Storage is used in PV systems to increase the amount of time that the PV system can be used to power a load. Batteries are the most common type of storage in a PV systems. However, in specific types of systems or applications, other storage components can also be used. For example, in water pumping systems, the amount of battery storage can be greatly reduced or eliminated if extra water is pumped and stored in a water tank for use in cloudy periods.
In stand alone electricity generating systems, some form of storage is needed unless the load is exactly matched to the time during which the sun is shining. (Such an exact match is rare and limited to a few types of systems - for example powering a fan for cooling or in some cases water pumping for irrigation). In stand alone systems, storage is needed not only to power loads at night, but also allow a load to operate during cloudy weather. The number of days of storage needed depends on the weather pattern at a particular location, with cloudier locations needing more storage. In systems with a large amount of storage, and additional utility of the storage system is that is can buffer the system against periods of low insolation, such as in winter. For example, in telecommunications systems that require high reliability, a large battery bank can allow high reliability without requiring the PV array to be sized to meet the worst possible insolation conditions. In general, the larger the amount of storage included, the less sensitive the system will be to periods of low insolation, and the more reliable the power availability will be. The figure below shows how the power availability increase with increasing storage.
In systems connected to the utility grid electricity supply, storage is typically not needed. PV power is used when the sun is shining, and at night or during periods of cloudy weather, the grid provides the electricity. However, even in grid-connected systems, storage can be included, not to increase the reliability of having power as in a stand-alone system but rather to increase the value of the PV-generated electricity. In the load seen by many utility companies, an air conditioning load occurring on summer afternoons increases the overall load that the utility must supply. These peaks in the load are significantly more expensive to supply power for. Since the power output from PV is typically largest during summer months, the output from the PV system can well-matched to the peak load of the generated electricity is stored for a few hours. The use of storage for this application is called peak shifting and is shown in the figure below.
Functions of Batteries
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 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 uninterruptable power supplies.
In addition to the different mode of operation, batteries in photovoltaic systems also must meet 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:
- efficiency of the battery
- how battery capacity and lifetime is affected by deep cycling and extended states of low charge
- the initial and ongoing battery costs
- the maintenance requirements of the battery.
Electrolysis of Water
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.
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 2.2V 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, 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 dependent, the voltage at which gassing of a battery changes with the battery temperature and on the details of the battery components.
Uses of batteries in PV systems
While the primary function of a storage system is to provide power when sunlight is not available, hence increasing the fraction of time the photovoltaic system provides electricity, the addition of batteries has numerous other advantages which mean that the batteries can be used for multiple purposes. For small systems consisting of one or two photovoltaic modules, batteries can act as a load-matching system. Alternately, in photovoltaic systems which contain a load with a large initial current draw (such as experienced by an inductive load, typically represented by a motor), the batteries can be used to provide initial start-up current. In grid-connected systems, battery storage can be used for peak shifting, in which the power generated by the sun is stored for several hours in order to better match when the peak load occurs.