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Solar Thermal |
Have you ever felt warm water
trickle out of a garden hose that’s been
sitting in the sun? If so, then you’ve witnessed
solar water heating in action.
Now imagine that
same water moving slowly though a system specifically
designed to heat and store water – that
is the essence of solar thermal water heating.
People have for centuries used water heated by
the sun and stored it for bathing, hand washing,
cleaning clothes, heating homes and much more.
The solar thermal systems used today combine the
most effecient techniques for capturing the sun’s
heat with modern plumbing systems to produce cost
effective hot water and reduce the need for gas
or electricity to heat water.
There are a number
of different solar thermal designs, but all are
based on the same simple principle as the garden
hose. Each has its pros and cons, and each is
suitable for a specific application. Consult with
your local installer to determine which is best
for your situation. |
Active solar thermal panel on the
Southface Energy and Environmental Resource Center
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What
are the different types of systems? |
Passive
vs. Active |
The terms
passive and active in solar thermal systems refer to
whether the systems rely on pumps or only thermodynamics
to circulate water through the systems. |
Passive
The simplest systems are
passive solar water heaters, also called batch
or breadbox collectors, they are most common in
regions that do not experience extensive periods
of below freezing temperatures. The water in these
solar collectors circulates without the aid of
pumps or controls.
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Active
Active solar water heaters
use pumps to circulate water or an antifreeze
solution through heat-absorbing solar thermal
collectors.
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Direct vs. Indirect
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An important
distinction among solar thermal systems is whether they
are of direct or indirect design. In a direct system,
the water used by building occupants to wash their clothes
or bathe is the same water that is pumped through the
solar collector. In an indirect system, an antifreeze
solution is pumped through the solar heat collector.
This warm solution is then used to heat the water used
by building occupants. In this case, water is indirectly
heated.
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Indirect Systems
In an indirect system, also known
as closed loop, a simple pump moves the antifreeze
solution through a loop into the solar collector,
through the collectors pipes, and out of the solar
collector. Then, the sun-warmed antifreeze solution
flows into a heat-transfer unit where it warms
the cool water heading into a conventional hot
water tank. The antifreeze solution then returns
to the pump and again flows into the solar collector
without ever mixing with the buildings water.
Indirect systems are encouraged in climates with
extended periods of below-freezing temperatures.
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Direct Systems
A direct system,
also known as open loop, is a little simpler.
There is no antifreeze solution; the water heated
directly by the sun is the same water used by
building occupants. A thermometer and controller
sense when the solar collector is warm and ready
to heat water. The controller starts a pump that
moves cold water into the solar collector, where
it is heated. The solar heated water is then stored
in a conventional hot water tank. It is typical,
especially during high use or periods of little
sun for the water to be kept warm through supplemental
gas or electricity. This type of system, because
it circulates pure, potable water through an outdoor
collector, is susceptible to freezing in many
climates, unless additional safeguards are added.
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In reality,
both direct and indirect systems are somewhat more complicated
than presented here. Differential thermostats, pumps,
sensors, and controls are used so the simple systems
illustrated work effectively and safely. However solar
thermal technology is mature and proven with few maintenance
requirements from the installed systems. |
Collecting
the Sun |
Solar thermal
systems also differ by the type of collector used to
gather and store the sun’s energy. Flat plate
collectors are the simplest and most common type. Copper
pipes wind back and forth through the flat plate collector,
which is painted black to absorb heat and covered with
glass, or “glazing,” to prevent heat from
escaping. Often the pipes are painted black and bonded
to the material of the flat plate collector to maximize
heat absorption. |
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Solar pool heating systems use a similar design, but
sometimes glazing is removed to save money and to prevent
the pool water from becoming super-heated. Some non-glazed
systems look like flat black mats. Inside the mats is
a network of headers through which the water slowly
passes. |
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More advanced systems like evacuated tube collectors
and parabolic trough collectors can heat water or other
fluids to much higher temperatures appropriate for industrial
needs. |
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Photovoltaics = Photo
(Light) + Voltaics (Electricity) |
Photovoltaic technology has come a long way since
Bell Labs produced its first functional solar
cell in 1953. But the basic theory is still the
same…
The sun’s
waves hit a photovoltaic cell and excites the
electrons within layers of the cell. The excited
electrons jump back and forth, creating electricity.
This electricity is captured by wires running
through the PV cells and sends the electricity
into your home. The electric current generated
by PV cells is direct current (DC), or the type
of current used in batteries. Most of the appliances
in the United States run off of alternating current
(AC), or the type of current that comes over power
lines. If you decide to use conventional appliances
in your building, the electricity from the solar
cells will now go into an inverter where it will
be turned into alternating current. From the inverter
the electricity will then be used by the appliances
and systems in your home or go out into the grid.
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Photovoltaic
cells are almost always arranged on a panel
to form a solar module. Modules are then
linked in series to form what is known as
a solar array. The size of a solar module
or array is most commonly given in terms
of its peak power production, or, Watts-peak
(Wp or just W.) Let’s say, for example,
that Solar Incorporated makes a 100 Watt
solar module, which is comprised of 50 cells
at 2 Watts each. This module generates 100
watts of electricity when fully exposed
to bright sun. If 10 of these modules were
combined in series, they would form a 1000
watt, or, 1kilowatt (kW) solar array. |
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The Grid
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Once you have
decided to use photvoltaics, you must choose whether
its power will be: |
a)
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connected to the
conventional electricity grid |
b)
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connected to the
building and a series of batteries that will supply
power during hours without sun or remote location |
c)
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a combination
of the two |
If the solar
array is supplying a home with access to the electrical
grid, it is recommended that the system be grid-connected
(also called grid-tied). In a grid-connected solar system,
all electricity generated is sent directly to the grid.
Your electricity bill will reflect your net electric
usage or the difference between the amount of electricity
your solar panels produced and the amount of electricity
you used.
Inverter and wiring hookups for
Southface Energy and Environmental Resource Center
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Solar photovoltaic panels produce
direct current (DC) electricity. Direct current
is one type of electrical current; alternating
current (AC) is another. In the United States
, the vast majority of residential and commercial
appliances and equipment use AC current. Power
plants produce AC current. The majority of DC
current usage is for devices that use batteries.
An inverter is
a key component of a photovoltaic system and is
used to turn DC current into AC current. Electricity
can then be directed back to the electrical grid.
In states like Georgia with net metering laws,
the power company must purchase electricity from
the PV array owner |
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