exergy analysis of assam

 

 

 

project submitted in partial FULFILLMENT of the requirements for award of the degree of

bachelor of mechanical engineering

 

By

 

Satyaki Das (roll no 03/90) 

 

Under the supervision of

 

H.K. Thakuria

Professor

 

 

 

 

 

 

Department of Mechanical Engineering

Assam Engineering College, Jalukbari

(Jan) 2007

 

 

ABSTRACT

 

 

 

 

 

Most of the energy resources being used at present are non-renewable ones, which are limited in quantity. So, need was felt for proper use and conservation of these resources for future use. Hence energy analyses are conducted regularly by the official statistical agencies of all developed countries, to keep a track of the usage and wastage of resources. Exergy analysis is one of several methods adopted for this purpose. This project is an effort to present the exergy transformation occurring in the Assamese Society, the wastage of exergy occurring here, and the possible ways to minimize this wastage.

 

 

 

 

INTRODUCTION:

 

 

EXERGY analyses are carried out at regular intervals by the official statistical agencies of all developed nations and included in the official statistical reports. These reports are of great importance in the fact that they help in keeping an exact record of the use of valuable   resources of the country. These resources may include tangible physical resources such as coal, electricity, wood, petroleum etc. Again, the resource might be something that doesn’t exactly fall under the general description of a resource, but is indeed one, like waste material (obtained on use of other resources) or even human resources.

Now, all these resources, their use and their value have traditionally been defined in terms of monetary unit. But this view may not always present the true picture.

 

An example of the above would be how the value of fossil fuels is usually given—the spot price is taken as the basis for calculations, without taking into account the effect of burning the fuel on the environment, or irreversible change associated with the burning.

As a result of this discrepancy existing in the analysis, need was felt for the use of a different metric, which would provide a more accurate result (or picture) of the utilization of resources; Consequently, Exergy was chosen as the basis for all calculations/analyses of resources of a country.

 

As we know, Energy is always conserved, what we need to conserve is Exergy.

Energy has two forms- Exergy and Anergy. Exergy is the useful form of energy, while Anergy is that part of energy that can’t be utilized. Any form of energy once wasted is converted into anergy, which can’t be further used. So, by analyzing the exergy of different energy forms, we can reduce the wastage of energy, and utilize it in a more proper way for future use.

Analyses of exergy use provide knowledge as to how effective and how balanced a society is in use of the natural resources. This type of knowledge can be used to identify areas in which technical and other improvements should be undertaken and also indicates the priorities, which should be assigned to conservation measures.

Exergy is the fuel for dissipative system, i.e. systems that are sustained by converting energy and materials, e.g. a living cell, an organism, an eco-system, the Earth’s surface with its materials cycles, or a cycle, or  a society. The exergy concept could therefore, in this sense, be systematically be used to describe such systems.

The concept is mostly used in energy engineering and in cases where one has to deal with energy of varying quality. However, its field of application can be extended to the totality of energy and material conversions in society. This approach yields a uniform description of the use of physical resources and the environmental impact in connection with this one.

Natural resources are traditionally divided into energy resources and other resources. This separation is often only approximate. Oil, for example, is usually looked upon is an energy resource and wood is regarded as a material resource. This distinction isn’t very meaningful however, because oil can be also be used in the production of useful materials and wood may be used as a fuel. It would be more appropriate to consider these resources together. Exergy is thus an adequate resource measure. 

 

LITERATURE REVIEW:

 

Among the many pressing problems facing mankind today, two particular problems involve that of excessive use of the natural resources, and also the consequent reliance on non-renewable resources as a result of increasing scarcity of high quality, finite (low entropy) energy carriers. Over exploitation of Nature’s energy resources has given rise to gradual depletion of the limited resources, and excessive use of these has also given rise to an environmental unbalance, which needs to be countered. Needless to say, these problems originate to a large extent, from the huge energy requirements of the present day society. As the developing countries progress, economical interests force them to imitate and assimilate the values of other modern (developed) industrial societies. With time, energy requirement increases, initially to obtain energy carriers of ever decreasing quality (increasing entropy), and then to dispose of them (now in the form of high entropy waste energy).

                                                The question arises whether Society can correct these problems by matching energy supply and demand with both the minimum depletion of the (low entropy) energy resources and the minimum production of entropy. Thermodynamics and more specifically, use of the concept of ‘Exergy’ indicate that distinct energy carriers mayn’t only be quantitatively different but also qualitatively diverse. In addition to the desirability of having a quantitative equilibrium between supply and demand, the efficient use of energy carriers also entail the matching the quality of the energy supplied to the quality of energy required for a given process or device.

