Re-calibrating Efficiencies vs. Utilization

The Green Grid released a Quantitative Efficiency Analysis Of Power Distribution Configurations For Data Centers document late last year, outlining the best case conversion efficiencies of various power distribution options from 480Vac, 3-phase input down to a 12Vdc intermediate voltage rail. As a member of the power task force that created this document, I can attest to the many hours spent analyzing various data sources and finding a way to present the information as accurately as possible. What we ended up with was a document showing best-case efficiencies of all components in this conversion chain, using the best data we could publish from either vendors or third-party sources.

As good as this document is, there are some obvious next steps to be done to further refine the analysis and make it more relevant to real-world conditions. Probably the biggest area that needs to be addressed is better information on load levels. When we look at a graph of efficiency vs. percentage load, it's all too easy to look at the upper part of this graph and discard the lower loads as meaningless. Unfortunately, it's in these lower load levels that most data centers tend to live. Let's look at an example:

• Suppose a data center (or a portion of a data center) is anticipated to ultimately need 800kW of actual peak capacity of critical load backed up by a UPS (uninterruptible power supply).
• That same data center will be filled up incrementally over the next three years, with the electrical load doubling each year while it is being filled.
• For high reliability, the UPS system deployed in 1+1 configuration, with the outputs routed to a common bus bar that feeds redundant PDUs (Power Distribution Units); the PDUs provide redundant feeds to chassis power supplies, which provide redundant power to the blades in the chassis.
• To meet electrical code and company policies, 20% headroom is required in the power system to handle any unanticipated loads or planning/projection oversights.

The 20% headroom on an 800kW critical load drives the requirement for a 1MW UPS. [If the next largest size UPS is 1.2MW, the UPS would start out with an extra 200kW of unusable capacity; for the purposes of this comparison, however, let's assume that there's a perfect match available with a 1MW UPS.] The 1+1 redundancy requirement means that two 1MW UPS units will be required; under normal circumstances, these will share the load between them. Thus, an 800kW load will at most drive a 40% loading of the UPS systems.

But wait, there's more! That 40% load is when all the servers, network gear, and other critical loads are simultaneously operating at their maximum power draw. This rarely, if ever, happens (though special events can cause it to happen, so you have to be able to handle those events without losing power). Under normal circumstances, the daily peak load could be 70% of that value or less. If traffic in that data center follows significant daily cycles (i.e., high demand during the work day and minimal traffic at night), power consumption in the data center could be less than 50% of the potential peak.

Utilizing 50% of that 40% maximum UPS load puts overall load in the 20% of rated capacity range. Given the projected population of the data center over time, the data center manager could find themselves with power infrastructure operating 5, 10, or 20% of rated capacity on average. All of a sudden those esoteric numbers at the left edge of the efficiency vs. load graphs start to become much more important.

My advice to facility planners is to ignore the right-hand 2/3 of the efficiency graphs. Even in a well-designed operation with redundant systems, you're likely to spend the bulk of your time at load levels less than 30% of the rated infrastructure capacity. Focus on the numbers on the lower end of the load curve.

--kb