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Types
of
Solar Electric Systems |
| Home |
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| Types
of Solar Energy Systems |
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Energy
Source
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Connected
to the electricity grid?
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Energy
storage device in the system?
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Examples
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| Grid-tied*
solar system |
Solar
Cells**
|
Yes
|
No
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Home
system that draws on the electricity grid at night and
exports excess power in the day
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| Stand
alone grid- tied* solar system |
Solar
Cells**
|
Yes
|
Yes
(batteries)
|
Home
or business system uninterruptible power (e.g. for computers,
servers). Still operates when the grid is down
|
| Stand
alone solar system without energy storage |
Solar
Cells**
|
No
|
No
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Water
pumping
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| Stand
alone solar system with energy storage |
Solar
Cells**
|
No
|
Yes
(batteries)
|
Remote
homes, lighting, TV, radio, telemetry
|
| Stand
alond off-grid Hybrid solar system |
Solar
Cells** in combination with another energy source (e.g.
diesel, wind)
|
Most
often not
|
No
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Remote
large scale communications,
industrial uses
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* also known as grid connected
** also known as photovoltaic cells
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| Grid-tied
solar system (alternating current) |
|
This
type of system requires no storage equipment (i.e. batteries).
The
crucial issue relative
to the PV Systems discussed below is the technical aspects of
tying into the electricity grid. In these applications, grid-tied
inverters must be used that meet the requirements of the utilities.
They must not emit "noise" which
can interfere with the reception of equipment (e.g. televisions),
switch off in the case
of a grid failure and
retain
acceptable levels of harmonic distortion (i.e. quality of voltage
and current output waveforms).
This
type of system tends to be an optimum configuration from an
economic viewpoint because all the electricity is utilized by
the owner during the day and any surplus is exported to the
grid. Meanwhile, the cost of storage to meet night-time needs
is avoided, because the owner simply draws on the grid in the
usual way. Also, with access to the grid, the system does not
need to be sized to meet peak loads.
Inverter and Controller
|
| Stand-alone
grid-tied solar system (alternating current) |
| This
type of solar energy system is the same as the grid-tied system
above except that battery storage is added to enable power to
be generated even when the electricity grid fails. The additional
cost to the customer can be quantified against the value of knowing
that their power supply will not be interrupted. |
| Stand-alone
off-grid solar system without energy storage (direct current) |
|
In
this configuration (i.e. without any energy storage device),
the PV system output is dependent upon the intensity of the
sun. The more intense, the greater the output. In this simple
system, the electricity generated is used immediately. Therefore,
the application must be capable of work on both direct current
(DC) and variable power
output.
This
type of system works well for water pumping or greenhouse ventilation.
Specialized solar water pumps are designed for submersible use
(in a borehole) or to float on open water. Usually, the ability
to store water in a tank means that battery power storage is
unnecessary. Each PV cell within a solar module generates a
small amount of electricity, so the cells are normally connected
together within modules, or panels, to produce a useful amount
of power.
Modules
are weather-proof and usually have an expected lifetime of twenty
to thirty years. A module can be as big as a window or as small
as your computer screen. PV modules, on their own, generate
direct current (DC) electricity like that from a battery.
For
any module with a defined peak power, the actual amount of electricity
in kilowatt hours (kWh) that you will get from it depends mainly
on how much sunlight it receives. The electrical power output
of a PV module is the current that it generates (dependent on
its surface area) multiplied by the voltage at which it operates
(a function of the active material in the PV cell).
The
bigger the module, or the solar array (an array is simply a
number of modules connected together), then the more power
is generated. A Linear Current Booster can be added which converts
excess voltage into amperage in order to keep a pump running
in low light conditions. An LCB can boost pump output by 40%
or more. For safety considerations, PV arrays are normally earthed
(grounded).
|
| Stand
alone off-grid solar system with energy storage (direct current) |
Where a customer requires the guarantee of a certain power output
at any time of the day or night, either some kind of storage
device is necessary or the PV system should be combined with
another energy supply (like propane, diesel generator).
The latter is known as a hybrid system (see below). Most off-grid
systems use batteries to store power during periods of low or
no sunlight. Rechargeable batteries are the most effective storage
mechanism available. Notwithstanding this, the electrochemical
conversion process of the battery utilizes about 20-25% of energy
produced, so storage is only about 75% efficient. The storage
capacity of batteries is rated in ampere hours, which is the
current that it delivers over a  set number of hours at a normal voltage
at a temperature of 25șC. Batteries must be protected from the
elements.
Most PV systems use lead acid batteries, either sealed or conventional
flooded batteries. Nickel cadmium batteries are usually the
choice where very high reliability is required. A charge controller
is needed to prevent over- and under-charging of the battery.
If the peak charging rate of the solar module is more than 1.5%
of the battery ampere hour capacity, a charge regulator is typically
necessary.
The quality of the regulator is a key factor in the reliability
of the overall system, because it will align the depth of discharge
with the battery temperature and the rate of discharge. Blocking
diodes perform the role of preventing reverse discharge of the
battery through the modules at times of low or no sunlight.
This prevents damage to the modules and reduces energy losses.
Monitoring current and voltage throughout the system is important
for safety and overall system performance. A voltmeter will
monitor the performance of the battery, while an ammeter monitors
the output of the solar modules. For lighting applications,
high quality compact fluorescent lamps are available with good
lifetimes; poor quality lamps will blacken quickly and their
light output will drop off.
|
| Stand
alone off-grid solar system with energy storage (alternating current) |
In addition to the previously mentioned equipment, an inverter
is required in this case to convert DC into AC electricity. The
conversion process will cause some energy losses, depending upon
the efficiency of the inverter. Inverters are not chargers.
Some inverters have a built-in charger to help compensate for
what the inverter will draw out of the battery when operating
an appliance. However, the benefit of AC systems is that AC components
(wiring, etc.) are usually cheaper
than DC components and AC products like TV's are in more common
use.
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| Stand
alone off-grid solar hybrid system |
To
meet the largest power requirements in an off grid location, the
PV system is sometimes best configured with a small diesel generator.
This means that the PV system no longer has to be sized to cope
with the worst sunlight conditions available during the year.
The diesel generator can provide the back-up power, but its use
is minimized during the rest of the year of the PV system, in
order to minimize fuel and maintenance costs. |
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