| How Solar Thermal and Photovoltaics
Work |
| 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.
Click to enlarge
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Active
Active solar water heaters use pumps to circulate water or an antifreeze solution
through heat-absorbing solar thermal collectors.
Click
to enlarge
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| Direct vs. Indirect |
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 collector’s 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 building’s 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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Click to enlarge |
Click to enlarge |
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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) |
connected to the conventional electricity grid |
| b) |
connected to the building and a series of batteries that
will supply power during hours without sun or remote location |
| c) |
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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