The Age Of The Megawatt Minute
Is it time to reconsider UPS energy storage requirements?
As is my usual practice for my year end column, I asked the official Hot Aisle Insight crystal ball for guidance on the latest developments and trends in data centers. When I queried the insightful orb about energy storage technology, it glowed as if it was trying to use facial recognition and emulate the iPhone X for a few seconds. It then apparently gave up and transferred the request to Apple’s “Siri,” who did not recognize my face, since I do not have an iPhone X.
- The Megawatt Minute
- Power Density VS Energy Density
- Super Capacitors (Super-Cap)
- Power For Edge Data Centers
- Grid Level Energy Storage
- The Bottom Line
Nonetheless, after a few seconds, Amazon’s “Alexa” responded with “Super Start-IT Lithium-Ion 12 volt 600 Amp Battery Booster” one day sale for only $59. What I found most interesting was it only weighed one pound and was small enough to fit in a glove box. Curious, I looked at the specifications, it was rated at 18,000 milliamp-hours (mah), so it appeared to have energy storage capacity of 216 Watt-hours (based 18 AH at 12V).
Wow, I was a bit surprised how far Lithium-Ion (Li) battery performance and prices had improved this past year. While I was not planning to buy a battery booster, however, with winter setting in, it sounded like too good a deal to pass-up, especially since it included free shipping!
Well, like almost all things that seem too good to be true, when it arrived it was a bit less than advertised when it came to the apparent energy storage ratings. Upon closer examination, it has a 12 Volt battery, however, it was not (18 AH) at 12 V, it was really a pseudo rating based on an equivalent 18,000 mah rating at 3.6 V, the typical internal battery voltage normally used to support 5V USB ports to charge cell phones. Essentially, it is only a 65 Watt-hour battery.
Nonetheless, it did a reasonable job functioning as its primary purpose: a 12 V battery with enough current to jump-start my car. While I did not measure it with an ammeter, it did deliver several hundred amps for about the 2 to 3 seconds that it took to crank and start the engine. So while I am not sure if it can deliver the claimed 600 amps, I can verify that it delivered about 10 volts during cranking at an estimated 300 to 400 amp load (3,000 to 40,000 Watts) in a 1 pound compact 7- x 3- x 1.5- in. package. Clearly, a fairly high-power weight/size density (not to be confused with energy density, which while good was 65 Watt-hours). When compared to the typical much larger sealed “Gel Cell” Lead Acid battery used in most vehicle booster units, typically weighing about 10 to 15 pounds, the Li battery had tremendous advantage.
While by now you are wondering, that was an interesting product review, but what does it have to do with data centers? Recently I saw a product announcement on LinkedIn that Piller offers a high-speed compact flywheel based UPS that can deliver a ride-though time of one minute at one megawatt, and I commented that we seemed to have entered the age of the “Megawatt Minute.”
While flywheel energy storage is not new to the data center market, here in the U.S., it has received limited acceptance compared to the more traditional battery backed UPS. More recently, Li battery technology seems to be making some in-roads, as evidenced by many of the major UPS manufacturers which are now promoting it as a factory supplied alternative to the classic lead acid battery, the long standing “industry standard.”
The use of flywheels for data center applications falls into two general categories. First, the Rotary UPS, a large heavy low-speed unit (directly coupled to an electric motor fed by the utility i.e., a motor-generator). There is also the diesel rotary UPS (DRUPs), wherein the AC motor-generator is also coupled to a diesel engine (via a clutch), all on the same shaft should power fail. The second category is the high-speed DC flywheel used to replace the battery on the DC bus of the UPS. Regardless of type, they typically store 15 to 60 seconds of energy, but can deliver a power in a much smaller space than a VRLA battery.
Last year I commented on a VRLA battery vendor promoting their UPS battery for applications of 5 minutes or less of ride-through, which seemed odd. So we seem to have some sort of divergence in the thinking regarding UPS back-up time. While short energy storage times (battery or flywheel) have been used by the internet hyperscalers, only a few enterprise and colocation providers such as DuPont Fabros (now part of Digital Realty), use flywheels for some of their U.S. facilities. One on the major advantages is that a high-speed flywheel has a high-power delivery density (size and weight), and it can repeatedly discharge its stored rotational kinetic energy in a very short period (15 to 60 seconds) without any chemical deterioration, unlike VRLA batteries.
Most people tend to use the words power and energy interchangeably, even those who should know the difference. Power is an instantaneous measurement (kW), whereas energy represents power delivered or consumed over a period of time (kWh). This distinction is particularly important when discussing battery characteristics. The density is expressed as physical size as well as weight. Depending on the design and chemistry, battery power and energy density do not directly correlate to each other. For example, a battery may be able to deliver a high amount of energy over a long period of time (i.e., 10 hours), yet its energy capacity will be more limited when delivering a high amount of power over a very short period (5 to 15 minutes). This is the common non-linear discharge characteristic of lead acid batteries; the amp-hour rating (AH) is typically based on a 10 hour a C/10 (or even 20 hour C/20) discharge rate.
