Sunlight is Earth’s most abundant energy source and is delivered free of charge. Indeed, the energy from sunlight reaching the land surface of the earth is thousands of times greater than the world’s current total annual consumption of energy.
Yet harnessing sunlight’s energy content directly—rather than indirectly in fossil fuels, wind, or hydroelectric power—makes only a small contribution to humanity’s energy supply.
Active solar techniques include the use of photovoltaic systems, concentrated solar power and solar water heating to harness the energy. Passive solar techniques include orienting a building to the Sun, selecting materials with favorable thermal mass or light-dispersing properties, and designing spaces that naturally circulate air.
As sunlight can be considered in two forms – heat and light – solar energy can also be divided into two main components – Solar Light Energy and Solar Heat Energy.
Solar Light Energy – In this, the main objective is to convert to electricity the energy contained in sunlight in the form of photons. This sector is known as Solar Photovoltaic or Solar PV
Solar Heat Energy – In this, the objective is to use the sunlight’s heat energy as thermal energy or to generate electricity. This sector is known as Solar Thermal.
Solar Thermal Energy (ST) is the most direct way to use the energy from sunlight – this method used the heat of the sunlight for heating and drying applications. Simple examples of solar thermal applications are solar water heaters and solar cookers. In some select cases, this thermal energy can also be used to generate power too.
The PV in solar PV stands for photovoltaic, which simply means light to power. Solar PV systems convert the light energy in sunlight (in the form of photons) to electricity, through the use of semiconducting materials. The photons excite the electrons in the semiconducting material, resulting in a flow of electricity.
SOLAR THERMAL (HEAT APPLICATION)
Solar thermal applications are many, and the process of heating can be direct or indirect. Depending upon the temperature of heating application, it is classified into – Low Temperature, Medium Temperature and High Temperature Heat Applications. Common examples include solar water heaters, solar cookers, solar dryers etc.
SOLAR THERMAL (POWER APPLICATION)
Solar thermal energy can be used to generate power too. This is done through concentrating sunlight through mirrors or lenses. The high temperatures achieved are used to run a steam turbine to generate power.
OFF – GRID SOLAR SYSTEM
Systems that are not connected to the gird and function independently are referred to as Off grid solar system. They can be in the form of rooftops solar system and other solar related products such as solar street light, solar lanterns, solar water heater etc.
DISTRIBUTED GRID CONNECTED
Solar power plants, owing to their modularity, are ideal for generation from distributed locations – on rooftops of residences or industries, on car parks and more. Such distributed solar power units can also be connected to the grid so that the system gets the twin benefits of a sustainable energy source and the reliability of grid power.
UTILITY SCALE GRID CONNECTED
Utility scale grid connected power plants are the large solar farms with tens of thousands of panels. These could range from 1 MW to upwards of 100 MW in capacity. The electricity produced by such a power plant is generally exported to the grid for sale to the local utility company or to a private company.
LOW TEMPERATURE SOLAR COLLECTOR
Low Temperature Solar Collector is a classification of Solar Thermal with Heat Application which has an operating limit of up to 80 deg C. The heat collection can take place either using Flat Plates Collector or Evacuated Tube Collector. Examples of Low Temperature Solar Collector are Solar Cooker, Solar Water Heater.
MEDIUM TEMPERATURE SOLAR COLLECTOR
In Medium Temperature Solar Collectors temperatures of up to 150◦C can be achieved. They are generally used in industries for specific heating or drying applications. Typically, Enhanced Evacuated Tubes and sometimes Parabolic Concentrators are used to raise the temperature up to 150◦C.
HIGH TEMPERATURE SOLAR COLLECTOR
High Temperature Solar Collectors are used to attain temperatures of up to 250◦C. In order to reach such high temperatures, large mirrors and lenses are used to capture the solar heat energy and concentrate the heat on to a small area. As these solar collectors achieve the high temperature through concentrating the sunlight, they are often referred to as Concentrated Solar Thermal (CST).
CONCENTRATED SOLAR POWER
Concentrated Solar Power (CSP) uses the heat from the sun to produce power. By means of large mirrors or lenses sunlight is concentrated onto a small area. This solar energy heats the water and produces steam. The steam is used to run a Steam Turbine and further on a generator to produce electricity.
