4th International Conference on Power Engineering, Energy and Electrical Drives
Istanbul, Turkey, 13-17 May 2013
Design and Implementation of a Low Cost Solar PV Backup System for Low Reliability Grids Ahmed Sanaullah, Nauman Qaiser and Hassan A. Khan# Department of Electrical Engineering, Lahore University of Management Sciences (LUMS), Lahore Cantt 54792, Pakistan #
[email protected]
Abstract— Electricity is the driving force in a country’s economy and the developing world needs reliable low cost electricity for its rapid growth. In light of the challenges faced by these countries, we have proposed a low cost PV based back-up system for critical loads in low grid reliability regions. In addition, the system can also be used in remote places where no access to grid is available. The proposed system extracts maximum available power from PV generator by employing a simplified MPPT algorithm, implemented through a low cost 8-bit microcontroller. The control mechanism ensures that the load and the system components are protected from sudden failures and fluctuating grid power. The control system also maintains the battery at a specified state of charge ensuring a longer battery life and an increased reliability of the system. The whole system was implemented and tested for up to 500W loads which showed highly satisfactory results. Keywords; PV, low cost, MPPT
I.
INTRODUCTION
Energy provides impetus to a country’s growth. Sustained economic growth of a country crucially depends on the longterm availability of energy from sources that are affordable, accessible and eco-friendly [1]. Due to urbanization and increase in population, the global demand of energy is ever increasing. It is estimated that the global energy demand will increase at the rate of 1.7% per year and the demand will reach 16.5 billion tons of oil equivalents (TOE) by 2030 [2]. Conventional resources, based on fossil fuels, are depleting and the costs of these fuels are continuously increasing. The solution lies in abundant and indigenous renewable energy resources which can effectively meet the energy demand of a developing country. Electricity generation from PV has traditionally been expensive with price in the range of 5$/W around a decade ago. However, in recent years a decreasing trend is observed for the price of PV modules with cost of as low as $0.7/Wp in 2011 [3]. The overall system cost of a grid-tied PV systems, including panels, electronics, parts and labor, has decreased to $1.67/Wp [3]. However, for developing countries it is imperative that indigenization of power electronic components occur at a low cost to keep the overall cost down [4]. In accordance with the utility of solar PV in developing courtiers, we have designed and implemented a low cost system which could lower the system cost even further. All the necessary control mechanisms have been implemented along
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with simplified implementation of Maximum Power Point Tracking (MPPT) for efficient operation. This type of system is very pertinent for the developing world where households have limited or no access to electricity. Therefore, this kind of technology could be highly beneficial for rural electrification projects as well as other stand-alone applications. II.
METHODOLOGY
A stand-alone photovoltaic (SOPV) system comprises of PV modules, charge controllers, battery bank, inverter and the control mechanism for the overall systems as shown in fig 1. This work is aimed at providing low cost optimum solution for regions which either have no access to the grid or have an unreliable and intermittent grid. Rural residential homes are rarely equipped with under/over voltage protection devices or timing devices that would protect appliances from rapid series of outages. The requisite system is, therefore, developed to work well in these regions. It is well-established that mass manufactured inverters cost around 0.05$/W. In addition, Valve-Regulated lead-acid batteries are used in PV systems which are cost effective and reliable if kept at a requisite state of charge (SOC). The actual design challenges include the implementation of a low cost MPPT system and a control mechanism what would control power output to the load while protecting the system and home appliances from sharp outages. This control should also ascertain that batteries are properly monitored and required SOC is maintained and the system should cease operation at low-battery conditions. A. Control System/Controller Circuit The control system includes designing and implementing a circuit that would monitor the power from the gird and the state of the battery bank. In an event of failure of grid it will power the load using the PV generator. To protect from sharp outages, the system involves a slight delay in transfer of power which not only provides adequate protection for the load but also ensures a prolonged life of the low cost inverter as sudden
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Figure 1: Basic topology for a back-up system incorporating power from both solar PV and the grid.
and rapid switchovers between inverter and utility may lead to sparking inside the main relay and can potentially damage the inverter. A delay is therefore a compulsory part of the algorithm and Figure 2 shows an overview of the controller mechanism implementation. The entire control implementation is based on low cost discrete components and a single quad operational amplifier chip is used to manage all operations inside the controller.
