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Proc. of Int. Conf. on Control, Communication and Power Engineering 2010

277

2009 ACEEE

Design Considerations of a Solar Powered StreetLight for Stand -Alone PV Systems

Mini Rajeev, Sreedevi .S. Nair, and Seema JadhavDepartment of Electrical Engineering, Fr .C .Rodrigues Institute of Technology, Vashi, NaviMumbai

[email protected],[email protected],[email protected]

Abstract This paper deals with the basic designconsiderations of a stand alone solar powered street lightingsystem using Light Emitting Diode as the light source.Selection of all the components of the solar powered streetlight which includes Photovoltaic panel, Battery, DC/DCconverter, Microcontroller and the Light detection circuitare discussed in detail. LED lighting, besides longer life andlower power consumption, compared to CompactFluorescent Lamp is considered to be absolute greentechnology.Use of solar powered street light reduces theglobal consumption of electricity, CO 2 emissions anddependence on fossil fuels.

Index Terms - PV panel, MPPT, Street Light, Boostconverter, LED.

I. I NTRODUCTION

Taking care of the environment is an issue that belongsto all mankind as every individual is the custodian of our environment. The rate at which the conventional energysources are diminishing is creating an alarming concern.

Not only these sources are diminishing but also theycontribute to greenhouse gas emissions thereby pollutingthe environment. India ranks sixth [1] in the world intotal energy consumption and the demand for energy isexpected to increase to keep pace with its developmentobjectives. In order to meet the increasing demand of

electrical energy and simultaneously reduce negativeenvironmental impacts, it is very important to reduce thedependence on non sustainable, nonrenewable fossil fuelsand to incorporate renewable energy as a source of electrical generation.

Renewable energy is the energy from a source that isreplaced by a natural process and is not subject todepletion in a human time scale. Most renewable formsof energy except geothermal and tidal energy, ultimatelycome from the sun [2].Among the renewable energysources, solar energy is the most important as it is clean,nonpolluting, inexhaustible and free. Photo Voltaic (PV)systems offer the possibility of exploiting suns energyavailable everywhere. The main applications of PVsystems are either stand-alone applications such as water

pumping, domestic lighting, street lighting, electricvehicles, and space applications or grid-connectedconfigurations such as hybrid systems and power plants.

A standalone PV system is the one which is notconnected to the power grid and consists mainly of a PVarray for converting sunlight to electricity, Battery for energy storage, DC/DC converter for interfacing the PV

panel and battery and Microcontroller for controlling theoperation of the DC/DC converter. If the load requires dc

power, battery can supply the power directly and if AC power is needed, battery can be connected to the load

through a DC/AC converter (i.e inverter) as shown in thegeneric block diagram of fig. 1.

V

I

Figure 1. Generic block diagram of the system

II. PV PANEL CHARACTERISTICS

The V-I and P-V characteristics of a 75W p, 12V PV panel which was tested at 28 oC at 11.30 am in the monthof January with variable resistance load [3] is shown infig. 2. It is clear from Fig.2, that the current delivered (I)

by the panel remains relatively constant over a widerange of voltage and the panel output power (P) will bemaximum when the voltage across the terminals of PV

panel is 15 V.

Figure 2. Characteristic of PV panel

This means that, with this particular operatingcondition of temperature and insolation, only if the panelis operated at this voltage of 15V, maximum power can

be drawn from the PV panel. By doing this, theconversion efficiency of the PV panel is improved.

