Sunday, 9 December 2012

Importance of shadow analysis and effects of shading

Abstract: As mentioned in my previous blogs, rooftops will become the 'next big thing' in India. Rooftops are available and so are the mounting structures however, there are some factors that should be taken into account when preparing and planning a rooftop system. One of the most important factors is shadow analysis, here in this paper, i will be discussing various shadowing conditions using special software tool and IEEE paper based evaluation on effects of shading (hot spots).

Looking for a rooftop installation? Well its a good decision, but how effective/suitable is your rooftop? Does your rooftop system integrator consider the various issues associated with shading and hot spot effects?

Shading is a problem, but this cannot be avoided in most cases. This is likely to happen in early mornings and evening but one has to be take these conditions into account during designing the plant. These are special ready made structures available in the market as shown in the simulation below, this was simulated exactly for a site in chennai at time 4.40pm in the month of July using a special software tool. From the below shadow analysis it is very clear that the panels placed on the rooftop of the building has been affected by shadowing just because of lack of planning. Here the panels where placed near the terrace walls and too low from the floor. 

Similarly on the below picture you can notice the shaded area on the multi-c-Si module marked in red, even these small shadow spots on the panels can greatly reduce the panel output. This can also lead hot spots on the modules in the absence/failure of bypass diodes. 

The hot spot risk of modules other than the partial shading also depends also from the climatic conditions such as,
  • Ambient temperature (worst case: high)
  • Irradiance (worst case: high)
  • Wind speed (worst case: low)

However, the highest hot spot risk is given by the wafer raw material, the process quality in the solar cell production, the string length in a given module design and the cell sorting.This prolonged heating without or damaged bypass diodes can cause hot spots on the solar panel. In the picture below you can clearly notice the hot spot damage on the solar panel. 

Case 1- With working by-pass diodes: What really happens in the shaded cell is, they are reverse biased by the unshaded cell and this mutually turns on the by-pass diodes to protect the shaded cells.

Case 2- With damaged or absence of by-pass diodes: There is a increasing reverse voltage and leakage current at the particular shaded area by the unshaded cells. And therefore power dissipation at the shaded cells happens and this finally leads to hot spots.

Lab experiment results: Given below are the results of thermal analysis, here one cell is completely shaded and the results are observed using a infrared camera over a time period.

The density of leakage current is very high in a small hot spot region compared to a large hot spot, therefore more damage due to rise in temperature. The graph below displays a relation between the size and the maximum temperature of the hot spot: A decrease of the hot spot size leads to higher maximum temperatures. The reason for this is attributed to a higher leakage current density. 

The 'easy solution' is to move the panels more towards the other side of the wall, but this can cause shading in the mornings and winter season. This is not recommended because it just gives a temporary solution and this solution can only be considered if the owner has already installed the rooftop system. 

The 'best solution' is to increase the height of the mounting structures greater or equal to the height of the rooftop walls. Therefore early morning shadowing or evening shadowing will be reduced. But if many many strings are placed one behind the other, this can still cause shadowing on the panels except the first row/string. In the screenshot below you can find the simulated images for the same conditions for a rooftop in chennai and you can also notice that the panels have avoided shading this time. This is just because of the tall mounting structures to avoid rooftop wall shading.

Stefan Wendlandt1,3, Alexander Drobisch1, Torfinn Buseth2, Stefan Krauter1,3,4 and Paul Grunow1
1 Photovoltaik Institut Berlin AG, Wrangelstr. 100, D-10977 Berlin, Germany