The home battery revolution is officially under way. Lithium-ion batteries are leading the charge, but they are not the only storage solutions on the block. Here we look at the range of battery technologies available and their various pros and cons.

The arguments in favour of home energy storage are so well worn by now that they hardly bear repeating. The price of grid power is rising, feed-in tariffs are falling, and all those solar panels are just sitting on the roof generating money for someone else. The only way to get value from your PV is with energy storage. Plus batteries are a great way to enable smarter integration of rooftop solar into the grid, so everybody wins.

It’s a persuasive case, and the only real flaw is that, for now at least, the sums don’t add up for most households and businesses.

Apart from a few early adopters, battery storage is still mostly the domain of off-grid and remote applications.

However, according to every research body, energy agency and fortune-teller out there, all that is about to change. With the rise of cheap, mass-produced lithium-ion batteries, skyrocketing economies of scale, and consequent falls in cost per kWh, residential and commercial energy storage is on the verge of going mainstream – if not in 2016 then very soon thereafter.

But just what are the options? There are new products coming onto the home storage market every month, with a wide range of competing technologies behind them. Each one claims to outperform the other, and it can be hard to cut through the hype.

At the moment, most of the talk is about lithium-ion (Li-ion) batteries, such the Tesla Powerwall and the Enphase AC Battery, but they are not necessarily the best solution in every case.

Indeed, each of the major technologies has its strengths and weaknesses, be it in price, safety, efficiency or just plain size, and the critical thing is to match the right technology to the right situation.

Here, we look at the main battery technologies vying for a place in the Australian storage market. It’s not a comprehensive list by any means, and there are new products and technologies emerging all the time. In five years, or even two, this list might look very different. But for now, these are the top contenders in residential and commercial storage technology.

LEAD-ACID

For a long time the world’s most pervasive battery storage solution, lead-acid is the steady old workhorse of the battery world. The technology has been around since the mid-1800s and, in the early years of solar PV, was the only real option for clean energy enthusiasts looking to go off-grid with solar and storage. (Of course, this was long before ideas such as time-shifting, energy arbitrage and smart grids came along.)

The technology has seen some advances in recent years, but it is still essentially the same as it was 100 years ago. Even so, lead-acid still has a big part to play in today’s energy storage market. Why? Because it’s cheap and it works. After decades of use in residential energy storage, lead-acid technology not only has a track record of safety and reliability, but it is also highly versatile and very well understood by the market. In off-grid systems, where the priority is simply to put away a large number of kWhs – and considerations of size and weight tend not to be so important – lead-acid batteries are still the preferred choice, and will likely continue to be so for some time.

Another key advantage of lead-acid is its flexibility. The batteries are available in many different configurations and capacities, and for almost any residential storage job there will be a wide range of compatible options to choose from.

Acid reigns

There are three main types of lead-acid battery: flooded, gel and AGM (Absorbed Glass Mat). The chemistry of all three battery types is essentially the same, with the differences lying in how the electrolyte is contained. In gel batteries, a thickening agent is added that turns the electrolyte from a liquid to a gel, while in AGM cells, the liquid electrolyte is contained in a porous glass-fibre mat.

These, more advanced, technologies are also known as sealed or valve regulated lead-acid (SLA or VRLA) batteries. They are more expensive than plain old flooded batteries, which remain the most popular for home storage (as well as being used in most cars). On the downside, flooded batteries require regular maintenance of the electrolyte, they must be kept upright to prevent leakage, and they must be kept in a ventilated environment.

There are many other variants on the lead-acid formula out there besides these – it is, after all, a mature technology with decades of R&D behind it. Among the newer lead-acid technologies are silicon batteries and graphite (carbon) batteries. There is also the next-generation lead-acid technology of the UltraBattery, which we treat separately.

PROS

  • Proven reliability and safety
  • Low upfront cost compared to Li-ion
  • Suitable for very large installations
  • Still the technology of choice for off-grid and commercial UPS systems
  • Highly recyclable

CONS

  • Very heavy and bulky
  • Can degrade rapidly in hot and humid conditions
  • Not suitable for regular deep cycling or fast charging
  • Lifespan can be compromised if correct charge/discharge parameters are not maintained
  • Lead content and electrolyte are environmentally unfriendly
  • Can present dangers due to toxicity of materials and emission of explosive gases

ULTRABATTERY

Claiming to combine the safety and reliability of lead-acid with the responsiveness and versatility of Li-ion, the UltraBattery is an innovative hybrid technology developed by CSIRO. The design combines two common energy storage devices: the lead-acid battery, stalwart of home energy storage, and the supercapacitor – a high-capacity, fast-charging device typically used for short-term energy storage or burst power delivery, such as in camera flashes. The result is a battery that boasts affordability, fast charging and a long lifespan.

The key advance made by CSIRO is to allow lead-acid batteries to operate efficiently in a partial state of charge without degrading the battery. This gives UltraBattery the flexibility to meet the erratic demands of a grid-connected solar household, where the battery is continually active in a partial state of charge. As such, the UltraBattery promises to support all the ‘smart’ energy management options of Li-ion, such as peak lopping and energy arbitrage.

