Abrasive Jet Machining (AJM) Process Advantages and Disadvantages

Abrasive Jet Machining (AJM) Principles 

In abrasive jet machining process, a focused steam of abrasive particles (of size 10 to 40 microns) carried by high pressure gas or air at a velocity of about 150 to 300 m/sec is made to impinge on the work surface through a nozzle, and the work material is removed by erosion by the high velocity abrasive particles. The inside diameter (ID) of the nozzle through which abrasive particles flow is about 0.18 to 0.80 mm and the stand-off distance (i.e. distance between nozzle tip and workpiece) is kept about 0.3 to 20.0 mm. The process can be easily controlled to vary the metal removal rate which depends on flow rate and size of abrasive particles. This process is best suited for machining super alloys and refractory type of materials, and also machining thin sections of hard materials and making intricate hard holes. The cutting action is cool because the carrier gas serves as coolant. 

When an abrasive particle (like Al2O3 or SiC) having sharp edges hits a brittle and fragile material with a high speed, it makes dent into the material and lodges a small particle from it by a tiny brittle fracture. The lodged out or wear particle is carried away by the air or gas. The operating elements in AJM are abrasive, carrier gas and the nozzle as schematically shown in the following Figure

Abrasive Jet Machining (AJM) Principles 

The distance between the nozzle tip and the work surface has great influence on the diameter of cut, its shape and size and also rate of material removal. The following Figure shows the variation in the diameter of cut with change in the stand off distance (SOD). It is evident that the SOD changes the abrasive particles spreads (i.e. covers wider area) on the work surface and consequently increases the diameter of the cut.

The basic Units of AJM

The basic unit is schematically shown in following Figure. It consists of gas supply system (compressor), filter, pressure regulator, mixing chamber, nozzle assembly and the work holding device. In the mixing chamber, the abrasive is allowed to flow into the gas stream. The mixing ratio is generally controlled by a vibrator. The particle and gas mixture comes out of the nozzle inside the machining chamber of the machine tool unit. The feed motion can be given either to the work holding device or to the nozzle. 

AJM setup

Please read :

• Water Jet Machining Process .
• Laser Beam Machining 
• Electrical Discharge Machining (EDM) Principles 
• Electrochemical machining process 

Abrasive Jet Machining (AJM) Advantages:

• This process is quite suitable for machining brittle, heat resistant and fragile materials like, glass, ceramic, germanium, mica etc. 
• It can be utilized for cutting, drilling, polishing, deburring, cleaning etc. of the materials. 
• The depth of damage to the surface is very little. 
• Holes of intricate shapes could be produced efficiently.
• The surface machined can have good finish (by controlling the grain size mainly). 

AJM Disadvantages: 

The materials removal rate is low. For example, for glass, it is 0.0164 cm3/min. 

• The tapering of hole especially, when the depth of the hole is more, becomes almost inevitable. 
• A dust collecting chamber is a basic requirement to prevent atmospheric pollution to cause health hazards.
• The abrasive particles may remain embedded in the work surface. 
• Abrasive particles cannot be reused.

AJM Applications:

• Abrasive jet machining is best suited for machining brittle and heat sensitive materials like glass, quartz, sapphire, ceramics etc,
• It is used for drilling holes, cutting slots, cleaning hard surfaces, deburring, polishing etc. 

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• Different types of welding defects.
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• TIG welding Principles in Easy language 
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Important terms used in air compressors

An air compressor is a machine used to compress the air and raise its pressure. The compressed air is used for many purposes such as for operating pneumatic drills, riveters, road drills, paint spaying, in starting and supercharging of internal combustion engines, in gas turbine plants, jet engines and air motors etc. It is also utilised in the operation of lifts , rams, pumps and a variety of other devices. In industry, compressed air is used for producing blast of air in blast furnaces and bessemer converters.

Important terms used in air compressors frequently 

The following important terms are frequently used in air compressors: 

Inlet Pressure: It is the absolute pressure of air at the inlet of a compressor. 

Discharge Pressure: It is the absolute pressure of air at the outlet of a compressor. 

Compression Ratio or Pressure Ratio: It is the ratio of discharge pressure to the inlet pressure. Since the discharge pressure is always more than the inlet pressure, therefore the compression ratio is more than unity. 

Compressor capacity: it is the volume of air delivered by a compressor and is expressed in cubic meter per minute or cubic meter per second. 

Free air delivery: It is the actual volume delivered by a compressor when reduced to the normal temperature and pressure conditions. The capacity if a compressor is generally given in terms of free air delivery. 

Swept volume: It is the volume of air sucked by the compressor during its suction stroke.

