Surgery and Healing in the Developing World - Dartmouth-Hitchcock

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78 Surgery and Healing in the Developing World
means private charities, churches, nonprofit foundations, etc. Since child vaccination
programs are a major objective of remote health care, organizations that support
such efforts are good candidates for providing financial assistance. In general,
charitable organizations are most likely to assist in PV deployment when the request
for aid is made in the context of extending health care, rather than in the context of
technology demonstration. There are some organizations, e.g., the Winrock Foundation
in Washington, DC, and Solar Energy International in Carbondale, Colorado,
that sometimes assist with in-country logistics even if they do not contribute
funding. The ministries of health and the embassies of individual countries are good
starting points for information about in-country help.
Equipment and Characteristics of Small Photovoltaic Systems
All solar cells produce direct current (DC). This is the mode of current that is
supplied by a battery, as opposed to the current from a wall outlet in a home or
commercial building. A wall outlet supplies 50- or 60-Hertz alternating current (AC).
If alternating current is desired from a PV system, a small electronic device known as
an inverter (current inverter) must be employed to change the direct current into
alternating current. Both DC-output and AC-output photovoltaic systems are common,
and the decision to configure the system one way or the other is based on the
types of electrical loads that will be powered by the system. It is advisable to avoid the
use of AC equipment if this is convenient, because the inversion process of DC into
AC is only about 90% efficient. However, some applications require AC power,
either because the given piece of equipment operates most reliably in an AC mode or
because DC compatible equipment is not available. This includes most tools with
electric motors and any piece of equipment that uses a built-in transformer. To provide
flexibility, many remote clinic PV systems are designed to supply both DC and
AC.
In any case, whether the object is supplying DC power or AC power, or both,
the system is always interfaced to a battery bank. That is, the DC output from the
PV modules serves the immediate purpose of charging batteries, and those batteries
in turn will supply power for DC appliances and for DC-to-AC inversion. Except
for a few applications such as daytime water pumping, one seldom finds small remote
PV arrays without a battery interface. There are two reasons for this that are
particularly germane to a clinic.
First, remote clinic applications require uniform power for lighting and medical
appliances. This result cannot be accomplished without a battery interface because
the output of a photovoltaic array is approximately proportional to the intensity of
the sunlight. Direct connection of the array to the loads will result in wildly varying
power as the sun is transiently obscured by clouds and (less quickly) as the sun
climbs and falls in the sky. Additionally, telecommunications and other equipment
are sensitive to small voltage variations and can be damaged if the voltage is not
regulated. A battery and charge controller combination will maintain a constant
output voltage. Secondly, a battery interface is necessary for PV energy storage so
that the clinic can function after dark when the modules have no output. The only
cheap and simple way to allow functionality after dark is with rechargeable batteries.
And there is also the related issue of autonomy, i.e., the ability of a PV system to
maintain power during extended periods of inclement weather. It is not unusual for
tropical environments to experience several continuous days of extreme cloudiness
and rain on a seasonal basis. Consequently, most PV systems intended for a tropical