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INTRODUCTION TO PHOTOVOLTAICS
TRAINING COURSE OUTLINE
SOLAR ROOTS
1) Energy
Is “The capacity to do Work”. (Also “The capacity to change conditions or circumstances”) (Also “The Total Work Done”)
Unit: KwHr (Also Joule and Btu among others).
Many Forms: Electrical/Chemical/Radiant/Mechanical/Thermal
Energy cannot be created or destroyed, only transformed.
There are always losses during transformation
Potential (stored) and Kinetic (movement) Energy
2) Present Global Situation
Expanding Global Economy - (Globalization)
Rapidly Rising Energy Demand
Climate Change and Greenhouse Effect
Population Growth
Peak Oil
Pollution
3) Non-Renewable Energy
Hydro Carbons - Fossil Fuels
Oil - Coal - Natural Gas: - One Time Use - Oil (Unique Energy Density) - Peak Oil - Unconventional Oil - Tar Sands/Shale - Boom or Doom?
4) Renewable Energy
Geothermal and Tide Power (Non-Solar)
Indirect Solar Energies:
Wind, Hydro, Wave Power, Biomass
Direct Solar Energies:
Solar Thermal: - Hot Water, Steam Turbines, Food Drying,Cooking and Photovoltaics: - Electricity from Light!!
5) Photovoltaics
Advantages: - Available everywhere, No moving Parts-Low Maintenance, Modular - Scaleable, Durable, No Fuel Cost - Ever!!!
Disadvantages: - High initial Cost, Solar resource variable, Energy must be stored, More expensive efficient appliances required, User education critical. Components List: - Panels/Controllers/Batteries/Inverters
3 System Types: - PV Direct/Battery Charging/Grid Tied.
6) The Solar Cell and Panel
Silicon: - Earth's second most abundant element comprising 20% of Earth's crust, ie sand. But PV uses naturally purified Silica or Quartz from mines. Other technologies include Copper Indium Diselenide, Gallium Arsenide and Cadmium Telluride, but today, crystalline silicon dominates 90% of the PV market.
3 Silicon Types: - Mono-crystalline, Poly-crystalline and Amorphous
Energy intensive Manufacturing Process-Efficiencies18,16,12%
Open Circuit Voltage: (Voc) is 0.58V for every cell, regardless of size.
Current (I): is proportional to cell size - 3 inch (3.6A) to 6 inch (7.2A)
Doped: with Boron (P-Type) and Phosphorous (N-Type)
How it works: - N-Type on top (Electron Flow), P-Type below (Hole Flow). Photons knock electrons free of their orbit around the nucleus, causing them to cross the P/N Junction, creating a potential difference and a current to flow.
Photons: - Particles of light - Unit of Electromagnetic Radiation.
Degradation: - 0.5% per year for Crystalline cells and up to 10% in the first year for Amorphous
The Solar Panel (Module) - 36 cells in Series for 12V Battery charging - Glass on Front, Plastic on Back.
Manufacture: Matched Cells, good solder connections and sealing are critical. Beware of panels with odd numbers of cells, such as 39 or 42, misaligned cells, sloppy application of sealant, or amateurish specification label. Always test Voc and Isc before purchase. Optimization
1) Orientation: - Azimuth Angle (The angle between True South [or N] and the direction the array is facing).
2) Tilt Angle: - Latitude (or sometimes Latitude + 15 degrees for Design Month). A Panel perpendicular to the Sun at noon on the equinoxes gives your latitude and ideal annual tilt angle.
3) No Shade: - Effect of shading is disproportional to area shaded. One cell shaded can knock out 75% of module's power. Very important in long Series strings.
4) Minimize Cell Temperature: - High temperature reduces Voltage by + or - 0.5% per degree C, and therefore, reduces Power. 5) Keep panels clean: - Losses from soiling can be from 2% to 7%.
6) Load Resistance: - The resistance of the Load determines the Operating Point of the Panel, (somewhere along the I/V Curve). A battery might operate between 11 and 14V, but a PV-Direct water pump might operate at 17V, as might a grid-tie inverter, both tracking the Maximum Power Point.
I/V Curve: - Maximum Power Point - (The largest rectangle that can fit under the curve)
Module Rating - Specification Label on Panel
Wp: - Maximum Power (Voc x Isc) (Also called Rated Pow
Isc: - Short Circuit Current (Zero Voltage)
Vmp: - Voltage at Max Power Point (Also called Rated Voltage)
Imp: - Current at Max Power Point (Also called Rated Current)
STC: - Standard Test Conditions (Irradiance of 1,000/m2, 25 degrees C cell temperature, 1.5 ATM spectral conditions.(Not real world conditions!) Fill Factor: - (FF=Wp/Voc x Isc) - Usually 70-75% - Ratio of area of MPP Rectangle/area of Rectangle formed by Voc and Isc
Reality Check: - PV modules are priced in $ per Watt. It is in the interests of PV manufacturers to have the highest Wp number possible, even if this is rarely achievable in the real world.
