Electricity from the sun

Photovoltaic cells convert sunlight directly into electricity. Photovoltaic cells are made from silicon which is a common substance found in sand. Although silicon is cheap, before it can be used in making photovoltaic cells, it needs to be processed into a pure from. Usually, silicon crystals are grown, sliced into a thin layers and then impregnated with chemical to create oppositely charged layers. When sunlight strikes the impregnated layers electrons are freed and an electric current is generated. The impregnated substance is called a semi-conductor. The semi - conductor sliced are connected together and covered with a sheet of strong glass for protection. This is called a photovoltaic (PV) panel or module.

Eskom's power stations operate 365 days a year. They have a total nominal
generating capacity of 39 154 MW which is spread across 20 power stations. Two thirds of that capacity is concentrated in just ten base loadcoal-fired power stations.

According to ESKOM the total amount of electricity sold in 1999 was 1,73422 x 10¹¹kWh.

Secondly, we need to know how much solar energy South Africa receives each year.

The average amount of solar energy received in South Africa per m² per day =5.28kWh.

In order to calculate the energy received by South Africa during one day we need to know South Africa’s surface area.

South Africa’s surface area =1 221 000 km²

Energy received by South Africa in one day

=5.28 kW/m² x 1.221 x 10¹² m²

                                                                        = 6. 45 x 10¹² kWh/day

Therefore energy received per year:

= 6.45 x 10¹² kWh/day x 365 days

=2.3 x 10¹⁵ kWh

How does this compare with the amount of electricity consumed in South Africa annually?

Great news... South Africa recieve more than 13000 times more energy from the sun than electricity energy used during 1998.

Therefore most of the solar radiation reaching the earth s surface is either reflected or radiated back into the atmosphere. For this reason the earth's surface does not get hotter and hotter.

POWERING DOMESTIC APPLIANCES

Let's begin in the kitchen.

Could a single PV module be used to power an electric kettle?

We need to know how much electrical is needed to boil a kettle of water. Assume we have an electric kettle with a power rating of 2 k W that takes 3 minutes to boil.

The electrical energy required to boil water is given by:

ELECTRICAL ENERGY =POWER x TIME

=2 kW x 0.049 hours

=0.1 kWh

An explanation of kWh as an electrical unit might be necessary. See 'ELECTRICITY COST IN OUR HOMES' for an explanation of this unit.

POWER PRODUCED BY A MODULE

PV modules are rated in 'peak watts', (Wp). This is the maximum power a PV module can produce under standardised test conditions-with the modules receiving 1000 w/m².

PHOTOVOLTAIC MODULE EFFECIENCY

Note all the sunlight (solar energy) is converted into electrical energy. Some get reflected off the surface of the photovoltaic module, others gets absorbed as heart. Even then only part of the remaining sunlight is effective in producing an electric current. The most 'common' photovoltaic modules convert between 4% and 13% of the sunlight received into electrical energy.

How many watts wold a single photovoltaic panel with an area of 0.5m² and efficiency of 10% produce in these conditions?

Up to 50 Watts or 0.05 Kilowatts.

How long would it take for single PV module to produce the amount of electrical energy needed to boil the kettle?

TIME =ENERGY /POWER

TIME =0.1 kWh / 0.050 kW

                                                                                        =2hours

DELIVERING GREATER POWER

Instead of using a single PV module, a number of PV modules can be connected in an array. The total electrical power of an array is equal to the sum of power from each module.

The disadvantage of using PV modules to power high power rated appliances is cost .The cost of buying additional modules makes the use of PV modules unattractive for powering high power appliances such as stoves to providing small amount of electricity for low-energy appliances such as lights, TV's, etc.

Are PV modules limited to supplying dc appliances.

Most domestic appliances, such as washing machines, vacuum cleaner, television sets and refrigerators are design to operate at the voltage at which grid electricity is supplied to homes at 240 V ac. PV modules are usually designed to produce 12 V dc.

In order to power an ac appliance such as a washing machine with PV modules, we would connect an electronic device called an inverter, between the module and the washing machine. The inverter converts dc voltage to ac.

COST OF PHOTOVOLTAIC MODULES

Although some PV modules are assembled in South Africa, most are imported from overseas countries such as France, Germany, Japan and USA. PV modules are priced in foreign currency prices. As PV module production methods have improved or become cheaper the cost of PV modules have dropped.

SUITABILITY OF PHOTOVOLTAIC MODULES

These modules are ideally suited to provide electricity to small scale users such as schools and clinics that are far from the national grid.

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