harnessing energy from the light of the Sun. It has been used in many



traditional technologies for centuries and has come into widespread

use where other power supplies are absent, such as in remote

locations and in space. Its use is spreading as the environmental

costs and limited supply of other power sources such as fossil fuels

are realized.

Energy from the Sun

Theoretical annual mean insolation, at the top of Earth’s atmosphere

and at the surface on a horizontal square meter .

Global solar energy resources. The colors in the map show the actual

local solar energy, averaged through the years of 1991-1993. The

scale is in watts per square meter.

The land area required to supply the current global primary energy

demand by solar energy using available technology is represented by

the dark disks.The rate at which solar radiation reaches a unit of area

in space in the region of the Earth’s orbit is 1,366 W/m , as measured

upon a surface normal (at a right angle) to the Sun. This number is

referred to as the solar constant.[1] The atmosphere reflects 6% and

absorbs 16% of incoming radiation resulting in a peak power at sea

level of 1,020 W/m . [2] [3] Average cloud cover reduces incoming

radiation by 20% through reflection and 16% through absorption.[4]

The image on the right shows the average solar power available on the

surface in W/m calculated from satellite cloud data averaged over

three years from 1991 to 1993 (24 hours a day). For example, in

North America the average power of the solar radiation lies

somewhere between 125 and 375 W/m , between 3 and 9 kWh/m

/day.

It should be noted that this is the maximum available power, and not

the power delivered by solar power technology. For example,

photovoltaic panels currently have an efficiency of ca. 15% and,

hence, a solar panel delivers 19 to 56 W/m or 0.45-1.35 kWh/m /day

(annual day and night average). The dark disks in the image on the

right are an example for the land areas that, if covered with solar

panels, would produce slightly more energy in the form of electricity

than the total primary energy supply in 2003. [6] That is, solar cells

with an assumed 8% efficiency installed in these areas would deliver

a bit more energy in the form of electricity than what is currently

available from oil, gas, hydropower, nuclear power, etc. combined.

It should also be noted that a recent concern is that of Global

dimming, an effect of pollution that is allowing less and less sunlight

to reach the Earth’s surface. It is intricately linked with pollution

particles and Global warming, and is mostly of concern for issues of

Global climate change, but is also of concern to proponents of Solar

Power due to the existing and potential future decreases in available

Solar Energy. The order of magnitude is about 10% less solar energy

available at sea level, mostly due to more intense cloud reflections

back into outer space. That is, the clouds are whiter and brighter

because the pollution dust serves as a vapor-liquid phase change

initiation site and generates clouds where otherwise there would be a

moisture filled but otherwise clear sky.

After passing through the Earth’s atmosphere, most of the sun’s

energy is in the form of visible and Infrared radiations. Plants use

solar energy to create chemical energy through photosynthesis.

Humans regularly use this energy burning wood or fossil fuels, or

when simply eating the plants.

Solar design in architecture

Solar design can be used to achieve comfortable temperature and

light levels with little or no additional energy. This can be through

passive solar, where maximising the entrance of sunlight in cold

conditions and reducing it in hot weather; and active solar, using

additional devices such as pumps and fans to direct warm and cool

air or fluid.

Solar heating systems

Solar hot water systems use sunlight to heat water. These systems

may be used to heat domestic hot water or for space heating but are

most commonly used to heat pools. These systems are basically

composed of solar thermal collectors and a storage tank. The three

basic classifications of solar water heaters are:

Active systems which use pumps to circulate water or a heat transfer

fluid.

Passive systems which circulate water or a heat transfer fluid by

natural circulation. These are also called thermosiphon systems.

Batch systems using a tank directly heated by sunlight.

A Trombe wall is a thermal mass that is heated by sunlight during the

day and radiates stored heat during the evening.

Solar cookingr

Pictured: Solar CookerA solar box cooker traps the Sun’s power in an

insulated box; such boxes have been successfully used for cooking,

pasteurization and fruit canning. Solar cooking is helping many

developing countries, both reducing the demands for local firewood

and maintaining a cleaner environment for the cooks. The first known

western solar oven is attributed to Horace de Saussure.

Photovoltaics

The solar panels (photovoltaic arrays) on this small yacht at sea can

charge the 12 V batteries at up to 9 Amps in full, direct sunlightSolar

cells, also referred to as photovoltaic cells, are devices or banks of

devices that use the photovoltaic effect of semiconductors to generate

electricity directly from sunlight. Until recently, their use has been

limited due to high manufacturing costs. One cost effective use has

been in very low-power devices such as calculators with LCDs.

Another use has been in remote applications such as roadside

emergency telephones, remote sensing, cathodic protection of pipe

lines, and limited “off grid" home power applications. A third use has

been in powering orbiting satellites and other spacecraft.

Total peak power of installed PV is around 5,300 MW as of the end of

2005. This is only one part of solar-generated electric power. For solar

reflector plants see below.

Declining manufacturing costs (dropping at 3 to 5% a year in recent

years) are expanding the range of cost-effective uses. The average

lowest retail cost of a large photovoltaic array declined from $7.50 to

$4 per watt between 1990 and 2005. With many jurisdictions now

giving tax and rebate incentives, solar electric power can now pay for

itself in five to ten years in many places. “Grid-connected" systems -

that is, systems with no battery that connect to the utility grid through

a special inverter - now make up the largest part of the market. In

2004 the worldwide production of solar cells increased by 60%. 2005

is expected to see large growth again, but shortages of refined silicon

have been hampering production worldwide since late 2004.

Solar fibers

A photovoltaic device not using silicon is currently in development.

The device consists of a “solar tape," containing titanium dioxide

(TiO2) in the form of a tape or fiber that could be combined with

clothing or building materials. The material has inferior efficiency to

conventional photovoltaics (5% for an initial commercial version to

“near 12%" in the lab as of 2004, versus 15-30% for conventional

cells). Its advantages are its low manufacturing cost, low weight,

flexibility, function in artificial light, and resulting versatility.

Written by: Tejus Mathew