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Solar energy made simple.

This is the second part of our major series on solar energy which we began in the July issue. Africa gets 365 days of glorious sunshine a year, and yet the majority of our people (at least 85% or so) who live in rural areas have no electricity. In this article, Kwabena Osei (left), our solar energy expert and engineer, takes us through the very basics of solar energy generation.

You can call this the ABC of solar energy. As in every field, the jargon can be technical and confusing. But I am going to do the best I can to break them all down to the lowest denominator for the ordinary reader to understand this very important subject (solar energy), which, to me, holds the key to Africa's second liberation. The "first liberation was political, the "second liberation" will be economic and how we use energy (of gain access to abundant and cheap electrical power) for economic development and domestic use.

Thus, in the piece, I will explain how a Photovoltaic (PV) system functions. I will also highlight the components that are brought together to make up the PV system.

Photovoltaic (PV) is the term used to describe the complete system for generating electricity from the sun. The basic components are the PV generator, the battery and the controller unit, and the cables that join all these components together.

A PV generator churns out electricity so long as the sun shines. To understand how solar cells work, we must first know something about the nature of the sun.

Sunlight is a form of electromagnetic radiation similar to radio waves and microwaves. The sun radiates simply because it is hot. The sun is a black body whose radiation is composed of a broad mixture of different wave lengths. If the sun were cold, it would appear black simply because it would only absorb radiation.

Solar cells are electronic devices without any moving parts, which convert light energy (sunlight) directly into electricity. The idea is so fascinating that Africans should make good use of this energy from the sun. Poor street lighting in most African cities can dramatically be improved. PV systems can be installed anywhere the sun shines.

A major component of this system is the solar cell, which is made from semiconductors such as silicon, usually in the form of thin slices (wafers) about 1/4 mm thick. The positive contact is a layer of metal on the back of the wafer, while the negative contact on the top of the cell collects the current but alto allows as much light as possible to enter the device. The top contact is made in the form of a grid.

To know how solar cells work, we must understand the term electricity. Electricity is a form of energy. It is simply the flow of electrons. All matter consists of atoms. The centre of an atom, known as the nucleus, contains both positively-charged particles known as protons and uncharged particles known as neutrons.

The nucleus is surrounded by negatively-charged particles called electrons. An atom is stable when there is a balancing force between the negative and positive charged particles.

The number of electrons in an atom is normally equal to the number of protons. When an outside force, for example light energy (sunlight), disturbs the balancing force, an atom may gain or lose an electron. A repeated upset by an outside force results in free movement of electrons and this represents an electric current.

A solar cell produces electric current and voltage by "photovoltaic effect"--a process in which two different materials in close contact act as an electric cell when struck by light or other radiant energy.

Light striking such crystals as silicon of germanium provides the energy needed to free some electrons from their bound condition, which result in electrons moving from atom to atom within the crystal.

A PV-generator generates direct current (DC), normally 12 volts, The system may contain a supplementary of back-up generator (for example a diesel generator) to from a hybrid system. We will now consider in detail each of the components mentioned above.

PV generator

It is the heart of the PV-system. This consists of a photovoltaic module made up of solar cells being the basic construction unit.

A large number of solar cells are connected in series and parallel to construct photovoltaic of solar arrays. The number of cell in a module determines the voltage of the module. It has been found that 36 cells in series ensure a reliable system operation, but for self-regulated systems the number of cells are usually 32 to 34 in a module.

The PV modules are interconnected to form a De power-generating unit. The term "array" is normally used to describe the physical assembly of modules with supports.

The nominal operation voltage of the system usually has to correspond to the nominal voltage of the storage system, Manufactures of PV modules have a standard configuration that works with 12V batteries,

The battery

Most PV applications require some sort of storage system. Solar cells converts solar energy into electric energy and since the solar energy supply is basically variable with time, some form of storage is needed for stand-alone PV systems.

The majority of stand-alone systems today use battery storage. These days lead-acid batteries are commonly used because of their cost effectiveness.

These batteries operate on the principle of changing electric energy into chemical energy by means of a reversible reaction. These batteries are rechargeable and can be used for a period of time.

The ordinary car battery is a typical lead-acid battery. A number of lead-acid battery designs have been developed for electric vehicles, such as forklift trucks, golf carts and as standby batteries for telephone systems and other uninterrupted power uses.

