Sun is the most abundant source of energy and the energy from the sun is commonly known as solar energy. Solar energy is the radiant (light and heat) energy which is produced by the sun. The solar energy that reaches the surface of the earth can be used to produce electricity or heat through the use of solar collectors and for many other useful purposes. Solar energy, radiant light and heat coming directly from the sun, has been harnessed by humans beings since ancient times using a range of ever-evolving technologies and thus advancing the technology slowly with time. Solar energy applications include solar heating, solar cooking, solar photovoltaic, solar thermal electricity, solar architecture and artificial photosynthesis, which make considerable contributions in solving some of the most urgent energy problems the world now faces an essential for human existence.

Solar energy is the most readily available source of energy. It does not belong to anybody and is thus free to use. The most important fact of the solar energy is that is an everlasting and auto-renewing source of energy and also very important non-conventional sources of energy because it is non-polluting and, therefore, helps in lessening the greenhouse effect.

Solar technologies are broadly characterized as either active solar or passive solar depending on the way they capture, convert and utilize solar energy to convert it into something useful. Active solar techniques include the use of photovoltaic panels and solar thermal collectors to harness the energy and convert it directly into electric energy. Passive solar techniques include orienting a building to the Sun, selecting some materials with favourable thermal mass or light dispersing properties, and designing spaces that naturally circulate air or space which helps in heating up during winters using solar energy.

Solar Energy from the Sun

When we hang out our soaked clothes in the sun, they get dried after some time. This takes place because of the solar energy. The wet clothes absorb energy from the sun which imparts their energy in wet clothes and helps to dry them. The light energy that passes through the clothes is absorbed by the clothes and utilized to heat the clothes and thereby drying them.

The Earth receives about 174 petawatts (PW) of incoming solar radiation at the upper atmosphere. Out of this 174 PW, approximately 30% is reflected back to space while the rest is absorbed by clouds, oceans and various other land masses on the surface of the earth. The spectrum of solar light at the Earth's surface is mostly spread across the visible and near-infrared ranges with a small part in the near-ultraviolet range. Earth's land surface, oceans and atmosphere absorb solar radiation and thereby raising their temperature due to solar energy. Warm air containing evaporated water from the oceans rises up and causes atmospheric circulation or more commonly known as convection. When the air reaches a certain altitude, where the temperature is low enough, and the water vapour condenses into clouds, which rain onto the Earth's surface, completing the water cycle. By photosynthesis, green plants convert solar energy into chemical energy, which produces food, wood and the biomass from which various useful fossil fuels are derived.

The amount of solar energy that is reaching the surface of the Earth is so vast that in just one year it is about twice as much as will ever be obtained from all of the Earth's non-renewable resources of coal, oil, natural gas, and mined uranium combined and petroleum etc. Solar energy can be harnessed at different levels around the world, mostly depending on the distance from the equator, which determines the position of the sun from a particular place and amount of sunlight that a particular area receives.

In the next few years, it is expected that millions of households in the entire world will be using solar energy as the trends in some developed countries like UK, USA and Japan show. In India too, the Indian Renewable Energy Development Agency and the Ministry of Non-Conventional Energy Sources are formulating a programme to have solar energy in more than a million households in the next few years and also giving away subsidy for buying solar products. However, the people’s initiative is essential if the programme is to be successful.

Applications of Solar Energy

Solar energy is a free form of an everlasting source of energy and is being utilized these days for plenty of purposes. Solar energy refers primarily to the use of solar radiation for practical ends. However, all renewable energies, other than geothermal and tidal, derive their energy from the sun. Solar energies can be broadly characterized as either passive or active depending on the way they capture, convert and distribute sunlight. Active solar techniques are those using photovoltaic panels, pumps, and fans to convert sunlight into some useful output while passive solar techniques are those which involve selecting materials with favourable thermal properties, designing spaces that naturally circulate air, and referencing the position of a building to the Sun. Active solar technologies increase the supply of energy and these may be considered as side supply technology, while passive solar technology would reduce the need for alternate resources and is generally considered demand side technology.

Though solar energy is utilized for a number of purposes, solar energy can also be used to meet our electricity requirements. Through Solar Photovoltaic (SPV) cells, solar radiation can easily be converted into DC electricity directly. This electricity can either be used directly as it is or can be stored effectively in the battery. This stored electrical energy that can be used whenever required. SPV can be used for a number of applications such as:

• Residential lighting purpose
• Street lighting purpose
• Industrial power requirement
• Agriculture (Green-house) Heating, cooling and ventilation
• Village electrification
• Water pumping
• Desalination of salty water
• Powering of remote telecommunication repeater stations and
• Railway signals

Solar Power

Solar power is basically the energy produced by the conversion of sunlight into electricity, either directly using photovoltaic (PV), or indirectly using concentrated solar power (CSP). Concentrated solar power systems use lenses or mirrors and tracking systems to focus a large area of sunlight into a small beam and converting it into a useful form. Photovoltaic convert light into electric current using the photoelectric effect in which by the use of semiconducting materials charge is generated utilizing light energy. Other technologies also exist, that work on the principle of solar power such as Stirling engine dishes which use a Stirling cycle engine to power a generator. Photovoltaic were initially used to power small and medium-sized applications to increase their mobility, like calculators etc. with time and technological advancements this technology has reached new heights today.

