array to meet peak power demand is discussed; the system is ... might cause an 18% reduction in output power. ... The power output of the cell is given as [20].
Automated Hybrid Solar and Mains System for Peak time Power Demand M. Habyarimana and C. Venugopal A solar system using PV arrays to convert solar radiation into direct current was a preferred renewable energy source compared to others. Fluctuations in a commercial grid power supply depended greatly on peak power demand load. It was, therefore, more convincing to utilise a PV array system to meet peak time demand. In this paper the design of a hybrid system using mains and a PV array to meet peak power demand is discussed; the system is completely automatic through design of an automatic source selector with a pre-scheduled panel cleaning system through use of a microcontroller. A LCD displays selection of source and provides for the overriding of automatic source selection; manual switching is included. During peak power demand, solar power and/or battery power would be used; at other times mains will be the power source. Moreover, emergency switching to solar or battery during mains failure was also provided. The MPPT design controlling a PWM signal for a boost converter to achieve maximum output is also discussed. The automated design makes it user-friendly. Although initial costs are high, this hybrid system could reduce the load on commercial grids during peak power demand. Abstract
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major factors motivating use of renewable energy were economic reasons, energy security and climatic change mitigation [4]. Solar systems using PV arrays to convert solar radiation into direct current were a preferred renewable energy source compared to the other renewable sources because of abundant annual solar radiation for at least eight months in South Africa. Also it was simple and cost effective for domestic use. It was, therefore, more convincing to utilise a PV array system to meet peak time power demand. In this paper, a hybrid system using mains and a PV array is discussed. A switching topology from mains to PV array during peak power demand or power failure is also featured. The size of solar panel and design of a converter was analysed to meet peak power demand of a coffee shop on the University of KwaZulu-Natal campus.
Daily profile or a typical Eskom high·consumption residential customer
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Index Terms-Boost converter. inverter. MPPT, peak time power demand, PVarray, solar charge controller, solar panel 1
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(5) Since the cell light current, It is directly proportional to the cell irradiance, the value of It (G) at any other irradiance; It (G) is given by,
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(6)
Where It (G) is known under standard conditions of G(s) at 2SoC; this gives the value of voltage and current produced by a cell. The power output of the cell is given as [20] P=FIV (7) Where, F is the cell fill factor and F can be written as:
F=
vo- ('0)n(�Q-) Ln(q:;+0.72) q Vo+q / kT q
Figure 4: Durban monthly global horizontal irradiance [in w/m2] (Jan 2012- Nov 2014) [21]
(8)
By solving the equations the output current and output voltage from the PV array can be obtained as function of time.
Monocrystalline cells provide greater efficiency than polycrystalline cells because they are made from one large crystal as opposed to many smaller ones. In addition, monocrystalline panels perform up to 10% better in high ambient temperature. Since monocrystalline panels are more efficient per area, the size of each panel was less than for the same wattage. A disadvantage of monocrystalline panels was their cost and shut down during leakage; polycrystalline panels were efficient under heat compared to monocrystalline panels and were cheap. But the polycrystalline panels were space inefficient, so monocrystalline panels were preferred for this proj ect.
