Photovoltaic principles are used in a solar power system to produce electricity using solar energy.
We use solar power to produce heat in our day to day activities, but electricity generation method by using solar power is totally different. To produce hot fluids or air we apply solar thermal principles but for producing electricity Photovoltaic principles are used. A solar panel is actually a photovoltaic panel (PV panel) which is made of the natural element, silicon. When this PV panel is subjected to sun light, it becomes charged.
This electrical charge is consolidated in the photovoltaic panel and directed to the output terminals and low voltage direct Current (DC) is produced. This DC current is usually 6 to 24 volts. The most common output is designed for nominal 12 V, with an effective output usually up to 17 V. A 12 V nominal output is the reference voltage, but the operating voltage could be 17 V or higher. There is a difference between the actual operating voltage & the reference voltage. Sun’s radiation intensity changes with the hour of the day, time of the year and weather conditions. The total amount of solar radiation energy is expressed in hours of full sunlight/m², or Peak Sun Hours.
Peak Sun Hours: The average amount of sun available per day throughout the year is represented by Peak Sun Hours. At “peak sun”, 1000 W/m² of power reaches the surface of the earth and 1 hour of full sun provides 1000 Wh/m² or 1 kWh/m² of power. The solar energy received in 1 hour on a cloudless summer day on a 1 m² surface directed towards the sun is 1 KW. The daily average of Peak Sun Hour is calculated based on either full year statistics, or average worst month of the year statistics.
Components used to provide solar power: The 4 primary components for producing common 110-120 volt AC power using solar power for daily use are:
- Solar panels
- Charge controller
Solar panels charge the battery, and proper charging of the battery is controlled by charge regulator. The inverter is used to convert the DC power to AC which is provided by battery. Where 220-240 volts AC power is required, we need to increase the voltage either using a transformer or connecting two identical inverters in series.
Solar Panels: The output of a solar panel is stated in watts, and the wattage is determined by multiplying the rated voltage by the rated current. The formula for wattage is –
WATTS = VOLTS * AMPS
So for example, a 12 volt 60 watt solar panel with a dimension about 20 * 44 inch has a rated voltage of 17.1 and a rated 3.5 amperage. If an average of 6 hours of peak sun/day is available in an area, then this solar panel will produce an average 360 Wh/day.
Solar panels can be wired in series to increase voltage or in parallel to increase amperage, and they also can be wired both in series and parallel to increase both voltage and amperage.
Series wiring: Positive terminal of one panel and the negative terminal of another is connected in series wiring which results voltage the sum of the two panels, but the amperage stays the same. So two 12 volt/3.5 amp panels wired in series produces 24 volts at 3.5 amps.
Parallel wiring: Parallel wiring refers to connecting positive terminals to positive terminals and negative to negative. The result is that voltage stays the same, but amperage becomes the sum of the number of panels. So two 12 volt/3.5 amp panels wired in parallel would produce 12 volts at 7 amps.
Series/parallel wiring: Series/parallel wiring is designed for increasing both volts and amps. If 4 panels each 12 V/3.5 Amps are wired, 2 in series and 2 in parallel we will achieve the desired output 24 V/7 Amps.
Charge Controller: This device monitors the battery’s charge state, when the battery needs charge and when the battery overcharged. A solar panel battery without a regulator can damage the battery. Charge controllers or charge regulator are rated based on the amount of amperage they can process from a solar array. A 20 amps controller can connect up to 20 amps of solar panel output current .
Pulse-Width-Modulation (PWM) charging principal is utilized by the advanced charge controllers, this ensure the efficient charging and extends the battery life .More advanced controllers utilize Maximum Power Point Tracking (MPPT) charging principal. This maximizes the amount of current going into the battery by lowering the panel’s output voltage, as a result the charging amps increase.
