Energy efficiency has become a key performance metric for many comms room, data centre and facilities managers faced with rising electricity costs and energy usage. ‘Eco Mode’ is a selectable operating mode available on most on-line three-phase uninterruptible power supplies. The option can improve operating efficiency by around 2-3% (for an average installation) and result in lower electrical supply costs. There are however downsides to Eco Mode UPS operation; principally a less secure power path to critical loads but the financial incentives can be very persuading.
Whatever the load or application, efficiency is essentially a measure of the total output (work) of a system as a ratio of the total inputs. A number of data centre-related energy efficiency measures have sprung up in recent years including Power Usage Effectiveness (PUE) and Data Centre Infrastructure Efficiency (DCIE). Green metric alternatives are now emerging such as GPUE which attempt to include a green element in the form of sustainable energy used and its carbon footprint. Whatever the metric chosen for internal Key Performance Indicator (KPI) monitoring or datacentre benchmarking, it is important to install the most suitable as well as the most efficient hardware systems and practices,
The On-line UPS Configuration
EN/IEC 62040-3 is the standard that defines operating topologies for uninterruptible power supplies. It is important when considering UPS within three-phase applications to understand how an on-line (Voltage and Frequency Independent) VFI system works. In an on-line (double or triple conversion UPS) the inverter is constantly powered and supports the critical load. The inverter draws its power from the rectifier which provides the right DC voltage level of power, derived from either a rectified AC mains power supply or the connected battery set (or alternative fuel cell, super capacitors or flywheel).
If the inverter output collapses due to a load short-circuit, overload or internal fault condition, the load is automatically transferred via the static switch to the AC mains power supply. The load will still be protected to a degree by built-in filtering circuits. A maintenance bypass (normally manually operated) may also be available either built-into the UPS or externally mounted to allow the UPS to be isolated for maintenance purposes.
This is the standard configuration for an on-line, double-conversion uninterruptible power supply and the schematic can in general represent both a transformerless or transformer-based UPS system.
When it comes to UPS efficiency, manufacturers of uninterruptible power supplies have invested heavily in their research and development programs. Advances in UPS design have seen a marked increase over the last decade in UPS operating efficiencies and the load range these can be achieved over. However, the rate of efficiency improvement has slowed to one where it is now marginal.
Older UPS systems would have a ‘sweet-spot’ range over which they achieved their highest efficiencies (85-90%). UPS developments in recent years have increased both the range (from 25-100%) and overall efficiency with many systems now achieving 95% or greater in efficiency. 95% efficiency means that 5% is ‘wasted’ but it is also important to consider how this lost energy is dissipated. For most UPS with high-frequency switching the losses are principally in heat which adds to local air conditioning demands.
Efficiency improvements have resulted from advances in UPS design including the use of IGBT (insulated-gate bipolar transistor) based inverters and rectifier/booster chargers and the adoption of high-efficiency transformers (in transformer-based UPS). Not only is less energy ‘wasted’ within the UPS itself through the use of more efficient components and sub-assemblies. The way modern UPS now draw power from the connected AC power supply has also helped to improve their operational efficiency. Principally here we are looking at UPS with a high input power factor of 0.9 to 1 (Unity) which more closely matches the waveform of the AC power supply itself.
Further potential to improve operational efficiency (to 98-99%) is also available from selecting Eco Mode operation as opposed to full on-line mode.
Eco Mode UPS
Eco Mode is a method of operation available with most on-line UPS systems today. It may be known as Smart-Boost, Green Boost or eBoost but essentially the mode of operation mimics that of a Voltage Independent (VI) of Voltage and Frequency Dependent (VFD) uninterruptible power supply as defined by EN/IEC 62040-3. This is more commonly known as Line Interactive (VI) or Standby (VFD) mode with VFD providing the least protection in terms of power quality protection and the longest transfer times.
