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High Voltage Direct Current
Power Solutions for Data Centers
Professor: Dr. Danny Silver
Acadia University
Student Name: Jim Nasium
Published: April 10th, 2011
1. Introduction
As the demand on data centers increase, the required processing power draws a
substantial amount of power. Most data centers still operate on standard 120-240 volts
alternating-current (VAC), but research and progress has been made to mitigate the
demand for electricity by redesigning the way power is carried to the servers. By
distributing electricity to the servers using direct current, significant power savings can
be attained. This paper will give a brief history of the modern power grid before delving
into the present research and results of using direct current in data centers. The work
required to standardize high-voltage direct current (HVDC) in data centers and the power
sources best suited to supply this power will follow the research results. It should be
noted that a significant portion of the results in this paper are directly associated with
research performed at the Lawrence Berkely National Laboratory as part of its “HighPerformance High-Tech Buildings” project1.
2. History
In the 1800’s when electricity was in its infancy, there was a great debate between the
viability of direct current versus alternating current. Thomas Edison’s direct current grid
proved to be challenging to distribute over long distances. Small generating stations were
built every few blocks because greater distances proved to have too be inefficient and
power losses too high. Early generators were limited to low voltages2, which resulted in a
high current and high losses. Nikola Tesla’s Alternating current system proved to be
efficiently distributed over long distances, making it the obvious winner. It wasn’t long
1 LBNL (2007) 2 EPRI (2006) before city streets were lined with electric lamps and homes furnished with incandescent
bulbs and electric heating.
3. Present Grid
The present grid is an elaborate web of power lines feeding every home and industry.
While lighting, heating, and motors are still the primary consumer of electricity,
electronics are becoming ever more thirsty. Electronics are unique from other devices
because they require direct current to operate. This poses an issue that hasn’t been
considered since the advent of the grid. To feed electronics its direct current, switchedmode power supplies3 (SMPS) convert alternating current to direct current and drop the
voltage to the required level. Consumers recognize these devices as the blocks called
power adapters for laptops, or the power supply attached to a power cable on a desktop
computer. By 2004, 2.5 billion SMPS were supplying direct current to devices of all
shapes and sizes in the United States alone. The power required for these devices totaled
207 billion KWh, or 6% of the total electricity demand in the United States.
4. AC Power in Data Centers
Data centers are built for the sole purpose of housing large arrays of servers. These
servers are the backbone for almost every large institution today. The power required for
a single data center can surpass 15 MW4. In California, data centers use 14.6 TWh in a
year5 and rising. Electricity enters the facilities at either 600 VAC or 480 VAC before
3 EPRI (2006) 4 EPRI (2006) 5 LBNL (2007) getting stepped down to either 208 VAC or 120 VAC for use by servers, lighting and
more.
Even though alternating current is pumped into the building and servers are plugged into
standard alternating current, it is not the sole intermediary. Six or more conversions are
required between AC and DC from the grid to the server6. The first conversion takes
place at the uninterruptable power supply (UPS). The UPS provides a grace period for the
generators to kick in and are in series between the grid and the servers in order to
maintain a constant flow of electrons. It is comprised of an inverter, a switch, controls
and batteries. Batteries are inherently DC, so the inverter converts the grid’s AC to DC
for the batteries.7 The electricity is then converted back to AC for distribution to the racks.
At this point, some racks convert the AC to DC for use in the servers, but as discovered at
the Bell Data Center in Halifax, this is still not common place. Instead, every server has
an internal power supply that converts the electricity back to DC of select voltages for
internal components.
4.2 Losses in AC Data Centers
Converting electricity between AC and DC is an inefficient process because even the best
inverters are only 90-95% efficient8. Approximately 12-16% of all energy consumed by
servers are wasted in the conversion between AC and DC9. Figure 1 illustrates the
severity of the loss within the server itself, since 131 watts of a 450 watt server is lost
simply in the conversion between AC and DC. It is the single largest user of power in the
server. In a 10-15 MW data center, 2-3 MW may be lost by power conversions alone.
6 LBNL (2007) 7 EPRI (2006) 8 LBNL (2007) 9 EPRI (2006) The energy that is lost in conversion is lost to the surrounding environment in the form of
heat. Removing the heat requires fans within the servers as well as large ventilation and
Figure 1 Power for a typical server consuming 450W (Leonard Energy, 2008). Figure 2 Typical data center power distribution (Leonard Energy, 2008). cooling units. Figure 2 shows how the cooling unit consumes almost as much electricity
as the servers themselves, at about 33% of all electricity. As mentioned earlier, the UPS
requires two conversions between AC and DC, which draws 18% of the power. The heat
lost from these conversions contribute to the 33% of electricity required for the chillers.
5. DC Power in Data Centers
Technology is emerging that could allow data centers to be designed to run exclusively
on DC power. By going DC, data centers could reduce their losses by 10-20%.10 In the
present era of blade-servers, data centers are faced with a growing challenge of powering
and cooling these tightly packed machines. Direct current offers a long-term solution to
the power density issues related to the emission of heat from power supplies.
10 EPRI (2006) The results from the Lawrence Berkeley National Laboratory are presented in Tables 1 &
2. Table 1 compares two AC systems to one DC system, all of which containing a top-ofthe-line, high-efficiency UPS, transformer, and server power supplies. Table 2 gives a
realistic evaluation because the equipment used in the test are closer to the equipment
used in a typical data center. As seen in the tables, a high-efficiency DC data center will
show a 7% improvement in efficiency over a high-efficiency AC data center. In
comparison, a standard-efficiency DC system showed a 28% improvement over a
standard-efficiency AC system.
