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Introduction :- India is a tropical country blessed with abundant solar energy. Almost about 5,000 trillion kWh per year is incident during longer hours per day and in great intensity almost around the year. Solar energy, therefore, has great potential as future energy source. It also has the advantage of permitting the decentralized distribution of energy, thereby empowering people at the grassroots level. Solar Energy is the radiant energy emitted by sun comprising of UV, Visible & IR radiation.

Solar Insolation is the amount of solar energy that strikes a square meter of the earth surface in a single day.
By knowing the insolation value of particular area, required size of collector and energy output can be calculated. The values are generally expressed in kWh/m2/day. India receives solar energy in the region of 5 to 7 kWh/m2/day for 300 to 330 sunny days. This energy is sufficient to set up 20 MW of solar power plant per square kilometre of land area.

Type of Solar Energy: Solar energy can be used through two different routes namely, Solar Thermal Energy and Solar Photo Voltaic Energy. Solar Thermal energy uses the sun heats convert and convert it into heat energy while photovoltaic system uses sun heat to convert it into electric energy.

Basics of Solar PV :
Solar Photovoltaic (PV) technology enables direct conversion of sunlight into electricity. Photovoltaic cells, commonly known as solar cells, are used to convert light (photon) in to electricity. Most of the commercially available solar cells are made from high purity silicon wafers. Solar cells can also be made from several materials such as silicon thin films both multi crystalline and amorphous, cadmium telluride (CdTe), copper indium diselenide (CIS), gallium arsenide (GaAs) etc.A number of solar cells are joined together to make a solar photovoltaic module. The electrical output of a solar cell / PV module is rated in terms of peak watt (Wp), which is the maximum power output that the PV module could deliver under Standard Test Conditions (STC) of:

  • Incident solar radiation of 1000 watts per square meter area
  • Spectral distribution of solar radiation as Air mass 1.5
  • Measurements being made at 25°C ambient temperature

A combination of solar modules in series/parallel combination, inverter, interface electronics, mechanical support structure, cable and switches etc. constitute a solar photovoltaic (PV) system.

Technology overview PV modules can be broadly divided into two main categories: 1. crystalline silicon 2. Thin film Crystalline silicon (c-Si) modules use cells of either mono-crystalline or multi- crystalline silicon. The cells are manufactured by cutting wafers from a solid ingot block of material. Crystalline technologies are commercially proven and have a track record of over 25 years in operation. Mono-crystalline silicon cells are generally the most efficient, but are also more costly than multi-crystalline cells. Thin film modules are made with a thin film deposition of semi-conductor onto a substrate. Although relatively new compared to crystalline technologies, some Thin Film technologies are being increasingly used in large scale installations. Thin film modules include semi-conductors made from amorphous silicon (a-Si), Cadmium Telluride (CdTe) and Copper Indium Gallium diSelenide (CIGS). Both PV technologies are being used globally for large scale utility power generation, but crystalline modules are preferred for KW scale solar PV projects. Crystalline modules are significantly more efficient than thin film modules, but they are also more expensive. Due to their lower efficiency, thin film modules require a larger area to produce the same nominal power than crystalline modules. On the other hand, Thin Film modules may be better suited for locations with a higher average temperature due to their lower temperature coefficient, when compared to crystalline silicon.

Technology Evaluation

In order to understand the best suited PV technology for limited area we have evaluated and compared the relative area requirement for crystalline & thin film technology. Efficiency of solar cell and solar module varies with type of cell material or technology used. As high the efficiency of the module the surface area required will be less. The table presented below shows the area required by the four commercially established solar PV modules among crystalline & thin film technologies for 1 kWp capacity of Solar PV system to be installed at rooftop.

Crystalline Technology
1. Mono Crystalline,

2. Poly Crystalline,

Thin film Technology
1. Amorphous Silicon (a-Si)

2. Cadmium Telluride (CdTe)

Cell Material Module Efficiency Temperature Coefficient Surface Area Need for 1 KWp of solar module Rooftop Area Need for 1 KWp of solar PV system
Mono crystalline Silicon 13-19% -0.45%/oC 6-8 m² 10 ~12 m2
Polycrystalline Silicon 11-15% -0.40%/oC 7-9 m² 10~12 m2
Thin Film Cadmium Telluride CdTe 9-11% -0.25%/oC 9-11 m² 15~18 m2
Amorphous silicon (asi) 5-8% -0.21%/oC 13-20 m² 20~30 m2

