Without batteries, off-grid PV systems (except some cases such as water pumping) would be meaningless, because their functionality depends on electrical energy storage.

The battery is an electrochemical device that transforms chemical energy into electrical energy, whose presence is necessary because solar modules only generate energy when light hits them.

In addition, sometimes battery provides an instantaneous power higher than that of modules (eg: for starting motors) and provides stable and constant voltage regardless of light incidence.

The battery determines modules operating voltage. Therefore a safety margin is required which will mean a small loss (about 10%) with respect to maximum power that module could provide at higher voltages.

There is no ideal battery. The choice is a compromise between economy and suitability starting from a minimum quality that provides reliability and long life to the system.

In a battery, we have to take into account 3 technical considerations:

1º The discharge capacity

It is the maximum amount of electrical energy that can be supplied from its full charge to its complete discharge. Measurement unit is the amp hour.

The loading and unloading ratio and the battery and environment temperature are factors that can make vary its capacity.

2º The discharge depth

In renewable energy systems, only deep discharge batteries are used (we refer to capacity percentage that is used in a cycle of loading and unloading).

Deep discharge batteries have an average discharge of 25%, and can reach 90%.

3” Cycles of a battery

It is the time from complete charge to discharge.

Battery life is measured in number of cycles it can handle.

Auto-discharge should also be considered as an additional consumption that daily demands a certain percentage of stored energy.

As damaging as excessive discharge is for a battery to too much load. Way to prevent this is by introducing a charge controller.

Every time battery is recharged, does not completely regenerate, resulting in a degradation that will determine battery life.

If discharge depths are respected and maintenance is correct, battery service life should be approximately 10 years.

For PV systems, batteries used are:

1. Lead-Acid: Characterized by their low cost and maintenance they require (need to be in a cool place and periodically check electrolyte amount).

Lead-antimony are the most used in medium and large systems and lead-calcium are mainly used in small systems.

There are also 2 types of sealed lead-acid batteries: Gelled (incorporating an electrolyte gel type) and Absorbed Electrolyte (electrolyte is absorbed into a microporous glass fiber or a polymer fiber web).

These batteries don´t require maintenance in water aggregate form nor develop gases, but both require less deep discharges during their service life.

2. Nickel-cadmium: offer better performance, but have a higher price.

The electrolyte they use is an alkaline, have a low self-discharge coefficient, good performance at extreme temperatures and the discharge they support is around 90% of their rated capacity.

They are recommended for isolated or dangerous access places.

They can´t be tested with same reliability as lead acid. Therefore, if it is necessary to control charge state, they aren´t the best option.

3. Lithium: they take up little space, they weigh less, they do not emit gases, they can be put anywhere, loading time is the fastest, total discharges can be made without affecting their useful life in a relevant way.

What is the disadvantage? Its very high price.

The manufacturer who can optimize them will have found the solar sector Holy Grail.

This is an extract of contents included in Technical-Commercial Photovoltaic Solar Energy Manual and Sopelia e-learning training.

All you need is Sun. All you need is Sopelia.

Cells are silicon in the most used modules, element which is the main component of the silica, the material of the sand.

The regional production capacity distribution differs significantly depending on product type and its value chain position.

Solar grade silicon production capacity is headed by the US; followed by Europe, China, Japan and the rest of Asia.

Silicon cells and modules production capacity is dominated by Chinese and Taiwanese manufacturers; followed by Europeans, Japanese and the US.

Thin-film manufacturers must still optimize production to reach optimal cost structure to be competitive.

A difficult task with much lower prices for polysilicon, resulting in a significant decrease in silicon modules prices.

In order to avoid scarcity or oversupply cases, it is of utmost importance to guarantee supply, demand stability, based on a sustainable market so that the industry can foresee the growth of the same and plan its capacities.

Photovoltaic systems demand depends to a large extent on general economic climate and, most importantly, on governments policies to support their development.

Tariffs, along with administrative procedures and grid connection simplification, as well as priority grid access are policies aimed to guaranteeing sustainable demand.

A silicon cell provides a voltage of about 0.5 V and a maximum power of between 1 and 2 W.

In module manufacturing process, a certain number of cells must be in series connected to produce voltages of 6, 12 or 24 V indicated for most applications.

To produce a 12 V module, you need between 30 and 40 cells.

Cells connecting process is done by a special welding that joins the back of a cell with the front face of the adjacent one.

After electrical interconnections are completed, cells are encapsulated in a sandwich structure (tempered glass laminate – EVA – EVA – polymer cells).

The structure varies by manufacturer.

Subsequently a vacuum sealing is carried out, introducing it in a special furnace for its lamination, making tight the assembly.

If they have a metallic support frame, module perimeter is first surrounded with neoprene or some other material that protects it.

Once positive and negative connections are mounted, following controls are performed to ensure a 20-year service life with acceptable performance levels:

– Thermal cycles (-40 ° to 90 ° C)
– Humidity cycles.
– Freezing cycles.
– Wind resistance.
– Mechanical strength.
– High electric shock resistance.
– Saline atmosphere test (for marine environments).

Manufacture, performance, electrical and mechanical characteristics of photovoltaic module are determined in product technical specifications provided by the manufacturer.

As in solar cell, following parameters are important:

– Module maximum power or peak power PmaxG.
– IPmax: Intensity when power is maximum or current at maximum power point.
– VPmax: voltage when power is also maximum or voltage at maximum power point.

Other parameters are:

– IscG short-circuit current.
– Open circuit voltage VocG.

These parameters are obtained under standard conditions of universal use according to EN61215. Established as follows and the manufacturer must specify:

* Irradiance: 1000 W / m2 (1 Kw / m2)
* Incident radiation spectral distribution: AM 1.5 (air mass)
* Normal incidence
* Cell temperature: 25ºC

Modules working conditions may be very different once installed, so it is advisable to know variations that can occur, in order to make calculations relevant corrections.

In practice, module power decreases by approximately 0.5% for each cell temperature increase degree cell above 25 ° C.

To avoid having to calculate radiation average intensities, we can assume that cell average working temperature is 20º higher than ambient temperature.

For this concept, yield drops to 90%. In not based on crystalline silicon technologies, yield lower is smaller.

This is an extract of contents included in Technical-Commercial Photovoltaic Solar Energy Manual and Sopelia e-learning training .

All you need is Sun. All you need is Sopelia.