However, as the load current (power) increases, the effective energy capacity is severely reduced. For the typical data center UPS application, a battery with 100 AH rating (10 amps x 10 hours) may be able to deliver only 25% to 30% of the C/10 rated capacity when it is tasked to discharge its stored energy in 5 to 15 minutes or less. Example: 120 amp current at 0.25 hours or 250 amps at 0.1 hours, discharges until it reaches its low-voltage cut-off point (typically a “2.0 Volt nominal per cell” will drop to approximately 1.75 volts). Result, a 12 Volt battery will go down to 10.5 Volts at UPS shutdown to avoid battery damage.
This energy capacity reduction is further exacerbated as the UPS battery voltage drops since the current demand increases to maintain the same power levels on the DC bus feeding the UPS inverters. Example: a 480 Volt-240 cellbattery string decreases 10% to 432 V (or lower) as it discharges. This will result in current draw increasing by 10% or more in order to deliver constant power to the load. And while these are simplified examples, this results in needing larger battery capacity to ensure enough energy storage capacity at high-power discharge rates for data center applications. In contrast, Li battery voltage will remain relatively constant under load until they near the very end of the discharge curve even under relatively high discharge rates. Also, this comparison does not factor in the age and condition of the VRLA batteries. This is why in many cases the requirement calls for two or more parallel strings of VRLA UPS batteries to minimize the risk of failure of a single string of batteries.
Li batteries are also being promoted as having a longer life than typical VRLA batteries as well as higher temperature ranges, promising lower long-term TCO, but at a greater upfront cost compared to VRLA installations. However, Li batteries are relatively new to the data center, and I, like many of you, have been waiting to see how well Li UPS batteries will perform over an extended time in actual data center operating conditions.
While Super-Cap technology has been around for a long time, it has not been given much serious consideration for data center applications, since like a flywheel it offers a relatively short ride-through time. However, some major UPS manufacturers such as Eaton offer Super-Caps as an alternate to conventional batteries. Their basic Super-Cap module is approximately 7- x 7- x 35–in. rated at 62 Volts and weighs 35 pounds, yet can deliver up to 2,000 amps. It can operate at much wider and higher temperature ranges (-40°F to +150°F) than VRLA and Li batteries with a predicted lifespan of over 15 years with little or no maintenance.
While I do not have any cost estimates, Eaton claims it has a lower TCO than flywheels with comparable power delivery and energy capacity. In comparison, a conventional VRLA battery cabinet would require a minimum runtime capacity of 5 minutes, with a minimum of two strings, to ensure it would reliably support a 100 kW load, especially when batteries near the end of 4 to 5 years of their typical operating life. Super-Caps, like flywheels, also promise lower TCO over a 15-year period since VRLA batteries would need to be replaced and recycled three or more times over that period. In the end, the philosophy of Super-Caps are similar to flywheels based on the assumption that the back-up generator will start and pick-up the load in 30 seconds or less. Both Super-Caps and flywheels could also be used to extend the operational life and reduce the sizing of VRLA batteries by buffering any short-term power anomalies, which would normally cause the UPS to frequently switch to battery, severely reducing battery life.
This past year there was a lot of attention on the micro and edge data centers. They are expected to meet the demand and optimize localized network traffic, expected from IoT devices, as well as the early deployment of 5G networks. They will need to incorporate some form of power backup in very dense packages, which will benefit from newer energy storage technology.
While I have been discussing short-term ride-through to cover utility outages until the back-up generators start and pick-up the data center load, there is a great deal of interest and deployments in grid-level energy storage as a method of improving grid peak capacity and overall reliability. In addition, they also improve the ability to incorporate sustainable, but intermittent energy sources such as solar and wind as they become more common.
This past year there have been multiple announcements of multi-megawatt grid energy storage using Li batteries to support peak loads, thus minimizing the need to fire-up natural gas “peaker” plants. An interesting example is the recent announcement by GE:
Southern California Edison (SCE) and General Electric have taken a grid edge concept — integrating natural gas-fired turbines and batteries. SCE started operations at the second of two of its new hybrid electric gas turbine (EGT) units — GE’s term for its combination of turbines, batteries, and power controls installed at the two sites. Each peaker plant is in the 50-megawatt range, and is outfitted with a set of batteries capable of providing 10 megawatts and 4 megawatt-hours of power.
Another grid scale energy storage technology that is being deployed is the Vanadium Redox Flow Battery, in which the energy is stored in a fluid (which flows between two tanks), as it charges and discharges, which can be seen in this IEEE article at .
China is building a 200 MW plant using this technology. While this example is about grid level energy storage, with colocation and web-scale data center 100 MW campuses regularly being developed on hundred acre sites, it does not seem very farfetched that they may shorten ride-though times for their UPS (or use no UPS) if they can have their own multi-megawatt energy storage system on-site or as a dedicated resource via contractual relationship with energy providers.
So while reliability and availability as well as the presumed number of “9s” have been the cornerstone mantra of the data center culture, will shorter UPS ride-though become more acceptable as new energy storage technologies become more reliable and commonly deployed?
My data center prediction for 2018 is simple: there seems to be a number of organizations battling over data center availability standards, see “Tier Wars”. So while not all data center operators may be ready for the “Megawatt Minute” for their UPS back-up time, every organization will need to view their data center requirements for “fitness for purpose” based on their own business requirements.
Finally, this is the time of the year when shopping can become a stressful and rushed experience. Be careful when speaking to your favorite voice activated personal assistant for any last minute gifts, and look out for drones before going out to your doorstep. In any event, have a Green Holiday Season and a Happy Sustainable New Year!