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down under conditions of utility voltage or frequency as specified by IEEE 929.
The typical small utility-interactive system of a few kilowatts consists of an array of modules selected
by either a total cost criterion or, perhaps, by an available roof area criterion. The modules are connected
to produce an output voltage ranging from 48 V to 300 V, depending upon the DC input requirements
of the PCU. One or two PCUs are used to interface the PV output to the utility at 120 V or, perhaps,
120/240 V. The point of utility connection is typically the load side of a circuit breaker in the distribution
panel of the occupancy if the PV system is connected on the customer side of the revenue meter. Con-
nections on the utility side of the meter will normally be with double lugs on the line side of the meter.
Section 690 of the NEC provides the connection and installation requirements for systems connected on
the customer side of the revenue meter. Utility-side interconnects are regulated by the local utility.
Since the cost of PCUs is essentially proportional to their power handling capability, to date there has
been no particular economy of scale for PV system size. As a result, systems are often modular. One form
of modularity is the AC module. The AC module incorporates a small PCU (H"300 W) mounted on the
module itself so the output of the module is 120 V AC. This simplifies the hook-up of the PV system,
since NEC requirements for PV output circuits are avoided and only the requirements for PCU output
circuits need to be met.
Medium- and large-scale utility-interactive systems differ from small-scale systems only in the possi-
bility that the utility may require different interfacing conditions relating to power quality and/or con-
ditions for disconnect. Since medium- and large-scale systems require more area than is typically available
on the rooftop of a residential occupancy, they are more typically found either on commercial or industrial
rooftops or, in the case of large systems, are typically ground-mounted. Rooftop mounts are attractive
since they require no additional space other than what is already available on the rooftop. The disadvan-
tage is when roof repair is needed, the PV system may need to be temporarily removed and then reinstalled.
Canopies for parking lots present attractive possibilities for large utility-interactive PV systems.
Stand-Alone PV Systems
Stand-alone PV systems are used when it is impractical to connect to the utility grid. Common stand-
alone systems include PV-powered fans, water pumping systems, portable highway signs, and power
systems for remote installations, such as cabins, communications repeater stations, and marker buoys.
The design criteria for stand-alone systems is generally more complex than the design criteria for utility-
interactive systems, where most of the critical system components are incorporated in the PCU. The PV
modules must supply all the energy required unless another form of backup power, such as a gasoline
generator, is also incorporated into the system. Stand-alone systems also often incorporate battery storage
to run the system under low sun or no sun conditions.
PV-Powered Fans
Perhaps the simplest of all PV systems is the connection of the output of a PV module directly to a DC fan.
When the module output is adequate, the fan operates. When the sun goes down, the fan stops. Such an
installation is reasonable for use in remote bathrooms or other locations where it is desirable to have air
circulation while the sun is shining, but not necessarily when the sun goes down. The advantage of such a
system is its simplicity. The disadvantage is that it does not run when the sun is down, and under low sun
conditions, the system operates very inefficiently due to a mismatch between the fan I-V characteristic and
the module I-V characteristic that results in operation far from the module maximum power point.
If the fan is to run continuously, or beyond normal sunlight hours, then battery storage will be needed.
The PV array must then be sized to provide the daily ampere-hour (Ah) load of the fan, plus any system
losses. A battery system must be selected to store sufficient energy to last for several days of low sun,
depending upon whether the need for the fan is critical, and an electronic controller is normally provided
to prevent overcharge or overdischarge of the batteries.
© 2001 CRC Press LLC
PV-Powered Water Pumping System
If the water reservoir is adequate to provide a supply of water at the desired rate of pumping, then a
water pumping system may not require battery storage. Instead, the water pumped can be stored in a
storage tank for availability during low sun times. If this is the case, then the PV array needs to be sized
to meet the power requirements of the water pump plus any system losses. If the reservoir provides water
at a limited rate, the pumping rate may be limited by the reservoir replenishment rate, and battery storage
may be required to extend the pumping time.
While it is possible to connect the PV array output directly to the pump, it is generally better to employ
the use of an electronic maximum power tracker (MPT) to better match the pump to the PV array
output. The MPT is a DC DC converter that either increases or decreases pump voltage as needed to
maximize pump power. This generally results in pumping approximately 20% more water in a day.
Alternatively, it allows for the use of a smaller pump with a smaller array to pump the same amount of
water, since the system is being used more efficiently.
PV-Powered Highway Information Sign
The PV-powered highway information sign is now a familiar sight to most motorists. The simpler signs
simply employ bidirectional arrows to direct traffic to change lanes. The more complex signs display a
message. The array size for a PV-powered highway information sign is limited by how it can be mounted
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