Page 167 - Applied Photovoltaics

x structural conditions
x insect trapping
x seismic recording
x scientific research
x auto-dial alarms
x water supply reservoir levels
x radiation and pollution monitoring.

With any monitoring system, power is required for the instrumentation and the data
communications equipment. Since these power requirements are typically quite low
(loads can often be measured in mAh per day), monitoring applications are ideal for
photovoltaic systems. Because of the simplicity and reliability of photovoltaic power
supplies, they are even sometimes replacing AC-powered battery chargers in grid-
connected areas. Even where high voltage grid power is immediately available, it can
be more economical to supply small loads from PV than to install step-down
transformers (Brooks, 2000).
It is important to include varistors in the photovoltaic system, to protect the data
acquisition equipment from surges, particularly due to lightning.
Rechargeable batteries (NiCd, or gelled electrolyte lead-acid batteries) are often
included in data acquisition equipment packages. If using NiCd batteries, self-
regulation is probably suitable and preferable.

8.15 DIRECT-DRIVE APPLICATIONS

PV systems can be connected directly to the load, typically using only one module
(Ball & Risser, 1988). Such direct-drive systems have considerable appeal, as they
avoid the use of batteries, blocking diodes, and all power conditioning circuitry. The
module must be able to produce the necessary current for the load, while the load
must be able to tolerate voltages as high as the module’s open circuit voltage. Many
complete packages that include photovoltaics are now available. Most of these are
portable systems. Typical applications include:
x ventilation fans
x portable radios
x toys
x solar tracking devices
x solar collector pumps
x water pumping systems.
The last of these is covered in detail in Chapters 11 and 12 and represents a major
application for photovoltaics in the future.

For direct drive applications in general, good correlation is obviously required
between power demand and solar intensity, since the systems can only operate during
daylight hours. As long as the load impedance is matched to the photovoltaic
module’s output, any small series-connected or permanent magnet (but not shunt
type) DC motor can, theoretically, be powered by a direct drive photovoltaic system
(Roger, 1979). Manual disconnect switches are recommended for fixed installations,

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