Photovoltaic background, technology

The photovoltaic effect

This is the ability to transform solar energy into electricity and is achieved by using photovoltaic (PV) cells.
A PV cell (see Fig. P1 ) is capable of generating voltage of between 0.5 V and 2 V depending on the materials used and a current directly dependent on the surface area (5 or 6 inch cells).

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Fig. P1: Photovoltaic cell manufactured in a silicon plate (source: Photowatt)

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Its characteristics are shown in a current/voltage graph as shown in Figure 2.

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Fig. P2: Typical characteristic of a photovoltaic cell

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The photovoltaic effect is dependent on two physical values (see Fig. P3)
– irradiance and temperature:

• As irradiance E (Wm²) increases, so does the current produced by the cell
• Conversely, as the temperature (T°) increases, the output voltage decreases.

In order to compare the performance of different cells, the standard has set out Standard Test Conditions (STC) for irradiance of 1000 W/m² at 25°C.

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Fig. P3: Irradiance and temperature influence the photovoltaic effect

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To make it easier to use energy generated by photovoltaic cells, manufacturers offer serial and/or parallel combinations grouped into panels or modules.

Photovoltaic modules

These combinations of cells (see Fig. P4) enable the voltage and current to be increased. To optimise the characteristics of the modules, these are made up of cells with similar electrical characteristics.

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Fig. P4: PW1400 photovoltaic module dimensions:1237 x 1082 x 45 mm (source: Photowatt)

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Each module providing a voltage of several tens of volts is classified by its power level measured in Watt peak (Wp).This relates to the power produced by a surface area of one m² exposed to irradiation of 1000 W/m² at 25°C. However, identical modules may produce different levels of power so the common standard variation for power is ±3% (see table in Figure P5). Modules with typical power of 160 Wp include all modules with power of between 155 Wp (160 -3%) and 165 Wp (160 +3%).
It is therefore necessary to compare their efficiency which is calculated by dividing their power (W/m²) by 1000 W/m².

For example, for a module of 160 Wp with a surface area of 1.338m² (*), the peak power is 160/1.338 which gives 120 Wp/m².
Therefore the efficiency of this module is: 120/1000 = 12%.

(*) The dimensions of these modules (L x W x D) in mm are:1237 x 1082 x 38.

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Encapsulation	Glass/Tedlar
Cell size	125.50 x 125.5 mm
Number of cells	72
Voltage	24 V
Number of bypass diodes	4 bypass diodes
Typical power	150 Wp	160 Wp	170 Wp
Minimum power	145 Wp	155 Wp	165 Wp
Voltage at typical power	33.8 V	34.1 V	34.7 V
Current at typical power	4.45 A	4.7 A	4.9 A
Short circuit current	4.65 A	4.8 A	5.0 A
Open wire voltage	43 V	43.2 V	43.4 V
Maximum circuit voltage	1 000 V CC
Temperature coefficient	α = (dl/l)/dt # + 0.032 %/°C β = dV/dt # - 158 mV/°C ς P/P = - 0.43 %/°C
Power specifications at	1000 W/m²: 25°C: AM 1.5

Fig.P5: Electrical characteristics of a PW1400 module (source: Photowatt)

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However when photovoltaic cells are connected in series, a destructive phenomenon known as the “hot spot” may occur if one of the cells is partially shaded. This cell will operate as a receiver and the current passing through it may destroy it. To avoid this risk, manufacturers include bypass diodes which bypass damaged cells. Bypass diodes are usually fitted in the junction box behind the module and enable 18 to 22 cells to be shunted depending on the manufacturer.

These modules are then connected in series to achieve the level of voltage required, forming chains of modules or “strings”. Then the strings are arranged in parallel to achieve the required level of power, thus forming a PV array.

A faulty module within a string must be replaced by an identical module and therefore it is important to choose a supplier which is likely to be in business in the long-term.

Since there are increasing numbers of PV module manufacturers throughout the world, it is important to consider the various options carefully when choosing equipment. Installers should also:

• Ensure the compatibility of the electrical characteristics with the rest of the installation (inverter input voltage).

• Ensure that they are compliant with the standards.
• Select suppliers likely to be in business in the long-term to ensure that faulty modules can be replaced as these must be identical to those already installed.

This final point is important as installers are responsible for the warranty granted to their clients.

Additional equipment: inverters or chargers

Photovoltaic generators only supply energy as direct current and when there is sunlight.

Therefore, if this energy is to be supplied to the distribution network, the direct current must be converted into alternating current using converters or inverters, and if it is to be supplied permanently, it must be stored in rechargeable batteries using a battery charger.