Water vapour permeation measurement for PV Solar Cell packaging

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With fears of carbon dioxide causing global warming and diminishing supplies of fossil fuels, a looming energy crisis is affecting all countries.  As a consequence, a great deal of world-wide effort is going into the development of alternative energy technologies.  At present, wind, hydro and geothermal are becoming well established with more niche roles for tidal and solar. The latter technology has two common forms, firstly solar water heating, and secondly photo voltaic (PV) solar electricity generation.

 Solar water heating

For domestic hot water needs, solar hot water is starting to make significant inroads in Europe and Turkey but very little impact in the rest of the world, with the exception of China. This country has more than 70% of the world’s solar water heating capacity, assisted greatly by government subsidy and promotion. (REN21 Renewables 2010 Global Status Report)  

 Photovoltaic Solar electricity

PV Solar from the above mentioned report shows a different picture, with Germany having the dominant usage at 47%, whereas China had less than a few percent at the end of 2009. However, whilst the power from PV in total is no more than about 15% of that of solar water, its use is spread more evenly throughout the world.  There is much research into new and improved PV technologies as well as rapidly increasing use of existing commercial products. Traditionally the cost per watt of this technology has far exceeded that of conventional power generation and not surprisingly the main development thrusts are increase in efficiency and reductions in cell costs.

The operation of a PV solar cell is based on the photovoltaic effect, when the capture of a light photon photoexcites an electron to a higher energy level. The cell is constructed as a PN junction from semi-conductor material, like the common diode, in which photoexcited electrons preferentially diffuse across the junction barrier and flow around an external circuit providing power. 

 Silicon single crystal solar cells

Traditional cells are made from single crystal Silicon or c-Si and produce about 0.5Volts per cell, with typical power efficiency of 15-17%. The maximum theoretical efficiency of conversion from light energy to electrical energy is about 30% and recent developments have produced examples exceeding 24%. Clearly, with an output of a mere 0.5V, numerous cells have to be connected together to produce enough voltage to work with.  This is done in the solar panel where the cells are arranged in a matrix, encapsulated in transparent EVA, usually behind a glass front cover and sealed into an outer frame.  The back of the panel is sealed with a polymer film backsheet, selected for its electrical insulation, toughness, and importantly, lack of water permeation. These devices are expected to last for 25 years or more and corrosion damage to the electrical contacts by water is one of the biggest threats.

 Thin film solar cells

At the time of writing, c-Si PV solar cells are the most dominant type in the market by far, occupying over 80% of the market.  However this position is likely to change soon with the advances being made with thin film solar cells, which form the remainder of the market.  These newer cells are made from other types of semiconductors and are based on an amorphous rather than crystalline material, which is assembled in thin layers allowing it to be flexible.  There are at least three different common types in production, these being, amorphous Si (a-Si), Cadmium Teluride (CdTe), and Copper Indium Gallium deSelenide (CIGS).  Mention should also be made at this juncture of dye sensitised solar cells (DSSC), which are also classified as thin film but are not PN junction based. These are referred to as photoelectrochemical cells, and were invented by Michael Grazel.  As mentioned for c-Si cells, this type also needs exemplary environmental protection, with emphasis on water ingress through permeation.

While thin film cells are in general less efficient than the c-Si variety, being in the 11-13% region at present, they have some major advantages over their predecessors.  Much larger single cells can be made, not being constrained by the size of a single crystal, at markedly lower cost, and their flexibility allows considerable diversity in mounting.  It is possible to print the materials, which can be made into a form of inks, in layers onto wide continuous strips a metre or more wide.  The resulting rolls can then be deployed in large solar collecting arrays and can even be used as an outer skin on buildings.  Not surprisingly, given the volume of research into improving these thin film variants, their market share is expected to soar over the next few years.

 Solar panel backsheets

Polymer film backsheets which are used for c-Si panels are often based on polyvinyl fluoride (PVF), which is frequently laminated with other materials for improved properties.  The DuPont PVF material known as Tedlar is a favourite and is used to sandwich a layer of polyester film to form TPT or combined with EVA as TPE.  Numerous alternative polymers are available offering trade-offs between performance/lifetime and cost.  Related materials are available for use on thin film solar products, with perhaps more emphasis on performance because of the need to keep thickness down to maintain flexibility.

 Measurement of water permeation

Clearly, the future for Solar PV electricity generation looks optimistic and importantly, a major growth area.  It is also likely to be a growth market for the solar cell manufacturers, as well as the equipment and instrumentation companies to support the cell production. The ability to provide definitive water vapour transmission rate (WVTR) data for production backsheet materials on a routine QA basis, is key to a reliable PV Solar Cell product.  Such WVTR measurement can be provided by the Systech Illinois 7000 water vapour permeation analyser, capable of measuring from as low as 0.002 g/m²/day.

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