7) The Solar Resource
Sun's Movements
Daily: - The Sun is due South in the Northern Hemisphere at 12.00 Noon every day. This is called Solar (or True) South and is always true for locations north of the Tropic of Cancer (23.5 degrees N). The exact reverse is true for the Southern Hemisphere. In the Tropics, the noonday Sun will either be due North or due South, or directly overhead, depending on location and season.
Seasonally: - Solstices and Equinoxes
Peak Sun Hours: - 1,000W/m2
Solar Window: - Azimuth Angle and Angle of Incidence
8) Electricity
Is “The Flow of Electrons in a Conductor”.
Simple Circuits: - Open and Closed
AC/DC: - Alternating Current and Direct Current
Voltage - Potential Difference (Electromotive Force)
Is “Electrical Pressure that forces electrons through the circuit”
Measured across the circuit, in Volts (V).
Current
Is “The rate of flow of electrons” (through a section of the conductor).
Measured through the circuit, in Amperes (A). Often represented by the letter I, for 'Intensity'
Power
Is “The rate at which Energy is generated or used”.
Power = Voltage x Current, measured in Watts.
W = V x A
Energy
Is “The capacity to do work”.
Energy = Power x Time
KwHr = Kw x Hours
Example: The Energy required to boil one liter of water is constant. But more Power is required to boil it in 2 minutes than in 10 minutes. Resistance
Is “The property of a conductor that opposes the flow of electrons through it and that converts electrical energy into heat”.
Voltage = Current x Resistance
V = I x R
Measured in Ohms
Water Examples - High tanks, low tanks, Big pipes, little pipes.
9) Series and Parallel Circuits
Series: - Voltages increases, Current stays the same.
Parallel: - Current Increases, Voltage stays the same.
10) Batteries
Lead Acid: - The most common type - 6 x 2V cells = 12V Battery.
Car starting (SLI) Vs Deep Cycle
SLI: - Car starting batteries are constructed using thin plates for maximum surface area exposed to electrolyte, with calcium for plate strengthening, (against road vibration). But calcium doesn't tolerate more than 25% DoD. Thin plates minimize weight. SLI are almost always at full charge and can last up to 8 years in this automotive duty cycle. Not recommended for solar systems. But if budget dictates their use, restrict DoD to 20% to get a 1 to 2 year lifespan.
Deep Cycle: - Use fewer, but thicker plates for long slow discharges, (and recharges). More room above (for more acid), and below, (for more debris) inside battery cases. Deep Cycle use Antimony for plate strengthening. These batteries are heavier and more expensive than SLIs, (up to 4 times the price). But they are purpose built for deep cycling, and should be the first choice for solar systems.
Flooded Vs Sealed
Flooded: - These batteries, also called wet cell, and have liquid electrolyte, (dilute sulfuric acid) that must be topped up occasionally with distilled water (only!). They are more rugged and less expensive than sealed batteries. But true deep cycle flooded batteries are not available everywhere. Sealed: - Also called Captive Electrolyte or Valve Regulated Lead Acid (VRLI) batteries. Sub-categories include Gel cells (electrolyte is not liquid, but rather, in the form of a gel) and Absorbed Glass Mat (AGM). These batteries are more expensive and less tolerant of under and over charge conditions. Sometimes also called 'Maintenance Free' batteries, but this is not a technical specification.
Rated Capacity in Amp Hours (Ahr): - But varies according to Discharge Rate (C/Rate).
Discharge Rate: - Manufacturers quote rates of C/20 and C/100 at 20 degrees C.
Example: 100Ahr battery is fully discharged at 5A for 20 hours - this is a C/20 rate.
Charge Rate: - C/20 is the minimum. C/5 is too fast. C/10 is about right. Total array amps should be about 10% of battery capacity. Rule of Thumb - At least 1 Watt of PV for each 1 Ahr of Battery Capacity, (both converted back to 12V).
Self-Discharge Rate: - 2 to 5% per month, depending on battery type and temperature.
State of Charge (SoC):
Can be determined by:
1) Specific Gravity of the electrolyte using a Hydrometer for Flooded batteries. This gives an accurate assessment of SoC.
2) Voltage reading of the battery at rest, that is, after at least two hours without charging or discharging. (Both Flooded and Sealed). Less accurate assessment.
Depth of Discharge (DoD):
For maximum battery life, DoD should be limited to:
20% on a daily basis
50% occasionally
80% rarely
100% never,ever!!
Example: 5 x 18W lights kept on for 3 hours = 270Whr, which in a 12V system is 22.5Ahr. Taken from a 150Ahr battery, this represents a DoD of 15%. (5 x 18 x 3 = 270/12 = 22.5/150 x 100 = 15%). Be sure to include system efficiencies, (see below), when calculating how many Ahr it will take to recharge this battery!
Battery Life Expectancy: is measured in Cycles - One Cycle =1 discharge + 1 recharge
Deep Cycle Battery - 2,000 cycles of 20% DoD, but only 750 cycles of 80%.
SLI Batteries - 500 cycles of 25% DoD .
