solar pv and stand-alone power systems in rural settings · solar pv and stand-alone power systems...
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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 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.
• 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?
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
• 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 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
Variability of generation and load
Variation of typical loads
Variation of wind
Variation of solar
Matching supply and demandP
ow
er
Time
MismatchSupply
Demand
Excess
Deficit
AggregationAggregation
Tota
l S
ignal
Time
1 signal
10 signals
1000 signals
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
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:
– 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
• Relatively cheap
System design process
• Load assessment
• Resource assessment
• 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:
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:
– 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 days)
– Take into account maximum depth of discharge (typically 50%)
Battery charge protectionBatteries 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….
• 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
• 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.
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 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
SystemNegative
Take +/- system connectionsfrom ‘opposite’ ends of the system.This will ensure each parallel battery is evenly charged and discharged.
Ensure very thick cable usedfor battery bank interconnections
Use a good qualitybattery 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 battery bank voltage drops too far.
• Sometimes added to a system.
Inverters
Vo
lta
ge
Time
Vo
lta
ge
Vo
lta
ge
Time
Time
Square Wave
Stepped Square Wave (or Modified ‘Sine’ Wave)
Sine Wave
Pure sine wavefor reference
• Convert DC into AC
• Benefits
– Can use readily and cheaply available products
• Problems
– Cost and usually not locally manufactured
– Added complexity
Thanks for your attention.
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:
• Test the solar PV module
• Test the battery
• Do a load and resource assessment
• Draw circuit diagram
• Build the system
Health and safety
Health and safety
• Using knives, drills and hammers
• Careful when using tools
• Be aware of others around you
REVIEW OF PV SYSTEMS
Problems experienced
Failure analysis of 421 systems over 3 years in Taquile, Peru
0
10
20
30
40
50
60
70
80
0 6 12 18 24 30 36
Months
Accu
mu
late
d f
ailu
res a
s %
of
tota
l
Modules
Batteries
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 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 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 experiencedShort 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
• Inadequate training
• Access to spare parts
• Political situations
• No long-term strategy
• Community organisation problems
• Funding
• Bad resource assessment
