Stand-Alone PV Systems
Stand-alone PV systems are used when it is impractical to
connect to the utility grid. Common standalone 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.
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 without becoming a target for vandalism.
Generally this means the modules must be mounted on the top
of the sign itself to get them sufficiently above grade level to reduce temptation.
This limits the array dimensions to the width of the trailer (about 8 ft) and
the length of the modules (about 4 ft). At full sun, such a 32-ft2 array, if
15% efficient, can produce approximately 450 W. Depending on location and time
of year, about 5 h of full sun is typically available on an average day. This
means the production of approximately 2250 Wh of energy on the typical day.
Taking into account system losses in the batteries, the
control circuitry, and degraded module performance due to dirty surfaces, about
70 to 75% of this energy can be delivered to the display, or about 1600 Wh/d.
Hence, the average power available to the display over a 24-h period is 67 W.
While this may not seem to be very much power, it is
adequate for efficient display technology to deliver a respectable message. If
the system is a 12 V DC system, a set of deep discharge batteries will need to
have a capacity of 185 Ah for each day of battery back-up (day of autonomy).
For 3 d of autonomy, a total of 555 Ah of storage will be needed, which equates
to eight batteries rated at 70 Ah each.
Hybrid PV-Powered Single Family Dwelling
In areas where winter sunlight is significantly less than
summer sunlight, and/or where winter electrical loads are higher than summer electrical
loads, if sufficient PV is deployed to meet winter needs, then the system
produces excess power for many months of the year. If this power is not used,
then the additional capacity of the system is wasted.
Thus, for such cases, it often makes sense to size the PV
system to completely meet the system needs during the month(s) with the most
sunlight, and then provide backup generation of another type, such as a
gasoline generator, to provide the difference in energy during the remaining
months.
Such a system poses an interesting challenge for the system
controller. It needs to be designed to make maximum use of PV power before
starting the generator. Since generators operate most efficiently at about 90%
of full load, the controller must provide for battery charging by the generator
at the appropriate rate to maximize generator efficiency.
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