grand necessity, then, for our bodies is to keep warm, to keep the
vital heat in us."
Henry David Thoreau, Walden
same solar heat that can burn your skin at the beach produces a
daily average of 4 to 5 kilowatts of energy on each square meter
of the earth's surface. Taken as a whole, our planet's surface receives
quadrillions of kilowatts, far more than we need for our energy
needs. All this energy can be directly captured or indirectly utilized
in the form of wind
and water power or biomass
fuels. Direct radiation from the sun can be used for solar
heating, cooking or crop drying, to power industrial processes,
to produce solar electricity, to pump or
distill water, and for many other purposes.
For a better
idea of the availability of solar, wind or water power see the Weather
Data tables later in this web page. You'll find information
on sun angle, average daily solar radiation, average temperature
and annual degree days, average wind speed, and annual precipitation,
in addition to longitude, latitude and elevation, for 168 cities
and towns throughout the U.S.
The sun's thermal
energy is commonly used for space or water heating and increasingly
for industrial processes. For these applications the radiant energy
of the sun is absorbed as thermal energy and then transmitted to
the liquid or gas to be heated. Because dark colors absorb more
of the sun's energy than light colors, the surface that receives
the solar heat is generally blackened or at least darkened.
is generally divided into passive and active applications.
space heating systems collect and utilize solar energy by
design changes and other natural means; it generally excludes
the use of mechanical power or electronic controls.
space heating systems utilize a collector, a circulator, and
The most common
application of solar energy is for solar water heating - for use
in showers, dish or clothes washing, swimming pools, etc.
solar water heating systems - using pumps, controllers and
valves - are usually more expensive than passive systems, but
they are more efficient and often easier to retrofit. However,
they will not operate during an electric power outage. If the
circulating fluid in either an active or a passive system is water,
then provision must be made to prevent freezing during the colder
months. In most active solar water heating systems the sun's heat
is trapped in flat plate collectors or concentrating
collectors that heat a fluid that is then stored in
solar water heating - relying on the natural convection of
heated and cooled fluid - is usually less expensive but often
less efficient. In this case the storage needs to be installed
above or quite close to the tank. The two primary types of passive
water heaters are batch heaters and thermosyphon systems.
heating is generally cost competitive when you account for the total
energy expenses over the life of the system. You can expect a simple
payback of 4 to 8 years on a well-designed and properly installed
water heater. Performance is dependent on how cold the water to
be heated is as well as how much solar radiation is available at
the site. Information on solar radiation, sun angles and temperature
in 168 towns and cities across the USA is given in the Weather
For an index
of manufacturers of solar collectors, glazing, heating and cooling
equipment, sun spaces and greenhouses, as well as solar water tanks
and water pumps, see the Renewable
Energy Manufacturers Index.
The sun's energy
can also be used to heat fluids to high temperatures in order to
produce solar thermal electric power.
A simpler method produces electricity by using photovoltaic cells
(PV cells), which convert the radiant energy of the sun directly
into electrical energy.
were initially quite expensive, PV cells were first used in remote
locations (where other sources of electricity were not available)
for applications such as water pumping, highway lighting or signs,
weather stations, maritime signals, and forest lookouts. On a small
scale, PV cells have proven to be very practical in powering millions
of watches, calculators, radios and other electronic devices. Medium-sized
modules can produce domestic electricity or charge electric automobiles.
At the other end of the spectrum, because of their modularity, PV
arrays can be joined to form small power plants linked to a public
utility grid. If these small power plants are distributed throughout
the grid, each one close to the source of demand, they can reduce
transmission losses and costs.
of PV cells in converting solar energy to electricity has risen
from about 4% when the first silicon cell was developed at Bell
Laboratories in 1954 to over 30% for some concentrating cells today.
During the same time period the cost of PV cells has fallen sharply
- so that we are now approaching the time when solar electricity
will be economically feasible for many homes and businesses. In
most sections of the U.S., about 30 square feet of roof space can
currently supply electricity for an average house. One cost-effective
approach is to integrate solar electricity into the building structure
by using PV roofing tiles, PV curtain walls, etc.
For an index
of manufacturers of photovoltaic cells, modules and equipment -
including equipment for water pumping, outdoor lighting, battery
charging and other applications - see the Renewable
Energy Manufacturers Index.
Browse our glossary
of renewable energy terms or our list of solar
energy organizations for more information.