The exergy concept is reviewed as a tool for resource accounting. Conversions of energy and material resources in the Assamese society are described in terms of Exergy. The necessary concepts and conversions are introduced. Energy losses in transformations of material resources and in conversions of various forms of energy into heat are described.

 

Energy and Exergy

 

Energy is defined conventionally as the capacity for doing work and overcoming resistance to do so. Because the concept of energy doesn’t contain a provision for the quality of the energy, it mayn’t be the most appropriate concept for energy planning and policy purposes. The problem with energy content is that it doesn’t distinguish between heat and work. Contents of an energy carrier can potentially be completely converted to heat, but in general, only partially to work. The work content of an energy carrier can theoretically be entirely converted to either heat or work. Therefore, there is a sharp difference between these two forms of energy. The work content is a more valuable form of energy and actually sets the ‘opportunity cost’ of energy, (define opportunity cost).

The concept of Exergy, on the other hand, incorporates the precepts of both the 1^st and the 2^nd law of thermodynamics. And thus is therefore more suitable for planning and policy purposes.

The most natural and convenient standard is the maximum work which can be obtained from a given form of energy using the environmental parameters as the reference state; this standard of energy quality is called exergy. Exergy is a measure of how a certain system deviates from equilibrium with respect to its environment.

The exergy E for a system in a large environment is given by

E = T_o (S_eq - S),

Where                                  T_o = temperature of the environment

  (S_eq - S) = deviation from equilibrium of the entropy of the                             system and its environment.

Another expression for the exergy is

E = U + p₀V – T_oS - S m_io h_i

Where,  U, V, S, h_i denote extensive parameters of the system.

             P_o, T_o and m_io are intensive parameters of the environment.

 

 A very useful formula for determining the exergy was given by the APS group [Berman in 1975].

E = U – U_eq + p_o(V-V_eq) – T_o(S-S_eq) - S m_io (h_i - h_i,eq)

 

For a substance, which has an exergy content deriving only from its concentration the following relation, holds:

E = nRT_oln(c/co)

Where, n  =  no of moles of the substance

                   R =  gas constant

       T_o = temperature of the environment

       c = concentration of the substance in the material

       c_o = concentration of the substance in the environment.

 

Let us illustrate the meaning of exergy by some very simple examples:

1. A system in complete equilibrium with its environment doesn’t have any exergy. There is no difference in temperature, pressure or concentration etc that can be used to drive any processes.

2.  The more exergy a system carriers, the more its deviates from the environment. Hot water has a higher content of exergy during the winter than it has during a hot summer day. Similarly, a block of ice hardly carriers any work in it during winter, but in summer, it does so.

3. When any form of energy loses its quality, this means that the exergy has been destroyed. The exergy is that part of the energy that is useful in the society and therefore it has an economic value, and is therefore worthy of being taken care of.

 

The sources of energy can be divided into two groups, viz. high-grade energy and low-grade energy. Examples of High-grade energy include Mechanical Energy, Electrical energy, while Low-grade energy includes thermal energy, heat of nuclear fission etc.

Most of the high-grade energy available to us is obtained from sources of low-grade energy, such as fuels, using a cyclic heat engine.

The second law of thermodynamics makes it clear that complete conversion of low-grade energy heat, into high-grade energy, i.e. shaft work, is impossible. So, that part of the low-grade energy, which is available for conversion, is referred to as available energy, while the part, which, according to the second law must be rejected, is known as unavailable energy.

 

 

 

 

 

 

 

 

 

Exergy associated with a heat transfer

 

The maximum work output obtainable from a certain heat input in a cyclic heat engine is called the available energy (AE), or the available part of the energy supplied. The minimum energy that has to be rejected by the second law is called the unavailable energy (UE), or the unavailable part of energy supplied.

                                                                                                                

 

 

 

In the above figure, T₁ = temperature of the source.

                               T₂ = temperature of the sink.

                               Q₁ = Heat supplied to the system.

                               Q₂ = Heat rejected from the system.

 

 

Now, Q₁ = A.E. + U.E.

          W_max = A.E. = Q₁ – U.E.,

           Where W_max is the maximum work that can be obtained from the system.

 

For the known temperatures of the Source and Sink,

 

We have,  

              h_rev = 1 – T₂/T₁.

 

As we can see, for a known value of T₁, hrev will increase with the decrease of T₂.

Lowest practicable temperature of heat rejection is the temperature of the surroundings, To.

 

Therefore,

                 h_max = 1 – T₀/T₁

and          W_max  = (1- T₀/T₁)Q₁.

 

This maximum possible work that can be thus obtained from a cyclic process is what we refer to as Exergy. Also, the unavailable part of energy, which can’t be used, is called Anergy.