III. NEED FOR MAXIMUM POWER POINT TRACKING

In the generic block diagram in fig. 1, if the PV panel is directly connected to battery without usingDC/DC converter, the PV panel will be forced to operate

PVArray

Inverter

DC/DCConverter

ACLoad

DCLoad

MicroController

Battery

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at the battery voltage. The PV panel is considered as acurrent source and hence voltage at its terminals dependson the load connected across the PV panel. Therefore if the panel is operated at the battery voltage which is notnecessarily the maximum power point (MPP) of the PV

panel, some of the power-generating capability of the PV panel is lost. Any deviation from this MPP will reduce

the conversion efficiency of the PV panel.In order to extract the maximum power which the PV panel is capable of delivering, an electronic system calledMaximum Power Point Tracking (MPPT) system [4] isrequired. Since the MPP of the PV panel depends on theinsolation, ambient temperature etc, MPPT mustconstantly adapt to maximize the power transfer from thePV panel. So a DC/DC converter is interfaced betweenthe PV panel and the battery for MPPT. With theincreased use of PV systems and in view of their highcost, more attention is given to the design of MPPT tooptimize the utilization of PV panel so that the size andcost of the entire system can be reduced.

IV. CONTROL ALGORITHM

Various methods have been proposed in the literatureto implement the maximum power point tracking [5] insolar powered applications. The MPPT typically uses aniterative process to find the maximum power point.Themost widely used methods are the Perturb & Observe (P& O) method and the method based on the IncrementalConductance (dp/dv method).

Figure 3. PV characteristics with dp/dv algorithm

A common problem in P&O algorithm is that the PV panel terminal voltage is perturbed every MPPT cycle;therefore when the MPP is reached, the output power oscillates around the maximum, resulting in power loss inthe PV system. Since the incremental conductancemethod offers good performance under rapidly changingatmospheric conditions that method is considered in this

paper. The dp/dv method is based on the fact that theslope of the P-V characteristics of the panel is zero at theMPP, positive on the left of the MPP and negative on theright as shown in fig. 3[6]. Once MPP has been reached,the operation of PV panel is maintained at this point untila change in operating condition is noticed.

Figure 4. Flow chart of the dp/dv algorithm

Flow chart of the dp/dv algorithm is given in fig. 4.The micro controller sends an arbitrary value of dutycycle to the DC/DC converter to initiate the process. Atfirst, it detects voltage and current values from the PV

panel and multiples it to obtain the power (P=V*I). Thenit gets the next values and calculates power. The present

power (P) and voltage (V) are subtracted from their valueat previous instants and thus sign of the slope, dp/dv isdetermined. Depending on the sign of the dp/dv slope, theduty cycle is decremented or incremented, therebychanging the voltage conversion ratio of the DC/DCconverter. This forces the panel to operate at the voltagewhere it delivers the maximum power.

To locate the operating point on PV curve, dynamicstep size concept is used here. In this, the step size isdetermined based on the sign of three previous iterationsof dp/dv. This helps in tracking MPP faster, therebyimproving transient response of the dc/dc converter.Another advantage of using dynamic step size is that theoperating point will not oscillate much near MPP.

V. DESIGN DETAILS

The bock diagram of the stand-alone PV system withLED lamp is shown in fig. 5. When sunshine is there,the solar panel charges the battery through the DC/DCconverter with the help of suitable light detection circuit.The same light detection circuit disconnects the batteryfrom the DC/DC converter and connects it to the loadduring the night time. The street light has dusk to dawnoperation for about 10hrs per day.The MPPT is mostcommonly constructed out of DC/DC converters that usePulse Width Modulation (PWM) technique to vary theduty cycle of the switch used in it. This duty cycle of theDC/DC converter is controlled by the microcontroller, sothat the PV panel gives the maximum power outputwhich it can deliver at that specific atmosphericcondition.

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Figure 5. Block diagram of the system

Design aspects of various components of the systemshown in fig. 5, are discussed below.

A. PV panel sizingSizing a PV system is, a quite complex issue because

several stochastic parameters having a significantcontribution, are involved like the meteorological data,variation of demand on daily and seasonal bases, andeconomical considerations.Mainly the panel sizing [7]depends on the total system load, which meansdetermination of wattage of the unit and usage hours of load. Here we are considering 18W lamp load which will

be used for 10hrs daily and hence total daily load becomes 180Whr. PV Panel Load= Wh per day/ ( B* C). (1)PV Array Size= PV Panel Load / (Insolation *0.85). (2)

The PV Panel Load and PV Array Size is calculated by (1), and (2) respectively where B is the batteryefficiency ( 80%) and C is the charge controller efficiency(90%) and 0.85 is the mismatch factor. The

average insolation during the least sunny days is5.2KWh/m 2/day in India [7]. From (1), and (2), a75W p, 12V solar PV panel is chosen. Table 1 shows datagiven by the manufacturer for 75W p, 12V PV panel. Therated peak power (75W p), is available only at StandardTest Conditions (STC) such as cell temperature of 25 oCand insolation of 1kW/m 2.

