Transcript Solar Home Systems Part 4

Marshall Islands March 31-April 11, 2008
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2-7. What about battery additives that
are supposed to “rejuvenate” old
batteries. Do they work?
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How are batteries damaged by excess
discharge?
• When a battery discharges the plates swell. The deeper
the discharge the more the plates swell. Batteries that
are not designed for deep discharge have flat plates
that have the active material pressed into pockets in the
plate. When the plates swell greatly due to deep
discharge, some of the active material is pushed out of
the pockets and falls to the bottom of the battery
causing loss of capacity and possible shorting of cells.
Batteries that are designed for deep discharge have
plates that have the active material wrapped in porous
membranes to prevent the swelling from causing the
active material to fall off. This adds considerably to the
cost but increased battery life.
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Choices for Solar Use
• Open cell batteries provide the best value and longest
life but electrolyte levels have to be checked and water
added when needed
• Tubular cell, deep discharge batteries provide the
longest life and should be used where access for
replacement is expensive or very difficult
• Valve regulated, sealed batteries are only
recommended where there is no one to properly
maintain an open cell battery
• Worst choice is an automotive type “maintenance free”
battery.
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Increasing battery voltage
• Add cells or batteries in series. Increments may be 2V
(single cells for large batteries), 6V (three cells in one
case – medium sized batteries), 12V (six cells in one
case, smaller batteries). No problems usually develop
because of series connections though a battery with a
shorted cell can create overcharging conditions for the
rest of the cells because the lower battery voltage that
results from a shorted cell makes the controller think
that the battery still needs charging so the charging
current is not shut off when the good cells do come to
full charge. The result is excessive water loss from the
battery, a definite symptom of a shorted cell in an SHS
battery along with voltage that is lower than expected.
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Increasing Ah capacity
• Put in a larger battery. It is always preferred to use a
single large battery than to connect batteries in parallel
• It is possible to parallel identical batteries just as it is
possible to parallel panels. However never should more
than two batteries be placed in parallel and even then
do not expect as long a life as a single larger battery. If
one cell of either battery loses capacity, both batteries
may rapidly develop sulfation problems. Note that many
battery manufacturers void battery warranties if more
than two are paralleled.
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Battery Safety
•
Most injuries relating to batteries are the result of dropping them or being
hurt somehow by their weight. Do not carry batteries by the connections,
always support the battery from the bottom or sides of the case. Preferably
use a special carry strap made for the purpose. For large batteries share
the load with another person. Many batteries have built in handles. Use
them
•
For open cell batteries, note that the electrolyte is dilute sulfuric acid and
can cause mild chemical burns and is toxic if swallowed. If the acid gets into
your eyes, immediate flushing with water is vital to avoid eye damage. For
that reason, keep a full bucket of water nearby when working with batteries
and battery acid.
•
Be sure cell caps have clear ventilation holes. A plugged ventilation hole will
cause pressure build up in the cell and will cause the battery case to swell
and may cause damage to the battery.
•
Never lay tools on top of the battery. A short circuit could occur and may
damage the battery, cause an explosion or cause burns.
•
Do not smoke around batteries that are charging. Explosive gas is present
in the cells.
•
Do not take the caps off battery cells when charging.
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Marshall Islands March 31-April 11, 2008
Don’t lay tools on batteries!
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2-8. Appliances
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Marshall Islands March 31-April 11, 2008
Marshall Islands March 31-April 11, 2008
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Appliances suitable for SHS
• High efficiency DC lights. Currently CFL type lights
have the highest efficiency but LEDs are getting close
and provide the major advantage of simplicity and very
long life plus they cause no radio interference.
• DC Appliances (battery operated radio/boom box, DC
Televisions)
• Small to medium AC Appliances (videos, fans,
refrigerators, washers)
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Appliances not reasonable for
SHS use
• High demand, low value appliances such as electric fry
pans, electric tea kettles, coffee makers, toaster ovens
• High demand major appliances such as electric
cooking stoves, electric dryers, electric water heaters
and air conditioners.
