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Performance measurements on a

Photovoltaic (PV) system

System integration project II

Student: Zhenxue Wu 0621034

Supervisor: Prof.dr.ir.R. J. Ch. van Zolingen

Eindhoven University of Technology

Department of Mechanical Engineering

Master program of Sustainable Energy Technology

Report number WET 2007.13

Date:

02-09-2007

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Summary

A Photovoltaic (PV) system is comprised of photovoltaic cells which convert light energy

directly into electricity. It offers consumers the ability to generate electricity in a clean,

quiet and reliable way. Both an autonomous and two grid-connected PV systems are used

for experimental purposes in this project.

In this project the focus was in the area of performance measurements on the PV system.

The goals include checking the characteristics of an autonomous PV system by testing

the battery and regulator’s performances; testing the relationship between the open circuit

voltage Voc and the cell temperature Tcell and testing the relationship between energy

output and ambient conditions, such as sunlight irradiance and cell temperature.

In order to test the battery’s performances, the internal resistance was checked and the

old battery was around 40~80 mΩ while the new one was around 20 mΩ. Besides, the

total amount of energy withdrawn from the battery during discharging was measured. For

the old battery the available energy was only 40 Ah while for the new battery was more

than 70 Ah. Therefore the old battery was considered to be aging and was replaced by the

new one.

The incorporation of a regulator in the PV system can be used as overcharge protection

and deep discharge protection for battery. For the regulator in our project the charge

status of battery will control the charging process by means of Pulse Width Modulation

(PWM) and an oscilloscope was used to observe the performance of it. By observing its

working waveforms we can know that the repetition frequency is fixed even when the

irradiance changes and the ratio between pulse duration (TD) and repetition frequency

(TR) increases with an increase of irradiance. However, with a higher State of Charge

(SOC) condition, this TD and TR ratio decreases as they are also determined by the

actual battery voltage.

As the performance of a PV module is dictated by several ambient conditions such as

ambient temperature, irradiance, wind speed, wind direction and humidity, tests were

performed to find if the open circuit voltage could be used as a measure of cell

temperature. Based on the actual open circuit voltage Voc measured, the ambient

temperature and a simple model, STCOCV , (Voc at Standard Test Conditions) can be

extrapolated. Next Voc can be used to estimate the cell temperature under operational

conditions, using STCOCV , . A conclusion was given that open circuit voltage can give an

immediate indication for the cell temperature only when the solar irradiance does not

change too fast.

In the grid-connected PV system in our project a maximum Power Point Tracking (MPPT)

is installed inside the inverters to extract the maximum power from a PV array. Through

measuring the energy output and ambient conditions we can find that with a higher

irradiance and lower cell temperature, the PV module can generate more energy.

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Contents Summary............................................................................................................................. 2

1 Introduction...................................................................................................................... 4

1.1 Background of project............................................................................................... 4

1.2 Project goals.............................................................................................................. 4

1.3 Structure of the report ............................................................................................... 4

2 Description of PV system ................................................................................................ 5

2.1 Energy of sun ............................................................................................................ 5

2.2 Principle of photovoltaic conversion ........................................................................ 6

2.3 PV systems................................................................................................................ 7

2.3.1 Autonomous PV systems ................................................................................... 7

2.3.2 Grid-connected PV systems............................................................................... 8

2.3.3 Hybrid PV systems ............................................................................................ 8

2.4 Description of actual experimental PV system......................................................... 8

3 Battery tests.................................................................................................................... 13

3.1 Working of battery.................................................................................................. 13

3.2 Properties of battery................................................................................................ 13

3.3 Test of the old battery ............................................................................................. 14

3.4 Test of the new battery............................................................................................ 16

3.4.1 Discharging the new battery (Charged by solar energy) ................................. 16

3.4.2 Comparison with charged by a conventional battery charger.......................... 19

3.5 Re-test of the old battery......................................................................................... 20

4 Regulator tests................................................................................................................ 21

4.1 Principle of regulator .............................................................................................. 21

4.2 Observation of regulator current waveform by oscilloscope.................................. 23

5 Relationship between open circuit voltage and cell temperature................................... 26

5.1 Extrapolation of Voc to Standard Test Condition (STC)........................................ 26

5.2 Calculation of cell temperature............................................................................... 27

5.3 Analysis of outcome ............................................................................................... 28

6 Energy output and ambient conditions .......................................................................... 30

6.1 Power output of PV system..................................................................................... 30

6.2 Analysis of the relationship between energy output and ambient conditions......... 30

7 Conclusions and recommendations................................................................................ 33

References......................................................................................................................... 34

Appendix........................................................................................................................... 35

A.1: Measured data charged by solar system................................................................ 35

A.2: Measured data charged by a conventional battery charger ................................... 36

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1 Introduction

1.1 Background of project

This project was performed by student of the Master program ‘ Sustainable Energy

Technology’ of the Eindhoven University of Technology by means of the course ‘ 0C921

Internal research traineeship’.

1.2 Project goals

A photovoltaic (PV) system is comprised of photovoltaic cells which convert light energy

directly into electricity. It offers consumers the ability to generate electricity in a clean,

quiet and reliable way. They are usually referred to the amount of energy they generate in

full sunlight, known as kilowatt peak or kWp. The greater the intensity of the light, the

more energy a solar cell generates.

For this project the focus was in the area of performance measurements on the PV system.

Accurate and consistent evaluations on it are critical for the continuing development of

the PV industry. The goals include the following: 1, checking the characteristics of

autonomous PV system by testing the battery and regulator’s performances; 2, testing the

relationship between the open circuit voltage Voc and the cell temperature Tcell ; 3, testing

the relationship between energy output and ambient conditions, such as sunlight

irradiance and cell temperature.

1.3 Structure of the report

In chapter 2 the actual system used for measurement experiments will be presented.

There descriptions about basic performance of solar cell system and the main components

of the system are given. Following in chapter 3 battery measurements which will test the

charging and discharging characteristics of both old and new batteries are carried out.

Theory of regulators (controllers) used in PV system will be introduced in chapter 4 and

its working waveform as can be observed by oscilloscope will be presented.

Extrapolation of Voc to Standard Test Condition will be given in chapter 5 and analysis

about the relationship between the open circuit voltage and cell temperature is included.

Following, experiments to test the relationship between energy output and ambient

conditions are presented in chapter 6 and conclusions and recommendations will be given

in the final chapter 7.

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2 Description of PV system

2.1 Energy of sun

Solar energy is a source of energy that uses radiation emitted from the sun. The sun light

can be considered as a flow of quasi particles, called photons. These photons have energy

as the equation shown below:

λ/hchvE ph ==

with

phE – Photon energy;

h – Planck constant, sJ ⋅× −3410626.6 ;

v – Frequency of the light;

λ – Wavelength of the light;

c – Speed of the light, sm /100.3 8× .

