Energy Storage News
A company called EOS is planning to manufacture zinc-air batteries for utility scale grid storage applications. Metal-air batteries which make use of oxygen in the atmosphere as one of the electrodes have long been known to have high energy density. Non-rechargeable zinc-air button cells are currently being commercially produced by a number of companies primarily for use in hearing aids. However, producing rechargeable zinc-air batteries has proved difficult. Among the problems with such rechargeable batteries are the formation of dendritric zinc structures on the anode during recharging (The dendrites eventually pierce the separator between the anode and cathode and cause the battery to short circuit.) and the clogging of the gas diffusion electrode by potassium carbonate evolved from the electrolyte solution containing KOH. According the this posting both of these problems have been solved in a laboratory setting by the use of non-aqueous electrolytes.
EOS, however, is using an aqueous electrolyte with an novel chemistry. They have gone beyond laboratory demonstrations and say that they have built prototype batteries which have demonstrated greater than 1000 charge/discharge cycles without significant performance degradation. EOS will be accepting pre-orders for their Aurora Zinc-Air battery system in 2012 with volume manufacturing to start in 2013.
Some details about the Aurora zinc-air battery taken from the technology data sheet posted on their web site are given below:
I was not able to find any information on the round trip efficiency of these batteries. The projected cost of $160/kWh is substantially cheaper than the cost of sodium sulfur (NaS) batteries (>$400/kWh) and the projected cycle life of 10,000 is more than twice that of NaS batteries (4500). If EOS can deliver on these projections then a market space may open up to these batteries that is currently unavailable to other battery technologies.
Zinc air batteries have high energy density because the bulk of the cathode is composed of atmospheric oxygen which is not a part of the battery structure. For example the Duracell zinc air button cell described this tech brief has an advertised energy density of 442Wh/Kg which should be compared to 160Wh/Kg for lithium ion batteries. EOS also claims high energy density for its rechargeable zinc-air batteries, but does not give a specific number. One might ask why, if their batteries are cheap and have high energy density, EOS is not pushing them for electric/hybrid vehicle applications?
The answer is the EOS is in fact planning to get into this market, but the power density (as opposed to energy density) of their batteries is too low for effective regenerative braking and for acceleration. They are planning to remedy this defect by add a second smaller battery to handle these functions. They are attempting developing a new form of lead-acid battery which will be able to endure many more cycles at deep discharge than current lead acid batteries. These new lead acid batteries are farther back in the development cycle than the zinc-air batteries, so EOS is tackling the stationary storage market first.
If one is hoping that zinc air batteries will enable a renewable energy smart grid which will eliminate the vast majority of carbon emissions by the middle of the century and allow 9 billion people to achieve OECD standards of living (not, in my opinion, a very likely prospect) then one needs to think about the scalability of this technology. The abundance of zinc in crustal rocks is 0.008% by weight which is comparable the abundance of copper at 0.007%. Zinc is currently the 4th most commonly used metal trailing only iron, aluminum, and copper. The global annual production of zinc is 12 million tonnes (1 tonne=1000kg).
How much zinc would it take to support a global smart grid for nine billion people living at OECD standard's of living? In an attempt to answer this question I will use some data from Duracell's tech brief on their zinc-air button cells:
Multiplying the current capacity times the voltage gives us the energy capacity:
anode energy capacity: 820Ah/Kg×1.18V = 968Wh/Kg
The United States average consumption of electricity per capita (for domestic and industrial uses) is 12,000kWh. Dividing by 365 we get the daily average electricity consumption per person 32.9kWh. Suppose that we wish two store 24 hours worth of electricity for each of 9 billion people in zinc-air batteries. The required amount of zinc is given by:
Total Zinc = 9×109×32.9 kWh/(0.968 KWh/Kg) = 3.06×1011 = 3.06×108 tonnes
The required zinc is 25.5 years worth of production at current global production rates. Directing this amount of zinc into a new market over the next several decades would be a non-trivial task.
Oct 09, 2011Energy Storage News
rogerkb [at] energystoragenews [dot] com