Battery storage for solar PV: Which battery to choose?
Posted by Chris Rudge on 30 March 2016 at 12:55 pm
Battery storage bolt-on options for existing renewable energy generation systems are now becoming the next 'must have' for people looking to either reduce their energy bills or increase their energy security at times of power cuts.
I have been providing different types of battery systems for customers over the last 10 years and have seen a huge upsurge in interest in the last 6 months. There has always been some bafflement over the best battery technology to use with different installations, which has escalated recently now that Lithium-Ion batteries are coming down in cost.
In this blog I'll outline the features and cost benefits of some currently available battery types offered for renewables.
The main battery types currently offered to customers wanting a battery add-on are based on either Lead Acid or Lithium-Ion.
These batteries have been available for a very long time and until recent years, would have been the only option available for anyone to use. Originally provided as what we refer to as 'flooded', you may be familiar with the need to keep these topped up with distilled water regularly in your car. Really big versions were made for fork lifts, submarines, commercial backup supplies, etc. Though this is still the case, as flooded batteries are quite robust when scaled up to commercial size, most Lead Acid offerings' for domestic and small commercial are either Sealed AGM or Gel types.
Without going into the internal technology, which you can easily discover on Wikipedia, the practicality for non commercial users mean that Sealed batteries do not need regular maintenance such as topping up with water. Sealed AGM's will usually be able to take a far higher charge current than Flooded, thus making them ideal for the Solar PV market where potentially a lot of power is available to be stored during peak times of the day. Likewise, Gel batteries have a good storage capability, but cannot be heavily charged.
Both types will have a lifespan of no more than 5 years and you will need to keep in mind that you'll need to replace them at this point or before. As either type contain a good amount of lead, they will attract a good price at the recycling centre. Typically around 20% of their cost as new. Lead Acid batteries are about the only battery type that can be recycled in this way. So disposal is straightforward through recycling and produces some income!
As with anything, the less you pay for your batteries, the lower the performance. No matter what the salesman tells you, a typical lifetime of an AGM or Gel battery is around 800 - 1000 charge discharge cycles to 50% depth of discharge. This is around 2.5 years if cycled to 50% every day of the year. For many households this will not be the case, thus the batteries will last much longer
This reminds me to mention that all lead acid batteries do not fare well if deeply discharged, which will considerably reduce their storage capacity and longevity. As part of the battery management which will be installed with your add-on storage system, it will be set up to ensure the batteries will not drop below 50%, this is critical for any system, as well as carefully charging the batteries according to their individual characteristics.
Another big advantage with any Lead-Acid battery type is that it comes at the lowest cost of any current storage type. You may find this aspect appealing, even though you will get much fewer charge/discharge cycles. You will know without heavy use you will get 5 years of life and a small payback for selling them at the end of this time. You can then ask your installer what the cost of other more efficient technologies are, which may have dropped in price by that time. See the chart below to get an idea of cost effectiveness at the moment.
These fairly recent storage devices have been hailed as the holy grail of battery technology and you will find it fitted in most portable devices, such as your phone, due to their far higher efficiency and space saving over all other types available to us.
Even more recently, Lithium-Ion has been offered to the renewable energy market in easy to use packages making it far safer than ever before to use. As Lithium-Ion has a really low internal resistance, allowing them to be charged really quickly if needed, they can also discharge very fast if given the opportunity and will overheat and cause a fire very quickly - the very reason for Hoverboard fires recently.
All approved Lithium-Ion battery sets contain a battery management setup which will shut them down with excess charge/discharge or fire, so these are nice and safe to use in the house.
The huge advantages over Lead Acid is the smaller space required, higher lifetime cycles and that you'll be able to discharge to 80% or more daily if you wanted with little lifetime effect.
Most Lithium Ion batteries will come with a 10 year or 6000 cycle warranty and will work across a far wider temperature range, far far better than Lead Acid. The Lithium Ion battery really is pretty much a fit and forget battery.
So, though Lithium Ion costs much more on initial purchase, the lifetime will be at least 3 times more, and easily in excess of this if you keep the discharge down to 60% or less. See the chart below for cost effectiveness against Lead Acid.
One of the drawbacks with Lithium Ion - there had to be one - will be that the recyclability is practically zero. Once the battery set is depleted you'll need to find a way of safe disposal, which you need to pay for as they are listed as a pollutant. Disposal will be in at least 10 years, so as Lithium is a limited resource, the situation may have changed by then, but right now it's a difficult disposal issue.
Aqueous Hybrid Ion batteries
For the real Eco warrior, the recent release of the Aqueous Hybrid Ion Battery is a major leap in clean power. Being based on a harmless saline solution with other natural products such as coconut husk, the Aqueous unit will never be a hazard in its own environment.
They can be deeply discharged and each 'module' will store 2kWh of power.
As you will imagine, this natural solution does come at a cost. It is more than either of the above per kWh, although currently not excessively so. As production ramps up the price will come down to make it a far more economic solution.
Aqueous hybrid ion battery charge and discharge rates are far lower than either Lead-Acid or Lithium-Ion, so are more suited to low capacity loads such as lighting and electronics. Though the 2kWh of storage is available in each cell, it will need around 5 hours to charge or discharge that amount. Obviously stacking more in parallel will double the power and storage at the same time.
This chart easily shows that though Lead-Acid wins on initial system outlay, for system economic longevity, Lithium-Ion wins hands down!
The Aqueous hybrid ion, which I have scaled to be representative of the others, is also great on long term economics, but you need to be careful the installation does not have a big current draw.
More information about Energy Storage on YouGen.
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About the author: Chris Rudge is a qualified electrician who specialises in renewable energy.
If you have a question about anything in the above blog, please ask it in the comments section below.
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