Besides, light also can be treated as an electromagnetic wave with time varying electric

and magnetic fields, which are propagating through space in such a way that always

perpendicular to each other and to the direction of propagation.

The spectrum of the radiation emitted by the sun is close to that of a black body at a

temperature of 5900 K. About 8% of the energy is in the ultra-violet region, 44% is in the

visible region and 48% is in the infra-red region [1].

While traveling through the atmosphere layer, solar radiation is partially depleted and

attenuated due to absorption, scattering and reflection. That is to say, the intensity of

sunlight at ground level varies with latitude, geographic location, season, cloud coverage,

atmospheric pollution, elevation above sea level, solar altitude, etc. In the Netherlands,

the intensity of the sun is about 1000 W/m2 at an optimally oriented surface at noon

during the summer under blue sky conditions [2].

As the efficiency of a solar cell depends on the spectral composition of the light, air mass

(AM) is introduced which characterizes the effect of a clear atmosphere on sunlight. AM

1.5 is a typical solar spectrum on the earth’s surface on a clear sunny day, with a total

irradiance of 1000W/m2 used as standard test irradiation in Standard Test Conditions

(STC, 1000W/m2 irradiance, 25

oC cell temperature, AM 1.5 spectrum) for solar cells.

The available irradiance is usually not equal to this value because of the diverse weather

conditions and the rotation of the earth. It may sometimes be higher and in most cases

lower than this value.

The amount of radiation that reaches the ground is very variable, and it depends on

various factors, for example weather conditions, the time of the year, the time of the day

and location. From the global yearly solar radiation we know that in the Netherlands, a

large difference occurs regarding the irradiation in summer and in winter. The average

irradiation on a south oriented plane with a tilt angle of 45 degrees in June and December

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is 5 kWh/ m2 per day and 1 kWh/m

2 per day respectively. In a poor December month the

average daily irradiation can even drop to 0.5 kWh/ m2 per day.

2.2 Principle of photovoltaic conversion

Photovoltaic (PV) conversion is the conversion of (sun) light by means of solar cells,

which use the photovoltaic effect of semiconductors to generate electricity directly from

sunlight. A simplified schematic diagram of a typical solar cell is shown in Figure 2.1.

Figure 2.1 Principle of operation of the solar cell [3]

From the schematic figure 2.1 we can see that a typical silicon PV cell is composed of a

thin wafer consisting of an ultra-thin layer of phosphorus-doped (N- type) silicon diffused

in a thicker layer of boron doped (P-type) silicon. An electric field is created at the

junction of the cell between the P-type and N-type material. When the sunlight falls on

the solar cell, photons with energy Eghv ∆≥ (the band gap of the semiconductor silicon)

are absorbed. During the absorption of such a photon an electron is transferred from the

valence band to the conduction band. A hole stays behind in the valence band. The

absorption of a photon generates an electron-hole pair, therefore. If the electron-hole pair

is generated near the p-n junction the electron and the hole will be separated by the strong

electric field which presents at the p-n junction. If the electron-hole pair is generated

deeper within the p-type semiconductor, then the electron can reach the p-n junction by

diffusion and reach the n-type area by the strong electrical field. Electrons are collected

by the contact grid and resulting in a flow of current when the solar cell is connected to

an external load.

Regardless of size, a typical multi-crystalline silicon PV cell produces about 0.6 volt DC

under open circuit, no-load conditions and about 0.5 volt DC in its maximum power point.

However, this voltage is too low for most practical applications. In autonomous PV

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systems, therefore, photovoltaic cells are connected electrically in series and/or parallel

circuits to produce higher voltages, currents and power levels. Photovoltaic panels

include one or more PV modules assembled as a pre-wired, field-installable unit and a

PV module often contains 36 cells in series in order to obtain a maximum power point

voltage of approximately 17 to 18 volt at standard test conditions. This voltage is

sufficient to charge a lead-sulphuric acid battery with a nominal voltage of 12 volt

adequately.

Conclusively, the current and the power output of a PV system depends on its efficiency

and size, and also depends on the intensity of sunlight striking in the surface of the cell.

Besides, ambient conditions which may change its operation temperature also have

influences on the electrical power generation of the PV system.

2.3 PV systems

PV systems are usually categorized into three types: autonomous (Off-Grid), grid-

connected and hybrid. The type chosen depends on the needs, location and budget.

2.3.1 Autonomous PV systems

Autonomous PV systems are PV systems which can produce electricity independently of

other energy generation systems. They are not connected to the main utility grid and in

most cases, batteries for storage are incorporated which store energy from the PV

modules during the sunny days and supply energy at night or in periods of low solar

radiation. Such a system consists of a number of PV modules, battery storage, regulator

and loads (as shown in figure 2.2).

Figure 2.2 Circuit diagram of an autonomous PV system [4]

In figure 2.2 we can see that PV module connected in series and/or parallel give a DC

current if the module is illuminated. The PV module charges the battery via a diode

which ensures that the battery can not discharge itself via the PV module at night. The

consumer receives its current directly from the PV module or from the battery when the

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irradiance is low. The incorporation of a regulator (also called controller) can be used as

overcharge protection and deep discharge protection for battery.

Autonomous PV systems are usually used in those areas where only small amounts of

electrical energy are required, where no public grid is available or where the cost of a

grid connection is too high such as remote homes, cottages.

2.3.2 Grid-connected PV systems

Grid-connected PV systems are systems which supply their energy to the public

electricity grid. The basic Grid-connected PV system design has the following

components:

PV modules: As in autonomous system;

Inverter: Inverts the DC power from the modules into AC power. The characteristics of

the output signal should match the voltage, frequency and power quality limits in the

supply network;

Loads: Energy consumption of the local house;

Meters: Account for the energy being drawn from or fed into the local supply network;

Local grid: Public grid which acts both as a sink for energy surplus in the PV system or

as a backup for low PV generation periods.

Figure 2.3 Circuit diagram of a Grid-connected PV system

When the PV system is connected to the grid it will produce electricity that will be used

directly in the house. If the system generates more power than needed by the home then

the excess can be sold back to the local electricity company via public grid network.

However grid connection of a PV system must have local electricity company approval

that ensure the system is properly installed and with the required degree of protection.

2.3.3 Hybrid PV systems

Hybrid systems receive a portion of their power from one or more additional sources. In

practice, PV modules are often paired with a wind generator or a fuel-fired generator.

2.4 Description of actual experimental PV system

Both an autonomous and two grid-connected PV systems in the Mechanical Engineering

laboratory are available for experimental purposes in this project. The PV panels are

mounted on the roof of the W-Laag building, TU/e, Netherlands (Latitude: 51.26’ N;

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Longitude: 5.29’E; altitude: 19 m). They are installed on one support structure which

holds the panels at an optimal angle of about 36 degrees measured from the horizontal

and south facing for effective capturing of solar irradiation (as shown in figure 2.4).

