Case Study

For a Case Study on "Greener Cities", click here.

 

Solar cell background information

What are solar cells, solar panels and solar arrays?

Solar cells are also known as photovoltaic cells. Photovoltaic (PV) cells are devices that convert solar energy directly into electrical energy. Solar energy is the energy we receive from the Sun.

A solar panel consists of a set of solar cells connected in series and/or in parallel to produce a desired voltage and current. The solar cells are set into a frame.

A solar array (also known as a PV cell array) is a set of solar panels connected in a grid like those in Figure 1. Solar arrays are used on the rooftops of buildings, including homes and schools, to help meet their energy requirements.

Figure 1 The solar array on the rooftop of Mossman State High School, Queensland. This STELR school is part of the Australian National Solar Schools Program (NSSP).

Sometimes solar arrays can generate more electricity than is required. The excess electrical energy produced can be sold back into the electricity grid.

 

Electrical energy

Energy is associated with any change. It can be thought of as the ability to make something happen. It is not a force! (A force is simply a push or a pull or a twist.)

Electrical energy is the energy possessed by electrically charged particles, which may be associated with an electric current or stored charge.

What unit is used to measure energy?

The international metric unit (SI unit) used for energy is the joule, symbol J. The joule can be used to measure all forms of energy.

Commonly used energy units based on the joule are listed in Table 1.

Table 1 Commonly used energy units on the metric scale

Unit of energy	Symbol	Relative value
joule	J	This is the standard international unit for energy.
kilojoule	kJ	1000 J
megajoule	MJ	1 000 000 J, i.e. 106 J

Electrical power

Electrical power is the amount of electrical energy delivered per second. It is usually measured in the units shown in Table 2.

Table 2 Metric units of electrical power

Unit of electrical power	Symbol	Relative value	Commonly used to measure:
watt	W	This is equivalent to 1 joule of electrical energy delivered per second. It is the standard international unit for power	The electrical power required to run household appliances such as light globes and microwave ovens
kilowatt	kW	1 kW = 1000 W	The electrical power delivered by a small-scale energy resource, such as a solar array
megawatt	MW	1 MW = 1 000 000 W             = 106 W	The electrical power delivered by a large-scale energy resource, such as a coal-fired power station
terawatt	TW Or TWe	1 TW = 1 000 000 000 000 W           = 1012 W	The total electrical power demanded across the world. Note:  The small 'e' in TWe is often used to distinguish electrical power from other forms of power, such as solar power. The prefix 'tera' comes from a Greek word meaning 'monster'.

In the STELR program you will learn about other energy units that are commonly used for electrical energy. These include the watt-hour and kilowatt-hour.

Meeting the world's demand for electrical power

Our global demand for electrical power is increasing all the time. In 2006 it was about 16 TWe; this is expected to rise to about 18 TWe by 2030.

Did you know that this global demand could be met by covering the areas shown as black in this map with solar panels?

Figure 2 A map of the world, coloured to show how much solar power reaches the Earth's surface in each region, measured in watts per square metre.
Source: http://en.wikipedia.org/wiki/File:Solar_land_area.png Accessed: 17 August 2010

 

For this claim, it is assumed that the solar panels covering the ‘black spot' regions of the world in Figure 2 would be operating at 8% energy efficiency. In other words, just 8% of the light energy reaching the panels would be transformed into electrical energy. (The remaining 92% of the light energy would be transformed into heat energy.) This is a typical value for the percentage energy efficiency of silicon-based solar panels, although new technologies have achieved greater efficiencies. You will learn more about energy efficiency in the STELR Program.

Three countries in particular have taken the ability of solar cells to meet their energy needs very seriously, and have extensive areas covered by solar panels, far more than all the other countries combined. They are Germany, Japan and the US.

 

How do solar cells work?

The general principles by which all solar cells work are:

• Light consists of little ‘parcels' or ‘packets' of energy called photons.
• When photons shine on a solar cell, they are absorbed by the cell. This causes the cell to release electrons. 
• The liberated electrons enter wires and travel around an electrical circuit. 
• The resulting electrical current is in the form of a direct current (DC). This is a current that flows in one direction only. 
• The more intense the light, the greater the number of electrons liberated per second and the greater the amount of electrical power produced.

 

How are rooftop solar systems set up?