Eco Mode relies more heavily on the static switch as the load is essentially powered directly from the filtered mains power supply. The Inverter is energised but only supports the load when the mains power fluctuates or fails completely. For most UPS there is a zero or minimal transfer time during which the voltage supply to the critical loads will drop but most server Switch Mode Power Supplies (SMPS) can easily accommodate this due to their inherent capacitance. Adaptive ‘intelligence’ within most modern UPS designs means that uninterruptible power supplies operating in Eco Mode will automatically select whether to stay in Eco Mode or full On-line Mode based on their assessment of the mains power supply characteristics and stability.
Operating in Eco Mode will lead to a higher electrical efficiency. The larger the UPS system the greater the energy savings and the faster the Return on Investment (ROI) against the initial capital cost. So why doesn’t every datacentre and facilities manager adopt this energy saving UPS mode?
The Downsides to Eco Mode UPS Operation
Firstly, operating in Eco Mode can lead to a more complex critical power path for electrical engineers and facilities managers. The mode may not be suitable for all locations and load types. To achieve the maximum energy savings the site must have a clean and stable electrical supply and low harmonic generating loads. Eco mode operation may not be available or suited to supplying loads with leading power factors such as Blade servers and other loads on site. Its implementation can also lead to ‘hunting’ and and poor generator synchronisation.
Secondly, Eco mode lowers the overall resilience of a critical load installation. For a Tier 1 installation (Uptime Institute) with a single UPS system available, there could be a back-up power transfer delay. In Eco mode, critical loads could be exposed to power breaks and extra consideration may need to be given to mains pollution in the form of surges, spikes and electrical noise suppression.
Thirdly, Eco mode has resulted from the UPS manufacturers seeking a competitive advantage they can publish. This is no different to any other supplier into the datacentre arena. Everyone supplies the ‘greenest’ solution. In the UPS world, efficiency improvements are very much like the lap-time savings in Grand Prix Formula 1 racing. They are marginal and each manufacturer adopts very similar design philosophies and practices.
There is simply no definition for a ‘high efficiency’ UPS system and so any figures provided have to be taken with a ‘pinch of salt’ unless they are audited by a third body. In the UK, the Carbon Trust can provide some assistance here in the form of their Energy Technology List (ETL). For inclusion on the List, UPS manufacturers have to provide efficiency data at 25/50/75 and 100% loads. Once on the ETL, they have a marketing advantage over competitor. Purchasers of their systems can claim enhanced capital allowances from the UK tax authorities that can allow them to write of the value of their UPS investment against corporation tax. The additional benefit to the UPS purchaser is that the efficiency figures are reviewed by the Carbon Trust and the manufacturer could be audited with the threat of being ‘struck off’ if they cannot justify them. Power Protection Consultants like EcoPowerSupplies can also provide some form of third party verification. Being independent means that a Power Protection Consultant is not tied to any one brand or system and simply wants to get the ‘right’ system installed to meet the complete selection criteria provided by the client.
Choosing Eco Mode Operation
If you are a datacentre operator, manager, designer or consultant just how to do you decide on whether Eco Mode UPS operation is right for you? Three factors are crucial to consider. Firstly, the transfer time of the UPS internal bypass and inverter arrangement. Without a ‘zero-transfer’ time critical loads will be exposed to potential power breaks. Secondly, the ride-through capacitance of your critical IT loads. Not all server loads can withstand the potential transfer time they could be exposed to. Finally, the quality and stability of the local AC mains power supply. If you site is has known power problems and suffers frequent power breaks, the UPS will automatically operate as a true on-line VFI system. Operating in Eco Mode will be limited to those periods when the mains power supply becomes more stable. Greater Energy savings from running in Eco Mode as opposed to On-line mode will therefore be minimal.
In summary, Eco Mode as a form of UPS operation can be useful for certain datacentres and facilities but this may not always be the case. Whilst a 2-3% saving in energy consumption can seem attractive, energy efficiency is just one part of the power protection mix. Overall reliability and long term operational and consumable costs (e.g. batteries, fans, capacitors) should also be taken into account. The benefits from selecting a UPS purely on efficiency can easily be lost if the installation suffers downtime or server failure from being exposed to a polluted mains power supply.