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General Configuration of Rooftop Solar power Plants

Classification of Solar Photovoltaic (PV) power systems is done according to the following:

  • Their functional and operational requirements,
  • Their component configurations, and
  • The connection of the equipment to other power sources and electrical loads.

The principal classifications in the solar PV system are grid-connected systems and stand-alone systems. On the basis of these two classifications, Rooftop Solar PV system can be further configured in the following ways:

  • Grid Tied Solar Rooftop PV (SRTPV) System
  • Off Grid/ Stand Alone Solar Rooftop PV (SRTPV) System
  • Grid Interactive Solar Rooftop PV (SRTPV) System/Hybrid System

Apart from these systems, other Solar PV System which can be configured on the basis of functional and operational requirement is Micro Grid System. Each SRTPV system has its own benefits and limitations. The following paragraphs will give an overview:

Grid Tied Solar Rooftop PV (SRTPV) System

Grid Tied SRTPV System

In a grid-tie system, electric loads in the home runs on solar power during the day(Sun shine hours). Any surplus energy that if produced by the system is then fed into the grid. In case of deficit of solar power, the differential power is drawn from the power grid. In this system, a bi-directional meter is installed to measure the net energy (Net metering). These grid tied systems allows to take advantage of the free electricity which is generated from the solar PV system as well as the electric grid. These systems are simple to install, less expensive and easy on maintenance. Grid connected systems do not require battery banks.

Off-Grid/Stand Alone Solar Rooftop PV (SRTPV) System

Off grid power plants are best suitOff-Grid/Stand Alone SRTPV Systemed for hilly and remote areas. It is the only solution for areas where state power grid is inaccessible or unfeasible. That is producing 100% electricity by ourselves in a clean and sustainable manner. This comes with a thought of never paying another electricity bill, and no suffering from grid blackouts.

An off grid unit can be a single light or can run into many kilo watts.  Off grid units or power plants invariably has battery banks to store energy. The power plants are designed and configured to service the loads directly by sun power during the sun shine hours. The excess energy will charge the battery bank. The energy stored in the battery bank is used during the non-sun shine hours.

The quality of power will be generally better than state grid power since latter has many distortions.

With the fall in battery prices and rapid advancement in battery technology, off-grid solutions are proving more feasible and profitable. It is predicted that very soon off grid solar power will be economically on par with grid power (Grid parity).

Grid Interactive Solar Rooftop PV (SRTPV) System/Hybrid System

A grid interactive solar PGrid Interactive SRTPV SystemV system uses solar modules as the power generation source. The power produced is fed into an inverter which changes the DC power output of the solar array to AC power compatible with the power grid.

Designs for these system allows for any onsite loads to be powered by the solar modules or by the combination of power generated by PV system, battery bank and power drawn from the mains power grid. Excess power generated by the PV system onsite will be used to charge the batteries.

Generally, there will be no export to the grid. However, if the state policy allows export, the latter can be enabled.

In other words, the system uses three sources of power Viz Solar, battery and Grid. System can be configured to prioritize the source of power to service the loads. Most common configuration is Solar-Battery-Grid. That is the loads are first serviced by the solar power. In case the loads exceed the power generated by the solar modules, then the battery bank takes over. In case the loads exceed the capacity of the battery bank, the required power is drawn from the grid.

Designs of Grid interactive system are very flexible and can be very optimally designed to suit the power requirement and the budget. However, the additional complexity of maintaining batteries and managing multiple energy sources have to be factored in the designs.

Micro Grid System

A large percentage of the population of rural India does not have access to electricity and relies on non-conventional and/or polluting sources of fuel such as firewood or kerosene for lighting.

These fuel sources not only have adverse effects on health and the environment but also, restrict economic growth. Non-availability of proper power plays a key role in the lack of economic development in rural India.

With the advances in Solar technology, villages can now have access to electricity by investing in their own micro-grid power plant. The requirement and size of a micro-grid can be calculated by adding the power needs of individual homes in the village that will be connected together.

Let us illustrate the technology with an example. A village with 50 homes would typically require 100 units of power a day (2 units per house hold per day). This roughly translates into 25kW of solar power plant. This would typically require an investment of about 25 Lakhs. This power plant would produce 365,000 units of electricity per year. Not only power, this clean power will also avoid 192 tons of carbon emission into the environment.

The micro-grid system regulates the balance between energy used and energy generated. The system controls and optimizes the generator, batteries and the load. These systems are modular, highly scalable and can be easily integrated into the grid at a later stage without any investment.

The illustration below may help in better understanding of the system.

Micro Grid

  1. Required number of Solar PV panels are connected in a particular order of series and parallel combinations to form an array. The connected PV array will generate electricity by converting solar radiation into electrical energy. The generated electricity is then transmitted to a large power inverter, which is known as the Power Conditioning Unit (PCU).
  2. The PCU acts as a central controller, which controls, regulates and directs the electrical energy transmitted from the array. Further, the PCU is connected to the Distribution Box (DB) on one hand, which supplies electricity directly to homes, shops, offices, street lights etc. and the battery bank on the other.
  3. The battery connected to the PCU will be useful during the day, in case, when the power generated is not used or surplus power is generated. The stored power can then be used in the late evening or at night. Further, the whole system is connected to a computer, which will keep an account of local power usage. A modem can also be used in addition to remotely access the information and which in turn reduces the need of manpower to monitor the system.
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