Surgery and Healing in the Developing World - Dartmouth-Hitchcock

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80 Surgery and Healing in the Developing World
cally manufactured batteries. More often than not, the clinic PV system will wind
up using conventional thin-plate automotive batteries. This saves a lot of initial
expense and is an easy way to proceed. However, the depth of cycling of the batteries
must be carefully controlled. These cheap batteries can be expected to last only one
year at the 20% discharge level, and maybe two years at the 10% discharge level,
with temperature at 25˚C. Overall, the system is not as reliable as with the
PV-dedicated batteries and has little autonomy.
Regardless of the choice of battery, the single biggest cause of PV system failure
is battery failure due to inadequate hydration. The battery is often the weak link in
the PV chain. The user needs to be sure the system integrator has carefully explained
exactly how the batteries are to be maintained. There are unfortunately many good
PV arrays in developing countries that have been abandoned due to unnecessary
battery failure. High temperature quickly degrades battery lifetime because of the accelerated
plate corrosion. Keep the batteries in a cool enclosure (20˚-25˚C) and be
sure to follow the manufacturer’s instructions for periodic hydration.
A word of caution: the room or enclosure holding the batteries must have some
ventilation. Toward the end of the charging phase, a flooded battery can produce an
oxygen/hydrogen gas mixture. There is the potential for explosion or acid dispersal.
Always use caution around batteries. Eye protection should be worn while servicing
the battery bank.
All PV systems are designed with some method for electrically isolating the battery
from the array at night. This is necessary so that battery energy is not wasted by
reverse flow of charge into the array when the sun goes down. Most systems allow
the user to program disconnect/reconnect times into the charge controller. Other
systems have a manual on/off switch.
Both solar cells and PV modules have peculiar characteristics that are helpful to
keep in mind when purchasing a system or when comparing one system against
another. One such characteristic of all solar cells is the dependence of their power
output with the load they are driving. That is, the power which is delivered into the
electrical load (lighting, medical appliances, etc.) is dependent on the size of that
load. (This is contrasted with internal combustion engines.) Figure 7 shows a pair of
typical current vs voltage curves at the terminals of the array. The two curves correspond
to different illumination levels. For each curve, there is a point near the knee
of the curve that maximizes the product of current and voltage, i.e., maximizes the
output power. This is called the maximum power point and is dependent only on
the size of the load. The efficiency claimed by a module manufacturer, or by a system
integrator for an entire system, is always in reference to the maximum power point of the
module (or system). In this regard, efficiency claims always assume an optimal load.
When a PV system is designed, the system integrator will configure the load so that
the array stays near the maximum power point.
On the module level, a curious characteristic is the dependence of power output
on module obscuration. The output of a module is drastically degraded by partial
shading. If even one cell in the module is completely shaded from the sun, the
output of the module can fall immediately by as much as 80%. This phenomenon is
reversible, and modern modules are expected to immediately regain full power output
when the partial shading is removed. To assure optimal performance, though, it
is important that all of the modules in an array receive full exposure during sunlight
hours. Common sources of shading are trees, fence posts, and bird droppings.
Heat build-up around an array also affects a PV system because of the degradation
of solar cell output with increasing temperature. The temperature effect does