Design and Simulation of an Independent Solar

Proceedings of the 2017 4th International Conference on Advances in Electrical Engineering (ICAEE), 28-30 September, Dhaka, Bangladesh

Design and Simulation of an Independent Solar Home System with Battery Backup A.K. Podder1, K. Ahmed2, N. K. Roy3 and P.C. Biswas4 Dept. of Electrical and Electronic Engineering, Khulna University of Engineering & Technology, Khulna, Bangladesh

E-mail: [email protected],[email protected], [email protected], [email protected]

Abstract— This paper presents a detailed design of an independent solar home system for a typical residential building in the southern part of Bangladesh, Khulna. The system is composed of a Photovoltaic array, Maximum Power Point Tracker (MPPT) Controller, DC-DC buck converter, charge controller, inverter and lead acid battery. The modelling is carried out by estimating the required load, selecting and determining the proper specifications of the components involved in the system. Various factors such as the geographic location, climate condition, solar irradiance and load consumption upon which the whole work depends are all considered in this work. The daily average load demand is found to as 8.730 kWh/day which can be met by an array of 17 solar panels with a backup of 12 units of the battery having a capacity of 3031.25 Ah. The system is implemented in MATLAB/SIMULINK platform with the solar radiation on a sunny and wintery day. Simulated results indicate that the proposed model meet the load demand and show satisfactory performance. Keywords—Photovoltaic array, Inverter, System Sizing, Standalone system.

I. INTRODUCTION Renewable energy sources exhibit an outstanding figure for producing electricity without any fuel consumption. Due to the continual declination of fossil fuel and to protect the environment from greenhouse gases which cause global warming. Sustainable power sources (solar, wind, etc.) are more drawing in as substitute sources than traditional energies. Among them, the solar power energy based on photovoltaic (PV) system is the most promising sources as its conversion and control is simple, clean, unlimited, easy to maintain, sustainable and eco-friendly. A PV solar system is becoming popular nowadays because of its high reliability, high modularity and pollution free characteristics [1],[2]. Based on their functional and operational requirements, PV systems can be categorized into two sections including the grid-connected system for decreasing the power from the utility and the other off-grid system for providing the load power without receiving any power from the utility grid [3]. A stand-alone PV power system is an interconnected system for converting solar irradiance directly into electricity and generally consists of the PV array, battery bank, charge controller, an inverter, protection devices and the system’s load. Since the aggregate sun oriented irradiance that achieves the surface of the earth differs with the time of day, season, area and climate

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conditions, a maximum power point tracker (MPPT) device is used between the array and load to trace maximum power output of the PV array and also for matching the impedance of the electrical load[4]. A buck converter is utilized here in order to provide a constant dc voltage. The stand-alone PV needs batteries as energy sources in case of stormy weather conditions and at night as they supply energy to the load. To prevent overcharging and deep discharge of the batteries, a charge controller is added in the system. These systems generally include an inverter, which converts the DC voltage of PV modules into AC voltage for direct use with the appliances. The other grid connected PV system is similar to stand-alone systems except for the connection of the system to the utility grid. Due to the interconnection with the utility grid, a system can sell the excess PV electricity production to the grid, charge battery system at off peak hours and buy power whenever the PV and battery power are deficient to feed the loads. Many research studies have been performed on solar off-grid PV system for residential unit [6]-[16]. The paper presents the detail calculation of a stand-alone photovoltaic power system for a typical residential building but does not provide the simulation results of the design [12]. The Paper exhibits the simulation result of the designed solar off-grid PV for a fixed irradiance system but does not show the effect of variations in irradiance on the system [14]. But in order to improve the performance of a PV system, it is necessary to overcome the effect of the variation in irradiance and also the system should have a battery backup to supply power while the PV system power is insufficient. That is why the main contribution of this work is sizing the off-grid PV systems i.e. an independent solar home system based on the specific residential load requirement and designing the system in MATLAB/SIMULINK platform with including closed loop dc-dc converter to overcome the effect of the variation of irradiance with a battery backup to validate the calculation. This paper is arranged in six sections. In section II, the geographical location and irradiance nature of the selected site is presented. In Section III, the residential load estimation and load demand during summer and winter days are given. Consequently, in Section IV, theoretical background for the modelling of the independent solar home system is mentioned. In Section V, simulation results are shown. Section VI brings a conclusion of this work.

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II. GEOGRAPHICAL LOCATION OF THE SELECTED SITE The first and the essential piece of the plan is the geological area of the establishment, site review and radiation investigation [14]. It decides if a remain solitary PV framework is reasonable or not. The review included the field visits to the sites in Khulna close to the campus of Khulna University of Engineering & Technology. The geographic area of the chosen site in Khulna, Bangladesh at 22ιͷͷǤ͸ͺʹͷᇱ latitude and 89ι͵ͲǤͻͻ͸ᇱ longitude makes it nearly sun-rich district with a sunlight based irradiance of around 600 Wh/m2 every day [5]. The normal surrounding temperature of around 25.66°C, though most extreme and least encompassing temperature are 28.25°C and 20.01°C, individually, if the area is without overcasts from close-by trees and structures [5]. The variation of irradiance throughout the day during summer and winter in the selected site is shown in Fig.1.

