Understanding Photovoltaic Cell and Module Level Efficiency

After a recent presentation during a government renewable energy conference, I received a number of questions regarding why there was such a large difference between crystalline solar cell efficiency and a fully packaged and weatherized module. For instance, a 19% efficient crystalline photovoltaic (PV) cell, when packaged into a module with 60 cells results in a panel that is roughly 15% – 16.5% efficient depending on the manufacturer. According to the NREL, the cell to module loss is in the 11% – 17% range for most manufacturers.

Solar module

Solar Module Packaging

The losses are a result of three distinct issues. 1) physical layout of the PV module and framing,  2) optical loss from encapsulation and glass, and 3) series loss from cell connections

The physical layout of the module affects the efficiency by having a large inactive area, meaning the space between cells, the edge of the module and width of the frame. The larger the inactive area of a module, the lower the efficiency.

The optical loss is a less straightforward problem and has a number of challenges resulting from the top glass and the encapsulation film.

The top glass needs to have low reflectivity so the maximum amount of solar radiation reaches the solar cells. The glass choice has to balance a number of factors including thickness, to meet hailstorm impact rating; tempering, to meet safety standards; and optical clarity, for maximum radiation absorption by the PV cells. A good, if technical overview here.

The EVA encapsulation film used to protect modules from moisture and the elements require a similar balancing act. These include letting the maximum amount of solar radiation reach the cells, while maintaining a near-100% moisture barrier with no significant expansion or contraction of the film over the 20+ year life of the module. And it needs to do this without creating an overheating of the module in hot climates.  A module with a high temperature coefficient (loss due to heat) is the

PV module ribbon series connection

Solar Cell PV Ribbon 

enemy of high solar power production.

The series loss is due to series resistance in the cells themselves and in the cell and string connectors. The cells themselves are made from silicon, which not as good as metal for transporting current, and its internal resistance is fairly high, resulting in current loss.  This loss is compounded by copper ribbon (silver looking ribbon between cells) interconnection loss, and the cells’ series configuration in the module. While cells are put in series to meet a target voltage for a given module, this results in loss from the large number of connections.

There are a number of efforts underway to reduce this cell-to-module loss to 5% or less with novel approaches in all 3 areas. While the reduction to 5% has been achieved in national laboratories in an academic environment, the challenge always is to translate these new methods into a highly efficient manufacturing production line where throughput speed and yield (sellable product) are not compromised.

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