DETERMINATION OF YEARLY PERFORMANCE AND DEGRADATION RATE OF ELECTRICAL PARAMETERS OF AMORPHOUS SILICON PHOTOVOLTAIC MODULE IN MINNA, NIGERIA

ABSTRACT

There is need for accurate knowledge of performance, degradation rate and lifespan of photovoltaic (PV) module in every location for an effective solar PV power system. Outdoor degradation analysis was carried out on amorphous silicon PV module rated 10 W using CR1000 software-based Data Acquisition System (DAS). The PV module under test and meteorological Sensors were installed on a metal support structure at the same test plane. The data monitoring was from 09:00 to 18:00 hours each day continuously for a period of four years, from (December 2014  to  November 2018).The experiment  was  carried  out  near  the Department  of Physics, Federal University of Technology, Minna (latitude 09o37’N, longitude 06o32’E and 249 meters above sea level). The sensors were connected directly to the CR1000 Campbell Scientific data logger, while the module was connected to the logger via electronic loads. The logger was programmed to scan the load current from 0 to 1 A at intervals of 50mA every 5 minutes, and average values of short-circuit current, (Isc), open-circuit voltage, (Voc), current at maximum power, (Imax), voltage at maximum power, (Vmax), power(P) and maximum power(Pmax) obtained from the modules together with the ambient parameters are recorded and logged. Data download at the data acquisition site was performed every 7 days to ensure effective and close monitoring of the data acquisition system (DAS). At the end of each month and where necessary, hourly, daily and monthly averages of each of the parameters-solar irradiance, solar insolation, wind speed,  ambient  and  module  temperatures,  and  the  output  response  variables  (open-circuit voltage, Voc, short-circuit current, Isc, voltage at maximum power, Vmax, current at maximum power, Imax, efficiency, Eff, and fill factor, FF) of the photovoltaic modules were obtained. Annual yearly averages of the performance variables were carried out to ascertain the performance, degradation rate and lifespan of the module. The module performance for the four years of study was compared with Standard Test Condition (STC) specifications. The maximum power achieved at 1000W/m2  for the four years of study were 0.652W, 2.186W, 2.078W, and 1.812W  representing  6.52%,  21.86%,  20.78%  and  18.12%     of  the  manufacturer’s  10W specification. Module efficiency at 1000W/m2  for the four years of study is 2.25%, 7.56%, 7.19%, and 6.27% respectively as against the manufacturers STC specification of 33%. Accordingly, Module Performance Ratios for the PV module investigated were 0.07, 0.23, 0.22 and 0.19 respectively. For the Rate of Degradation (RoD), it was observed that Open-Circuit voltage (Voc), Short-Circuit Current (Isc), Power-Output (P), Maximum Power (Pmax), had an average yearly degradation rate of 0.73V, 0.010A, 0.040W, 0.050W respectively for the four years  of  study.  To  also  determine  the  lifespan  of  the  module,  an  empirically  determined statistical model given as YEAR = 3.36 – 0.237 Voc (v) – 71.5 Isc (A) + 8.07 Power (W) was fitted to the observed data to predict the lifetime of the module at any given year.

CHAPTER ONE

1.0 INTRODUCTION

1.1 Background to the Study

A photovoltaic cell is an electrical device that converts the energy of light directly into electricity by the photovoltaic effect. It is a form of photoelectric cell, defined as a device whose electrical characteristics, such as current, voltage, or resistance, vary when exposed to light. Individual solar cell devices can be combined to form modules, and modules are combined to form solar panels. In basic terms a single junction silicon solar cell can produce a maximum open-circuit voltage of approximately 0.5 to 0.6 volts.

Solar cells are described as being photovoltaic, irrespective of whether the source is sunlight or an artificial light. They are used as a photo detector such asinfrared detectors, detecting light or other electromagnetic radiation near the visible range, or measuring light intensity. The operation of a photovoltaic (PV) cell requires three basic attributes:

          The absorption of light, generating either electron-hole pairs or excitons.

          The separation of charge carriers of opposite types.

          The separate extraction of those carriers to an external circuit.

In contrast, a solar thermal collector supplies heat by absorbing sunlight, for the purpose of either direct heating or indirect electrical power generation from heat. A “photo electrolytic cell” (photo electrochemical),  on  the  other  hand,  refers  either  to  a  type  of  photovoltaic  cell  (like  that developed by Edmond Becquerel and modern dye-sensitized solar cells), or to a device that splits water directly into hydrogen and oxygen using only solar illumination.(Ezewonra, 2016).

