Download Solar Home Systems Part 3

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2-5. Controllers
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Marshall Islands March 31-April 11, 2008
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Why have a Charge Controller
• Open cell batteries can lose water quickly if
overcharged. If electrolyte falls below the top of the
plates damage occurs.
• Sealed batteries may be ruined if frequently
overcharged
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Why have a Discharge Control?
• To prevent damage to batteries from excessively deep
discharge
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Shunt (Parallel) Controller
•Placed between the panels and
the battery
•To prevent overcharge, the
output from the panel is shorted
by the controller using a
semiconductor switch
•Because the panel wires go to
the battery, a blocking diode has
to be installed or shorting the
panel output would also short the
battery
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Advantages of the Shunt Controller
• Simple and cheap. Lends itself to local production
• Less likely to be damaged by excess current flow than
series switching control.
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Disadvantages of Shunt Control
• All the panel power is shorted through the
controller and converted to heat. So large
panels generate a great deal of heat and it is a
problem to get rid of it. Shunt controls are
therefore best suited to small (under 50Wp) PV
systems.
• Sensitive to lightning
• Requires a blocking diode with its attendant
power loss
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Series (switching) Charge Control
• Placed between the
panels and the
battery
• A transistor switch or
a relay is used to
disconnect the panel
from the battery so
overcharging cannot
occur
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Advantages of Relay Type Switching
Controller
•
Relay switching is very resistant to lightning strike damage
•
Very low voltage drop and internal losses
•
There is no heat dissipation problem so it can be used with any
size of PV system without problem.
•
Typically has lower power losses than the shunt controller since
there is no blocking diode
•
Simple circuitry that can usually be repaired locally
•
No microchips required so less vulnerable to moisture and
corrosion
•
No high frequency circuitry so less vulnerable to dirt and salt
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Disadvantages of relay type
controllers
• Usually more expensive than semiconductor controllers
• Difficult to incorporate other than on/off type of
switching. Semiconductor switches can do high
frequency pulse charging, tapered charging and other
more sophisticated charging methods.
• May draw more current for its internal operation than
semiconductor circuits though good designs minimize
this potential problem.
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Relay type controller
High reliability, simple relay type controller
designed by S.P.I.R.E. and constructed in
Kiribati by the Kiribati Solar Energy Co.
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Disadvantages of Semiconductor type
Switching Controllers
• Semiconductor switching type of controller is sensitive
to lightning damage (not the case for relay types)
• Easy to damage by excess current flow through the
semiconductor controller (not true for relay types)
• Higher internal voltage drop than relay controllers
• In the Pacific, the experience has been that the
operating life of relay controllers is around twice that of
all but the best semiconductor controllers
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Discharge controller
• Placed between the battery and the loads
• All are of the series switching type. Often use a relay
even if the charge controller uses semiconductors but
many use a semiconductor switch
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Causes of Controller Failure
• Nearby lightning strikes. The controller switch is directly
in the path of induced voltages in the long wire
connecting the panel and the battery.
• High temperatures caused by blocking the ventilation
around the controller or placing it in a hot location
• Damage by technicians and users
• Insect damage
• Corrosion of circuit boards due to poor design
associated with salt exposure, high humidity and high
temperatures
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Choosing a controller
•
Reliability is the result of a simple design
•
Avoid complex extra “features” such as many indicator lights, LCD
screens, micro-processor controls
•
Choose a control that fits the type of battery being used especially
if it is for a sealed battery since they require specific controller
features for maximum life.
•
Be sure there is adequate current capacity for both charging and
for operating the loads
•
Low power loss with small internal voltage drop and low internal
energy use.
•
Housing prevents insect entry and water entry
•
Good lightning protection
•
Proper voltage of operation
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Important Installation Note
• There should NEVER be more than 2 meters (6 feet) of
wire connecting the controller to the battery and if
possible it should be shorter than that. The wire should
be the same size as the panel wire or larger.
• Longer wires between the battery and controller result
in inaccurate sensing of battery voltage by the
controller and improper operation.
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2-6. Batteries for Solar Home Systems
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Marshall Islands March 31-April 11, 2008
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Types of Batteries
• Lead-Acid
– Cheapest, mature technology, readily available
– Easily damaged by improper discharge control,
some types require periodic maintenance
• Nickel-Cadmium
– Expensive, mature technology, not readily available
– Long life, minimal maintenance
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Types of Lead-Acid Batteries
• Open cell automotive starting battery (NO GOOD)
• “Maintenance Free” starting battery (NO GOOD)
•
“Solar Modified” automotive battery (OK)
• Traction battery (powers vehicles) (GOOD)
• Stationary (intended for back up power) (POOR)
• Tubular cell deep discharge (EXCELLENT)
• Valve-Regulated sealed battery (GOOD)
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General Battery Characteristics
• Nominal voltage (number of 2V cells)
• Capacity in Ampere hours
• Open cell or sealed
• Liquid or Gel electrolyte
• Cycle life
• Acceptable repeated depth of discharge
• “Starting Amps” “Number of Plates” or “Starting
Minutes” not useful for solar specifications
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Charge/Discharge Rate
• Rate of charge or discharge is based on the
capacity of the battery in Ah and the time taken
to bring it to full charge or full discharge
• Designated as Cx where C represents the
capacity in Ampere Hours and x represents the
number of hours to fully charge or discharge
the battery at a fixed number of Amperes. Thus
C10 means that the full capacity of the battery is
discharged in 10 hours at a fixed rate.