Fig.1: Variation of irradiance during summer and winter days at Khulna

III. RESIDENTIAL LOAD ESTIMATION The study involved the field visits to the sites in Khulna. Four families were chosen randomly for the vitality review. Meetings were led to set up the number of hours the different appliances were most likely to be kept on. The estimated load demand during the summer and winter days in the selected site is shown in Figs.2 &3. The electrical gadget accessible at the living arrangement are recorded together with their energy evaluations and time of operation amid the day to acquire the normal vitality request in watt-hour every day has appeared in Table I. The total average energy consumption is used to determine the size of the equipment that is related to the proposed independent solar home system.

Table I Table for the estimation of the residential load Wh Load Quantity Watt Total * AC Watt Use AC h/d 3 60 180 8 1440 Ceiling Fan Iron

1

1000

1000

0.4

400

Computer& accessories

1

150

150

4

600

Light(CFL)

10

22

220

6

1320

Television

1

110

110

7

770

Refrigerator

1

350

350

12

4200

Total connected load

8730 Wh

IV. INDEPENDENT SOLAR HOME SYSTEM MODELLING The proposed independent solar home system that gives the imperative power for a residential unit consists of PV array with MPPT controller, charge controller, buck converter, battery, inverter and load. The system is outlined such that it can recharge the storage battery and the battery can likewise supply the obliged energy to the load when the sun based irradiation is not adequate. The system voltage is considered as 24V and an inverter is utilized to provide AC power to the residential AC load. A. PV Array Sizing A PV array is a combination of several solar cells. A single module of solar cell rarely provides the amount of the required energy needed for a residential area. The modules are linked together for getting the desired energy. Generally, the modules in a PV array are connected in series to obtain the desired voltage and the individual strings are connected in parallel to produce the current as desired. For estimating the size of the PV array, the required average peak power (Ppv) has to be calculated which can be done using the given equation (1) [12-13]. ܲ௣௩ ൌ

Fig.2. Variation of load demand in summer days during 24 hours

2010 W

ாವ ܵ ఎೇ ఎೃ ்ೞ೓ ி

(1)

where, ED= average daily energy demand, ߟ௩ = inverter efficiency, ߟோ = efficiency of the charge controller, Tsh= peak sun hour and SF= safety factor. The total dc current required can be determined by dividing the average peak power with the system dc voltage as given in equation (2) ௉೛ೡ

‫ܫ‬ௗ௖ ൌ ௏

೏೎

(2)

Finally, the total number of series ሺܰ௦௠ ሻ and parallel modulesሺܰ௣௠ ሻ to form the PV array can be evaluated by applying the equation (3)

ܰ௧௠ ൌ Fig.3. Variation of load demand in winter days during 24 hours.

௏೏೎ ௏ೝೡ

ூ

 ൈ ூ೏೎  ൌ ܰ௦௠ ൈ ܰ௣௠ ೝೌ

(3)

B. Battery Sizing A Battery is an important part of the designed solar home system, as it should supply the adequate power to run all the loads

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at night, overcast and dusty days. The battery used in this model is a lead acid battery. The total retention of the battery (Ct) can be evaluated by applying the given equation [12]. ஽

ൈாವ ್ ൈఎಳ

ೌೠ೟ ‫ܥ‬௧ ൌ ஽ை஽ൈ௏

(4)

where, Daut = no. of autonomy days, DOD= depth of discharge, Vb = battery voltage,ߟ஻ = battery efficiency. The size of the battery bank can then be easily figured out. The total number of batteries can be calculated by dividing the total retention of the battery (Ct) by the retention of the individual batteryሺܰ௕ ሻ.The number of batteries in series (ܰ௦ ) and parallel (ܰ௣ ) to figure out the size of the battery bank can be assessed by applying the given equations. (5) ܰ௕ ൌ ‫ܥ‬௧ Τ‫ܥ‬௕ ܰ௦ ൌ ܰ௣ ൌ

௏೏೎ ௏್ ே್ ேೞ

E. Inverter Modelling The majority of the appliance in a domestic building generally use AC current, whereas PV module and battery bank are power wellspring of DC current. The major purpose of the inverters is conversion of DC power into AC, adjusting the frequency of the output AC power, and controlling the effective value of the output voltage. The inverter in this model must be able to handle about 2010-W at 220-Vac. Therefore, Latronics inverter, LS- 3024, 3000-W, 24-Vdc, 220-Vac is selected as the requisite inverter. The Inverter is built by using PWM technique. A fullbridge configuration designed in MATLAB with four Insulated Gate Bipolar Transistors (IGBTs) as shown in Fig.5 which provides an ac output to the load. A transformer is used to get desired AC voltage level (220V).

(6) (7)

C. DC-DC Buck Converter A DC/DC converter plays the vitally important role in PV system application. The major function of a DC-DC converter is to convert the unregulated voltage into regulated voltage. Without DC/DC converter, designing inverter control will be complicated and the performance of PV system will not be good. The DC voltage input to the inverter will not be constant and will vary with the switching of the inverter. Therefore, it will be difficult to control the power flow. In this act, a constant 24V DC/DC converter is designed as shown in Fig. 4 to supply the constant dc input voltage to the inverter. The performance of the DC-DC converter is improved by incorporating closed loop control for providing the constant voltage to the inverter.

