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Currently, humanity requires coal, oil, and natural gas to meet most its energy needs and, therefore, the dependence on fossil fuels is a great problem. Fossil fuels are a limited resource, and its consumption leads to environmental pollution. Due to these circumstances, people began looking for alternative sources of energy. Renewable energy sources represent an energy type that is constantly renewed in the biosphere of the Earth. This type of energy is classified in the following categories, such as solar energy, water, wind energy, biomass, biogas, and geothermal energy. Renewable energy sources are also called green energy. The sun is the most powerful source of energy. The idea of using solar energy originated many centuries ago but there were no technologies to turn this idea into reality. Nowadays, there are photovoltaic panels and solar collectors that aggregate thermal energy and turn it into electrical energy. Wind energy is used to generate energy by converting air mass into electricity and heat. It is believed that wind is the most affordable source of energy. Nonetheless, it is worth noting that its character is changeable and unpredictable. The main device that converts the wind flow into mechanical energy is a wind turbine. These days, many people worldwide prefer these renewable energy sources. It is associated with the fact that the use of non-renewable energy resources is dangerous for the planet and because such resources are scarce. Thus, the use of sun and wind energy will soon become even more popular and widespread. The purpose of the paper is to examine principles of operation and features of wind electricity systems and photovoltaic systems, their advantages, disadvantages, and capacities, as well as to study successful wind and solar power projects.

Principles of Operation and Features of Photovoltaic Systems

Solar power plant is an engineering construction, which converts solar radiation into electrical energy. Methods for conversion of solar radiation are different and depend on the design of the power plant. The principle of modern solar power stations operations is based on the collection of concentrated solar energy through mirrors and the reflection of sunlight on receivers, which gather solar energy and transform it into heat. Such energy can be used to generate electricity by heat engine or the steam turbine, which launches a generator. Production of electricity from the sun has long been used around the world. The main task of the scientists is to improve existing technologies to increase their efficiency. Production of electricity from solar energy is a relevant and interesting topic for many countries. Small solar power plants can provide electricity to houses and public buildings. They preserve the richness of the bowels of the earth. Large solar energy systems are able to produce an unlimited amount of electricity and promote the development of electric power industry on a global scale.

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Photovoltaic cells are referred in the scientific environment to as the solar cells. These are devices made from semiconductor materials that are used to generate electricity. Photovoltaic cells can be of different sizes, volumes, and shapes. They are often combined with each other in photovoltaic modules and modules are combined in a photovoltaic array. Photovoltaic elements, photomodule, and devices convert sunlight into electrical energy. The concept of photovoltaic or generation of power from solar energy can be literally described as light and electricity. The main components of photovoltaic cells are solar panels, inverter, which converts the direct current to alternating current, storage batteries, voltage regulators, as well as other electrical equipment.

Solar cells convert radiant energy of the sun into electricity. Power, which produces solar battery, depends on the amount of solar radiation captured by the surface. For example, if the power of 250 W is available in the solar panel, it means that it provides 250 watts of electrical energy when the solar radiation is 1000 W / m2 (mean when the sky is clear and the sun is at its zenith). Solar radiation reaching the solar battery depends on two factors, such as the angle between the rays of light and the plane of the receiving surface, as well as on the density of clouds and the thickness of the atmosphere, through which it passes. During the sunset, when the angle between the solar plane and the rays of light is 30 ° and 60% of radiant energy is absorbed and reflected in the atmosphere and the clouds, the arrival of solar radiation will be 300 W, and the electrical energy produced by solar panel of 250 W will be equal to 72 watts. Every day, the system generates a different amount of electricity depending on weather conditions.

Principles of Operation and Features of Wind Electricity Systems

The sun heats the atmosphere unevenly. Thus, some areas are warmer while others are colder. Air moves from warmer to the colder areas, thus creating a wind. The power of the wind is used by wind generators, such as wind power stations. Wind blows the screw of the wind turbine thereby causing it to move. To rotate the screws, wind has to have the speed of about 25 km/h. It is relatively a weak flow of wind.

There are different types of wind generators

They differ in the number of blades, materials, from which blades are manufactured, arrangement of the axis of rotation to the ground surface, and a stepper feature of the screw (Tong, 2010). In general, all wind generators can be divided into horizontal and vertical. Wind electricity systems with a vertical axis of rotation have an undeniable advantage, since their components are concentrated at the bottom. One of the generators in this group is Savonius (Tong, 2010). It includes two half-cylinders on a vertical axis of rotation. Despite the direction of the wind, such wind generator easy rotates around its axis producing energy. Its main drawback is the low use of wind energy. It is explained by the fact that the blades half-cylinders operate only in a quarter of a turn, while the rest of the rotation of a circle slows down their movement speed. It uses only one third of wind energy. There are also vertical wind generators with a rotor Darrieus, which has two or more flat blades (Tong, 2010). Blades are made from simple elastic tape without any profile. They are easy to manufacture and install. It starts to unwind due to the difference in aerodynamic drag of the hump and the pocket tape, reaching high-speed and forming its circulation. However, the rotational momentum is small and in the starting position of the rotor parallel and perpendicular to the wind, it is absent. Thus, self-promotion is possible only with an odd number of blades. Another type of the wind generator with vertical axis of rotation is the one with helicoid rotor (Tong, 2010). It is able to rotate uniformly due to swirling of blades. The advantage is that it reduces the load on the bearing and increases the service life. However, because of the complex technology, it is too expensive. Finally, there are wind generators with multi rotor (Tong, 2010). It is one of the most effective types of vertical wind generators.

