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One increasingly popular way of using photovoltaics is what
is known as "grid-attach". In this configuration, the consumer
stays attached to the electric company (the "grid"). The PV system forces
whatever it produces directly into the building's AC lines.
If the PV system is producing more than the building uses,
the meter runs backwards. If the building is using more than
the PV system is producing, the difference is made up by
energy from the electric company.
This type of attachment has a number of advantages:
- It is cheaper, because there is no need for batteries.
- It needs less maintenance, again because of the lack of
batteries.
- It makes the most effective possible use of the energy
that the PV generates: because the grid is always able to
accept any surplus, the PV panels can always be operated
at their most efficient point.
The obvious disadvantage is that there is no backup (however,
some inverters support both battery backup and attachment to
the grid).
Typical Grid Attached System
A grid-attached PV system is about as simple as could
be imagined. An array of PV panels produces DC, which is
wired into the DC side of an inverter, separated only by
a DC disconnect, which may also contain a fuse. The AC side
of the inverter is then connected to the house AC, separated
only by an AC disconnect box and a fuse or circuit breaker
(the breaker is often a "backfed" breaker in the building's
main AC circuit breaker panel).
Grid Attached PV in Texas
For anyone wanting to implement a grid-attached system
in Texas, the news is partially good and partially bad.
The PUC (Public Utility Commission) has a set of rules that
state that the company that delivers your electricity
must allow you to attach to their lines, and feed electricity
back into it. Every line company is required by the PUC to designate
a contact person for distributed generation connection, and
publish the contact information on their web site.
The bad news is that, since deregulation, the rules for
accounting for that electricity have become fuzzy. The
ideal situation would be an enforced "net metering" rule,
whereby you would be allowed to run the meter backward
for your surplusses, using the grid as a giant battery in
effect. In practice, many people are doing net metering, but
in the absence of definitive rules, "anything goes".
Another piece of bad news is that electric co-ops are
not governed by the PUC rules. They do what they want.
Same with municipal utilities. Some are PV friendly, some
less so.
Discussion of Net Metering
There are many ways of looking at the question of
small generators pushing electricity onto the grid and
running their meter backwards. Here are some points of
view (remember that in Texas, there are consumers, line companies,
and generating companies):
- Lines company: "If we allow this, we are letting people
move electricity across our network for free. We stay in
business by charging for that service".
- Generation company: "We generate electricity, we don't
buy it".
- Consumer: "PV is hard enough to justify financially,
without having to effectively give electricity away".
- A balanced point of view: In fact, net metering, at
least when combined with a PV system, is a 3-way win, for
consumer, generator, and line company:
- In practice, the electricity produced by the customer
goes to go either to the customer, or to a near neighbor.
It is thus a totally different situation from energy that
is being moved from a power plant to where it is used.
In fact, from the line company's point of view, locally
generated PV electricity is indistinguishable from demand
reduction, not something that they ordinarily get paid for.
- The PV system produces its greatest output during
the times when the electric grid is most heavily loaded.
These are the times at which electricity is trading at
a premium, because of the extra capacity that generators
are having to put online to meet peak demand. The presence
of the PV system reduces required generating capacity
during time times that additional electricity is most
expensive to the generator. This "peak shaving" also
benefits the line company.
- The consumer benefits from the availability of what
amounts to a nearly perfect, and infinite, battery. This
dramatically improves the economics of PV.
A workgroup from the Texas Renewable Energy Industries
Association (TREIA) is currently (9-2002) working on this
issue, trying to get net metering rules establshed that
are favorable to small PV and wind electricity producers.
Safety Issues
One obstacle that grid-attached PV has encountered has
been the perception by many in the power industry that
the presence of these distributed generators presents a
safety hazard to electric workers: what if an inverter keeps
running during a power failure and keeps the lines energized
while utility workers are trying to work on them?
There has been a great deal of work in this area.
All UL-listed "utility-intertie" inverters implement a
complex checking algorithm to make sure that they disconnect
when the grid goes down, and are not capable of "islanding".
It must always be remembered that PV systems are capable
of generating electricity on a lethal scale. To ensure safe
and reliable operation over their entire lifetime (which
is likely to be very long) They should always be
installed according to the requirements of the National
Electrical Code. This is not something to be
taken lightly.
Resources
Here are some places to look for more detailed discussion
of grid-attached PV systems:
Home Power Magazine
Connecting to the Grid, A
Guide to PV Interconnection Issues,
from the Interstate Renewable Energy Council.
Photovoltaic Power Systems and the National Electrical Code, published
by Sandia Laboratories.
If you have a hard time sleeping, all of the rules of the PUC
are available on line. Relevant to this discussion are
Rule 25.211,
Rule 25.212, and
Project 22318.
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