Surgery and Healing in the Developing World - Dartmouth-Hitchcock

Establishing Electrical Power in Remote Facilities for Health Care
parison, the visible spectrum is between 400 and 700 nm.) Because of the great
distance between the earth and the sun, the intensity of the sun’s radiation is greatly
attenuated by the time it reaches the earth. At the earth’s surface at sea level on a clear
day, when the sun is near the zenith, the incident power density (watts/cm 2 ) is about
100 mW/cm 2 . This is equivalent to 1 kW/m 2 . At elevations of 2000 meters above sea
level, the power density can be as high as 120 mW/cm 2 . As the elevation increases,
more and more of the solar insolation is in the energetic blue end of the spectrum.
(This is why people easily get sunburns when they travel into mountainous regions.)
In remote locations with frequent sunny days, the generation of electric power through
photovoltaics becomes very attractive. As an example, in central Uganda, the average
daily solar insolation (illumination intensity) on an annual basis is about 6.5 kWh/
m 2 . If that sunlight falls on a 1 m 2 photovoltaic system with an overall system efficiency
of 10%, a total of about 0.65 kWh of energy per day would be recovered for
useful work. This would be sufficient to drive five 20-watt high-efficiency compact
fluorescent lamps for six and a half hours per day. Many sunny (not necessarily tropical)
parts of the world are well-suited for PV remote electrification.
Certain tropical areas can experience less average sunlight during the year than
many cooler areas on the globe. An example is the coastal region of Equador. Though
it is positioned on the equator, this region averages only 4.0-4.5 kWh/m 2 /day due to
inclement weather. By comparison, southern Mongolia at 45 o north latitude is cool
and windy, but averages 5.0-5.5 kWh/m 2 /day because of the very dry cloudless conditions.
Maps of average daily incident solar energy are readily available. 5
The heart of any photovoltaic system is the solar cell. Solar cells are large-area
semiconductor devices. They are governed by physical principles similar to those
seen in transistors and other devices common to the microelectronics industry. The
first solar cells were fabricated and extensively studied in the mid 1950s at Bell
Telephone Laboratories. Those cells were made from silicon wafers and had
sunlight-to-electricity power conversion efficiencies of about 6%. Modern cells are
about 15% efficient. Today, solar cells are used to generate electric power in a variety
of remote terrestrial environments. 5 The most common applications are water pumping
in isolated villages (Fig. 2), as well as village lighting and telecommunications
(Fig. 3). In 2003, worldwide production of photovoltaics for terrestrial use came to
about 744 megawatts. The photovoltaics industry worldwide is growing quickly.
Because the power density of sunlight is low, the power output of an individual
commercial silicon solar cell is only about 3 watts per cell. And the voltage output of
an individual commercial cell is only about 0.5 to 0.6 volts. These characteristics are
insufficient for most practical applications. To deal with this, cells must be electrically
connected together to yield a useable output power and voltage. This connection takes
the form of a module (Fig. 4). Most modules have 33 to 72 cells. However, the largest
module now on the market has over 100 cells and produces over 300 watts of power.
The module structure also provides mechanical support for the cells and protection
from such environmental hazards as hail stones and water ingression.
The module is the minimum deployable photovoltaic unit. Commercial photovoltaic
power is always sold in the form of modules, and never as individual cells. A
group of modules is referred to as an array (Fig. 5). In their brochures and marketing
literature, photovoltaic module manufacturers quote prices in terms of cost per watt
($/W) for the module. The end-user can buy modules directly from a module manufacturer;
but more commonly, business is done by way of a system integrator. A system
integrator obtains the modules and all of the associated module mounting,
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