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As to horizontal wind generators, there are one-two-three and multi-bladed types. The main advantage of horizontal generators is their high efficiency in comparison to vertical ones (Tong, 2010). The main disadvantage is the need for a wind vane device to constantly search for the wind direction. Besides, when it turns to the wind, the speed of rotation slows, which reduces its efficiency.

A variable pitch-blade system is a modern technology, which gives a wind turbine an opportunity to keep a permanent peak output even during the strongest wind (Tong, 2010). Every blade of the wind generator has a centrifugal hammer. It follows the speed of the turning generator. Every blade is pitched and, thus, generator’s speed is controlled. In such a way, this system is highly effective, as it produces more energy to use.

Currently, almost all wind turbines have a direct drive system of the generator (Tong, 2010). However, there are also wind turbines equipped with a geared drive system. These systems have a number of differences. First, geared drive systems are regarded less powerful in comparison to the direct drive systems. Another difference consists in the fact that a direct drive system is better than generators with a geared drive system in relation to energy yield and losses. Due to these reasons, direct drive systems of the generator are more popular nowadays than geared drive systems.

It is necessary to know how to interpret the data provided by producers to choose a system that will give the best performance for a particular area. For this purpose, it is important to interpret the characteristic performance curve (Tong, 2010). Each generator produces electricity at different levels depending on the rotation speed. When specifying the output against the speed, a curve occurs. If the original driving force of the wind is known, it is possible to construct a graph that gives a curve of power of the turbine installation. Peak power for the wind generator is rather different. In most areas, speed of the wind, at which the unit generates a peak power, occurs only during a very short period. Thus, when focusing attention on the peak data, more exaggerated expectations of energy will be received (Tong, 2010). An attempt to compare one wind generator with another using the power curve is another common mistake because people look only at the peak data. If the graph accurately predicts the performance, it can make sense to compare the installation by looking at the minimum data of the curve. Good performance at low wind speeds is the most important indicator about a wind generator since it is the load, with which it will operate most of the time.

Wind stations can be both onshore and offshore. Both of them have their advantages and disadvantages. An onshore wind farm is currently the most popular type. Wind turbines are set on hills and uplands. In order to construct such farm, a proper road to the construction site is necessary. The station is connected through the cable with the transmitting electrical network. The largest wind farm is located in Roscoe, Texas. The station consists of 627 wind turbines (Tong, 2010). The author states that the full power is about 780 megawatts. The area of the wind farm is more than 400 kilometers. Thus, it can be seen that onshore wind farms occupy a large territory. It is their main disadvantage. Offshore wind farms are built in the sea, 10-12 kilometers from the coast (Tong, 2010). They have several advantages in comparison to the onshore farms. They are almost not visible from the shore. They do not occupy the land. They have a higher efficiency due to regular sea winds. Offshore wind farms are built on the shallow parts of the sea. Electricity is transmitted to land via underwater cables. An offshore plant construction is more expensive than the onshore analogues. Generators require higher towers, larger number of cables, and a massive foundation. Salty seawater can cause corrosion to the metal structures. It reduces the service life of such stations.

Energy Potentials and Operational Efficiencies

The potential of the solar energy reaching the Earth is enormous. The power of solar radiation absorbed by the atmosphere and the earth’s surface is 1017 W (Sumathi, Kumar, & Surekha, 2015). It is considered the most promising kind of alternative energy. Nevertheless, the use of solar energy is minimal these days. The development of highly efficient methods to convert solar energy can provide the rapidly growing energy needs of people for many hundreds of years. Wind energy, similarly as solar, also has a great potential. Indeed, the wind potential is huge. It is about 2,000 TWh amounts to power of the wind flow in the atmosphere (Sumathi, Kumar, & Surekha, 2015). The use of even a small part of this capacity can lead solve modern energy problems faced by humankind. The potential of wind energy is at least five times greater than current energy consumption in the world. To date, thousands of solar and wind farms operate around the world. Wind and solar power out of all other alternative sources of energy grow extremely rapidly. Despite this fact, there are certain obstacles to using solar and wind energy. First, there are geographical and seasonal variations. It is necessary to locate wind and solar farms in the areas where these resources are available. The unique feature of these types of renewable energy is the fact that they obscure one another especially in summer. Thus, if there is a strong wind, there are more clouds and less sun exposure. However, in light winds, there is greater intensity of solar radiation. In sunny weather, when there are no clouds, usually strong winds are absent. Therefore, wind and solar farms cannot be located on one territory.

