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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Plea

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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.

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Solar Concentrator Classification: Advantages of Solar Concentrator

Introduction: Definition of Solar Concentrators:

Solar Concentrator is a device which concentrates the solar energy incident over a large surface onto a smaller surface. The concentration is achieved by the use of suitable reflecting or refracting elements, which results in a increased flux density on the absorber surface compared to that existing on the concentrator aperture. In order to get a maximum concentration an arrangement for tracking the sun’s virtual motion is required.  An accurate focusing device is also required. Thus a solar concentrator consists of a focusing device, a receiver system and a tracking arrangement. Temperature as high as 3000 degrees Celsius can be got from a solar concentrator. So they have potential applications in both thermal and photovoltaic utilization of solar power at high temperatures.

Solar concentrating devices have been used for a long time. In Florence as early as 1695, a diamond could be melted by solar energy. Lavoisier carried out a number of experiments with his double-lens concentrator. The knowledge concentrator dates back even time of Archimedes, whose book "On Burning Mirrors" is an evidence of this fact. Many uses of concentrators were reported in the eighteenth and nineteenth centuries, particularly in heat engines and steam production. The advantages of concentrator are as follows: 

The advantages of solar concentrator

1. It increases the intensity by concentrating the energy available over a large surface onto a smaller surface (absorber)

2. Due to concentration on a smaller area, the heat loss area is reduced. Further, the thermal mass us much smaller than that of a flat plate collector and hence transient effects are small.

3. The delivery temperatures being high, a thermodynamic match between the temperature level the task occurs.

4. It helps in reducing the cost by replacing an expensive large receiver by a less expensive reflecting or refracting area.

Disadvantages of Solar Concentrator

However, concentrator is a optical system and hence the optical loss terms become significant. Further it works on beam component of solar radiation, resulting in loss of diffuse component. Although the basic concepts of flat plate collectors are applicable to concentrating systems, a number of complications arise because of non-uniform flux on absorbers, wide variations in shape, temperature and heat loss behavior of absorbers and finally the optical considerations in the energy balance conditions. It may be noted that higher the concentration of the collector, higher is the precision of optics and more is the cost of the unit. In addition to the complexity of the system, the maintenance requirements are also increased.

Classification of solar concentrator

Solar concentrators may be classified as (i) tracking type and (ii) non-tracking type. Tracking may be continuous or intermittent and may be one-axis or two -axes. As the sun may be followed by moving either the focusing part or receiver or both; concentrators can be classified accordingly. Further the system may have distributed receiver or central receiver.

The concentrators may also be classified on the basis of optical components. 

They may be 

1. Reflecting or refractory type

2. Imaging or non-imaging type

3. Line focusing or point focusing type

The reflecting or refracting surface may be one piece or a composite surface, it may be a single stage or two stage type system and may be symmetric or asymmetric. In practice however hybrid and multistage systems, incorporating various levels of the features, occur frequently.

Types of solar concentrators

There are a number of methods by which the flux radiation on receivers can be increased. Some of them have been discussed here:

• Tracking Concentrators
• Non-tracking Concentrators 

   Tracking Concentrators classification: 

   Tracking Concentrators can be further classified as

• One-axis tracking concentrators (Follow the link for the description)
• Two-axes tracking concentrators (Follow the link and know about all types of two-axes tracking concentrators) 

    Concentrators with one axis tracking 

    These are used to achieve moderate concentration. A few of them have been described below.

i.    Fixed Mirror Solar Concentrator (FMSC)

ii.  Cylindrical Parabolic Concentrator

iii.  Linear Fresnel Lens/reflector
         (Follow the link of one axis tracking concentrator for getting the description of all three concentrators)

Concentrators with two-axes tracking 

           In order to achieve a high concentrators 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: 

i. Paraboloidal dish concentrators 

ii. Central Tower Receivers 

iii. Circular Fresnel Lens 

iv. Hemispherical bowl mirror

Non-tracking concentrators classification 

These are classified as follows: 

i. Flat Receiver with booster mirror 

ii. Tabor- Zeimer Circular Cylinder 

iii. Compound Parabolic Concentrator 

iv. V- tough  

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Renewable Energy Sources: The Great Green Hope for clean environment

Fossil fuel use is the prime cause of global warming and ocean acidification. Then the obvious solution to that problem is that:

• Either some way is found out to use fossil fuels, but not let the resulting CO₂ escape to the atmosphere, in other words to sequester (catch/lock up) the CO₂ , or
• To find other fuels which do not release CO₂ (or other greenhouse gases) 

Renewable Energy Sources: The Great Green Hope 

The world is trying both the options. The sequestration of CO₂ is very much possible theoretically, it is as of now uneconomical. Moreover, quantities of CO₂ that must be sequestered is so huge that there is no way to store it without serious risks to environment and public safety.

Realizing the limitations of sequestering fossil fuel CO_2, great hope has been pinned on the renewable energy sources. The year 2008 has seen more funds to be allocated globally to renewable – based power generation than ever before. The year also witnessed renewables getting higher share of funds than conventional energy sources for the first time ever.