Istanbul, Turkey, 13-17 May 2013
B. Low Battery Latch The low battery latch is implemented through operational amplifier as shown in Figure 3. This particular implementation allows for the battery voltage to be compared with a reference voltage level. A feedback loop is implemented to change the reference level immediately if the battery voltage crosses that reference. Once the system experiences a battery low situation, and the inverter is shut down, it will not instantly turn back online due to rise in open terminal voltage of the battery. As soon as the comparator saturates towards zero volts, the reference voltage at opamp’s negative input rises. This allows for a variable reference value and allows for the battery to be charged to another specified voltage before it can start delivering power to the load.
Reference Voltage
Battery Voltage
R3
+ _
Output
LM 324
R2 R1
Figure 3: Implementation of Low battery latch.
Figure 2. Control mechanism implemented for the system.
The controller for the system implements safety features for generator and the load attached. A small transformer is used to continuously monitor the utility supply. In presence of utility supply, the load is powered using the grid. In the event of utility supply failure, a timer is initiated that waits for a period of 5 seconds (configurable) before the inverter is turned on. When utility power becomes available again, the inverter is turned off and another timer initiated that would wait a further 5 second before the load resumes its power from utility. In this way, any sharp fluctuations in the utility power will not potentially affect the system components. At every point in time, a battery sensor senses the battery voltage, if the battery voltage is below a specified low voltage disconnect (LVD) level, the inverter is shut down. Once latched into low battery state, the inverter will not be turned on unless the panels charge the battery up to a specified battery level at which inverter is turned on automatically again if utility is still offline.
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C. Timer Implementation The timer functionality is also implemented through same IC LM 324 as shown in fig 4. This form of a mono-stable timer allows us to implement timing feature while still utilizing only a single op-amp chip to implement all the functionalities of the control system. The timer operates by charging a timing capacitor in the RC circuit. Once the capacitor’s voltage rises above the reference, the op-amp saturates. For an instant reset of the timer, the PNP transistor allows for a discharge of C1 through a resistor. Utility Sensor R1 Reference Voltage
+ _
Timer Output
LM324
R2 + _C
R3
Figure 4.Implementation of timer circuit for turning the back-up power ON in case of a grid failure.
III.
LOW COST MPPT IMPLEMENTAION
MPPT charge controllers are complex devices that are designed to maximize the power output from a PV module. They employ several techniques in hardware and software to
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4th International Conference on Power Engineering, Energy and Electrical Drives
perform the task with varying degrees of efficiency. Some of these rely on real time processing in a closed feedback loop in which the point įPPV/įVPV= 0 is tracked by the system, which is the global maxima of the curve between PPV (output power of the PV module) and VPV (output voltage of the PV module). Examples of such techniques are the Perturb and Observe (P&O)[5-6] and the Incremental Conductance[7-8]. Other novel methodologies [9] involve estimating the position of the Maximum Power Point (MPP) based on the offline characteristics of the PV Module and thus avoid real time processing. Real time techniques are popular because of the efficiency with which they are able to track the MPP. However, due to the complex mathematical operations involved, a high end microcontroller or equivalent is required which increases the cost of the processing hardware. Techniques requiring lesser processing such as [9] face a trade off in terms of the error they face which adds restrictions on the type of system with which they can be used. In this work, we present a technique of tracking the MPP that utilizes a semi real time approach which is aimed at availing the efficiency of a real time algorithm without incurring the associated cost in hardware and mathematical complexity. A. DC-DC Conversion MPPT controllers convert the output dc-voltage of the PV Module to a desired value while conserving the total power in this process. To achieve this, we have employed a single switch buck converter [10]due to the reduced hardware complexity it offers and is shown in Figure 5. In order to drive the converter, an 8 bit microcontroller is utilized to generate a Pulse Width Modulated (PWM) signal. Operating at a frequency of 8MHz, the microcontroller is able to generate a PWM of 31.25 kHz using an 8 bit Counter according to eq. (1).
