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Experiment 1a:
Recording the characteristics curve of a diode

To understand the function of a solar cell and its characteristic curve, one should first take a look at the diode.  A diode allows a current to flow in only one single direction.  If a direct voltage in forward direction is created, a current flows which depends on the level of the external voltage.  The basic materials used define the characteristic diffusion voltage values at which the current increases significantly.  If the voltage is increased still further and the maximum current of the diode is surpassed, the diode will be thermally destroyed.


Experiment 1b:
Recording the characteristic curve of a diode series connection


The solar cell is the smallest unit of a solar generator.  Its characteristic curve is similar to the one of diode.  Because of the low voltage and power range of a cell, several cells are combined in a series connection so they can be used in technical applications.  Due to the similar characteristic curves of a solar modules simply by combining diodes in a series connection.

Experiment 2:
Recording the characteristic curve of a solar module
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Solar cells convert sunlight to electricity.  To use the solar cells in technical applications, they combined in a series connection so as to achieve a higher operation voltage.  In addition to the voltage, the series connection of the cells also increases the power of a solar module.  The cell surface defines the level of the possible current flowing in case of  a corresponding irradiation.  The combination of several cells is called solar module.  There are crystalline and amorphous cells, with the crystalline cells divided further in mono-and multi-crystalline. Just as the behaviour of a diode, the behaviour of a solar modules can be described by a UI characteristic curve.

Experiment 3:
The effect of the angle of inclination on the power output of a solar module


The angle of inclination has a major effect on the power output of a solar module. 
The irradiation is ideal if the module is at all times perpendicular to the irradiation light.  This is not possible with the modules with a fixed mounting, as the distance of the sun is determined by daily and annual cycles.  However, moving systems make sense only where the share of direct irradiation is lager than that of indirect irradiation. For generators with fixed mounting there is an optimum tilt angle, depending on the latitude, which yields the largest annual amount of energy.

Experiment 4:
The effect of the temperature on the characteristic curve of a solar module


The cell temperature of a solar module depends largely on the irradiation, the ambient temperature and the wind. It is known from semiconductor technology that the conductivity increases along with an increasing temperature. While the current increases slightly, the voltage behaves the opposite way.  Therefore the manufacturer of the modules marks their products with temperature coefficient which describe this behavior.  The coefficients depend on the material.

Experiment 5: Recording the characteristic curves of a solar module with different amounts of irradiation. 

 The characteristic curve of a solar module depends largely on the influencing parameters of temperature and irradiation.  The amount of irradiation has the larger effect on the two.  It is the module current in particular that shows a direct proportional interdependence.  Next to the effects of the weather (clouds, fog…), the suboptimal adjustment of the irradiated surface can also lead to reduced irradiation.

Experiment 6:
Series connection of solar modules


Just as solar cells within a solar module can be connected in series, solar modules themselves can also be connected in series.  The voltage of a solar generator increases along with the number of modules while the current remains constant.  This makes voltages of several hundred volts possible.  The series connection of solar modules in a strand can help increase the power of the solar generator.

Experiment 7:
Recording a daily progression for summer winter

The use of photovoltaic equipment is partly subject to seasonal changes defined largely by the change of the day and night, the season and climate.  In case of a sudden change in the weather pattern, two subsequent may bring completely different results with regard to the energy yield.  This also applied to long-term variations.  For example, in the summer semester, the energy yield may be much greater than the winter semester.  To be able to bolster these changes, storage devices are used in isolated photovoltaic systems.

Experiment 8:
Parallel connection of solar modules

If the power of a solar generator is to be increased, the number of modules must be increased first.  This can either be done by connecting individual solar modules in a parallel connection.  If the current remains constant, a series connection will increase the system voltage, whereas a parallel connection work exactly the opposite way.  The parallel connection is the only possibility to increase the power especially a photovoltaic units with small system voltages (e.g. 12V). It is possible to connect not just individual modules but whole strands of them.

Experiment 9:
Shading of solar modules without bypass diode


Even if the shading of a solar is locally limited, there can be unreasonably high energy yield losses unless bypass diodes are integrated into modules.  The shaded part of the cell behaves like a resistor in a closed circuit.  The power converted in the shaded cells can lead to the part of the cell getting warmer.  In the worst case this could result in the destruction of individual cells.

Experiment 10:
Shading of solar modules with bypass diode


To minimize the losses in the shading case, bypass modules are integrated into solar modules.  These are usually two diodes in one standard module.  Their purpose is to “bypass” the current around the shaded area in the shading case.  Bypass diodes are connected to the solar cells in an anti-parallel connection.

Experiment 11:
11.1  Photovoltaic unit for grid-connected system operation.

In photovoltaic units for grid-connected system operation, the solar current generated by the solar generator is fed to the mains via an inverter.  A surplus is transferred directly to the mains supply.  A storage is there fore not necessary.  The major component of this type of unit are solar generator, generator terminal box, inverter and meter.  The operation of such a unit may show different energy flow directions, depending on the energy supply and consumption.

11.2  Measuring the inverter efficiency

Efficiency is the ratio of power output to power input.  This is often an important criterion when buying an inverter.It describes how efficiently the conversion of the energy generated by photovoltaic processes can take place. However, manufacturers often just specify the efficiency for the nominal power. But since the devices are mostly operated below the nominal power, a good efficiency is not so much important in the partial load area. Most devices today reach an efficiency of more than 90% in the nominal power range.

Experiment 12:
Island-grid photovoltaic systems

Photovoltaic systems used as separated from the mains system, i.e. not connected to it are called island-grid systems. Isolated systems supplied by photovoltaic technology can differ largely with regard to their structure. Their complexity depends largely of the requirements posed by the load to be supplied.

Experiment 12.2:
Photovoltaic unit for island-grid system operation


Small photovoltaic units for island-grid operation are mostly designed for the supply of direct current loads. If alternating current loads are also to be supplied in addition to the direct current loads, an inverter will have to be used, converting the direct current to an alternating current. The curve of the output voltage can have different shapes in such a case. As these devices cause additional losses and represent a possible source of faults in the system they should be used only if absolutely necessary.

 
 
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