Surgery and Healing in the Developing World - Dartmouth-Hitchcock

Establishing Electrical Power in Remote Facilities for Health Care
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Figure 6. A bank of batteries in an
acid-proof container stores energy
from a 1-kW PV system. (Photo by S.
Thornton, courtesy NREL/PIX.)
clinic will be designed with enough battery storage to allow at least several days of
operation without any sunlight at all. The more critical the application, the greater
is the storage capacity that must be built into the system.
Several different battery technologies have been used for PV storage, with the
nominal 12-volt flooded lead-acid battery being the most common. This is the same
type of battery found in automobiles. However, unlike automotive batteries, a flooded
lead-acid battery specifically designed for PV purposes is able to survive many hundreds
of deep discharge cycles—up to 80% discharge—without loss of storage capacity.
As indicated above, these batteries are known as deep-cycle or PV-industrial
batteries. The 80% discharge survivability feature is important from the standpoint
of allowing the system to have several days of autonomy, on a frequent basis, without
ruining the batteries. During good weather, the battery will be brought back to
full charge every day. For 50% depth of discharge, a really good deep-cycle PV
battery will survive over 3000 cycles at 25˚C. With provision for a few 80% discharges
every now and then, that’s about nine years of service. Automotive batteries,
by comparison, will fail after only a few 50% discharge/charge cycles.
Like an automotive battery, the special PV battery tends to lose water through the
outgassing of hydrogen and oxygen toward the end of the charging phase. Periodic
addition of water is necessary to maintain the correct electrolyte level (measured
through specific gravity) around the plates. This is easily checked with a hydrometer.
If funds are available and the batteries can be readily obtained, the best approach
for the PV powered clinic is to use the deep-cycle flooded lead-acid battery described
above (Fig. 6). From a practical standpoint, though, this is often not possible.
Most developing countries do not manufacture PV-dedicated batteries. The
battery expense and import duties can be prohibitive, and government restrictions
can be very frustrating. Some countries, e.g., India, require the use of only domesti-
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