Figure 2.4 PV panels mounted on the roof of W-Laag building

Besides these PV panels, a number of other components such as batteries, regulator and

measured meters are needed to complete the system. The wiring diagram of the whole

system is shown in figure 2.5.

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Figure 2.5 Wiring diagram of this PV system [5]

On the left side of modules array in figure 2.4 there are three PV modules that are rated at

100Wp each. One of these modules is idle which is mounted there for future research

(module 3 in figure 2.5) and the other two modules (module 1 and 2 in figure 2.5) are

connected to their own inverter that converts the DC current to AC current. The power is

then fed into the grid system by plugging a socket into the grid and the total power

transmitted will be recorded by two energy meters (energy meter 1 and 2 in figure 2.5).

These two PV panels form a grid-connected PV system without battery storage.

On the right side of these modules array there are two 36 Wp modules (module 4 and 5 in

figure 2.5) which are connected in parallel. The power output of them, which is used to

charge a 100 Ah battery, varies with variation of the radiation intensity incident on the

modules. These two PV modules form an autonomous system with battery storage. As

crystalline silicon cell produces approximately 0.5 V in its maximum power point, each

of these two modules has 36 cells connected in series, which makes sure that the 12 V

batteries can be charged completely. For this autonomous system the nominal system

voltage is 12 volt, therefore.

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On the most right of these modules is the smallest one which is taken as a reference

module. A reference module is a specially calibrated PV module which is used to

measure irradiance. It is connected to an ampere meter which effectively short circuited it.

When solar irradiation of intensity 1000 W/m2 is incident on the panel under standard test

condition, a short circuit current of 1.5 A is measured by the meter.

As we mentioned previously, besides the PV modules, there are other components such

as battery, regulator, meters and external loads and they are placed inside the lab (figure

2.6) to protect against the environment erosion, i.e. wind, rain, sunshine and dust.

Figure 2.6 Indoor devices of the PV system

On the left of figure 2.6 is a battery for storage. The battery used for experiment is a

Varta E34s, 12 V, capacity 100 Ah lead acid batteries. In the center is the PV charge

regulator and to the right are the meters that measure the current, voltage, irradiance and

power. The charge regulator used in this project is a 12 A charge controller of Steca make.

It is used to protect the circuit from over charge and too deep discharge. The loads here

include three 35 watt incandescent lamps and one 7 watt fluorescent lamp and all of these

lamps are connected in parallel.

On the left side of top aluminum box is the voltmeter that indicates the battery voltage.

To the immediate right of the voltmeter is an ampere meter that is used to show the

charging current from the PV modules. To the right of charging current ampere meter is

another ampere meter that measures the current drawn by the loads. These meters

together with switches for the loads were encased in a small aluminum box for protection

against the external environment.

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In the center of the bottom box is the ampere meter that measures the irradiance intensity

which is based on the short circuit current of the reference panel. On two side of this

meter there are two energy meters which are used to show the present delivering power

and to record the amount of power that have been delivered to the grid since installation.

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3 Battery tests

3.1 Working of battery

Batteries are often used in autonomous PV systems for the purpose of storing electrical

energy produced by the PV array during the day, and supplying it to electrical loads as

needed (during the night and periods of low irradiation).

The lead sulphuric acid battery is the most common battery used in PV systems, mostly

because it has high performance compared to its cost. They are made up of a number of

cells and each cell has a nominal voltage of 2 volts. Therefore for a 12 V battery, it means

it was made up by six cells connected in series. Each cell contains a number of ‘positive’

and ‘negative’ plates immersed in a sulphuric acid solution diluted with de-ionized water

(the electrolyte).

During discharging both the lead oxide and the lead are converted into lead sulphate and

the density of the sulphuric acid decreases. During charging the lead sulphate is

converted back to lead oxide and lead and the sulphuric acid solution reaches its

maximum density when the battery cell is fully charged again. The reactions are as

shown below:

OHPbSOSOHPbPbO 24422 222 +→++ ……………….Discharging reaction

42224 222 SOHPbPbOOHPbSO ++→+ ………………Charging reaction

These batteries are charged by the PV modules in periods with high irradiation and have

different characteristics compared with being used in other fields:

1, the batteries are charged and discharged daily;

2, most of the time the batteries are not fully charged due to short charging time available;

3, exposed to low state of charge conditions for a long time, especially in the winter

period, or exposed to a high ambient temperature in summer, can shorten their life time

and accelerate their aging process.

3.2 Properties of battery

Some important properties of a battery:

Capacity:

The capacity of a battery is the quantity of charge (expressed in Ah) which can be

withdrawn from a fully charged battery. The capacity depends both on the size of the

discharge current and on the temperature of the battery. As the discharge current

increases the capacity decreases and as the temperature increases the capacity increases,

the capacity of a battery must therefore be defined at a specific discharge current and a

specific battery temperature.

State of Charge (SOC):

The State of Charge (SOC) of a battery indicates the relative quantity of charge available

in a battery. The SOC is 100% when the battery is fully charged; the SOC is 0% when the

battery is fully discharged.

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Self-discharge:

The self-discharge of a battery indicates what part of the battery charge is lost if the

battery is not used. The self-discharge increases sharply when the temperature rises. A

typical value for the self-discharge of a lead acid battery with an antimony content of 2%

is 3% per month at 25 oC and for a lead acid battery in which the antimony is replaced by

calcium it is 1% per month.

Floating voltage:

When the batteries are used as back-up systems, the batteries are kept fully charged by

compensating for the self-discharge. This is done by keeping the battery voltage at the

float voltage under trickle charging. If the floating voltage setting is too high, the

floating current is also too high, which will accelerate corruption of the grid and shorten

the life of the battery. If the setting is lower, the battery can’t be kept in fully charged

state, which will crystallize PbSO4, decrease the capacity and also shorten the life of the

battery.

Ageing mechanisms:

Some of operating parameters are used to indicate aging effects because the actual

capacity of the battery is subjected to change during service life such as the loss of active

material, corrosion of the conducting elements, and the gradual loss of water.

Grid corrosion at the positive electrode is the most critical parameter during the float

charge of lead acid batteries. Increased grid corrosion can be induced by overcharging at

too high voltage, which may increase internal resistance of the battery and therefore

increase voltage drop and decrease the amount of energy withdrawn from battery.

Besides, if the battery is discharged too deeply or if a battery cell is exposed to a low

charging condition for too long, the amorphous lead sulphates can change to

polycrystalline lead sulphates. During charging it is virtually impossible to convert this

form of lead sulphate. This is known as sulphating and leads to a permanent decline in

the capacity of the battery. Moreover, the cyclical stress, differences in acid density and

exposure to high temperature also will speed up the aging of battery.

Because of these characteristics and high sensitivity to over-charging and deep

discharging, some forms of services such as regulators are needed to ensure the service

life of batteries by controlling its performance during charging and discharging process.

3.3 Test of the old battery

As we mentioned before, the battery used for experiment is a 12 V, capacity 100 Ah lead

acid battery. As it had been used for quite a long time, testing its performance is

necessary to see if it was still suitable for the project.

