Case Studies: Enhancing Reliability and
Efficiency Using Locally Generated DC Power –
The Hybrid Building.
Mr. Mark Robinson, LEED
VP Sales and Marketing
Nextek Power Systems
See the powerpoint presentation
Edison’s day Alternating Current (AC) and Direct Current (DC) have co-existed
by necessity: AC to make the trip from the generating plant and DC to power
electronic loads. This has resulted in billions of electrical compromises in
the form of the ubiquitous power supply, or a rectifier that must stand in
front of DC loads to convert AC to DC. As Arthur
Rosenfeld, California Energy Commissioner calls them, our “Energy Vampires.”
now, many buildings are generating power of their own, usually Direct Current
energy. Is this wasteful back-and-forth conversion really necessary? Just
as the automobile industry has advanced to the hybrid
car, buildings can use multiple sources of power to achieve dramatic
increases in efficiency.
Edison and Westinghouse fought the
AC/DC battles around the turn of the century. AC won because Tesla’s
transformer allowed AC voltage to be boosted for easy transmission from Niagara
Falls into the city. Edison’s DC network required unpopular ‘backyard’ DC generation
stations every few miles.
But where AC won in the transmission,
it’s DC that is now used inside almost all of our devices. You see, only DC can
be precisely regulated to get the exact voltages we need for sensitive
electronics. So our current building electrical systems are fed with AC that is
converted to DC at every fluorescent ballast, computer system power supply,
phone system, and other electronic device.
This model works fine until we bring
back Edison’s original idea and buildings begin generating power of their own –
usually DC power such as solar, fuel cell, and wind. The inefficiencies
involved with inverting to AC, matching grid frequencies, and protecting
linemen from hazards are all avoidable by creating a Hybrid Electrical System.
Inefficiencies in an
inverted solar system.
The inverter model of the
traditional solar system has several flaws:
– Inverter Efficiency.
Rated inverter efficiencies rated between 90%
and 95%, Actual field efficiencies are even less. Many inverters consume power
at night. Several models do not turn on in low light conditions.
For the protection of utility line
workers, inverters are required to shut down in the event of grid failure. This
means that, for most solar systems, there is no energy production during a
power failure (when we need it the most).
– Net Metering.
Power sent back into the grid is not
always repurchased at full cost. Sending excess power back into a sometimes
overburdened grid may not be the best way to manage the resource. Net-Metering,
as a business practice for utilities, is not sustainable and is likely further
erode the value of power sent back to the grid. Net-Metering agreements and the
meters that they require can be expensive.
– Reconversion losses.
Now that we’ve suffered the losses
of inverting, additional losses are incurred converting back to DC in the
electronic devices like fluorescent ballasts, computers, and more.
The Hybrid Solution
The theory is simple: In a building
that produces DC power of its own, use the DC power for the DC devices and use
the AC power of the grid for everything else. If more DC power is needed then
is available, take some grid power, convert it to DC, and use it to supplement
the local source.
- Efficiency gains come from the
fact the locally generated DC power is never converted and AC power from
the grid is only converted when necessary.
- The system is more reliable
because there are redundant sources of power. It is not necessary to shut
down a DC system during a grid failure like it is with an AC system.
- The system is simpler because
no net-metering or utility interconnection agreements are necessary. The
utility cannot even ‘see’ the system and, in most cases, does not even
need to be notified.
Drawbacks of the Hybrid
The only drawback of the hybrid
system is that there is no efficient provision to store excess electricity. It
cannot be sent back into the grid and re-purchased later and storing excess
power in batteries can be too expensive to justify.
The solution is to identify base DC
loads that will always be on when the system is generating. If solar panels are
the local DC source, then the local DC loads need to be on all day every day.
An ideal example of this is commercial fluorescent lighting.
Example of a Hybrid Lighting
In this example (which can be seen
live at http://www.DirectCoupling.com)
solar panels are connected to DC ballasts in the lighting.
Daytime: The solar power from the
panels is sent directly to the lighting, with no conversion, at nearly 100%
efficiency. Wiring losses are the only significant losses. The system is
designed so that, at full sun, about 90% of the power needed for the lighting
is supplied by the panels. The additional 10% is taken from the grid, converted
to DC at the NPS1000 power gateway.
Clouds: When clouds reduce the PV production,
more power is taken is taken from the grid and converted to DC. The system is
using all available power from the panels (the least expensive source) and
using the grid as the backup.
Night: When there is no solar power available,
all the power is taken from the grid and converted to DC. As we discussed
previously, a typical AC lighting system takes AC power from the grid and it is
converted to DC at every ballast. In this DC system the conversion is handled
centrally. The number of conversions is not increased.
Power Failure: In the event of a grid outage,
the lighting system continues to be powered by the solar panels and, if needed
by optional batteries. In a traditional inverter based solar system the
inverter is required to shut down, shutting off the lights.
Other suitable DC loads in
commercial buildings include telephone systems, motor controllers, computer
server systems, and more.
Current installations include
grocery stores, offices, big-box retailers, and, most recently, a Frito Lay
Distribution center in Rochester, New York.
Whole Foods, Berkeley
This 30k system powers the lighting
and was installed with Powerlight Photovoltaics. One of the primary benefits of
this system, besides increased efficiency, is the reliability aspect. Power
failures are extremely expensive for grocery stores, not because of the
freezers, but because 200 people with shopping carts full of frozen food
abandon the carts and leave the store. This creates an expensive emergency for
the store as the staff need to scramble around, reshelving the food by opening
coolers which should really stay closed. This, as well as the lost sales cost
the average small grocery store over $8,000 per five minute failure.
Shortly after the installation,
Whole foods experienced a brief power failure. The lights were powered by the
solar panels and did not shut off. Customers remained in the store.
Frito Lay, Rochester
One of the other benefits of low
voltage DC ballasts is the ease at which they can be controlled. Each ballast
has a phone wire-type connector which can be used to provide DC power to, and a
light switch for an occupancy sensor. This reduces the installation cost of an
occupancy sensor for $200.00 each to $75.00 each.
Target Stores, El Cajon,
This system uses the Nextek system for part of the store,
and an inverter for the rest. This allows us to monitor each of the systems and
compare the efficiency of both. Initial readings illustrate that the Nextek
System is providing over 20% more power than the inverter based system.
The most efficient way to utilize
locally generated power is to consume it all, where, when, and how it is
generated. We can accomplish this by identifying DC devices in a building and
powering them with the locally generated energy and use the grid as a backup.