Mean Effective pressure: It is the ratio if the work done per cycle to the stroke volume of the compressor  

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Biomass-Advantages and Disadvantages

Biomass - Important facts 

Biomass advantages and disadvantages are associated with some very important facts. Day by day the conventional energy sources like fossil fuels is reducing. Finding sources for conventional energy is a very complicated and lengthy process. But with the increasing population, the need for new energy sources is also rapidly increasing in developing countries and in rural areas. Sometimes the grid power is not economically feasible to expand in those areas. In many south Asian countries like India, Bangladesh and Pakistan biomass is the first choice when it comes to the production of heat for cooking. Here you can find the impact of renewable energy on the global warming in India. The most important biogas plant found in India is Fixed Dome Type Janata Model. 

image courtesy: http://renewableenergyhub.com/

Definition of biomass fuel?

Biomass is naturally produced from plants and animals. It is natural and organic. Biomass is one of the most cheapest non-conventional energy sources because it is produced from micro organisms. Biomass stores energy from sunlight by the photosynthesis process. Though it is cheap and clean it is not a popular form of energy source like fossil fuels. Especially in the developed countries biomass has lost its popularity. But in the rural areas biomass is widely used in small industries and for cooking purposes. Some examples of biomass fuels are wood, manure, animal and human wastes, crops, manure etc. In the last two decades biomass has become very common alternative energy. 

Please read : 
Renewable energy the great green hope ! 
Renewable energy sources - graphical representation  

Classifications of biomass 

Biomass is generally classified in two big classes 

1. Solid biomass  like - weeds, agricultural residues, coconut shells, cotton stock con cob etc. 
2. Powdery biomass like - rice husk, ground, cofee husk, sugarcane smashes parts etc. 

Biomass advantages and disadvantages 

If you talk about the advantages and disadvantages of biomass energy then we will see that there are very few cons. There are different techniques for the energy harnessing. One of the biggest disadvantage of biomass energy is that sometimes some biomass fuels are needed direct burning. Wood or dried cow dung cakes are some of the sources which need direct burning. And this can cause some pollution. So if not taken care of, biomass can produce severe environmental problems as it releases a lot of carbon. As a result the balance in the ecosystem can be hampered. 

We cannot deny the fact that biomass can be a good alternative fuel to fossil fuels. Biomass can be collected from various sources. So if this energy is harvested in a way that creates negligible harm then it can dispel some of the energy crisis of the world. So for the proper use of the biomass energy , appropriate policy should be made. Awareness about the potential biomass energy advantages and disadvantages should be created. All the energy sources have their own advantages and disadvantages. With proper guidelines and policy biomass can be a great sustainable and low cost clean energy. Like any other energy sources there are advantages and disadvantages of using biomass energy. Let's discuss about them: 

Advantages

• Biomass energy is renewable or alternative. 
• Biomass is carbon neutral. 
• It is  inexhaustible fuel source. 
• Biomass produces very low amount of  carbon compared to fossil fuel energy;
• The environmental impact is minimum if the direct burning burning is avoided. Instead of that fermentation or pyrolysis can be used 
• Alcohol and other forms of fuels produced from biomass is very clean burning and environment friendly. 

Disadvantages

• Fossil fuels are more efficient than biomass fuels. 

• Sometimes biofuel production can be proved a little expensive. 
• Some biomass production plants need a lot of space to grow the raw materials of the plant (crops and plants)
• Direct burning can produce pollution by particulate emission. 

Conclusion

In developing and rural areas biomass is still used in heating purposes and also for cooking. Using biomass farmers can do their agricultural activities. These days almost 16% of the world's total energy supply from biomass. In developing countries 40% of the rural supply come from the biomass energy. A number of environmental groups are strongly opposing the wide use of forest biomass use because of high carbon emission. For the better realization of biomass advantages and disadvantages, I think this post will be very helpful. Biomass fuels can be great option to replace the fossil fuels as a source of  power generation in rural, developing as well as developed countries. 

Please Read :

• Pros and Cons of Geothermal Energy 
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• What is the cheapest renewable energy source - biomass 
• Solar pond pump or solar fountain pump advantages and disadvantages 
• Wave energy pros and cons 

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Tidal Energy - Advantages and Disadvantages on electricity generation

Tidal Energy - How Tidal energy works- An introduction 

All flowing water carry with them kinetic energy. When such water encounters a turbine, part of the momentum of the flowing water is transferred ton to the turbine, causing it to rotate. The rotation of the turbine cam then be used to generate electricity. So for electricity generation purpose we must know about tidal energy advantages and disadvantages.  Whether the water is om the open ocean, an estuary or a river, its motion can thus be utilized in generating energy. The tides occurring in the oceans are one such source of energy based on the movement of the water. If we analyze the tidal energy pros and cons then we will find that tidal energy is renewable and the output of this energy varies with the variation of the tidal range.

Now How tides are generated ?

• Tides are generated by the gravitational forces of the sun and moon on the oceans. 
• by the spinning of the earth around its axis 
• and the relative positions of the earth, moon and the sun. 

 What are tides and tidal current ? 
Tides are the periodic vertical rise and fall of ocean water. The period between consecutive high tides is 12.5 hours. The tidal rise and fall of water is accompanied by periodic horizontal and to and fro motion of water called tidal currents. Tides and tidal current is intimately related. So we are already a getting an idea about the pros and cons of tidal power.

Difference between Tidal Energy and Wave Energy 

• Tidal energy differs from wave movement. Waves have a period of only about 6 seconds, whereas tides have a period of about 12.5 hours. 
• Waves are caused by surface winds, whereas tides are caused by the gravitational forces of sun and moon on ocean water. 

Have a look - 

• Waves energy's advantages over tidal energy 

The amplitude of tides covers a wide range from 25 cm to 10 m. The speed of the tidal currents is in the range of 1.8 km/h to 18 km/h. The tides and and tidal currents possess renewable energy. The rise and fall of water follows a sinusoidal curve.

Please Read :

• Renewable energy - The great green hope 

Advantages and disadvantages of tidal energy to generate electricity 

Advantage 

1. The biggest advantage of tidal power, besides being inexhaustible, is that it is completely independent of the uncertainty of precipitation (rain). Even a continuous dry spell of any number of years will have no effect whatsoever on the tidal power generation.  
2. A great environmental advantage of tidal power generation is - its free from pollution because it does not use in any fuel and also does not produce any unhealthy waste like gases, ash or atomic refuse. 
3. Electricity generation from tidal power  do not require large areas of valuable lands because they are mainly on the seashore, 
4. Peak power demand can be effectively met when it works in combination with thermal or hydroelectric systems. 

Disadvantage and solutions to tidal energy cons 

There are a number of reasons why the power generation is still novelty, rather than a normal source of energy, The reasons are -

1. The fundamental drawback to all methods of generating tidal power is the variability in output caused by the variations in the tidal range. 
2. The tidal ranges are highly variable and , thus the turbines have to work on a wide range of head variation. This is a great disadvantage of using tidal energy to generate electricity as this affects the efficiency of the plant. 
3. Since the tidal power generation depends upon the level difference in the sea and an inland basin, it has to be intermittent operation, feasible only at certain stage of tidal cycle. This intermittent pattern could be improved to some extent by using multiple basins and a double cycle system. 
4. The tidal range is limited to a few meters. As bulb turbine technology was not well developed for this range, use of conventional kaplan runners was the only alternative. This was found to be unsuitable. Now, with the development of reversible flow bulb turbines, this difficulty is overcome. 
5. The duration of power cycle may be reasonably constant, but its time of occurrence keeps changing, introducing difficulty in the everyday planning of the load sharing grids. This handicap can be removed now with the help pf computerized programming. 
6. Sea water is corrosive and it was feared that the machinery may get corroded. Stainless steel with a high chromium content and a small amount of molybdenum and the aluminium  bronzes proved to be good corrosion resistant at La Rance project. The vinyl paint exhibited good results. 
7. Construction in seas or estuaries is found difficult. 
8. Cost is not favorable compared to the other sources of energy. 
9. It is feared that the tidal power plant would hamper the other natural uses of estuaries such as fishing or navigation. 

Please Read:

• Pros and Cons of Geothermal Energy 
• Solar energy pros and cons 
• Different renewable energy sources - graphical presentation 
• What is the cheapest renewable energy source - biomass 
• Solar pond pump or solar fountain pump advantages and disadvantages 
• Biomass - pros and cons 

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Advantages and Disadvantages of Wave energy

Wave Energy - A steadier Renewable energy than Wind 

Waves are increasingly generated in oceans and large lakes; at times the waves are strong enough to overturn large ships or toss them ashore.

Gravitational attraction of the sun and the moon as well as the force of wind are mainly responsible for the wave being created. Oceans are efficient collectors of wind energy; indeed waves are a source of energy steadier than the wind, because once wind generates a wave, the latter conveys the energy it had derived from the wind over immense distances with only moderate dissipation. It is another matter that wind energy has always proved far more easier to harness than wave energy. Wave energy may vary from a few watts to kilowatts per meter which is fluxed in the open sea or against coasts . Greatest power is achieved in winter and smallest in the summer, mainly in the zones of the prevailing westerlies and trade winds. 

Both vertical as well as horizontal movement of the water contributes to wave energy. Every particle of water experiences almost a circular motion moving up and down reaching the crests and troughs.  

Please Read :

• Geothermal energy pros and cons 
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• What are the cheapest sources of non-conventional energy - Biomass  

Wave energy Advantages and Disadvantages 

Advantages 

1. Wave energy has this advantage over solar or wind energy that the energy has been naturally concentrated by accumulation over time and space and transported from the point at which it was originally present in the winds. 

2. A much greater amount of power is concentrated in the waves than in the wind. If we compare the power concentrated in a good wind energy to the corresponding area having wave energy then we will find that wave energy is 100 times greater than wind energy.

3. It is a free and renewable energy source. 

4. Wave power devices do not need huge land masses like solar energy wind energy. 

5. These devices are almost pollution-free. After removing the energy from the waves waters are left in a placid state.

6. No wastes or greenhouse gases are produced in the process. In my opinion this is the most important advantage of wave energy. 