The test was started by disconnecting the PV module from the fully charged battery (or

SOC> 90%). This was done by removing the PV modules’ connections from the

controller. The connections of the battery to the controller were retained to prevent the

battery from too deep discharge. The loads were switched on and the voltage and current

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were measured at a certain time intervals until the voltage regulator disconnected the load

automatically due to low energy left inside the battery. Consequently battery voltage

versus time, and battery voltage versus discharging current characteristics were obtained

and the energy (Ah) drained from the battery can be determined by multiplying the

current drawn from the battery and the number of hours this current flows ( circuit

diagram as shown in figure 3.1).

Figure 3.1 Simple circuit diagram of discharging process

During the first time of discharging test, three 35 Wp lamps in parallel were used as loads.

The battery terminal voltage was 13.1 V before connected with loads. When the lamps

were turned on, the battery terminal voltage dropped to 12.8 V. The discharging current

was kept around 7.9 A. However, only after 20 minutes, the lamps were turned off

automatically, with a terminal voltage of battery of 12.05 V. This was done by regulator,

which can control the system running and avoid deep discharging when the terminal

voltage of battery drop to the lowest allowed working voltage. Based on this the total

energy of 3.1 Ah withdrawn in this test was calculated out, which was quite far away

from the rated energy of 100 Ah. This may due to several possibilities: the dry of distilled

water immersed in the sulphuric acid solution; the battery hadn’t been used for quite a

long time and need to be charged and discharged for several times to activate the

materials; aging of battery.

In order to find out the reasons of low withdrawn energy, first the de-ionized water inside

the battery was checked. As the distilled water was still sufficient, the cause of distilled

water shortage can be eliminated. After that the battery was repeatedly charged and

discharged several times and tried to activate the reaction materials. However, when

discharging test was taken again, still only 10 Ah of energy was withdrawn. Activating

the reaction materials didn’t success, therefore.

When considering the aging of battery, we know that if a battery ages and loses capacity,

its vital internal components undergo unavoidable degradation, which can cause an

increase in the battery’s internal resistance. Therefore the internal resistance is usually

measured to determine the health of a battery. A healthy, fully charged 12 V, 24 Ah lead

acid batteries would have an internal resistance of 20 mΩ [6] and it goes up during

discharge process. This change is caused by the decrease of the specific gravity, a

depletion of the electrolyte as it becomes more watery. Therefore under different state of

charge, the internal resistance would be different.

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One way to measure the internal resistance is to compare the difference between open

circuit voltage of the battery and voltage of battery when connected with load. The

internal resistance was then considered as the ratio between the difference of voltage and

the discharging current when connected with load. The internal resistance of the battery,

however, is a dynamic nonlinear parameter that is continuously changing along with the

temperature and discharge state. In that case using this method to measure the internal

resistance of battery under discharging is not accurate due to the status change from a still

state to a discharging state. Therefore we determined the internal resistance under

discharging state by following steps:

1, Connect a load to the battery and quickly measure the battery terminal voltage and

discharging current at the same time, U1, I1; Here as the load will heat up very quickly,

which in turn will falsify the voltage readings- so conduct the whole measurement within

max.5 s and disconnect the load immediately;

2, Wait for 4~5 minutes for the voltage to stabilize, and determine the terminal voltage

U2 and discharging current I2 with another different load;

3, Internal resistance is then calculated according to Ohm’s law, R=∆U/∆I, where ∆U is

the voltage difference and ∆I is the current difference for these two sets data (as shown in

table 3.1).

Table 3.1 Calculation of internal resistance

Loads connected Terminal voltage of

battery(V)

Discharging

current(A)

Internal resistance(mΩ)

Set 1

1*35 Wp lamp 12.86 2.85

1*7 Wp lamp 12.97 0.45 46

Set 2

1*35 Wp lamp 12.99 2.8

1*7 Wp lamp 13.18 0.45 81

From the data above we can see that the internal resistance is a little high.

3.4 Test of the new battery

As the total energy withdrawn from the old battery was too low, a new battery which is

also a Varta E34s, 12 V, and capacity 100 Ah lead acid batteries was used to continue the

experiments of this project.

3.4.1 Discharging the new battery (Charged by solar energy)

This test was discharging the new battery which had been charged by solar energy and

reached a SOC of >90%. First of all the internal resistance of this new battery was

measured and the value is around 20 mΩ, which is much less than that of the old battery.

The initial terminal voltage of battery was 11.98 V before connected to any PV panels.

As we know that the new batteries often do not give their rated capacity when received

from the manufacturer. This is due to the methods of making the plates. The plates are

usually made by applying oxides of lead, mixed with a liquid, which generally is dilute

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sulphuric acid, to the grids. These oxides must be subjected to a charging current in order

to produce the spongy lead and lead peroxide. After the charge, they must be discharged,

and then again charged. This is necessary because not all of the oxides are changed to

active material on one charge, and repeated charges and discharges are required to

produce the maximum power available. In this case, we repeated charge and discharge

the new battery several times first. The production of discharging, however, was still

very low, with energy withdrawn of 4 Ah. It was strange for a new battery and there must

be some problems in this system.

As the system was monitored by the regulator, when checking the connection wires

between battery and controller we found that the resistance of the cable wires was higher

than 0.15 ohm, which may cause a voltage drop of 1.64 V at full load. In that case even if

the terminal voltage of battery is still very high, for example 12.2 V, the voltage

measured by regulator terminal is only 10.6 V, which was considered by the regulator

that it was near the lowest allowed voltage. Therefore the controller would disconnect the

loads automatically to protect the battery from deep discharging. In order to increase the

energy withdrawn by the loads, we displaced the original wires with some other thicker

ones, with resistance of only 0.0066 ohm and the voltage drop turns to be only 0.05 V.

During the discharging process, however, we noticed that the loads were usually turned

off for 3~5 seconds and then turned on again automatically, even though the voltage of

battery was still high then. That means the reason to disconnect the loads by the regulator

was not because of the protection of deep discharge, but the other reasons. As we found

that during that period the regulator surface was very hot, and the LED information light

turned to be red, which means the current state of regulator was under abnormal

condition. When the lamps were turned on automatically after several seconds, at the

same time the LED lights changed back to green color again, which indicated it was

under normal condition then. As we know that besides the deep discharging protection

and over charging protection functions, the regulators still have other mechanisms such

as self-protection under high temperature, current compensation, short circuit protection

and other functions. For our case as the temperature of regulator exceeded the setting

protection temperature, the regulator cut off the discharging process for several seconds

and then was recovered immediately when the temperature cooled down. Sometimes,

however, even if the temperature of regulator was low, it happened as well. It means that

the system was under instable condition. When checking the fuse of controller, we can

see that the fuse had broken and connected with thin copper wire directly. As the

resistance of copper wire is higher than fuse material, the current flowing through these

wires generated much more heat and increased the temperature of regulator consequently.