Lead-acid batteries are all designed for specific purposes, thus a lead-acid battery designed for one use might not necessarily work well in another application. So knowledge of the different types is necessary in order to make the proper selection. For the purposes of this article, the type we are interested in is called the "deep cycle battery".

Solar cells require deep-discharge batteries which have the ability to be fully discharged and recharged up to 500 times in smaller types and about 2,000 times in bigger batteries.

The rate at which these batteries are discharged determines how long the battery lasts and how much power you will get out of it. These batteries can last from 5 to 15 years and some even 20 years. Of course depending on the battery's quality and size and user's need for electricity

The size of a battery is expressed in Ampere Hours (AH). This is the total amount of electricity that can be drawn from a fully charged battery until discharged to a specified battery voltage given at a specified discharged time. Thus, considering a car battery of 100-AH capacity, this could theoretically deliver one ampere for 100 hours of 100 ampere for one hour.

The three most important characteristics of lead-acid batteries to be considered when designing and sizing battery storage systems are:

1. The voltage output, which is a function of temperature and the state-of-charge.

2. The useful capacity of" the battery increases significantly with a reduction in temperature.

3. A fully charged lead-acid battery will slowly discharge on stand-by This self-discharge rate is also a function of temperature and battery design.

The above characteristics of lead-acid batteries are important when it comes to systems designed for use in Africa, where high ambient temperature will automatically influence the battery's voltage output.

In my next article, I will put forward some suggestions of how to get the best out of your battery. In practice, the slower the discharging rate, the greater the capacity.

Finally, battery manufacturers are making batteries specifically designed for PV power systems, An example is the DELCO 2000 PV battery. Similar batteries are made by other manufactures such as Gould and Exide.

Power conditioning & control unit

A range of electronic devices are used to accommodate the variable nature of power output from the PV generator, to avoid the malfunction of the system, or to convert the DC current produced by the PV generator into AC output.

A solar cell generates direct current and since most electrical appliances available today works with alternative current, some form of power conditioning and control are necessary--a kind of monitoring device.

The power conditioning and control elements make it possible to convert the generated DC power to AC, protect the battery from overcharge or excessive discharge, and optimise the energy transfer between the PV generator and the battery or load

There are many types of monitoring device ranging from the simple to the very sophisticated, with prices ranging from $100 to $400. The more sophisticated devices are capable of monitoring almost all the parts of the system. Some even allow you to download information to your computer via a modem, giving you a table of your energy consumption over a day, a week, a month or a year.

Since it is possible to monitor almost all parts of the solar system, in case of any malfunctioning you will be able to find out where the problem is. It all depends on how much money you are willing to invest initially in the components. So make sure you understand what and how much you need to monitor.

The cables

Tire cable, to me, is as important as the PV generator. The cable transports tire electricity for use. Every aspect of the cables is therefore important: the size, the material they are made of, the length and even the colour:

Using a wrong cable size can result in inadequate power supply From the PV-system. It is extremely important to use the right wire size to let all the electricity more easily through to avoid the loss of electricity within the cable.

The wire size should be sufficient to carry the peak current produced by the array. Bigger conductivity will not harm the system, but will rather add more mechanical strength to the system.

The conductor size of wire is measured in numbers of AWG (American Wire Gauge). The type and size are printed or engraved on the cable. The smaller the size number, the bigger the conductor.

If you look at the printing on the cable, it might look like this: Size AWG 10-2 UF-B Sunlight Resistant. The first number (10) is the size of the conductor. The second number (2) tell you that there are two conductors in The cable

The letters UF stands for underground feeder, which means the cable can be buried directly in the ground.

The next letter (-B) refers to the temperature the conductor in the cable is rated for, which in this case is 90[degrees]C.

Telephone wires ranges from size 18 to 2. AWG. Your house wiring is mostly size 14 o 12 AWG, As a rule of thumb: number 14 cal carry 15 amps, number 12 20 amps, number 10 25 amps, number 8 35 amps, and so on There are rabies and charts to help you choose the right cable size.

It is important to know that a low voltage distance Becomes a crucial factor. Every centimetre counts because you have very little pressure of volts available.

In the next article, we will look at the systems designed for domestic applications.

This will be one of the most important part of our series on solar energy, because it will give us practical ideas of how solar energy can be used in our homes, offices, etc. We will also look at load calculations as well as system upgrading which will give us an idea of the cost of solar electricity.
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Article Details
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Author:Osei, Kwabena
Publication:New African
Geographic Code:60AFR
Date:Dec 1, 2003
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