Photovoltaic Cell Functioning Principle

Published Date: 02 Nov 2017

Chapter 1 - Introduction

History…………………………………………………………………………………………2

Chapter 2 â€" The science of Photovoltaic Cells

2.1 Photovoltaic cell functioning principle………………………………………………………..

2.2 Conditions……………………………………………………………………………………..3

2.3 The photovoltaic cell in an electrical circuit…………………………………………………..3

2.4 Types of solar cells…………………………………………………………………………….

2.4.1 Thin film(amorphous) modules.…………………………………………………….

2.4.2 Monocryistalline modules……………………………………………………………

2.4.3 Polycrystaline modules………………………………………………………………

2.4.4 Shapes of the photovoltaic panels……………………………………………………

2.4.5 Concerns……………………………………………………………………………...

Chapter 3 â€" The installation of Photovoltaic Panels

3.1 Types of photovoltaic installations……………………………………………………………

3.1.1 Stand-alone ………………………………………………………………………….

3.1.2 Grid-connected………………………………………………………………………

3.2 PV Array………………………………………………………………………………………

3.2.1 Sizing

3.2.1 Nominal array power……………………………………………………………….

3.2.2 Temperature dependance of array power output……………………………………

3.2.3 Module orientation………………………………………………………………….

Chapter 4 â€" Example of a Stand-Alone PV System

Chapter 6 â€" Bibliography…………………………………………………………………………...

Chapter 1 â€" Introduction

History of Photovoltaics

Solar comes from the latin word “Sun” â€" the most powerfull energy source used to heat, and light our lifes, homes, buildings. There are lots of ways to convert the energy from the Sun intro usefull energy for us. These are solar water heating, passive solar design for space heating and cooling, and solar photovoltaics for electricity.

Solar cells, also called photovoltaic cells (PV) convert the energy of the Sun into electricity at the atomic level. PV gets its name from the process of converting light (photons) to electricity (voltage), which is called the PV effect.

The photoelectric effect was first discovered in 1839 by a 19 year old French physicist, named Edmund Bequerel, who found out that certain materials immersed in a liquid are producing a small amount of electric current when they are exposed to light. Later in the 1870’s W.G. Adams and R.E. Day, two english investigators observed that it is possible to generate and maintain an electrical current in selenium using candlelight. After a few years later, C. E. Fritts, an American inventor, made the first selenium based photovoltaic cell wich converted sunlight into electric current. The discovery was confirmed by Werner Siemens, a German inventor. However, the results were not satisfying. But at the time, many researchers and phisicists felt that the production of energy without the consumption of pure matter was a violation of the laws of physics and the work and discovery were not pursued.

Photovoltaics was considered an acceptable area of study only after the concept of photons and electrons was introduced with the beginnings of quantum mechanics. Scientific models were finally equipped to deal with the concept of light absorption. In 1931, the German Scientist, Bruno Lange, rediscovered the selenium-based solar cell. The promise for inexhaustible, pollution-free energy created a lot of excitement. However, the selenium-based solar cell converted less than 1% of the incoming sunlight into energy, which was far too low to justify it as a practical power source.

Researchers and the public did not get excited about photovoltaics again until the 1950’s with the beginning of silicon transistor technology. In 1954, Calvin Fuller and Gerald Pearson, of Bell Labs, discovered that the efficiency of silicon rectifiers (which convert AC to DC) changed depending on their purity as well as the lighting conditions. They soon discovered that the rectifiers converted 4% of the incoming sunlight into electrical energy. Along with Darryl Chapin, a colleague, Calvin Fuller and Gerald Pearson started a team to work on what they named the Bell Silicon Solar Battery.

Further research increased the efficiency to 15% and soon limited markets for silicon solar cells were discovered. They were first used as a power source for a telephone relay system in an isolated rural area, however an economic analysis later revealed that a conventional power supply would have been better. Plans were laid out to use them at national forest lookout posts and Coast Guard buoys, but government agencies anticipated that small nuclear power systems would be better.

At this point, no commercially viable applications could be found for silicon solar cells on earthâ€"but they were soon found above the earth, in space. The space race began in the late 1950’s and designs for satellites called for a long-term power source that was compact and lightweight. Conventional fuels systems and batteries were far too bulky and heavy; in addition, fuel was expendable. Depending on the altitude and direction of the orbit, satellites could be exposed to the sun almost continuously. Because of these advantages the U.S. space program created the now viable silicon solar cell industry. Most of this history was obtained from reference.

