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Energy storage has been a hot topic in the last years. Two years have passed since I’ve started following their developments and I keep finding new options. A very interesting observation is that the new inventions put forward, they are everything but new. Practically they have been discovered long ago, as long as mid of 20th century, yet investment has been poured to them only recently.
Among the possibilities in the sector, batteries are the main focus of many established providers and start ups with wonderful developments to show. For the infrastructure, i.e. electricity on the main land, Li-on batteries and flow batteries are the front runners. On the other hand for the transportation sector, where size and weight are paramount, lithium batteries & fuel cells dominate.
In a nutshell, lithium ion batteries are leading. Unfortunately they are also intertwined with adverse environmental mining conditions and child labour, at least the ones that use cobalt.
A type less prominent and one that I personally haven’t researched so much before, is the salt water batteries. Despite the fact they have been invented back in the 70s, they didn’t take off as much till last decade. They got a lot attention with the establishment Aquion, as it was start up that attracted investors like Bill Gates and produced high quality products. Unfortunately it was declared bankrupted in 2018 and was bought at a fragment of its initial investment, luckily it remains in business.
Just to clarify, as the name can be deceiving, we are not referring to every day saltwater, but rather to an aqueous solution, enriched with sodium based salts. They are structured similarly to lead acid batteries having a carbon anode & an oxide cathode that supports the passing of the electrolytes from the sodium ion, through the separation film during charging and discharging, thus energy storage. A simple charge diagram is shown below.
From environmental perspective it could not be better, natural resources are readily available and recycling couldn’t be easier. Looking at safety and they get another gold point. BlueSky Energy has a wonderful video of a battery over fire for 30 minutes, with no adherent outcome, In comparison to lithium ones, who’s explosion instances have necessitated all of us to transfer laptops in the cabin of an aircraft, it is a dream. Aquion, previously mentioned manufacturer, was the first battery manufacturer to receive cradle-to-cradle certification due to its safe transfer and environmentally friendly manufacturing procedures.
Well by now one is already thinking if they are so great, how come and I haven’t I heard of it? Where is it used?
The main drawback of saltwater batteries, are their energy density and specific density. Energy density is the energy provided in respect of volume, given for batteries in kWh/l and specific density is the respective energy per mass, in kWh/kg. Watt is the unit of power common for electricity, yet for more scientific explanation, it is a unit of energy per unit of time, while pure energy unit is Joule. My apologies for the geeky part, but it is necessary to explain the next part.
Electrical potential energy is proportional to the voltage (potential difference) and the charge. Looking at the water, it is electro chemically stable up to 1.23V, compared to 3.7V for lithium. The limiting voltage therefore is translated to limited energy potential directly depicted in the specific density which salt water batteries have one-third to the Li-On ones. Moreover due to their energy density, the salt water batteries that end up being double the size of their counterparts.
Adding to the energy density and specific density, poor power capability is another adherent factor, as they support merely one fifth of the battery capacity. Meaning that if you require 2MW power, you need to have 10MW battery installation.
As they are only a handful manufacturers in this market and they are not mass produced, their prices are anything but competitive. Surely their life cycle is equivalent to the Li-on and their depth of discharge superior. Depth of charge for salt water batteries can reach 100%, while for Lion not more than 80%. State of charge for li-on can change over time, no different to saltwater batteries.
By now I trust it is clear why they haven’t taken off as yet, but many researches are happening. For example EMPA in Switzerland has managed to create a solution that remains electrochemically stable up to 2.6V, twice the voltage of present aqueous solutions. At this moment l am not aware of commercial application for this, but I would be very happy to hear about one.
As a conclusion, we can remember initial prices lithium batteries when they were originally introduced and foresee a similar pattern with salt water ones when they’d start being mass produced. Environmental and safety aspects support investing on them. For ships, or other vehicles on their own standing, it is too soon. Yet for homes or industrial facilities, where weight & size are not as restrictive and safety is, then it would make sense to store your own produced energy with saltwater batteries.