Using copper wire to replace the fuse is very dangerous as the fuse actually doesn’t have

protection function any more. The regulator could be easily damaged, especially under

high current. Besides, as the wire aged and was oxidized, the instability of wire

connection was happening. This could be the reason of automatic alternation between

turning off and turning on the loads. So here we replaced the broken fuse with a new 10

fuse and tested again.

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The new measurement data of discharging are shown in appendix A.1. Figure 3.2 and 3.3

are the terminal voltage of battery versus discharging time graph and terminal voltage of

battery versus discharging current graph. The loads used for this test was ( 3*37 Wp +1*7

Wp ) lamps.

battery terminal voltage versus time

10.4

10.6

10.8

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11.2

11.4

11.6

11.8

12

12.2

12.4

12.6

12.8

13

0 0.05 0.484 0.884 2.217 2.717 3.767 4.717 5.717 6.717 7.55 8.05 8.21

time(h)

Vo

ltag

e (

V)

Figure 3.2: Battery terminal voltage versus discharging time

From the figure 3.2 we can see that the voltage of battery drops gradually until reached

the setting lowest voltage value.

19

discharging current versus battery terminal voltage

7.8

7.9

8

8.1

8.2

8.3

8.4

8.5

8.6

8.7

8.8

8.9

9

9.1

12.72 12.36 12.13 11.97 11.74 11.46 11.3

Voltage (V)

dis

ch

arg

ing

cu

rren

t (A

)

Figure 3.3: Discharging current versus battery terminal voltage

The figure 3.3 is the discharging current versus battery terminal voltage characteristics

curve. The discharging current decreased slightly while the voltages of battery drop.

When regulator turned off the lamps automatically, the terminal voltage of battery was

11.3 V while the terminal voltage of regulator was 11.24 V, which had a 0.06 voltage

drop on the wire connections.

As the energy (Ah) drawn from the battery can be determined by multiplying the current

drawn from the battery and the number of hours this current flows, based on the

measured data we got the energy withdrawn from the battery is 70.3 Ah in this test.

3.4.2 Comparison with charged by a conventional battery charger

This time the battery was charged by a conventional battery charger without the regulator

control.

The battery charger used is Cargador de battery, 230V; 50/60 Hz;12/24 V; 10/20 A,

which means it can convert 50/60 Hz, 230 V grid energy to charge 12 V or 24 V battery

with a charging current of 10 A or 20 A. For this test we set the charging current was 10

A and charged it almost 15 hours, which can ensure that the battery had been fully

charged. The discharging data for this test are shown in appendix A.2.

Comparing the total withdrawn energy in case of charged by solar irradiance and by

battery charger (table 3.2), we can see that the withdrawn energy charged by the battery

charger is higher than charged by solar irradiance. It is the same situation for terminal

voltage of battery before starting discharged measurement.

20

Table 3.2 Two sets data of withdrawn energy

Terminal voltage of

battery before starting

discharged

Terminal voltage of

battery when deep

discharge protection of

controller working

Withdrawn

energy

Charging by PV system 13.21 V 11.3 V 70.3 Ah

Charging by battery

charger

13.43 V 11.27 V 78.7 Ah

This can be explained by the function of regulator. Once the battery had been charged to

a reasonable amount (>90% SOC), the full charging current was not applicable any more

and the regulator is resorted to reduce the charging current to prevent from over-charging.

Therefore it is not easy to reach a fully charged state by PV system charging mechanism

due to the working of regulator. It is the same for the result of terminal voltage of battery

when finishing charging. As there is no such protection while charging with battery

charger, the terminal voltage reaches is relatively higher than charged by PV system,

which consequently obtained a higher energy withdrawn. From this we can see that more

energy can be withdrawn without the regulator mechanism. The regulator is

indispensable, however. If the battery undergoes over charged and deep discharged for

several times, it would greatly decrease the performance of battery and accelerate the

aging of battery. Therefore regulator is considered to be a very important component to

protect the PV panel systems.

3.5 Re-test of the old battery

The reason for us to change the old battery with the new one because we thought it was

aging and can not be suitable for the experiments any more. However, based on the

previous experiments done we can see that the low of energy available from the old

battery also may due to the high voltage drop on the wires connection between battery

and regulator or the instability of regulator. In order to make sure it was indeed aging, the

old battery was tested again with the new mechanism and the amount of energy obtained

from battery was around 40 Ah. Although compared with the data measured before (only

around 10 Ah) the energy increases a lot, it is still far away from the rated capacity value.

So for the old battery it is really aging and should be replaced by the new one.

21

4 Regulator tests

4.1 Principle of regulator

The controller, also known as a regulator, is used in PV power supplies with battery bank.

The charge controller monitors the charge status of the battery, controls the charging

process and the connection/disconnection of the loads. Besides, the controller can provide

other functions:

-Temperature compensation (as the gassing voltage of a battery decreases with increasing

temperature, the settings of the overcharge protection circuit have to be adjusted

according);

- External voltage sensing (in case of a rather large value of voltage drop between

controller and battery);

-Current compensation (to compensate the dependence of the battery voltage on the

discharge current);

-Short circuit protection (this can either be an electronic fuse or a conventional fuse

suitable for DC-current. If the output current becomes higher than as prescribed value

than the load is switched off automatically).

All these functions allow the battery use to be optimized and its service life to be

significantly extended. The system connections of the regulator used in the project (a 12

A charge controller of Steca make) are shown in figure 4.1.

Figure 4.1: Overview of PV system connections

22

The LED 1 is used to indicate the operating status and fault message. It flashes green

during normal operation. A red color indicates that there is a fault. The reason of fault

could be module current too high, controller overheated, battery voltage too high, battery

voltage too low, no battery connected or fuse faulty, and so on. The color of the right

LED indicates the charge status of the battery. The color changes from red (approx. 0%

charge status, 10.8 V) to yellow (approx. 50%, 12.0V), to green (approx. 100%, 13.2 V).

Besides, the flash of right LED is used to indicate deep discharging warning. To prevent

from deep discharging, the device is disconnected with loads automatically when the

charge status reaches 30 %( 11.2 V). The load is automatically reconnected when the

SOC reaches 50% (12 V). When the battery had been charged to a reasonable amount

(>90% SOC), the regulator will be resorted to restrict the charging current. When the

battery is fully charged and the PV module is still producing energy, the charge controller

will disconnect the battery automatically. This can be done by opening the circuit

between the battery and the module. If during discharge process the battery voltage

reaches a specified voltage value as low energy left, the load will be switched off

automatically by the deep discharge protection. This is done either by a relay or a solid

state switching device.

Usually, the State of Charge of the battery is being determined by means of measurement

of the battery voltage and the battery current and temperature were measured as well as

correction factors. For the regulator in our project the charge status of battery will control

the charging process by means of Pulse Width Modulation (PWM) to ensure gentle

charging of the battery.

