The electrical energy produced by the PV generators and turbines needs to be stored for use at night or generator outage in special batteries, which supports a large number of cycles and operate at constant current discharge. Lead-acid batteries are the oldest type of rechargeable battery. Despite having a very low energy-to-weight ratio and a low energy-to-volume ratio, their ability to supply high surge currents means that the cells maintain a relatively large power-to-weight ratio. These features, along with their low cost, make them attractive for use in applications that require high currents.
There are two main categories of lead-acid batteries, flooded batteries and sealed batteries. The sealed batteries are called VRLA battery (valve-regulated lead-acid battery). The VRLA battery are free maintenance, the oxygen evolved at the positive plates will largely recombine with the hydrogen ready to evolve on the negative plates, creating water and so preventing water loss. The valve is a safety feature in case the rate of hydrogen evolution becomes dangerously high. In flooded cells, the gases escape before they have a chance to recombine, so water must be periodically added.
Compared with flooded lead-acid cells, VRLA batteries offer several advantages. The battery can be mounted in any position, since the valves only operate on over pressure faults. Since the battery system is designed to be recombinant and eliminate the emission of gases on overcharge, room ventilation requirements are reduced and no acid fume is emitted during normal operation. The volume of free electrolyte that could be released on damage to the case or venting is very small. There is no need (nor possibility) to check the level of electrolyte or to top up water lost due to electrolysis, reducing inspection and maintenance. Compared to flooded batteries, VRLA batteries are more sensitive to high temperature environments and more vulnerable to thermal run-away during abusive charging conditions.
AGM batteries use saturated absorbent glass (boron silicate) mats between the plates instead of a gelled or liquid electrolyte. AGMs batteries are more expensive than the flooded (liquid) batteries but offer a number of advantages. Gel batteries are VRLA batteries with gelified electrolyte. The sulfuric acid is mixed with silica fume, which makes the resulting mass gel-like and immobile. Unlike a flooded wet-cell lead-acid battery, these batteries do not need to be kept upright. Gel batteries reduce the electrolyte evaporation, spillage (and subsequent corrosion issues) common to the wet-cell battery, and boast greater resistance to extreme temperatures, shock, and vibration.
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