Photovoltaics are a clean and a free way to harvest electrical power from the sun's rays through the use of solar cells. Solar power as we know is a never-ending supply of electricity that can greatly reduce our dependency on conventional electric power and considerably reduce our electric bill. This free power source isn't really free though and may involve a quite high initial investment, but benefits would be clearly visible in long run. Once you have invested in, you would enjoy a lifelong supply of solar energy at almost free rates and also these solar cells are pollution-free and can be mounted in areas easily accessible or areas that keep them free from anyone coming into contact with them. They can be mounted in yards or atop the roof of your house or any other suitable place for its proper functioning. These solar cell panels are connected together to form a layout of panels that produce all or part of your power that your home requires. Solar cells produce direct current (DC) and that is stored in batteries or is converted to usable alternating current (AC) that takes care of daily domestic power requirements.

Practical Usage of Solar Energy

• Architecture Planning for Residential Buildings: Sunlight is an important aspect of architectural planning these days. In early days only thing considered was about utilization of sunlight for lighting during the day, but now vision has changed a lot. Sunlight has influenced building design since the beginning of architectural history, so this concept is basically not new; just the application level has advanced to a high level. Advanced solar architecture and urban planning methods were first employed by the Greeks and the Chinese, who started with orienting their buildings toward the south to provide light and warmth. With time there were many more advancements added to the concept for much effective utilization of sunlight.
  The common features of passive solar architecture are oriented relative to the Sun, a low surface area to volume ratio, selective shading overhangs and thermal mass. When these features are in accordance with the local climate and environment they can produce well-lit spaces that stay in a comfortable temperature ranges. The most recent approaches to solar design use computer modelling controlling together solar lighting, heating and ventilation systems in an integrated solar design package as one whole system. Active solar equipment such as pumps, fans and switchable windows can complement passive design and improve the performance of the system.
• Commercial Buildings: Solar energy can effectively be used in office buildings and help to reduce hefty electricity bills. On an office building, roof areas can be covered with glass PV modules, which can be semi-transparent to provide shaded light, whereas, in a factory or warehouse, large roof areas are the best location for solar modules. If the roof is flat, then arrays can easily be mounted using techniques that do not breach the weatherproofed roof membrane. Also, skylights can be partially covered with photovoltaic. The vertical walls of office buildings also provide several opportunities for photovoltaic incorporation, as well as sun shades or balconies incorporating a PV system for power generation. Sunshades may have the PV system mounted externally to the building, or have PV cells specially mounted between the glass sheets comprising a window.
• Solar Energy for Household Power Requirement: It is possible to power your home with solar energy. The system needed is not really a complex one when you examine the devices that you’ll need. A simple step is to add solar panels to collect sunlight and convert it into electricity. DC power (direct current) is then sent to an inverter, which converts DC power into AC power, which in turn take to fulfil your domestic power requirements. Through the use of transfer switches and other safety devices, this clean and renewable power source is capable of powering your home or any other building for that matter. Apart from power requirement solar energy can also be used for water heating, indoor lighting and cooking purpose by the use of solar cookers.
• Solar Energy for Agriculture and Horticulture: Solar energy is also very useful in the agriculture sector. Apart from providing the sunlight for the process of photosynthesis, there are even more advancements using solar energy to optimize the production. Techniques such as timed planting cycles, tailored row orientation, staggered heights between rows and the mixing of plant varieties can improve crop yields to a great extent. During the short growing seasons of the Little Ice Age, French and English farmers employed fruit walls to maximize the collection of solar energy and were found to be quite an effective way, these walls acted as thermal masses and accelerated ripening by keeping plants warm. Early fruit walls were built perpendicular to the ground and facing south, but over time, sloping walls were developed to make better use of the sunlight.
  The greenhouse effect is a commonly used advancement for effective production. Greenhouses convert solar light into heat, enabling year-round production and the growth (in enclosed environments) of speciality crops and other plants not naturally suited to the local climate. The greenhouse effect was first used during Roman times to produce cucumbers year-round for the Roman emperor Tiberius. The first modern greenhouses were built in Europe in the 16th century to keep exotic plants brought back from explorations abroad. Greenhouses remain an important part of horticulture today.
• Solar Energy for Industries: Solar energy also finds its application in industries. For past many years, solar energy has been the power supply choice for industrial applications, especially at places where power is required at remote locations. Because solar systems are highly reliable and require little maintenance, they are ideal in distant or isolated places where there is no other effective source of power.

Limitations of Solar Energy

• Initial cost involved is very high.
• Proper sunlight is desirable for these systems to work effectively and efficiently.
• You cannot depend completely on these solar-driven systems.
• This system does not work in clouds or at night. In case of rain, these fail to give power.
• Along with these power systems, there is also a need for power backup plan, as you cannot completely rely on solar energy.
• These systems large need space for installation on roofs or areas where there is the abundant availability of sunlight.
• Since many big solar plants are set up in desert areas, there are no energy grids and hence the cost of power transmission also adds up.
• Time to time repair needed by these solar panels also adds to cost, and also to get them repaired you need an experienced energy technician.
• Solar panels are made up of materials that are harmful to the environment and human. If some parts are replaced then old parts can’t be simply thrown away. They need proper disposal process.