Backup Battery Technology – Can We Park a Tesla in the Plant Room?

I was kind of excited by Elon Musk’s announcement of Tesla’s Powerwall energy storage systems. Why? Because most of what I do in my working life as a specialist in critical power systems is a complete mystery to my friends and family at home. What’s more, the subject has tended to draw blank looks on the rare occasions I start talking about it! But thanks to Tesla, all of a sudden, they’re interested. The lights are on, so to speak!

In case you were off the planet or in a news lockdown, Tesla’s range was announced with a great fanfare. It’s a compact, wall mounted Lithium Ion battery with a relative high storage capacity – ideal for domestic installations where it can store up to 10kWh of energy generated by wind or solar systems. If you want to know more, Wired magazine wrote a nice and accessible explanation of battery technology which is available online.

Powerwall solves the problem of intermittency – a common challenge for renewables – so that electricity can be generated when conditions are favorable and stored for use when it’s required. It’s not unique – other energy storage systems are available – but none come with Elon Musk’s personal guarantee that you’re buying into a solution which will contribute to making the world more sustainable.

The announcement of Powerwall, which also comes in a 100kW industrial form, prompted me to revisit the various ways that we ensure back-up power is available for the critical applications I work with. Because of the nature of the systems they support – where complete power loss could also put data, money or even lives at risk – regulations or common sense dictates that they should not depend upon a single back up power technology.

Typically such systems comprise a technology – some form of UPS – to provide ride through power to support the load during shorter power events, or until a secondary power supply (usually a diesel powered generator) comes online if the outage is more serious. What Powerwall does is ask us to address how we ensure that ride through power is available. In industry, a lead acid battery array is the most common way of doing this – but why is this, and what other options exist?

In recent years, UPS have been developed which utilize ultra-capacitors to provide energy backup. The advantages here tend to be weight (important if there are floor loading constraints), the speed of recharge and environmental considerations – capacitors don’t use heavy metals in their manufacture and remain effective in almost any hot or cold conditions. On the downside is the expense, their relative rarity in these sorts of applications, and their suitability for any use above a few seconds’ ride through capacity.

In certain markets, data centers notably, the use of rotary or flywheel UPS has been promoted as a “green” alternative to traditional static systems. Flavors of flywheel systems have a relatively low market penetration globally. They are not scalable and therefore mostly only suited to large facilities where the eventual power capacity requirement is predictable. They are extremely heavy, have high CAPEX and maintenance needs, as well as cooling requirements above those of static UPS. Additionally, their use tends to have implications for secondary power systems which must always be started in any power event.

As a side note, we want to minimize the use of diesel generators wherever possible as they cannot be simply switched on and off quickly. They take time to come up to power, and they also take time to be cooled down and brought to a halt. All the while they are consuming diesel oil and creating emissions.

And so we come to lead acid battery based UPS systems. And you might ask why we can’t simply replace these rather old fashioned cells with something more techy, like Powerwall’s Lithium Ion. Or Lithium polymer, NiCad or NiMH batteries such as those used variously in power tools, cellphones and laptops. The advantages and disadvantages of these various chemistries requires a fairly technical explanation which I won’t attempt in a short blog. However, two major advantages lie with lead acid batteries; one is cost and the other is ubiquity.

Today, lead acid systems benefit from being commonly in use and therefore there are also strong existing supply chains for new installations as well as maintaining existing arrays. The systems themselves are inert, and if subject to regular maintenance programs, will provide years of reliable and cost-effective use to protect the load in the case of an emergency. If you want to find out more, my colleagues have written a white paper – Comparing Data Center Batteries, Flywheels and Ultracapacitors – which you can download at no cost. I’ll have a look at the why’s and how’s of maintenance in separate blog posts.

Thanks for reading.

Tags: Battery back up, flywheel, Power Management, UPS, white paper