HYBRID GRID CONNECTED SYSTEM
These refer typically to distributed solar power systems that, in addition to being grid-connected, also are connected to another source of power such as a battery or diesel power generator among a few. The advantages of such a system are that they provide uninterrupted power supply and can be programmable according to the available source of energy and the needs of consumer.
A grid tied system refers typically to distributed solar power systems that are connected to the grid and no other source of power such as batteries. In case of a power outage, this system would disconnect from the utility grid, owing to technical and security reasons. In the absence of the grid, the system stops generating electricity.
ROOFTOP SOLAR SYSTEM
A rooftop solar system generates electricity and supply it to the household based on a solar panels mounted on top of the roof.
CONCENTRATED SOLAR THERMAL
Concentrated Solar Thermal, that can provide heat up to 250 deg C, produces thermal energy by using the heat from the sun. Such a high temperature is achieved through large mirrors or lenses that concentrate the heat energy onto a small area. The resulting heat is used directly or indirectly for heating and drying purposes.
SOLAR WATER HEATER
A solar water heater captures the heat of sunlight through a thermal collector and uses it for water heating. The heat energy harnessed can also be stored in a storage tank and use it for later purpose.
Solar cooker uses the heat of sunlight to heat and cook food and beverages. Solar cookers use reflecting surfaces to concentrate sunlight onto a small area to prepare food items.
Solar farms refer to grid connected power plants that produce electricity on MW scales. These are usually ground mounted and export all the power to the grid, for sale to the local utility or to a private consumer.
Lanterns that are powered by solar power are referred to as Solar Lanterns. These lanterns charge their batteries from solar panels during the day and can be used at nights. They generally use LEDs or CFLs. These lanterns are commonly used in rural areas to replace the kerosene lamps.
Solar Streetlights are outdoor light fixtures that work on solar energy. During the day these streetlights get charged and the energy is stored on to a battery. At nights, these solar streetlights switch on to light up the streets.
SOLAR WATER PUMP
Pumps that work on the electricity generated by the photo-voltaic panels are called Solar Water Pumps. These pumps are highly efficient and useful in places where power from the grid is either unavailable or is unreliable. The agricultural sector is the main end user of solar water pumps.
Some important Products and Devices in detail:
SOLAR WATER HEATER
Solar water heaters typically consist of two components – a solar collector and storage tank.
Solar Collectors are devices that collects heat by absorbing sunlight. The heat absorbed is then transferred to the working fluid (water or anti-freezing liquid). These collectors consists of an absorber which are generally made up of several narrow metallic strip. It also consists of a metallic tube through which the working fluid flows.
Storage tank as the name suggests is used to store hot water. These Storage tanks are either connected horizontally on to the solar collector or are ground mounted. In some cases, a heat exchanger is present inside the storage tank. There are two types of solar collectors that are available in the market:
- Flat Plate Collector
Flat plate collectors (FPC) is a metallic box with a completely transparent glass or plastic cover. Inside the box is a series of dark colored heat absorbing plates generally made of copper. The sides of the FPCs are covered with insulating materials to keep the heat loss to a minimal.
Evacuated Tube Solar Collector
It consists of several glass tube, with each tube being evacuated to reduce the heat loss within. Inside the glass tube are curved or flat metallic tube that is attached with a fin to absorb the heat.
Cooking using solar cooker is done by means of UV rays from the sun. A solar cooker lets the solar UV rays in and converts them to Infrared rays. These Infrared light rays has the right amount of energy to make water, fat and protein particles in food to vibrate and in turn heat them up. On the contrary to what people think about the direct heat from the sun being used to cook food, it is the rays from the sun that are converted to heat energy which contribute maximum for cooking of food in the solar cooker. This heat energy is retained in food by covering the lid of the vessel.
There are three major types of solar cookers that are used –
Box Solar Cooker
Box solar cooker is the most common and simplest type of solar cooker. It is based on the design of traditional modern ovens, where the food in the vessel is placed inside an insulated box which is covered with a transparent glass, to allow the sun rays to pass through it on to the vessel. The solar UV rays are converted to longer Infrared rays that heat the vessel. Some UV rays that do not get converted into longer Infrared rays, are retained inside the box to build up the temperature and make the box a desirable place to cook food. Reflecting panels are also used to concentrate the sun rays towards the vessel for higher cooking temperature and effectiveness.
