solar pv and stand-alone power systems in rural …...pv practical split into 5 or 6 groups each...
TRANSCRIPT
Stand-alone power supply
systems?
• SAPS are small electricity supply systems based
upon a single generator (sometimes a few
generators) that is physically close to the loads.
• No long transmission system, hence lower • No long transmission system, hence lower
voltages used
• No clear distinction between a national grid and a
large stand-alone power system
• Typical sizes: 10’s watts up to 10’s of Kilowatts
• Far from any national grid.
• Un-economic to connect to
a national grid.
Why use SAPS?
a national grid.
• National grid does not exist.
• Many reasons to use a stand-alone systems.
• Focus on community power supplies for developing world
applications, but the principles apply to all.
Why use SAPS?
• Around 1.6 billion people do not have access to an
electrical grid – mainly in developing countries.
• To locally supply electricity SAPS are used, with the vast
majority of SAPS are based on diesel generators.
• Depending upon the size of load and the distance to the
national grid, there is a point at which a stand-alone
system becomes economic.
• Can have lower investment costs, but may prove
expensive in the long term due to fuel and maintenance
costs.
Benefits of access to electricity
• Reduced hours spent doing labour intensive tasks
• Improved lighting
– Extended working hours can be economically beneficial
– Less indoor air pollution
• Improved communications
• Improved healthcare
– Access to refrigeration for medical supplies– Access to refrigeration for medical supplies
• Improved education
– Reduced time spent on labour intensive tasks
– Better lighting so more time for study
Diesel generator based SAPS
• Majority of SAPS are based on a diesel generator.
Problems with the use of diesel generators include:
Cost of fuel and its transportation
Local environmental effects
Global environmental effectsGlobal environmental effects
Security of fuel supply
Inefficient when partially loaded
Start-up response time
Noise
Require a high level of maintenance
Benefits of renewable energy
�Inherently distributed.
�Usually some form of renewable energy
locally available.
�Buffer from fluctuating fuel costs.
�Local environmental benefits.
�Local health benefits.
�Global environmental benefits.
Problems of integrating renewable
energy
• Variations in supply
• Variations in demand
• Aggregation• Aggregation
Variability of generation and
load
Variation of wind
Variation of typical loads
Variation of solar
Matching supply and demandP
ow
er
MismatchSupply
Demand
P
Time
Excess
Deficit
AggregationAggregation
Tota
l S
ignal
1 signal
10 signals
1000 signals
Time
Large networks with many generation units and loads benefit from aggregation
PV based water pumping
PV based battery charging
PV based battery charging
Wind - PV hybrid systems
Sitio Buli, Lubang Island, Philippines
1 kW wind turbine / 300 Wp solar
Potable water pumping system
Installed July 2007
Wind - PV hybrid systems
Wind based SAPS
• Barangay Lamag,
Quirino, Ilocos Sur,
Philippines
• 500 W wind turbine
• Electrification of church
building and rectory
• Installed March 2006
Wind based SAPS
SOLAR PV BASICS
DC based systems• Main components:
– Energy source
– Energy storage
– Regulator/controller
System voltage
• System voltage is the DC voltage of the energy store, the
generators and loads.
• System voltage is the most important system parameter.
• Affects currents flowing through the system.
• Depends upon a number of factors:• Depends upon a number of factors:
– Cost of cable
– Cost of wind turbine rectifier, charge controller and
inverter.
– Availability of components rated at the system voltage
– Voltage requirement of the loads
Energy storage• Majority (99% of renewable energy based SAPS) use lead-
acid batteries as energy storage.
• Lead-acid batteries store energy in the form of chemical
energy using a reversible reaction.
• Why use them?
• Easily obtainable• Easily obtainable
• Relatively cheap
BATTERY TEST
System design process
• Load assessment
• Resource assessment
• Battery bank sizing• Battery bank sizing
• Wiring diagram
Load assessment• Need to know
– Loads on the system (ALL of them)
– Time the loads run for
– Power rating of loads
• Write a list:• Write a list:
Load Power Time Energy
Lights 25W 2hrs/day 50Whs
Pump 100W 5hrs/day 500Whs
TOTAL 550Whs
Resource assessment• Need to know the resource available.
• Depends upon the renewable energy source.
• Also depends upon size of renewable energy
collector.
Example for PV:Example for PV:
– 5 sun-hours per day available at location
– 50Wp solar panel
Total energy per day = 5 x 50W = 250Whs
Battery sizing
• Must store enough energy to cope with
variations in supply and demand.
• Number of days of autonomy
– Use number of days with no input (typically 4 – Use number of days with no input (typically 4
days)
– Take into account maximum depth of
discharge (typically 50%)
Battery charge protection
Batteries are used:
must maintain them for the longest lifetime.
• Typical lifetimes: 5-7years (well maintained), 2-4 years (average?)
