Introduction: On and off-grid systems and the need for batteries
Photovoltaic systems can be classified as on-grid and off-grid. On-gridsystems are hybrid systems where the electricity grid and the PV system both act as the sources of power. Mostly, residential PV systems are on-grid systems. If the solar panels are producing more power than required by the PV system, the excess power is supplied back to the grid. Likewise, if the panels are producing less power than required, the grid supplies to the system in order to compensate the rest of the required power. Off-gridsystems are standalone systems where the PV system is the sole supplier of power. For example, payphones and charging stations which are powered by a single PV module are usually standalone systems. In standalone systems the excess power generated needs to be stored. This excess power is stored in a battery.
Solar batteries store the energy that is produced by the PV panels so that it can be used later. The amount of energy a battery can store depends on the capacity of the battery. Batteries can also be integrated into on-grid systems. This way the excess power stored by the PV system can be stored in the battery instead of being fed back to the grid. This energy can then be used later at a time when there is no electricity being produced by the modules. In on-grid systems with batteries, the only time the electricity is fed back to the grid is when the battery is fully charged. And the only time electricity is drawn from the grid is when the battery is completely discharged. This proves useful especially at night and at times of power-cuts.
This article highlights how a battery works, the different characteristics of batteries, and finally selecting an appropriate battery to use in PV systems.
How does a battery work?
A battery converts chemical energy into electrical energy using a set of reactions called Redox Reactions (Reduction + Oxidation = Redox). The chemical energy is stored in the chemical bonds of the material that is inside the battery. Redox reactions are chemical reactions where an electron is either produced (in the case of oxidation) or required (in the case of reduction) by the chemical reaction. Oxidation and reduction occur in pairs as oxidation produces the electron that is required for reduction. The following video explains redox reactions in a more comprehensive way:
To understand how redox reactions take place in a battery, let us consider the image below:
The image above shows a Voltaic Cell which is an application of redox reactions and explains how redox reactions can create an electric potential. In the beaker on the left, there is zinc and zinc ions, while in the beaker on the right there is copper and copper ions. These need to be physically separated for a cell to be formed. The two are connected with a wire. A redox reaction occurs and the electrons transfer through the wire, hence electricity is generated in the setup. The tube-like structure in the image above that connects the two beakers is called a salt bridge. It contains a solution of an ionic compound whose ions move to both the sides of the voltaic cell. This helps maintain the balance of charge. In the absence of a salt bridge, there can be a charge imbalance.
The operation of a voltaic cell explains that of a battery. The main aspect of a battery that distinguishes it from standard oxidation/reduction reactions is that the oxidation and reduction are separated from each other physically. This allows for a load to be inserted between the two reactions. The electrochemical potential difference between the two electrodes of the battery corresponds to the voltage of the battery that drives the load. The exchange of electrons between the two reactions corresponds to the current that passes through the load. The selection of the electrolyte (solution in the beakers) and the electrode determines many of the properties of the battery. In the following section we will see the different characteristics of a battery.
Characteristics of Batteries
PV systems have different types of requirements from the batteries than other applications. Most importantly, the batteries that are used in PV systems need to be able to handle a lifetime of fully discharged conditions as the PV system may not always be able to generate power. This is not the case with common rechargeable batteries. For instance, car batteries are at full charge for most of their lives. Let us look at the common characteristics of batteries:
- Battery Efficiency: The overall battery efficiency is based collectively on two factors: Columbic Efficiency and Voltage Efficiency. Columbic efficiency of a battery is the ratio of the number of charges that enter the battery during charging to the number that can be taken out from the battery while discharging. There can be losses in the columbic efficiency. These occur due to the loss of charge due to the reactions which are secondary. These refer to electrolysis of water or other reactions in the battery. Generally, the columbic efficiency of a battery may be high (more than 95%). The voltage efficiency is determined by the voltage difference between the charging voltage and the voltage during discharging.
- Energy and Power Density: Energy density is mostly used to compare the types of battery systems with each other. It is the capacity of the battery divided by either the weight of the battery, giving the gravimetric energy density (Wh/kg), or by the volume, giving the volumetric energy density (Wh/dm3). The higher the energy density, the lighter the battery. In conventional PV systems, the energy density is not considered too important. But the cost of transporting batteries to remote locations is quite high. So it is advantageous to have a high energy density battery. The power density is related to the energy density of a battery. It is also related to the ability of the battery to discharge quickly. The power density is not critical in photovoltaic systems. However, it is important in some applications such as transport.
- Battery Capacity: This is a measure of the charge stored in a battery. It is determined by the mass of the active material in the battery. The unit of battery capacity is typically Amp-hr. Battery capacity represents the maximum amount of energy that can be taken out from the battery under specific conditions. However, it can vary significantly as the battery capacity depends a lot on the age of the battery, the charging or discharging routines of the battery and the temperature. If a battery is discharging relatively fast, that means that the capacity of the battery is lower as the energy that can be extracted has reduced. Temperatures also affect the capacity of a battery. At higher temperatures, battery capacity is higher. However, manually increasing the battery temperature is not effective as it decreases the battery lifetime.
