Mismatch in Solar Cells & Modules
13th March 2018
The fundamental unit of a solar module is the solar cell. It works on photovoltaic effect i.e. it produces electricity when light falls on its surface. A typical solar cell may produce output of around 4 to 5Wp (depending on technology and efficiency). The solar cells are assembled in different series and parallel combinations in order to meet our power requirements (voltage and current). As the basic concept of electricity goes (Figure 1), any two (or more) load connected in series shall operate at same current with its operational voltage being added up. Similarly any two (or more) load connected in parallel shall operate at same voltage with its operational current being added up. The same concept is applicable to generators i.e. (here) the solar cells, however there is a catch. The voltage and current output of solar cell may vary marginally due to manufacturing limitations. Such cells when interconnected in a module may lead to accounted loss in power output. Additionally such modules when inter-connected with each other may expedite the mismatch losses. Such mismatch while small in number (in terms of power output), may adversely affect the solar cell and the solar module. This article hence aims to educate its readers on such affects and its possible remedies.
Figure 1: Concept of series and parallel connection (Source: Google Images)
Mismatch in series connection
The solar cells (60 or 72 full cells) in general, are (usually) connected in series in a solar module. As we know, the cells connected in series should have the same operating current flowing through it for fully realizing the module's power output. However solar cells when in operation may be shaded due to leaves, clouds, bird droppings, etc. This would lead to mismatch in generation characteristics (i.e. current and voltage output) from a solar cell. When connected in series, the mismatch in voltage output does not impact the power output as such different voltages are just added up.Say in a string of 12 solar cells, 11 have voltage output of 0.6V whereas 1 cell has output of 0.55V, the total output voltage of the string is as follow: 11 x 0.6 + 1 x 0.55 = 7.15V. It is the mismatch in current output of the solar cell which is fatal for the string (and for the solar module). Say in a string of 12 solar cells, 11 cells have current output at maximum power of 7.5A whereas 1 cell (say cell X) has output of 7A. As the string operates at a single current value, the current of the cell with the lowest value (7A)presides over the string. In such case, the excess current (0.5A) from the remaining cells (11 in this case) is forced onto the cell X. This causes the operating temperature of cell X to shoot up which may lead to hotspot (A condition where the temperature of part of/entire cell rises up so much that it may rupture)on the cellin the long run (Figure 2).
Figure 2: Hotspot in a solar cell (Source: Google Images)
An effect similar to it can be observed in the solar module connected in series. Due to (say) shading, one of the solar module in an array may not able to generate the desired current (and hence the power output). Such module forces the entire array of modules to operate at a lower current. The excess power produced by the remaining modules is dumped into the partially/fully shaded module. This dumping leads to an increase in temperature of the shaded (and already stressed) solar module (Figure 3). If such shading is not mitigated, it may adversely impact the module causing hotspot and reduced power generation capacity of solar module. A single shaded module can typically bring down the power output of a plant from around 10% to as high as 40%. Additionally with formation of hotspot, the life of the solar module may be reduced to around 10 years (depending on the external conditions).
Figure 3: Hotspot in solar module due to current mismatch (Source: Google images)
In order to avoid such issues, the solar module are fitted with bypass diodes. These diodes are connected in opposite polarity in parallel to the string (usually 2) in the module (Figure 4). In normal operation, these diodes are inactive and do not participate in the generation. However if any of the string (or cell/cells in the string) is/are shaded, the entire string operates at lower current than the other strings which forces the shaded string to act as a load rather than generator (i.e. it operates in reverse bias mode). This reverse biasing (or reverse polarity) causes the bypass diodes to be in active state. They perform the following main functions:
Figure 4: Typical configuration of solar module along with bypass diode (Source: Waaree Energies)
Mismatch in parallel connection
The mismatch in parallel connection is applicable in the array of solar module. The array has various strings of solar module connected in parallel.A voltage mismatch occur when string producing different voltage (when measured independently) are inter-connected and hence the entire array operates at the voltage equivalent to lowest performing string (preliminary leading to loss of power output). However a more severe problem occurs when there is high voltage mismatch between the inter-connecting strings. Such intense mismatch causes the modules to operate at a point far off from its Maximum power Point (MPP). Such operation in addition to loss of power also causes the inverter to work outside its ideal voltage range for a longer amount of time which in turn affects the inverter efficiency. It is recommended that the mismatch in the operating voltage within the parallel string should not exceed more than 7 ~ 10%.
While mismatch happens in both series and parallel type of connections, it is the mismatch in series connection that effects performance the most. One reason that can be attributed to this is that (currently) at module level, all the solar cells are inter connected in series (to obtain desired voltage) and similarly at the power plant level, various modules are connected in series to ensure that the output voltage is within the input voltage window of the inverter. The effect of series mismatch can only be minimized and not nullified totally as due to varied manufacturing process, climatic conditions, etc. so the mismatch is bound to happen.
Waaree Energies ensures that all its modules are passed through rigorous quality check. This enables its modules to be almost error free. Additionally we carry out few techniques which enables us to understand our modules better. One such technique is current sorting, where the modules are sorted according to their current output and labelled accordingly. This ensures that our customers (and the installers) are well informed about the performance of our modules and could position the modules accordingly to maximize the plant output.
Let us all pledge to make solar energy the primary source of energy in the near future.
RAHE ROSHAN HAMARA NATION