Introduction

Introduction to Solar energy technology :

All energy, with the exception of geothermal resources, is derived from the sun. Fossil fuels are
just solar energy stored in organic material converted to hydrocarbon fuels by pressure and
temperature over geologic time. Unlike hydrocarbon fuels, solar energy is pollution free and for
all practical purposes, inexhaustible–the sun will continue to shine for the next billion years or
so. The principal technologies used to extract energy from the various natural processes
generated from the radiant energy of the sun include:
• Solar photovoltaic
• Solar thermal

Most of these technologies can produce mechanical and thermal energy directly, but they are
being used more frequently to produce electric power. Each technology and how it is used to
produce electric power are briefly described here.
Solar radiation is used to generate electricity using two technologies; photovoltaic (PV) and solar
thermal. Photovoltaic systems convert sunlight directly into electricity, without converting it to
heat first. Conversion efficiencies are typically in the 12% - 20% range without concentrators,
and 22% - 28% with concentrators. These systems generate electricity directly from solar cells
made from semiconductor materials. The solar cell is specially treated to give one layer (the n-
layer), a negative charge and the other layer (the p-layer) a positive charge when sunlight enters
the cell structure. This sets up a cell barrier between the semiconductor layers, creating a current
and a voltage across the cell. Sunlight striking the cell excites electrons, which move across the
cell. The electron flow is conducted by metallic contacts placed on the cell in a grid-like fashion.
PV cells are electrically and physically linked together into modules. The entire structure, along
with the supporting elements, is called an array. Each module is given a peak power rating
according to the output under standard test conditions. Arrays are typically designed to meet the
electricity requirements for specific applications.

Modules usually provide electricity in the form of direct current (DC) at 12 or 24 volts. The
relative simplicity, lack of moving parts, and the promise of little or no maintenance make
photovoltaics an attractive technology.
Ideal conditions for generating electricity using
photovoltaic cells are long, clear, cold, sunny days. In order to provide electricity during the night
and on cloudy days batteries are used to store some of the electricity generated during the
daylight hours. Complete systems including batteries, inverters and control systems are needed
for most applications as shown below


Research and development advances have lowered PV costs dramatically. Costs are still high, but
continue to drop. Capital cost is currently US$4,500 - 10,000/kW (compared to wind at about
$1,000/kW). Cost is sensitive to the size of the order. Because energy costs (per unit) are usually
very high, at about US$0.40 /kWh, PV is not widely deployed. Yet, its advantages often make it
appropriate in off-grid applications.
Solar Thermal systems are comprised of a collection component, working fluid circulation
system, storage component, and controls. These function together to convert solar radiation into
heat, which can be used in various applications. Collection component types include:
Flat-plate hydronic collectors
Flat-plate heat-pipe collectors
Evacuated-tube heat-pipe collectors
Parabolic-trough collectors
Paraboloidal-dish collectors
Power-tower collector systems
Eneregy Generated by the soiar thermal is directly used by the end u
user, such as in domestic hot water
heating or space heating or cooling.
Sometimes in larger applications
energy can be stored and used for other
applications, such as mechanical