. Here we notice that further research is needed to improve its efficiency to its maximum possible value. When we come to the advantages of solar energy we conclude here some of them here, they are non-polluting with no detectable emissions or odours. They can be stand-alone systems that reliably operate unattended for long periods. They require no connection to an existing power source or fuel supply. They may be combined with other power sources to increase system reliability. They can withstand severe weather conditions including snow and ice. They consume no fossil fuels - their fuel is abundant and free. They can be installed and upgraded as modular building blocks - as power demand increases; more photovoltaic modules may be added. D 5ROHRI0337 LQ6RODU(QHUJ\6\VWHP
Figure 1: T ypical forward bias I V characteristics of a PV cell
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Here is the mathematical expressions of PV cell output current, voltage and current generated by PV cell at typical temperature and isolation level [1]. ݍൈ ൫ܸ௩ ܫ௩ ܴ௦ ൯ ቋ െ ͳ ܫ௩ ൌ ܰ ܫ െ ܰ ܫ ቈ݁ ݔቊ ܰ௦ ܶܣܭ
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Above expressions results can be verified only if there is no shading effect on PV Cell while operating. But the value deviate a lot when shading effect happens while operating on PV Cells [2]. The PV array is normally shaded by disturbances like branches of trees, passing clouds, poles and buildings, etc., which result in partial shading of PV systems as shown in Figure 2. In a PV array the cells configured in series will render constant current whereas the shaded cells will operate with a reverse bias resulting reverse power polarity leads to net power drop and thereby reducing the net power conversion efficiency [3].
In the above figure one cell shaded and it current source becomes zero after shading [5]. Voltage drop across ܴ ݄ݏas current flows through it causes the diode to be reverse biased so diode current is also zero. It means that entire module current will pass through shaded cell of Rs and Rsh and it output actually reduces instead of adding voltage in series [6]. It mean that the output of entire module Vsh with one cell shaded will drop to …here Rp=Rsh ܸ ݄ݏൌ ܸ݊െͳ െ ܫሺܴ ܴ ݏሻ...... (i) With all n cells in the sun and carrying I, the output voltage was V of the bottom n-1 cell will be ݊െͳ
ܸ݊െͳ ൌ ሺ
݊
ሻܸ...... (ii)
Combining (i) & (ii) ݊െͳ
ܸ ݄ݏൌ ቀ
݊
ቁ ܸ െ ܫሺܴ ܴ ݏሻ…… (iii) ଵ
οܸ ൌ ܸ െ ܸௌு ൌ ܸ െ ቀͳ െ ቁ ܸ ܫሺܴ ܴ௦ ሻ...... (iv)
οܸ ൌ
ܫሺܴ ܴ௦ ሻ….. (v)
Since the parallel resistance Rp is so much greater than the Rs, so expression becomes following οܸ ؆ ܴܫ …… (vi)
Figure 2: PV system under partially shaded condition
Figure 4: Shading impact on one cell in n-Cell mode
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Figure 3: Circuit model of an array consisting of n series connected array
Here we see in the above figure that we need a bypass diode to nullify the effect of PV cell acting as a reverse biased diode and drop all the potential across it, when it is partially shaded [4]. If there no bypass diode considered in the above diagram than we will describe an expression here below.
There are number of MPPT techniques available for power harvesting from PV Cells, which were proposed by different researchers. When we thoroughly studied many techniques we noticed that these techniques have disadvantages as well as after advantages [7]. For example fuzzy logic control, artificial neural network and Particle Swarm method, these techniques are expensive and not easy to implement. There are two main techniques P&O and Incremental Conductance are used for this purpose, these are not only cheaper techniques but also easy to implement and have remarkable efficiency [8]. But there is one problem in both these techniques, these are for two different weather conditions, i.e. for slow and fast weather changing conditions. The conventional P&O algorithm is easy to implement and is most commonly used in battery charging with
commercial PV modules. In this method, the operating voltage or current of the PV module, is perturbed and then the power obtained is observed to decide the direction of further changes in the voltage or current [9]. If the power is increased by the perturbation then voltage or current is kept on changing in the same direction until the power begins to fall. The algorithm measures the instant voltage (Vt) and current (It) to calculate the power (Pt) and then compares it with last calculated power (Pt-1) [10]. The algorithm continuously perturbs the system if the operating point variation is positive, otherwise the direction of perturbation is changed. Here is the flow chart for the conventional P&O algorithm.
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Referring to (viii), the solution of (vii) is zero at MPP, positive on the left of the MPP and negative on the right of the MPP. So, (vii) can be rewritten as οܫ
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οܫ οܸ
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ܸ
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ܽ ܲܲܯ݂ݐ݂݈݁ݐǥ ǥ ሺሻ
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Thus, MPP can be tracked by comparing the instantaneous conductance to the incremental conductance [16]. It is the same efficient as P&O, good yield under rapidly changing atmospheric conditions. Here, also the same perturbation size problem as the P&O exists and an attempt has been made to solve by taking variable step size [17]. But, it requires complex and costly control circuits.
Figure 5: Flow Chart of P&O Algorithm
The second one is the conventional Incremental Conductance (INC) method. The incremental conductance algorithm of MPPT was developed by K. H. Hussein, I. Muta, T. Hoshino and M. Osakada, however the concept technique was developed by O. Wasyneczuk. They used derivative of conductance to determine the maximum power point (MPP) [11]. The MPP is determined by comparing instant conductance I/V to the incremental conductance ΔI/ΔV and the INC technique is based on the fact that slope of P-V curve is zero at MPP as shown in Figure 4. This algorithm performs better than P&O algorithm in rapidly varying environment and is robust to the rapidly varying solar radiation [12]. The MPPT speed and accuracy was improved by introducing automatically adjustable variable step size to conventional INC technique. When MPP is far from operating point, the step size is large for fast tracking while during operating point closer to MPP, the step size becomes small to reduce steady state oscillation[13]. The algorithm is modified and the derivative of resistance (dV/dI) is used in place of derivative of conductance [14]. The modified algorithm is variable step-size incremental resistance (INR) algorithm is based on the fact that slope of P-I curve is zero at MPP, positive on the left of MPP, and negative on the right of MPP [15]. For a PV system, the derivative of panel output power with its voltage is expressed as
Figure 6: Flow chart of Incremental Conductance Algorithm
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Here we see that P&O algorithm looks at dP/dV>0 and in the same way INC algorithm looks for dI/dV>-i/v. and somehow they appears to be equivalent [18]. We know that P&O algorithm looks for increase or decrease in overall in power while in the same way INC algorithm looks for wether change in current over voltage (i.e. change in inductance ) [19]. However when we compared both these two algorithm, we noticed that both these two algorithm have different behaviour while harvesting power from PV cell. Here we see their behaviour in Figure 7,8,9.
Figure 7: PV voltage output
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