Thin film modules are significantly less efficient as compared to crystalline silicon modules and as a result cover up to 30% more surface area than crystalline silicon modules in order to achieve the same output. The crystalline solar PV modules cover a maximum surface area between 6m² to 9m² per kWp while thin films cover an area between 9m² to 20m² per kWp depending upon the specific technology used. This therefore entails increased cost of installation, support frames and cabling. Although thin film modules perform favourably in diffused light conditions and at higher temperatures (due to lower temperature coefficient), however due to the limited technological experience about thin film technology on field, long term rate of degradation for many thin film modules may not be available. In that case it may be better to go in for Crystalline Silicon Technology. Actual area required at the rooftop depends on the tilting angle of the modules and the orientation of the rooftop. Including inter row spacing to avoid the shadow and other area for safe operation & maintenance at the rooftop, it is generally taken as 1.5 times the surface area of the modules. So crystalline technology would require rooftop area of about 10m² to 15 m² where as for thin film it will vary from 15 m² to 30m² for 1 kWp of Solar PV system depending on the cell material as shown in the table above. For estimation of potential at the rooftop consider 15m² shadow free area for 1 kWp solar PV system.

Cells Semiconductor device that converts sunlight into direct current (DC) electricity
Modules PV modules consist of PV cell circuits sealed in an environmentally protective laminate and are the fundamental building block of PV systems
Panels PV panels include one or more PV modules assembled as a pre-wired, field-installable unit
Array A PV array is the complete power-generating unit, consisting of any number of PV modules and panels.

PV Benefit:
• Solar power provided by photovoltaic systems lower your utility bills and insulate you from utility rate hikes and price volatility due to fluctuating energy prices • Installing a solar system increases property value and home resale opportunities • Purchase of a solar power system allows you to take advantage of available tax and financial incentives • Because they don't rely on miles of exposed wires, residential PV systems are more reliable than utilities, particularly when the weather gets nasty. • PV modules have no moving parts, degrade very, very slowly, and boast a life span that isn't fully known yet, but will be measured in decades. • Solar electric systems are quiet, reliable, fossil-fuel free • Unlike mobile power generators, avoids greenhouse gas emissions

Type of PV Systems:
1. Grid Tied System: Grid-Tied systems are connected to your grid and require no battery storage.

2. Grid Interactive System:These are essentially grid tied systems with an additional battery bank which can store power. They can provide electricity for critical loads in case of a utility power failure thus providing a certain protection against short term black outs.

Key components of solar system

Solar PV plant (off grid) systems, directly connected to the battery bank and then inverter to user end electrical distribution network. The one described below, have a relatively straightforward configuration. The PV array is connected to the through junction boxes and then to Battery bank. The output of battery bank is connected to the inverter, which is connected to the electrical distribution network. PV array with racking, Junction box and DC distribution box or DC disconnect and battery bank on DC side system and Inverter, AC distribution box or AC disconnect, main distribution panel and metering system on AC side. Metering system configuration will be selected as either gross metering, which includes uni-directional meter to be installed nearby to existing utility meter or net metering, which includes single bi-directional meter along with manual disconnect switch on line side. The following figure gives an idea about the key components of solar rooftop system,

Solar Panel: -
A solar panel (also solar module, photovoltaic module or photovoltaic panel) is a packaged, connected assembly of photovoltaic cells and it is used as a component of a rooftop system to generate and supply DC electricity.). Solar panels usually form a part of a large solar array (multiple panels connected together). Each panel is rated by its DC output power under standard test conditions, and typically ranges from 100 to 350 watts

Junction box: -
An electrical junction box is a container for electrical connections, usually intended to conceal them from sight and tampering. A small metal or plastic junction box may form part of an electrical conduit wiring system in a building, or may be buried in the plaster of a wall, concealed behind an access panel or cast into concrete with only the lid showing. It also includes terminals for joining cables. Its protection degree should be IP65.

DC Combiner box: -
DC Combiner box provides a means of combining multiple source circuits from a PV array into a single DC Source. Each source circuit is fused separately according to the requirements of the National Electric Code (NEC). The combiner box allows for fail-safe operation of the system in the unlikely event that a problem with a source circuit leads to abnormally high current. In addition, the combiner box provides a convenient means of diagnosing the DC portion of a PV system for routine maintenance and troubleshooting

Charge Controller:-
A charge controller, charge regulator or battery regulator limits the rate at which electric current is added to or drawn from electric batteries. It prevents overcharging and may prevent against overvoltage, which can reduce battery performance or lifespan, and may pose a safety risk. It may also prevent completely draining ("deep discharging") a battery, or perform controlled discharges, depending on the battery technology, to protect battery life. The terms "charge controller" or "charge regulator" may refer to either a stand-alone device, or to control circuitry integrated within a battery pack, battery-powered device, or battery recharger.