The technology is being brought to market by Sydney-based company Ecoult, starting with the UltraFlex in late 2015. A medium-scale battery aimed at the industrial, agricultural, off-grid and business markets, the UltraFlex offers more than 11 kWh of usable storage and 25 kW of peak power, with a retail price of $19,900. This will be followed in April 2016 by the UltraPod, a 5 kW small-scale unit for residential storage, with a target price of $5,000.

While this all sounds great, the UltraBattery is still a new and to some extent unproven technology compared to VRLA and Li-ion.

Ecoult has done its best to demonstrate the strengths of its products – they have been tested by major independent labs around the world, as well as being installed in grid and hybrid electric vehicle projects – but it will be some time before claims about long lifespans are entirely validated. As such, grabbing market share from the more established players may prove difficult.

PROS

  • Safe, reliable, recyclable (as with other lead-acid technologies)
  • Suitable for the high cycling and irregular charge and discharge patterns of residential usage
  • Long lifespan – two to three times longer than a regular VRLA battery
  • Fast charge/discharge rates
  • Still cheaper than or comparable to Li-ion

CONS

  • A new technology, not yet proven in the real world
  • Very heavy and bulky (almost 1,200 kg for UltraFlex)
  • Lead content and electrolyte are toxic and environmentally unfriendly

LITHIUM-ION

And so we come to the real star of the home storage revolution, lithium-ion batteries. If lead-acid is the slow and steady workhorse of storage, then Li-ion is the nimble gundog – lighter, faster and packing in more energy per square inch. In fact, with its high power density and huge weight advantages, Li-ion has already triggered one battery revolution, being the technology of choice for almost every portable electronic device on the market, as well as in many electric cars. As such, it is well understood and accepted by the general public.

Home energy storage is still something of a new frontier for Li-ion, but it is fast gaining supremacy here as well, as manufacturing scales up and prices come down. The poster child for Li-ion is, of course, the Tesla Powerwall – the battery that’s getting everyone worked up about residential storage – but there are dozens of other highly competitive options on the market, and many more in the offing.

Admittedly, there are some safety concerns with Li-ion, including the prospect of thermal runaway leading to fires or explosions. This has been highlighted recently by media reports of products such as ‘hoverboards’ catching alight. However, unless you’re planning on putting wheels on your home storage unit and riding it around the streets, there’s probably not much to worry about. With residential Li-ion systems, most problems are likely to arise from poor battery management (or substandard installation), and can be mitigated by a good-quality battery management system.

Against these safety concerns stand the many benefits of Li-ion batteries: they are smaller and lighter than lead-acid batteries, they offer greater depth of discharge (DoD), and they have a higher cycle life. As a result, their ascendancy seems assured, despite relatively high prices.

Ions in the fire

Although we often talk about Li-ion batteries as if they are all the same, there are actually many different types of lithium battery with different chemistries in each. Dozens of them, in fact, each with strengths of weaknesses appropriate to different applications. The most popular for residential storage presently seems to be lithiumiron-phosphate (LiFePO4), which is the safest and most stable of the main Li-ion variants (as used in the Enphase AC Battery). Other popular types include lithium-nickel-manganese-cobalt (as used in the 7kWh Tesla Powerwall and the LG Chem RESU range) and lithiumnickel-cobalt-aluminium (as used in the 10 kWh Tesla Powerwall).

One of the latest variants, which is not strictly Li-ion, so is really in a new category of its own, is the lithium-sulfur battery currently being developed by UK company OXIS Energy. This technology promises to outperform Li-ion at almost every level, including higher energy density (leading to lighter, more compact batteries), greater safety and reliability, and reduced environmental impact. However, the first Li-S storage products will not start to emerge until later in 2016.

PROS

  • Established and proven technology
  • Compact and lightweight, with high energy density
  • Greater depth of discharge than lead-acid (and lower self-discharge)
  • Greater cycle life compared to lead-acid
  • Low maintenance

CONS

  • Most effective for time-shifting and peak shaving rather than back-up power
  • Needs active cooling to control temperature
  • Some risk of fire if not managed correctly
  • Repeated deep cycling can shorten battery life and damage battery
  • Not cost-effectively recyclable; no recycling facilities currently exist in Australia

FLOW BATTERY

Though less well known than the likes of lead-acid and Li-ion, flow battery technology is no newcomer to the storage scene, and has been around in some form since the 1970s. It was developed by scientists at NASA, who were looking for a way of storing energy from – guess what? – a PV array. It is now a mature and proven technology, known for providing cost effective, safe energy, and is widely used in large-scale storage systems.

The basic operating principle of flow batteries is simple: you have two discrete liquids containing electrochemical components dissolved in solution (the electrolyte). Between them you have a membrane that prevents the solutions from mixing, but allows an electrochemical reaction (ion exchange) to occur across it. The two liquids are pumped through the cell from storage tanks, and as the current flows through the membrane, electricity is extracted. The battery is then recharged by reversing the reaction, achieved by applying an opposite voltage to the electrochemical cell.