The ever increasing world energy demand, the fast depletion of fossil fuels and the unpredictable weather pattern due to global warming have prompted the world to look for alternative source of energy. New manufacturers have come up and new technologies have emerged to meet the high energy demand by consumers. While more manufacturers and new technologies are emerging, the reliability of solar PV modules becomes a critical performance measure for the success of the industry (Ronget al., 2011). The performance of PV modules has been observed to gradually decrease with operation  time (Dunlop and  Halton, 2006).  It is important to investigate the performance parameters of the modules. In order to have maximum sunlight conversion, the tilt and orientation of the modules should be maximized (Akachuku, 2011).

With rapid economic growth and improvement in living standards, there has been a marked increase in energy consumption in many third world countries. Most countries use fossil fuel, hydroelectric power and nuclear power as a source of energy. Nuclear and fossil fuels have adverse effects on the environment such as large amounts of greenhouse gases emissions and pollution from the burning of fossil fuel (René, 2005, Azhar and Abdul, 2012).

Since fossil fuel and nuclear sources of energy are not renewable, it is necessary to explore other sources of energy that are cost effective especially in the developing countries that rely heavily on imported fossil fuel. Renewable energy such as sunlight, wind tides and wave can be particularly suitable for developing countries especially in rural and remote areas where transmission  and  distribution  of  energy  generated  from  fossil  fuels  can  be  difficult  and expensive. Producing renewable energy locally can offer a viable alternative.

Technology advances are opening up a huge new market for solar power.  Even though they are typically poor, these people have to pay far more for lighting than people in rich countries because they use inefficient kerosene lamps and stoves. Solar power costs half as much as lighting with kerosene. An estimated three million households get power from small solar panels (Duke et al., 1999).

The energy conversion efficiency of a PV module or array as a group of electrically connected PV modules in the same plane is defined as the ratio between electrical power conducted away from the module and the incidence power of the sun (Rakovect and Klemen., 2011). This conversion efficiency of photovoltaic (PV) modules by manufacturers is done under Standard Test  Conditions  (STC).  The  Standard  Test  Conditions  are  module  temperature  of  25oC, Irradiance of 1000W/m2 and Air mass of 1.5.

The orientation of PV modules determines their power output. This orientation is described by its azimuth and tilt angle. For fixed modules, the azimuth angle is the angle the modules make with the true North, when measured in a clockwise direction. The tilt angle is the angle that forms between the horizontal and the vertical axis of the PV module. It is the latitude at a given location. Many investigations have been carried out to determine the best tilt angle for PV systems.

Different PV module technologies now exist in the market. These include crystalline modules such as mono/single crystalline, poly/multi crystalline and amorphous modules. The modules available are rated by manufacturer depending on their power output such as 5Watts, 10Watts, 15 Watts. The choice of the module to use depends on the power output needed by the consumer and its efficiency. Photovoltaic (PV) modules are often considered as the most reliable elements in PV systems. However, PV module reliability data are not shown on commercial data sheets in the same way as it is with other products such as electronic devices and electric power supplies. Conversely, the high reliabilities associated with PV modules are indirectly reflected in the output power warranties usually provided in the industry, which range from 25-30 years. As a matter of fact, PV modules have a low return time, the exceptions being the catastrophic failures. The performance of PV modules decreases when deployed outdoors over time. After several years of operation, this decrease will affect PV module reliability (Manuel and Ignacio, 2008).In this study, it is therefore necessary to determine the yearly degradation rate of electrical parameters of amorphous silicon PV module and the findings can be used to investigate its stability and the reliability in any location.

1.2 Solar Energy

The earth is constantly lightened and warmed by the electromagnetic radiation from the sun. This energy warms the earth and atmosphere and sustains life on our planet. It is the most abundant source of energy that can be technically exploited by mankind with what we know today. The amount of solar energy reaching earth equals 4 x 106exajoules (EJ) per year. This is about ten thousand times the primary energy use of the world in 2007 (Chen, 2011).