(Estimates are for batteries available in the developing world. Very variable!!).
Battery life is shortened by 50% for every 10 degrees C above the ideal of 25 degrees C.
Electrodes:
Positive Electrode (Anode) made of Lead Oxide
Negative Electrode (Cathode) made of pure Lead.
Charged State: - Electrolyte is a solution of Sulfuric Acid + Water at about 25% concentration and exists as Ions (charged molecules) - 2 Hydrogen Ions and 1 Sulfate Ion. Voltage at rest is around 12.7V.
Discharged State: - Electrolyte is almost pure water. Voltage at rest is under 11V.
Discharge Cycle: - Both electrodes are gradually plated with Lead Sulfate, which is an electrical insulator. Towards the end, Voltage drops sharply and Internal Resistance rises abruptly.
3 Levels of Lead Sulfate bonding
1st - Reversible bonding during normal cycling.
2nd - After I month chronic undercharging, only heavy overcharging can break bonds.
3rd - Deep bonds are formed that cannot be broken. Large permanent crystals form. This is called Sulfation.
Rapid voltage increase and decrease is a sure sign of Sulfation.
Aging: - Never add a new battery to an old one after 6 months.
Equalization Charge: - For Flooded batteries only. This is a “controlled overcharge” that brings the 6 battery cells back to an equal charge. It also breaks up stratification of the electrolyte.
11) Controllers - 4 Types:
Series:- On/Off - Almost never used now
Parallel Shunt: - Gradually reduces charge through a resistor
PWM: - Bulk charge to 14.5V, Float charge at 13.5V
MPPT: - Uses voltage normally unavailable to batteries and turns it into charging current.
Better quality controllers can have:
1)LED indicators of State of Charge
2)LCD displays of Voltage and Current
3)Low Voltage Disconnect (LVD) to protect the battery and loads against excessive low battery voltage.
12) Inverters - 3 Types:
Square Wave: - Almost never used now
Modified Sine Wave: - Lower quality, Cheaper, Lower Standby Losses.
Pure Sine Wave: - High quality, More expensive ,Higher Standby Losses.
13) Sizing - 6 Steps:
1) - Load Calculations in Watt Hrs:
AC Loads x Inverter Efficiency + DC Loads/DC System Voltage = Average AHrs per Day.
2) - Battery Calculations: AHrs per Day x Days of Autonomy x % Depth of
Discharge/Battery AHr capacity = Batteries in Parallel. DC System Voltage/ Battery Voltage = Batteries in Series. Batteries in Series x Batteries in Parallel = Total Batteries.
3) - Array Calculations: Ahrs per Day x % Battery Efficiency/Peak Sun Hours = Array
Peak Amps. Array Peak Amps/Rated Amps(Imp) per Module = Modules in Parallel.
DC System Voltage/Nominal Module Voltage = Modules in Series.
Modules in series x Modules in Parallel = Total Modules.
4) - Controller Calculations:
Module Short Circuit Current x Modules in Parallel x 1.25 = Array Short Circuit Amps.
5) - Inverter Calculations:
Look at AC total connected Watts and 3 x surge current of largest load to select Inverter.
6) - Wire Size Calculations:
NEC Requirement:
Isc of Module x # of Modules in Parallel = Total Amps x 1.25 x 1.25 = NEC Required Amps.
Voltage Drop Requirements:
From System Voltage, Amperage and Distance get Wire Size from Voltage Drop Table.
Is this equal to or greater than NEC requirement?
14) Wire Size and Voltage Drop (Power Loss)
Is “The loss of Voltage and subsequently, Power, due to the resistance of the wire to the flow of electricity in long runs of cable”.
3 Factors are important:
1) The current flowing through the wire
2) The wire size (gauge)
3) The length of wire
Large currents, small wire sizes and long cables together are a receipt for voltage drop. For circuits on the PV side of the battery (source circuits), 2% is considered the maximum voltage drop. For circuits on the AC side of the inverter, 5% is an acceptable maximum. For voltage drop, less is always better.
See Voltage Drop Charts for acceptable perameters.
Solutions:
1) Increase System Voltage at the design stage from 12V to 24V, thereby cutting currents in half.
2) Use a MPPT controller, which allows the Array to operate at a higher voltage than the battery bank.
3) Invest in larger gauge wire, which will easily pay for itself over the life of the system.
15) Efficiency
Is “Power Output/Power Input as a Percentage”
No machine or transformation is 100% Efficient.
The Internal Combustion Engine is less than 25% efficient. The incandescent bulb is about 5% efficient. (The rest is lost as heat).
The silicon cell is 15-18% efficient and the solar module is 12-15% efficient.
Typical PV system efficiencies:
Modules 90% (mismatched cells)
Batteries 80% (when new)
Inverter 90%
Wiring 97%
Total 63%
So, in reality, a 100Wp PV module, through the battery and inverter, will only be able to deliver 63W to your AC loads, (if you are lucky!) Appliances:
It is essential to use the most efficient appliances you can afford with your PV system. Use only LED and compact or tube fluorescent lights, never use incandescent lights. Use a laptop computer instead of a desktop. New ultra- efficient refrigerators and freezers are now available, though at a high cost.