As with the other battery types, there are many variations on the flow battery based on different chemistries. Vanadium redox batteries or VRBs are the current frontrunners, but other examples include zinc-cerium, iron-chromium and bromine-polysulphide. There are also hybrid batteries that use flow battery architecture but include solid storage materials rather than just liquids.

The main limitation of flow batteries is their low energy and power density compared to Li-ion and lead-acid, along with the need for large storage tanks for the liquid electrolytes. This has so far restricted their use to grid-scale and industrial storage projects, mostly in the 100kW to 10MW range. On the other hand, scaling up the storage capacity of a flow battery up is simple – you essentially just make the storage tanks bigger – so they are ideally suited to storing large amounts of power for the grid.

The reds are here

In Australia, flow battery technology is more or less synonymous with Brisbane-based battery maker Redflow, which sells Australian-designed zinc bromide flow batteries in a range of sizes and configurations.

Redflow’s batteries are actually hybrid flow batteries, as the chemical process stores energy by plating zinc metal as a solid onto the anode plates in the electrochemical stack.

To date, Redflow has focused on the large-scale market, as well as niche applications such as off-grid telecoms towers. However, the company is set to challenge the perceived limitations of the flow battery with the launch of a compact 10 kWh residential energy storage product. The modular residential unit should be available for pre-order by the time of print, with first installations due before June. This will put Redflow’s zinc bromide battery in direct competition with the leading Li-ion solutions.

Redflow claims some distinct advantages over the competition. Unlike the sealed units of an Li-ion or lead-acid cell, you can take a flow battery apart and service or replace many of the components. Also, the stored energy can be held almost indefinitely without self-discharging – while lead-acid batteries require a trickle charge to prevent damage to the battery. And because the battery does not degrade due to daily cycling, Redflow claims its batteries will have a much longer useful lifespan than Li-ion, which could potentially make it a more economical option.

PROS

  • Cost competitive with Li-ion and VRLA
  • High temperature tolerance and no active cooling required
  • Suitable for deep cycling, with inherent 100% depth of discharge
  • Long expected lifetime, as battery does not degrade due to daily cycling
  • No risk of thermal runaway
  • Serviceable, refillable and recyclable

CONS

  • Relatively new technology, not yet proven for small-scale applications
  • Electrolyte materials such as bromine are corrosive and toxic
  • Many possible points of failure, including pumps, flow controllers, storage tanks
  • Some parasitic losses involved in having to run pumps

SALTWATER BATTERY

Something of a wildcard in the storage mix, saltwater batteries are claimed to be the cheapest, safest and most environmentally friendly batteries ever devised. They’ve only been on the market for a few years, but have already caused a stir with their combination of performance, safety and long lifespan.

The idea of using saltwater as an electrolyte is not a new one – in fact, it’s around 200 years old. However, it’s only recently that the first commercial saltwater batteries have emerged for residential and industrial applications, from US-based battery manufacturer Aquion.

Founded by a former NASA scientist, Aquion has developed a unique saltwater battery technology that it calls Aqueous Hybrid Ion or AHI. The battery chemistry comprises a saltwater electrolyte, a manganese oxide cathode, a carbon composite anode, and a synthetic cotton separator.

Because of their low energy density and relative bulk, saltwater batteries are being positioned more as a successor to lead-acid than a potential Tesla-beater. However, the advantages offered by the technology are many.

For a start, all the components are non-toxic and non-corrosive. The batteries contain a safe, water-based electrolyte, meaning they are virtually non-combustible. They also require very little maintenance – there is no need to maintain fluid level in the batteries or clean the terminals.

In terms of performance, AHI batteries offer longer system life than lead-acid, in both deep discharge and partial state of charge applications. Aquion says they can be completely discharged to the maximum depth at least 3,000 times without damaging their ability to hold a charge.

What’s the catch?

At present, AHI batteries are more expensive than equivalent lead-acid batteries, at least in terms of upfront costs. Over their lifetime, however, AHI’s combination of longer product life and better overall performance can put them ahead.

The main thing holding the technology back is recognition. Although more than 2 MWh of AHI battery capacity has already been installed in Australia, and more than 10 MWh worldwide, that is still a drop (of saltwater) in the ocean compared to the amount of installed lithium-ion and lead-acid capacity. (By way of perspective, a South Korean company recently connected 40 MW/15 MWh worth of Li-ion storage to the electricity grid at a single stroke.)

So, while saltwater technology has plenty to recommend it, it clearly faces a tough battle against the more established technologies – and this in turn means it will be hard to achieve the economies of scale necessary to deliver large price reductions. Nonetheless, there is plenty to like here – and it’s way too soon to be picking winners and losers in this battle.

PROS

  • Very safe: not flammable, explosive, or corrosive
  • No dangerous or toxic components
  • Good temperature tolerance
    Suitable for deep cycling, with 100% depth of discharge
  • Very high cyclic life

CONS

  • Low energy density, and therefore very heavy and bulky
  • Not yet well known in market
  • May not attain the market share to achieve economies of scale
  • Good for storing energy but not delivering power – they have a limited charge and discharge rate compared to other chemistries.