- Battery State of Charge: State of Charge (SoC) of a battery is defined as the fraction of the energy (or battery capacity) of the battery that has been used to the total available from the battery. SoC gives the ratio of the amount of energy presently stored in the battery to the rated capacity of the battery. SoC can be measured by measuring the voltage of the battery and then measuring the voltage of the battery when it is fully charged. This gives a rough idea of the SoC as the temperature affects the capacity.
- Depth of Discharge (DOD): In many battery types, it is impossible to fully discharge them without causing physical, irreparable damage. The DOD determines the fraction of power that can be withdrawn from the battery. For instance, if the DOD provided by a manufacturer for a battery is 40%, then only 40% of the power can be withdrawn from the battery. The reason why this happens is because extracting the battery’s full capacity makes a major impact on the battery lifetime towards reduction.
- Charging and Discharging Rates: One way to specify the capacity of a battery is to use the battery capacity as a function of time. This can be the time it takes to fully discharge a battery. This is called the C-rate of the battery. The notation of C-rate is Cx where x is the amount of time (in hours) it takes for a battery to discharge. For instance, Cx = Z, where Z is the battery capacity as a function of x, which is the time in hours. The discharge rate when discharging the battery in x hours is found by dividing the capacity by the time. The charging rate is the amount of charge that is added per unit time (Coulombs/second or Ampere).
- Battery lifetime: There are different ways in which battery lifetimes are specified. For PV systems, it is the most appropriate measure of lifetime to count the number of charge and discharge cycles through which the battery can maintain a given fraction of its capacity. This is because batteries used in PV systems need to be charged and discharged again and again. Battery life given in years if it remains fully charged. If not, then battery life is given in number of cycles under a given set of conditions that include temperature and DOD.
- Maintenance requirements: The amount and type of maintenance that a battery requires depends on the type of battery. The reactions inside some types of batteries generate gases. This changes the volume of the contents inside the battery. Hence, the battery needs to be sealed properly. In the case where an effective way of sealing is not possible, water or some other solution is added depending on the type of battery to compensate for the gases. Certain batteries are hermetically sealed. These require less maintenance. Batteries such as lead acid batteries require a lot of maintenance.
Based on these characteristics and parameters, battery selection is done. In the following section, we will see how to select a battery for a PV system and which type(s) of battery is/are the most suitable for the use of PV systems.
Battery selection for PV systems
There are two types of batteries: primary and secondary. In primary batteries, the conversion of chemical energy into electrical energy is an irreversible process. This means that a primary battery cannot be recharged. For instance, alkaline battery is an example of primary battery as it cannot be recharged. In secondary batteries, the process is reversible. The chemical energy that is converted into electrical energy can be converted back to chemical energy. This allows the battery to be recharged. Some examples of secondary batteries are lithium-ion batteries and lead-acid batteries.
For PV systems, the battery being used must be secondary batteries. Choosing the right battery is very important as this ensures a longer lifetime, less maintenance and better performance of the battery and the PV system on the whole. Other than the characteristics and parameters mentioned in the previous section, another important factor to select a battery is the quality of the manufacturer and hence, the battery. If outdated raw materials are used by the manufacturer for instance, this compromises the battery’s performance and life. Also, checking the warranty for the battery is also important as this can save money in a catastrophe where the battery fails due to freezing or shorting.
This brings us to the most important question: Which is the best battery for PV systems?
The following are the best types of batteries that can be used in PV systems:
- Lead acid: Lead acid batteries reliable as they are tested pieces of technology. They have been used in off-grid energy systems for quite some time now. They have a relatively shorter life and a lower DOD than other battery types. A very big advantage that lead-acid batteries have is that they are one of the least expensive options as of now in the market. Therefore, for off-grid PV systems, lead-acid is a very good option.
- Lithium ion: A major fraction of new residential energy storage technologies use lithium ion chemical composition of some type. The reason for the popularity of lithium ion batteries is because they are relatively lighter and more compact than lead acid batteries. Compared to lead acid batteries, they also have a higher DOD and a longer lifespan. The disadvantage of lithium ion batteries is that they are more expensive than lead acid batteries.
- Saltwater: This is a relatively new technology. Saltwater batteries don’t contain heavy metals. As the name suggests, they rely on saltwater electrolytes. They are easily recycled, while heavy metal batteries such as lithium ion and lead acid batteries need to be disposed off with special care and processes. The disadvantage they have is that they are relatively untested being a new technology.
We have now seen how there is a need for battery in both on-grid and off-grid solar systems in order to store the energy generated by the PV panels. To understand how a battery stores energy, we have seen the chemical process of redox reactions and how they apply to a voltaic cell. The functioning of a voltaic cell has explained how redox reactions take place inside a battery. We then saw the different characteristics of batteries and how they help us make a selection. We then looked at primary and secondary batteries and how solar PV systems require secondary batteries. We finally looked at the different types of secondary batteries and which are the best options to choose for solar PV systems based on their characteristics and pricing.