Disadvantages 

1. The major demerit of wave energy, in comparison to wind, is that the energy is available in the ocean. So the equipment needed for the extraction of wave energy must operate in a marine environment. The lifetime and reliability is a great factor which should be taken care while constructing the equipment. The transportation of energy is a great factor because the energy produced needed to be transferred to a great distance from the shore. 

2. Wave energy converters must be capable of withstanding very severe peak stresses in storms. 

3.  Finding a proper site for the extraction of energy from the wave is pretty tough because wave energy is totally related to ocean! 

4. Devices needed for the harnessing of the wave energy are very complicated.  

5. Many economic factors are important in the installation of a wave energy based power plant. These factors are capital, maintenance cost, repair cost as well as replacement cost. For the power generation companies economic factors can play as the major disadvantage of wave energy.

For a better and green house effect free world, renewable energy sources are the most preferable option. So after going through the advantages and disadvantages of wave energy the leaders of the world, power generation companies along with engineers should take some initiative to make the best use of wave energy.

image source: http://www.our-energy.com/ocean_energy.html

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• Renewable Energy: the great green hope for clean environment
• Solar Air Heater Classification and Advantages
• Solar pond pump-features of solar fountain pump
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Pros and Cons of Solar Energy

The energy from the sun reaching the earth per day ranges from about 600 Btu (British Thermal Unit) / square feet for Northern Europe to about 2000 Btu/square feet for arid regions near the equator. More energy from sun strikes the earth in one hour than all energy consumed on the planet in a year! A summer day’s sun at noon provides a square, meter of land with a power input almost a kilowatt. One might wonder why solar energy has not replaced the conventional energy sources, not even a significant fraction. So there are definitely some pros and cons of solar energy. In this article I will discuss about the pros as well as cons of this vast energy source.

Pros of Solar energy

No pollution

There is no pollution while producing energy from the solar panels. Only pollution we can talk about is the pollution that can happen while making the solar panels.

Great for household use 

Theoretically the sunlight falling on a typical single house can easily provide from one third to one half of the heating needs of that house anywhere in hot countries even in the cooler climates and even when there is persistent cloudiness.

Zero dependence on the fossil fuels 

The dependence on the fossil fuels can be reduced a great deal. In one day the solar energy falling on India alone is greater than the energy in their present annual coal consumption. 

Reasonable Cost 

One installed the cost of electricity production is very low though the cost of producing solar cells are high. And in the remote areas the installation cost will be comparatively very low.

Solar panels can be set up in the house roof tops. So there is no requirement of finding spaces for installing the solar panels.

Availability 

Most of the areas of the world are exposed to the sun’s rays and capable of using this energy very easily.

No transportation cost of fuels 

No need of transportation of fuels from one place to another. The fuel is everywhere!

The harnessing of electricity is very much easy in the remote areas when these areas are not connected to the national grid. 

Cons of Solar energy

Cost

The solar energy has not been the prevailing one because of its cost. The primary reason is clearly the cost of solar energy based technology, especially in comparison to the lower priced commercial fuels.

Some adverse environmental effects of solar energy

Large scale utilization of solar energy is associated with adverse environmental effect. To produce an electrical equivalent of 10 terra watt of solar electricity a land area as big as 220000 square kilometer will be required. An area of this size cannot be covered with solar energy collecting devices without making any adverse environmental effect.

Storage of captured energy

The devices for storing solar energy are of low efficiency and high costs. This can cost thousands of dollars for a buyer to set up a solar panel in houses as well as in factories. 

Though there are some cons of solar energy in the near future we must find a way to properly utilize solar energy because the fossil fuel storage are at the verge of getting finished. And the harm occurred due to the use of these solar energy is irreparable. So Solar energy pros and cons should be properly studied.

Please Read:

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• What are the cheapest sources of non-conventional energy - Biomass  
• Solar Air Heater Classification and Advantages
• Solar pond pump-features of solar fountain pump
• Hydroelectric Power Facts - Advantages and Disadvantages 
• Alternative energy sources graphical representation  
• Biomass - pros and cons 

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Impact of Renewable Energy Sources on Global Warming in India

 A brief story of global warming- global warming skepticism 

Of all the forecasts of global environmental disasters made from time to time, the one which aroused the most widespread skepticism was about global warming. Even as several well-meaning people took it seriously- some rather too seriously- a whole lot of other scientists and policy-makers dismissed it as over-exaggeration. During 1970s through 1990s, those who believed in global warming talked of it as soon as they felt a day was hotter than the previous one. Others believed that even if the world continued to produce gases which cause greenhouse effect, our oceans would keep assimilating them and no long-term global warming will ensue. They dismissed episodes of extreme climate as the usual fluctuations that occur in nature. Indeed during the late 1960s and early 1970s, it was the forecast of global cooling that had captured public imagination (Gribbin, 1975; Thompson, 1975; Peterson et al, 2008).