For a lkW solar plant, an inverter with a minimum power rating of lkW, 48V was required; also a minimum of two 24V, 300Ah batteries connected in series. More batteries provided back-up but required more charging time. Solar charge controllers were required to couple the solar panel to the batteries, regulating fluctuating output from the panels. It could also protect any battery over- or under charging. Also, reverse current flow from battery to solar panel during night and on overcast days was prevented by the solar charge controllers. The lkW solar array charged a battery bank of 48V; charge controller size was 1,000/48 20.83amps. Considering a safety factor of 1.25 to account for variable power outputs, a charge controller of 26 amps was imported. =
3
DESIGN OF CONVERTERS
The proposed hybrid solar power and mains supply system is shown in Figure 5. In this research, the hybrid of solar power system was used to supply power during peak power demand for a lkW load in a coffee shop on Howard College Campus, University of KwaZulu-Natal, Durban. The solar power system was also used as an alternate source to supply power when the mains were not available. It was automated through switching between solar and mains via a microcontroller. A self-panel cleaning system with schedules programmed also by microcontroller was included to maximise power output from the solar panels. f-----EB----->I
Panel cleaning robot
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Figure 6: Solar panel I-V curve [22]
The most commonly used methods are Perturb and Observe (P&O) method and Incremental Conductance method as they offered most efficiency [23], The incremental conductance method offered better efficiency than the P&O method, but was difficult to implement. Therefore, for this design the P&O method was used. The P&O algorithm compared current output power with previously measured output power from the boost converter and checked the solar panel output voltage, The PWM signal of the boost converter and/or the position of the solar panel were adjusted accordingly. The algorithm was tested using a MatLab-Simulink toolbox, The PWM signal generated by the MPPT algorithm for maximum power and minimum power conditions are shown in Figures 7 and 8 respectively, It can be seen from Figure 7 that the width of the PWM output increased when current input power was less than previous output power. Figure 8 indicates the width of PWM output was reduced when the current output power was higher than previous output power; the algorithm could be used for practical applications via a microcontroller
Figure 5: Block diagram of complete design
3.1
Design of Maximum Power Point Tracking (MPPT) system
The efficiency of the solar panels could be increased by tracking maximum power at any time. The MPPT system achieved maximum output by varying the operating point on the cell I-V curve shown in Figure 6. The four common MPPT methods: i. Perturb and Observe Method ii. Incremental Conductance Method iii. Constant Voltage Method iv. Current Sweep Method
Figure 7: PWM signal for maximum input power
3.3
Design of Inverter
The simulation model of single phase DC-AC inverter is shown in Figure 10. The Pulse Width Modulation method was used to select the switching states of the Mofset. The simulated result of a DC - AC inverter is shown in Figure 1 1. It can be seen from Figure 1 1 that the switching of the Mofset produced an alternating square wave. A maximum of a 220V AC square wave is generated from 220V dc input supplied by the boost converter. Figure 8: PWM signal for minimum power input
3.4
Design of switches for automatic selection of source during peak power demand time
3.2
Design of Boost Converter
The simulation of boost converter is shown in Figure 9. The input of 48V was fed to the boost converter from the solar panel through the charge controller. The PWM signal of the boost converter was adj usted according to power output from the solar panel. The boost converter operated in continuous conduction mode. Boost Cooverter
..
Figure 9: Simulation model of boost converter
o
Figure 10: Simulation model of DC to AC inverter
The automatic power source selector of the hybrid system was designed using a Proteus Virtual System Modelling simulator. The flow code software was used to write the algorithm which enabled the user to download the programme into any microcontroller and was also user friendly. The design automatically selected solar panel power either from battery or from a panel directly to supply a lkW load during peak power demand as shown in Figure 1. Also, during mains shut down solar power was automatically selected. The system automatically checked availability of sources to switch between solar and mains during peak and off-peak times. At night with mains off and during grey days the batteries powered the load. The programme was also user-friendly and could be changed to requirement. Also, for emergency needs, manual switching was provided to change the source. The programme was written based on Eskom's off and peak time demands. Figure 14 shows the programme's flow chart. The designed system worked continuously; monitoring was on a second-to-second basis. Implementation of the hybrid system is shown in Figure 12; LCD display, giving selection of source, is shown in Figure 13.
The cleaning cycle depended on the environmental dust accumulation rate. This varied greatly according to seasons even day-by-day. Variability made it difficult to obtain a deterministic value, so a reasonable cyclic cleaning mechanism was used. In this proj ect the number of cycles used was 10 and cleaning done once a week (Sunday from 2pm-2.30pm). A weekly cleaning frequency was chosen to work on the worst area where the panel tilt angle was low (almost horizontal), usually littered with an accumulation of dust, dirt, pollen, bird droppings and other debris. A study by M. Mani and R. Pillai found accumulated dust on the surface of photovoltaic solar panel could reduce system efficiency by up to 50% [24]. The timing was strategically selected to target the time of day after which maximum solar irradiation was experienced.
Figure I I: Output voltage of inverter
This gave the battery bank time to charge before powering the cleaning process. The cleaning process did, however, proceed regardless of the batteries' charge levels. In the absence of solar irradiation at cleaning time, the process waited for the next scheduled cleaning cycle.
Figure 13: LCD display showing source selection
3.5
Self-powered panel cleaning robot
Solar panel power was also used for the cleaning robot - a 48V DC motor controlled to run forward and in reverse directions. When the motor ran, a sweeper, brushes or squeegees connected to its shaft, cleaned the panels.
Figure 16: Panel cleaning schedule in LCD display
Power Source Selector & Fuse
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Figure 12: Proteus simulation model of automatic power source selector
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