Lesson Notes By Weeks and Term - Junior Secondary School 3

SUBJECT: BASIC TECHNOLOGY   

CLASS:  JSS3

DATE:

TERM: 3RD TERM

Reference Materials:

1. EVANS, Introductory Technology For JSS, Book 3.
2. NERDC, Introductory Technology For JSS, Book 3.

WEEK 1

TOPIC: ENERGY SOURCES- Solar Cells and Solar Electricity 

CONTENT

• Solar Energy         
• Solar cell    
• Solar Electricity 

INTRODUCTION

Solar energy produces heat energy along with light energy reaching the earth from the sun. The sun, with a surface temperature of about 67620C radiates about 353 kW of solar power to an area of 1m2 on the earth’s atmosphere.

SOLAR CELL

While the STPP utilizes the heating effects to produce steam, which drives the turbine to generate electricity, there is a device called PHOTOVOLTAIC CELL OR SOLAR CELL, which converts the sunlight directly to electricity. Photovoltaic cells are capable of converting from 3% to 20% of the incident solar energy into DC (direct current) electricity. It behaves very much like a low –voltage (≈ 0.65V) battery.

Principle of operation 

The working principle of photovoltaic cell is quite simple. Sunshine comes in little bundles of energy called photons. This photons enters the semi-conducting material (e.g. high purity silicon), it is absorbed. The absorbed photon breaks the electronic bond of the material to release electron. The entity which is left after the electron is releases is called a hole. It is positively charged. Thus, the absorbed photons create free electrons (negative charges) and holes (positive charges), which move freely in the semiconductor material. By carefully a small quantity of another material called the dopant to the semiconductor material, free electrons in the semiconductor material are separated to one side and holes to the other side. Thus, the ends of the semiconductor material serve as the positive and negative terminals respectively. These semiconductor materials can be connected in parallel and series as required.

Evaluation 

1. State the two forms of energy emitted from the sun
2. Define the following: (i) Dopant (ii) hole (iii) pole
3. Explain the principle of operation of a solar cell

SOLAR ELECTRICITY

Solar electricity is the electricity generated from the energy beamed from the sun to the earth. Solar electricity utilizes the heating effect of the sun to generate electricity. This is called thermal collection. 

Principle of operation  

In the thermal collection process, the solar heat is used heat a fluid such as water, Freon etc, to a very high temperature. The high temperature fluid at high pressure uses its appreciably mechanical energy (just like the mechanical energy possessed by the falling water from a dam) to turn, for example, the turbine of an electric generator, which operates in this manner is called Solar Thermal Power Plant (STPP) One of the major. To attain a high temperature, concentrating system is used to concentrate the solar energy at a point, so that the temperature of the point will be very high.

Solar Thermal Power Plant

Evaluation

1. What form of energy is used by solar electricity?
2. Explain the principle of operation of a solar electricity 

Reading Assignment 

Read “RENEWABLE ENERGY –SOLAR ENERGY”

Reference Materials:

• EVANS, INTRODUCTORY TECHNOLOGY FOR JSS, BOOK 3, pages 185-190

Weekend Assignment 

1. _____ converts solar directly to electricity (a) Generator  (b) Fan (c) Photovoltaic cell  (d) inverter
2. Photocells are made from _______ materials (a) hydrogen (b) oxygen (c) semiconductor (d) proton
3. Photocell absorbs ____ from the sun (a) holes (b) poles (c) photons (d) electrons
4. Solar electricity uses the following devices except (a) turbine-generator  (b) concentrating system (c) boiler  (d) photocell 
5. Sun generates both (a) light and heat  energy (b) mechanical and light energy (c) sound and heat (d) light and kinetic energy

Theory 

1. Explain the principle of operation of a solar cell
2. Explain the principle of operation of a solar electricity