How tough it is to find replacement which will be as efficient as petrol and diesel, and yet be non-polluting, can be gauged from figure below. It show s that the volumetric energy density, in other words energy packed in each liter of diesel is as much as double of that of ethanol and three times higher than liquid hydrogen. Petrol is more energy-dense than bio-diesel; also ethanol, hydrogen, etc. This means that if vehicles are to be run on liquid hydrogen- which is ultimate aim of most of the R&D- they have to have their tanks three times larger than what they were for the diesel engine. To put in other words, even if we succeed in finding fuels which are cleaner than petrol and diesel, we have little hope of finding fuels which are as efficient as the twoRenewable Energy Sources: The Great Green Hope

Fossil fuel use is the prime cause of global warming and ocean acidification. Then the obvious solution to that problem is that:

Either some way is found out to use fossil fuels, but not let the resulting CO₂ escape to the atmosphere, in other words to sequester (catch/lock up) the CO₂ , or

To find other fuels which do not release CO₂ (or other greenhouse gases) 

The world is trying both the options. The sequestration of CO₂ is very much possible theoretically, it is as of now uneconomical. Moreover, quantities of CO₂ that must be sequestered is so huge that there is no way to store it without serious risks to environment and public safety.

Realizing the limitations of sequestering fossil fuel CO_2, great hope has been pinned on the renewable energy sources. The year 2008 has seen more funds to be allocated globally to renewable – based power generation than ever before. The year also witnessed renewables getting higher share of funds than conventional energy sources for the first time ever.

How tough it is to find replacement which will be as efficient as petrol and diesel, and yet be non-polluting, can be gauged from figure below. It show s that the volumetric energy density, in other words energy packed in each liter of diesel is as much as double of that of ethanol and three times higher than liquid hydrogen. Petrol is more energy-dense than bio-diesel; also ethanol, hydrogen, etc. This means that if vehicles are to be run on liquid hydrogen- which is ultimate aim of most of the R&D- they have to have their tanks three times larger than what they were for the diesel engine. To put in other words, even if we succeed in finding fuels which are cleaner than petrol and diesel, we have little hope of finding fuels which are as efficient as the two. 

Figure: Fuel and Power Sources

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Solar Air Heater Classification and Advantages

This post provides the description and analysis of different types of solar air heaters used in space heating and drying purposes. The Solar air heaters have the following advantages over other solar heat collectors.

Solar Air Heater Advantages 

• The need to transfer heat from working fluids to another fluid is eliminated as air is being used directly as the working substance. The system is compact and less complicated.
•  Corrosion is a great problem in solar water heater. And this problem is not experienced in solar air heaters.
•  Leakage of air from the duct does not create any problem.
• Freezing of working fluid virtually does not exist.
• The pressure inside the collector does not become very high. 
• Thus air heater can be designed using cheaper as well as lesser amount of material and it is simpler to use than the solar water heaters.

Solar Air Heater Disadvantages 

• Air heaters have certain disadvantages also the first and foremost is the poor heat transfer properties of air. Special care is required to improve the heat transfer. 
• Another disadvantage is the need for handling large volume of air due to its low density. 
• Air cannot be used as a storage fluid because of its low thermal capacity. 
• In the absence of proper design the cost of solar air heaters can be very high. 

Solar Air Heater Applications  

The applicability of the solar air heater depends on various factors like high efficiency, low fabrication cost, low installation and operational cost and some other specific factors regarding specific uses. Extensive work in solar air heaters has been done. Various geometries have been proposed and their theoretical investigation is carried out. But it needs commercial exploitation.
 

Solar Air Heater Classification

A conventional solar air heater is essentially a flat plate collector with absorber plate, a transparent cover system at the top and insulation at the bottom and on the sides. The whole assembly is encased in a sheet metal container. The working fluid is air, though the passage for its flow varies according to the type of air heater.
Material for construction of air heaters are similar to those of liquid flat plate collectors. The transmission of solar radiation through the cover system and its subsequent absorption in the absorber plate can be given by expressions identical to that of liquid flat plate collectors. Selective coating on the absorber plate can be used to improve the collection efficiency but cost effectiveness criterion should be kept in mind.
 

Non-porous Type solar air heater

In non-porous type, air stream does not flow through below the absorber plate but air may flow above and/or behind the plate.

In first type, no separate passage is required and the air can flows between the transparent cover system and the absorber plate. ( see the figure). In this heater as the hot air flows above the absorber, the cover receives much of the heat and in turn, loses it to the ambient. Thus a substantial amount of heat is lost to the ambient and hence this air heater is not recommended.
The non-porous type with air passage below the absorber is most commonly used. A plate parallel to the absorber plate is provided in between the absorber and the insulation, thus forming a passage of high aspect ratio.
In another variety of non-porous type air heater, the absorber plate is cooled by air stream flowing on both sides of the plate.  

Depending on the type of the absorber plate, the air heater can be non-porous and porous. Figure below shows the non-porous absorber type air heaters.

It may be noted that the heat transfer between the absorber plate and the flowing air being low, the efficiency of air heaters is less. The performance, however, can be improved by roughening the absorber surface or by using a vee-corrugated plate as the absorber plate. Turbulence induced to the air flow helps increase the convective heat transfer.
The radiative loss from the absorber plate are significant, unless selective coatings are used, decreasing the collector efficiency. Also, the uses of fin may result in a prohibitive pressure drop, thus limiting the applicability of non-porous type.
 

Porous type solar air heater    

The second type of air heaters has porous absorber which may include slit and expanded metal, overlapped glass plat absorber and transpired honeycomb.

The sir heater with porous type of absorber has the following advantages: 

Advantages of porous solar air heater 

• Solar radiation penetrates to a great depth and is absorbed along its path. Thus the radiation loss decreases. Air stream heats up as it passes through the matrix. 

• The pressure drop is usually lower than the non-porous type. 

It may be noted however, that an improper choice of matrix porosity and thickness may cause reduction in efficiencies as beyond an optimum thickness, matrix may not be hot enough to transfer the heat to air stream.

Wire mesh porous bed formed by broken bottles and overlapped glass plate are some examples of porous type absorbers used in Solar air heaters. 

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