16) Installation
1) - Solid mounting of panels and BOS: The panel mounting structure must be solidly attached to the roof or pole to avoid damage or injury during windy weather. The Balance of System (BOS) parts must also be firmly mounted to the building structure. Wiring should be neat and orderly.
2) - Correct wire size and type: Wire size should selected based the amperage it is expected to carry and the length of the wire (to avoid Voltage Drop). See Voltage Drop Tables in the Supplementary Pages. Wire is rated for Indoor or Outdoor use - select the appropriate one.
3) - Over-current protection: Too much current passing through a wire creates heat and can cause the insulation to melt or even start a fire. A Fuse or Circuit Breaker should be included in every circuit, especially those connected to the battery. A battery can instantaneously release thousands of amperes in the event of a short circuit. This excess of current can easily start a fire, injure personnel or cause the battery to explode. DC rated fuses and breakers are more rugged than AC ones but are difficult to find in the developing world. At the very least, include an old-fashioned fuse block, an automotive fuse or AC fuse in the circuit.
4) - Grounding: (a wire connection the Earth) is very important in electrical systems, including PV systems. The Grounding Electrode is usually a copper rod 4 to 6ft long, driven into the ground, a 20ft length of thick copper wire buried, or a connection to underground metal water pipes of structural building steel. On the DC side, the panel frames, the panel mounting and the Negative electrode of the battery are grounded. On the AC side, the chassis of the inverter, the Neutral terminal and the ground wire of the interior wiring circuits are grounded. This latter protects personnel and facilitates the fast blowing of fuses and circuit breakers in the event of a short circuit.
5) Lightning Protection: It is very important to connect the panel frames and the panel mounting structure to the Grounding Electrode with a thick copper wire as this conducts any current from lighting activity to ground, avoiding damage to expensive electronic equipment including controllers and inverters.
17) Maintenance
1) Keep solar panels clean. Wash them at least three times during the dry season.
2) Keep battery terminals “tight and bright”. Check once a month for signs of corrosion - (a blue/white crust).
3) Check that plants and trees haven't grown up to partially shade the array.
4) Add only Distilled Water to Flooded batteries, (just after charging is complete).
5) Keep all equipment manuals and records in a safe place.
18) Energy Management
1) Become familiar with your system's voltage profile. That is, keep a record of voltage readings taken at various times of day and under various weather conditions. Voltage, read first thing in the morning, after the battery has been at rest for several hours, is your best indicator of State of Charge for sealed batteries. When the system is newly installed, be sure to conscientiously record voltage readings during the first sunny month. That will give you a benchmark against which you can compare future changes.
2) Unplug Phantom Loads. That is, appliances that are plugged in, but are not in use, although they still draw power. Examples are any device with a remote control, such as TVs or DVD players. Or, any appliance with a digital clock, such as a microwave.These devices can consume large quantities of energy when left plugged in 24/7.
3) Let daily energy usage be determined by weather conditions. That means, reduce energy consumption during cloudy weather.
4) Keep the battery fully charged, as much of the time as possible. A full battery is a happy battery!
5) Keep the Depth of Discharge to a minimum - See above.
6) Watch out for “load creep”, which is the tendency for consumption to increase with the passing of time, as new devices are added. 7) Use as efficient lights and appliances as the budget will allow. They will pay you back in the end.
19) Troubleshooting
Most problems in solar systems relate in one way or another to the battery - it is the “weak link”. The main problems stem from the lack of training for users, the lack of understanding about State of Charge, Depth of Discharge and how they are affected by weather conditions. The most common scenario is that the battery is habitually over-discharged, (same thing as chronically under-charged) and it bounces back and forth between the Low Voltage Disconnect set-point and about 20% State of Charge. After several months of this abuse, the Sulfation is irreversible and the battery will become completely unusable in the next 6 to 12 months. A clear sign that a battery is sulfated is that, from a state of discharge, it reaches “Full Charge”, (according to the Controller), very quickly, in less than an hour. However, at night after one or two hours of modest use, it is completely discharged again. This indicates that its plates are almost entirely covered in sulfate crystals and there is very little surface area left to interact with the electrolyte. The battery must be replaced. The basic procedure to identify a problem is as follows:
1) If you do not know, check with local people what the recent weather has been like. Cloudy?
2) Check the array for shading or excessive dirt.
3) Check all fuses and circuit breakers.
4) Check system wiring for loose connections and corrosion.
5) Check panels and batteries for correct Series/Parallel configuration.
6) Check system wiring for correct polarity.
7) Check for proper system voltage and current.
Using a Digital Multimeter (Volt-Ohms Meter - VOM)
For detailed instructions on using a multimeter, see Supplementary Pages “Troubleshooting Using a Multimeter” pages 1,2,and 3.
For a complete list of possible PV system problems and solutions see Supplementary Pages “Troubleshooting List” pages 1-5.
