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Solar Photovoltaics

Source: Rfassbind (Own work.) [Public domain], via Wikimedia Commons
Source: Rfassbind (Own work.) [Public domain], via Wikimedia Commons

Components of a Typical Solar PhotoVoltaic System

Solar photovoltaics are a combination of panels containing a number of solar cells which convert the incident solar energy into usable electricity. These panels can be placed at any place which receives abundant amount of sunlight. The solar cells are made up of semiconductor materials, such as crystalline silicon, which includes monocrystalline silicon, polycrystalline silicon, ribbon silicon and mono-like-multi silicon, and thin films, which include cadmium telluride, copper indium gallium selenide, silicon thin film and gallium arsenide thin film. The broad difference between the two technologies is that the former consists of slices of solar-grade silicon assembled together whereas the latter consists of thin layers of semi-conductors deposited in cheap materials like glass, polymer, or metal.

The process of generation of electricity from solar cells is a two – step process. The first step is the physical process and involves the photoelectric effect in which the photons strike the metal surface and provide energy to the electrons in the metal. The next step involves the electrochemical process in which the excited electrons are arranged in a series, thereby creating an electric voltage and generating electric current. The generated electricity can either be consumed instantaneously on site, stored in batteries for later use, or sold to power utilities according to local government regulations and prevalent tariffs.

Solar Energy Deployment

Based on the availability of space and capital, solar energy can either be generated offsite at utility level, or onsite at small scale. Both types of generation mechanisms are guided by their own rules and policy frameworks.

  • Utility driven solar project development – These are large scale centralized solar power plants which generate electricity to be sold to power utilities. These plants require large tracts of land and considerable capital. These plants usually have a long term Power Purchase Agreement (PPA) with the power utility and usually serve to fulfil their Renewable Purchase Obligations (RPO).
  • Customer driven solar project development – These are small scale decentralized solar power plants installed by electricity consumers in their own premises. These type of projects require less area and capital investment. These systems are further divided into two parts:
    • Grid Connected Systems – Grid connected PV systems are designed to work in conjunction with the utility grid. Such systems can either supply the complete generated electricity to the grid or can use the electricity for building use and supply only the excess power to the grid.
    • Stand – alone systems – Stand – alone PV systems are designed to operate within the context of the building and are not connected to the grid. The electricity generated is consumed by the building and excess energy generated can be stored in the batteries for future use.

Solar electricity generation system

A complete solar electricity generation system consists of components to produce electricity, convert generated DC into AC that can be used by equipment installed in the building, and store excess generated electricity (for those systems which do not intend to sell excess generated electricity to grid).

  • Solar PV panels – Solar panels are the basic part of a solar electricity generation system. These panels consist of numerous solar cells which are made up of a semiconducting material. These solar cells are responsible for conversion of incident light into usable electricity. Although the sizes may vary according to generation capacity, location and budget, the typical length of the solar panel ranges between 65 inches and 77 inches and the breadth ranges between 35 inches and 39 inches. The typical depth of solar panels ranges from 1.4 inches to 1.8 inches.
  • Inverter – The inverter converts the DC produced by the solar panels into AC that can be fed into the grid or used for the operation of electrical appliances. Additionally, the inverter acts as a safety valve between the PV system and the electricity mains.
  • Storage Batteries – Storage batteries are used to store excess electric energy generated by the PV system for future use. Batteries are typically employed in PV systems which do not intend to sell excess electricity to power utilities.
  • Electricity Meter – The meter counts the number of units of electricity generated by the PV system. They are essential for calculating the proceeds from the sale of electricity to the grid.