For any panel 60 watts capacity with 17.2 volts and 3.5 amps, if the voltage is reduced to 14 volts then the amperage increases to 4.28 (14v X 4.28 amps = 60 watts), which results 19% increase in charging amps. Low Voltage Disconnect (LVD) and Battery Temperature Compensation (BTC) are also some optional feature for many controller. The LVD feature permits connecting loads which are voltage sensitive. If the battery voltage drops to a certain range the loads are disconnected to prevent potential damage to both the battery and the loads. BTC adjusts the charge rate based on the temperature. Batteries are sensitive to temperature variations above and below about 75 F degrees.
Battery: In solar power system Deep Cycle battery is used, capable of discharged and then re-charged
hundreds or thousands of times. These batteries are rated in Amp Hours (ah) which refers to the amount of current in amps and this amount of current can be supplied by the battery over the period of hours. As an example, a 350 Ah battery is able to supply 17.5 amps of current over 20 hours or 35 amps for 10 hours continuously.
we can calculate total power from the battery rating, if a battery rating is 6 volt 360ah total power will be 360ah times of 6 volts equals 2160 watts/2.16 kWh (kilowatt-hours). Batteries are also wired in series and/or parallel for increasing voltage and current to the desired level like solar panels.
Battery bank size: The battery with sufficient capacity is capable of supplying required power during the longest expected “no sun” period or extremely cloudy weather. where back-up power source is available , like a standby generator with a battery charger, it is not required to design the the battery bank considering the worst weather conditions. Required battery bank size depends on –
- The storage capacity
- The maximum discharge rate,
- The maximum charge rate
- The minimum temperature at which the batteries will be used.
During battery bank design phase all of these factors are needed to be considered. One of the biggest common mistakes is observed among the users is not understanding the relationship between amps and amp-hour requirements, which is effect of AC items on their DC low voltage batteries.
As an example, A 24 volt nominal system powering a load of 3 amps, 120VAC, has a duty cycle of 4 hours/day. So the battery rating is 12 amp hour (3 amps * 4 hrs=12 ah).
But the true drain in this battery is, (120V/24V)*12 ah = 60 ah
So from the calculation it is clear 60 ah drained from this battery instead of 12 ah. We also calculate this true drain another simple way , dividing total watt-hours by nominal voltage.
Total watt-hours of 120VAC device = 3 amps * 120 volts * 4 hours = 1440 watt-hours.
So, true drain= 1440 watt-hours/24 DC volts = 60 amp hours.
In PV systems Lead-acid batteries are the most common because of their lower initial cost and they are readily available everywhere in the world. There are many different sizes and designs available for lead-acid batteries, but deep cycle batteries are most popular. they are available in both wet-cell which requires maintenance and sealed which does not required any maintenance. Some other maintenance free batteries are also popular , like AGM and Gel-cell deep-cycle batteries.
Inverter: An inverter is a device which convert DC power stored in a battery to AC power. DC current is generated in solar power system, which is stored in batteries. But all lighting, appliances, motors, etc., are designed for AC power, so inverter is used to convert battery-stored DC to required power (120 VAC, 60 Hz). In an inverter, DC current convert into alternating current (AC), then it passes through several processes like transformed, filtered, stepped, etc. to get desired output waveform.
Types of inverter: Inverters has two basic output designs –
- Pure Sine wave inverter
- Modified sine wave inverter
Most 120VAC devices use the modified sine wave, but there are some exceptions. Devices like laser printers which use triacs and/or silicon controlled rectifiers might damaged by modified sine wave power. Motors and power
supplies generally run warmer or less efficiently using modified sine wave power. However, modified sine wave inverters are very efficient fro converting power from DC to AC and relatively inexpensive.
Pure sine wave inverters can virtually operate anything. if utility company provides sine wave power, sine wave inverter is equal to or even better than utility supplied power. This can “clean up” utility or generator
supplied power because of its internal processing.
Internal features: Inverters have various internal features and permit external equipment interface. Internal battery chargers is the most common internal features, this can rapidly charge batteries when an AC source when generator or utility power is connected to its INPUT terminals. Another feature is Auto-transfer switching , which enables switching from either one AC source to another and/or from utility power to inverter power for designated loads. other significant features are Battery temperature compensation, internal relays to control loads, automatic remote generator starting/stopping etc.