A rooftop solar system is the name given to the solar array on the roof of a building, together with the electrical circuit that must be set up to link the solar array to the electrical circuitry in the building.

As you have just leaned, the electrical current produced by a solar cell is a direct current. However, electrical appliances operate on an alternating current (AC), a current which continually switches the direction in which it flows. For this reason a device known as an inverter must be inserted into a rooftop solar system to convert the direct current into an alternating current, as shown in Figure 3.

Figure 3 How a PV (solar) array and an inverter are connected into household wiring. (Note: A utility pole is another name for a power pole.)

Source: http://www.cobaltpower.com/images/house_conceptual.jpg Accessed: 24 August. 2010

The electrical energy generated that is not needed at the time can be:
1 ‘Stored' in a bank of batteries that are connected into the solar system, so it is available at night; or
2 ‘Fed' into the power grid (if the building is connected into one).

When a building is in a remote area, and is not connected into a power grid, the bank of batteries is necessary to store the electrical energy for night-time, when the solar panels cannot generate electricity.

Batteries store electrical energy by transforming it into chemical energy. When the battery is connected to an electric circuit, the chemical energy is transformed back into electrical energy.

When electrical energy is fed back into power grid, a meter measures the electrical energy that has been supplied.

Types of solar cell

How a solar cell actually works depends on whether it is a silicon-based solar cell or another type of solar cell, such as an organic solar cell (which is made up of plastics), or a dye-sensitised solar cell (also known as a Grätzel cell).

Different research teams across the world are doing some very exciting work using different designs and different technologies, including nanotechnology, in the hope of developing solar cells that:

• are more efficient (convert a greater the proportion of the energy from the Sun into electrical energy);
• can be used in a wider variety of applications;
• are more environmentally friendly (made from less harmful substances, consume less of the Earth's resources, produce less wastes when manufactured);
• cost less. 

Some research projects involve developing flexible, light solar cells that can be part of clothing or a back pack used by people such as hikers and field workers. Others involve developing windows and roofs that can act as solar panels.

We will only discuss conventional silicon-based solar cells in this article, because they are the main ones being used across the world today.

Silicon-based solar cells

Even silicon-based solar cells are not all the same, in fact. There are a number of different types, and researchers are developing new technologies all the time in order to improve their energy efficiency.

What they do have in common, however, is that the material that releases electrons when light shines on it is mostly made from the element silicon. This is a very abundant element on Earth. Sand, for example, is made from silicon.

One kind of silicon-based solar cell has two wafer-thin layers of silicon sandwiched together inside the cell, as shown in Figure 4.

Figure 4 Inside one kind of silicon-based solar cell.

Source: http://www.pbs.org/wgbh/nova/solar/images/insi-01.gif Accessed: 24 August 2010

The two silicon layers you can see in the cell in Figure 4 are both made from highly purified silicon. Of these two layers, the top layer is the one exposed to the light. This is made up of a material that releases electrons when it absorbs light energy.

 

 

What are some advantages and disadvantages of solar panels?

Table 3 Some advantages and disadvantages of solar panels

Advantages of solar panels	Disadvantages of solar panels
They are a renewable energy resource. Solar energy will be available for millions of years, and there is more than enough to supply all of the world's energy needs.	The amount of electrical power they generate varies all the time, due to the changing position of the Sun in the sky, changing cloud cover, dust and so on.
Solar energy is free and solar panels have a long life. (They can last for up to 50 years.) Therefore they are a good long term investment.	They are expensive to install. Many people cannot afford this.
Whilst operating, solar panels do not produce greenhouse gases or other pollutants. And as they last for up to 50 years, they soon more than compensate for the greenhouse gases emitted in making them, especially if they replace polluting forms of lighting such as kerosene lamps.	Silicon-based solar panels contain some toxic materials, and energy is required to extract and transport the raw materials, and to manufacture, transport and install the panels, which means that greenhouse gases and various pollutants are produced at these stages.
They operate without noise.	Some people do not like their appearance on roofs (although work is now being done to incorporate them into the structure of new buildings).
They can be used in remote areas, where there is no access to an electricity grid.	They do not produce electrical power at night, the time when most electrical power is needed in homes.
The heat energy also produced can be used to heat water.	They have low energy efficiency.