TABLE ISPECIFICATIONS OF 75W P, 12V PV PANEL (STC)

Sl. No.

Parameters Values

1 Short circuit current 5A2 Open circuit Voltage 21V3 Maximum Power 75W

4 Maximum Power Voltage

17V

5 Maximum Power Current

4.4A

6 No. of solar cells 36

B. Battery sizingStreet Lighting systems typically use either lead

acid or alkaline batteries because of their high energydensity to cost ratio and ability to function in a widerange of temperatures [8]. Car batteries cannot be used asthey are not designed for repeated deep discharges.

Nickel-Cadmium batteries can be deeply discharged

many times without harm and are less affected bytemperature changes than lead-acid batteries, but aremore expensive and very expensive to recycle. As a result,their use is primarily restricted to applications wheremore reliability and low maintenance is required. In India,with PV systems the most commonly used batteries arelead-acid batteries.

Battery storage capacity is generally rated in ampere-hours (Ah). This is the amount of current that a batterywill deliver over a given number of hours at its normalvoltage and at a temperature of 25C. Battery sizing isdone depending on the the voltage of the battery, thewattage of the load, the duration of operation and theambient temperature of the batteries.

Battery Sizing = (Daily Load *2) /(Nominal Voltage*0.5)(3) In the calculation two days of reserve backup is

considered. Based on (3), 24V, 30 Ah lead acid batteryis selected for the application. The value 0.5 is themaximum allowable discharge of the battery. Thisenhances the life of the battery.

C. DC/DC Converter The converter configuration is chosen based on the

ratings of the PV array and the battery voltage. Boostconverter configuration is preferably used becausecompared to buck converter, boost converter can track theMPP in almost all conditions [9].

Boost converter shown in Fig.6 is used to step up thevoltage of the PV panel to the voltage suitable for battery.10% of the input current is considered as ripple ininductor current ( I) and 1% of the output voltage V o istaken as ripple in the output voltage ( Vo) for the boostconverter [10].A switching frequency of 25 kHz is chosenfor the design and a MOSFET of appropriate rating isselected as the switch (S).

F igure 6. Boost converter circuit

L V i *D *T / (2* I)(4)

C = D *I o * T S / Vo(5)

Here Ts and D are the switching time period and dutycycle of the switch S used in the converter. Vi is theoutput voltage of the PV array and Vo is the voltage of the battery. A value of 1 mH of inductor and 100Fcapacitor was designed for this work.

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Choice of diode D is a trade off between breakdownvoltage, speed and forward voltage. Higher the forwardvoltage more power will be dissipated and lost. If the loadis suddenly disconnected, there is a possibility of largevoltages and hence the diode must have a high

breakdown voltage. As the switch S is operated at highfrequency, the diode also should operate faster and hence

reverse recovery time of the diode also should be less. D. Control Circuit

The system is built around 80C51 Microcontroller which has a powerful instruction set, with built inmultiply function [11]. The controller uses two Analog toDigital Converters (ADCs) for reading the voltage andthe current as shown in fig. 7.These values are used for calculating the instantaneous power output of the PVarray. Based on dp/dv algorithm which is fed in to themicrocontroller, digital signals are generated which areconverted to analog signals by using Digital to AnalogConverter (DAC).These analog signals are fed to PulseWidth Modulator (PWM) and its output is connected tothe gate terminal of MOSFET in the DC/DC converter tocontrol the duty ratio of the boost converter.

