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Grid Tie and Net
Metering A bit of Renewable Energy History
15 years ago before utility
grid tie technology became advanced, most solar and wind systems had
batteries. Batteries were the weak link in the system. They required
maintenance and were as much as 1/3 the cost of the system. These were stand
alone systems. With stand alone systems you had to use the energy at or near
the time of production or the excess energy would be wasted. At times of
high demand and low production backup energy from a generator was often
needed. Therefore, the system had to be designed to maximize production at
the time of greatest demand. Homes generally have the greatest demand for
energy in the winter. Therefore, solar panels were often tilted up to steep
angles to be perpendicular to the low winter sun angle. The peak production
of solar energy, however, happens in the summer. Batteries can not store
energy from one season to the next. They can only store for a few days.
Then came highly efficient
grid interactive battery-less inverters and net metering. Net metering is an
arrangement with your electric utility where you can send power to the
utility for which the utility credits you at the same retail rate that you
pay them. During times of over production during the day if your loads shut
down the meter spins or records backwards and automatically credits you
directly reducing your bill. At other times when you need more than you are
producing you buy energy from the grid as usual. During the summer over
production happens more than in winter. Thus you can build credits during
the summer towards your winter bills. This is like having a giant annual
storage battery without the cost and reduced efficiency of batteries. The
down side is that during a utility power outage the system can not back feed
a dead grid for safety reasons and must automatically shut down even if it
is sunny. Therefore during outages you do not have power from the system
unless you install optional batteries. If you choose to have batteries for
backup power from the system you have to realize that you loose efficiency
and the system will not save as much on your electric bills as without them.
Batteries will need to be replaced at least twice during the life of the
system. So it is best to get your backup power from a generator and delete
batteries unless you believe the utility company is soon going the way of
Iraq. Batteries can also be helpful if you have critical loads that must be
powered without even momentary interruption.
The design philosophy of
grid tied solar PV systems becomes completely different from that of stand
alone systems. Unrestrained by the limitation of batteries one can design to
maximize annual production. It turns out that the ideal tilt angle for a
grid tied solar array in Central New York is a low tilt that favors summer sun. One great
advantage is that this tilt is similar to the standard roof tilt. This
favors summer sun during the peak production time of year building more
credits toward winter bills and actually increases the total annual
production of energy. Also it is more aesthetic to have panels parallel to
the existing roof plane than trying to raise them up.
We now
have Net Metering available in New York. This is an arrangement with the
power company that allows you to return excess produced power to the utility
grid at the same retail rate that you pay them. The next advance for
government encouragement of renewable energy technologies will be the
introduction of the Feed in Tariff. This is an arrangement where the power
company is required to purchase your excess production at a rate greater
than retail. This has been highly successful at encouraging the free market
to deploy large quantities of renewable resources technologies very rapidly
in such places as Germany and Spain. It has recently been introduced here in
the USA in Washington and Vermont. In Canada the Province of Ontario has a
Feed-in-Tariff.
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