Why such rapid industrial shifts with busbar?
At the crux of solar plant, the entity which is responsible for producing power output is the solar cell. While there has been a considerable technology shift in the material used for solar cell, the industry has always demanded on extracting the maximum power output from the available cell to maximize their returns. The solar cell as we know is made from combination of various materials and numerous processes. Hence, to realize an increase in power output (and hence the efficiency) requires proper process and material optimization (at each and every step). The cell efficiency depends on various factors such as its thickness, junction properties, screen printing, doping, etc. to name a few. One of the heavily researched (and hence considerably easy) option is to increase the number of busbar while screen printing on the silicon wafer. The commercial market has seen an up gradation in busbar from a 2 busbarsolar cell (and module) to (currently available) 4/5 busbar module. This article aims to educate its readers on the basics of a busbar, the rapid shift in increasing number of busbar and what lies ahead for the market.
Figure 1: A typical 3 busbar solar cell (Source: Google images)
Conventional solar cellare metallized with a rectangular and thin shaped strips on both the (front and rear) sides which are known as busbar. Considered as one of the most critical component, their purpose is to collect the generated electrons from solar cell via the grid (or more often known as fingers) which are interconnected by the busbar (figure 1). The development of busbar in solar cell has been significant. Typical solar cell has shifted from a 2 busbar cell to 3 and then 4 to the most recent 5 busbar cell (Figure 2). A key reason for such shift was the increasing demand for power output from the same cell.It was hence important to enhance the efficiency of solar cells. One of the major losses in solar cell is the inefficient current collection and/or current accumulation at the solar cell. This was because the current carriers (i.e. the number of busbar) in the (conventional) solar cell were limited. Hence, it was necessary to increase the number of busbars in the solar cell (figure 3). Such increase would result into increase in number of contact points in both front and rear side of solar cell. It would further result into increased current collection and hence increased power output from the solar cell. Additionally, with better current collection, the probability of hotspot formation also reduces.
Figure 2: Currently in use 4 busbarcell (on left) and the advanced 5 busbar solar cell (on right)
Figure 3: Effect of no of busbar on cell efficiency (Data source: Braun S, et. al. "The multi-busbar design" Energy Procedia 43 (2013) 86 - 92)
As we increase the number of busbars, it should also be noted that we don't compromise the active area of the solar cells, hence optimizing the width of busbars is mandatory. The width of busbar varies from 1 mm to 2 mm based on the number of busbar the cell uses. The primitive solar cell (2 busbar) used a thicker busbar compared to the cell currently in use.While the (most) ideal design of busbar is a rectangular, many a times different producers (of solar cell) in order to save on material cost differ from the rectangular design. In many cases the distance between empty spaces of busbar varies (Figure 4) reducing the cost for printing but would also lead to substantial loss in current collection. It is important to optimize shading and metallization due to busbars. Due to this reason we believe that now instead of gradual increase in number of busbars, the market would move to a multi busbar technology. Such technology uses special type of cells and busbar whose design is optimized in such a way that both the power and cell efficiency are optimum.
Figure 4: Varying designof busbar (Source: Google Images)
Waaree has already understood this market trend and had introduced Merlin® technology (Figure 5) which uses a patented grid technology over the cell. These are highly flexible 20 busbar modules which have optimized grid over the solar cell. This would enable maximum current collection even in case of micro cracks (and sometimes cracks) giving them the extra edge over its competitors. It is the same grid which also allows the module to be flexible while maintaining high efficiency and product reliability. Waaree has been manufacturing such modules for both national and international markets.We would continue this innovative drive to deliver the best of class products and service to our consumers.
Figure 5: Waaree's next generation multi busbar flexible modules
Let us all pledge to make solar energy the primary source of energy in the near future.
RAHE ROSHAN HAMARA NATION