INTRODUCTION TO PHOTOVOLTAICS
TRAINING COURSE OUTLINE
SOLAR ROOTS
1) Energy
Is “The capacity to do Work”. (Also “The capacity to change conditions or circumstances”) (Also “The Total Work Done”)
Unit: KwHr (Also Joule and Btu among others).
Many Forms: Electrical/Chemical/Radiant/Mechanical/Thermal
Energy cannot be created or destroyed, only transformed.
There are always losses during transformation
Potential (stored) and Kinetic (movement) Energy
2) Present Global Situation
Expanding Global Economy - (Globalization)
Rapidly Rising Energy Demand
Climate Change and Greenhouse Effect
Population Growth
Peak Oil
Pollution
3) Non-Renewable Energy
Hydro Carbons - Fossil Fuels
Oil - Coal - Natural Gas: - One Time Use - Oil (Unique Energy Density) - Peak Oil - Unconventional Oil - Tar Sands/Shale - Boom or Doom?
4) Renewable Energy
Geothermal and Tide Power (Non-Solar)
Indirect Solar Energies:
Wind, Hydro, Wave Power, Biomass
Direct Solar Energies:
Solar Thermal: - Hot Water, Steam Turbines, Food Drying,Cooking and Photovoltaics: - Electricity from Light!!
5) Photovoltaics
Advantages: - Available everywhere, No moving Parts-Low Maintenance, Modular - Scaleable, Durable, No Fuel Cost - Ever!!!
Disadvantages: - High initial Cost, Solar resource variable, Energy must be stored, More expensive efficient appliances required, User education critical. Components List: - Panels/Controllers/Batteries/Inverters
3 System Types: - PV Direct/Battery Charging/Grid Tied.
6) The Solar Cell and Panel
Silicon: - Earth's second most abundant element comprising 20% of Earth's crust, ie sand. But PV uses naturally purified Silica or Quartz from mines. Other technologies include Copper Indium Diselenide, Gallium Arsenide and Cadmium Telluride, but today, crystalline silicon dominates 90% of the PV market.
3 Silicon Types: - Mono-crystalline, Poly-crystalline and Amorphous
Energy intensive Manufacturing Process-Efficiencies18,16,12%
Open Circuit Voltage: (Voc) is 0.58V for every cell, regardless of size.
Current (I): is proportional to cell size - 3 inch (3.6A) to 6 inch (7.2A)
Doped: with Boron (P-Type) and Phosphorous (N-Type)
How it works: - N-Type on top (Electron Flow), P-Type below (Hole Flow). Photons knock electrons free of their orbit around the nucleus, causing them to cross the P/N Junction, creating a potential difference and a current to flow.
Photons: - Particles of light - Unit of Electromagnetic Radiation.
Degradation: - 0.5% per year for Crystalline cells and up to 10% in the first year for Amorphous
The Solar Panel (Module) - 36 cells in Series for 12V Battery charging - Glass on Front, Plastic on Back.
Manufacture: Matched Cells, good solder connections and sealing are critical. Beware of panels with odd numbers of cells, such as 39 or 42, misaligned cells, sloppy application of sealant, or amateurish specification label. Always test Voc and Isc before purchase. Optimization
1) Orientation: - Azimuth Angle (The angle between True South [or N] and the direction the array is facing).
2) Tilt Angle: - Latitude (or sometimes Latitude + 15 degrees for Design Month). A Panel perpendicular to the Sun at noon on the equinoxes gives your latitude and ideal annual tilt angle.
3) No Shade: - Effect of shading is disproportional to area shaded. One cell shaded can knock out 75% of module's power. Very important in long Series strings.
4) Minimize Cell Temperature: - High temperature reduces Voltage by + or - 0.5% per degree C, and therefore, reduces Power. 5) Keep panels clean: - Losses from soiling can be from 2% to 7%.
6) Load Resistance: - The resistance of the Load determines the Operating Point of the Panel, (somewhere along the I/V Curve). A battery might operate between 11 and 14V, but a PV-Direct water pump might operate at 17V, as might a grid-tie inverter, both tracking the Maximum Power Point.
I/V Curve: - Maximum Power Point - (The largest rectangle that can fit under the curve)
Module Rating - Specification Label on Panel
Wp: - Maximum Power (Voc x Isc) (Also called Rated Pow
Isc: - Short Circuit Current (Zero Voltage)
Vmp: - Voltage at Max Power Point (Also called Rated Voltage)
Imp: - Current at Max Power Point (Also called Rated Current)
STC: - Standard Test Conditions (Irradiance of 1,000/m2, 25 degrees C cell temperature, 1.5 ATM spectral conditions.(Not real world conditions!) Fill Factor: - (FF=Wp/Voc x Isc) - Usually 70-75% - Ratio of area of MPP Rectangle/area of Rectangle formed by Voc and Isc
Reality Check: - PV modules are priced in $ per Watt. It is in the interests of PV manufacturers to have the highest Wp number possible, even if this is rarely achievable in the real world.
