Effective energy generation is an issue, which all states try to solve. The problem is complicated by the fact that people’s demands are constantly growing. Energy is needed in all spheres of life to provide comfortable existence. Scholars reasonably relate the problem of energy generation to numerous environmental externalities. Energy production should be as clean as possible and should not affect the environment, thus non-renewable energy sources such as coal or natural gas do not meet this requirement. From the environmental and the economic point of view the future belongs to renewable energy which is much safer. This paper analyzes the potential of photovoltaic and wind energy generation to satisfy consumers’ demands and concludes that substantial efforts and investments should be made to expand usage of these energy sources.
Principles of Operation and Features of Photovoltaic Systems
While discussing any energy system, it is important to know the principles of its operation. Solar energy aroused scientists’ interest some time ago and a wide range of technologies to convert solar energy appeared. One of these technologies is photovoltaic. Sunlight is converted into electricity through semiconducting materials, which creates photovoltaic effect. Photovoltaic system consists of solar panels which comprise a number of solar (photovoltaic) cells. An electric current is generated as a result of ionizations of these cells’ crystalized atoms. This electrochemical process is caused by the photoelectric effect.
Photovoltaic cells currently receive new designs. One of the variants of creating a solar cell can be shaping n–p junction in a certain form, where n- type silicon has excessive electrons and p-type silicon possesses excessive holes with elements of cadmium sulphide and cuprous sulphide. An alternative design as described may be a solar cell, which has a thin n-type layer, which is exposed to solar radiation and located on top of a p-layer. Solar cells can also be obtained from other materials, namely metal (cadmium telluride and copper indium diselenide) and semiconductor in their structure or multicrystalline materials. There is a method of placing cells on top of each other to receive stacked cells and use amorphous or organic materials for producing solar cells. The best solar cell design should be oriented towards generating maximum energy and minimal waste.
With the purpose of understanding how each piece of equipment works, it is important to evaluate its performance curve. The curve at figure 3 shows the performance of the PV device in standard conditions of sunlight and device temperature.
Abbreviations are used to show the performance of the PV module:
- Isc is the maximum current or short circuit current,
- Voc is an open circuit voltage, which occurs, when there is a break in circuit.
- Vmp is the maximum power voltage
- Imp is the maximum power current
Looking at the corresponding photo, it is possible to state that a solar module produces its maximum current, when there is not any resistance in a circuit. The power received from a solar module at any point of the curve is the product of current and voltage at the same point and is expressed in watts. The maximum power point is located on the knee of the curve, where the voltage is 17 volts and the current is 2.5 amps.
Principles of Operation and Features of Wind Electricity Systems
Photovoltaic energy generation is not the only source of renewable energy. Another option is wind energy. This device is called a wind turbine and the principle of its operation is as follows: wind flows rotate the blades of a turbine; blades start gears of generator, which is located behind blades in turbine; the generator converts this kinetic energy into electric, which is received by electric transformer and distributed to consumers. This process is simple, if properly organized.
People tried to use wind energy since ancient times. The simplest example is movement of wooden ships, which was caused by the wind. However, for the purpose of making wind energy commercial, wind generators should be designed in a certain way. Sørensen writes that scientists have performed a significant number of experiments, looking for the best model of a wind turbine. Slow rotation of a shaft is usually increased by a gear box and received electrical output is taken by cables. Since the beginning of the 20th century, wind turbines became bigger in size (blades of them are 60 m in length and rotor diameter of 120 m). There has always been a discussion of whether to design wind turbines as horizontal or vertical structures. Some vertical turbines with H-shaped rotors have been tested, but horizontal wind energy generators work much better nowadays from a commercial point of view.
Another important aspect of constructing a wind turbine is blade pitching. If blades are pitched, lift forces on it will be reduced. Wind turbines which manage their output by pitching the blades are called pitch-controlled, and mechanisms which limit a turbine output by the process of stall are named stall regulated. Comparing direct drive mechanisms for wind turbines with geared drive for generators, energetics find the first option capable of being more profitable for wind energy production, because their installation is much cheaper and assembling is quicker. Scientists consider different options to find the most productive wind turbine.
As operation of wind turbines depend on the speed of the wind, it is possible to evaluate the level of power output with a wind being steady on the corresponding performance curve. Looking at figure 6, at a very low wind speed, when the torque produced by the wind is insufficient to make blades rotate, turbine cannot produce electrical power. While the speed of the wind increases, it starts to perform its main function. The speed at which blades start to rotate is called cut-in speed (usually 3-4 meters per second). Rated output power is the maximum level of power output, which an electrical generator can produce (usually at the wind speed of 12 and 17 meters per second). The wind speed at this moment is called the rate output wind speed. All turbines are designed in a way to limit the power to its maximum level. The cut-out speed is usually 25 meters per second, when the breaking mechanisms of the turbine start to prevent the damage.
Recognizing the potential of wind energy generation, wind farms are built on the continent and in the ocean. Currently offshore farms have become more popular. Though their construction requires more time and efforts to cable the turbines than onshore farms, they do not spoil the visual image of area and are not so vulnerable to lightning. Stakeholders are ready to spend more money on the construction of wind turbines to ensure their proper functioning.
