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How grid interactive roof top solar PV systems work?

Many countries in the world are using conventional energy sources to fulfill their electricity requirements. Conventional power generation involves burning of fossil fuels that releases Greenhouse gases (GHGs) resulting in global warming and climate change.  Growing concern about climate change, countries around the world are searching for best alternative to fossil fuels. Renewable energy is the solution but current state of technology and cost factor makes it the least suitable option as an alternate choice. However, since the past one decade, the technology has evolved in such a way that we can see utility scale renewable energy power projects based on Solar, Wind and other renewable technologies coming up. For countries like India where coal is the dominant fossil fuel to generate electricity, the least GHG emission intensive option is the major requirement. Rooftop Solar PV system is the best option nowadays.

Unlike MW scale solar PV projects, rooftop system uses very less space and is also cost effective. We all know that the major issue behind utility scale solar PV project is availability of land. It takes around 5 acres of land for installing 1MW of solar PV plant. In India, where the population is growing enormously, we require considerable land area for growing food grains to support the growing population. Agricultural land conversion for solar project does not seem a viable option. Another issue is the availability of grid. Rural population in India is still struggling for  electricity as they don’t have access to power grid. In addition, there is acute shortage of electricity generation in India due to non/ inadequate availability of coal resulting in frequent power cuts.

Rooftop Solar PV system: How it works?

Solar PV rooftop system is basically a small power plant at your rooftop. The Grid interactive Roof Top Solar Photo Voltaic (PV) mainly consists of three major components. These are the solar PV modules, mounting structure for the modules and the inverter or power conditioning units. Solar PV modules form an array and it requires a mounting structure to hold PV modules at the required angle for maximized generation. The solar panels convert solar energy in the form of light into electricity in DC form (Direct Current). The DC electrical energy is converted to AC (Alternate Current) power by the inverter/power conditioning unit which is connected to the power grid through AC distribution board. The AC power output can be measured through metering panel connected to it. The 415 V AC output of the system can be synchronized with the grid and the electricity can be exported to the grid depending upon solar power generation and local consumption.

Grid synchronization

Electrical inverters convert direct current generated from solar PV modules to alternating current. Therefore, solar modules need to be connected to inverters. Nowadays we have smart Grid-interactive inverters. These inverters can produce AC power that matches voltage and frequency of the grid and the power line it connects to. There is another requirement of adding an isolation transformer. For safety purposes, isolation transformer is used to transfer electrical power from inverter to the connected load while isolating the load from the power source. In addition, the injection of DC power into the grid can be avoided by using an isolation transformer at the output of the inverter.

Power generated from the rooftop solar system during the daytime can be utilized fully by powering the building loads and feeding excess power to the grid as long as grid is available. Whenever, solar power is not available due to shadow or a cloudy day, the building loads can be served by drawing power from the grid or DG sets. Smart inverters automatically understand the power situation and always give preference to power generated from solar modules.

Metering

If the project location has feed-in-tariff applicable, the separate feed-in-meter (export meter) is suitable. Feed-in-tariff meter records gross generation from the rooftop solar system. However, conventional energy meter (Import meter) record electricity consumption of the building. Currently, the state of Gujarat have feed-in-tariff scheme and hence feed-in-tariff meters are applicable in Gujarat. However, where feed-in-tariff provision is not applicable, project developer can have net metering system (bi-directional meter).

Solar PV rooftop single line diagram-Feed in metering
Fig 1: Feed in metering schematic

Key facts about rooftop solar systems;

  1. The solar panels should be south facing to maximize the production (In India). The angle of panel should be 25-30 degrees with respect to the land surface
  2. The weight exerted by the Solar Panels and the structure is around 10 Kgs per square meter.
  3. The solar installation require about 100-150 square feets of area per Kilowatt.
  4. No frequent maintenance is required for the solar system except regular cleaning of surface of the panels.
  5. There are two types of rooftop solar installations. First is for captive use and second is for feed in tariff based grid connected system. In captive use, rooftop solar system installer himself consumes all electricity generated by the system. However, in second type i.e. feed in tariff based grid connected system, installer can also feed/sale excess power to the grid.
  6. In captive rooftop solar systems, there are two types. The first one is standalone systems and other is grid connected system.
Solar rooftop connecting diagram-Net metering
Fig 2: Net metering schematic

Benefits of Rooftop Solar PV systems

  1. Cost of electricity is increasing. Electricity utility companies are bound to increase the cost per unit of electricity due to increase in cost of fuel.
  2. Many regions in the country are facing severe power cuts extending to 12 -14 hours in a day.
  3. Grid tied solar PV systems are the most common systems for captive power generation. It is technologically feasible to have hybrid systems such as Solar PV +Grid or Solar PV +Diesel Generator
  4. It cost about Rs. 2 Lakhs/KW with battery storage and 1.5 Lakh/KW without battery storage. It seems high as compared to conventional diesel power generators. However, operational cost for diesel generator is very high. Considering the replacement cost of power generation from diesel generators, solar PV systems provide attractive returns.
  5. MNRE provides capital subsidy to the off grid solar PV power systems.
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