IEEE Electrification Magazine - June 2016 - 30

diode (LED) lighting and increasing loads from electronic devices such as computers and displays. Some devices, including fluorescent lighting and variable speed
drive motors, convert ac to dc (and then to high-frequency ac), so are also dc internally. A large portion of
the remainder of loads in buildings could readily be
converted to dc at equal or greater efficiency than with
ac. Vehicle charging with dc is not
the norm, but it is available.
DC distribution today is highly
successful in specific niche applications but is rarely used outside of
them. It is used in 12- and 24-V distribution in vehicles, USB for mobile
device charging, Ethernet for desktop phones in office buildings, and
off-grid buildings; these are all very
specific applications in which dc
distribution and use have advantages over ac. In data centers, 380-V dc
is seeing increasing uptake. The use
of dc may not be competitive today
for mainstream or generic use, but
that is beginning to change with the rise of lighting
powered by Ethernet.
The use of dc distribution in buildings is growing, primarily at the very edge of the building power-distribution
tree. Power over Ethernet port shipments has consistently
risen during the past decade, from under 10 million/year
in 2005 to more than 100 million/year in 2015. With the
expected increase soon in Ethernet power to nearly 100 W
(more than six times the original power limit), many more
devices will be brought into the scope of being dc powered, and shipments should thus grow even more rapidly.
USB annual port shipments are approaching 5 billion.
With USB recently enhanced to provide 100 W, it also can
now support many more product types than before. With
this capability, USB can now provide 40 times as much
power as it did when originally introduced more than
20 years ago.
There are three basic approaches to power-native dc
end-use devices, with two involving dc distribution. Many
buildings contain two or even all three of these architectures in different places. In the present paradigm of building operation (ac distribution), the source of all power is

originally ac from the grid, with conversion to dc occurring
inside of each dc device (with an internal or external
power supply). There is no alteration or addition to the
building wiring. This is shown in Figure 1(a). The second
option, central dc distribution, moves the location of the
ac-dc conversion to a central device and distributes dc to
end-use devices, as shown in Figure 1(b). This may be
more efficient [i.e., save energy
compared to the scheme of Figure
1(a)] in some uses because it reduces
the number of front-end components
that must engage in power conditioning to deal with variations on the
input ac bus and provide stable dc
output. Central dc distribution is often
used to obtain cost savings, greater
reliability, and convenience. If any
local power generation and/or storage
are present, they are connected to the
ac infrastructure.
More interesting and useful are
power architectures in buildings with
local generation [most commonly,
solar photovoltaic (PV)] and local storage, as shown in the
right of Figure 1(c). Direct dc enables power to flow from
generation to end use-through storage as needed-without ever having to be converted to or from ac. This saves
energy from the avoided conversion losses and saves capital by requiring less conversion hardware, thus saving
money from both. Direct dc increases reliability by avoiding potentially unreliable conversion hardware and by the
presence of battery storage, because storage is less expensive to include in an all-dc system. Savings estimates
from direct dc vary dramatically, from 2 to 14%, in part
due to differences in the application contexts and baseline assumptions.
The ac-dc conversion in direct dc can be bidirectional,
to enable exporting excess power out of the dc domain;
this provides operational flexibility. A simple alternative
is to install a unidirectional interface, as shown in
Figure 1(c), in which electricity is never exported from the
dc domain; we call this "semidetached direct dc." Power is
imported across the ac-dc link as needed, but for long
periods of time no power may be flowing across this link.
In this model, a unidirectional link means that dc generation and storage are effectively
invisible to the utility grid; the dc
system is just another load. This
ac Grid
dc Generation
ac Grid
ac Grid
approach should reduce permitting
and regulatory burdens of installing
Storage
direct dc systems. In cases in which
more generation exists than can be
ac Loads
ac Loads
ac Loads
stored or used for ordinary purposdc Loads
dc Loads
dc Loads
es, inexpensive resistance heating
(a)
(b)
(c)
of space or water could productiveFigure 1. The (a) ac distribution, (b) central dc conversion, and (c) direct dc.
ly use excess power.

Savings estimates
from direct dc vary
dramatically, from 2
to 14%, in part from
differences in the
application contexts
and baseline
assumptions.

30

I E E E E l e c t r i f i c ati o n M agaz ine / j un e 2016



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