Why choose Nickel-Iron in an isolated site?
"The current growth of lithium batteries in isolated sites is necessarily part of the race to the overall performance of the storage market. High-tech batteries, certainly ever more compact, more powerful, but also increasingly dependent on strategic metals, monitoring electronics, and extremely delicate manufacturing processes. This logic leads to the manufacture of batteries that are too technical, too complex, non-repairable and intrinsically fragile, which inevitably stresses their recyclability and longevity, not to mention the rarefaction of rare metals. Against the current, we believe that technically sustainable batteries must tend towards "low-tech". Simple, reliable, durable. Since then, Nickel-Iron alkaline chemistry appears as the ideal candidate. "
Nickel-Iron batteries use an alkaline aqueous electrolyte (potash) whose density does not change during cycling. Unlike lead acid batteries, the electrolyte thus preserves the internal elements. Non-toxic, without heavy metals, the solution can be changed entirely over long periods of use (> 10 years) to revive performance.
The tubular design of the battery, or the materials of the electrodes are enclosed in finely drilled tubes, assembled on plates, makes it possible to have a very high mechanical resistance (vibration, shocks ...). They are almost indestructible.
Nickel-Iron batteries are very tolerant of electrical abuse: deep over-charges, extended discharges at 0%, accidental short-circuiting. Their non-flammable aqueous electrolyte acts as a stabilizer in case of over-charging, so thermal runaway is impossible.
Conventional Nickel-Iron batteries are often criticized for their lower performance than lead or li-ion batteries. Depending on the SOC, it actually varies between 72 and 86%, and requires a properly configured MPPT charge controller. In the same way, self-discharge in float does not exceed 0.1% per day. It is necessary, however, to recharge the electrolyte every 3 months to compensate for water losses.
Nickel-Iron batteries, as well as their Nickel-Cadmium variant, are considered the most durable on the market. With a life expectancy of more than 30 years in daily cycling, there are documented cases of Ni-Fe batteries put into service after more than 70 years of inactivity. They can be stored empty for very long periods without damage.
Mechanisms of degradation
The structural degradation of the electrodes being almost non-existent (the solubility of the elements in the electrolyte is zero), there are no documented cases of sudden death of Ni-Fe or significant degradation, in a normal use (temperature ambient 25 ° +/- 5 °) which can not be corrected by repackaging (change of the electrolyte).
Lithium (NMC, LFP)
Since lithium is highly reactive, the electrolytes used are necessarily aprotic (without water) generally containing a lithium salt (LiPF6) dissolved in an organic solvent (often carbonates) and sometimes additive. The major disadvantages: the electrolyte can not be changed, is inherently flammable, poses obvious recycling problems, and can deteriorate in the long run.
The materials used are either lithium iron phosphate (LiFePO4), or a mixture of lithium oxides of manganese, cobalt and nickel (NMC) associated with a negative mass of graphite. Various protection systems designed to limit the risk of thermal runaway are incorporated in the cylindrical type of cell, which are assembled in series (in the event of physical damage, or over-charging, etc.)
Lithium batteries are vulnerable to overload, are not storable in the long-term when empty, and require proper parameterization between the BMS and the inverter-charger to ensure perfect operation and constant monitoring.
Lithium systems being non-aqueous, there are no parasitic reactions during the charge-discharge, the yields are therefore extremely high (> 96%) which gives very efficient and effective batteries. On the other hand, the absence of secondary reactions means that they are not self-regulated, which forces the addition of an ad-hoc electronics (BMS) to ensure the thermal stability and to avoid any damage related to voltages of inappropriate charge.
Although there is still a slight decline, the advertised calendar life of the best current lithium systems (Sony Fortelion LFP or Samsung SDI NMC) is theoretically around the 20 years.
Mechanisms of degradation
There are two mechanisms currently known. First, the progressive increase of the internal resistance of the cell, related to the increase of a layer of elements called SEI on the electrode which is formed following the dissolution of metallic elements of the electrode (especially at high temperatures,> 40 °). Secondly, the oxidation of the electrolyte and the alteration of its formula in the long term. These mechanisms, underpinned by complex chemical reactions, are still the subject of academic study to try to model more precisely the lifetimes of lithium systems.
From design to commissioning
The technical reference for Nickel-Iron batteries
With our technical knowledge (partnerships with companies Encell et BOCHEMIE) and our practical experience of Nickel-Iron batteries (totaling close to 1 mWh of storage in service), we advise you in the implementation of a solution Nickel-Iron best adapted for off-grid & micro configurations -grids of all sizes. Consult us for your project.