Stand-alone systems can
be more cost-effective than connecting to the grid in remote
locations.
There are
several approaches that can be taken to generate and use electricity
generated by Solar Photo-Voltaic (PV) cells and Wind Turbines. The
following sections describe each of these approaches:
For many people, powering their
homes or small businesses using a small renewable energy system that is
not connected to the electricity grid—called a stand-alone system—makes
economic sense and appeals to their environmental values.
In remote locations, stand-alone systems
can be more cost-effective than extending a power line to the
electricity grid (the cost of which can ran range from $5,000 to $10,000 per
mile). But these systems are also used by people who live near or on
the grid and wish to obtain independence from the power provider or
demonstrate a commitment to non-polluting energy sources.
Successful stand-alone systems
generally take advantage of a combination of techniques and technologies
to generate reliable power, reduce costs, and minimize inconvenience. Some
of these strategies include using fossil fuel or renewable hybrid systems
and reducing the amount of electricity required to meet your
needs.
Connecting Your System
to the Electricity Grid
While renewable energy
systems are capable of powering houses and small businesses without any
connection to the electricity grid, many people prefer the advantages that
grid-connection offers.
A grid-connected system
allows you to power your home or small business with renewable energy
during those periods (diurnal as well as seasonal) when the sun is
shining, the water is running, or the wind is blowing. Any excess
electricity you produce is fed back into the grid. When renewable
resources are unavailable, electricity from the grid supplies your needs,
thus eliminating the expense of electricity storage devices like
batteries.
In
addition, power providers (i.e. electric utilities) in most states now
allow ' net metering'
, an arrangement where the excess electricity
generated by grid-connected renewable energy systems "turns back" your
electricity meter as it is fed back into the grid. Thus, if you use more
electricity than your system feeds into the grid during a given month, you
pay your power provider only for the difference between what you used and
what you produced.
Grid-Connection Requirements from Your Power
Provider
Currently, requirements for connecting distributed generation
systems—localized or on-site power generation systems like small renewable
energy systems—to the electricity grid vary widely.You will need to contact your power provider directly
to learn about its specific requirements. If your power provider does not
have an individual assigned to deal with grid-connection requests, try
contacting your state utilities commission, state utility consumer
advocate group (represents the interests of consumers before state and
federal regulators and in the courts), state consumer representation
office, or state energy
office.
Metering
and Rate Arrangements for Grid-Connected Systems
Net
purchase and sale
Under this arrangement, two uni-directional meters
are installed—one records electricity drawn from the grid, and the other
records excess electricity generated and fed back into the grid. You pay
retail rate for the electricity you use, and the power provider
purchases your excess generation at its avoided
cost (wholesale rate). There may be
a significant difference between the retail rate you pay and the
power provider's avoided cost, though a number of states have implemented legislation
through their Public Utilities Commission (PUC) mandating that the purchase and
sales price for power from a homeowner be the same.
This is great news since it means you are able to sell
your power back at retail !
As of June 2003 the
following states have mandated equality: CA, AZ, NM, MN, FL. More
states have legislation under way (table of net-metering rules by
state
). In addition, many smaller
utilities and co-ops have aggressive policies regarding renewable energy
systems. If you are on such a small electricity co-op call them
and ask if they allow net-metering and how you can check what their
policies are.
Net
metering
Net metering provides the greatest benefit to you
as a consumer. Under this arrangement, a single, bi-directional meter is
used to record both electricity you draw from the grid and the excess
electricity your system feeds back into the grid. The meter spins
forward as you draw electricity, and it spins backward as the excess is
fed into the grid. If, at the end of the month, you've used more
electricity than your system has produced, you pay retail price for that
extra electricity. If you've produced more than you've used, the power
provider generally pays you for the extra electricity at its avoided
cost. The real benefit of net metering is that the power provider
essentially pays you retail price for the electricity you feed back into
the grid.
Some power providers will now let you carry over
the balance of any net extra electricity your system generates from
month to month, which can be an advantage if the resource you are using
to generate your electricity is seasonal. If, at the end of the year,
you have produced more than you've used, you forfeit the excess
generation to the power provider.
Solar PV-Wind Turbine Hybrid Systems
The optimal renewable energy solutions will utilize a
combination of Solar and Wind resources. Solar electric,
heated air, and hot-water generation techniques can be coupled with
Wind Turbines to take full advantage of renewable energy resources.
Wind Turbines and Solar PV cells use the same storage
(battery/grid), charge control, inverter, and monitoring equipment,
so realizing additional power savings by adding a wind turbine is an
obvious next step in many areas.
Off-Grid Systems with Generator
Back-Up
What
about situations in which remote or non-grid-connected power is needed,
but that power must always be available — for example, to keep vaccines
cold, or a rural clinic's lights on, or communications equipment running
continuously? Or the times when users know they'll occasionally need a
larger amount of power than a PV system can supply alone — say, in a
national park camping ground, where it's difficult to predict how much
power will be needed? In those cases, PV is still a practical choice. We
just need to add an electric generator that can work effectively with a PV
system to supply the load.
During the day, the PV
modules quietly supply daytime energy needs and charge batteries. If the
batteries run low, the engine generator runs at full power — its most
cost-and fuel-efficient mode of operation — until the batteries are
charged. And, in some systems, the generator makes up the difference when
electrical demand exceeds the combined output of the PV modules and the
batteries.
Systems that use
several types of power generation have the advantages of each one. Engine
generators can produce electricity any time. So, they provide an excellent
backup at night or on cloudy days for the PV modules, which produce power
only during daylight hours. The advantages of a PV system are that it
operates quietly and does not pollute. As to the rather high initial cost,
we can select a smaller PV system if we operate it with a generator.
Where no other form of
power generation is available, the PV array and the battery storage have
to be large enough to supply nighttime electrical needs. However, having
an engine generator as a backup means fewer PV modules and batteries will
be necessary to supply power whenever it's needed.
Including a generator
makes designing a PV system more complex, but it's still easy to operate.
In fact, modern electronic controllers allow these kinds of systems to
operate automatically. Controllers can be set to automatically start the
generators, to supply AC or DC loads, or to do some of each. Wind
generators, small hydro plants, and any other source of electrical energy
could also be added to make an even larger hybrid power system.
Equipment Required
In addition to purchasing
photovoltaic panels, a wind turbine, or a small hydropower system, you
will need to invest in some additional equipment (called
"balance-of-system") to condition and safely transmit the electricity to
the load that will use
it.
The amount
of equipment you will need to buy depends on what you
want your system to do. In the simplest systems, the current generated by, for
example, your wind turbine is connected directly to the load. However,
if you want to store power for use when your
turbine isn't producing electricity, you will want to purchase batteries
and a charge controller. Depending on your needs, balance-of-system equipment could account
for half of your total system costs.
Learn about the major
balance-of-system equipment for stand-alone, grid-tie and PV-Wind Hybrid systems: