Figure 1 Schematic parabolic concentrated solar power (Source adopted: Chu, 2011) 5
Figure 2 Systematic mechanism of Dish/Engine System 12
Figure 3 Illustration of section-A of figure-2 12
Recommendation Report: Dish/Engine Type-The Best Concentrated Solar Power Technology
Introduction
The sun has provided the earth and everything in it with immense energy for years, and lots of these energies are left untapped. Even with the unprecedented advancement in technology today, only a little solar power is tapped. The studies reported that the side of the Earth having facing towards the Sun, a square kilometer at the outer edge of the atmosphere gets solar power up to 1,400 MW per minute (“Solar Energy.”(2009). Chu (2011) also mentioned that solar energy falls on the earth surface at the rate of 120 petawatts. Thus, the solar energy falling to the earth in a day can satisfy the world energy demand for a period of 20 years. Most of the energy goes untapped, and some of the energy is used for several other purposes such as photosynthesis, heating up of the planet and so forth. However, with further advancement in technology, the solar energy reaching the earth can be better utilized.
Solar energy is a sustainable as well as renewable energy. It is one of the most effective solutions for the energy demands of the world today and to reduce the immense pollutions and contamination of the environment caused by fossil fuel. Fossil fuel releases the greenhouse gas, carbon dioxide, to the atmosphere. This causes the depletion of the ozone layer resulting in the melting of the polar cap, heating up of the planet, global warming and so forth.
Solar energy is used in various ways to generate electricity. The two most common methods are PV and concentrated solar power technology. Concentrated solar power technology is of various kinds. This report examines concentrated solar power technologies as an effective way to harness solar power and also recommends the best-concentrated solar power technology.
Background
Since the time immemorial, concentrated solar power has been to perform some tasks. Professor Giovanni Francia built the first concentrated solar plant in 1968, and it set the pace for the modern day concentrated solar plants. Braun, (2010) defined concentrated solar power (CSP) as systems that use mirrors or lenses in order to concentrate a huge amount of solar thermal energy on a small area. When the concentrated light is transformed to heat that is further used to drive a heat engine or steam turbine connected to electric power generator and consequently electric power is generated.
The studies reported CSP as a type of renewable energy that employs solar energy in generating electricity and heat. Furthermore, the plant can be applied to solar fuels and also in desalinization purposes, mostly in large-scale use (“Concentrating Solar Power.” 2014).
With improving technology, the cost of concentrated solar plants is dropping rapidly and thus making the technology much more welcoming in the world today. However, CSP is not without drawbacks especially in its environmental impacts. Chu (2011) mentioned that although solar energy is clean and renewable and also some of the materials used in producing the device are safe. Even though the application of solar energy technologies can be very dangerous under several conditions. The leakage of coolants and lubricants of concentrated solar plants could be a problem. Energypedia (2014) further elaborated that CSP causes atmospheric pollution from fuel combustion, has impacts on flora and fauna and consumes water, energy and materials. This produces noise in the cooling tower when applying air or evaporation cooling, produces obnoxious stench and can pollute soil and water. Systems using the hazardous material like synthetic oil heat transfer fluid may be in danger of fire hazard (“Concentrating Solar Power.” 2014).
Concentrated Solar Power Technology is of various kinds and categories. These include Parabolic Trough Collectors (PTC), Linear Fresnel Collectors (LFC), Parabolic Dish Reflectors (PDR) and Solar Tower (Heliostatic field collectors). These various types of Concentrated Solar Power Technologies have a wide range of applications, and it is important to state emphatically that CSP is the least expensive way of generating large-scale electricity from solar energy. This makes it very efficient in producing solar energy at competitive rate ( Chu, 2011; “Solar Energy”, 2009).
Technology: Description of Different Types of CSP
IRENA, (2012) categorized concentrated solar plants depending on the mechanism of the solar collectors that concentrate rays of the sun along a focal line or in a single focal point. Figure 1 is illustrating concentrated solar Thus; two classifications exist for concentrated solar power.
- Line-focusing systems include linear Fresnel plants and Parabolic trough
- Point-focusing systems includes solar tower plants and solar dish systems.
Figure 1 Schematic parabolic concentrated solar power (Source adopted: Chu, 2011)
Line-Focusing Systems
Line-focusing systems are concentrated solar power plants that have single-axis tracking system.
Parabolic Trough Collectors (PTC)
In this type of parabolic trough collector, reflective materials are formed into the shape of a parabola (Chu, 2011; IRENA, 2012). The particular materials used in making this type of concentrated solar plant include solar collectors (mirrors), support structures and heat receivers. The sunlight entering into the collector is reflected onto a central receiver tube at the focal line of the collector. IRENA (2012) pointed out that in a PTC, the array of mirrors used could be as long as 100m and have a curved aperture having range between 5-6 meters. Furthermore, as a means to direct the solar collectors and heat receivers towards the sun, a single-axis tracking mechanism is employed. Another important characteristic feature of the parabolic trough collectors is that they are positioned in the North-South direction and hence track the sun moving from East to West. This technique is employed so as to make the system more efficient in collecting energy.
The option of orientation of the trough depends on the application and whether more energy is needed during the summer or during winter (Chu, 2011). In order to collect the energy produced in a CSP, a heat transfer fluid is circulated through the absorber tube. This energy is then transferred to the steam generator or heat storage system. Usually, synthetic oils are used as heat transfer fluid. It is important to consider that concentrated solar power technology is the most commercially operating CSP available (IRENA, 2011).
Linear Fresnel Collector Technology (LFCs)
In linear Fresnel collector, an array of mirrors is used in concentrating the light to a fixed receiver mounted on a linear tower. IRENA, (2012) explained that this type of concentrated solar power is similar to parabolic trough collectors, but the major difference is of its uses in a series of long flat. In some cases, it is slightly curved, and mirrors usually positioned at different angles in order to direct the rays of the sunlight to the sides of a fixed receiver. The major benefit of the LFCs is that the flat or elastically curved reflectors they use are cheaper than the parabolic glass reflectors used in parabolic trough collectors (Chu, 2011). In a linear Fresnel system, water flows through the pipe running through the focal point of the mirrors. Unlike the parabolic trough system, heat transfer fluids are not necessary in the LFCs because it is designed to make quicker direct steam generation (“Concentrating Solar Power.” 2014).
Point-Focusing Systems
Point-focusing systems differ from line-focusing systems because they include two-axis tracking systems used in concentrating the power of the sun. There are two types of point-focusing system. They include solar dish systems or Dish/Engine system, and solar tower plants.
Dish/Engine System
In a dish/engine system as described by Energypedia, (2014), parabolic dishes are used to concentrate the sun’s rays at a focal point in the center of the dish. The system tracks the sun in two axes. The energy of the sun is then transferred to a heat transfer fluid heated up to 750 degree Celsius (gas transfer fluid can also be used). In order to generate electrical power, the fluid or gas is used to drive an external combustion engine or a micro-turbine engine usually attached to the receiver. Chu (2011) reported that the Stirling engine is the most common type of the heat engine used in dish-engine systems. According to Braun (2010), the dish system provides the highest solar-to-electric efficiency among all available CSP technologies and also scalability is ensured by its modular nature. The Stirling dish technologies have the capacity of achieving the highest efficiency of all the types of the concentrated solar power technologies (IRENA, 2011). This makes it hold great prospect among the known solar power technologies.
Solar Tower Technology
IRENA (2011) described solar tower technology as a CSP that uses field of mirrors placed at ground to focus direct solar irradiation to a receiver. This receiver is installed at the top of a central tower where light is captured that is transformed into heat. In order to generate electric power, the heat produced is then used to drive a thermo-dynamic cycle such as a water-steam cycle. A lot of computers controlled mirrors known as heliostats are used in the solar field to track the sun individually in two axes. One major feature of this CSP is that it can attain higher temperatures than linear Fresnel and parabolic trough systems. This is because since more sunlight can be focused on a single receiver and also heat losses can be greatly reduced. The system can generate heat up to 1,000 degree Celsius (“Concentrating Solar Power.” 2014).
Analysis of the Concentrated Solar Power Technologies
The discussion in the above section shows that the various kinds of concentrated solar power technologies have individual peculiarities. Each of these technologies has distinctive features that can be used in identifying it. In addition, they have varieties of applications and find usefulness in various fields.
Parabolic trough, for instance, is commercially proven to operate at 400 degree Celsius and has a net efficiency of 14%. Some other advantage of this CSP includes its modularity, good land-use factor, lowest materials demand, and excellent proven hybrid concept and storage capacity. However, this CSP employs oil-based heat transfer media which poses a disadvantage of restricting the operating temperatures to 400oC (the system has potential operating temperature of up to 500oC). As a result of this low operating temperature, the system produces moderate steam qualities (“Concentrating Solar Power.” 2014).
Linear Fresnel systems are often used in conventional thermal power. They are stationary devices that are not dependent on the focusing devices of the plant. However, they are readily available and also the manufacturing cost is greatly reduced since it uses flat mirrors that can be bought and bent on site. They also have very high space efficiency around solar noon and can incorporate hybrid operation. However, these are relatively new in the market and hence are not used in most projects (Energypedia, 2014; IEA, 2010).
Dish/engine or parabolic dish systems are simply stand-alone systems, usually off-grid power systems that have the highest single unit solar capacity. Its major benefits include high conversion efficiencies modularity. It's solar to electric conversion is more than 30%. The term demonstrates project of the system that has operational excellence. It is easily manufactured in large quantity. However, the system is not available in commercial forms, and it is difficult to integrate the system for grid integration. Additionally, there are no hybrid receivers yet available for the system.
Power tower systems are grid connected plants that have the highest single unit solar capacity of all the concentrated solar power available. It has a versatility in its benefits including good mid-term prospects for high conversion efficiencies. The potential operating temperature of the system is 1000oC, but the proven operating temperature for a 10 MW scale system is 565oC. In addition, hybrid operation is possible in the system, and it is better suited for dry cooling concepts than the line focusing CSPs namely linear Fresnel and parabolic trough systems.
Concentrated solar power is an excellent source of energy today. Though the technology is still in the phase of development, however, it is the best solution for electric power today. IEA, (2010) pointed out that in countries with strong direct normal irradiance (DNI), concentrated solar power provides low-carbon and renewable energy. This solar technology has a great edge over photovoltaic systems in terms of cost. Solar photovoltaic systems are very costly to deploy, but concentrated solar power (CSP) is relatively cheap. In addition, when it comes to large scale or commercial deployment, photovoltaic systems fall very short. The implementation of these systems are very expensive, but CSPs are affordable. Braun, (2010) mentioned about a study conducted by Greenpeace International and the International Energy Agency’s SolarPACES group on the future and potential of CSP discovered. That concentrated solar power could account for about one-fourth of the energy needs of the world by the year 2050. The features of concentrated solar power, as well as its potentials, clearly show that it is the best solution for the world energy needs.
Recommendation
The analysis presented in the previous systems shows that the various types of concentrated solar power technologies have their various peculiarities and features. However, one of the most wonderful concentrated solar power technologies is the dish/engine type. This solar power technology is classified as a point-focusing system. The attributes and features of this type of system were outlined in the previous section. The systems are versatile, have high efficiency and also hybrid operation. In the context of comparison with other solar technologies, the dish/engine type CSP has higher efficiency. This higher efficiency is because of its high power densities and low cost. The system could require between 1.2 to 1.6 hectares of land per MW depending on the system and the site. The installed system costs range from $12,000/KW, corresponding to solar-only prototypes, to $1,400/KW, corresponding to hybrid systems used in mass production.
The high efficiency of the dish engine is the major reason for its low-cost potential. This can be concluded from the analysis that the dish/engine system has the highest net annual solar-to-electricity efficiency and also the highest plant peak efficiency. The operating temperature of the system is between 550oC and 750oC. It has the best suitability for air cooling and requires only between 0.05 and-0.1 m3/MWh of water, which corresponds to mirror washing. This simply implies that no water is required for cooling the dish/engine type system but in some cases very little quantity of water may be required for mirror washing. The system has a collector concentration of more than 1300 suns and lots of other interesting features (IRENA, 2011; Solar Dish Engine, 2005). These features and important facts proved that the solar dish/engine system is the best-concentrated solar power technology available out there.
Figure 2 Systematic mechanism of Dish/Engine System,
Source adopted: (“Solar Dish Engine”, 2005)
Figure 3 Illustration of section-A of figure-2
Source adopted: (“Solar Dish Engine”, 2005)
Conclusion
This report examined the technological application of solar energy. Solar energy is a sustainable energy, capable of providing the ultimate solution to the needs of energy on the globe. This is a renewable energy source and therefore it has no such limitations like fossil fuel. More so, solar energy is a clean energy and would effectively replace fossil fuel that produces greenhouse gasses like carbon dioxide when burnt. Our planet is in a great danger from the pollutants released into the atmosphere on a daily basis on burning fossil fuel.
Concentrating solar power (CSP) is presented as one of the specific ways in which solar energy can be harnessed. CSP is unlike photovoltaic system as it simply concentrates sun's rays using arrays of mirrors or lenses onto a small area which is then converted into heat and used in driving the engine. Four types of CSP were identified including linear Fresnel system, parabolic trough system, solar tower system and dish/engine system. These systems are unique in their ways but on examining their features the dish/engine system stands out as the best solution and most excellent concentrated solar power technology available today. Its features were clearly examined in the report.
References
Braun, F.G. (2010). Concentrated Solar Power (CSP). Retrieved November 16, 2014 from http://www.rets-project.eu/UserFiles/File/pdf/respedia/09%20Concentrated%20solar%20power/09-Solar-energy---Part-III---Concentrated-Solar-Power_EN.pdf
Chu .Y., (2011). Review and Comparison of Different Solar Energy Technologies. Retrieved November 16, 2014 from http://www.geni.org/globalenergy/research/reveiw-and-comparison-of-solar-technologies/Review-and-Comparison-of-Different-Solar-Technologies.pdf
Concentrating Solar Power (CSP) – Technology (2014) Energypedia, Retrieved November 16, 2014 from https://energypedia.info/wiki/Concentrating_Solar_Power_(CSP)_-_Technology
International Energy Agency (IEA), (2010). Technology Roadmap – Concentrating Solar Power. Retrieved November 17, 2014 from http://www.iea.org/publications/freepublications/publication/csp_roadmap.pdf
IRENA, (2012). Renewable Energy Technologies: Cost Analysis Series. Concentrating Solar Power. Volume 1(2/5). Retrieved November 16, 2014 from http://www.irena.org/documentdownloads/publications/re_technologies_cost_analysis-csp.pdf
“Solar Dish Engine” (2005). Retrieved November 17, 2014 from https://www1.eere.energy.gov/ba/pba/pdfs/solar_dish.pdf
“Solar Energy”. (2009). In Microsoft Encarta 2009 Encyclopedia [CD-ROM]. Redmond, WA: Microsoft Corporation.
