How much a 100watt module can produce energy.

The 100watt module will produce only 67watt. The efficiency can be increased to 77watt by adopting high efficiency modules and minimize other factors. In general energy calculation we take efficiency 77%

The PV solar plant consists of series and parallel connected modules. The specifications printed on the module are based on STC condition. Suppose module rated power is 120watt, it says that module will produce 120 watt power at 25°C cell temperature, 1.5 air mass and 1000watt/m^2 solar irradiance. In the actual practice this can not be achieved. There are so many factors which affects the plant efficiency.

1.     Module tolerances:  On the name plate of module “+/-5%” is written. It means that 100 watt module can generate 105watt or 95watt,  still the module is called 100watt module. For safety factor 95watt is considered. Its efficiency factor is 0.95

2.     Temperature: The output power of module decrease as temperature increases. In the summer season when the ambient temperature is 50°C than the module cell temperature will be 80°C on the roof top of building (30°C above ambient temp.). For crystalline module the temperature reduction factor is 0.89. It varies from manufactured technology of module.

3.     Dirt and dust: Bird spikes and accumulation of sand on the on the module surface, blocking some of the sunlight and reducing the output. A typical annual dust reduction factor is 0.93.

4.     Mismatch and wiring losses: The maximum output of the total array is less than the sum of individual modules. The difference is a result of slight inconsistencies in performance from one module to the next and is called module mismatch factor and amounts to at least 2%. Power is also lost in resistance of wiring. The wiring loss 3%. The total loss factor is 0.95.

5.     Dc to Ac conversion losses: The Dc power generated by modules converted in Ac power by inverters, this again causes losses anf loss factor is 0.9.

The total losses are products of 1 to 5 factors and are equal to:

 0.95x0.89x0.93x 0.95x0.9 =67watt. It means that 100watt module will generates only 67watt.

     6. If battery is used for back up an additional loss of 6 to 10%

7. Array oriental losses: Array gives maximum power in south facing and lilt equal to  latitude. But compromise  is made to avoid shading effect and old odd pitched roofs. These again lossess 2 to 6%.

How to size Wiring and Cabling for your System

WIRE VOLTAGE DROP CALCULATIONS EXAMPLE
Solar pv stand alone system operates at nominal voltage 12-volt, 24-volt & 48-volt. The Dc power is equal to voltage multiply by current & voltage is equal to current multiply by resistance. As the size of power plant increased, the current increased. For example in 240 watt power plant at 24-volt the current is 10 amp, but for 480 watt power plant it will be 20 amp. The resistance is measured in ohms per 1000 feet. From it is clear that as the current and wire length increases the voltage drop will be increases and energy will lost in heat. This increase in voltage drop will drop the power.
The solar energy is very costly so voltage drop is kept minimum 3% to 5%, by keeping the wire length minimum..
Allowable % voltage drops are: -
For Dc system: total: 1 to 6 = 4.18%
1. Module wiring : 0.69%
2. Array to Junction box: 0.72%
3. Junction box to Combiner box: 1.82%
4. Combiner box to Charge controller: 0.43%
5. Charge Controller to Disconnects: 0.26%
6. Disconnects to Inverter: 0.26%
For Ac Circuits: total: 1&2 = 0.36%
1. Inverter to disconnect: 0.06%
2. Disconnect to service panel: 0.3%
Over all Dc & Ac =4.18%+0.36% =4.54%
Example: Calculate wire size between the Junction box to combiner box at a distance 40 feet at a site. Consider the 24-volt Dc system, module Imp current =7.0 amp (printed on module).
From above point no 1, voltage drop from junction box to combiner is 1.82%
Voltage drop/system voltage =% loss in voltage
Voltage drop = 1.82x24/100=0. 4368
Voltage drop = 2 x L x Imp x resistance of wire.
Where L is distance between Junction box to Combiner box.
Resistance of wire = Voltage drop/(2x40x7) =0.00078 ohms
The resistance of wire is measured in ohms/1000Feet.
So the resistance of 1000 Feet wire is 0.78 ohms.