In case of a controller with Pulse Width Modulation (PWM) (as is the case with the Steca

regulator) the charge current is modulated by means of a PWM technique. The charge

current is composed of a series of pulses of which the pulse duration can be varied. The

typical fixed repetition frequency (1/TR) is between 0.5 and 50 Hz. The pulse duration

TD depends on the battery voltage or charge status of the battery. As shown in figure 4.2

when the charge status is low the pulse duration TD is equal to the repetition time TR.

When the battery voltage or the State of Charge becomes high enough the pulse duration

TD will reduce (in other words the duty cycle TD/ TR of the pulses will become smaller

than 1). This way a quasi-linear control is used to adapt the duty cycle such that the float

of battery voltage is kept in a certain narrow range.

23

Figure 4.2: Current versus time for a PWM-controller. TD is pulse duration time, TR is

repetition time

4.2 Observation of regulator current waveform by oscilloscope

Even though a quasi-linear control is performed under high charging status, it is difficult

to notice the variation of charging current by using a low sensitive analog current meter.

It is only possible to observe that the indicator needle shakes under high irradiance.

In order to observe the performance of regulator, an oscilloscope is required. An

oscilloscope is a general-purpose instrument for examining electrical waveforms. It can

be used for various sets of measurements depending on how it is has been set up.

When noticing the shake of needle in charging current meter, we considered that the

current state of charge is higher than 90% and the regulator started to control the charging

current. Between the connection of battery and controller, a wire cable with a resistance

of 17 mΩ had been added in series. When the charging current is controlled by the

regulator, the voltage drop of this wire will change consequently. In this way, we can

obtain the status of charging current by observing the status of voltage drop on this wire

cable and get the working performance of the regulator.

24

Figure 4.3: Current versus time for a PWM-controller when irradiance =613 W/m

2

When irradiance was 613 W/m2, we can read the charging current value from current

meter of 3.3 A. However this is not the actual charging current due to the interference

from controller. From the oscilloscope we can see that pulse duration TD is 98 ms and

the repetition time TR is 106 ms. From the waveform indication the magnitude of voltage

drop was 26 mV and therefore the magnitude of charging current was equal to voltage

drop/wire resistance=26mV/17 mΩ =1.53 A (as shown in figure 4.3). Based on the

waveform we also can know that the repetition frequency is equal to 1/repetition time=

1/0.106s = 9.48 Hz.

Using the same way we observed the working waveform under different irradiance and

obtained different outcome as shown in table 4.1.

Table 4.1 Working data of oscilloscope

From the data in table 4.1 we can see that the repetition time is fixed, which is 106ms and

consequently with a fixed repetition frequency of 9.4 Hz. While the irradiance increases,

the proportion of pulse duration TD increases.

As the regulator starts to control the charging current when SOC >90%, we tried to

observe how the performance of regulator changes when SOC changes. Six days after the

first time’s observing of the working waveform of regulator, we checked the waveform

again and compare these two sets data. When irradiance was 295 W/m2, the pulse

duration time TD was 64 ms and the repetition time TR was 108 ms, the ratio between

TD and TR became 0.59. From this we can conclude that with a higher SOC, the

repetition keeps the same around 106 ms. However, the ratio of TD and TR decreased

compared with previous measured data.

Irradiance

value

(W/m2)

Pulse

duration

TD (ms)

Repetition

time TR

(ms)

Repetition

frequency

(Hz)

Ratio between

TD and TR

333 94 106 9.43 0.887

613.3 98 105.5 9.48 0.93

873.3 102 106 9.43 0.96

966.7 103 106 9.43 0.96

25

As mentioned above, when irradiance was around 600W/m2

the magnitude of voltage

dropped on the shunt resistance was 26 mV and the magnitude of charging current can be

obtained as 1.53 A.

Consequently, the mean voltage drop of wire resistance is:

mVmV

VT

TDV

T

TDdtVdtV

Tdttv

TV

T

TD

TDT

15.2426*106

98

)1(*)(1

)(1

minmaxmin0

max0

==

−+=+== ∫∫∫

As the shunt resistance of wire cable is 17 mΩ, the mean charging current should be:

AmmVRVI 416.117/15.24/ =Ω==

Based on the principle we know that the working of regulator only changes the pulse

duration time, in other words, the ratio between TD and TR, which consequently changes

the mean charging current. For the magnitude of charging current, however, it mainly

depends on the irradiance and will not change due to the change of State of Charge.

Usually, the magnitude of charging current can be up to 3 A under 600 W/ m2 in our

system. Nevertheless, as we shown above, the magnitude value we obtained was only 1.5

A, which was quite low compared with the expected data. This means the oscilloscope

waveform can not right indicate the actual voltage value and consequently the current

value can not be obtained correctly. This may due to the ambient noise or the capacity of

the wire, which might cause the voltage measured to be a mixed signal. Even though the

oscilloscope used in this project can correctly show a pure AC or a pure DC signal, it can

not indicate the signal which combining DC component with low frequency AC

component successfully.

After observing the performance of regulator, another discharging test was carried out

and a total energy of 74.7 Ah was withdrawn from the battery. This value was higher than

the withdrawn energy of 70.3 Ah, which was measured following the first time’s

observation of regulator’s working performance. This energy data also proved the state of

charging this time is higher than the state of charging last time and therefore the energy

withdrawn increased.

26

5 Relationship between open circuit voltage and cell temperature

The performance of a PV module has a direct connection with cell temperature, which is

dictated by several ambient conditions such as ambient temperature, irradiance, wind

speed, wind direction and humidity. The purpose of this chapter is to determine whether

the open circuit voltage Voc can be used to estimate the cell temperature, without taking

into account ambient conditions like wind speed and ambient temperature. The work we

did in this chapter include following: (1) extrapolate STCOCV , under different ambient

conditions; (2) calculate the cell temperature based on actual open circuit voltage value

and extrapolated open circuit voltage value; (3) analyze the relationship between them.

5.1 Extrapolation of Voc to Standard Test Condition (STC)

Under different condition the measured characteristics could be different. Therefore, it is

difficult to compare the performance among different panels directly based on the

measured data obtained. It is also very hard to judge the working capability of a panel as

the measured data might be diverse under different actual on-site conditions. Actually,

they are usually extrapolated to Standard Test Conditions (STC) before compared.

Standard test condition is the condition under which a module is typically tested in a

laboratory: (1) irradiance intensity of 1000 W/m2; (2) AM 1.5 standard reference

spectrum; (3) cell temperature is 25 oC. If the panel is under good condition, the

extrapolated data should be similar to the factory given data. It is usually used to detect

the possible difference between different modules or test the possible performance

degradation of modules and arrays. [7]

In the international standard IEC 1829 [8], extrapolating the measured data of crystalline

silicon photovoltaic array characteristics to standard test condition (STC) or other

selected temperatures and irradiance values are introduced. Such a extrapolating

evaluation can provide data on power rating; verify of installed array power performance

relative to design specifications; detection of possible differences between on-site module

characteristics and laboratory or factory measurements; detection of possible

performance degradation of modules and arrays with respect to on-site initial data.

