• Introduction to Solar Power Generation

    Solar power is energy from the sun. "Solar" is the Latin word for "sun" and it's a powerful source of energy. Without it, there will be no life. Solar energy is considered as a major source of energy for many years because of the vast amounts of energy that is made freely available, if harnessed by modern technology. Unfortunately, the sun does not available in the night, and in some days, clouds and rains and other natural conditions prevent the sun's powerful rays to reach us. This means that it is not always available. That is why we cannot rely on solar energy alone.

    Solar power is the conversion of sunlight into electricity, either directly using photovoltaics (PV), or indirectly using concentrated solar power. Concentrated solar power systems use lenses or mirrors and tracking systems to focus a large area of sunlight into a small beam. Photovoltaic cells convert light into electric current using the photovoltaic effect.

    Photovoltaics were initially solely used as a source of electricity for small and medium-sized applications, from the calculator powered by a single solar cell to remote homes powered by an off-grid rooftop PV system. As the cost of solar electricity has fallen, the number of grid-connected solar PV systems has grown into the millions and utility-scale solar power stations with hundreds of megawatts are being built. Solar PV is rapidly becoming an inexpensive, low-carbon technology to harness renewable energy from the Sun. Solar power plants use one of two technologies:


    1. Photovoltaic:

    A solar cell, or photovoltaic cell (PV), is a device that converts light energy (photon energy) into electric current using the photovoltaic effect. The first solar cell was constructed by Charles Fritts in the 1880s. These early solar cells cost 286 USD/watt and reached efficiencies of 4.5–6%, each cell converts the light of the sun into electrical energy, which can then be used to power electrical devices. Efficiency of small solar panel is about 6% while that of large solar panel is 14-17%. Note that monocrystalline PV module is more efficient than polycrystalline module.

    The array of a photovoltaic power system or PV system, produces direct current (DC) power which fluctuates with the sunlight's intensity. For practical use this usually requires conversion to certain desired voltages or alternating current (AC), through the use of inverters. Multiple solar cells are connected inside modules. Modules are wired together to form PV arrays then tied to an inverter, which produces power at the desired voltage, and for AC, the desired frequency/phase.

    Many residential PV systems are connected to the grid wherever available, especially in developed countries with large markets. In these grid-connected PV systems, use of energy storage is optional. In certain applications such as satellites, lighthouses, or in developing countries, batteries or additional power generators are often added as back-ups. Such stand-alone power systems permit operations at night and at other times of limited sunlight.

    Figure: Photovoltaic


    2. Concentrated Solar Power (CSP):

    Concentrated Solar Power (CSP) uses thousands of solar tracking mirrors called heliostats to reflect and concentrate sunlight onto a large heat exchanger called a receiver that sits atop a 550-foot tower. Within the receiver, fluid flows through the pipes. This fluid absorbs the heat from the concentrated sunlight. The fluid utilized here is molten nitrate salt, which has superior heat transfer capability, energy storage capability and can be heated upto 1,100 0F (≈ 600 0C).


    Molten nitrite salt is an ideal heat capture medium and maintains its liquid state even above 1,000 0F, allowing the system to operate at low pressure for convenient energy capture and storage. After passing through the receiver, the molten salt then flows down the piping inside the tower and into a thermal storage tank, where the energy is stored as high-temperature molten salt until electricity is needed. When electricity is required by the utility, day or night, the high-temperature molten salt flows into the steam generator, as water is piped in from the water storage tank, to generate steam.

    The hot molten salt generates high-quality superheated steam to drive a standard steam turbine at maximum efficiency to generate reliable, non-intermittent electricity during peak demand hours.

    After the molten salt passes though the steam generator, it flows back to the cold tank and is re-used throughout the life of the project. The cold molten salt stored in cold tank will then send back to the receiver where it is reheated and the process is repeated again and again. The exhausted steam is condensed back to water and returned to the water tank, where it will flow back into the steam generator. This steam generation process is identical to the process used in conventional gas, coal or nuclear power plants, except that it is 100% renewable with zero harmful emissions or waste.