From NEC table the resistance of stranded wires per 1000 Feet is as follow.
1. 14AWG = 3.140 ohms 2. 12AWG =1.980 ohms 3. 10AWG = 1.240 ohms
4.8AWG= 0.778ohms 5. 6AWG=0.491ohms
By comparing, 8SWG wire is suitable for our application.

Wire sizing between inverter & battery in stand alone pv solar system

In a stand alone PV system, inverters are frequently used to change the direct current (DC) from a battery bank to 230-volts, 50-Hertz(Hz) (In USA and Canada 120-volts, 60 Hz) alternating current(AC).The conductors between the inverter and the battery must have properly rated over current protection and disconnect mechanisms [NEC 240,690-8b(3), -8(b)(4),-15]. These inverters frequently have short duration (tens of seconds) surge capabilities that are four to six times the rated output.
For example, a 2500-watt inverter might be required to surge to (2500x4)10,000 volt-amps for 5 seconds when a motor load must be started. The NEC requires the ampacity of the conductors between the battery and the inverter to be sized by the rated 2500-watt output of the inverter. Consider a 24-volt system, a 2500 watt inverter would draw 134 amps (2500/0.85/22) at full load (85% inverter efficiency and 22 volt battery charge. The ampacity of the conductors between the battery and inverters must be 125% of the 134 amp or 167amp.
This 167amp current in conductor is at the ambient temperature 30°C. Normally the inverter operates at temperature at 40°C to 45°C so derating of cable be must. Go to NEC table 310.16 and select 60°C column, where you find derating correction factor 0.71 in the right hand side of 41-45 ambient temperature. Devide 167 by 0.71 you get corrected 235 amp and conductor size 300 kcmill.
The length of conductor should be kept small to avoid voltage drop. The conductor should be tested for voltage drop also (continue).

Conduit, conduit fill & Conduit derating

(Under construction)

SOLAR PV MODULE WIRE DESIGN EXAMPLE

Example: Crystalline silicon module 120 watt at STC (standard testing conditions)
• Voc=21.0V, Isc = 7.2A, Vmp = 17.1V, Imp = 7.0A, Pmax =120 watt.
• Voltage temperature coefficient = -0.5%/°C
• Module Dimensions 660mmx1420mm
• Weight 11kgs.
Circuit Current: It is also called Continuous Current is 125% of short circuit current. In our module specifications, Isc is 7.2A, so Circuit Current or System Current is 7.2x1.25= 9.00 A(A= Ampere). Solar PV module is current limitted device so it's current will not increase beyond 9.00A.
Over Current Device Rating: These devices are fuses or breaker used to save the module and module wiring. This designed 156% of short circuit current or 1.25% of continuous current. In our example it is 1.56xIsc=1.56x7.2= 11.23A. This current limit is only applicable if this device operates below 40°C ambient temperature .
Cable sizing: It's ampacity is designed 156% of short circuit current at 30°C conductor temperature. In our example it is 11.23A
Cable Derating: In actual conditions the module is operated above 30°C to ambient temperature. If the ambient temperature is 48°C, than the module cell temperature will be =48+30=78°C (If module is placed on the roof top) & if 3" to 6" space is provided between the module and roof for air circulation than the cell temperature will be 20°C above the ambient temperature, =48+20=68°C. The cable will work at temperature so always select the 90°C wet rated cable. Selection of cable based on the location
1. Free air: For free air USE-2 single conductor cable 90°C wet- rated & sunlight resistant) is used for exposed applications. 2.
2. Type TC Multi conductor cable for exposed conditions with THWN-2, or XHHW-2, or RHW-2 or equivalent 90°C wet- rated conductors in the cable.
3. Race way or through conduit: Conductors type THWN-2, or XHHW-2, or RHW-2, or equivalent 90°C wet- rated conductors in high rated conduit (Conduit rated for a minimum of 75°C wet conditions.)
From our example four PV modules are connected to make 24 volt circuit. (Two modules are connected in series). Two module sets are connected to junction box in parallel. The wires are already supplied with modules. Now the wires are going from junction box to combiner box through conduit.