Fig.4. MATLAB model for buck converter

D. Charge controller The fundamental task of the charge controller is to manage and control the current flow between PV array and battery. It restrains overcharging and shields battery from voltage changes. A decent charge controller must have the capacity to withstand the array current as well as the full load current and must be designed to match the voltage of the PV array as well as that of the battery bank. In this work, a proper charge controller is designed to control the flow of power either from the PV solar array or the battery to meet the demand and also, monitor the charging and discharging of the battery.

Fig.5. MATLAB model for inverter

V. SIMULATION RESULTS The models of PV array, buck converter, battery, controller and inverter are connected to make the proposed independent solar home system model. Fig.6 below shows the independent solar home system designed in MATLAB/Simulink platform. The Samsung SDI LPC235SM-02 module is selected as the PV panel which is of monocrystalline type. The specification of the selected panel is demonstrated in Table II. Table II Table for the specifications of the PV array [17] Electrical Characteristics Parameters Values Power at Standard Test Condition (STC) 235 W Power at PVUSA Test Condition (PTC) 207.8 W Voltage at maximum power 29.97 V Current at maximum power 7.84 A Open circuit voltage 37.24 V Short circuit current 8.43 A Nominal Operating Cell Temperature 48.8Ԩ Open circuit voltage Temp Co-efficient -0.348 ΨȀԨ Short circuit current Temp Co-efficient 0.053 ΨȀԨ Maximum Power Temp Co-efficient -0.46 ΨȀԨ Number of Cells 17 Number of Cells in Series 1 Number of Cells in Parallel 17 Mechanical Characteristics Length 1630.0 mm Width 982.0 mm Module area 1.6 m2 PV area 14 m2

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The PV array used in MATLAB/Simulink is a built-in module which uses the equations of the equivalent circuit model of the solar cell. The PV array is designed according to the requirement of the model. The voltage and power waveforms at the PV

terminals during summer and winter days are shown in Fig.7 which indicates that the voltage and power from the PV array varies with the variation of the irradiance.

Fig. 6. The Proposed independent solar home system in MATLAB/Simulink Platform

(a)

(b)

Fig.7 Variation of Voltage and Power at the output terminals of PV array due to variation of irradiance in (a) summer and (b) winter days

In order to remove the variation of output voltage of PV array, a DC/DC buck converter with closed loop control is utilized. The system voltage is considered as 24 V which is provided by the properly designed DC/DC converter. The output voltage waveform at the DC/DC Buck converter terminal is shown in Fig. 8.

Fig.8. Output voltage of the buck converter

A charge controller is incorporated in the model in order to control the flow of power either from the PV array or from the battery. A combination of two series and six parallel i.e. 2× 6 batteries with a terminal voltage of 24 V and a capacity of 3031.25 Ah are connected in this model to supply the power to the load when the solar irradiance is not sufficient. The batteries are arranged to charge from the PV array when the load is less and excess power is generated by the array. When the PV array fails to generate power, the battery discharges through the load. The state of charge of the battery is monitored to prevent under discharging and when the state of charge of the battery is less than 30%, the battery is disconnected from the load by the breaker. The model is simulated for 24 hours, where each hour is considered as 0.1s and run for 2.3s to monitor the behavior of the model. The state of charge of the battery during summer and winter days is shown in Fig. 9 which indicates the battery is discharging i.e. supplying power to the load during night time and when the PV array got proper irradiance from the sun i.e. during the day time, the PV array charges the battery as well as supply power to the load.

430

with a proper charging and discharging system is employed to meet the load while the PV array fails to provide the required power. Simulation results found in MATLAB/ SIMULINK platform were verified by comparing the calculated results which give satisfactory output. This study will be extended in focus of grid connected PV system in future research. REFERENCES [1] [2] Fig.9. Charging and discharging of the battery during summer and winter days

An inverter is used here to supply ac power to the residential appliance. The output voltage and power across the load during summer and winter days is shown in Figs.10 & 11 which shows the proposed independent solar home system can supply 2010W while the battery is in operation i.e. during night time and can supply 750W while the PV array is in operation i.e. in day time which is enough to meet the load demand.

[3]

[4] [5] [6] [7]

[8]

[9] [10] Fig.10. Voltage vs Time curve across the load [11] [12] [13]

[14]

Fig.11. Power vs Time curve across the load during summer and winter days

[15]

VI. CONCLUSION This paper presents a simple but an efficient independent solar home system in MATLAB/SIMULINK environment that can fulfill the residential daily load demands. A generalized mathematical description for the determination of the size of PV array, battery, charge controller and inverter has been followed in order to model the system. The average daily load demand of a residential unit considered here is 8.73 kWh/day. In order to accomplish this load demand, an array of 17 solar panels are utilized. A battery storage system having a capacity of 3031.25 Ah

[16]

[17]

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