The power of the electromagnetic radiation on a surface is called irradiance and is measured in watts per square meter (W/m2). Irradiance depends on the radiant power of the sun, the distance between the earth and the sun, the angle at which it strikes the earth and on the atmospheric conditions.

The irradiance of the sun outside our atmosphere is fairly constant and changes only slightly dueto variations in the distance between sun and earth.The solar energy that reaches the earth’s surface varies greatly, depending on the geographical location, time, date and atmospheric conditions. This solar energy can be partially converted into electricity.

Solar energy is created at the core of the sun when hydrogen atoms are merged into helium by nuclear fusion. The core takes up an area from the sun’s centre to about a quarter of the star’s radius. At the core, gravity pulls all of the mass of the suns interior and produces strong pressure.

This pressure is much more adequate to force the fusion of atomic masses. For each second of the solar nuclear fusion process, 700 million tons of hydrogen is converted into the heavier atom helium. Since its formation 4.5 billion years ago, the sun has used up about half of the hydrogen found in its core. The solar nuclear process also produces enormous heat that makes the atoms to release photons. Temperature at the core are about 15 million degrees Kelvin (15 million 0K). Each photon that is created travels about one micrometer before being absorbed by an adjacent gas molecule. The radiative surface of the sun, or photosphere, has an average temperature of about 5,800 Kelvin. A good number of the electromagnetic radiation released from the sun’s surface sits in the visible band positioned at 500 nm (1 nm = 10-9  meters), though the sun also emits considerable energy in the ultraviolet and infrared bands, and small amount of energy in the radio, microwave, X ray and gamma ray bands. The total quantity of energy emitted from the sun’s surface is about 63,000,000 Watts per square meter.

The sun hits the earth’s surface at various angles ranging from 00 (just above the horizon) to 900 (directly overhead), as the earth is spherical in shape. The earth’s surface obtains most of the radiant energy when the sun’s rays are perpendicular. When the sun’s rays are further slanted, they pass through the atmosphere to a longer distance, becoming more scattered and dispersed. As the earth is spherical, the cold Polar Regions do not receive high solar insolation, and also due to the slanting axis of rotation, the regions receive little or no sunlight throughout the year. The earth’s surface gets more solar energy when the sun isinan elliptical orbit and is closer to the earth. The earth is closer to the sun when it is summer in the southern hemisphere and winter in the northern hemisphere. The 23.5 degrees tilt in the earth’s axis of rotation is a very important factor in determining the amount of sunlight striking the earth at a specific place. Generally, the amount of insolation received at a particular place and time depends on the following factors:

i.      Distance from the sun

ii.      Duration of daily sunlight period

iii.       Solar elevation or inclination of the solar rays to the horizon

iv.          Transparency of the atmosphere towards the radiation and

v.      Output of solar radiation

The first three of these factors are closely connected with revolution of the earth. It is to be noted here that the earth revolve around the sun in elliptic orbit and make one complete revolution in 365 days; simultaneously it spins about itself and completes one rotation in 24 hours. The average distance of the earth from the sun is 149.5 million km. The duration of daylight also varies with the latitude and season. The longer the daylight, the greater is the insolation received. In the solar region (i.e. region within solar activities of flares and winds) the duration of daylight is  24  hours  during  summer  and  minimum  of  zero  in  winter  season

1.3 Solar Cells and Applications

The photovoltaic effect was first experimentally demonstrated by French physicist A. E. Becquerel. In 1839, at the age of 19, experimenting in his father’s laboratory, he built the world’s first photovoltaic cell. However, it was not until 1883 that the first solid state photovoltaic cell was built, by Charles Fritts, who coated semiconductor selenium with an extremely thin layer of gold to form the junctions. In 1888 Russian Physicist AleksandrStoletov built the first photoelectric cell based on the outer photoelectric effect discovered by Heinrich Hertz earlier in 1887.

Albert Einstein explained the underlying mechanism of light instigated carrier excitation – the photoelectric effect – in 1905, for which he received the Nobel Prize in physics in 1921. Russell Ohl patented the modern junction semiconductor solar cell in 1946, which was discovered while the researcher was working on the series of advances that would lead to the transistor.

The first practical photovoltaic cell was developed in 1954 at bell laboratories by Daryl Chapin, Calvin Souther Fuller and Gerald Pearson. At first, cells were developed for toys and other minor uses, as the cost of the electricity produced was very high; in relative terms, a cell that produced

1 watt of electrical power in bright sunlight cost about $250, and comparing to $2 to $3 per watt for a coal plant.