    Types of solar concentrators

    1. Parabolic Trough Collectors (PTCs): PTC systems function through a parabolic reflective surface that concentrates sunlight over an evacuated metal tube located at the focal point of the surface. The tube carries water or molten salt which transfers the heat away from the point of concentration, either to a thermal energy storage facility or to a water boiling facility, where the heat generates steam to drive steam turbines and produce electricity.


    2. Solar Towers: Solar towers are tall structures surrounded by a field of mirrors, which are called heliostats. These can track the movement of the sun by moving on two axes, and they focus sunlight at a point on the tower, which houses an evacuated pipeline. The pipeline carries a fluid (such as water or molten salts), which gets heated by the concentrated sunlight. It is then transferred to a water boiling facility for steam generation or to a thermal energy storage facility (in the case of molten salts).


    3. Linear Fresnel Collectors (LFCs): Linear Fresnel Collectors are very similar to PTCs, except that they employ plane mirrors to concentrate sunlight on a fluid carrying metal tube. The principle of operation for both technologies remains the same, but LFCs are easier to produce and install on account of their use of plane mirrors and the complex supporting structures are not required.


    4. Stirling Dish Technology: A Stirling Dish is a parabolic dish-shaped concentrator that focuses solar radiation onto the focal point of the dish, which houses a receiver. The receiver can be a micro-turbine or a stirling dish in which hydrogen gases are heated. Hydrogen gas expands and drives piston of engine. Stirling cycle engine coupled with generator generate ac voltage. Stirling dishes can be designed to track the sun through two axes, can generate very high temperatures and are usually employed for small, off-grid applications.


    Advantages of Solar Power Generation

    The solar energy is free and renewable resource to generate electricity, but requires collectors and some other equipment for conversion of solar energy into electrical energy.

    1. Solar cells used for power generation causes no noise. Whereas generators or turbines of other methods causes noise pollution.

    2. It does not cause much pollution compared to other power generating methods such as a thermal power plant, nuclear power plant, and so on.

    3. The solar cells do not consist of any moving parts and hence requires a little maintenance for their operation.

    4. It can be used in remote areas for generating and utilizing power in that locality, where the transmission of electricity is too expensive.

    5. The solar power offers energy security by avoiding the general power system in which there is a possibility of power theft.

    6. In general, calculators and some low-power-consuming electronic devices can be energized using solar energy effectively.

    7. The solar energy can produce 50% of the power required to house by installing the solar panels.

    8. In long term usage of solar energy, the solar power setup investment can be regained at maximum levels as solar energy is free of cost.

    9. It is an everlasting infinite renewable energy source compared to other limited energy sources such as nuclear energy, coal, etc.

    10. It can be considered as a job creating power house; if once the installation or construction of solar power stations is initiated, then it will offer more job opportunities for many engineering students.


    Disadvantages of Solar Power Generation

    The installation cost of the solar panels to use solar energy is very expensive, and the initial investments can be covered only after long-term (many years) utilization.

    1. The solar power energy generation entirely depends on the sunlight incident on the solar panels and which in turn depends on the climatic conditions.

    2. The solar energy can be harnessed in a limited period as the sunlight is available only during the day time and sunny days; thus, power can be generated only in limited time period and the power has to be saved in batteries for later usage.

    3. The batteries used to store solar power are very costly, huge sized and need to be replaced from time to time.

    4. The efficiency of solar power system (conversion of solar energy into electrical energy) is around 20% and thus large areas are required to capture more Sun light and produce adequate electricity.

    5. As in all renewable energy sources, solar energy has intermittency issues; not shining at night but also during daytime there may be cloudy or rainy weather.

    6. Consequently, intermittency and unpredictability of solar energy makes solar energy panels less reliable a solution.

    7. Solar energy panels require additional equipment (inverters) to convert direct electricity (DC) to alternating electricity (AC) in order to be used on the power network.

    8. For a continuous supply of electric power, especially for on-grid connections, Photovoltaic panels require not only Inverters but also storage batteries; thus increasing the investment cost for PV panels considerably.