Xylem SIT65 SATELLITE INTERNET TELEMETRY - Manual (Page 11)

Global Water Instrumentation
11
Typically, a system is designed to make sufficient power available during the worst
times of the year. In these cases, solar panels should be oriented to gather the most
light when the sun is lowest in the sky. In areas with snow, they may even be
pointed lower to allow the snow to slide off. This reduces the efficiency but overall,
will probably produce more total current. Clouds can reduce the output current of a
panel by 90% or more, even a small cloud will reduce the output to almost nothing as
it goes by. Shade is a very big factor and clouds create shade, even if it seems very
bright out.
Solar systems designed to provide enough power in the winter will produce far too
much in the summer. The result is the battery will become overcharged, reducing
the capacity to hold stored energy. When the following winter arrives, the battery
may not provide enough storage capacity to keep the system operating as it was
designed. For this reason, a charge controller should always be used. This device
senses the battery voltage and adjusts the charging current to match the demand.
The charger is selected based on the amount of solar capacity that is connected to it.
High ambient temperatures may make it necessary to over-rate the charger.
Many other factors affect the ability of a battery to maintain a charge. A solar system
that is under-rated will cause the battery to make large voltage fluctuations. Typical
batteries used in these applications are the sealed lead acid type, which slowly lose
capacity when charged and discharged by large amounts over long periods of time.
The larger the typical voltage fluctuations are, the more the effect. If the battery is
allowed to become severely overcharged or discharged, the capacity can be
significantly reduced. Low ambient temperatures also reduce available power
capacity but this capacity returns when the temperatures rise again.
The solar power output and battery capacity are the key design features. When
simplified; the capacity of the batteries is chosen based on the amount of time the
system needs to be running in the absence of sunlight, while the size of the solar
panels is chosen for their ability to recharge the batteries at the end of that time with
whatever sunlight may be available. What the worst case scenario is depends on
the application, but designing the system for that possibility is the ultimate goal.
There are too many applications to make estimates of what is typical. The total
power drain depends on transmission interval and system configuration; while
recharging limitations depend mostly on the environment. Two things to consider
are that an extra solar panel is usually cheaper than lost data, and the cost of a fresh
replacement battery is much less when someone is already in the field.