A quick introduction to solid state semiconductors is needed to understand the operating principles of a solar cell. A semiconductor is a material with an electrical conductivity between a conductor and an insulator. Its conductivity is defined by the ‘band gap’, the difference between the valence energy of bonded electrons and the conduction energy of free electrons. The most common semiconductor material is Silicon, and it forms the basis of most electronics (Twidell& Weir, 2006).

The current flow in intrinsic, or “pure” semiconductors is low. The conductive properties of thesilicon changes if certain impurities are added . Adding certain impurities is called “doping”. The added impurities create either an electron surplus (n-type) or an electron deficit (p-type). The junction of a p-type and an n-type doped material creates so called diode, enabling conduction of electrons in one way but not the other way. A positive voltage applied to the junction will increase the electrical current, while a negative voltage will decrease the current.

The relationship between current and voltage is described by the Shockley ideal diode equation (Twidell& Weir, 2006):

Where Iis the current through a p-n junction, I0 the saturation or recombination current, qtheelementary charge, V the applied voltage, kthe Boltzmann constant and Tabsolute temperature. A solar cell produces both a voltage and current when it is exposed to light. Electrons are knocked from their valence bonds leaving a ‘hole’ behind. A voltage pulls the electrons in one direction, which then migrate through the cell and through an electrical circuit after which they recombine with holes at the back contact of a solar cell. Under illumination, the total current,Iis the difference between the light generated current ILand equation 2.1

The short-circuit current Iscis the current that would be measured, if there was no voltage across the cell. It is the maximum current that can be generated by a solar cell but without voltage no power is produced. The short-circuit current changes proportionally with irradiance and increases slightly with temperature.

If the voltage is increased, more and more recombination occurs within the cell. The maximumvoltage is called open-circuit voltage (Voc) and at this point no power is produced because there is no current and all generated carriers recombine within the cell. When power is drawn from the cell the voltage decreases because fewer electrons accumulate. The open-circuit voltages changes only a little with irradiance but is very sensitive to temperature changes. A typical Vocfor silicon solar cells is 0.6V. The Vocdepends on temperature and on the ratio of the light-generated current and the recombination current as can be seen in equation 2.3 (Twidell & Weir, 2006),

Tleads  to  a  decrease  in  the  open-circuit  voltage  because  of  the  strong  dependence  of  the saturation current I0 on temperature. The I0 depends on the intrinsic carrier concentration, which is proportional to the third power of absolute temperature (Mulleret al; 2003). A rise in temperature leads to a strong increase in the intrinsic carrier concentration which increases the saturation current which ultimately leads to a drop in the Voc.For each solar cell there is a current-voltage combination that has the highest yield. This is calledthe maximum power point (MPP). This point is characterized by the MPP voltage multiplied with the MPP current. A graphical representation of the current voltage relationship of a PV cell is shown in Figures 1.1

The current-voltage, or I-V curve represent the combinations of current and voltage at which different mechanism influencing the I-V of  a PV module or string can operate (Solmetric, 2011). A normal I-V curve can be divided into three areas: A very low slope starting at zero voltage, a

’bend’ or curve around the MPP and a steep slope between the MPP and the Voc. The I-V curve can look different, due to various reasons. An example can be seen in the upper plane of Figure

The current can be lower than expected, for example due to soiling, shading or degradation ofthe module packaging. Another reason can be a decreased shunt resistance leading to current leaking away.  The  bend  or  curve  around  the  MPP  can  change,  for  example  due  to  corrosion  and increased series resistance. The voltage can be lower than expected, for example due to increased temperatures caused by shaded cells (Solmetric, 2011).

The fill factor (FF) is also an important parameter in determining module performance. It is theratio of the MPP voltage times MPP current, and the open circuit voltage times the short circuit current:

1.4 Statement of the Research Problem

Though we have lifetime of solar modules in literatures, in most cases, it is a projection from laboratory conditions which are quite different from real outdoor conditions. Even when the lifetime is from actual outdoor conditions, it is usually from foreign climatic zones other than ours. Consequently, laboratory conditions are suspect and contestable and actual outdoor evaluation, as regards degradation and lifetime are lacking in our local environment.These have necessitated the study of degradation rate and lifetime in our local environment since the atmospheric parameters that are responsible for the degradation change with locations. The result of this study will assist solar energy installers, planners and designers to have first hand information that will help in establishing an effective solar power system. Manufacturers do not state rate of degradations among their specifications, therefore, knowing their lifespan will help designers to design a good solar power system suitable for our local environment.

