Guidelines to Quality Technical Specifications

The Technical Specifications to be considered can be directly identified from the PV module datasheet include:
  • Technology type (mono-, poly-crystalline Silicon, thin-film)
  • Efficiency (%)
  • Peak power and tolerance (W)
  • Temperature coefficients (%/°C)
  • Nominal Operating Cell Temperature (°C)
  • Bypass Diodes
  • Package type
  • Dimensions/Weight
  • Manufacturer specific patented technology

Technology

Photovoltaic (PV) is the solar technology which produces electricity directly from sunlight, using cells which are connected and encapsulated in panels called PV modules. At present, the three main technologies available in the market are:
• Mono-crystalline silicon
• Poly-crystalline silicon
• Thin-film: amorphous silicon (a-Si), cadmium-telluride (CdTe) and copper-indium-diselenide (CIS).

Efficiency (%)

Is the ratio of the electrical output a photovoltaic cell or module to the energy input from the incident sunlight, provided as a percentage. Typical commercial efficiency values of PV modules vary from 4-17 % depending on the technology.

PV Technology Module Efficiency Surface area for 3 kWp system
Mono-crystalline silicon 13-16 % 21-27 m2
Poly-crystalline silicon 11-15 % 24-27 m2
Copper-indium-diselinide (CIS) 12-14 % 24-32 m2
Amorphous silicon (a-Si) 4-8 % 48-60 m2

Peak Power and Tolerance (W)

The electrical power output of a PV module is strongly dependent to the environmental conditions and mainly to the incident sunlight. The maximum power a module can produces is defined as the Peak Power or Rated Power (Watts) and is the power output achieved by the module under ideal environmental conditions which are defined as the Standard Test Conditions (STC) - 1000 W/m2 solar irradiance, module temperature 25 °C and an Air Mass (which describes atmospheric thickness that influences the composition of the spectrum of sunlight) of 1.5.
The Power Tolerance provides a range within the PV module will over- or under- perform compared to its Peak Power. For example if the Peak Power of a module is 250 W and the power tolerance is +/-5 %, this means that the Peak Power output could be either 262.5 W to 237.5 W under rated conditions. Obviously a high positive Power Tolerance is to the advantage of the end user.

Power Temperature Coefficient (%/°C)

Unlike other solar technologies, PV modules produce power by converting sunlight to electricity while the heat component of sunlight decreases the power output. The loss in power production due to high operating temperatures that PV modules can experience especially in Cyprus, is an important loss factor that can be described with the temperature coefficients. The temperature coefficient of power (%/°C) provides a measure of the decrease in produced power due to temperature increase. When selecting between modules it is preferable for improved performance at elevated temperature to choose a module with a less negative power temperature coefficient. For example, the output of a PV module decreases more at elevated temperatures (over 25 C) when the temperature coefficient is -0.47 %/°C, compared to another module with a power temperature coefficient of -0.43 %/°C

Nominal Operating Cell Temperature (°C)

Another important technical parameter associated with the thermal operation of a PV module is the Nominal Operating Cell Temperature (NOCT). The NOCT (°C) of a PV module provides an indication of the temperature that will build up within a PV module due to its packaging arrangement and materials, when the module is exposed to solar irradiance of 800 W/m2, prevailing wind of 1 m/s, ambient temperature of 20 °C and when the module is mounted at an open rack at an inclination of 45 degrees from the horizontal. A typical values of the NOCT is 45 °C, and when selecting amongst PV modules it is important that the NOCT is as low as possible as a module with an NOCT value of 47 °C is expected to heat up more at the warm conditions in Cyprus compared to a module with an NOCT value of 45 °C.

Bypass diodes

All PV modules must include Bypass Diodes in order to protect the cells of a module in the event that the module is partly shaded, either from soiling or a distant obstacle. Bypass diodes included within PV modules allow the non-shaded cells to produce power while protect in parallel shaded cells from being overheated and damaged. Typical PV modules include three Bypass Diodes.

Busbar topology

PV cells have a network of electrical grid contacts called fingers and busbars, that are necessary in order to collect the current produced by each cell. Usually older PV cells had two busbars , and in order to improve the efficiency of each cell manufacturers have at the moment added an extra busbar (three busbars) to each cell, improving also in this way the performance. The three busbar topology allows efficient current collection while also is an indication to the end-user that the module is of a recent production line and not an old production line module.

Package type (frame/glass/encapsulant/backsheet)

Typically PV modules are composed of a series interconnection of PV cells that are encapsulated and electrically isolated using an encapsulant material and enclosed between a front glass surface and back plate material. Most modules also have frames that provide a rigid stricture to the module.
Encapsulant: The encapsulant is used to provide adhesion between the cells, the top and the rear surface of the PV module. The quality and endurance of the encapsulant is important for the lifetime performance of the modules as it should not discolourize and detach from its structure throughout the guaranteed lifetime of the module.
Glass: The glass used for PV modules allows sunlight to pass while also protects the cells from environmental conditions. Variance in photovoltaic efficiency has been presented by manufacturers stating improved performance due to glass layers with improved optical (anti-reflection technologies) and anti-soiling performance.
Backsheet: The back sheet is the outermost layer of the PV module and is designed to protect the inner components of the module. Many manufacturers design backsheets with improved thermal dissipation capabilities in order to achieve improved outdoor performance.
Frame: The frame is the structural component of the module which provides a rigid structure to the module. The frame structure should not corrode and have no projections which could result in the lodgement of water and soiling.

Dimensions/Weight

PV modules come in a variety of sizes depending on the technology. Typical crystalline Silicon PV modules with Peak Power 250 W have dimensions: 1665 x 991 x 30 mm (Length x Width x Height). The weight of such modules ranges from 18 to 25 kg.

Manufacturer specific patented technology

Some manufacturers have developed patented technologies either throughout the manufacturing process or to improve the outdoor performance (either in terms of special coatings, glass layers, contacting system etc).