In a solar installation dedicated to self-consumption with storage, the battery bank represents a significant part of the initial investment. The service time of the self-consumption system and the payback period will be strongly impacted by the choice of storage technology, and the chosen size of the battery banks and therefore should be carefully considered.
It is necessary for the system installer to ask certain questions before installing a lithium battery for a user:
- How much excess production will be available to charge the lithium batteries? (It is necessary for this to estimate the solar production and consumption profiles of the site.)
- How much power should lithium battery provide?
- What percentage of autonomy does the user want?
- Which batteries are compatible with the solar inverter I want to use?
It is important to note that the inverter selected for the project must be able to communicate with the BMS (battery management system) of the battery. Different manufacturers use different communication protocols, therefore not every inverter is compatible with every battery.
There are several chemistries one can come across:
Lithium Iron Phosphate (LiFePO4) or LFP:
The University of Texas discovered in 1996 that phosphate could be used as material for the cathode. The main advantages of a phosphate cathode are:
- they support high charge and discharge currents,
- they have a long lifespan,
- they have good thermal stability, important for safety aspects
- they tolerate being subjected to high stresses over long periods of time.
The main disadvantages of LFP cells are:
- they have a lower energy density than any of the other types of lithium batteries, consequence of a low nominal voltage, lower than 3.2V,
- as with most batteries, a cold temperature alters the capacity of LFP cells and a hot temperature alters the lifespan of Lithium Iron Phosphate cells,
- they have a slightly higher self-discharge rate than other types of lithium batteries.
For applications like self-consumption with storage, several manufacturers of lithium batteries have designed LPP modules with a nominal voltage of 48V, which can be directly integrated into standard 19-inch racks. It is possible, in most cases, to put these LFP modules in parallel in order to increase the capacity of the battery bank. LFP batteries are one of the best compromises for self-storage solar installations.
Below is a table giving a score out of 5 to Lithium LFP batteries for different criteria.
Lithium Nickel Manganese Cobalt Oxide (LiNiMnCoO2) or NMC:
The main advantage of this type of battery is the ability to provide power while having a high energy density. Because of this advantage it is the preferred technology currently for electric mobility. The downside of NMC lithium batteries is that they are less safe to use than LFP batteries. As the energy density aspect is of little importance for stationary self-consumption applications with batteries, NMC cells are not the most used for this type of application.
Below is a table giving a score out of 5 to Lithium NMC batteries for different criteria.
Lithium Manganese Oxide (LiMn2O4) or LMO :
The discovery of this type of cell was published in 1983 in the Material Research Bulletin. The first commercialization of LMO cells dates back to 1996. The LMO cell has a lower internal resistance compared to other lithium cells. The main advantage of this type of cell is that it has a very high thermal stability, which makes it a good choice from a safety perspective. However, LMO cells have a shorter lifespan and will perform fewer cycles than other types of lithium batteries.
Below is a table giving a rating out of 5 to LMO Lithium batteries for different criteria.
Lithium Cobalt Oxide (LiCoO2) or LCO
Due to its relatively short lifespan, LCO cells are not among the first choices for solar self-consumption installations. However, its high energy density allows the manufacturers of smartphones, laptops and digital cameras to use it regularly. The LCO lithium battery consists of a Cobalt oxide cathode and a graphite carbon anode. During the discharge of the battery, the lithium ions move from the anode to the cathode and they perform the reverse path during the charging phase.
LCO lithium cells have the advantage of high energy density but have the disadvantage of a relatively short lifespan.
Below is a table giving a rating out of 5 to Lithium LCO batteries for different criteria.