IEEE Electrification Magazine - June 2016 - 35

comparison to that in the dc grids. This is convenient in
that the ac infrastructure is changed minimally.
A variant of this is to carve out a portion of the ac
infrastructure that can be powered from the dc side during times of grid failure, through the inverter, as shown in
Figure 6(c). This adds an islanding switch to assure that
power does not flow to the utility grid when it is not operating. There does need to be a mechanism to extend the
price-based management of the dc grids to the reliable ac
domain, to ensure that supply and demand can be best
balanced. In all of these cases, dc systems are reliable,
and easy reliability is a key benefit. In LPD, the degree of
reliability can be varied from nanogrid to nanogrid and
device to device. Changes to the ac infrastructure are
minimized as much as possible to minimize costs. A
long-term principle that can be adopted is that all devices
for which power reliability and quality are of particular
concern should be dc powered. This
would mean that no capital or other
expense would need to be expended
to make ac infrastructure reliable,
and over time, the utility grid itself
could optimally be tuned to lower
reliability and quality levels, saving
considerable capital and energy.

DC Deployment Paths

all at once. The evolutionary approach allows modest
investment and risk to allow dc to prove itself in cost
effectiveness and other benefits, so that financial and
other risks are minimized. DC technology is very much in
flux, and so for many end uses in many buildings it is premature to convert to dc. However, this need not preclude
installation of dc devices for which the merit of doing so is
clear today. DC devices can be introduced with external
central or local ac-dc conversion, and dc sources added
later to produce direct dc.
This model of technology deployment is familiar for
information technology (IT) systems, where conversion of
functions from analog to digital, or upgrading or adding
new functions, is generally done on an ad hoc basis rather than through single large-scale upgrades. The nature of
IT networking is such that swapping in new hardware
and connectivity is not as burdensome in the way that it
is for modifying traditional electrical
systems. This is another way in which
networked dc is inspired by architectures and capabilities of IT.

The economics
of daisy-chaining
devices on a power
cable are significant,
and developing ways
to accomplish this in
the context of LPD
will be valuable.

One approach to dc deployment is to
install a large amount of direct dc
infrastructure at one time. This is
possible for new construction or
major renovations, but for most
buildings it is impractical in the near
term. An alternative is to slowly evolve toward major
use of dc and direct dc in a building, through a series of
many steps. DCs can thus be deployed incrementally
and organically and can be introduced as opportunity or
need dictate. The evolutionary approach can be used in
circumstances such as those in the following examples:
xx
spot reliability; when there is a need or desire to make
some devices reliable in the face of a grid outage,
installing direct dc infrastructure can be done for
those particular devices, rather than for a large part of
the building
xx
modest remodeling projects; small projects can be
used to introduce dc hardware
xx
large device replacement, such as an appliance or climate control system; although it may not make economic sense to replace ac devices that are functioning
properly, it is a much smaller hurdle to shift to a new
dc device rather than buy a new ac device
xx
occupancy changes; occupant needs change over
time, as do the occupants themselves, introducing
more opportunities for changed infrastructure.
A building owner may be interested in dc power but
reluctant to move a lot of infrastructure over to it at all or

Reliable Communications

Another current opportunity for dc
distribution is in reliable communications. The U.S. Federal Communications Commission (FCC) has been
concerned in recent years with ensuring residential communications continuity during utility grid outages. For
many decades, telephones were reliable in such circumstances because
they were very low-power devices
powered through the communications wiring from the
telephone central office. Today, communications is more
commonly a combination of text messaging from mobile
phones, e-mail from PCs, and voice-over IP phone calls,
requiring a combination of modems, handsets, network
equipment, mobile phones, and computers, all of which
must be continuously powered or periodically recharged.
It is possible to supply ac power to these devices via a
generator (or battery), but this would be cumbersome and
relatively expensive, particularly given the relatively low
power levels involved and the expense of hardware to
automatically make the switch and ensure continuous
power delivery. Another alternative is to place batteries
into each end-use device, but this would make them costlier, bulkier, and more failure prone. A better solution is to
power all devices needed for communication via USB
from a central hub, eliminating ac-dc conversions within
each device-because all of the involved devices are dc
internally. The hub could have a battery recharged from
the ac power and, even better, have a link to a PV panel to
provide power when the utility grid is down and displace
grid power when it is up. USB could add a mechanism
to  indicate when power is scarce so that the end-use
IEEE Electrific ation Magazine / j une 2 0 1 6

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