7) The Solar Resource
Sun's Movements
Daily: - The Sun is due South in the Northern Hemisphere at 12.00 Noon every day. This is called Solar (or True) South and is always true for locations north of the Tropic of Cancer (23.5 degrees N). The exact reverse is true for the Southern Hemisphere. In the Tropics, the noonday Sun will either be due North or due South, or directly overhead, depending on location and season.
Seasonally: - Solstices and Equinoxes
Peak Sun Hours: - 1,000W/m2
Solar Window: - Azimuth Angle and Angle of Incidence
8) Electricity
Is “The Flow of Electrons in a Conductor”.
Simple Circuits: - Open and Closed
AC/DC: - Alternating Current and Direct Current
Voltage - Potential Difference (Electromotive Force)
Is “Electrical Pressure that forces electrons through the circuit”
Measured across the circuit, in Volts (V).
Current
Is “The rate of flow of electrons” (through a section of the conductor).
Measured through the circuit, in Amperes (A). Often represented by the letter I, for 'Intensity'
Power
Is “The rate at which Energy is generated or used”.
Power = Voltage x Current, measured in Watts.
W = V x A
Energy
Is “The capacity to do work”.
Energy = Power x Time
KwHr = Kw x Hours
Example: The Energy required to boil one liter of water is constant. But more Power is required to boil it in 2 minutes than in 10 minutes. Resistance
Is “The property of a conductor that opposes the flow of electrons through it and that converts electrical energy into heat”.
Voltage = Current x Resistance
V = I x R
Measured in Ohms
Water Examples - High tanks, low tanks, Big pipes, little pipes.
9) Series and Parallel Circuits
Series: - Voltages increases, Current stays the same.
Parallel: - Current Increases, Voltage stays the same.
10) Batteries
Lead Acid: - The most common type - 6 x 2V cells = 12V Battery.
Car starting (SLI) Vs Deep Cycle
SLI: - Car starting batteries are constructed using thin plates for maximum surface area exposed to electrolyte, with calcium for plate strengthening, (against road vibration). But calcium doesn't tolerate more than 25% DoD. Thin plates minimize weight. SLI are almost always at full charge and can last up to 8 years in this automotive duty cycle. Not recommended for solar systems. But if budget dictates their use, restrict DoD to 20% to get a 1 to 2 year lifespan.
Deep Cycle: - Use fewer, but thicker plates for long slow discharges, (and recharges). More room above (for more acid), and below, (for more debris) inside battery cases. Deep Cycle use Antimony for plate strengthening. These batteries are heavier and more expensive than SLIs, (up to 4 times the price). But they are purpose built for deep cycling, and should be the first choice for solar systems.
Flooded Vs Sealed
Flooded: - These batteries, also called wet cell, and have liquid electrolyte, (dilute sulfuric acid) that must be topped up occasionally with distilled water (only!). They are more rugged and less expensive than sealed batteries. But true deep cycle flooded batteries are not available everywhere. Sealed: - Also called Captive Electrolyte or Valve Regulated Lead Acid (VRLI) batteries. Sub-categories include Gel cells (electrolyte is not liquid, but rather, in the form of a gel) and Absorbed Glass Mat (AGM). These batteries are more expensive and less tolerant of under and over charge conditions. Sometimes also called 'Maintenance Free' batteries, but this is not a technical specification.
Rated Capacity in Amp Hours (Ahr): - But varies according to Discharge Rate (C/Rate).
Discharge Rate: - Manufacturers quote rates of C/20 and C/100 at 20 degrees C.
Example: 100Ahr battery is fully discharged at 5A for 20 hours - this is a C/20 rate.
Charge Rate: - C/20 is the minimum. C/5 is too fast. C/10 is about right. Total array amps should be about 10% of battery capacity. Rule of Thumb - At least 1 Watt of PV for each 1 Ahr of Battery Capacity, (both converted back to 12V).
Self-Discharge Rate: - 2 to 5% per month, depending on battery type and temperature.
State of Charge (SoC):
Can be determined by:
1) Specific Gravity of the electrolyte using a Hydrometer for Flooded batteries. This gives an accurate assessment of SoC.
2) Voltage reading of the battery at rest, that is, after at least two hours without charging or discharging. (Both Flooded and Sealed). Less accurate assessment.
Depth of Discharge (DoD):
For maximum battery life, DoD should be limited to:
20% on a daily basis
50% occasionally
80% rarely
100% never,ever!!
Example: 5 x 18W lights kept on for 3 hours = 270Whr, which in a 12V system is 22.5Ahr. Taken from a 150Ahr battery, this represents a DoD of 15%. (5 x 18 x 3 = 270/12 = 22.5/150 x 100 = 15%). Be sure to include system efficiencies, (see below), when calculating how many Ahr it will take to recharge this battery!
Battery Life Expectancy: is measured in Cycles - One Cycle =1 discharge + 1 recharge
Deep Cycle Battery - 2,000 cycles of 20% DoD, but only 750 cycles of 80%.
SLI Batteries - 500 cycles of 25% DoD .
