POLYCRYSTALLINE SOLAR PV MODULE

The power generated by a single cell is small and therefore several cells are interconnected in series/parallel combination to get the required voltage and current. When a number of solar cells are connected in series to get a specific voltage the unit so formed is called as Solar Module. Charging batteries is the primary use of SPV module. Therefore normally 36 cells are joined in series to form a standard module, which is capable of charging 12 volts battery

CONSTRUCTION OF PV MODULE

Solar photovoltaic or ‘PV’ panels are made using the 6 main components described below and assembled in advanced manufacturing facilities with extreme accuracy. In this article we will focus on panels made using silicon crystalline solar cells which are by far the most common and highest performing solar technology available today. There are other solar PV technologies available such as thin film and screen printed cells but we will not be discussing these as they have limited use or are still in development.

The 6 Main components of a solar panel

• Extruded Aluminium frame
• Tempered Glass – 3 to 4mm thick
• Silicon PV cells
• Encapsulation – EVA film layers
• Polymer rear backsheet
• Junction box – diodes and connectors

Many well known solar panel manufacturers are ‘vertically integrated’ which means the one company supplies and manufactures all the main components including the silicon ingots and wafers used to make the solar PV cells. However many panel manufacturers assemble solar panels using externally sourced parts including cells, polymer back sheet and encapsulation EVA material. These manufacturers can be more selective about which components they chose but they do not always have control over the quality of the products so they should be sure they use the best suppliers available.

PV Cells

A Monocrystalline solar cell (click to enlarge)

Silicon photovoltaic cells or PV cells convert sunlight directly into DC electrical energy. The performance of the solar panel is determined by the cell type and characteristics of the silicon used, with the two main types being monocrystallineand polycrystalline silicon. Additionally the base of the cell can be built using different additives to create either a positive p-type silicon or negative n-type silicon. However there are several different cell configurations available which offer different levels of efficiency and performance. See more information in the complete solar PV technology review.

Most residential solar panels contain 60 cells linked together via busbars in series to generate a voltage between 30-40 volts depending on the type of cell used. Larger solar panels used for commercial systems and utility scale solar farms contain 72 or even 90 cells and in turn operate at a higher voltage. The electrical contacts which interconnect the cells are known as busbarsand allow the current to flow through all the cells in a circuit.

Basic construction diagram of a common (P-type) silicon solar cell.

Glass

The front glass sheet protects the PV cells from the weather and impact from hail or airborne debris. The glass is typically high strength tempered glass which is 3.0 to 4.0mm thick and is designed resist mechanical loads and extreme temperature changes. The IEC minimum standard impact test requires solar panels to withstand an impact of hail stones of 1 inch (25 mm) diameter traveling up to 60 mph (27 m/s). In the event of an accident or severe impact tempered glass is also much safer than standard glass as it shatters into tiny fragments rather than sharp jagged sections.

To improve efficiency and performance high transmissive glass is used by most manufacturers which has a very low iron content and an anti-reflective coating on the rear side to reduce losses and improve light transmission.

Frame

The aluminium frame plays a critical role by both protecting the edge of the laminate section housing the cells and providing a solid structure to mount the solar panel in position. The extruded aluminium sections are designed to be extremely lightweight, stiff and able to withstand extreme stress and loading from high wind and external forces.

The aluminium frame can be silver or anodised black and depending on the panel manufacturer the corner sections can either be screwed, pressed or clamped together providing different levels of strength and stiffness.

EVA Film

EVA stands for ‘ethylene vinyl acetate’ which is a specially designed polymer highly transparent (plastic) layer used to encapsulate the cells and hold them in position during manufacture. The EVA material must be extremely durable and tolerant of extreme temperature and humidity, it plays an important part in the long term performance by preventing moisture and dirt ingress.

The lamination either side of the PV cells provides some shock absorption and helps protect the cells and interconnecting wires from vibrations and sudden impact from hail stones and other objects. A high quality EVA film with a high degree of what is known as ‘cross-linking’ can be the difference between a long life or a panel failure due to water ingress. During manufacture the cells are first encapsulated with the EVA before being assembled within the glass and back sheet.

Backsheet

The backsheet is the rear most layer of common solar panels which as acts as a moisture barrier and final external skin to provide both mechanical protection and electrical insulation. The backsheet material is made of various polymers or plastics including PP, PET and PVF which offer different levels of protection, thermal stability and long term UV resistance. The backsheet layer is typically white in colour but is also available as clear or black depending on the manufacturer and module.

‘Tedlar’ PVF material from Dupont is known as one the leading high performance back sheets for PV module manufacturing.

Dual glass panels – Some panels such as bifacial and frameless panels use a rear glass panel instead of a polymer backsheet. The rear side glass is more durable and longer lasting than most backsheet materials and so some manufacturers offer a 30 year performance warranty on dual glass panels.

Junction Box

The junction box is a small weather proof enclosure located near the top on the rear side of the panel. It is needed to securely attach the cables required to interconnect the panels. The junction box is important as it is the central point where all the cells sets interconnect and must be protected from moisture and dirt.

Bypass diodes

The junction box also houses the bypass diodes which are needed to prevent back current which occurs when some cells are shaded or dirty. Diodes only allow current to flow in one direction and a typical 60 cell panel has 3 rows of 20 PV cells and in turn there are 3 bypass diodes, one for preventing reverse current to each of the 3 sets of cells. Unfortunately bypass diodes can fail over time and may need to be replaced, so the cover of the junction box is usually able to be removed for servicing, although many modern solar panels now use more advanced long lasting diodes and non-serviceable junction boxes.

Number of Cells in Module

Number of cells in a module depends upon the standard voltage requirement per module. In 1980’s solar modules were mainly manufactured for charging 12 Volt batteries. But for charging a 12 Volt battery it is required to have sufficiently higher output voltage of the module than 12 Volt. It was standard practice to design a solar module with maximum voltage rating (Vm) of 15 Volt. This module of 15 Volt becomes standard module from those days.

The number of solar cells to be connected in series to achieve standard voltage output depends upon the open-circuit voltage (Voc) of the individual cells. The Voc of a solar cell depends upon mainly its manufacturing techniques. The table below shows the open-circuit voltage of different solar cells at standard test conditions.

Solar Cell Types Open Circuit Voltage at STC
Mono Crystalline Silicon Solar Cell 0.55 to 0.68 V
Poly Crystalline Silicon Solar Cell 0.55 to 0.65 V
Amorphous Silicon Solar Cell 0.7 to 1.1 V
Cadmium Telluride Solar Cell 0.8 to 1.0 V
Copper Indium Gallium Selenide Solar Cell 0.5 to 0.7 V
Gallium Indium Phosphide/ Gallium Arsenide / Gallium Solar Cell 1 to 2.5 V

For a crystalline solar cell the open-circuit voltage, is about 0.5 V, as shown in the table above. The voltage Voc is mentioned at 25oC but at the temperature higher than 25oC the value of this voltage drops nearly by 0.08 V.

So at normal operating temperature the voltage available across the terminals of each crystalline solar cell is Now, it is standard to make a solar module which can give 15 V open-circuit voltage at any condition.

Hence, the required number of solar cells to construct such solar module is, So, 36 numbers of crystalline solar cells are required to build a standard solar module of 15 V.