Table 1 High Efficiency AC vs. DC Efficiency Comparison (LBNL, 2007).
Table 2 Typical AC vs. DC Efficiency Comparison (LBNL, 2007).
5.2 Rack-Level DC
One way to reduce the heat produced within the racks is to have one inverter in each rack
that converts 208/120 VAC to 380 VDC for all the servers contained within it. 11 This
concept is great for business who are still using AC because it allows for the gradual
migration to a DC environment as more racks are installed in the facility. While the racks
remove the heat dissipation dilemma within the racks, nearly the same amount of heat
must be carried by chillers to the outside environment. Furthermore, the UPS system
remains shrouded by an AC environment where multiple AC/DC conversions are
required. Eventually the majority of the data center will contain servers designed to
operate on DC, at which point it would be a simple process to flip the center to a facility
level DC system.
5.3 Facility-Level DC
A facility-level DC system is the most efficient design because only one AC/DC
conversion is required. In this design, 480 VAC is converted into 380 VDC before
reaching the UPS and distributed throughout the facility12. Potentially, the one inverter
could be placed in a weather-proof enclosure and placed outside so less cooling is
required within the building. The DC system is potentially more reliable than AC because
there is only one inverter to maintain (plus a redundant inverter), as opposed to over the
six conversions required in an AC setup. From an electrical engineering standpoint,
distributing DC power eliminates harmonics and concerns surrounding AC’s power
factor in servers.13 These terms can best be explained as occurrences centered around the
11 LBNL (2007) 12 LBNL (2007) 13 Leonard Energy (2008) conversion of AC to DC where the quality of AC is diminished as demand from the
inverters grow.
6. Integrating Alternative Energy
Integrating alternative DC energy sources such as photovoltaic panels, and fuel cells can
be integrated into the system without any AC/DC conversions required between the
power source and the server. As a result, the clean energy source becomes even more
efficient by not requiring the use of an inverter. Traditionally, these alternative power
sources would require an inverter to convert the DC to AC14. The inverters would cost
25% of the installation and would need to be replaced every 5-10 years. With no inverter
required, these alternative energy sources become more viable for data centers and can
reduce cost, increase reliability, and increase the useable power produced by them.
6.1 Solar Panels
Solar panels may be used to supplement the power from the grid. While they would not
be viable to provide the sole source of electricity to a data center, they could significantly
reduce the building’s carbon emissions. Not only would this help the business save its
carbon credits, photovoltaic arrays
can prove to cut down the energy
costs for a building if installed in
areas of frequent and intense solar
radiation. Figure 3 illustrates the
setup of a photovoltaic array
14 EPRI (2006) Figure 3 Photovoltaic integration (EPRI, 2006).
installed without the use of an inverter. Instead, a DC/DC inverter (which far surpasses an
AC/DC inverter for efficiency), is implemented to convert the output voltage of the array
to the input voltage of the UPS.
6.2 Fuel Cells
Bloom Energy recently unveiled a fuel cell that has a net efficiency of over 50%.15 It
creates electricity from gas such as Natural Gas or Biogas as its fuel source. Its current
model has a rated output of 100kW at 480 VAC. The modular design allows for easy
expansion to multiple units. While it may seem
outlandish to think that fuel cells are affordable
enough to compete with grid power, over 25
companies have bought Bloom Boxes- including
Google and Ebay. As seen in Figure 4, a
significant portion of the box is dedicated to
Figure 4 Bloom Box (Bloom Energy, 2010) converting the DC power produced by the fuel
cells to AC. If a model were available that
replaced the AC/DC inverter with a DC/DC inverter, it would increase the net efficiency
of the Bloom Box considerably.
7. Advancements Required for DC Implementation
While there are servers available at 48 VDC because of the standard 48 VDC
telecommunications equipment, there is currently no 380 VDC native servers on the
market.16 Part of the challenge is that there is no standardization agreed upon by the
15 Bloom Energy (2010) 16 LBNL (2007) industry. After agreeing upon a distribution voltage, a power cord and plug must still be
agreed upon. Circuit breakers designed for 380 VDC are scarce because of the lack of
demand for them, and very few have been certified by UL.
8. Conclusion
Building or retrofitting a data center to run its UPS and servers exclusively on DC has
proven to be possible and economically viable. A data center running a HVDC system
can save upwards of 10-20% on its electricity bill. Not only is there less of an initial
investment because of the fewer parts required in a DC system, but the elimination of
AC/DC inverters means there is less work for the chillers. If a data center is already built
using AC infrastructure, new racks should be built with DC servers installed. After all the
servers are replaced with DC servers, or if a new data center is being built, a facility-level
DC system should be considered. If the demand is there, new renewable energy sources
(such as the Bloom Box) will rise to the challenge of providing an entirely DC data center.
The more demand there is for DC servers, the more manufacturers will produce DC
equipment.
References: Bloom Energy. (2010). Product Data Sheet. ES-5000 Energy Server. Downloaded from
http://www.bloomenergy.com/products/data-sheet/. Page viewed 2011-04-09
Electric Power Research Institute. (2006). An EPRI White Paper. DC Power Production, Delivery and
Utilization.
Jong & Vaessen, Leonardo Energy. (2007). Briefing Paper. DC Power Distribution for Server Farms.
William Tschudi, Lawrence Berkeley National Laboratory. (2007). DC Power for Improved Data Center
Efficiency.