Global Warming A Scientifically Accepted Fact  

During the last 20 years, the balance of evidence has gradually and decisively shifted towards global warming. It is now a scientifically accepted fact that global warming is indeed occurring and that it will have long-ranging impacts on the earth’s ecosystems. There is no longer a disagreement on the existence of global warming; if there are disagreements, they are on the extent of harm global warming will cause. There is also near-complete consensus that use of fossil fuels is the principal cause of global warming and unless the emissions to atmosphere of CO₂ and other greenhouse gases are drastically reduced, global warming will progressively increase and lead us to our doom.

Adverse Effects of Global Warming 

      So, global warming has already hit us, and it is beginning to hit us harder. It has also produced another monster which may me even more destructive than temperature rise and ocean acidification .Given that 70% of the world is covered by oceans, any disturbances in oceans can have bigger and ‘deeper’ effects on earth than the disturbances in the rest of the 30% of the world!

Global Warming and Renewable Energy Source: India and the world 

Once again, there is a groundswell of demand for ‘alternative energy sources’, particularly the ‘renewable’. Even before global warming became an accepted reality in the post-modern era, fossil fuels were almost universally perceived as highly ‘unclean’ fuels responsible for numerous forms of gross pollution, including acid rain. On the other hand, non-conventional energy sources, especially the renewable energy sources, have created a ‘clean’ image regarding environment impacts. But obviously there is exception, one of the major exceptions is the large hydropower projects. Past experience showed us that they can be catastrophic for the environment. Now it is believed that mini-hydel and micro-hydel projects can be proved harmless for alternatives.

• Hydroelectric Power Facts - Advantages and Disadvantages 

The tide has turned so strongly in favour of renewable that for the first time ever since the dawn of the fossil fuel era over two hundred years ago, renewable energy technologies have attracted more investment globally, ($140 billion) in 2008, compared with $110 billion for fossil fuel-based technologies according to figures released by the United Nations, June 2009 (Macalister, 2009). Wind energy has attracted the highest new worldwide investment, $51.8 billion, followed by solar at $33.5 billion. Biofuels are the next popular investment, winning $16.9 billion. There is as much as a 27% rise to $36.6 billion in developing countries led by China, which pumped in $ 15.6 billion, mostly in wind and biomass plants. India’s overall spending on renewable energy has risen to $4.1 billion 2008, 12% up on 2007 levels.

India has started Spending on Renewables to reduce global warming and pollution 

         According to India’s Ministry for New and Renewable Energy (MNRE, 2009), India has a potential of generating over 82000 MW (8.2 GW) of power from just wind, small hydro and biomass (Table 1.1). Of this, only 6100 MW, i.e., a mere 7.4% of the potential is presently being realized. There is a similarly vast potential for dispersed units, but only a small fraction has been realized (Table 1.2). Which is why India has stepped up its spending on renewable, just as the rest of the world has. It as if the world is preparing to stake it’s all on renewable in the hope that renewable will save it from the looming disaster of global warming and irreversible pollution.

Table 1.1 : Potential of Power Obtainable from wind, small hydro and biomass in India vs its actual realization at present (MNRE, 2014)

Table 1.2 : Potential utilization of biogas and dispersed solar energy, biomass and wind energy systems in India and the present state of its realization (MNRE, 2014)

These links gives glimpses of the renewable energy sources. It also addresses these tricky questions: are renewable energy sources really as benign as is widely believed? Are they really a sure answer to the problem of global warming?

• Renewable Energy: the great green hope for clean environment
• What are the cheapest sources of non-conventional energy - Biomass  

Is there any major proof that Renewables are environment friendly ? 

One may say that for thousands of years when humankind was dependent almost totally on renewable, the world was much less polluted and there was no global warming. Is this itself not a major proof that renewable are environment-friendly?

Sadly, it isn’t!

The reason is that till the mid-18^th century the global population and the per capita energy consumption, hence the total global energy consumption, were small fractions of what they are today. Had we used fossil fuels at the rate renewable sources were being used till 1850, we would not have experienced global warming. But at the present rate of population growth and per capita consumption no source of energy, however, clean it may be, can bail us out of rapidly increasing global warming and other forms of pollution. 

India, in its National Electricity Policy 2005 (MNRE, 2009), has set for itself the goals of, among other things: (a) access to electricity for all and (b) increase in per capita availability to over 1000 units by 2012. In other words, we want to greatly enhance energy consumption. In doing it, we will have to face the inevitable consequence of more serious pollution. And, as we will bring out in the next articles that renewable aren’t as squeaky clean as are popularly believed. Nor is the use of renewable energy sources on a large scale an insignificant burden on the environment.

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Non-tracking Concentrators Classification and Working Principles

Tracking concentrators provide high delivery temperatures but require accurate tracking device and fine surface accuracies and hence are expensive. However, for medium temperature operation, less expensive concentrators have been designed, without the tracking required.

