User Manual

Global Water Instrumentation
11
Typically, a system is designed to make suffic ient power available during the worst
times of the y ear . In these cases, solar panels should be oriented to gather the most
light when the sun is lowes t in the sky. In areas with snow , they may even be
pointed lower to allow the snow to slide of f. This reduces the efficiency but overall,
will probably produce more t otal current. Clouds c an r educ e the output current of a
panel by 90% or more, even a small c loud w ill r educ e 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 prov ide enough power in the wint er w ill pr oduc e far too
much in the sum mer. The result is the battery will become overcharged, reducing
the capacit y to hold stored energy. When the following winter arr iv es , the battery
may not prov ide enough s torage capacity to keep the syst em operat ing as it was
designed. For this reason, a charge controller should always be used. This device
senses the battery voltage and adjust s the charging current to match the demand.
The charger is select ed based on the amount of solar c apac ity that is connected to it.
High ambient tem per atures may mak e it neces sary to over-rate the charger.
Many other factors affect the ability of a batter y t o m aintain a charge. A solar system
that is under -rated will cause the battery to make large voltage fluctuations. Typical
batteries us ed in these applications are the sealed lead acid type, w hic h s lowly los e
capacity when c har ged and dis c har ged by large amounts over long periods of time.
The larger t he typical voltage fluctuations are, the more the effect. If the bat tery is
allowed to become sev er ely ov er c harged or dischar ged, the capacity can be
significant ly r educ ed. Low ambient tem per atures also reduce available power
capacity but this capacity returns when the temperatures rise again.
The solar power output and battery capac ity are the key design f eatures. When
simplified; the capacity of the batteries is chosen based on the amount of t ime the
system needs to be running in the absence of s unlight, while the size of the solar
panels is chosen f or their ability to r ec har ge the batteries at t he end of that time with
whatever sunlight may be available. What the worst cas e s c enar io is depends on
the application, but designing the sy s tem for that possibility is the ultimate goal.
There are t oo m any applications to make estim ates of what is typical. The total
power drain depends on transmission interval and system configurat ion; while
recharging limitations depend mostly on the environment. Two t hings to consider
are that an ex tra solar panel is usually c heaper than lost data, and the cost of a fresh
replacement battery is much less when someone is already in the f ield.