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Understanding UPS Efficiency for Data Centers: Part 1
In our technology-driven world, data centers are the backbone of industries like e-commerce, finance, healthcare, and communications, relying on uninterrupted power to safeguard operations and data. A critical component in maintaining this reliability is the Uninterruptible Power Supply (UPS) system. Despite its essential role, UPS efficiency often goes overlooked, even though it significantly affects operational costs and sustainability. This blog delves into the importance of UPS efficiency, how it is calculated, and the key factors influencing it in data centers.
In today's technology-driven world, data centers play a crucial role in supporting a variety of industries, including e-commerce, finance, healthcare, and communications. As the backbone of digital infrastructure, data centers require reliable, uninterrupted power to avoid costly downtime and protect valuable data. One key aspect of ensuring this uninterrupted power is through Uninterruptible Power Supply (UPS) systems. Despite its crucial role, the efficiency of UPS systems is often overlooked, even though it has a significant impact on both operational costs and sustainability.
This blog will explore what UPS efficiency is, how it is calculated, and what factors influence it in the context of data centers.
What is UPS Efficiency?
In the context of UPS systems, efficiency refers to the ratio of energy delivered to the load compared to the energy absorbed by the UPS from the mains grid or battery system (or other energy storage system). In terms of technical performance, efficiency measures how effectively the UPS system utilizes the absorbed energy to power and protect the load. The higher the efficiency, the less energy is wasted during conversion, leading to lower energy costs and less heat generation.
This UPS efficiency metric is essential for data centers, which operate 24/7 and have high energy demands. Even a small increase in efficiency can lead to substantial cost savings over time, especially for large-scale data centers. Moreover, improving efficiency aligns with the growing industry emphasis on sustainability and reducing the carbon footprint of operations.
Over the past 40 years, advancements in power electronics have resulted in substantial improvements in UPS efficiency. These improvements are reflected in the design and development of new UPS systems, which are more energy-efficient and capable of handling increasing demands in data centers.
Image 1: Cost Savings Due to High-Efficiency UPS Systems in Data Centers
How is UPS Efficiency Calculated?
Calculating UPS efficiency is straightforward; it’s the ratio between the output power and the input power, typically expressed as a percentage. However, it depends on whether the system is running on mains power or batteries.
On Mains Power: When a UPS system is powered by the mains, some input energy is used to charge the system's batteries. However, this phase is temporary, and usually excluded from efficiency calculations. For state-of-the-art UPS systems, the energy used for battery charging is minimal and has negligible impact on overall efficiency.
On Battery Power: When the UPS runs on battery power, all the energy is drawn from the batteries. In this case, efficiency is calculated based on how much energy is delivered to the load compared to how much is taken from the batteries.
These values are typically measured using high-precision watt meters connected to both the input and output of the UPS system when it is running. Manufacturers usually provide data for both modes of operation in their technical documentation to help operators assess efficiency.
The efficiency is calculated as:
‘Output Power’ is the actual power delivered by the UPS to the connected load (in watts or kilowatts), and ‘Input Power’ is the power drawn by the UPS from the utility source or battery system.
Factors Influencing UPS Efficiency
Several factors can impact UPS efficiency:
Load Level and Type: UPS systems tend to operate most efficiently when running at a higher load (closer to their capacity). Efficiency decreases when systems are underutilized, as idle components still consume power.
Power Factor: A power factor close to unity means the system uses power more effectively. UPS systems with a higher power factor reduce reactive power, improving overall efficiency.
Ambient Temperature: Temperature affects the performance of the UPS and its components. Higher ambient temperatures can reduce efficiency by putting thermal stress on the internal electronics, leading to increased cooling requirements.
Component Aging: As components such as capacitors, fans, and batteries age, their performance declines, reducing the system's efficiency over time. Regular maintenance and timely replacement of worn-out components are essential for optimal efficiency.
Comparing UPS Topologies for Efficiency
Different topologies offer different levels of efficiency due to the various internal architecture and running modes. It is important to highlight that the topology with higher efficiency often corresponds to lower levels of protection and energy conditioning (disturbance filtering).
Off-Line (VFD): This is the simplest topology:
On Mains Run: The load is directly connected to the input, so the efficiency is very high, but the UPS does not take action to protect the load from voltage and frequency disturbance.
On Battery Run: The load is supplied by the inverter, which absorbs energy from the batteries. In this case, the efficiency is the ratio between the power supplied to the load and the power absorbed by the batteries.
Line-Interactive (VI): This topology is like the Off-Line but with the capability to stabilize the output voltage on the nominal value even if the input voltage has some variations.
On Mains Run: The load is connected to the input through an Automatic Voltage Regulator (AVR), so the efficiency is related to the efficiency of this AVR; the UPS protects the load from voltage variations but not from frequency disturbance. In this case the efficiency is still high but lower than the Off-Line Topology.
On Battery Run: The UPS works like the Off-Line Technology, and the load is supplied by the inverter, which absorbs energy from the batteries. In this case, the efficiency is the ratio between the power supplied to the load and the power absorbed by the batteries.
On-Line Double Conversion (VFI): This topology is more complex than the previous two. In this topology, there is the input stage (rectifier), which converts the AC input voltage to DC voltage, and the output stage (inverter), which re-converts the DC voltage from the rectifier to the AC Voltage, which feeds the load.
This architecture allows for the filtering of all the disturbances in voltage and frequency and recreates a very stable AC output voltage. In this topology, the bypass circuit connects the load directly to the input in case of malfunctions or overload. This means that there can be three running modes:
On Mains Run: Both rectifier and inverter run, so the efficiency is related to the performance of these two converters.
On Battery Run: The load is supplied by the inverter, so the efficiency depends on the performance of the DC/AC conversion.
On Bypass Run: Similar to the off-line topology, but the efficiency depends on the bypass circuits, in particular on the static bypass switch.
The on-line double conversion topology provides higher protection to the load compared to the previous two topologies, but its efficiency is lower because two electronic converters are continuously running.
In some cases, it is possible to reduce the protection level to achieve higher efficiency by using ECO Mode. In ECO Mode, the UPS uses the bypass circuit until the input voltage is stable and suitable for the load; as soon the input voltage goes out from the frequency and voltage safe range, the UPS immediately comes back to On-Line mode.
For critical IT applications, especially in 3-phase (3PH) systems, the need for continuous and high-quality power typically makes the online double conversion topology the preferred choice.
Image 2: Comparison of UPS Topologies
Conclusion
Understanding UPS efficiency is critical for optimizing energy use, lowering operational costs, and ensuring reliable power delivery in data centers. As technology evolves, so do UPS systems, which continue to improve in efficiency while maintaining robust protection against power disturbances.
In the next part of this blog series, we will explore the advanced technologies driving UPS efficiency and the practical benefits for data center operators.
Need help identifying the best UPS solution? Contact Legrand for an online demo or consultation here.