 The description of some of these concentrators is given below:

Classification of Non-Tracking Concentrators 

Broad classification of the solar concentrators involves tracking and non-tracking types 

Tracking Type Concentrators are mainly of two type. For having a descriptive knowledge of those type, please follow these links. 

1. One axis tracking concentrators

2. Two-axes tracking Concentrators  

 Non Tracking Solar concentrators are mainly of 4 types. These are described below: 

Flat receiver with booster mirror

Figure shows a flat receiver with plane at the edges to reflect additional radiation into the receiver. Mirrors are also called booster mirrors. The concentration ration of these concentrators is relatively low, with a maximum value less than four. As the solar incidence angle increases, the mirrors become less effective. For a single collector, booster mirrors can be used on all the four sides. When the sun angle exceeds the semi angle of booster mirrors, the mirror actually starts casting shadow on the absorber. In case of an array of collectors, booster mirrors can be used only on two sides.  The efficiency of a boosted flat plate system can be increased if the angle of the flat mirrors can be changed several times during the year. The advantage of such a system is that it makes use of the diffuse radiation in addition to the beam radiation. The attainable temperature and collection efficiency will be higher than that of a flat plate collector of the same collection area. 

Tabor-Zeimer circular cylinder

Figure shows such a concentrator. It is a very simple cylindrical optical system which consists of an inflated plastic cylinder with a triangular pipe receiver. The cylinder has a clear portion on the top to permit radiation to enter and fall on its rear portion which is aluminized to act as a mirror. The incident radiation is reflected by the mirror and is focussed on the absorber near the bottom of the cylinder.

A concentration of about 3 can be achieved without tracking. It can be placed along East-West axis and requires only seasonal tracking. The concentrator uses, in addition to beam component, some diffuse radiation. The delivery temperatures and collection efficiencies are higher than that possible with an ordinary flat plate collector.

Compound parabolic concentrator

This concentrators is a non-imaging one and belongs to a family of concentrator which has highest possible concentration permissible by thermodynamic limit for a given acceptance angle. Further, it has a large acceptance angle and needs to be intermittently turned towards the sun.

The first design of a compound parabolic concentrator (CPC) was found independently by Winston (1965) and Baranov (1966). It consists of two parabolic segments, oriented such that focus of one is located at the bottom end point of the other and vice versa (Fig.8.14). The axes of the parabolic segments subtend an angle, equal to acceptance angle, with the CPC axis, and the slope of the reflector surfaces at he aperture plane are parallel to the CPC axis. The receiver is a flat surface parallel to the aperture joining two foci of the reflecting surfaces.  Rays incident in the central region of the aperture undergo no reflection whereas those near the edges undergo one or more reflections. The number of reflections depend on the incident angle, collector depth and concentration ratio (Rabl, 1976). To reduce cost of the unit, the CPC can be truncated in height to half, without any significant change in concentration.

Extensive investigations on this concentrator have led to several modified designs of the ideal CPC. The salient modifications can be listed as follows:

• The use of receiver shapes such as fins, circular pipes for better optical thermal performance.
• Truncation of CPC height to reduce the physical size and cost.
• Asymmetric orientation of source and aperture to deliver seasonal varying outputs.
• Design of CPC as a second stage concentrator. 

In view of the above modification the reflecting surface of all resulting concentrators may not be parabolic, but still belong to nonimaging group of concentrators. 

The CPC can be used in a non tracking mode for concentration ratios of about 6. However for higher ratios the reflector surface area becomes very large and hence cannot be used.  

V-Trough

Figure shows schematically such a concentrator. It consists of highly reflecting side walls which reflect solar rays to a receiver plate placed at the base of the trough. The trough is aligned in East-West direction. So as to avoid diurnal tracking these concentrators provide higher concentration (of the order of 3 in straight wall case) than flat plate collectors with booster mirrors, because in the latter case the acceptance angle is very large and so the concentration is low. Different combinations of depth to base-width ratios and cone angle are possible for optimum performance depending on the frequency of seasonal tilt adjustments.  The average number of reflections in a V-trough is more or less the same as that in a CPC. For low concentrations, performance of both is comparable. For high concentrations, these appear impractical. The performance of these can be improved by using more than one mirror element in each side wall at suitable angle thus resulting in polygonal troughs.   

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Construction and Working Principle of Two-axes tracking Concentrators

In order to achieve a high concentration for high temperatures solar processes, concentrators with double curvatures are used. These requires two axes tracking of the sun. Some of these have been described below: 

Please Read: 

•  One axis tracking concentrator classification, construction and working principle 
• Elaborate classification, advantages and disadvantages of solar concentrators 

Find about : Non-tracking solar concentrators.  

Construction of Two axes tracking Concentrator 

The construction and working principle of these types of collectors are described below: 

Paraboloidal disc concentrator construction and working principle: 

A paraboloid is produced when a parabola ratates about its optic axis. When it is used to concentrate solar radiation, a high concentration ratio is achieved. 

