Australasian Science: Australia's authority on science since 1938

Off the Grid

dmfoto12/Adobe

Credit: dmfoto12/Adobe

By Cameron Shearer

Australians have taken to solar energy, but much of the electricity they generate cannot be stored and is returned back to the grid. However, commercial residential battery systems are now available, with new technologies on the horizon.

An increasing number of Australian households now produce their own electricity through rooftop solar panels. During a typical day, the electricity generated will be used to run some appliances, and any power left over is returned to the electricity grid with the homeowner receiving a feed-in tariff for the electricity they return. The plan of the homeowner is for the initial cost of the solar panel installation to be slowly paid back through lower power bills and feed-in tariffs.

The drawback of many renewable energy sources is that the power produced is intermittent and peak energy generation rarely matches peak usage. Solar energy is no different, with peak solar panel output occurring around midday while peak household electricity usage occurs in the evening. This mismatch in peak output and usage would not be a problem if the cost of electricity was equal for both the feed-in tariff and what is charged by the electricity provider.

The recent announcement by Tesla of the Powerwall, a lithium ion-based residential battery storage system, has many people considering going off the grid and relying upon their solar panels to generate their electricity and then storing any excess in their own battery and using it on demand.

Why hasn’t this been done before? The answer to this question lies in the technological advancement of the rechargeable battery, their energy-to-weight ratio and their drawbacks.

But the development of rechargeable batteries has progressed rapidly recently due to the demand for light batteries in portable electronic devices such as laptops and phones. This development has focused on increasing the energy-to-weight ratio, with less focus on safety and volume.

This article will review the various battery technologies available for residential energy storage, and review the developments in battery technology that may become available in the future. The average Australian household of four people uses approximately 20 kWh per day, so comparisons will be made for the size and weight of batteries required to produce 20 kWh.

Lead–Acid Batteries

The original rechargeable battery consists of concentrated sulfuric acid as the electrolyte, and lead and lead dioxide on both the anode and cathode. Used in automobiles, caravans and in some electric relay grids, lead–acid batteries have very high recyclability (the number of charge/discharge cycles) and hence have a long lifetime. Slow charge and discharge significantly reduces the life of lead–acid batteries, and it is often recommended to discharge them to only ~60% of maximum capacity.

Although lead is toxic and sulfuric acid is corrosive, the battery is very robust and rarely presents a hazard to the user. However, hydrogen gas can be produced if the battery becomes overcharged, and this is potentially explosive. With increasing battery size required for residential storage, the amount of each material will increase, as will the hazards. Developments in lead–acid batteries have minimised the loss of the sulfuric acid electrolyte by first sealing the battery and then absorbing the acid into solid materials such as a gel or fibreglass mat.

An example of a commercially available lead–acid battery is Battery Energy’s SG1000. By combining 24 x 2V batteries to create a 48V battery pack (with 100% discharge of 32 kWh), the battery pack is capable of 2300 cycles of 19.2 kWh when discharging to only 60% of its maximum. The entire system would weigh 1296 kg and take up 0.5 m3 in volume (Table 1).

Lithium-Ion Batteries

The current leader in rechargeable batteries is based upon the movement of lithium ions between a porous carbon anode and a lithium–metal oxide cathode. The electrolyte contains some free lithium ions in a liquid electrolyte.

The composition of the cathode has a great effect on the performance and stability of the battery. Currently a lithium–cobalt–oxide cathode has superior charge capacity but is more susceptible to breakdown than lithium–titanate or lithium–iron–phosphate cathodes.

Common breakdown pathways are related to the swelling of the cathode as lithium ions become intercalated within its structure, and the plating of the anode with lithium metal (which can become explosive). The chance of breakdown can be reduced by limiting the charge/discharge rate, but instances of laptop or phone batteries exploding or catching fire are often reported.

The lifetime of the battery also depends heavily on the anode, cathode and electrolyte composition. Generally the lifetimes are superior to lead–acid batteries, with Tesla reporting a lifetime of 15 years (5000 cycles) for the 7 kWh Powerwall when discharged for 5 kWh per cycle. The average four-person household would need four of these units connected in series (discharging 5 kWh each), and this would weigh 400 kg and take up 0.8m3.

The Tesla Powerwall is the lightest but takes up the most volume of the commercial systems compared in Table 1. The design of the system is visually appealing, and it’s made to be displayed in the home. Tesla has been using lithium-ion batteries in their automobiles for a number of years now, so consumer confidence in their safety is high.