Factors affecting generation of electricity

Solar cell efficiency is the ratio of electrical output to the incident solar energy. Major factors efficiency of solar cells are:

  • Location, tilt, and orientation – The incident solar radiation varies significantly with longitude and orientation. Within a particular longitude and orientation, maximum solar radiation is available in a particular tilt angle based on sun path.
  • Over shading – Site characteristics like geography, neighbouring buildings, self-shading, cloud factors etc. affect the useful solar radiation falling on the PV panels. System design should be done to minimize panel area affected by shading.
  • Temperature – An increase in panel temperature due to solar radiation can affect the PV module performance especially in crystalline silicon modules. It is estimated that for every 1 0C increase in ambient temperature above 25 0C, the PV module performance decreases in the range of 0.4 – 0.5%. Special considerations are given to design air flow over the backs of the PV modules to avoid higher temperature by excessive heat gains.
  • Panel efficiency – Panel efficiency is an estimate of successful conversion of incident solar radiation to electric energy. Panel efficiency depends on PV module technology, manufacturing techniques, and system design. A crystalline silicon based module has an efficiency in the range of 12-14% where as a thin film based module has an efficiency in the range of 10 – 11%.

Harnessing solar energy on site is one of the easiest ways for an NZEB to reduce its dependence on conventional electricity. Solar panels can be placed in almost any part of the site which receives abundant sunlight and is free of shade. Moreover, apart from conventional rooftop solar systems, NZEB owners can also look at innovative technologies such as Building Integrated Photovoltaics (BiPV). This system integrates the PV panels with building facade and serves the dual purpose of building envelope material and power generator, thereby reducing the energy demand of the building.

The initial cost of installation is high for a solar PV system. However, lower maintenance cost, along with Government’s subsidies and easy finance, makes solar PV an attractive investment with decent payback period and returns.

  • The efficiency of solar panels is tested by National Institute of Solar Energy under Standard Test Conditions. For India, Ministry of New and Renewable Energy mandates the following certifications for solar panels.
    • For crystalline silicon PV modules – IEC 61215 / IS 14286
    • For thin film PV modules – IEC 61646
  • Maximum solar radiation is received by states of Gujarat and Rajasthan and area of Ladakh in Jammu and Kashmir and these areas have the maximum potential for generation of solar electricity (see map below).
  • Space required for 1 kW of solar PV installation is in the range of 7 – 10 sq. m.
  • Wall space of approximate size 3’X3’ is required for inverter and an AC disconnect. A clear area of width 3’ is also required in front of the system for maintenance.
  • Electricity generated from 1 kW of solar PV in Delhi climate is in the range of 1400 – 1550 kWh/year.
  • Obstructions to the solar panels, such as mechanical and exhaust vents, and roof shading devices such as chimneys, should be kept clear of the area. Roof shading elements should be at least twice as far from the panels as their height.
  • Shading effect of any future expansion of nearby buildings and growth of trees should be analysed before finalizing the location of PV panels.
  • Maximum efficiency of solar panels is realized when they are installed facing the south direction. However, the PV system can be installed even if the orientation is not true south, with the help of specific structure systems. Solar trackers can also be installed to orient the panels towards the sun.
  • The cost of 1 kW of solar PV installation varies from Rs. 75,000 to Rs. 1, 30,000 depending upon the efficiency/ structure design/ inverters types/synchronization etc. of the system.
  • The payback of the solar system comes to 7-8 years in normal conditions. If DG is used more than 30% of the time it may come down to 4-5years.
  • The approximate life of PV modules is 25 years. However, the life of supporting equipment like batteries have an approximate life in the range of 10 – 15 years.
  • The PV modules require very less maintenance and need to be cleaned with pressure washers every 15 days. Batteries increase the maintenance cost of the system and they need to be replaced within the lifespan of the modules. Consequently, it is cheaper over the complete life cycle to install PV systems without batteries.
  • Provide DC isolators and AC isolators to safely disconnect the PV system in cases of emergency and to carry out the maintenance work.
  • Inverters should be located in a cool and well-ventilated space since they generate heat during their operation. They can be located in basements at a location having direct vertical connection to the solar panels.
  • PV panels result in an additional dead load of 30 – 40 kg/sq.m. on the building. The structure of the building should be sufficiently designed to accommodate the additional load.
  • Restriction on height and location of PV panels should be checked according to local bye – laws.

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