1.5 Aim and Objectives of the study

The aim of this research is to determine the rate of degradation of electrical parameters of amorphous silicon Photovoltaic modules as a result of atmospheric parameters in Minna, Niger State. The objectives are to:

i.characterise and evaluate the performance variables of the amorphous module using four years data.

ii.compare yearly performance variable of the amorphous module and deduce the rate of degradation using four years data

iii.deduce empirically determined model for prediction of yearly performance and life time of the amorphous module in our environment.

1.6 Justification of the Study

There are several types of photovoltaic (PV) modules from different manufacturers available in the market. The specifications provided in the manufacturers’ data sheet indicate high performance and high reliability. These specifications are always measured at Standard Test Conditions (STC: module temperature = 25o  C, Irradiance = 1000W/m2  and Air mass = 1.5). That is not very representative of the real conditions in which the PV devices have to operate. Therefore, knowing the yearly performance together with the degradation rate and lifespan of amorphous PV module will assist researchers, policy makers, PV energy designers and installers in designing an effective PV power system best suited for our local environment. It will equally give the consumers first hand information on what to expect from their PV power system before installation, comparative cost advantage and also energy payback time.

1.7 Study Area

Solar energy availability on the earth’s surface is site-dependent and varies throughout the year. It is only worthwhile installing solar radiation-based energy equipment in areas where one can be reasonably assured of adequate supply of such radiation. Minna is located on a location whose latitude is 09o37’N and longitude 06o32′ E, at altitude 249 metres above sea level and one of the Northern states of Nigeria that lie partially within the semi-arid Sahelian belt of West Africa. The climate of this zone is characterized by two distinct and well-defined seasons, namely wet and dry seasons. These seasons correspond to northern hemisphere summer and winter respectively.

The annual onset and cessation of the dry and wet seasons follow the quasi-periodic north-south to-and-fro movement of the inter-tropical convergence zone (ITCZ). The ITCZ demarcates the dry dust-laden north-east trade wind from the moisture-laden south-west wind. The dry season in the Sahel zone of Nigeria sets in about October each year and persists till about May of the next year. This is the period when the ITCZ is displaced

Source: Niger State Ministry of Land and Housing to the south and the prevailing north-east trade wind transports large quantities of dust and smoke from biomass burning into the atmosphere over the entire region (Anuforomet al., 2007).

Dust and smoke aerosols’ affect the climate system at local, regional and global scales in a number of ways. Due to its radiative impact, dust aerosols affects atmospheric temperature, thereby modifying the vertical temperature distribution in the troposphere as a result of the changes in heating and cooling rates at different altitudes (Carlson and Benjamin, 1980; Quijano, 2000). The stability of the atmosphere is thus affected. Some studies have suggested that dust and smoke aerosol in the atmosphere affects also photo synthetically active radiation (PAR) from the sun.

Average wet and dry season ambient temperature in Minna is 28.0oC and 30.7oC respectively, relative humidity is 70.8 and 39.9 respectively and wind speed is 1.68m/s for wet season and 1.82m/s for dry season.

However, Minna is endowed with annual average sunshine hours of about 9.00 hours. Similarly, it has an annual average daily solar irradiation of about 7.00kWh/m2/day of energy from the sun (Balaet al; 2000). Therefore, Minna has the capacity for solar energy equipment.

1.8 Scope and Limitations

Photovoltaic (PV) modules degradation rates are location dependent because the atmospheric parameters that influence their degradation vary with location. Solar irradiance,ambient temperature, wind and relative humidity are the atmospheric parameters known to affect the performance of PV modules in any location on the earths’ surface and the performance variables of a PV module are; open-circuit voltage, short circuit current, voltage at maximum power, current at maximum power, power output and maximum power.

However, different photovoltaic modules (monocrystalline silicon, polycrystalline silicon and amorphous silicon) respond differently to different atmospheric parameters. The study is only limited and within the context of yearly degradation rate of electrical parameters of amorphous silicon PV module in Minna.

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