(Estimates are for batteries available in the developing world. Very variable!!).
Battery life is shortened by 50% for every 10 degrees C above the ideal of 25 degrees C.
Electrodes:
Positive Electrode (Anode) made of Lead Oxide
Negative Electrode (Cathode) made of pure Lead.
Charged State: - Electrolyte is a solution of Sulfuric Acid + Water at about 25% concentration and exists as Ions (charged molecules) - 2 Hydrogen Ions and 1 Sulfate Ion. Voltage at rest is around 12.7V.
Discharged State: - Electrolyte is almost pure water. Voltage at rest is under 11V.
Discharge Cycle: - Both electrodes are gradually plated with Lead Sulfate, which is an electrical insulator. Towards the end, Voltage drops sharply and Internal Resistance rises abruptly.
3 Levels of Lead Sulfate bonding
1st - Reversible bonding during normal cycling.
2nd - After I month chronic undercharging, only heavy overcharging can break bonds.
3rd - Deep bonds are formed that cannot be broken. Large permanent crystals form. This is called Sulfation.
Rapid voltage increase and decrease is a sure sign of Sulfation.
Aging: - Never add a new battery to an old one after 6 months.
Equalization Charge: - For Flooded batteries only. This is a “controlled overcharge” that brings the 6 battery cells back to an equal charge. It also breaks up stratification of the electrolyte.
11) Controllers - 4 Types:
Series:- On/Off - Almost never used now
Parallel Shunt: - Gradually reduces charge through a resistor
PWM: - Bulk charge to 14.5V, Float charge at 13.5V
MPPT: - Uses voltage normally unavailable to batteries and turns it into charging current.
Better quality controllers can have:
1)LED indicators of State of Charge
2)LCD displays of Voltage and Current
3)Low Voltage Disconnect (LVD) to protect the battery and loads against excessive low battery voltage.
12) Inverters - 3 Types:
Square Wave: - Almost never used now
Modified Sine Wave: - Lower quality, Cheaper, Lower Standby Losses.
Pure Sine Wave: - High quality, More expensive ,Higher Standby Losses.
13) Sizing - 6 Steps:
1) - Load Calculations in Watt Hrs:
AC Loads x Inverter Efficiency + DC Loads/DC System Voltage = Average AHrs per Day.
2) - Battery Calculations: AHrs per Day x Days of Autonomy x % Depth of
Discharge/Battery AHr capacity = Batteries in Parallel. DC System Voltage/ Battery Voltage = Batteries in Series. Batteries in Series x Batteries in Parallel = Total Batteries.
3) - Array Calculations: Ahrs per Day x % Battery Efficiency/Peak Sun Hours = Array
Peak Amps. Array Peak Amps/Rated Amps(Imp) per Module = Modules in Parallel.
DC System Voltage/Nominal Module Voltage = Modules in Series.
Modules in series x Modules in Parallel = Total Modules.
4) - Controller Calculations:
Module Short Circuit Current x Modules in Parallel x 1.25 = Array Short Circuit Amps.
5) - Inverter Calculations:
Look at AC total connected Watts and 3 x surge current of largest load to select Inverter.
6) - Wire Size Calculations:
NEC Requirement:
Isc of Module x # of Modules in Parallel = Total Amps x 1.25 x 1.25 = NEC Required Amps.
Voltage Drop Requirements:
From System Voltage, Amperage and Distance get Wire Size from Voltage Drop Table.
Is this equal to or greater than NEC requirement?
14) Wire Size and Voltage Drop (Power Loss)
Is “The loss of Voltage and subsequently, Power, due to the resistance of the wire to the flow of electricity in long runs of cable”.
3 Factors are important:
1) The current flowing through the wire
2) The wire size (gauge)
3) The length of wire
Large currents, small wire sizes and long cables together are a receipt for voltage drop. For circuits on the PV side of the battery (source circuits), 2% is considered the maximum voltage drop. For circuits on the AC side of the inverter, 5% is an acceptable maximum. For voltage drop, less is always better.
See Voltage Drop Charts for acceptable perameters.
Solutions:
1) Increase System Voltage at the design stage from 12V to 24V, thereby cutting currents in half.
2) Use a MPPT controller, which allows the Array to operate at a higher voltage than the battery bank.
3) Invest in larger gauge wire, which will easily pay for itself over the life of the system.
15) Efficiency
Is “Power Output/Power Input as a Percentage”
No machine or transformation is 100% Efficient.
The Internal Combustion Engine is less than 25% efficient. The incandescent bulb is about 5% efficient. (The rest is lost as heat).
The silicon cell is 15-18% efficient and the solar module is 12-15% efficient.
Typical PV system efficiencies:
Modules 90% (mismatched cells)
Batteries 80% (when new)
Inverter 90%
Wiring 97%
Total 63%
So, in reality, a 100Wp PV module, through the battery and inverter, will only be able to deliver 63W to your AC loads, (if you are lucky!) Appliances:
It is essential to use the most efficient appliances you can afford with your PV system. Use only LED and compact or tube fluorescent lights, never use incandescent lights. Use a laptop computer instead of a desktop. New ultra- efficient refrigerators and freezers are now available, though at a high cost.
