Micro Energy Systems

Solar cells are connected together to form solar modules. Crystalline solar cells are generally sandwiched between a toughened glass front and a poly vinyl back sheet surrounded by an aluminum frame.

Solar modules have traditionally been sized to suit lead acid batteries, with outputs of 12 volts and relatively large current flows available for battery charging applications. Later grid connect technology has resulted in modules which tend to be physically larger with a higher voltage output, usually between 24V & 40V and with a lower current. This higher open circuit voltage and lower short circuit current module design means a greater efficiency is realised with less electrical losses and a power output curve more applicable to converting to grid mains connection.

Lower current higher voltage module configurations also means that solar cell interconnections do not have to carry as much current, making manufacturing processes cheaper and the results more reliable.

What are the different types of solar modules?

There are two main different applications for solar modules and several different types.

Traditionally solar modules where used in a stand alone application usually for charging batteries which supplied the energy for a load. As mentioned above these modules tend to be physically smaller, have a lower power output and a lower voltage output. These styles of modules still have applications for small stand alone applications such as remote area pumping. Telemetry and lighting systems.

When utilized for a grid connect application however, the relatively low voltage and high current configuration of the battery charging module is inefficient.

Grid connect modules have been developed especially for grid connect applications, the final array size is smaller and lighter compared to battery charging modules for the same application, the system losses are lower and the maximum power point (MPP) is in the best place on the output power curve to best suite a higher voltage application.

As mentioned traditionally solar power has been used in remote areas in conjunction with battery backup systems. These systems have voltages of 12 volts for smaller systems and multiples of 12 volts for larger systems. This lead to the vast majority of solar modules being made with a nominal voltage of 12 volts and these modules are still being manufactured. However more recent developments in Solar technology suited for RAPS (Remote Area Power Systems) has made the grid connect or AC grid configured system much more suitable for most stand alone power generation applications. For Stand Alone systems requiring 1KW or more of PV the AC grid connect modules utilized in an AC grid configuration prove to be more economical, providing greater efficiency and flexibility.

Module Types

Currently there are three main types of Module commonly used for GC PV applications. All use silicon as a base, manufactured into a module in three different styles.

Mono Crystal: As the name suggests Mono crystal is a cell shaved from a single crystal. This style is more expensive to manufacture requiring more rigorous controls and refined techniques. It produces the most efficient output however and is currently the predominant style for modules used in higher power outputs applications where reliability is essential.

Polycrystalline: This is a cell manufacture technique which uses many smaller crystals grown together to form the basis for a single cell. Poly crystal cells are cheaper to manufacture but have a slightly lower efficiency than mono crystal cells. This means that more are required to obtain the same power output than modules constructed using mono crystal cells.

Amorphous: Amorphous silicon is a departure from the above two types of cell manufacture. Amorphous materials, by definition, lack a crystalline structure and can be created by melting and then rapid cooling a crystalline substance as is done with a plasma vapor deposition process. Amorphous modules are cheaper to manufacture than crystalline silicon cell modules and have several advantages. They are however the least efficient in converting sunlight to energy and require about double the array area size to produce a similar power output to crystalline technologies. This means that they are impractical for many urban grid connect applications.

One of the advantages that the amorphous silicon module has is a more consistent output under high temperature conditions. It’s an irony of the technology that photovoltaic cells loose efficiency when subject to higher temperatures. This has made amorphous modules more popular in places like far North Queensland where relatively high temperatures are the norm. Amorphous or thin film modules also require much less silicon to manufacture.

This makes the process less dependent on a world resource which is currently experiencing a shortage due to high demand. Amorphous modules found popularity amongst several national Grid connect sales companies in Australia over the last few years during a period when PV grade silicon supply became a critical factor in the economics of solar energy.

What should I look for in a module?

STANDARDS

As a minimum you should ensure that any solar panel that you are considering is certified to the IEC (European) or UL (American) standards. These standards are developed to ensure a minimum level quality and safety in solar modules. Modules which do not carry an IEC or UL number will not be eligible for certification by an Australian quality assurance standard. Modules must also be certified by an Australian quality standard, listed with the Clean Energy Council before being eligible for any government rebates or RECs application.

TOLERANCE

Solar modules are rated to a certain power which is the power that they should produce under standard test conditions (STC). They will also have a tolerance rating, typically ±10% of the rated power which shows the power range that the module will lie in. For example a 100W module with a tolerance of ±10% will have a power output of somewhere between 90W and 110W at STC. Another 100W module with a tolerance rating of -5% to +15% will have a power output of between 95W and 115W at STC. Generally, lower tolerances are preferable as they mean that all panels of a certain model will have similar power outputs which is important in high voltage systems.

RAW MATERIALS

The quality of the raw polysilicon silicon feedstock used to make crystalline solar cells has a strong impact on the actual power output of the finished module (as opposed to the ‘rated’ power).

There is currently a world shortage of the polysilicon feedstock that is used to make solar cells due to the rapidly growing solar industry outstripping supply. The larger solar cell manufacturers have typically responded by entering into long term supply contracts with the major polysilicon feed stock manufacturers, ensuring both availability and quality of their raw materials. Smaller cell manufacturers, especially new entrants to the market don’t have the long term certainty to do this however and tend to buy silicon feedstock on the spot market, buying what they can get within their acceptable price range. This means that they are using highly variable raw materials, often of lower quality, which in turn affects the power output of these modules.

Although you can’t check the quality of the raw materials by looking at a finished solar module, you can ensure against low quality products by using products manufactured by reputable manufacturers with a track record in the solar industry.

WARRANTY

Virtually all solar modules carry a warranty of at least 10 years, with most reputable manufacturers offering warranties of 20 or 25 years in Australia. With such long warranty periods, it is important to consider the ability of the manufacturer, or importer in the case of an overseas manufacturer, to honour their warranty. In particular, a 25 year warranty is worthless if the company offering it goes broke or disappears in 5 years time.

FRAME

Most solar modules have an anodised aluminium frame to give them strength. Unframed modules break much more easily when hit by hail or other impacts. Furthermore, different manufactures use different types of frames. The better modules, have a double wall containing a boxed section that gives the frame much more strength than a single walled frame.