If you make the SHS big enough, you can run any
household appliance, but the enormous cost of the
large solar installation to run the appliance usually is
not worth it.
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Appliance Energy Comparison
•
•
•
•
•
•
•
B&W TV ≈ 1 10W light
Desk fan ≈ 5 lights
Color TV ≈ 7-10 lights
Ceiling fan ≈ 10 lights
Full Size Refrigerator ≈ 25 lights
Chest Freezer ≈ 40 lights
Room air conditioner ≈ 75 lights
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Converting DC to AC
• Inverter characteristics
– Type of output wave shape (square wave, modified
sine wave, sine wave) must fit the type of appliance
being powered.
 AC motors usually require sine wave inverters
 Electronic equipment usually works well with
modified sine wave inverters
 Square wave inverters should not be used.
– Continuous power rating
– Surge power rating (for motor starting mainly)
– Protection against over voltage, under voltage, over
current, over heating, reverse polarity
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Why not use inverters with all SHS?
•
Cost (up to US$1 per Watt capacity)
•
Power loss (10-15%)
•
Safety (120/220V can be lethal)
•
AC appliances tend to be much lower in energy efficiency than DC
appliances made for solar systems
•
Easy to overload systems by plugging in more appliances
Probably the best approach is to have a dedicated inverter for
each AC appliance. That ensures the right type of inverter is used
and the losses are minimized. Be sure that the inverter is turned
off when the appliance is turned off.
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What about a refrigerator or freezer?
• Around 500 Wp of panel capacity needed just for the
refrigerator. More for a freezer.
• A sine wave inverter is needed with sufficient surge
capacity to start the compressor and sufficient
continuous capacity to run the compressor. Typically
the surge may be 400 Watts for 5 seconds then a
running requirement of 150 Watts continuously. This
varies with the size of the refrigerator or freezer.
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What about power for
a video system
• About 200Wp of panel is needed just for the video
(color TV screen and video player)
• Modified sine wave inverter of 150 W continuous rating
dedicated to powering the video player and associated
color TV.
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2-9. Wiring
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Marshall Islands March 31-April 11, 2008
Characteristics of wiring
• Number of copper conductors per cable (1 or 2 needed
– never use aluminum wire)
• Stranded or solid wire
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• Type of insulation (indoor only, outdoor, direct burial)
• Size of conductor (mm2 or AWG)
• Length of wire
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Choose the correct insulation
• If indoor type wire is used to connect to the panels, the
insulation will crack and fall off after a few years
because of ultra-violet degradation of the plastic by
sunlight. If you must use indoor wire, put it inside a
plastic or metal pipe (solid plastic or metal conduit or
flexible water pipe) to protect it from the sun.
• If you bury the wire, always use wire with insulation
certified for burial since earth fungus and moisture will
gradually cause other types of insulation to fail. If nonburial wire must be used, it has to be placed inside a
plastic or metal pipe before burial and the open ends of
the pipe sealed with silicone rubber to prevent water
and insect entry. Burial type wire will be damaged if
exposed to sunlight. Where it comes out of the ground
it should be enclosed in plastic pipe and sealed with
silicone rubber at the end of the pipe.
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Connections
• Because 12V is very low pressure electricity,
the wiring connections must be of very low
internal resistance. Otherwise the voltage drop
will be excessive.
• The connections need to be ones that will not
change internal resistance over time due to
corrosion or loosening
• Only screw type connections or properly
soldered connections should be used. Note
that most field soldered connections are “cold”
solder joints and are not adequate. Solder must
be fully melted and flowing onto the wires.
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Improper connections
• Twisting wires together will quickly result in high voltage
drops. DO NOT USE
• Using “twist lock” connectors that twist the two wires
together in a plastic holder will be better than just
twisted wires but will gradually increase in voltage drop.
DO NOT USE
• Crimped lug connections are ok only if the proper tool
is used by a person well trained in the proper technique
for crimping. Rarely are crimped lug connections made
in the field good for more than a year or two. DO NOT
USE
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Marshall Islands March 31-April 11, 2008
Marshall Islands March 31-April 11, 2008
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Switches
• Switches often greatly increase in voltage drop with
age, particularly cheap “slide contact” type switches
commonly used for 220V installations.