The Battery backup connection: –
An electric battery is a device consisting of one or more electrochemical cells that convert stored chemical energy into electrical energy. Each battery consists of a negative electrode material, a positive electrode material, an electrolyte that allows ions to move between the electrodes, and terminals that allow current to flow out of the battery to perform work. In case of large rooftop solar PV plants the electricity is generally exported to the electrical network through battery bank and consumed by a wide variety of consumers who may be located on the 415 V voltage level and are typically located in the ground floor at the building’s premises.

DC Breaker Panel:-
The circuit breaker contacts must carry the load current without excessive heating, and must also withstand the heat of the arc produced when interrupting (opening) the circuit.

Inverter (Power Conditioning Unit): -
The Power Conditioning Units (PCU) used in SPV systems consists of an Inverter and other electronics for MPPT, Synchronization and remote monitoring. As the PV array output varies with the solar radiation the inverter has to cope with the same. The main functions carried out by the PCU are as follows:
Change the incoming DC received from PV modules into AC with suitable power quality.
The inverter produces sinusoidal AC wave forms with low harmonic distortion.
The inverter also has to act as a protective device of the system. It needs to trip out if the voltage, current or frequency goes outside acceptable ranges.

AC Breaker Panel:-
The circuit breaker contacts must carry the load current without excessive heating, and must also withstand the heat of the arc produced when interrupting (opening) the circuit. Contacts are made of copper or copper alloys, silver alloys and other highly conductive materials. Service life of the contacts is limited by the erosion of contact material due to arcing while interrupting the current. Miniature circuit breaker (MCB) and moulded-case circuit breaker (MCCB) are usually discarded when the contacts have worn, but power circuit breakers and high-voltage circuit breakers have replaceable contacts.

In electricity supply systems, an earthing system or grounding system is circuitry which connects parts of the electric circuit with the ground (electricity), thus defining the electrical potential of the conductors relative to the Earth's conductive surface. The choice of earthing system can affect the safety and electromagnetic compatibility of the power supply.In particular, it affects the magnitude and distribution of short circuit currents through the system, and the effects it creates on equipment and people in the proximity of the circuit. If a fault within an electrical device connects a "hot" (unearthed) supply conductor to an exposed conductive surface, anyone touching it while electrically connected to the earth will complete a circuit back to the earthed supply conductor and receive an electric shock.

Lightning arrester: -
A lightning arrester is a device used on electrical power systems and telecommunications systems to protect the insulation and conductors of the system from the damaging effects of lightning. The typical lightning arrester has a high-voltage terminal and a ground terminal. When a lightning surge (or switching surge, which is very similar) travels along the power line to the arrester, the current from the surge is diverted through the arrestor, in most cases to earth.

Auto Changeover switch: -
Usually the solar plant is not large enough to meet the full load of the building for 24 hours, therefore an external auto change over switch can be provided and programmed such to supply electricity by solar energy for a fixed period of time everyday or for providing backup electricity whenever there is a power failure. These power supplies from the solar energy system or from the grid power can be controlled by Auto change Over Switch. This ensures optimum use of power supply everyday and also ensures that the system is not so much over loaded that the inverter burns out or power backup is not extended for too much time so that the battery gets deep discharged.It also provides a protection system for the inverter, battery and the solar power plant which also ensures that maximum power generated everyday is used in the building instead of grid power. This switch also called as an automatic transfer switch (ATS) continuously monitors electric utility power. After backup voltage and frequency stabilizes, the transfer switch brings the system online. Once electric utility power is restored, the switch goes back to its normal position. The switch has two inputs and one output. Here the two inputs are, one from solar energy back up source and other grid supply. It can be installed near to the meter location.

Solar Opportunity
Solar energy presents an exciting opportunity for India as a renewable energy source. Though the current contribution of solar energy to the total India’s energy needs is insignificant, by observing the current trend, it is expected that solar energy will contribute significant component to the energy mix of the country in the near future. The Electricity Act 2003 emphasises the promotion of energy generation from renewable energy sources. The Government of India has taken several steps and enacted laws, policies and regulations to promote renewable sources of energy. The government has created fiscal incentive instruments and also market based instruments to promote energy generation from renewable sources. The Honourable Prime Minister of India, announced the NAPCC (National Action Plan on Climate Change) in June 2008 emphasising the seriousness of the government to mitigate the climate change and impacts. The key target of NAPCC is to have 15% of the total energy consumption in the country from renewable sources of energy by 2020. Earlier the set target was 20 GW till 2020 which in now 100 GW by 2020 out of which 40 GW will come through rooftop only.

Challenges in development of Solar PV projects
The main challenges in the development of Solar PV projects are mentioned below:
• High capital cost: The initial capital investment of solar PV projects is very high compared to other conventional power projects and significant financial assistance from government is needed to the developers and operators of new plants.

• Low Capacity Utilization factor: The total units generated are low compared to other electricity generation systems, because of the limited hours in a day plant that the plant receives solar radiation.