Flow Batteries

An alternative battery architecture is the flow battery. This consists of two storage tanks filled with different electrolytes separated by a membrane that allows the flow of electrons and ions but restricts the mixing of the electrolytes in the storage tanks. Examples of these include vanadium–vanadium, zinc–bromine and bromine–hydrogen.

The electrolytes in flow batteries must have different and stable oxidation states. In a zinc–bromide flow cell, the zinc bromide electrolyte is pumped past two electrode surfaces that are separated by a microporous barrier. During charge, the anode converts zinc ions in solution to zinc metal, which forms a coating on the electrode, while the cation converts bromide ions to bromine. The reverse occurs during discharge.

Flow batteries have very long lifetimes and are very stable. They can be upscaled almost indefinitely but require a pump to cycle the electrolyte around the storage tank.

Redflow’s ZBM2 module has a 10 kWh capacity but is capable of 100% discharge without affecting its performance. Two ZBM2 units are thus required to create a 20 kWh system that weighs 480 kg and takes up 0.55 m3. Redflow gives a warranty of 20,000 kWh, which equates to 2000 cycles.

Many other rechargeable residential power storage systems are currently available. Prospective residential battery purchasers need to consider what suits their environment and budget, and look into all options to find the correct solution. The situation is further complicated by new battery formats under development that could disrupt the battery industry in the future.

Improvements to Lithium-Ion Batteries

Research labs around the world are working to improve the specific energy, lifetime and safety of lithium-based batteries. Major areas of research include different ratios or chemical structures in the cathode, and the use of graphene and carbon nanotubes in both the cathode and anode.

Graphene and carbon nanotubes have a higher surface area, conductivity and mechanical stability than activated carbon and graphite used in current electrodes. While the exact composition of most anodes and cathodes is currently a trade secret, commercial production levels of carbon nanotubes hint that most phone and laptop batteries already have carbon nanotubes as part of their electrodes.

Carbon nanotubes and graphene can be used as scaffolds to hold active nanoparticles on either electrode. The small size of the nanoparticles will allow faster inter­calation and de-intercalation of lithium-ions during charge/discharge, while the carbon nanomaterials provide a fast pathway for electrons to migrate to the nanoparticles.

Lab-based batteries have shown incredibly storage capacity, but often the materials used are expensive or the process used is difficult to scale to industrial processes. With further reductions in material costs and further simplification of synthesis there is no doubt that the application of nanomaterials will continue to improve the capacity, lifetime and safety of lithium-based batteries.

Lithium–Air and Lithium–Sulfur

Lithium–sulfur and lithium–air batteries are alternative designs with a similar underlying principle of lithium-ion movement between two electrodes with much higher theoretical capacities. In both cases the anode is a thin sliver of lithium while the cathode is Li2O2 in contact with either air (in lithium–air batteries) or active sulfur (in lithium–sulfur batteries).

Since the anode in these batteries is lithium metal, the amount of lithium metal required for a residential-scale 20 kWh battery pack (total weight of 18 kg and 36 kg for lithium–air and lithium–sulfur, respectively) may limit their use to smaller devices in the short–medium term.

Sodium-Ion, Magnesium-Ion and Aluminium-Ion

Lithium has an atomic number of 3 and sits in the first row of the periodic table. Directly below it is sodium (atomic number 11).

Sodium-ion batteries are viable alternatives to lithium-ion due to the relative abundance of sodium. The cathode consists of a sodium–metal oxide (e.g. sodium–iron–phosphate) while the anode is porous carbon. Due to the size of sodium ions, graphite cannot be used in the anode so carbon nanotubes and graphene are being developed as anode materials. Since the mass of sodium is greater than lithium, the charge capacity per unit mass is generally lower.

Magnesium sits to the right of sodium on the periodic table (atomic number 12), which means it can exist in solution as Mg2+ (compared with Li+ and Na+). With double the charge, magnesium is capable of producing twice the electrical energy for a similar volume. However, the double charge makes Mg2+ more sluggish as it moves through the electrolyte, slowing the charge rate.

What’s Next?

In the short term it’s expected that lithium-ion batteries will continue to be improved. Future technologies have the capability to deliver higher specific energy and/or energy density but are expected to enter the market in smaller devices before moving toward residential energy storage.

In the meantime, older technologies such as lead–acid and flow batteries are available with commercial products that exhibit similar performance to lithium-ion and are expected to remain competitive in the short term.

Cameron Shearer is a Research Associate at Flinders University’s Centre for Nanoscale Science and Technology.