Solar Thermal Energy
When
solar energy is mentioned, most people think of photovoltaic energy
systems that are mounted on the rooftops of homes sprinkled throughout
their community. What isn’t in the forefront of most people’s thinking
is solar thermal energy.
Solar thermal energy (STE) is the technology of harnessing solar energy
for the purpose of creating thermal energy (heat). It’s been around for
hundreds, and even thousands of years. The government continues to give
out tax incentives to encourage individuals to install a
home solar system.
One of the
advantages photovoltaic has over solar thermal technology is that it
produces electricity that can be used for a variety of applications.
Solar thermal energy is primarily used to heat water. Another
disadvantage of solar thermal energy is there aren’t as many uses for
hot water in the summer time as there are in the winter. The days are
also shorter during the months hot water is needed the most. Shorter
days also equate to less solar energy being absorbed by the solar
panels.
Solar energy
has been used for decades for hot water solar systems. If the system is
for heating a swimming pool, in many areas of the world, you can get an
additional two to four months of swimming in as a result.
Solar hot water systems are
cost effective, environmentally friendly, and for the most part
maintenance free. They can now be installed on your roof to blend in
with your home’s architecture. Of the 1.5 million homes and businesses
throughout the country that have solar hot water systems, surveys have
indicated that more than 94 percent of those owners consider their
investment to be a good one. SolarHot is a manufacturer of these systems
that boasts of having the highest rating of any freeze-protected solar
hot water systems that use less than 95 square feet of collector area.
Their OG-300 system uses only 64 square feet of collector area. The
Solar Energy Factor (SEF) rating they received was 86%, far above many
of their competitors.
Most solar
hot water systems with flat plate solar collectors are installed facing
south to south-east on a rooftop. Did you ever wonder why a shallow lake
is normally warmer than a lake with deep water? That is because the
sunlight heats the lake bottom in the shallow areas. This in turn, heats
the rest of the water. This is nature’s way of using
solar heat. The sun can also
be used in this same way to heat water that is used in buildings or
swimming pools.
When hot
water systems are used for buildings, there are normally two main parts.
The first is the solar collector, which is normally called a flat-plate
collector that is mounted on the roof. This collector consists of a
thin, flat, rectangular box with a transparent cover that is facing the
sun. Through this thin box run small tubes which carry the fluid. That
fluid is either water or some other fluid, such as an antifreeze
solution. These thin tubes are attached to an absorber plate which is
black in color to absorb the heat. At the heat builds up inside the
collector, the fluid that is passing through it is also heated. The
second part of this kind of system is a storage tank that holds the hot
liquid. This container is usually larger than a regular water heater and
is very well insulated. Systems that use something other than water have
a coil of tubing inside the tank. As the fluid passes through this
tubing, it heats the water.
There are
two types of solar water heating systems. The active type of solar hot
water heating system relies on a pump to move the liquid between the
collector and the storage tank, while the passive system relies on
gravity to circulate the water.
With a
swimming pool system, the pool’s filter pump is utilized to pump the
water through the solar collector. That solar collector is normally made
of black plastic or rubber tubing. The pool is what stores the water in
this case.
The United
States Energy Information Administration classifies solar thermal
collectors in three separate categories. Those categories are
low-temperature collectors, medium-temperature collectors, and
high-temperature collectors. Low-temperature collectors are generally
used to heat swimming pools and use flat plate technology.
Medium-temperature collectors are also normally flat plates, but are
used to heat water or air for residential and commercial use.
High-temperature collectors concentrate sunlight using mirrors or
lenses, and are generally used for electric power production.
STE is quite
different from photovoltaic
energy, which converts solar energy directly into electricity. They
are also much more efficient. Another use for solar thermal energy is to
power smelting furnaces. A solar furnace is a structure that uses an
array of curved mirrors that acts as a parabolic reflector. This
concentrates light onto a focal point somewhat like a magnifying glass
does using sunlight to start a fire. The primary difference is the
smelting furnaces use reflected light which has a small focal point. The
smelting furnace in Odeillo, France reaches a temperature of 3,500
degrees. The curved mirror for this furnace is as tall as an eight story
building. The focal point is about the size of a cooking pot. This
system has been in operation since the 1970s. The salt flats, located in
the Western United States use solar thermal power to evaporate the
water, which allows them to retrieve the salt. There is a similar solar
oven located in Parkent in Uzbekistan which melts down aluminum. This
smelting oven is called the “physics-sun.” The biggest smelting oven in
the world is found in Rehovot on the terrain of the Weizmann Institute
of Science in Israel. It is used for scientific experiments. There is
one other big solar furnace called the Solar Two Furnace located near
Barstow in California (U.S.A.).
Solar Thermal Energy For Electrical Production
Believe it
or not, but solar thermal energy is actually more efficient than the
traditional photovoltaic cells when creating electricity from sunlight.
Photovoltaic energy converts photons of the sun directly into electrons.
Solar thermal energy uses heat from the sun to power a turbine, which in
turn powers a generator. This generator creates electricity in a similar
to coal or nuclear fired plants. One of the biggest advantages of solar
thermal energy over photovoltaic is that reserve energy can be stored as
heat much easier than it is to store electricity. Instead of needing
large banks of expensive batteries, thermal energy is stored in heated
sand or heated liquid. This energy can be stored overnight. As a result,
you have the ability to create a supply of electricity around the clock,
24 hours a day.
Most of the
large scale solar thermal plants are now being constructed overseas in
such places as Australia or Spain. There are also some large-scale solar
thermal energy projects being constructed in the United States.
A
New Solar Hybrid Breakthrough
Now it’s
possible to have a solar panel that produces both photovoltaic energy
and thermal energy at the same time. There is a team of researchers from
MIT and Boston College that have developed a hybrid flat panel that is
capable of doing both. These panels are eight times more efficient than
the previously developed solar thermoelectric generators and are more
cost effective. With this new technology, solar thermal energy could
become a much more valuable investment. The energy output has increased
significantly without much of an increase in cost.
The team
used nanotechnology to combine solar absorbers that are
spectrally-selective with a high performance thermoelectric material.
This new product actually produces both electricity and hot water. It is
touted that this system will give a much faster payback on the
investment. They are saying it could shorten the time by more than 30%.
The problem
most PV panels have is a drop-off of efficiency when the solar panel
heats up. A photovoltaic panel loses about one percent of its efficiency
for every 3.5 degrees Fahrenheit temperature increase. Solimpeks, a
solar panel manufacturer, has also addressed this problem with this
hybrid technology. This is how it works. Their panels address this
problem by using water to absorb this excess heat. This keeps the panels
cooler which equates to keeping the production at a higher level. The
heated water is then used to supply hot water to the building. With this
technology, it has been shown that there is a 20 percent improvement
over similar sized electric-only PV arrays. What’s better yet is that
traditional PV systems don’t provide any hot water. Now that’s a nice
added benefit. Another benefit to this hybrid system is it extends the
life of the panels, since heat is the one factor that can shorten their
life-span.