Delaying to use the alternative and renewable energy? – Know detail about electricity storing system in a solar PV system

Earlier I have discussed on the necessity of using alternative clean and green energy such as solar photo voltaic (PV) panels for the resources of clean and green energy. Fossil fuel and other resources of power and electricity has limit and will be finished off with in near future. In early version I have discussed on PV panels and how they work. Not only in my early article; there are many readings and books are available to take knowledge on solar PV panels in market now a days. Many analyst and expert had discussed on solar PV panels in different magazine and news paper in recent time. There are some more components and equipments are using in a solar powered electric system. Those equipments are very important role to play in a solar powered system. I think people should know about these for their knowledge and usability. One important component in the PV panels is the storage system. It is required to have a storage system in a solar powered electricity generating system. The major role of this storage system is to store the electricity and release it when ever required for. As we know Electric power is generated only during day time by PV panels. But we need light at night. So some sort of storage systems has to be used with PV system. Although various types of batteries are available in the market, lead-Acid batteries are widely used in PV systems. Many of us may know that In 1859 French physicist Gaston Planté invented this rechargeable battery. Despite having a very low energy-to-weight ratio and a low energy-to-volume ratio, they have the ability to supply high surge current and relatively large power-to-weight ratio. These features, along with their low cost, make them attractive for use in motor vehicles and now in PV systems. Their huge Ampere-Hour capacity makes them suitable for PV system. The lead acid batteries may be divided in to some parts. Parts of a Lead-acid Battery: A battery consists of a number of cells and each cell of the battery consists of (1) positive and negative plates (2) separators (3) electrolyte and (4) the container. Different parts of a lead-acid battery are described below: (1) Plates. A plate consists of a lattice type of grid of cast antimonied lead alloy which is covered with active material. The grid not only serves as a support for the fragile active material but also conducts electric current. Grids for the positive and negative plates are often of the same design although negative- plate grids are made some¬what lighter. (2) Separators. These are thin sheets of a porous material placed between the positive and negative plates for preventing contact between them and thus avoiding internal short-circuiting of the battery. A separator must, however, be sufficiently porous to allow diffusion or circulation of electrolyte between the plates. These are made of especially-treated cedar wood, glass wool mat, micro porous rubber (mipor), micro porous plastics (plastipore, miplast) and perforated p.v.c. In addition to good porosity, a separator must possess high electrical resistance and mechanical strength. Separators: (1) and (2) Miplast type (3) Perforated type (3) Electrolyte. It is dilute sulphuric acid which fills the cell compartment to immerse the plates completely. Battery container Followings are the parts of the lead acid battery picture shown as parts separately. 1. Negative plate 2. Separator 3. Positive plate. 4. Positive group 5. Negative group 6. Negative group grooved support block 7. Lug 8. Plate group 9. Guard screen 10. Guard plate 11. Cell cover 12. Plug washer 13. Vent plug 14. Monoblock jar 16. Supporting prisms of + ve group 16. Inter-cell connector 17. Terminal lug 18. Screw 19. Washer 20. nut 21. Rubber packing 22. Sealing compound. Discharging: When the cell is fully charged, its positive plate or anode is Pb02 and the negative plate or cathode is Pb. When the cell discharges i.e. it sends current through the external load, then H2SO4 is dissociated into positive H2 and negative S04 ions. As the current within the cell is flowing from cathode to anode, H2 ions move to anode and SO4 ions move to the cathode. At anode (Pb02), H2 combines with the oxygen of Pb02and H2SO4 attacks lead to form PbS04. CHARGING When the cell is recharged, then H2 ions move to cathode and S04 ions go to anode and the following changes take place: Hence, the anode and cathode again become PbO2 and Pb respectively. It will be noticed that during charging : The anode becomes dark chocolate brown in color (Pb02) and cathode becomes grey metallic lead (Pb)Due to consumption of water, specific gravity of H2SO4. is increased There is a rise in voltage, Energy is absorbed by the cell Efficiency: Ideally, the charging and discharging processes of the lead-acid system should be reversible. In reality, however, they are not. Some of the electrical energy intended for charging is lost in the internal resistance