 

Exergy associated with a steady stream of matter

 

Exergy of a steady stream of matter is equal to the maximum amount of work obtainable when the stream is brought from its initial state to the dead state by processes during which the stream may interact only with the environment.

Now exergy components of a stream of matter can be divided into distinct components, which include kinetic, potential, physical and chemical exergy.

We will now look into physical and chemical exergy.

 

Physical and chemical components of exergy

 

Due to disorderness, entropy dependent nature of these forms of energy (chemical, physical, kinetic etc), the corresponding exergy components can only be determined by considering a composite, two-part system, the stream under consideration and the environment.                            

                                                                        In principle one could determine the total exergy derived from disordered energy forms in one idealized device where the stream would undergo physical and chemical processes while interacting with the environment. It is convenient, however, to separate physical exergy and chemical exergy, enabling calculation of exergy values using standard chemical exergy tables.

Now, physical exergy is equal to the maximum amount of work obtainable when the stream of substance is brought from its initial state to the environmental state defined by P0, T0 (environmental state), by physical processes involving only thermal interaction with the environment.

         Chemical exergy is equal to the minimum amount of work necessary to synthesize, and to deliver in the environmental state, the substance under consideration from environmental substances by means of processes involving heat transfer and exchange of substances only with the environment.

 

Exergy as a general resource concept:

                        Exergy is the fuel for dissipative systems, i.e. systems that are sustained by converting energy and materials, eg.  A living cell, an organism, an eco system, the earth’s surface with its material cycles, or a society. The exergy concept could therefore in this sense be systematically to describe such system scientifically.

The exergy concept has mostly been used within heat and power technology, where one works with heat of varying qualities. The field of application can be extended to the totality of energy and material conversion in the society. This yields a uniform description of the use of physical resources and the environmental impacts in connection with this use.

Natural resources are usually divide into energy and other resources. This is however an approximate separation. For example, oil is usually regarded as an energy resource and wood as a material resource. But, oil can be used for producing material products and wood too can be used as a fuel. It is therefore more meaningful to consider these resources together. The exergy content of these resources can be used for evaluation, by simply multiplying the energy content by an ‘Exergy Conversion Factor.’ Values of this conversion factor are given for some energy forms in the table below:

 

                  Quality of Some energy forms*

 

 

                               

                            

* Table taken from ‘EXERGY USE IN THE ITALIAN SOCIETY’ by GORAN WALL.

 

  Power

 

 

The present capacity of the Assam State Electricity Board (A.S.E.B.) is MW, comprising the power stations shown in table I.

 

 

                                                              

                                                                   Table I

 

 

 

However of the above, only the first two are presently in operation, limiting the effective capacity to 253.5 MW, which is equivalent to 912. MU.

 

Table II below shows generation of electricity in Assam divided under different heads.

 

                                                           

Table II

 

As the generation of electricity is quite less compared to the installed capacity, A.S.E.B had accumulated losses over the years to the extent that it was not possible to run it profitably or get rid of its liabilities.

 

 

 

 

Table III below gives a break up Power availability in Assam

 

 

 

TABLE III

 

In order to meet the domestic demand, the state continues to purchase power from other sources.

 

 

TABLE IV

 

 

Table below presents the sale of electricity by type of consumption.

 

 

 

 

 

 

 

 

EXERGY ANALYSIS

 

 

The effective capacity of the power plants of Assam is 912.6 MU, of which 746.094 MU (2002-03), 710.669 (2003-04), 756.435 MU (2004-05) were respectively generated. But the demand is far greater than the supply (generated power). In order to meet the domestic demand, the state continued to purchase power form other sources, and the amount of energy purchased has been gradually increasing.

Table III shows energy purchased and produced for the last three years.

 

Since electrical energy is a high grade energy, it is exempted from the limitations of the 2^nd law of thermodynamics, i.e. the conversion factor is 1(one).

 

So, the availability (in MU) for the three years are

 

 

Now, for proper utilization of the availability, we must get the output in terms of the monetary value consumption.

From table IV, the total consumption is

 

 

So, out of the total availability, we have been able to properly utilize only 55.88%   (2002-03), 57.94 % (2003-04) & 58.82 % (2004-05) respectively.

A large part of the available exergy, viz – 44.12% in ’02-03, 42.06 % in ’03-04 & 41.18 % in ’04-05 has been wasted due to various reasons.

 

 

 

 

Bibliography:

 

 

 

1. Kotas T. J.’ “The Exergy Method of Thermal Plant Analysis” 1985, Anchor Brendon Ltd, Tiptree, Essex.
2. Nag P. K. “Basic and Applied Thermodynamics”.

 

 

 

 

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