Panel Cookers consists of a darkened cooking vessel, an oven cooking bag and reflecting panels. These panels are made of aluminum foil or from tin or sheet metal that are polished to high sheen. Even mirrors are used as reflecting panel. The reflecting panels concentrate the sun light on the cooking bag. The cooking bag in turns allows the UV rays to penetrate on to the cooking vessel, in trapping the energy and prevent it from escaping. The Panel Cooker works similar to Box Solar Cooker, however it much simpler and economical to build and results in the same effectiveness.
Parabolic Cookers are also known as curved concentrator oven. In comparison to other solar cookers, parabolic cookers create higher temperature which results in food being cooked faster. They reflect the sun’s rays onto the cooking vessel by means of the curved reflecting surfaces. They are much more expensive when compared to Box and Panel solar cookers. It also requires close supervision and regular adjustments for the cooker to work effectively.
Different Types of Solar Cookers
Concentrating solar power
Concentrating Solar Power (CSP) uses large mirrors to concentrate the heat energy from sun to drive a traditional steam turbine. This in turn is used to create electricity. CSP broadly consists of large reflecting mirrors called Heliostats and a receiver.
There are four types of CSP –
- Trough System
Trough systems consists of large U-shaped reflectors with an oil filled receiver present at the center of the reflector or the focal point. These reflectors are tilted towards the sun to harness maximum solar energy. When sun light falls on these reflectors, they reflect it towards the receiver. Because of the concentrating solar power on the receiver, the oil in the receiver reaches temperatures of 750 °F. This hot oil is used to heat the water, which produces steam to run a steam turbine or generator.
- Power Tower System
Power Tower System also called central receivers use large number of reflecting mirrors that track the sun in two directions. All the sunlight that falls on the reflecting mirrors are reflected on to a receiver which is placed on top of a tower. Inside the receiver is molten salt, which is heated to temperatures of 1050°F. The heated molten salt is used to produce steam by heating the water. Steam so produced is used to run a steam turbine/generator, which in turn helps in producing electricity. Molten salt can retain the heat obtained for 3-4 days. This helps in electricity being produced even during nights when there is a shortage of sunlight.
- Dish Engine System
Dish Engine Systems consists of large parabolic reflecting mirrors that are used to focus the sunlight on to the receiver which is placed at the focal point of the mirror. To capture maximum heat from the sun, the mirrors track the sun across the sky. The receiver is integrated with an external combustion engine. This engine has a thin tube consisting of Hydrogen or Helium gas that run along the outside of engine’s cylinder. Due to the heating of the receiver, hydrogen or helium gas expand inside the cylinder which enables the piston to be driven. The piston in turn is connected to the generator to produce electricity.
- Linear Fresnel Reflector
Linear Fresnel Reflectors work on the principal of Trough System, but with the use of long parallel rows of low cost flat mirrors. The sunlight is reflected from the mirrors on to the elevated receiver tank. This receiver tank consists of a system of tubes through which water flows. The concentrated heat, boils the water to produce steam which is us to run the generator.
When photovoltaic panels are installed on the roof of commercial or residential building to generate electricity for themselves, it is called Solar Rooftops. Photovoltaics panels are made up of semiconducting materials. The generation of electricity through PV panels is based on the concept of Photovoltaic effect – wherein, photons (Sunlight) fall on the panels, exciting the electrons in them. Due to the excitation, the electrons start to flow resulting in electricity. The electricity so produced is sent to the inverter, to convert DC to AC. From the inverter the Alternating Current is sent directly to various load that require electricity. In the case of battery, the electricity is provided to both the load as well as the loads. The presence of battery enables the electricity to be stored for later usage.
Broadly the solar rooftop systems are classified into two – off grid and on grid. In the case of off grid, the system works completely independent of the grid. These systems are effective only in places where there is no electricity. In the case of on-grid, houses are connected to both the rooftop as well as the gird. When there is surplus of electricity being produced, it is sent to the grid. Also in the case of a shortage of electricity being produced, electricity from the grid is used to over the shortage. On grid system too may have batteries for backup.