and is converted to heat. When hydrogen is lost, it also represents an energy loss. Typically, the charging process is about 95% efficient. The discharge process also results in some losses due to internal resistance of the battery, so only about 95% of the stored energy can be recovered. The overall efficiency of charging and discharging a lead-acid battery is thus about 90%. Since battery losses to internal resistance are proportional to the square of the current, this means that high current charging or high current discharging will tend to result in higher internal losses and less overall performance efficiency. Amount of stored energy: The amount of energy stored in a battery is commonly measured in ampere hours. While ampere hours are technically not units of energy, but, rather, units of charge, the amount of charge in a battery is approximately proportional to the energy stored in the battery. If the battery voltage remains constant, then the energy stored is simply the product of the charge and the voltage. Effect of DOD on Life-Cycle: The following Figure shows how the depth of discharge affects the number of operating cycles of a deep discharge battery. The PV system designer must carefully consider the trade-off between using more batteries operating at shallower discharge rates to extend the overall life of the batteries vs. using fewer batteries with deeper discharge rates and the correspondingly lower initial cost. Lead-acid battery lifetime in cycles vs. depth of discharge per cycle Vented and nonvented batteries: In certain lead-calcium batteries, minimal hydrogen and oxygen are lost during charging. This means minimal water is lost from the electrolyte. As a result, it is possible to seal off the cells of these batteries, making them essentially maintenance free. The trade-off, however, is that if these batteries are either purposely or inadvertently discharged to less than 75% of their maximum charge rating, their expected lifetime may be significantly shortened. Lead-antimony electrodes, on the other hand, may be discharged to 20% of their maximum rating. This means that a 100 Ah lead-calcium battery has only 25 Ah available for use, while a 100 Ah lead-antimony battery has 80 Ah available for use, or more than 3 times the availability of the lead-calcium unit. However, the lead-antimony unit produces significantly more hydrogen and oxygen gas from dissociation of water in the electrolyte, and thus water must be added to the battery relatively often to prevent the electrolyte level from falling below the top of the electrodes. Water loss can be reduced somewhat by the use of cell caps that catalyze the recombination of hydrogen and oxygen back into water, which returns to the cell. Chemistry of the Nickel Cadmium Storage Battery: Ni-Cd batteries use nickel oxi-hydroxide for the anode plates and finely divided cadmium for the cathode plates. The electrolyte in the Ni-Cd system is potassium hydroxide (KOH). The NiOOH anode is generally made of nickel fibers mixed with graphite- or nickel-coated plastic fibers. Small quantities of other materials such as barium and cobalt compounds are also added to improve performance. The cathode is also frequently made of a cadmium-coated plastic fiber. If the cathode is not a coated plastic, then it is commonly mixed with iron or nickel. The fiber structures of anode and cathode maximize the surface area while minimizing the amount of relatively expensive nickel and cadmium required for the electrodes. On discharge, the NiOOH of the positive plate is converted to Ni(OH)2 and the cadmium metal of the negative plate is converted to Cd(OH)2. The basic reactions are: At the positive plate: NiOOH + H2O + e- = Ni(OH)2+OH At the negative plate: Cd + 2OH- = Cd(OH)2 + 2e- Overall: 2NiOOH + Cd + 2H20 = 2Ni(OH)+Cd(OH) The voltage of the fully charged cell is 1.29 V. Unlike the lead-acid system where the specific gravity of the electrolyte changes measurably during discharge or charge, the KOH electrolyte of the Ni-Cd system changes very little during battery operation. Ni-MH: (Nickel Metal Hydride) ` Another technology that is becoming very popular, particularly in smaller applications such as camcorders and laptop computers, is the nickel-metal hydride (NiMH) battery. This battery replaces the cadmium cathode with an environmentally benign metal hydride cathode, allowing for higher energy density at the cathode and a correspondingly longer lifetime or higher capacity, depending on the design goal. The anode is the same as in the Ni-Cd cell and KOH is used as the electrolyte. The overall discharge reaction is MH + NiOOH = M + Ni(OH)2 Lithium Ion (Li-Ion) Battery: Lithium-Ion battery is an advanced battery. Lithium ion batter has high energy density, low self discharge and no memory effect. These batteries are used where smaller and light weight batteries are required such as cell phones and laptop computers. However the main disadvantages of these batteries are the high price and fewer charge/discharge cycles