Performance Degradation in Solar Plants

Lars Podlowski and Daniel Hundmaier

A

A solution to prevent potential induced degradation, a recently discovered new trend in high-voltage solar systems throughout the world, has been researched by SOLON SE's Dr Lars Podlowski and Daniel Hundmaier.

B

In photovoltaic (PV) modules, an initial drop in efficiency is a well-known phenomenon. Known as light induced degradation, it has long been included in the performance guarantees offered by producers in the industry or the calculations of project developers and plant operators. Light induced degradation can cause an approximate 2 per cent decrease in system performance in the first few hours of operation of any new PV installation.

C

In 2006, a new form of performance degradation began to be noticed. The effect was first discovered in solar plants whose modules used a certain type of high-performance cells. Initial suspicions turned to the specific technology of these cells, which differed substantially from industry-standard cells. In these cases, a concentration of charge carriers at the cell surface was suggested to be the cause of the potential difference between the cells and the ground potential.

D

It has now been established that this new type of degradation - known as potential induced degradation (PIO) or high-voltage stress - is indeed promoted by the special technology used in these cells. However, the phenomenon has also been observed in standard and thin-film cells. As these panels are serially interconnected modules of all cell types, the more modules you connect, the higher the voltage gets. It is the high voltage that causes PIO.

E

In order to understand PIO, it is important to understand how a solar cell works and how it interacts with other materials in the module. In simple terms, a standard cell consists of a thin film of negatively doped (polarised) silicon on top of a thicker layer of positively doped silicon. When exposed to sunlight, so-called electron-hole pairs are produced in the space between the two layers - the depletion zone or space-charge region. Positively charged holes move in the direction of the positively doped superconductor, whereas negatively charged electrons move to its negatively doped counterpart. The charge carriers are then conducted to the next cell.

F

The serial array of the cells means that the voltage increases from cell to cell in the module. The same applies to the individual modules in the system, also connected in series. The maximum voltage in such a system can easily reach up to 1000 volts - this is basically a positive effect, because the higher the voltage is, the lower the electrical resistance. In this way, high voltage helps to increase the capacity of the system.

G

At the same time, these high system voltages can lead to unwanted leakage currents between the solar cells, the bedding materials, glass, and the grounded module frame. This allows a positive charge to build·up on the anti-glare coating at the surface of the cells. The result is a temporary short circuit in the affected cell$, which means a decrease in cell voltage and a drop in efficiency - an effect that is reinforced by high temperatures or humidity around the modules.

Look below at the diagram of a standard solar cell and try to complete it with between one and three words from the text in each gap. The questions would not ordinarily be in the text, but are designed to help you find the answers.

Standard solar cell

Gap 1:
Gap 2:
Gap 3:
Gap 4:
Gap 5:
Gap 6:

1. Gap 1 shows something from outside the cell feeding into the cell. What powers the solar cell?
2. Gap 2 has the symbol -. What does this symbol usually mean?
3. Gap 5 has the symbol +. What does this symbol usually mean?
4. Gap 3 has both symbols, - and +. Can you find a word in the text that refers to two things together?
5. The top layer of the cell is labelled negatively doped silicon. What, then, do you expect the lower layer (Gap 6) to be labelled?
6. Gap 4 shows the space between the layers. How is this referred to in the text?