• Batteries must be protected from over charging and over
discharging – to do this we use a….discharging – to do this we use a….
• Charge regulator
• Power electronic device to prevent over-charging
• Must have well trained and knowledgeable operator.
• Battery bank maintenance is essential
• Daily monitor
• Monthly maintenance
Wiring diagrams
Electricity basics
• Need to know:
• Voltage
• Current
• Resistance• Resistance
• Power – instantaneous rate of doing work
• Energy - the total work done
• V=IR and W = IV
Using a multi-meter
• Measure V in parallel
• Measure A in series
• DO NOT measure A in parallel with battery
Good installation practice
• Want the system to be reliable and robust.
• The solar panel will last over 20 years,
hence the system must be installed to last
the same length of time.the same length of time.
DC cable sizing• Cables must be correctly sized for the current they will be
required to carry
• Problem is voltage drop
• Due to resistance of cable and current flowing
• Resistance = (ρ x L) / A
• Voltage drop = Resistance x Current• Voltage drop = Resistance x Current
• Voltage drop must be kept within reasonable parameters
(typically 5%)
• Design for the highest current
Cable interconnections
• Good reliable connections especially
important in low voltage DC systems.
• Must keep the connection clean and dry.
Fusing• Every cable must be protected by some form of fuse or
breaker
• Due to the battery installed:
– Can supply 1000s of amps
in short circuit
– Damage to components
– Fire risk
• Many types available
• Ensure correct rating
• Without it many other much more expensive problems
can happen
System monitoring
• Why monitor?
• Operation
• Maintenance
• Knowledge
• What to monitor?
• System voltage
• Currents flowing
• Battery bank state of charge
Battery bank connectionsBattery Bank Design
SystemPositive
Ensure very thick cable usedfor battery bank interconnections
Use a good qualitybattery clamp
SystemNegative
Take +/- system connectionsfrom ‘opposite’ ends of the system.This will ensure each parallel battery is evenly charged and discharged.
battery clamp
Ensure battery bankinterconnections are kept short
Low voltage disconnects
• Prevents over-discharge of the battery
bank.
• Voltage controlled switch, which will
disconnect non-essential loads if the disconnect non-essential loads if the
battery bank voltage drops too far.
• Sometimes added to a system.
Inverters
Vo
lta
ge
Time
Vo
lta
ge
Time
Square Wave
Stepped Square Wave (or Modified ‘Sine’ Wave)
Pure sine wavefor reference
• Convert DC into AC
Vo
lta
ge
Time
Sine Wave
• Benefits
– Can use readily and cheaply available products
• Problems
– Cost and usually not locally manufactured
– Added complexity
PV System Practical
PV PracticalSplit into 5 or 6 groups
Each group will be given parts to build a solar PV
power supply system
Each will have a different load to power
Each group should:Each group should:
• Test the solar PV module
• Do a load and resource assessment
• Draw circuit diagram
• Build the system
Health and safety
• Using knives, drills and hammers
• Careful when using tools
Be aware of others around you• Be aware of others around you
Problems experienced
Failure analysis of 421 systems over 3 years in Taquile, Peru
50
60
70
80
Accu
mu
late
d f
ailu
res a
s %
of
tota
l
Modules
Batteries
0
10
20
30
40
50
0 6 12 18 24 30 36
Months
Accu
mu
late
d f
ailu
res a
s %
of
tota
l
Regulators
Lamps
Fuses
Problems experienced
• Incorrect fusing
Problems with lead-acid
batteries• They self-discharge at a rate of 1-5% total energy per
month
• Temperature affects both capacity and lifetime
• Capacity is dependant upon current
• Lifetime is dependant upon discharge cycles and depth of • Lifetime is dependant upon discharge cycles and depth of
discharge
• Their storage density is approximately 30 to 40 Wh/kg
• A periodic equalisation charge is required
• Lead-acid batteries cost around £40 per kWh
Problems with lead-acid batteries
• The technology has been around for over 100 years so
there is little potential for cost reduction.
• Often cheap car batteries with lead sponge plates are
used rather than deep-cycle batteries especially designed
for stand-alone operation which exacerbates these for stand-alone operation which exacerbates these
problems.
If possible try to avoid using lead-acid batteries
• Some applications (such as water pumping) can be directly
coupled to the renewable energy source. The energy is
then stored as potential in the water.
Problems experienced
Crazy
wiring
Incorrect cable
sizing
Problems experienced
Short circuits and burned out components
Problems experienced
Insects
Problems experienced
Lightning!
Problems experienced• Battery failure
• Systems changed by operator for ‘better’ operation
• Cable voltage drop
• Flooding
• Access to engineering skills• Access to engineering skills
• Inadequate training
• Access to spare parts
• Political situations
• No long-term strategy
• Community organisation problems
• Funding
• Bad resource assessment
Thanks for your attention.