Solar Power: The Future Energy Resource For Africa-Part 2

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Kwabena Osei

The aim of this article is to provide my fellow Africans the information necessary for acquiring PV-systems for domestic use. We will in this article explain how PV-systems function. We will also highlight components that are brought together to made-up the photovoltaic system.

 

Solar cells are electronic devices that, without any moving parts, convert light energy (sunlight) directly into electricity. The idea is so fascinating that Africans can, with on doubt, make good use of this. Poor street lighting in most big African cities can seriously be improved. PV-systems can be installed anywhere the sunshines.

 

Solar cells are made from semiconductors such as silicon, usually in the form of thin slices (wafers) about ? 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 also allow as much light as possible to enter the device. The top contact is made in the form of a grid.

 

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

 

Photovoltaic generator generates electricity so long as the sun shines, so in order to understand how solar cells work, one first need to 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 is it hot. The sun is a black body. This black body radiation is composed of broad mixture of different wavelengths. If the sun were cold, it would appear black simply because it would only absorb radiation.

 

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 charge particles known as protons and uncharged particles also know as neutrons. The nucleus is surrounded by negatively charge particles called the electrons. An atom is stable when there is a balancing force between the negative and positive charge 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 free movement of electrons and the free movement of these electrons represents an electric current.

 

A solar cell produces electric current and voltage by ?photovoltaic effect?. Photovoltaic effect is 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 or 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. The process by which the absorption of light in a solar cell produces DC electrical power is represented in the figure below. PV-generator generates direct current (DC), normally 12volts.? ?Movement in electric field
PV-systems consist of the photovoltaic generator, batteries, power conditioning and control equipment. The system may contain a supplementary or back-up generator (for example a diesel generator) to form a hybrid system.

PV-systems

 

We will in the following consider each of the components mentioned above.

 

Photovoltaic generator

The heart of the PV-system is the photovoltaic generator. This consists of photovoltaic module made up of solar cells being the basic construction unit. The diagram below represents the photovoltaic hierarchy.

Photovoltaic

 

A large number of solar cells are connected in series and parallel to form module. Panels are made up of modules while the assembling of panels will construct photovoltaic or 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 reliable system operation, but for self-regulated systems the number cells are usually 32 to 34 in a module.

 

The photovoltaic modules are interconnected to form a DC power-generating unit. The term Array is normally used to describe the physical assembly of modules with supports ? see the above diagram.

 

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

 

The battery

Most PV applications require some sort of storage system. Solar Cells convert solar energy into electric energy and since the solar energy supply is basically variable with time, some form of energy storage is needed for stand-alone photovoltaic systems. The majority of stand-alone systems today use battery storage. The type of batteries commonly use is the lead-acid batteries. These batteries are available and cost effective. 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 design have been developed for electric vehicles, such as forklift trucks, golf carts and as standby batteries for telephone systems and other uninterrupted power uses. The lead-acid batteries are all designed for their 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.

 

Like I said earlier, there are various types of lead-acid batteries. The type we are interested in is called the deep cycle battery. Solar cells require deep-discharge batteries. This type of battery has the ability to be fully discharged and recharged up to 500 times in smaller types and about 2000 times in big batteries. The rate at which these batteries are discharged determines how long the battery last and how much power you will get out. These batteries can last from 5 to 15 years and some even 20 years. Of course depending on the battery?s quality, usage parting, size and user?s need for electricity.

 

The size of a battery is express in Ampere Hours (Ah). This is the total amount of electricity that can be drawn form a fully charged battery until discharged to a specified battery voltage given at a specified discharged time. Thus, considering an automobile battery of 100-AH capacity, this battery could theoretically deliver 1 ampere for 100 hours or 100 ampere for 1 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 of lead-acid battery is a function of temperature and state-of-charge. 2. the useful capacity of the battery decreases significantly with a decrease in temperature. Finally, 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. These characteristics of lead-acid batteries are important when it comes to system design for use in Africa, where high ambient temperature will automatically influence the battery?s voltage output. We will come with some suggestions of how to get the best out your battery in the next article. In practice, the slower the discharging rate the greater the capacity.

 

Finally, batteries manufacturers are making batteries specifically designed for PV power systems. An example of such a battery is the DELCO 2000 photovoltaic battery and similar batteries are made by other manufactures, such as Gould and Exide. The picture below is a deep-cycle battery, Exide Renewable Energy Series PHv-DL.

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 makes 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 simple to very sophisticated with prices ranging from $100 to $4,000. The more sophisticated devices are capable of monitoring almost all parts of the system. They monitor incoming power, -voltage and other incoming sources. Some even allow you to download information to your computer via modem, giving you a table of your energy consumption over a day, a week, a month or perhaps a year.

 

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

 

The cables

The cable, to me, is as important as the PV generator. The cable transports the 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 and to the other parts of the system. It is extremely important to use the right wire size to let all the electricity move easily through to avoid electricity lost as heat 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 number of AWG (American Wire Gauge). The type and size are printed or engraved on the cable. The smaller the size numbers, 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) tells 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 90oC. The last following words speak for it self.

 

Telephone wires ranges from size 18 to 22 AWG. Your house wiring is mostly size 14 or 12 AWG. As rule of thumb: number 14 can carry 15 amps, number 12 is for 20 amps number 10 is for 25 amps number 8 for 35 amps and so on. There are tables and chart to help you choose the right cable size. These two formulas are useful when considering the cable size: Amps x Volts = Watts and R=E/I.

 

It is important to know that at low voltage the distance becomes a crucial factor. Every centimetre counts because you have very little pressure or volts available. The higher the voltage the longer the run can be without too many losses.

 

In the next episode we will look at the system designing for domestic applications. We will go through load calculations as well as system upgrading. This will be one of the most important parts of this solar article series because it deals directly with your present and future cash flow. I will give an idea of what solar electricity costs now.

 

I hope this has been an interesting read.

 

Kwabena Osei

My contribution towards sustainable development in Ghana

Kooseicv@hotmail.com.

0249050653

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