Recent Development of Batteries used in PV Systems
Batteries have become an integral part in any P-V (Photovoltaic) systems. Batteries are used in storing the energy produced by the Solar module throughout the day, and supplying the loads at household. The storing of charge ensures there is a continuous supply of electricity even during night when the panels do not produce any energy, or in the period of blackout of grid. Some of the major functions of batteries are listed below:
- Storage: The primary function of battery is storage, batteries store charges and supply it during need. It can be used to power various loads that are necessary throughout the household, hospitals, industries.
- Starting current: Another application of batteries is it can be used to supply initial starting current which are used by loads, which require high current to start initially like motors.
Recent battery types used in P-V systems
There are generally two sub divisions of battery, that is 1) Primary battery and 2) Secondary battery, primary batteries cannot be re-charged hence, it is not used in the solar PV systems. Secondary batteries can be recharged for that reason it is widely used in P-V systems.
There are two types of battery system that have been the most popular form of backup system, they are:
Lead acid: Lead acid batteries has been the most popular option for battery backup system for solar. It has been used in off-grid system for decades, its advantage of being rugged, durable, low cost. Being one of the cheapest systems it has been the chosen battery backup system for developing countries. The disadvantage is it is big and heavy and not environmentally friendly.
Some of the types of lead acid batteries are:
- SLI batteries: Starting, lighting and ignition are the batteries that is used for shallow cycle services.
- Motive or traction batteries: it is a popular choice of batteries for a PV system, it has a deep discharge cycle, long life.
- Stationary batteries: Stationary batteries have same characteristics as both SLI and traction batteries and used in uninterruptible power supply (UPS) to provide backup to loads.
Batteries are used in uninterruptible power supply (UPS) to provide backup to loads.
Lithium-Ion battery: Lithium-Ion battery while primary being developed for consumer electronics, have shifty taken a large chunk of Photo voltaic market in the developed countries, Tesla, LG chem are some examples of companies that have been developing Lithium-ion based solar storage system. Lithium-ion is projected to take over the market of solar because of decrease in cost of Lithium-ion battery every year. The main advantage of Lithium-ion battery over a lead acid battery is that, it has greater Depth of Discharge at 80%, greater energy density and longer lifespan.
There are two core Lithium-ion battery technologies
- Lithium Iron phosphate (LiFePO)
- Nickel Manganese Cobalt (NMC)
NMC batteries are better suited for Electric Vehicle application whereas LiFePO is better suited for residential storage application.
Future of batteries
There has been constant improvement on batteries ever since their development, there is always more room for improvement for storage systems, lot of financial resources have been allotted for research in the field of battery, the development works that are being done is either, improvement of previous batteries technologies like Lithium-Ion, or searching for a breakthrough with completely new technology and chemistry. Even though lithium-ion battery technology has been one of the most successful batteries technologies that exists boasting high energy density and great depth of discharge, some experts believe that it may be too expensive for Grid-scale applications, some of the promising technologies are:
- Lithium- air: Lithium-air batteries first proposed in 1970 has been a promising alternative for lithium-ion due to the promise of high energy density. Li-Oxygen batteries are being considered due to both elements being light materials and promising of high energy densities.
- Lithium- sulfur: Although lot ofresearch have been put into the development of Lithium- sulfur, its promise of high energy densities (2000 Wh/kg), there is still a gap between theoretical academia and practical commercialization.
- Solid-state lithium: Unlike lithium-ion liquid electrolyte, Solid state battery promise a solid electrolyte which in terms provide longer lifespan and inflammable electrolyte in turn provides a greater environment safety.
- Flow Batteries: Flow batteries are suitable for storage application and might as well be used in P-V systems in near future, durability and long lifespan is an attractive feature of these batteries. Flow batteries are based on the principle of Redox and store energy in electrolyte. Some of the flow batteries that are already available are Vanadium redox flow, which has higher cycle life (>10,000) compared to lithium-ion batteries, but they still lack in energy density compared to lithium-ion.
- Salt water Batteries: Salt water batteries are battery that uses saltwater tostore energy. The attractive traits of this battery are that it has high level of safety and longer lifespan. It is also very eco-friendly as they can be easily recycled. Still this battery needs to have higher energy density it is to compete with lithium-ion technology.
- Sodium-ion Batteries: Sodium- Ion batteries will use sodium which is a very abundant resources compared to Lithium. Abundance of materials means that the cost would be much cheaper in long run.
- Graphene Batteries: Graphene batteries are made up of carbon atoms that are tightly bound in hexagonal order, it is promising technology due low weight, high electrical conductivity and high flexibility. Since, graphene batteries allows for higher electrical conductivity than Lithium-Ion it can be charged much faster.
Which of the listed battery technology can be best used for solar backup systems, in grid’s application can be up for debate, but it is crucial we must experiment with new technology to take the battery storage up to grid-scale, up until now pumped hydro has been most common way of storing energy, but we should not limit ourselves to just that technology for the future. Constant innovation on batteries and storage system is key to supply power on demand and not wasting any generated energy, reducing energy wastage is a key to efficient smart cities and it cannot be done unless there is a good backup storage system to store energy.