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The effect of discharge rates
• The more hours taken to discharge a battery,
the more energy can be transferred because
with a slow discharge the chemical process
that produces electricity is more efficient. So a
battery delivers more Ah at C100 than at C10 by
a quite significant amount. Note that C100
means that the battery takes 100 hours to
discharge fully while C10 means it only takes 10
hours to discharge fully. A 100Ah battery at C100
may become a 65 Ah battery at C10
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Battery Ah ratings
• A 100 Ah battery at C100 may be a 65 Ah battery at C10. So to
compare batteries, the battery Ah rating must include the Cx rate
for the stated capacity and you must compare at the same Cx rate.
• Manufacturers of solar batteries, particularly those of questionable
quality, often give Ah ratings using a C100 discharge rate. That
gives a substantially inflated Ah value but that capacity is never
reached in practice.
• Always compare battery capacities at the same discharge rate,
preferably C10 or C20 (C20 represents the typical solar discharge
rate for SHS and is best though C10 comparisons are commonly
done and are ok. Just be sure all comparisons are at the same Cx
rate.)
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Cycle Life
• Cycle life is the number of times that a battery can be
expected to be fully discharged then immediately fully
charged before losing 20% of its rated capacity.
• Partial cycles add up to full cycles. For example, five
days of 20% discharge/charge add up to be the
equivalent of 1 full cycle. So with a 20% average daily
discharge, a 300 cycle life battery will work 1,500 days
(about 4 years) before the cycle life is exceeded.
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Measuring the level of charge
• Battery voltage. About 10.5V represents full discharge.
Full charge voltage for a battery with no current flow in
or out will be around 12.6V. When charging, full charge
is reached at about 14.2V
• Electrolyte specific gravity. This is an indication of
acid concentration and is measured using a
hydrometer. The higher the value the greater the
charge (1.26 to 1.28 is full charge, 1.0 to 1.1 is about
fully discharged).
Neither battery voltage nor specific gravity is an
accurate measure of charge, especially in old batteries.
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Accuracy of charge estimation
• The use of voltage to determine level of charge is not
very accurate because the rate of charge or discharge
affects the voltage too. As the battery gets older the
accuracy of voltage readings as an indicator of state of
charge during charging or discharging gets less and
less because the battery’s internal resistance goes up.
• The use of specific gravity is accurate when a battery is
new but as the battery ages, the hydrometer tends to
show a lower charge than is actually present due to
increasing sulfation
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Causes of Battery Failures
• Sulfation – Most common problem. Sulfation is where
part of the cell becomes resistant to charge. Caused by
the battery remaining at partial charge for long periods.
May be offset by equalizing charges when cells are
seen to have unequal specific gravity.
• Internal corrosion – results in high internal resistance
and open circuits. Caused by cheap design, adding
acid instead of water and stratification of the acid in
some types of batteries
• Internal shorts – results in one or more cells not
producing voltage. May be due to cheap construction,
overheating or mechanical damage
• Loss of active material from plates – caused by
excessive depth of discharge and age. Mostly a
problem with cheaper batteries.
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What is Sulfation?
• When a battery discharges, Lead Sulfate is created.
When the battery is recharged, the Lead Sulfate is
supposed to dissolve. But if the Lead Sulfate is not
dissolved after a week or so because the battery is not
fully charged, it tends to form a mass that is very
difficult to dissolve when charging does take place.
Over time the amount of Lead Sulfate increases and
the battery loses its ability to be charged fully. The
effect is a loss of capacity. A 100Ah battery may
become a 50Ah battery after serious sulfation has
occurred.
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What are the signs of sulfation?
• When battery voltage indicates a full charge but the
hydrometer reading indicates a partial charge, that is a
strong indicator that serious sulfation has occurred in
the battery.
• Lead Sulphate is white in color. When looking at the
plates in a battery, if the battery has been fully charged
and the plates look light in color or have white sections,
that is an indication of sulfation
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Failure modes of batteries
• Total loss of power. Zero volts, cannot charge. Caused
by an internal open circuit. This may be because of
corrosion eating through a cell connector or mechanical
damage
• Gradually decreasing capacity. The time to charge and
discharge gets shorter and shorter. Caused by sulfation
or loss of active material from the plates or both.
Accelerated by deep discharge conditions and
operation at partial charge levels for weeks at a time.
• Reduced voltage at full charge. Cannot get the battery
to charge to more than about 10V. Caused by a short in
a cell making one cell inoperative. Excessive discharge
and mechanical damage are typically the reasons for
this mode of failure.
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