R1 L
PV MODULE
C RSHUNT
Istanbul, Turkey, 13-17 May 2013
ଵ
The minimum step in duty cycle in this case is ൈ ͳͲͲΨ ൌ ଶହ ͲǤ͵ͻͳΨ . The fact that the generated PWM is of a relatively lower frequency than those typical of buck converters (50-100 KHz) means that the high speed switching limitations of NPN transistors can be ignored. This implementation is cost effective as no expensive driver IC or complex bootstrapping circuit is needed to implement the switching. Traditionally, NPNs have been used in inverters and other power applications in most developing countries and thus are both readily available and less expensive than other possible alternatives, for example, Power MOSFETs and IGBTs. B. Voltage and Current Sensing Sensing the voltage and current values is a major aspect of an MPPT as these values are used to calculate the power being drawn from the PV panel. Low-cost ADCs require a reference voltage (usually in the range of 5V) higher than the input voltage of the panel. Therefore, VPV is scaled down to a lower voltage through a potential divider and is fed into ADC. Current sensing is somewhat more complicated. Typically, shunt resisters of known values are used and the potential difference across the resistor is measured and then scaled by the resistor value to obtain the value of current. Shunt resisters are typically expensive and so an alternate method of sensing current is employed. It should be noted that the potential difference across RSHUNT is merely the node voltage and knowing the exact value the shunt resistor is not imperative. In our MPPT algorithm, the circuit tracks the maximum value of power relative to the value of power at any other duty cycle. Therefore, the absolute value of power at any point is not important and therefore instead of an expensive shunt resistor, any ordinary piece of wire could be used. However, the length of this shunt should be short to ensure that the resistance of the wire was relatively constant over a large variation in current. In addition, the resistance of the wire should be higher than that of a normal copper wire to ensure a potential drop significant enough to be easily registered by the ADC. Therefore, a new quantity “Relative Power” (PRELATIVE) is defined that is to be maximized by the MPPT circuit. PRELATIVE is given by eq. (2) ܲ௧௩ ൌ ܸ ൈ ܸ௦௨௧
A/D
PWM
A/D
x 8 Bit Microcontroller Figure 5. MPPT Structure implemented for the system. An input filter capacitor is also employed parallel to PV module but not shown in the figure for simplicity.
݂ௐெ ൌ
ೀೄಽಽಲೀೃ
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ଶ
ൌ
଼ ଶఴ
ൌ ͵ͳǤʹͷ݇ݖܪ
(2)
Figure 6(a) shows the path taken by the current when the switching transistor was switched ON while Figure 6(b) shows the current path when the switching transistor was switched OFF. In the ON stage, the current supplied by the PV Cell is used to charge both the battery and the capacitor. This current leads to an increase in VSHUNT and a decrease in VPV. In the OFF stage, no current flows from the PV Module which means that VPV reached its peak value while VSHUNT became zero.
(1)
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4th International Conference on Power Engineering, Energy and Electrical Drives
through the system, at this voltage, is zero which represented by a minimal value of VSHUNT. As d increases, the output voltage of the PV generator is reduced which is manifested in the form of higher drop at RSHUNT (due to increased current in the circuit).
R1 L
PV MODULE
Istanbul, Turkey, 13-17 May 2013
C
30
RSHUNT
0.6 25
20 0.4
A/D
x 8 Bit Microcontroller
15
10
0.2
Vshunt(V)
PWM
VPV(V)
A/D
5
Figure 6(a)
0
0.0 1
10
100
Duty Cycle, d(%)
Figure 7: Variation of VPV and VSHUNT with varying duty cycle d in MPPT implementation.
R1 L
PV MODULE
1.00
C Relative Power, PR(W)
0.75
RSHUNT A/D
PWM
A/D
x 8 Bit Microcontroller
0.50
0.25
0.00 1
In order to compute VPV and VSHUNT at any duty cycle, eq. (3) and eq. (4) are used. ௨௧௬௬ ଶହ
ܸ௦௨௧ ൌ
ൈ ൛ܸǡ ൟ
௨௧௬௬ ଶହ
ሺଶହହି௨௧௬௬ሻൈ୫ୟ୶ሼುೇǡ ሽ
ൈ ሼܸௌுே்ǡ ሽ
ଶହ
ሺ͵ሻ
(4)
Where VPV,in is the instantaneous value of the terminal voltage of the PV generator at the given duty cycle and VSHUNT,in is the instantaneous value of the scaled current through the PV generator at the given duty cycle.VPV is the weighted average of the terminal voltage of the PV generator at the given duty cycle and VSHUNT is the weighted average of the scaled current through the PV module at the given duty cycle. Figure 7 illustrates the variation of VPV and VSHUNT with the duty cycle (d). It can be seen that the output voltage of the panel at low d is actually its open circuit voltage. The current
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Figure 8. Variation of power output from the panel with the varying duty cycle of the switch.