Energy Potentials and Operational Efficiencies
Both photovoltaic and wind energy generation are recognized as good substitutes of non-renewable energy sources. However, their potential should be objectively evaluated. Photovoltaic commercial modules have 20% efficiency, but they are costly for manufacture. Scientists and producers are sure that within the next ten years photovoltaic cells will be able to produce electricity at the same level as nuclear plants on condition of technologies, being much more developed than now. They need thin film nanocrystalline hybrid cells to be used in solar panels. Future perspectives of solar energy generation are positive, but only in case certain tasks are fulfilled by researchers.
There are also some details concerning wind energy, which officials and producers should take into consideration choosing this type of energy generation. CO2 emissions from wind turbine are much lower than from burning coal or gas. However, there are certain limitations for producing wind energy, conditioned by unreliable wind power resources. Sunlight in certain areas can be more predictable than wind that can change by seasons, years, days and even seconds (turbulence). Wind turbines cannot be transported from one location to another and the only possible solution for this case is to provide reliable weather forecasting and site examination. Thus, wind energy also has a potential for the future as a type of green energy, but the research of a place to locate a wind turbine should be highly accurate.
There are exact values, which confirm the operational efficiency level of photovoltaic and wind energy. Freris and Infield mention that in the European Union wind energy efficiency is estimated as 4800 TW h/year (approximately 20 % of the required electricity and by 2030 it will cover 22,6% of the consumer demand. Operational efficiency of solar panels is a little lower than of wind turbines, but is also expected to grow in the future.
Advantages and Disadvantages of Photovoltaic Systems and Wind Generators
As it is clear from the previous parts of the paper, both photovoltaic and wind energy generators have their advantages and disadvantages. A number of advantages, which characterize these renewable energy sources, are the following: (1) their location may stay unchanged for a very long period of time and only some force major circumstances can change it; (2) they reduce dependency on imported fuels; (3) they ensure the lowest possible level of emissions while assembling and operating; (4) these systems are not big in size and can be located close to the point of energy distribution; (5) wind and photovoltaic energy systems are ideal for developing countries, where a system based on renewable sources of energy can be installed much easier than nonrenewable energy sources.
On the contrary, disadvantages of photovoltaic and wind energy systems are: currently costs of electricity of these renewable energy sources can be higher than of electricity received by some conventional means; as discussed sources are distributed, system of electricity supply should be restructured. Advantages of renewable energy production significantly outweigh existing disadvantages.
Besides enumerated advantages and disadvantages of considered systems, which are similar for all renewable energy resources, there are some other negative impacts of them. Thus, photovoltaic energy systems demonstrate following limitations: (1) if the mode of construction, technology and volume of investment in building a photovoltaic energy plant is not substantial and appropriate, the process will generate significant environmental pollution; (2) if a plant occupies a big land area, scholars may start discussing the question of land degradation; (3) though a generator does not use any water or hazardous materials for producing energy, some of these resources are needed to build it and this fact arouses public concerns. Thus, there are some negative environmental influences from photovoltaic energy generation, but they are minimal compared to conventional methods of energy production.
Some of the negative effects of wind energy generation coincide with the impacts of photovoltaic generators, namely in the point, which relate to the resources and hazardous materials, used for construction. However, a specific issue, which is attributed to wind turbines only is the fact that, actually, they occupy a small part of territory, but the height and the size of their turbines, positioned in rural areas, make them “visually offensive”. However, Sørensen state that, actually, it is a matter of taste. Another negative characteristic of wind energy systems, which affect people, who live close to them, is the noise, produced by turbines, which cannot be reduced, when a turbine is working. In addition, there are some periods in operation the wind turbines, when they can produce electromagnetic interference and affect television transmission and even microwaves. The last negative impact, also taken into consideration, is numerous injuries and killings of birds, which cannot objectively evaluate the danger that blades bare for them. It is not possible to correct certain negative sides of wind generators, and moreover positive characteristics of this renewable energy significantly outweigh them. The only possible recommendation, which can be given to engineers and producers of photovoltaic and wind energy generators is to improve the design and implement new solutions of mitigating the negative effects of these energy sources.
Successful Examples of Implementation
Renewable energy sources are used in different parts of the world and there are some successful examples of implementing them. One of the most powerful solar power stations – Shams Solar Power Station – is situated close to Madinat Zayed, Abu Dhabi, the United Arab Emirates. Shams uses parabolic trough technology. Shams’ solar field aperture is more than 620 m2 and the number of loops is 192. The capacity of the generator is 100 megawatts.
Another successful implementation is the Tafila wind farm in Jordan. The project helps to increase a general power capacity of the state by three percent. Statistical data of the discussed farm states that installed capacity of energy generator is 117 megawatts, its energy production is 390 gigawatt-hours annually and emission reductions are 235,000 t annually. It already powers 83,000 homes. Success of discussed projects confirms the fact that renewable energy source will be an integral part of energy system of any state in the future.
It is worth noting that all people should understand the direct connection between energy generation and environmental problems. Everybody wants to lead a comfortable life and use modern technological devices. However, feeding them with electricity means producing additional CO2 emissions. Renewable energy sources, namely photovoltaic and wind generators, are verified solutions for this issue. Their construction is expensive and may promote the necessity to restructure energy system of the entire state. At the same time they can produce sufficient quantity of “clean” electric power. Hopefully, financing of such projects will be commonly raised for all states to ensure that the future of growing generations will be healthy and environmentally friendly.