In 1958, six small silicon solar panels, providing 100 mW of power were included on the satellite Vanguard I. Since then, the global photovoltaic production has gone from 100 mW to over 200 MW in 1999. Terrestrial applications make up the most of this as space systems consist of less than 1% of the industry. Two common photovoltaic applications are remote area power supplies (RAPS) and building-integrated photovoltaic (BIPV) systems. A RAPS system is one that is not connected to a standard power grid and therefore needs to store energy in batteries for use at night. These system are used for single homes, farms and local rural communities. BIPV systems integrate the solar cells into the design of the building by replacing roof tiles or façade material. They are connected to standard power grids so shortfalls in the power drawn by the sun can be made up by taking energy from the connected grid, and surpluses in energy can be sold back to the standard power grid.

Chapter 2 â€" The science of Photovoltaics

2.1 Photovoltaic cell functioning principle

http://www.energie-verde.ro/images/stories/sisteme-fotovoltaice/cum-functioneaza-panourile-solare.gif

1. Light (photons)

2. Front surface

3. Negative layer

4. Insulator layer

5. Positive layer

6. Back surface

The electrical energy is being produced as lond as the panel is exposed to Sun light. The solar cell absorbes only a part of the light particles which fall directly on its surface. Every photon contains a small amount of energy. When a photon its absorbed, it gives away some energy. And because every part of a solar cell is connected to a cable, a current will pass through it. The cell will produce electricity wich can be used instantly or can be stored in acumulators.

2.1 Conditions

In order for a material to convert the sun light into electrical energy, it must satisfy two conditions: the first: it needs to be able to absorb incident photons through the promotion of electrons to higher energy levels and second, it must contain an internal electric field that accelerates the promoted electrons in a particular direction, resulting in an electrical current.

2.2 The photovoltaic cell in an electrical circuit

In a basic point of view, a photovoltaic cell can be thought of as any deviceâ€"when exposed to lightâ€"that causes current to flow in an electrical circuit with a given load resistance. An example is shown in Figure 1. The voltage drop across the front and back contact and the current in the circuit can be measured with a voltmeter and ammeter. The magnitude of the electrical current depends on the intensity of the incoming light. In addition, the current also depends on the load resistance of the circuit.

Figure 1

For the following discussion, let us fix the intensity of the incoming light as Tl, and just vary the load resistance. If the resistance is infinite, the current will be zero. This is called the open circuit condition. In this case, the photons continue to generate pairs of electrons and holes within the photovoltaic material. The internal electric field separates the electrons and holes and accelerates them in opposite directions creating a voltage difference on either side of the photovoltaic cell. The magnitude of this voltage drop is called the open circuit voltage, VOC. The open circuit voltage scales with the intensity of light, but for a given intensity, the voltage remains constant with time. The reason it remains constant, despite the fact that photons are continually absorbed, is that the generated electron-hole pairs recombine at a certain rate at defects and surfaces. Because of this, the open circuit voltage is a good measure for the quality of the photovoltaic cellâ€"the higher the open circuit voltage, the more efficiently the cell can convert photons into electrical energy.

If the load resistance in the circuit shown in Figure 1 is zero, then under a given intensity of light, the current that is generated will be at its maximum attainable value. Under these conditions the photovoltaic cell is said to be in its short circuit configuration and the current is called the short circuit current, ISC. Note that through V = I R, the voltage drop is also zero. The short circuit current is also limited by the recombination rate of the material and is a good measure of the quality of the photovoltaic cell. In order to maximize the efficiency of a

photovoltaic cell, researchers and engineers attempt to maximize the open circuit voltage and short circuit current by removing recombination sites.

In between these two extremesâ€"when there is a finite resistance in the circuitâ€"the current and voltage are both less than their maximum values and are related to the load resistance through Ohm’s Law, V = I R. The power that is supplied as a result of the load resistance (e.g. the light bulb) is the product of the current and the voltage. For a higher load resistance, the voltage is increased but the current is decreased. For a lower load resistance, the voltage is decreased but the current is increased. Figure 2 shows how the voltage and current of a circuit can vary as the load resistance is changed for a given amount of intensity of light. Rather than obtaining this curve by putting in various size resistors, the voltmeter, ammeter and resistor can all be replaced by a voltage/current source. By applying a voltage and measuring a current the same curve can be reproduced.

Figure 2

Photovoltaic cell functioning principle

The radiation quantity wich reaches the earth is variable. This quantity depends on the moving of the Sun and olso on the climatic changes. From these reasons the photovoltaic panels are built in direct relationship to the measurements token in the building place.

The current-voltage characteristic equation on a photovoltaic cell is :

Where :

Iâ‚’ - saturation current

UT

2.4 Types of solar cells

There are three types of solar cells : amorphous or thin-film, monocrystaline and polycrystaline modules. They distinguish one from another by the type of the crystal used to produce them.