Figure 6. Current flow paths when the switched (a) ON (b) OFF.
ܸ ൌ
10
Duty Cycle, d(%)
Figure 6(b)
Figure 8 shows the normalized plot of PRELATIVE against duty cycle. The generated power from the generator has a unique global maxima which is stored in the microcontroller. The controller ensures that the value of this d is maintained until another global maximum arrives for different input sunlight or ambient conditions. C. Implemented Algorithm The proposed close-loop algorithm, shown in fig 9, is simple to implement as does not require any expensive processing hardware unlike most real time algorithms that require complex computations and high end microcontrollers and processors. The algorithm works by performing a full scan over the entire duty cycle and computing the power at each point. At each value of duty cycle, after computing the power, it compares it with a variable that contains the maximum power computed thus far. If the current power is greater than the value stored, the variable is updated with the value of the current power and the duty cycle at that point is stored.
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4th International Conference on Power Engineering, Energy and Electrical Drives
Istanbul, Turkey, 13-17 May 2013
in switching between power through the PV and the utility thus reducing the problems which could occur due to sharp gird outages (a common occurrence in low reliability grids). The Validity of the system was verified through loads up to 500W which showed very satisfactory results. ACKNOWLEDGMENT The authors would like to thank Mr Saad Pervaiz for his valuable technical input to this study. The authors would also like to thank the support from ‘Mitsubishi Endowment Fund’ and the Cleaners Product Institute (CPI) for this project. REFERENCES [1]
[2] [3] Figure 9.Implemented algorithm in the microcontroller for MPPT implementation.
After the scan, it sets the duty cycle to value which yielded the MPP and goes into a wait state. This duty cycle is maintained for the duration of the wait state before the scan is repeated. IV.
[4]
OVERALL SYSTEM
The entire system is shown in figure 10 which incorporates all the control features and necessary features at the output. The system was tested in all possible power input conditions and it showed highly satisfactory results for up to 500W loads.
[5]
[6]
[7]
[8] Figure 10.Implemented system prototype with control and MMP mechanism.
[9]
CONCLUSION A Low cost PV-based system was presented in this paper. The design was based on a simple control circuit and the MPPT implementation was done through low cost 8-bit microcontroller with simple PWM enable timer/counter as the algorithm did not require extensive processing for achieving MPPT. This system could be highly useful for low-cost low power PV applications in remote regions or where the grid is not reliable. The proposed control system incorporated delays
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[10]
R. Janarthanan, et al., "Development of low cost, efficient residential utility interface for modular PV system with improved power quality," in TENCON 2004. 2004 IEEE Region 10 Conference, 2004, pp. 179-182 Vol. 4. K. H. Solangi, et al., "A review on global solar energy policy," Renewable and Sustainable Energy Reviews, vol. 15, pp. 2149-2163, 2011. W. D. Grossmann, et al., "Distributed solar energy generation across large geographic areas, Part I: A method to optimize site selection," Renewable and Sustainable Energy Reviews, 2012. H. A. Khan and S. Pervaiz, "Technological review on solar PV in Pakistan: Scope, practices and recommendations for optimized system design," Renewable and Sustainable Energy Reviews, Vol. 23, pp 147-154, 2013. E. Koutroulis, et al., "Development of a microcontroller-based, photovoltaic maximum power point tracking control system," Power Electronics, IEEE Transactions on, vol. 16, pp. 46-54, 2001. D. Sera, et al., "Optimized Maximum Power Point Tracker for Fast-Changing Environmental Conditions," Industrial Electronics, IEEE Transactions on, vol. 55, pp. 2629-2637, 2008. K. Yeong-Chau, et al., "Novel maximum-powerpoint-tracking controller for photovoltaic energy conversion system," Industrial Electronics, IEEE Transactions on, vol. 48, pp. 594-601, 2001. L. Fangrui, et al., "A Variable Step Size INC MPPT Method for PV Systems," Industrial Electronics, IEEE Transactions on, vol. 55, pp. 2622-2628, 2008. V. Scarpa, et al., "Low-Complexity MPPT Technique Exploiting the PV Module MPP Locus Characterization," Industrial Electronics, IEEE Transactions on, vol. 56, pp. 1531-1538, 2009. B. Divakar and D. Sutanto, "Optimum buck converter with a single switch," Power Electronics, IEEE Transactions on, vol. 14, pp. 636-642, 1999.
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