The energy efficiency of your data centre UPS is important for two critical reasons. The first is running costs and the second is related to the battery runtime available when the mains power supply fails. An undersized UPS is often immediately obvious as load is applied on start-up but may not always be so. Intermittent overloading can occur if there is marginal capacity for extra load. Oversizing, can also be an issue whilst providing what can prove to be a false sense of security.
Today’s online datacentre UPS are highly energy efficient compared to those of 10-20 years ago. In terms of static UPS, datacentre operators typically choose to install one of three types when looking at applications under 1MVA:
For larger installations from 1MVA upwards, rotary UPS systems are an alternative option. This is a dynamic type of power supply using kinetic and stored energy to power the load when the mains power supply fails.
A datacentre-type uninterruptible power supply can have a working life of 10 years or more provided that the device undergoes regular annual maintenance and cyclical refurbishment.
Uninterruptible power supplies include ‘consumable’ items which require regular placement and within their design life. UPS batteries are the most common example and are generally of the lead acid 5-year or 10-year design life, requiring replacement around years 3-4 or 7-8 respectively and when used within a 20-25˚C ambient environment. Cooling fans are another example of a UPS consumable which should also be inspected annually and replaced according to fan manufacturer recommendations.
Around year 8 of operation a UPS system should be considered for refurbishment. The principle works here include replacement of the AC and DC capacitors and cable harnesses. The capacitors require replacement as they can dry-out and their self-healing characteristics be weakened to the point of a potential critical failure occurring. Capacitors use flammable polypropylene and at high internal temperatures can experience thermal runway and case a fire inside the UPS.
If your datacentre power system is at the 8th year of operation, it may be worth considering a complete UPS upgrade rather a refurbishment and especially if your legacy system is a transformer-based type.
An upgrade to one of the latest triple-conversion designs found in transformer-less monobloc and modular UPS systems can see energy efficiency improvements from 15-20% or more dependent on the load percentage and profile supported.
Most users associate energy efficiency with running costs, and this is certainly true with a datacentre-type uninterruptible power supply. The higher its operational energy efficiency the lower the amount of energy wasted. Energy efficiency savings also mean lower heat output and therefore less load onto the local cooling system.
Right sizing the uninterruptible power supply and load profile also helps to ensure the UPS is supporting a load profile within its optimum range. On an old transformer-based UPS system, the energy efficiency may only be 80-85% and within a highly loaded system. This is because in this older type of UPS system the efficiency profile tails off quite dramatically as soon as the load is less than around 70-80% of the UPS rating. In a more modern UPS design, a load as low as 25% can still achieve high operating efficiency. The use of a modular UPS design also helps to right-size as the ‘correct’ number of UPS models can be used in this high efficiency design to meet the load and any N+X requirements.
The energy efficiency of a UPS and its load size also affects the size of the battery required to deliver a specific runtime or backup autonomy. The formula for battery sizing is:
Battery Load (kW) = (UPS kVA x Power Factor) / UPS Efficiency
The energy efficiency of a UPS system plays a large role in sizing a UPS battery. The greater the efficiency the smaller the battery load.
Oversizing the UPS battery leads to a bigger installation space and investment cost than may be required. Under-sizing has its own downsides, not least of which is less runtime provision than may be required.
Care should be taken when assessing the stated efficiency of a UPS system. UPS manufacturer ratings should be verified to confirm whether the UPS has been tested in full on-line or eco-mode. A benchmark on-line energy efficiency rating is around 95-96% with eco-mode running at 98-99%. The issue with eco-mode UPS operation for most datacentres and server room is the lower grade of power protection provided and with potentially a momentary break in supply. On-line UPS mode provides a no-break supply and digitally generated sinewave output from a constantly running inverter to which the load is connected.
Some UPS manufacturers can also power down parts of their UPS system to improve energy efficiency. A battery charging circuit can be turned off and the battery allowed to self-discharge to a pre-set level before the charging circuit reengages.
Sizing an uninterruptible power supply should be relatively straight forward. If there is an existing load, it can be measured. However for new installations, the load needs to be estimated and this can be difficult, especially if ratings are pulled from datasheets and manuals. This is because the power drawn is nearly always shown as being the full-power rating of the power supply. In the case of a server with a 1500W power supply this could be misleading as the 1500W is the maximum power that can be supplied and not that which will be ultimately drawn. Taking this as a typical example, the surge current on power up may equate momentarily to only 80% of the 1500W rating and the true running current drop down to as low as 25-30%.
If the load is ‘steady’ with little anticipated change over time, then a monobloc UPS may be the most appropriate for an installation. The alternative is to choose a modular UPS system which by design can be more easily right-sized for any given load. A modular UPS system can also automatically adjust the number of operational modules required to support a given load at any time, thereby increasing energy savings.
UPS systems should be monitored over an Ethernet/IP network for alarm conditions. Continuously monitoring the UPS can establish load patterns over time and provide enough time to respond to issues that could affect energy efficiency and operational resilience. Two specific alarms that can indicate energy efficiency and operational issues include:
If the battery set is within a separate cabinet or room, the installation of a separate battery temperature sensor is recommended.
One other aspect to remember when selecting whether to upgrade a legacy installation or to deciding on the right UPS for a new datacentre. Capital incentives (first year tax breaks) are available for products listed on the Carbon Trust’s Energy Technology List (ETL). To be included on the ETL each product within its category must be capable of reaching threshold levels of energy efficiency in order to qualify. For uninterruptible power supplies, the tax-related incentive is available for static systems from 10kVA up to 1MVA.
More Information at: https://www.carbontrust.com/etl/products/uninterruptible-power-supplies/
Over the last two decades there has been a significant improvement in the datacentre UPS energy efficiency levels. The use of IGBT-based rectifiers and inverters has helped to dramatically lower fixed energy efficiency losses and flatted the efficiency curve out over a wide load range, from as low as 25% to 100% loaded. There have also be improvements in transformer design and the widespread introduction of modular datacentre UPS by many of the leading UPS manufacturers. Energy efficiency levels have risen from around 70-80% to around 96% for most online systems and 97-98% could eventually become the norm. However, there will always be the need to correctly size both the UPS and its battery set, monitor installations and ensure that they are correctly maintained if the design calculations are to be proven in use.
Within any power protection system, the type of battery installed can critically impact on the overall performance of the uninterruptible power supply. Aside from different battery brands, sizes, performance specifications and design lives, there are different technologies including valve regulated lead acid (VRLA) and lithium-ion. Whichever is selected, the battery installation must be reliable and cost-effective if it is to provide sufficient power to the UPS system when required and provide the best possible Total Cost of Ownership.