Evaluating DC vs. AC Power in Data Centers

While attending a battery-centric trade show recently, the discussion turned to DC power transmission. Some of my colleagues feel that DC power should be 24 Vdc instead of 380 Vdc, which many people prefer. They are, of course, referring to the safe, low-voltage standard for commercial and residential buildings, as opposed to the high voltage necessary for data centers and other critical facilities. Interestingly though, the conversation wasn't about the possibility of DC replacing AC - it centered on deciding what DC voltage would be become the standard for data centers.

Historically, there have been advantages of DC transmission over AC transmission, including lower line resistances than with AC, fewer losses and no reactance in the line, all resulting in higher power transfer capability. The frequency of a DC system is zero Hz, eliminating synchronization procedures when connecting to the power grid. High-voltage DC can transmit large amounts of power over long distances with potentially lower capital costs and fewer losses than AC. Depending on voltage level and construction details, losses can be a mere three percent per 620 miles with DC systems.

DC transmission is already used in the power grid to share power between two AC grids and simplify power transfer. Already, the eastern and western regions of the United States use DC links to share power without any difficulty, even if phase differences

occur between the two AC grids. Additionally, DC power can help stabilize a power grid by preventing cascading failures from spreading from one part of the grid to another. DC power allows for simpler integration of renewable energy, such as photovoltaic, because there are fewer places where a conversion is necessary to transfer from one voltage to another or from AC to DC. This results in higher efficiency, smaller size, and simplified connection of energy storage devices. However, DC is still less efficient than AC if a change in voltage levels is required, primarily because DC needs to perform a DC-to-AC rectification, an AC-to-AC transformation, and a DC-to-AC conversion. Another challenge with high-voltage DC is that it must be converted to low power once it reaches residential and industrial consumers so it can be used safely. To address this need, the EMerge Alliance developed the 24-VDC standard for these consumers – the voltage that some of my colleagues debated at the trade show.

Good use of DC power

The Internet has forced companies to get involved in data centers – whether a simple storage server room or an entire server farm – these data centers require lots of power. They also generate lots of heat – from servers and networking equipment to power distribution systems, UPS, power supplies, DC/DC converters and cooling systems. The more efficient the system, the less heat (and the less cooling) it will require, so companies are focused on implementing more efficient power distribution systems.

This leads us to DC systems - a data center using AC power typically converts power five times while a DC system usually requires only two conversions. Using DC power in a data center can help eliminate conversion steps, increase efficiency and lower electricity costs. Click on these links for a line interactive UPS from companies such as STMicroelectronics, a variety of manufacturers for AC/DC power supplies, and a TDK-Lambda DC/DC converter. 

When to make the switch

DC power to buildings and data centers is still in its infancy. So when should an engineer consider making the switch? To make manufacturers and customers more comfortable adopting this as a new technology, the EMerge Alliance and the Electrical Power Research Institute (EPRI) formed technical standards for buildings and data centers.

Test facilities, such as Lawrence Berkeley Labs, have also worked to demonstrate the value of DC by showing significant savings when using DC systems for data centers. Lawrence Berkeley Labs compared a DC system to a top quality, high-efficiency AC system and a typical efficiency system. The results showed that DC-powered servers can provide the same level of functionality and computing performance as a similarly configured and operating server containing AC power supplies.

The demonstration equipment included storage units and DC network equipment that can use a variety of DC voltages. The test project also clearly demonstrated efficiency gains by eliminating multiple conversion steps in the delivery of DC power to server hardware. Results were measured and documented from two sets of DC delivery systems and compared to two sets of AC delivery systems.

In both cases, the DC delivery system showed a minimum of five to seven percent efficiency gains without significant optimization over two best in class AC distribution systems that are more efficient than most systems found in today’s data centers. The efficiency measurements did not include additional gains from reducing cooling loads, which can potentially result in additional savings.

Research is starting to show how using an electric distribution system that is entirely DC has advantages over AC. However, DC is still in its infancy and many challenges are ahead, including securing approval and listings from safety agencies and ensuring the availability of DC vendor products. Like any new technology, there are advantages and disadvantages. DC systems can potentially save costs because they are more efficient than AC systems, but the provider and customer will need to make an upfront investment in time and money to convert from an AC system to a DC system.