Due to the compound curvature with perfect optics and a point source of light, theoretically, light is focussed at a point in a paraboloid. However, an image is produced due to the finite angular subtense of the sun. Also the surface may not be strictly parabolic so that the image will be enlarged  due to misdirection of the light rays by misaligned surface elements. A degraded image will be enlarged due to is off-axis. The rays from the edges travel a larger distance, resulting in a spread of the image. Thus a three dimensional image of the sun in the shape of an ellipsoid is formed. 

The thermal losses from paraboloid are primarily radiative and can be reduced by decreasing the aperture area. This however results in a smaller intercept factor. The optimum intercept factor is about 0.95 to 0.98. The larger the surface errors, the larger must be the absorber sixe to achieve the optimum beam intercept. 

High collection efficiency and high quality thermal energy are the features of paraboloid or parabolic dish type concentrator. The delivery temperature being very high, these devices can be used as sources for a variety of purposes. 

Central Tower Receiver two axes tracking concentrator 

The system consists of a central stationary receiver to which the solar radiation is reflected by heliostats. A heliostat is composed of a large erray of mirrors fixed to a supporting frame. This frame can be used to track the sun, as desired. 

The heliostats are installed in the open space and together they act like a dilute paraboloid. They focus solar radiation on a central receiver, which is stationary. Concentration ratio as high as 3000 can be achieved. The absorbed energy can be extracted from the receiver and delivered at a temperature and pressure suitable for driving turbines for power generations.  The advantage of this system s that it eliminates the need for carrying the working fluid over large distances. As a result the heat losses are reduced significantly thus eliminating the need for insulation. There are also many problems associated with this system. A majority of them are due to heliostats and receiver. A large number of heliostats are required to focus solar radiation requiring large free space. The heliostats may be provided with proper tracking arrangement amd may be arranged such that self shading is avoided; at the same time achieving desired concentration. In addition to the cost, the cleaning of mirrors to remove dust and dirt poses another problem. Further the heliostat arrangement must be strong enough to withstand extreme weather condition. The receiver must be able to interrupt the focused radiation, adsorb this heat and transfer this as energy to the working fluid with minimum heat loss. Several designs have been proposed for the receiver. Heat transfer fluids like steam, hitec and liquid metals are suggested for use. Some problems arise due to reflection and transmission losses and thermal stresses in the receiver.   

Circular Fresnel lens two axes tracking concentrator

Lenses are usually not used in solar energy applications due to cost and weight. However, these are used where high temperature is required as in solar furnace, Figure shows the principle of this lens. Optically, the lens is equivalent to a thin lens approximation. It is divided into a number of zones which are spaced at a few tenths of a millimeter; the space can also be few centimeters. Within each aperture zone , the tilt of the lens surface is so adjusted that its optical behaviour resembles that a conventional spherical lens of the same focal length. The focus of the annulus need not to be curved, but is required to have the correct tilt so as to refract the light to the focus. This is because, the absrobing surface is usually much larger than the width of a Fresnel zone on the lens. Meinel (1977) has given the equation for the tilt of the facet as a function of the aperture zone and focal length. 

The circular Fresnel lens provided very good concentration. For a precise plastic lens, the brightness concentration is as high as 2000. Hence, such concentrators are usually used with silicon and gallium arsenide solar cells for high flux. In solar cell applications, the lens has to track the sun since it is required to keep small solar image centered on the receiver. It may be noted that the brightness cocentration in this case is smaller than that in case of parabolic mirrors. 
The transmitting system has an advantage over the reflecting system in that it absorbs certain wavelenght of incident beam, which may result in heating of the focus. 

Hemispherical Bowl Mirror 

Figure shows another type of fixed mirror and movable receiver type concentrator, independently proposed by Steward (1973) and Meinel (1973). The major components of this concentrator are a fixed hemispherical mirror, a tracking linear absorber and a supporting structure. The hemisphere produces a hemispherical mirror, a tracking linear absorber and a supporting structure. The hemisphere produces a highly aberatted optical image. However, because of its symmetry all rays entering into the hemisphere after reflection cross the paraxial line at some point between the focus and mirror surface. Therefore, an absorber pivoted about the center of the curvature of the hemisphere interrupts all reflected rays. The absorber is to be moved so that its axis is always aligned with solar rays passing through the center of the sphere. This requires two axis tracking. Though this motion can be set in a number of ways, the simplest one is to adopt equatorial mount in which the absorber is driven around a polar axis, during the day, at a constant angular speed of 15degree/hour. Through a slow continuous motion or a periodic motion adjustment about an axis normal to polar axis, correct declination can be maintained. It may be noted that this type of concentrator gives a lesser concentration, owing to spherical aberration, than that in paraboloids.

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One-axis tracking concentrator: classifications: working Principles

Concentrators with one axis tracking are used to achieve moderate concentration. A few of them have been described below.

