Solar Cells Theory

Solar Energy – Electricity

Solar energy is the energy we receive from the Sun. The Sun emits the full range of the electromagnetic spectrum. This page only considers transforming solar energy into electrical energy.

FIGURE 1: THE ELECTROMAGNETIC SPECTRUM

Solar Cells

Solar cells are also known as photovoltaic (PV) cells. Solar cells are devices that transform light energy directly into electrical energy.

FIGURE 2: FLOW CHART AND SANKEY DIAGRAM OF THE ENERGY TRANSFORMATIONS OF A SOLAR CELL

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.
• If the photons have enough energy they cause the cell to release electrons.
• If the photons do not have enough energy, their energy is transformed into heat energy.
• The released 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.
• If the light is more intense (brighter light) more electrons will be released each second and the electrical current will be bigger. The voltage of the cell will stay the same.

The video below, which was produced by the University of New South Wales, also shows how silicon-based solar cells work.

Types of solar cell

FIGURE 3: A DYE-SENSITISED SOLAR CELL USES A PROCESS LIKE PHOTOSYNTHESIS TO TRANSFORM LIGHT ENERGY INTO ELECTRICAL ENERGY. (COURTESY DYESOL)

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).

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 have developed flexible, light-weight 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.

FIGURE 4: NEW TECHNOLOGIES LEAD TO NEW APPLICATIONS.

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 5.

FIGURE 5: INSIDE ONE KIND OF SILICON-BASED SOLAR CELL

The two silicon layers you can see in the cell in Figure 5 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.

FIGURE 7: A SOLAR PANEL IS MADE UP OF SOLAR CELLS CONNECTED IN SERIES TO GIVE A DESIRED DC VOLTAGE.

Solar panels and solar arrays

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 single solar cell has an output voltage of about 0.6 V DC. In a solar panel there are modules of 60 – 72 solar cells connected in series. This gives a nominaloutput voltage of 24 V DC. The maximum voltage could be greater than 36 V.

A solar array (also known as a PV cell array) is set of solar panels connected in a grid like those in Figure 7. Solar arrays are used on the rooftops of buildings, including homes and schools, to help meet their energy requirements.

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

FIGURE 8: SOLAR PANELS ARE CONNECTED IN PARALLEL TO EACH OTHER IN AN ARRAY.

Solar electricity for buildings

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.

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.

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.

FIGURE 9: A PV (SOLAR) ARRAY IS CONNECTED TO AN INVERTER BEFORE THE ELECTRICITY IS FED INTO THE HOUSEHOLD WIRING.

Advantages and disadvantages of solar panels

FIGURE 10: SNOW BLOCKS LIGHT FROM REACHING THE SOLAR CELLS.

Advantages 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.
• 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.
• 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.
• They operate without noise.
• They can be used in remote areas, where there is no access to an electricity grid.
• The heat energy also produced can be used to heat water.

Disadvantages of solar panels

• Variable light intensity due to:
• Cycle between day and night
• The path of the Sun across the sky
• Clouds
• Shadows
• Changing angle of the incoming sunlight  to the face of the panel, due to the changing position of the Sun in the sky
• Dirt or other obstructions
• They do not produce electrical power at night, the time when most electrical power is needed in homes.
• How they look
• Distance to grid connections (for solar farms).
• 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 have low energy efficiency.