In this video we’ll look at the seismic change that has happened in battery technology for home storage over the last ten years. Towards the latter half, we’ll reveal a technology that will be a game-changer. It’s not something that is up-and-coming. It’s already here and it’s not Tesla’s power wall, so let’s begin:
In the not-so-distant past adding batteries to your domestic PV system did not make any economic sense because it was one component that was never going to pay for itself but things have changed over the last ten years.
Ten years ago, when there weren’t many residential solar PV installations, lead-acid batteries were the dominant batteries for domestic energy storage. If there was a need, there were nickel metal-hydride batteries and nickel-iron or Edison batteries, too, but because of their drawbacks they were few and far between.
With the development of lithium-ion battery technology, home solar energy storage is an affordable and viable option. In the foreseeable future, super-capacitor based lithium hybrid batteries will ensure battery backup with a life exceeding the 25 to 40 year span of solar panels.
Despite the low cost of lead acid batteries they was little financial gain from energy storage. It was only for self-sufficiency in areas where there wasn’t any grid electricity. In other places they were being used to provide limited backup power in case of power cuts.
Now many people at the time did use heavy duty vehicle batteries for greater backup times but automotive batteries are not really suitable for domestic energy storage applications. They aren’t built for providing a small amount of power for a long amount of time. In fact, they’re designed to do the opposite, that is, provide high bursts of power for a short amount of time. Very obviously, because they were designed to start an engine.
There were some designs of lead acid batteries though that were more suitable for energy storage and re-use in a solar PV setup. These were the deep-cycle lead-acid batteries. They were used in motor homes and transmission towers. However, even with them, their short shelf life remained an issue.
How did we progress to a place where now there are dozens of home battery manufacturers but none of them used lead acid batteries? We’ll have to back up further in time to find out the answer.
What are the advantages of lithium batteries?
In 1991 Sony introduced the first commercial lithium-ion battery. It was expensive, so small-scale applications like cameras, watches and other electronics was its niche. Now that sector was already at the time occupied with alkaline and nickel metal hydride batteries. Rechargeable alkaline batteries had as low as 10 charge cycles while nickel metal hydride self-discharge very quickly and this is where lithium ion had a distinct advantage over the other two, besides having a very high energy density, meaning lithium ion could absorb and hold a lot more charge than any other.
It also had much longer cycle life. Lithium batteries kept pushing nickel metal hydride batteries out of the consumer electronics sector. In the year 2000 almost 50% of all portable rechargeable batteries sold in Japan were nickel metal hydride but this dropped to just 22% by the year 2010.
Now batteries have always been the bottleneck in electric cars. The rapid development of lithium-ion batteries which were long lasting and more charge bearing presented an opportunity for the whole automobile sector. A keen technologist by the name of Elon Musk was watching and grabbed it with both hands.
In 2008 he rolled out the Tesla Roadster which was the first-ever highway capable electric car to use lithium-ion batteries. Tesla Roadster wasn’t just a car. Its launch was a turning point in history. A new market had been created and the race to make affordable electric cars had begun.
Lithium ion batteries skyrocketed. Safer and more powerful batteries since then have been created. From 2010 to 2020 the energy density of lithium-ion battery has almost doubled. The best cell in 2010 had the energy density of 160 watt hour per kg. The best cell today has an energy density of 300 watt hours per kilogram.
The lithium-ion battery price since then has dropped consistently by at least 8% every year. With dropping prices and superior efficiency another market was created. This was not for home energy storage but for smart home energy storage. By the year 2015 the residential solar installations had reached almost 500 megawatt in capacity in the USA alone.
That meant that the number of homes with solar PV were in hundreds of thousands, with most of them also grid tied. Those who had installed solar PV were facing an issue – during most part of the day when the solar panels were producing electricity there was little of it being used in the home and most of it was being exported.
Is grid tie solar worth it?
We know that the home energy consumption mostly peaks in the late evening. It’s the time when solar panels are not producing electricity, therefore people had to buy back the electricity from the grid during those peak times.
As the feed-in tariffs got slashed, many were buying the energy at a higher price than the rate at which they exported it to the grid. This did not make sense. What did make sense was to save the excess electricity before sending it to the grid. What also made sense was to use the battery electricity first, particularly at times when the grid electricity tariff was high.
This called for not just a simple battery system but rather a smart one. The market was ready and yet again it was Tesla who capitalized by releasing the first iteration of power wall in 2015. Today there are dozens of smart home energy systems in the market.
What is a smart solar battery?
Almost all of them are lithium-ion based. They can be programmed with information on grid electricity tariffs. This allows the battery management system to use itself in the most financially sensible way. For people owning an electric vehicle, home energy storage makes greater sense as it allows them to charge the car anytime without the worry of higher tariffs.
Note that unless the battery system is sized properly and used in an optimal fashion, there is a strong possibility that even with a smart system it will not pay itself off. For example, Tesla Powerwall is a kilowatt hour battery. By comparison to others it is the largest battery for home energy storage in the market.
It however requires a solar PV system of five kilowatt or higher capacity, and higher energy consumption of eight kilowatt hour or more per day to be utilized in a fashion that will eventually help it to pay for itself. For a smaller battery a smaller PV system might be more suitable.
What does the future hold for home energy systems? To make the batteries more widespread its price has to come lower. In home systems the energy density does not matter. What matters is life. Lead acid batteries at best could be stretched to a couple of years. Lithium-ion batteries can be stretched to 10 years without losing capacity.
What is a super capacitor battery?
Bear in mind that a PV system lasts for over 25 years and if a battery life matches that then it would make it a more appealing and lucrative option. In light of this the upcoming super capacitor based energy storage presents a very attractive solution.
There is at least one company that is providing a super capacitor based energy storage solution. For those who aren’t aware, super capacitors have a usable life of a million charge cycles, which means they can go well beyond the life of your PV panels.
Large power from them can be discharged with ease and without causing harm to their usable life. One thing that made lithium-ion prominent was its high round-trip efficiency of 90%, meaning that for every one unit of electricity you put in, you get 0.9 of it back.
This compared to lead acid round trip efficiency of 70 to 80% was much higher. Super capacitors can go a step further than lead acid batteries and lithium batteries, and can achieve round trip efficiency of 95%. The area where super capacitors do lack is their energy density.
Reportedly, the best super capacitors can hold a tenth of the charge that a lithium-ion battery holds. For non-mobile application, such as for home storage, that may not be an issue, because it can be compensated with larger unit size.
It also has to be noted that ongoing research on super capacitors is helping to accelerate their energy density.
There have been claims made for instance by a company called Shenzhen – two men say they are already producing hybrid super capacitors with energy density comparable to lithium-ion batteries of two hundred to two hundred and sixty watt hours per kilogram.
These claims have been verified by University of Munich. We’ll keep you posted on this development. When capacitors will approach the energy density of batteries it could revolutionize, not just the home energy storage sector but also the whole transport sector.
Interestingly, Tesla last year in May 2019 also acquired Maxwell technologies, which was one of the most established super-capacitor manufacturers.