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Currently, wind generators are the main option of providing power in the places where there are persistent strong winds. Wind turbines have become incredibly popular around the world both for private and industrial consumptions. The same can be said about photovoltaic systems. Both systems do not require fuel (Sumathi, Kumar, & Surekha, 2015). Almost infinite energy of wind and the sun is used, while the maintenance is reduced mainly to a periodic inspection. Since the electricity is produced by direct conversion of light and wind energy, there is absolutely no noise generated by these systems. It should be noted that unlike photovoltaic systems, power is fixed in wind generators (Sumathi, Kumar, & Surekha, 2015). Sunlight is present practically everywhere. In some cases, it is a critical advantage of photovoltaic systems.

Advantages and Disadvantages of Photovoltaic Systems and Wind Generators

Photovoltaic systems and wind generators have both advantages and disadvantages. Their main advantage consists in the fact that the sun and wind are renewable energy. It means that the earth provides wind and sun constantly for free and without harming the environment. This type of energy replaces energy produced by thermal power plants thereby reduces greenhouse gas emissions. Wind and sun energy is available virtually anywhere on the planet (Sumathi, Kumar, & Surekha, 2015). Production and operation of wind turbines and photovoltaic systems create new work places. These stations reduce the dependence of companies and individuals on oil and gas monopolies. Therefore, they create competition, which is good for end-users. Another advantage is its extensive application. Photovoltaic systems and wind generators have a wide range of applications. It includes generation of electricity in the regions where there is no connection to centralized electricity system, water desalination in Africa, and even the energy supply for satellites in the Earth orbit (Fletcher, 2013). Thus, both these systems have many advantages that prove their superiority.

Despite a great number of benefits, there are still certain disadvantages of these systems. The first disadvantage relates to their cost (Fletcher, 2013). It is rather expensive to install and maintain wind generators and photovoltaic systems. In their installation, it is necessary to use expensive and rare components. For example, the issue of thin film solar panels requires the introduction of cadmium telluride, which is rare and expensive. This fact entails a rise in the cost of an alternative energy system as a whole. Another disadvantage is impermanence (Fletcher, 2013). In different parts of the Earth, wind blows differently. The same relates to the sun. Because there is no sunlight at night and on cloudy and rainy days, solar energy cannot be the main source of electricity. However, these systems have more advantages than disadvantages and they are the future of the planet.

Applications and Impacts of Topic Systems

Off-grid photovoltaic systems and wind generators are used where there are no centralized power supply networks. To provision energy at night, during periods without bright sunlight, or when there is no enough wind, grid-tied systems are needed. A grid-tied system is still considered the most simple and common system of interaction of photovoltaic panels or wind generators with the local power supply and the consumer of electricity. A grid-tied system consists of two components, such as an array of PV modules and the inverter network. Off-grid systems include a battery pack with charge controllers for storage of unused electricity.

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As it has been already mentioned, both photovoltaic systems and wind generators have disadvantages. They have a certain negative impact on the environment. Photovoltaic systems are still insufficiently studied and, thus, their reference to the environmentally friendly power plants is not entirely justified. Solar concentrators cause large area shading (Fletcher, 2013). It leads to strong changes of soil conditions and vegetation. The use of low-boiling liquids and their inevitable leakages in photovoltaic systems during prolonged use can lead to significant pollution of drinking water. To mitigate these effects, it is necessary to control the used liquids and in the future, it is necessary to make everything possible to decrease the area of photovoltaic systems.

Wind energy does not require liquids. However, it produces noise, requires land area, and materials for construction. Thus, systems of energy storage generated by solar and wind stations should be developed in the nearest future to mitigate the negative impacts of photovoltaic systems and wind generators.

A Successful Solar Power Project

The market of solar energy is one of the most actively developing resources in the world. Arab countries especially benefit from solar energy. For example, in the UAE, there is a powerful solar station. It started working in 2013. The total area of solar panels of the station is about 2.5 square kilometers. There are 258048 parabolic reflectors (Sumathi, Kumar, & Surekha, 2015). The station is located in Abu Dhabi. It is considered the biggest solar power station in the world.

A Successful Wind Power Project

In Arab countries, less importance is given to wind power plants because there are practically no powerful winds. Nevertheless, some countries of the region are interested in development this type of alternative energy. It is especially true for Egypt. The first wind power plant was built in 2000 (Sumathi, Kumar, & Surekha, 2015). It is still highly powerful. Its capacity is 600 MW. It operates in an extremely windy city where the speed of the wind is about 9 m/sec. Thus, it is a perfect location for the wind power plant.

Conclusion

Environmental pollution with the products of combustion of nuclear fuel, primarily coal, and fossil fuels is the main cause of deterioration of ecological conditions in the world. In addition, all these resources are exhaustible and, thus, they are likely to diminish in several decades. In such a way, people try to find other sources of energy. The sun and wind are the most popular renewable energy sources. Their potential is enormous. However, solar and wind energy have features that significantly hinder their widespread use such as seasonal and geographical variations. Despite this, solar and wind energy have a great number of advantages. There are many successful projects of wind and solar power stations operating in many countries worldwide and in Arab countries, in particular.

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