Case Study
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Solar cell background information
Solar cells are also known as photovoltaic cells. Photovoltaic (PV) cells convert solar energy (light) directly into electrical energy. When light hits the semiconductor it releases electrons that are picked up by a grid of wires and form an electrical current. Many cells may be grouped together to form a solar panel.
Did you know: Photovoltaic combines two words; photo meaning light and voltaic meaning they produce a voltage.
Watch this video to see how photovoltaic (solar) cells transform light energy into electrical energy. Alternatively,you can download the video in mpeg format directly from STELR.
Silicon based solar cells give a maximum (open-circuit) voltage of about 0.6 V. Solar cells with a large area have the same maximum voltage output as solar cells with a small area. They are able to provide bigger currents and more power as a result.
Solar panels
Solar panels can be made up of a number of individual solar cells connected together to give a desired maximum output voltage.
Solar cells can be connected in series to give a larger output voltage.
The total output voltage is theoretically sum of the individual voltages;
VT = V1 + V2 + …
Current is the SAME throughout a series circuit;
IT = I1 = I2 = …
Solar cells connected in series circuits
(The cells are connected one after the other. The “+” terminal of one cell is connected to the “‒” terminal of the next.)
When you connect solar cells in series, the maximum theoretical voltage output is a multiple of the voltage of a single cell. For example: 4 cells connected in series give a theoretical maximum voltage of 4 x 0.6 = 2.4 V
The Photovoltaic panel from Solar Educational kits usually have two solar cells connected in series to give a maximum theoretical output voltage of about 1.2 V. Look carefully through the plastic covering of a photovoltaic cell and see if there are one, two or more black wafers with grids of silver conductors on them. These are the cells in your panel.
Solar cells are not perfect conductors; they also have resistance. This means that some voltage will be lost when a current passes through them. This means that the actual maximum output voltage is less than the theoretical value. A 12 V solar panel will probably have 20 or more solar cells connected in series.
This solar panel has two silicon photovoltaic cells connected in series. The maximum theoretical voltage is 2 x 0.6 = 1.2 V, but the practical output of the panel is rated at 1 V. When a voltmeter is connected across a photovoltaic panel with no load, negligible current flows through the panel and the voltmeter gives a reading of the maximum theoretical voltage.
Solar cells connected in parallel circuits
(The cells are connected side by side. The “+” terminal of one cell is connected to the “+” terminal of the next. The “‒” terminal of one cell is connected to the “‒” terminal of the next.)
When solar cells are connected in parallel, there is no increase in maximum output voltage, but their total resistance is reduced and the current is increased. This means there is an increase in power delivered to the circuit.
Uses of photovoltaic panels
Solar panels are used in remote applications such as boats, railway signals, mining towns, isolated houses and farms, freeway signs, parking ticket machines, telecommunication installations and the international space station. Energy can be collected during daylight hours and stored in batteries for use when required.
Solar panels are also used in schools, homes and other buildings to reduce the amount of electricity taken from the power supply grid. If the electrical energy from the photovoltaic panel exceeds that used in the building, the excess can be fed back into the supply grid. This reduces the power produced at the power station and reduces the amount of greenhouse gases released into the atmosphere.
Photovoltaic panels are sometimes combined with other systems like wind turbines to meet energy requirements.
The efficiency of photovoltaic cells
The efficiency of a photovoltaic (PV) cell is a measure of how well the cell can convert the light energy falling on it into electrical energy. If all the light energy falling on a cell was converted into electrical energy the efficiency of the cell would be 100%.
Photovoltaic cells can never be 100% efficient. This has to do with the nature of light and the way electrons are bound to the atoms in the solar cells.
Light can be modelled as packets of energy called photons. When a photon hits an electron in a PV cell many things can happen.
- The photon will not have enough energy to remove the electron from its atom. The photon’s energy will be transformed into thermal energy. The cell heats up.
- The photon has just enough energy to remove the electron from its atom. The electron will contribute to the current of the cell.
- The photon has more than enough energy to remove the electron from its atom. The electron will contribute to the current of the cell, but the photon’s excess energy will again be transformed into thermal energy. The cell heats up.
- The photon does not interact with an electron. The photon’s energy will be transformed into thermal energy in the cell.
Power
Power (P) is the rate at which work is performed or energy is transmitted. The unit for power is the watt (W). The power ‘used’ by a device in an electric circuit can be calculated using this formula:
Power = Voltage x Current or P = V * I
Where the voltage (V) is measured in volts (V) and the current (I) is measured in amperes or amps (A).
Efficiency
The efficiency of a device is a measure of how well the device transforms energy from one form to another. Efficiency is usually given as a percentage. The efficiency = the useful energy coming out of the device divided by the total energy put into the device times 100. It can also be calculated using the input and output power of the device.
Efficiency = (Output Power / Input Power) x 100%
The power of sunlight
The power of sunlight on a clear sunny day is approximately 1000 W for every square metre of area at right-angles to the direction of the sunlight. This figure will vary for different locations on the Earth and for different times of the day.
If using solar cells in direct sunlight, the Input Power ≈ 1000 W/m2 x A (m2)
(A = area of cell or panel = length x width. Area is measured in m2. Remember to convert cm2 to m2, if necessary.)
Intensity of sunlight
The intensity of light is a measure of how much light energy falls on an area. The maximum intensity of sunlight occurs when the surface is directly facing the Sun. That is, the area is at right-angles to the direction of the light. The amount of energy hitting a solar panel therefore depends on the angle of the panel’s surface to the direction of the Sun’s rays, as shown in the diagram. The position of the Sun in the sky changes through the day and through the year. This means that fixed solar panels are seldom in a position to receive the maximum available power from the Sun.
Sun Angles
The apparent movement of the Sun across the sky it is due to the movements of the Earth, not the Sun.
The changes in the seasons are caused by the tilt of the Earth’s axis. The direction of the Earth's axis of rotation remains fixed at 23.5 degrees to the plane of the Earth’s orbit as it moves around the Sun. This affects the times of sunrise and sunset, and the variation in the Sun’s altitude in the sky throughout the day.
In December, the South Pole is pointing towards the Sun. The southern hemisphere has longer periods of daylight than the northern hemisphere. Summer begins in the southern hemisphere. The sun rises early and sets late. The light reaching the surface of the southern hemisphere is generally more concentrated. This means that there is more solar energy available in the southern hemisphere.
In June, the South Pole is pointing away from the Sun. The southern hemisphere has shorter periods of daylight than the northern hemisphere. Winter begins in the southern hemisphere. The sun rises late and sets early. The light reaching the surface of the southern hemisphere is spread out over a larger area of the Earth’s surface. This means that there is less solar energy available in the southern hemisphere.
Atmospheric reduction of sunlight
The intensity of light is also linked to the distance that the rays travel through the atmosphere.
As light travels through the atmosphere,
- some is can be absorbed by the gases, causing them to heat up
- some is scattered or diffused
- some is reflected by clouds
The amount of sunlight absorbed and scattered depends on the distance the light travels through the atmosphere. For example, the maximum intensity at the top of mountains is higher than at sea level, and is greater at the equator than it is near the poles.
At any place on the Earth, the intensity of light changes as the apparent pathway of the Sun across the sky varies throughout the year.
The apparent movement of the Sun as seen from a point on the Earth's surface
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