Figure 7. Interfacing circuitThe voltage generated by the PV panel can be

anywhere between zero and twenty one volts. Thisvoltage range needs to match that of the maximum inputvoltage range of the microcontroller, which is zero to fivevolts Voltage sensing is done with the help of simple

potential divider network and is then given to ADC. Ashunt is placed in series in the return path and is used tosense the PV panel current.Since this voltage drop is verysmall this voltage is amplified before giving to the ADC

by suitable operational amplifier circuit.

E. Light Detection Circuit Light dependent resistor circuit (LDR) could be used

as light detection circuit based on the fact that when lightfalls on the LDR, it gives low resistance and when it

becomes dark, LDR offers high resistance. Using LDR circuit battery will be connected to the boost converter inthe morning and in the night, it will be disconnected from

boost converter and connected to the lamp load.

F. Lamp Load A comparative study was done between retrofit type

Compact Fluorescent Lamp (CFL) which is an excellentreplacement of incandescent lamp with Light EmittingDiode (LED) light source. The CFL considered has aluminous efficiency or efficacy of 60 lumens / watt andLED lamp has an efficacy 93 lumens/watt. Since thelamp has dusk to dawn operation, 10 hrs of lighting per day is taken for the calculation of energy consumed. Itwas found that there will be a saving of Rs.2260/- when

50,000hrs of operation [12] is considered if a single CFLis replaced by LED light. Hence it is preferable to useLEDs as the light source because it has many advantagessuch as longer life, suitable for frequent on/off use,absence of trace of mercury and high reliability comparedto CFL.

VI. CONCLUSION The design considerations of all the components of a

solar powered street light are discussed in detail. Theincremental conductance algorithm used for MPPT,automatically adjusts the step size according to theoperating point of the PV panel and this helps to track maximum power point faster. The implementation of thesolar powered street light with Light Emitting Diode asthe light source mentioned here, is in the process and will

be used to replace the conventional grid dependent streetlights of our campus.

ACKNOWLEDGMENT The authors wish to thank Nitin Gawde and et al, the

students (2005-2009) of the Department of ElectricalEngineering of Fr.C.R.I.T, Vashi who helped us in thiswork.

R EFERENCES [1] Bureau of Energy Efficiency, Government of India,

Ministry of Power.[2] K.Sidhu, Nonconventional Energy Resources, PEC

Campus, Punjab State Electricity Board, Chandigarh.[3] PV Labnotes, Elec3205, Electrical Energy Systems &

Management, School of Electrical and InformationEngineering, University of Sydney

[4] Brian Johnson, Thesis submitted on Power Conditioningin Photovoltaic systems, Texas State University-SanMarcos, 2007.

[5] T. Esram, P.L. Chapman, Comparison of Photovoltaicarray maximum power point techniques , IEEE Transactions on Energy Conversion, vol.22,no.2,June2007,pp.439-449.

[6] D.P.Holhm,M.E.Ropp, Comparitive Study of MaximumPower point Tracking Algorithma, Progress InPhotovoltaics: Research And Applications, page 47-56,January 2003

[7] Shirish Sinha, Anand Shukla, Nandita Hazarika Fromsunlight to electricity solar photovoltaic application,Winrock International, 1998, New Delhi.

[8] Rakesh Reddy, Maximum Peak Power Trackers increaseefficiency of solar panels in Street Lights, CypressSemiconductor Corp..August 2009

[9] Geoff Walker Evaluating MPPT Converter Topologiesusing a MATLAB PV Model, Department of Computer

Science and Electrical Engineering, University of Queensland, Australia [10] L.Umanand, S.R Bhat, Design of magnetic components

for switched mode power converters, New AgeInternational (P) Limited,2001, New Delhi

[11] E. Koutroulis, K. Kalaitzakis, and N. C. Voulgaris,"Development of a microcontroller-based, photovoltaicmaximum power point tracking control system," IEEE Trans. Power Electron. , vol. 16, pp. 46-54, Jan.2001.

[12] Mini Rajeev, Divya M, Ruchi H,A Study of ReplacingCFL with LED Lamp for Solar Powered Street Light usedfor Stand Alone PV Systems , NCNTE-2010.

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