Solar PV - increasing efficiency

A new 215 watt (W) high efficiency SunPower solar PV module has been released on the Australasian market, boasting the world’s highest efficiency module available today.

The solar photovoltaic (PV) modules have a total module efficiency of 17.3 per cent - with field-tested reliability. The modules are certified to International Electrotechnical Commission (IEC) and UL standards and are now available in Australasia through Solar Sales.

SunPower’s goal is to continually develop modules with world-leading efficiency, reliability, and aesthetics. Field-proven encapsulation materials and design features are used which take advantage of the unique back-contact cells that have cell efficiencies of 21 per cent.

The benefits of these modules to the industry include:

  * Increased PV module efficiency - enabling more power to be generated in space-limited applications. It also reduces the installation cost per kilowatt for all applications which is important as installation is a significant part of the total cost of PV systems
  * Module aesthetics - important for building-integrated and residential applications
  * Reliable performance over the 25 year plus module life

Key design features of the SunPower high efficiency modules

Diode protection

Typical crystalline silicon modules use bypass diodes to protect cells against large reverse-bias voltages that can occur as a result of partial module shading or a problem with a cell. SunPower cells have advantageous breakdown behaviour at -5.5 volts (V) that is reversible and uniform, thereby eliminating the need for bypass diodes to protect the cells. While bypass diodes are not required for cell protection, diodes are included in modules to increase energy delivery under certain partial shading conditions and to provide comfort to customers who are used to diodes being present. Three diodes with 8A rating are included in the 72-cell module in a junction box tested under the present and the recently-announced IEC testing standards.

Cell-cell interconnect cloak

The cell-to-cell interconnect in the modules is visually shielded from the front (patent pending), because SunPower cells have no metal or other visible features on the front side, the use of an interconnect cloak results in a uniform appearance with no harsh reflections.

Cell-cell interconnect design

Traditional solar cells use ribbon to connect the front of a cell to the back of the adjacent cell. For back-contact cells, the interconnect extends only from the back of one cell to the back of the next cell. This gives the advantage of requiring interconnect placement and soldering only from one side. This also eliminates the front-to-back bend which was the location of the failure for some interconnects.

Busbar design

To minimise wasted module space, the busbars (which connect strings to each other and to the junction box) are located underneath the cells. Insulators are used to prevent shorting of the busbars to the cells. This approach allows a symmetric module with uniform edge gap all around. An edge gap of 16 mm was chosen for the 72-cell module to be compatible with the expected increase in the IEC-mandated gap for modules rated for 1,000 V applications.

Encapsulation materials

SunPower cells generate power from a very broad spectrum of light (330-1,170 nanometer), so high-quality, low-iron glass with solar transmittance of more than 91 per cent was selected to minimise absorption in the glass. Lightly-textured glass is used to eliminate harsh reflections from the front surface of the glass. The glass has a stippling pattern on the other side, which is oriented to maximize adhesion.

Ethylene vinyl acetate (EVA) from Specialized Technology Resources was chosen because it is a field-proven and accelerated-tested encapsulant. A Tedlar/Polyester/EVA backsheet was chosen that has a 125 micrometer thick polyester layer to provide excellent protection against environmental stress and the ability to receive 1,000 volt IEC certification. The extruded EVA layer of this backsheet provides excellent adhesion to the EVA encapsulant layer.

The 72-cell high efficiency module, which is 1,559 mm by 798 mm, was chosen to be a high power module that can still be easily handled by one person. The 72-cell modules are produced with two colour options currently available in Australia using a+/-5 per cent output tolerance:

  * The use of a white backsheet and white interconnect cloak gives a white-background module with a power rating of 215 W. The present rating this module (area=1.244 m²) corresponds to a total-area efficiency of 17.3 per cent. Solar Sales says that to its knowledge, this is the highest efficiency available for production modules in the world today.
  * The use of a black backsheet and black interconnect cloak gives a 200 W-rated module with a completely uniform appearance that is ideally-suited for certain residential applications.

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