Inverter and PV System Technology
Industry Guide 2013

The temperature coefficient of voltage – and consequently also the module output determined by voltage times current – is negative. This means that the module output and voltage (when compared to the data sheet and nameplate capacity) decrease at high temperatures (higher than the reference temperature T=25°C under STC). Conversely, they increase at low temperatures. The temperature coefficient of current is both very small and positive, so currents will only alter to a very small degree as a result of temperature fluctuations, therefore only exerting very little influence on module output.

Here is an example with some typical values: Under STC, a given solar module with crystalline silicon solar cells has a nominal output of 200 watts peak (Wp) and the temperature coefficient of output is –0.5%/kelvin (K). This means that the output of this module would decrease by 5% for every temperature increase of 10 K. If this module were to reach a temperature of T=55 °C, the output would drop by 15%, i.e. the 200 Wp module would “only” supply 170 Wp. Inversely, at a module temperature of T=5 °C, its output would increase to 220 Wp. Thin-film modules are characterized by a lower temperature coefficient of output, typically –0.3%/K. This means that at a module temperature of T=55 °C, the solar module would only show a drop in output of 9%.

Insolation can heat PV modules to as much as 70 °C. For this reason, they are installed so as to ensure that air can circulate to provide sufficient rear ventilation. Where rear ventilation is not possible, for instance if the modules are integrated into the roof or façade of a thermally insulated building, thin-film modules are better suited as their actual output is less dramatically impaired by high temperatures.

Since a single solar cell is only able to generate around 0.5 volts (V), a number of cells within a module are connected in series to form a string. This has the disadvantage of making the module extremely sensitive to partial shading because when a shadow is cast on a cell, e.g. from a chimney, dormer or an antenna, the cell can no longer generate power, turning it from power generator to power consumer. As the weakest link in the chain, the cell restricts the power output of the entire string.

Shaded cells do not generate electricity, while the other, fully illuminated cells in the string remain completely active and drive their power through the shaded cell, which converts that power into heat. In extreme cases, this leads to a “hot spot” being created in the cell, which can melt a hole in the cell material. A bypass diode, which bypasses the module string containing the shaded cell, is therefore used to steer the electricity past the passive cell.

A bypass diode usually bypasses 20 to 24 cells. Today, modules consisting of 60 to 72 cells are often equipped with three bypass diodes which are located in the module junction boxes. As each diode bypasses a part of the module, in the case of very slight shading, only some of the output of all the series-connected cells making up the module will be lost.

It would therefore be ideal if each solar cell could be equipped with a bypass diode. Unfortunately, the junction box does not provide enough space for this. To get around the problem, several manufacturers have started to laminate “string bypass diodes” into their modules. This allows a greater number of diodes to be used than will fit in the junction box, and shading tolerance is noticeably increased as a result.

Overall, shading has the same effect as sharply reduced insolation: a decreased flow of current. This applies in principle to both crystalline and thin-film modules. However, the latter benefit from the strip-like arrangement of their solar cells, as it is relatively uncommon for long, narrow, thin-film solar cells to become completely shaded. The reduction in output of a thin-film module is therefore usually proportionate to the shaded area.

Where losses are expected due to high operating temperatures or shading, thinfilm modules are often given preference over crystalline silicon models.

As efficiency increases with greater radiation intensity, the efficiency of solar cells can also be raised by concentrating the sunlight that falls on them with mirrors or lenses. In theory, multiplying the concentration of sunlight by 100 produces a 20% increase in output.

Concentrator cells fitted with Fresnel lenses can be combined relatively easily into modules. Modules that are ready for mass production achieve an efficiency of around 25% with a concentration factor of 500.

As diffused sunlight reaches the lens system from all directions, it cannot be focused on the cells. Consequently, con centrator modules only utilize the part of the global insolation that reaches them directly and must follow the sun on a dual-axis tracking system. Their output is therefore at its highest along the earth’s sun belt.

Module junction box

Module junction boxes connect solar cells to the outside world by joining the connection cables of the cell strings and interconnecting them with the bypass diodes and the module connection cables. To prevent moisture from entering the module junction box, it is waterproofed and often sealed with silicon.

Little by little, electrical functions are being incorporated into the module junction box to provide additional safety or increase the yield. DC/DC converters ensure that the voltage output is optimal for the inverter, irrespective of shading and temperature. Even performance boosters (e.g. power optimizers – see “Inverters and PV Plant Yield”) can be incorporated into the junction box. Each module will then have its own MPP controller. PV generator safety can be increased by automatic fire prevention systems, also located in the module junction box.

Reflection losses

In order for yield to be increased even further, reflection losses must also be taken into account. Modules with anti-reflection glass are already in use, but are relatively expensive. Reflection losses can, however, be virtually eliminated if the PV generators are equipped to track the sun’s movement on a dual axis, though this involves relatively high additional expense for the mechanical system. Such outlay is really only worthwhile if adequate additional yield can be achieved, i.e. if the PV system is installed at a site with a high proportion of direct insolation, preferably along the earth’s sunbelt. This applies similarly to concentrating sunlight with mirrors or optical lenses.

Yield can also be increased by active cooling. Here, cooling modules on their rear side produces warm water or warm air in addition to electricity. All in all, the advantages of this method are, however, too few for it to have become well-established.

Aging processes

Since they contain no moving parts, solar modules normally age very slowly. As long as their materials (glass, solar cells, plastics, aluminum) have been carefully selected, they are also sufficiently weather resistant. If a system is installed in such a way that corrosion cannot take hold, it can achieve a service life of 20 years or more. The assembly frame should be designed to ensure that there are no corners or niches where dirt, leaves and other deposits could collect, and standing water should also be avoided. Different metals may only be used together if it can be guaranteed that no electrochemical reaction will take place. This particularly applies to the screws and clamps in the support frame that holds the PV generator.

In the early days of PV technology, the transparent conductive oxide (TCO) coating, applied to the illuminated upper face of most thin-film modules to conduct current, was often damaged by corrosion. TCO corrosion is irreversible and leads to severe output losses. Such damage predominantly occurs in the event of high voltages caused by earth leakage currents. Grounding the generator’s negative pole can prevent TCO corrosion, though it also precludes the use of several inverter types.

The modules are connected in series to form a string. The cumulative voltage of the individual modules gives the string voltage, which must be calibrated to the system voltage of the inverters. Strings of equal length are then connected in parallel to make up the PV generator, where the output power of the strings is cumulative. Multiple string cables from the PV generator are consolidated using Y-adapters or joined in a GJB.

The GJB is located close to the modules and connects several strings in parallel, meaning that only one positive and one negative cable – albeit with large cable cross sections – must be laid from each junction box to the downstream inverter. It can also perform additional safety-related functions, such as those of string fuses or overvoltage conductors. If thin-film modules are used which are not reverse current proof, blocking diodes must also be employed. In addition, there are certain components which may be positioned in several different locations within the system. For example, the main DC switch could be a part of the GJB or could be integrated into the inverter.