16) Installation
1) - Solid mounting of panels and BOS: The panel mounting structure must be solidly attached to the roof or pole to avoid damage or injury during windy weather. The Balance of System (BOS) parts must also be firmly mounted to the building structure. Wiring should be neat and orderly.
2) - Correct wire size and type: Wire size should selected based the amperage it is expected to carry and the length of the wire (to avoid Voltage Drop). See Voltage Drop Tables in the Supplementary Pages. Wire is rated for Indoor or Outdoor use - select the appropriate one.
3) - Over-current protection: Too much current passing through a wire creates heat and can cause the insulation to melt or even start a fire. A Fuse or Circuit Breaker should be included in every circuit, especially those connected to the battery. A battery can instantaneously release thousands of amperes in the event of a short circuit. This excess of current can easily start a fire, injure personnel or cause the battery to explode. DC rated fuses and breakers are more rugged than AC ones but are difficult to find in the developing world. At the very least, include an old-fashioned fuse block, an automotive fuse or AC fuse in the circuit.
4) - Grounding: (a wire connection the Earth) is very important in electrical systems, including PV systems. The Grounding Electrode is usually a copper rod 4 to 6ft long, driven into the ground, a 20ft length of thick copper wire buried, or a connection to underground metal water pipes of structural building steel. On the DC side, the panel frames, the panel mounting and the Negative electrode of the battery are grounded. On the AC side, the chassis of the inverter, the Neutral terminal and the ground wire of the interior wiring circuits are grounded. This latter protects personnel and facilitates the fast blowing of fuses and circuit breakers in the event of a short circuit.
5) Lightning Protection: It is very important to connect the panel frames and the panel mounting structure to the Grounding Electrode with a thick copper wire as this conducts any current from lighting activity to ground, avoiding damage to expensive electronic equipment including controllers and inverters.
17) Maintenance
1) Keep solar panels clean. Wash them at least three times during the dry season.
2) Keep battery terminals “tight and bright”. Check once a month for signs of corrosion - (a blue/white crust).
3) Check that plants and trees haven't grown up to partially shade the array.
4) Add only Distilled Water to Flooded batteries, (just after charging is complete).
5) Keep all equipment manuals and records in a safe place.
18) Energy Management
1) Become familiar with your system's voltage profile. That is, keep a record of voltage readings taken at various times of day and under various weather conditions. Voltage, read first thing in the morning, after the battery has been at rest for several hours, is your best indicator of State of Charge for sealed batteries. When the system is newly installed, be sure to conscientiously record voltage readings during the first sunny month. That will give you a benchmark against which you can compare future changes.
2) Unplug Phantom Loads. That is, appliances that are plugged in, but are not in use, although they still draw power. Examples are any device with a remote control, such as TVs or DVD players. Or, any appliance with a digital clock, such as a microwave.These devices can consume large quantities of energy when left plugged in 24/7.
3) Let daily energy usage be determined by weather conditions. That means, reduce energy consumption during cloudy weather.
4) Keep the battery fully charged, as much of the time as possible. A full battery is a happy battery!
5) Keep the Depth of Discharge to a minimum - See above.
6) Watch out for “load creep”, which is the tendency for consumption to increase with the passing of time, as new devices are added. 7) Use as efficient lights and appliances as the budget will allow. They will pay you back in the end.
19) Troubleshooting
Most problems in solar systems relate in one way or another to the battery - it is the “weak link”. The main problems stem from the lack of training for users, the lack of understanding about State of Charge, Depth of Discharge and how they are affected by weather conditions. The most common scenario is that the battery is habitually over-discharged, (same thing as chronically under-charged) and it bounces back and forth between the Low Voltage Disconnect set-point and about 20% State of Charge. After several months of this abuse, the Sulfation is irreversible and the battery will become completely unusable in the next 6 to 12 months. A clear sign that a battery is sulfated is that, from a state of discharge, it reaches “Full Charge”, (according to the Controller), very quickly, in less than an hour. However, at night after one or two hours of modest use, it is completely discharged again. This indicates that its plates are almost entirely covered in sulfate crystals and there is very little surface area left to interact with the electrolyte. The battery must be replaced. The basic procedure to identify a problem is as follows:
1) If you do not know, check with local people what the recent weather has been like. Cloudy?
2) Check the array for shading or excessive dirt.
3) Check all fuses and circuit breakers.
4) Check system wiring for loose connections and corrosion.
5) Check panels and batteries for correct Series/Parallel configuration.
6) Check system wiring for correct polarity.
7) Check for proper system voltage and current.
Using a Digital Multimeter (Volt-Ohms Meter - VOM)
For detailed instructions on using a multimeter, see Supplementary Pages “Troubleshooting Using a Multimeter” pages 1,2,and 3.
For a complete list of possible PV system problems and solutions see Supplementary Pages “Troubleshooting List” pages 1-5.