Please read:

• Classification, construction and working principle of two-axes tracking concentrator
• Classification and Working Principle of non tracking solar concentrators 

For knowing about the other concentrators follow the link:

• Classification of Solar Concentrators and their advantages 

One Axis Tracking Concentrator Classification 

           Fixed Mirror Solar Concentrator (FMSC) Construction and Working Principle 

          This type of  concentrator consists of a fixed mirror associated with a tracking receiver system. The fixed mirror is made of long, narrow, flat strips of mirror. These mirrors are arranged on a chosen circular cylinder of an arbitrary radius R.

          The width of the mirror strip is synchronized with the diameter of the absorber pipe. The angle of each element is set in a way that the array has a focal distance twice the radius of the reference cylinder. The array produces a narrow focal line that lies along the same circular path with the diurnal motion of the sun. The focal line can be easily tracked by the movable receiver pipe that is made to rotate about the center curvature of the reflector module. Thus the delicate part (the mirrors) of the system can be rigidly fixed.

           The image width at the absorber is ideally is same as the projected width of the mirror element. Thus the concentration ratio is exactly the same as the number of mirror elements, ignoring the solar beam spread. As the aperture is fixed and concave in shape, the mirror strips results in shading with very high or very low sun altitude angles. Also, due to strips, edge loses occur during reflection. However, mirrors can be suitably designed to have less than 10% of the total energy lost over a year time. Some (Fixed Mirror Solar Concentrator) models have shown overall efficiencies in the range of 40-50%.

Cylindrical Parabolic Concentrator Construction and Working Principle

           A cylindrical parabolic trough is a very common optical imaging instrument which is used as a solar concentrator. It is composed of a cylindrical parabolic reflector associated with a metal tube receiver at its focal plane. The outside surface of the receiver is blackened and is covered by concentrator. And it is rotated about one axis to track the sun’s diurnal motion, absorber tube carries the heat transfer fluid. And thus absorber tube transfers the heat. 

          To define the concentrator aperture diameter, rim angle, shape and the absorber size are generally used. Mild steel or copper may be used to make the absorber and is coated with a black paint which is heat resistant. Different coatings may be used for different cases for better performance. According to the need of temperature different heat transfer liquids may be used. Anodized aluminum sheet or aluminized Mylar or curved silvered glass may be used to make the reflector. Mirror strips are sometimes used in the shape of parabolic cylinder because it is difficult to curve a very large glass. A lightweight structure is used on the reflecting part . The concentration ratio for this kind of  cylinder absorber is in the range of 5 to 30.

       
          The major energy losses:
       
           The main losses are during the reflection from the reflecting part and there is also convective losses from the receiver to the surrounding environment.
           High reflecting materials are generally used for reducing convection. Twisted tapes are used to transfer heat from the absorber to the working fluid.
           East-West, North-South or polar. are the three directions in which a cylindrical parabolic trough may be oriented. The first two arrangements, although simple to assemble, have higher losses due to incidence angle cosine losses. The polar configuration gives the best performance. It intercepts more solar radiation per unit area in comparison to other models. 

           Linear Fresnel Lens/reflector Construction and Working Principle 

           A linear Fresnel lens solar concentrator is shown in figure. It consists of linear grooves on one surface of the reflecting material. The groove angles are chosen with reference to a particular wavelength of incident beam so that the lens acts as a converging one for the light which is incident normally. Although both glass and plastic can be used as refracting materials for fabricating Fresnel lenses, glass is seldom used because it is difficult to mold and has large surface tension. Plastic lenses on the other hand, are economical and the mold last for an appreciable amount of time. Plastic Fresnel lenses  with 20 grooves per mm have been molded.

           The Fresnel lens may be installed with either the groves facing the sun or the grooves facing downwards. In the first case, the ineffective facts of the grooves prevent a part of the input light from being transmitted to the focus (according to Snell’s law the refracted light is deviated away from the normal on moving from a denser medium to a rarer medium). Also dust is accumulated in these grooves results a reduced performance. In the second case, the concentrator has a higher surface reflection loss and large off axis aberrations. While reflection loss causes low efficiency, the aberrations result in a low concentration ratio. The fact that the beam is not incident normally also affects. The focal length of the lens varies rapidly with the change of the angle of incidence. So, for a better performance, the optical system needs to track the path of the sun.

           Fresnel Reflectors can be used as concentrating devices. Figure shows such a configuration, which is made up of smaller flat or curved components. It consists of a number of mirror elements mounted suitably, so that all incident parallel rays of light after reflection, are focused at a common point. Ideally, mirror elements must be parabolic in shape, but to simplify the manufacturing process and assembling problems, flat mirrors are generally used.

Please Read: 

           Solar Pond Pump Features
Renewable Energy The Next Green Hope 
Renewable Energy: Graphical Representation 
Geothermal Energy Pros and Cons 
Hydroelectric Power Plants Advantages and Disadvantages 
Solar Air Heater Classification and Advantages 
Cheapest Source of Renewable energy - Biomass 
Fixed Dome type Janata Power plant construction 

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