Based on this standard document, the standard corresponding extrapolated Voc,STC can be

obtained by calculated together with reading of ambient temperature TA and of irradiance

G, the equation as shown below:

)]25(**)/1000ln(*[*, ASOCSTCOC TGBGANVV −+−+= β …….………(1)

Here Voc is the open circuit voltage measured under on-site condition;

STCOCV , is the standard corresponding extrapolated value;

Ns is the number of cells in series in the array; in this project the Ns is 36.

27

Voc value under ambient temperature usually depends on Voc,STC, irradiance G and cell

temperature Tcell. That is

OCV = f(Voc,STC, G, Tcell).

As )1ln(s

ph

ocI

I

q

kTV += , a coefficient value can be used to represent the relationship

between OCV and irradiance.

In equation 1, )/1000ln(* GA refers to the voltage value influenced by irradiance. Here

A is the product of the thermal voltage and the non-ideality factor. The value usually is

38 mV/cell.

As Tcell value is usually determined by Tamb and irradiance.

),( GTfTcell A=

The influenced factor caused by ambient temperature can be represented as in equation 1:

AT−25(*β ).

Here 25 is the standard cell temperature. For example if the ambient temperature is lower

than this standard cell data, the measured Voc could be higher than STCOCV , value. β is

the temperature coefficient of the open current voltage, usually - 2.2 mV/ o

C /cell for

crystalline cells.

For the irradiance part which influences the cell temperature, it can be described as in

equation 1: - GB * .

When solar irradiance increases, under illumination, the cell temperature increases too.

Therefore at high irradiance Voc will drop and a positive factor is needed to increase Voc

to get STCOCV , . Here B = dGdTJ /*β . Where dGdTJ / is usually about 0.03 oC /W.m

-2,

which means the increased cell temperature value corresponding to irradiance. The

production of β and dGdTJ / means the voltage cell temperature which refers to the

heating of the cell respecting to irradiance.

5.2 Calculation of cell temperature

As there is a good heat transfer coefficient for the PV panels, solar cell temperature is

usually considered to be the same as its junction temperature TJRO.

According to IEC 1829[8] we know that the junction temperature of PV panel can be

calculated as:

CVkVTTo

STCOCOCJROcell 25/)*('

,

'+−== β (5-1)

Here β is the voltage temperature coefficient of the referenced PV device (usually -2.2

mV/ oC);

k is a coefficient taking into account the irradiant deviation between measurement

conditions and 1000 W/m2:

28

k=1.000 for 1000 W/m2 irradiance;

k=0.996 for 900 W/m2 irradiance;

k=0.989 for 800 W/m2 irradiance;

k=0.983 for 700 W/m2 irradiance.

As '

OCV used in the equation (5-1) means the open circuit voltage of one cell and '

,STCOCV

means the extrapolated value of one cell, the equation needs to be altered as shown below:

CVkVTo

STCOCOCcell 25/)36/)*(( , +−= β (5-2)

Here OCV and STCOCV , are measured open circuit voltage and extrapolated data for total 36

cells in a panel. Therefore they need to be divided by 36 to obtain the voltage value for

one cell and then calculate the cell temperature.

5.3 Analysis of outcome

Based on this calculation, several sets of the measured data, extrapolated data and cell

temperature are shown in table 5.1.

Table 5.1 Measured Voc versus extrapolated STCOCV , versus cell temperature

Ambient

temperature (oC)

Voc (V) Irradiance

(W/m2)

STCOCV , (V) Cell

temperature

(oC)

31-05-2007

19 17.57 1133.33 19.62 50.84

19 17.33 1073.33 19.31 49.98

20 17.28 1033.33 19.29 50.43

20 17.51 833.33 19.34 48.15

28-06-2007

16 18.98 813.33 20.48 41.37

16 18.95 833.33 20.47 41.91

16 19.05 833.33 20.57 41.90

17 18.64 1066.67 20.45 48.57

From table 5.1 we can see that the extrapolated value STCOCV , obtained are around 19.4 V

with the measured data on 31st, May, 2007, under different ambient temperature and

irradiance. However, instead of keeping around 19.4V, the extrapolated value STCOCV ,

with measured data on 28th

, June, 2007 are around 20.5 V. It doesn’t conform to what we

expected. The expected extrapolated value STCOCV , should be more or less the same as all

of them should be close to the laboratory or factory data under STC tests, regardless of

ambient conditions.

This could be explained that although ambient temperature and irradiance are two major

factors that affect the deviation between actual Voc and STCOCV , , many other factors such

as wind speed, wind direction, humidity, clouds, also would more or less affect the

29

preciseness of deviation. However, the calculation in IEC 1829 doesn’t take these factors

into account. When checking the wind speed on 31st, May, it was 22 km/h while was 32

km/h on 28th

, June [10]. With a higher wind speed, the generated heat can be blown more

quickly; therefore the cell temperature reduced and open circuit value increased. Besides,

the tolerances of these measured devices also need to be taken into account.

For the cell temperature we can see that even Voc is higher, it does not mean that the cell

temperature is lower or higher as the irradiance also will cause the variation of cell

temperature, such as measured data on 31st, May. For those data which have the same

irradiance, like 833.33 W/m2 in the table, we can see that a higher Voc value can be used

to reflect a lower cell temperature existing. Based on these measured data, therefore, we

can conclude that the open circuit voltage can estimate the cell temperature only when the

solar irradiance does not change too fast and the other ambient conditions like wind speed

does not vary too much to ensure the stability of extrapolation data of STCOCV , .

30

6 Energy output and ambient conditions

6.1 Power output of PV system

The performance of a PV module has a direct connection with operating temperature. For

example, the power output of a mono or poly-crystalline silicon module decreases at

about 0.5%/oC. The operating temperature of a module could be dictated by several

factors, including module’s design characteristics, module installation type and ambient

conditions such as ambient temperature, irradiance, wind speed, wind direction and

humidity.

The characteristics of a Photovoltaic depend on fluctuations of the cell temperature and

irradiation. In our grid-connected PV system a maximum Power Point Tracking (MPPT)

is installed inside the inverters to extract the maximum power from a PV array, convert

this to alternating current and transmit the energy to the power grid.

A MPPT mechanism is a high efficiency inverter that uses maximum power point

tracking technology to harvest the maximum power from the PV array, which will change

according to the variations in parameters such as solar radiation and cell temperature. The

maximum power increases with increasing radiation at constant cell temperature the

maximum power decreases with increasing cell temperature at constant irradiance.

