The electric power taken off the grid is high-voltage alternating current (AC), typically 600V or 480V, but the power used by individual components is low-voltage direct current (DC). There are usually several steps in the conversion process. Power distribution units (PDUs) do the step down to 208V or 120V AC for distribution to the racks where internal power supplies convert to the final DC voltages. If an Uninterruptible Power Supply (UPS) is used, additional conversions from AC to DC and back may take place.
Typical server power supplies operate at roughly 65% to 75% efficiency, meaning that 25% to 35% of all the energy consumed by servers is wasted (converted to heat) within their power supplies. The technology exists to achieve efficiencies of 80% to 90% in conventional server power supplies.1 Improved efficiencies obviously translate into a lower heat load from the server. Moving this heat source away from the server allows the cooling efforts to be focused on the computing elements.
Converting from one form of power to another in a computing environment may not be performed efficiently, especially at the server level, and even then the resulting waste heat may be deposited in the rack or computer room at a point that requires further effort to dispose of it with the air handlers. Unfortunately, there is disagreement in the community over how to address these inefficiencies. Advocates of an improvement of AC power conversion and distribution would eliminate inefficient AC components and maintain operations of well-understood AC infrastructure (e.g., American Power Conversion [APC] approach). Opponents maintain DC infrastructure holds the promise of larger power savings but requires aggressive infrastructure modifications that are not yet well understood. Engineers at the Lawrence Berkeley National Laboratory (LBNL) have shown that DC power distribution in a data center can save up to 15% or more on energy consumption and cost.2 Using efficient conversion and locating the conversion outside the data center require further consideration.
Substituting DC power in data centers as a replacement for conventional AC power has not yet made significant inroads into may data centers because the technology is unfamiliar to many facility engineers. Despite the wide-spread use of DC power in telecommunications, there is reluctance within the computer industry to switch to new technologies without field experience showing that the switch could be done safely and would have operational and economic benefits without causing unanticipated problems.
Understanding the trade-offs involved in the use of either technology in the data center is a primary goal of the Energy Smart Data Center (ESDC). Specifically, there is a need to evaluate the challenges of DC high current distribution, operations maintenance, and cost impact. The studies conducted so far do not consider heat extraction of power conversion and distribution electronics co-located in the data center facility.
Understanding the impact of a DC conversion and distribution grid could speed up adoption. Understanding the impact on the thermal management of the facility could enable further cost savings.
- Power Supplies – High-Performance Buildings for High-Tech Industries (LBNL)
- Burt, J. 2006. “Engineers: DC Power Saves Data Center Dough.” eWeek. August 10, 2006.