Batteries are a common feature in most types of PV systems that are not connected to the utility grid. In addition to providing storage, batteries can also be used for several other functions:
Storage. Batteries store energy being produced by a given generating source, and when this source is unavailable this energy can be be used by the load. The inclusion of storage in any energy generating system will increase the availability of the energy.
Start-up current. Batteries can provide higher currents to the load than the array alone can provide. This is especially useful if a particular load has a high current draw on start-up. Many motors initially have a high current requirement.
Power conditioning.Batteries can function as power conditioning. Two cases where this feature is used is in directly coupled systems, such as water pumping, and in uninteruptable power supplies.
In addition to the different mode of operation, batteries in photovoltaic systems also must bee several other criteria. As reliability and low maintenance are desirable in photovoltaic systems, the batteries must also have a long lifetime. Further, since batteries will often be a substantial fraction of the total cost of a PV system, cost is a significant factor in batteries for PV systems. In general, batteries manufactured for other applications are not well suited to photovoltaic energy applications. The key characteristics of a battery in a renewable energy system are:
The previous pages have focused on relatively ideal battery systems, containing only a single oxidation and reduction reaction with a large surface area electrode. In practice, several affects in a battery further alter the performance and operating conditions of the battery, including the presence of additional chemical reactions (which causes battery gassing, corrosion of the electrodes and self-discharge), and changes in the shape of configuration of the electrodes and electrolyte (which may cause shedding or sulfation of the battery (in lead acid batteries), or stratification of the electrolyte. (electrode changes – also caused by solubility in the electrolyte).
In battery solutions in which a component of the electrolyte is water (such as in lead acid batteries), the possibility of electrolysis water must be taken into account when charging a battery. The electrolysis of water, which is breaking water into oxygen and hydrogen, consists of the following reaction:
xxx.
According to the standard potentials, the voltage of this reaction is 1.23V. However, the activation overpotential of this reaction is large, and hence it does not proceed at a significant rate (and can therefore be neglected in battery charging or discharging) until voltages on the order of xxV are reached in the battery. During high charging rates, the charging voltage may exceed this voltage, and hence two reactions will proceed in such a battery: one the charging of the battery and the second the electrolysis of water. As the electrolysis of water gives of hydrogen and oxygen, both of which are gases, the battery is said to be gassing. The electrolysis of water has several impacts on the battery. Firstly, it leads to water loss in the battery, which must be replaced. Further, the evolution of hydrogen gas forms a potential safety hazard if released in an improperly ventilated area, or can overpressure the battery case. Both of these issues may be minimized or circumvented by preventing the gases, particularly the hydrogen from escaping from the battery. Batteries using this approach are called sealed or recombinant batteries. Despite the potential maintenance and safety problems associated with gassing, it may also have beneficial impacts. For example, in lead-acid batteries gassing can be used to mix the electrolyte, thus preventing regions of higher sulfuric acid concentration (which is denser) from sinking to the bottom (an effect called stratification).
The electrolysis of water is affected by the presence of small amounts of impurities in the lead acid batteries, and hence batteries with additives to the lead (for mechanical strength or other practical purposes) can experience significantly different gassing voltages. Further, since the activation energy is temperature dependant, the voltage at which gassing of a battery changes with the battery temperature and on the details of the battery components.
Pix: maybe from battery book showing activation overpotetial (IV curves). Alternately, a pix showing gassing of battery of ventilation of a battery room.
Corrosion consists of a set or reduction/oxidation regions in which both the reactions take place at the same electrode. For a battery system, corrosion leads to several detrimental effects. One effect is that it converts a metallic electrode to a metal oxide. In some battery systems, such as the lead acid batteries, xxx. Additional detrimental effects are the change is the state of the electrode xxx.
All chemical reactions proceed in both the forward and reverse direction. In order for the reverse reaction to proceed, the reactants must gain enough energy to overcome the electrochemical difference between the reactants and the products and also the overvotlage. Usually in battery systems the probability of the reverse reaction occuring is small, since there are few molocules with a large enough energy. Although small, however, there are some particles that do have sufficient energy. In a charged battery, a process exists by which the battery can be discharged even in the absence of a load connected to the battery. The amount a battery discharges upon standing is known as self-discharge. Self-discharge increases as temperature increases because a greater fraction of products will have enough energy for the reaction to proceed in the reverse direction.
xxx- picture of energy diagram and distribution of carriers, showing the portion of carriers that can have the reverse reaction.
An ideal set of chemical reactions for a battery would be one in which there is a large chemical potential which releases a large number of electrons, has a low overvotlage, spontaneously proceeds in only one direction and is the only chemical reaction which can occur. However, in practice there are several effects that degrade battery performance, due to unwanted chemical reactions, to effects such as the change in phase of volume of the reactants or products and also to the physical movement of reactans and products within the battery.
While undergoing chemical reactions, many materials undergo a change either in phase, or if they stay in the same phase, the volume, density of the material moay be altered by the chemical reaction. Finally, the materials used in the battery, primarily the anode and cathode, may change their crystallility or surfae structure, which will in turn affect the reactions in the battery. Many components in redox reactions undergo a change in phase during either oxidation or reduction. For example, in the lead acid battery, sulfate ions changes from being in solid form (as lead sulfate) to being in solutions (as sulfuric acid). If the lead sulfate recrystallizes anywhere but the anode or cathode, then this material is lost to the battery system. During charging, only materials connected to the anode and cathode can participate in electron exchange, and therefore if the material is not touching the anode or cathode, then it can no longer be recharged. The formation of a gaseous phase in a battery also presents special problems. First of all, the gaseous phase will usually have a larger volume that the intial reactants, thus giving rise to a change in pressure in the battery. Secondly, if the intended products are in the gaseous change, they must be confinied to the anode and cathode, or they will not be aboe to be charged.
``A change in volume will also usualy be determinetal in battery oepration.