The working principle of it is based on the characteristics of photovoltaic cells that have

a single operating point where the values of the current and voltage of the cells result in a

maximum power output. The objective of maximum power point tracking is to move the

PV array operating voltage close to the maximum power point under changing

atmospheric conditions in order to draw the maximum power from the array. Maximum

power point trackers utilize some type of control circuit or logic algorithm to search for

this point. If a trial shows that what the best power is, it will move control point to use

new PV panels’ voltage and current output [9].

Because of this, we need to find out the relationship between cell temperature, irradiance

and energy output of PV systems, which can be considered as a reflection of MPPT

working performance.

6.2 Analysis of the relationship between energy output and ambient conditions

In order to find out the relationship between these data, firstly we need to obtain the

corresponding value. As mentioned in chapter two, two energy meters are available to

record the amount of kWh that had been delivered to the grid since installation and the

current delivering data. The power meters used in this project are 9024 power meter of

PeakTech make. This energy meter allows measuring the energy consumption of

electrical appliance, and by entering the electricity rate, to calculate the total cost of

31

appliances consumption. It also detects the overload condition and usage information. So

we can obtain energy generated currently from the power meters.

By using the same approach as mentioned in previous chapter, the cell temperature is

determined by measuring Voc and comparing the Voc with STCOCV , using the equation (5-

2) from section 5.2. In table 6.1 eight sets of data which were measured with an interval

of 15 minutes are shown.

Table 6.1 Data of ambient parameters and energy output

Based on the table data we can see that when the irradiance is the same, like the data of

set 4 and set 7, the module with a lower cell temperature (47.15 oC) generated more

energy, which fits in with what we mentioned previously that the maximum power

increases with decreasing cell temperature at constant radiation. While comparing the

data of set 5 and set 6, we can find that, with a higher irradiance and lower cell

temperature, the PV module generated more energy. Therefore, a good ventilation design

of the system which can help to lower the cell temperature is very important to insure the

energy output.

Moreover, as the peak power of the grid-connected system used in our project (under

Standard Test Conditions) STCP is 108 Wp, the actual peak power of a grid-connected PV

system (under on-site condition) actP can be described by:

PRPP STCact ×=

Here PR is called performance ratio, which takes into account all the losses that occur at

systems level, apart from the losses that take place in the solar cell itself at STC. In grid-

connected PV systems important system losses are mainly including: inverter losses;

losses because the cell temperature deviates from 25 oC and losses because the irradiance

on the cell is in general lower than 1000 W/m2. For the system the invert loss is normally

taken as 9%; for a crystalline module typically the temperature loss coefficient is around

-0.45 %/oC.

When testing the quality of energy output by set 1 data, we find that the on-site peak

power actP can be estimated as:

Sets

Voc (V) Irradiance (W/m

2)

Cell temperature (oC)

Left side power meter display (W)

Right side power meter display(W)

Actual Peak power (Wp)

1 19.05 833.33 41.90 76 79 75.7

2 18.98 813.33 41.37 76 80 74.0

3 18.98 866.67 42.83 78 84 78.3

4 18.55 1006.67 47.15 90 93 89.1

5 18.61 1053.33 48.25 95 98 92.7

6 18.54 1013.33 50.71 91 94 88.1

7 18.28 1006.67 50.55 88 91 87.6

8 18.22 966.67 49.65 85 89 84.5

32

actP= STCP PR× = STCP × inverter loss factor× temperature loss factor × (

2/1000 mW

G

)

= 108 W× (1-9%)× (1+ (41.9 oC -25 oC) × (-0.45 %/ oC)) × (2

2

/1000

/33.833

mW

mW

)

= 75.7 Wp .

Comparing between this estimated peak power value and the actual energy delivered to

the grid which we obtained from energy meters, we can see that as the working of MPPT,

the power generated was kept around the on-site peak power point. The same way to test

the other sets data and we get that the MPPT in our system is under good condition and

always keeps the solar systems running near the maximum power point.

As the peak power value can be obtained by using irradiance and cell temperature

parameters and cell temperature can be estimated by open circuit voltage, we can

approximate the energy power that will be delivered to the public power grid. With good

performance of MPPT, therefore, the irradiance and open circuit voltage are considered

to be very good parameters to reflect the energy output.

33

7 Conclusions and recommendations

From the experiments we did for this project, conclusions are given as following:

* As one of the important components in PV system, testing the battery’s performance is

very necessary to check if it is suitable for the research. The total amount of energy

withdrawn from the old battery in our project was only 40 Ah while for the new one was

more than 70 Ah. Therefore we conclude that the old battery was aging and are needed to

be replaced by a new one.

* Based on the measurements of the repetition frequency and ratio between TD and TR

by an oscilloscope we know that the repetition frequency is fixed even when the

irradiance changes. Meanwhile, the ratio between TD and TR increases with an increase

of irradiance. However, with a higher SOC condition, this TD and TR ratio decreases as

they are also determined by the actual battery voltage.

* For the ideal extrapolated value STCOCV , , the calculated data should be similar to each

other. However, the deviation we obtained is a little larger which we conclude as the

effect of the other ambient factors and the tolerances of the measured devices. For the

relationship between cell temperature and open circuit voltage we can get that open

circuit voltage can give an immediate indication for the cell temperature only when the

solar irradiance does not changes too fast and the other ambient conditions like wind

speed does not vary too much to ensure the stability of extrapolation data of STCOCV , .

* Via the measurement of energy output and ambient conditions we can find out that with

a higher irradiance and lower cell temperature, the PV module can generate more energy.

Besides, the open circuit voltage and irradiance can be used to estimate the energy output

in our grid-connected system as the MPPT trackers are performed under good condition.

By performing the experiments during this project some recommendations can be drawn

as following:

Repeated discharging and charging can help to activate the reaction materials and

therefore increase the battery’s capacity, especially for those that have not been used for a

long time.

For the regulator used in the system we need to care the voltage drop on the connection

wires, especially the wires connecting between battery and regulator. Thick and low

resistance cables are recommended to make sure the voltage measured in the regulator

side is as close as possible to the actual battery voltage.

During the experiments process, it is very important to pay attention to the order to

disconnect or connect with these components. Usually remove the PV modules’

connections from the controller first, and then disconnect battery and regulator. Never

connect the PV panel with regulator directly, without the connection of battery. It will

break the regulator easily.

34

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3, S.O.Kasap,(2001). Optoelectronics and Photonics: Principles and Practices. Published

by prentice-Hall, Inc.

4, R.van Zolingen .Shell Solar PV course (version 3), chapter 5;

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20th

European photovoltaic solar energy conference, p2051-2054.

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10, Weather online, retrieved 2nd

, 08, 2007 from

http://www.weatheronline.co.uk/cgi-

bin/aktframe?TYP=windspitzen&ART=karte&RUBRIK=akt&JJ=xxxx&MM=01&TT=1

8&TIME=1800&KEY=NL&LANG=en&SORT=2&INT=06

35

Appendix

A.1: Discharging data (charged by solar system)

36

A.2:Discharging data (charged by a conventional battery charger)