• For long life, use snap action “toggle” type switches
that use a spring to snap the contacts together,
preferably a type that is designed for DC use.
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Check Switches Annually
• Voltage drops on switches should be checked at least
once a year. To do that, turn the switch on so the load is
operating and measure the voltage across the switch
terminals. It should not be over 0.05V.
• If the cheap “slide contact” switches are used, they
should be replaced every time the battery is replaced to
ensure that the voltage drop does not become
excessive.
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2-10. Installation of SHS
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Marshall Islands March 31-April 11, 2008
Marshall Islands March 31-April 11, 2008
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Panels
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Free of shade 0900-1500
•
At least 60 mm (2.5”) of space behind the panel for ventilation, preferably
100mm (4”)
•
Facing the south and tilted at about the latitude angle but greater than 10°
•
Mounting uses non corroding screws or bolts preferably of marine grade
stainless steel
•
Mounting meets the requirement for strength under storm conditions
•
Wire fastened to the mount in a way that does not put a strain on the
connection boxes.
•
Terminal screws include lock washers and are tightened properly
•
Wire is big enough to keep losses at less than 0.5V between the panel
and battery at maximum current from the panel, usually 10AWG or 4mm2
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Battery
• Sits level on a solid surface that will not deteriorate if
exposed to acid (not wood or metal)
• Cool site not exposed to the sun
• Good ventilation for disposal of explosive gasses
• Covered to reduce dirt accumulation on the top of the
battery but readily accessible for service
• Located as close to the loads as practical to minimize
wire lengths.
• Located in a place not likely to be a play area for
children.
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Controller
• Must be within 2 meters (6 feet) of the battery and
preferably less
• Located in a cool place with adequate ventilation to
carry away generated heat
• Firmly affixed to the building with non-corrosive screws
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Appliances
• Place lights where the user prefers
• Arrange wires so the minimum length is needed
• Use the right size wire to fit the length used. Usually AWG 12 or
2.5mm2
• Mount lights for maximum lighting of the area desired by the user
with minimum loss of light in other directions
• Use non corrosive screws for mounting. Do not hang lights by
connecting wires (unless the light is designed for that type of
mounting) or tie lights to beams with cord.
• Provide a connection point in a convenient location for radio or
B&W TV use.
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Fuses
•
Fuses are intended to protect components (mainly the controller) and to
prevent overheating of wiring and possible fires.
•
Fuses must have low internal resistance that remains low for the long term.
•
Glass type cartridge fuses have not worked well with PV systems and are
not recommended. Automotive type clip-in fuses have worked satisfactorily
and are recommended
•
Circuit breakers have not worked well in small PV systems and are not
recommended.
•
At least one fuse located near one of the battery posts is best. It should
have an Ampere capacity about the same as the maximum safe carrying
capacity of the load wiring.
•
For larger systems with many circuits or systems with several larger loads,
a fuse may be needed for each load circuit that limits the current to the safe
capacity of the wiring in that circuit. Safe capacity is the wire’s rated
“ampacity”.
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Wiring
• Ensure wire sizes are appropriate for the load and length of
wire
• Install an automotive type plug-in fuse in the wire near one
battery connection
• All wire junctions and connections should use screw type
connectors with non-corroding screws.
• Fix wires to the building with clips at least once per meter
(3 feet).
• Place clips within 15 cm (6") of all connections
• Place clips within 15 cm (6") of each side of a right angle bend
in the wire
• Avoid circuit breakers, use automotive fuses instead.
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Lightning Protection
• For SHS where there is no AC power from an inverter,
no grounding or specific lightning protection is needed.
• Where there are AC appliances, the inverter should be
properly grounded and usually so is the negative side
of the battery circuit.
• All connections to earth should be at the same spot.
• Some believe that a large copper wire connected to the
panel frame and run directly to the earth connection
adds lightning protection. There is no clear evidence
whether this works or not.
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