Generation or Energy Efficiency?
In the past, Photovoltaic Solar Panels have been a key
part of a distributed generation program. It seems obvious,
doesnt it? New technologies, though, are enabling
solar electric panels to become a part of energy efficiency
programs. This distinction can have a significant impact
on the funding of, and the future of solar installations.
The typical solar electric system consists of solar
panels which create Direct Current (DC) electricity
and an inverter which changes the DC to Alternating
Current (AC) to be compatible with the grid. When the
solar panels generate more electricity than the building
is using, unused electricity is sent back to the grid.
Utilities usually pay for this electricity through net-metering
programs. In effect, this system uses the grid as a
place to store unused electricity.
It turns out that this storage of unused
electricity is quite expensive. The cost of the inverter
and its maintenance is a factor, as is the efficiency
losses of the inverter. In addition, the net metering
programs are expensive themselves and may not be sustainable
for utilities in the future.
Inverting DC electricity to grid-compatible AC electricity
is complex and expensive. To be compatible with the
grid, the AC produced must meet strict requirements
and the inverter itself must be capable of shutting
down instantly in the event of a power failure. This
regulation, called anti-islanding protects
linemen who might be working on a downed power line
but also shuts off the whole solar electric system when
you need it the most; during a power failure. Typical
inverters consume up to 15% of the solar power generated
and carry warranties of only five years, a quarter of
the estimated life of the solar system.
Net metering is the great advantage of an inverter
because it allows a building owner to sell back unused
power. Systems can be designed so that, over the course
of a year, the electric bill nets out to
zero. But is net-metering sustainable? Is it fair to
the utilities to mandate net-metering? In effect, were
telling the utilities that they have to buy their own
product from their customers at retail. Could a grocery
store survive if it had to buy vegetables from local
gardeners at the retail price?
Many utilities have gone to a more reasonable avoided
cost structure. This means that, if you generate
electricity and send it back to the grid, the utility
will credit you whatever it costs them to generate electricity,
or wholesale cost. Its as if the grocer were paying
you for your vegetables whatever they pay the farms.
True, this sounds fair, but frankly, as a gardener,
it would make more sense for me to eat my own broccoli
then sell it to the grocer at half of what Ill
need to buy it back for later.
The first point here is that storage is expensive.
The most effective way to deal with power you generate
is to avoid storage altogether and use it all, where
and when it is generated. This means that an optimal
solar electricity system will never generate more power
than will be used. The challenge with this is that building
electricity usage changes throughout the day, as does
the availability of sun (except in California where
its always sunny).
The solution, at least for most commercial office
and retail buildings, is lighting. In most offices and
almost all large retail establishments, the fluorescent
lighting is on all day, every day, and often
uses as much as 60% of the total buildings electricity.
The optimal solar system, then, provides just enough
electricity to power the fluorescent lights.
The second point here involves the fluorescent lights
themselves, and a fact that few realize. Each fluorescent
light ballast contains a small, rather inefficient,
AC to DC converter. This means that the fluorescent
light itself is a DC device and can be powered directly
from the solar cells without an inverter. If we can
do away with the inverter (which is unnecessary anyway
because were not trying to put AC power back into
the grid), we can avoid inverter losses, maintenance
costs, and complexity. And because we dont have
to shut the system down to comply with anti-islanding
laws, we can keep the lights on during a power failure!
The concept is called Direct Coupling
of DC generation to the load. Heres how a system
works. It uses power where, when, and how (DC) it is
generated: DC power from the solar panels is sent through
a power router directly to DC fluorescent
ballasts in the lighting. When there isnt enough
solar power being generated, the power router
takes electricity from the AC grid, converts it to DC,
and adds it to whatever is being produced by the solar
panels. The power router takes all the electricity from
the solar panels and whatever else is needed from the
grid to keep the lights operating during the daytime,
on cloudy days, and at night. If the grid fails, then
power from the solar panels and, optionally, batteries,
is used to keep the lights on.
A system designed like this is less expensive initially
because the solar array tends to be a little smaller.
Its ideal for retail use because it keeps the
lights on (and customers in the store) during a power
failure. Utilities tend to support the idea because
it doesnt involve complex and expensive bi-directional
interconnection to the grid; to them its an energy
efficiency measure, not energy generation. Its
more efficient during the day because all of the solar
energy gets used and its at least as efficient
at night because the centralized AC to DC conversion
in the power router is better than a similar conversion
at each fluorescent ballast.
It may be that the best way to design a photovoltaic
system in a commercial building is to direct couple
the lighting load. This system will have lower up-front
costs, be more efficient, keep customers in the store
during a power failure, and save the occupant the largest
portion of his electrical expenses.
A graphic demonstration of this technology can be
found at www.DirectCoupling.com
and at Nextek Power Systems website at Direct
Mark Robinson is VP Sales & Marketing of Nextek
Power Systems. Formerly, he was involved in the design
and service of inverters for solar systems. He is a
licensed master electrician and a LEED accredited professional.
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