ENERGY EFFICIENCY OF THERMAL SYSTEMS: An overview

BY

Prof. S.C.Kaushik*

* Centre for Energy Studies ,IIT Delhi

Energy can be transformed from one form to another

and never destroyedBut it does not distinguish between different forms of

energy on the basis of qualityIt can not differentiate between the loss during a

process and irreversibility lossesNot able to say what quality is required at which

temperature levelA high grade energy may be used for a low grade

application and leading to national loss of resources and

loss to environment

First law of thermodynamics

A higher grade energy can be converted into a lower

grade energy but the reverse is impossible without

spending additional energy.

It can analyze the system so that irreversibility losses

(controllable and non controllable) are identified.

A comparison of actual performance can easily detect

the controllable losses of the system and extends the

scope of improvement.

Matching the grade/quality of input energy required.

Second law of thermodynamics

A process is thermodynamically ideal

If

The available energy,

i.e. exergy remains constant.,

And it fails short of the thermodynamic ideal to

the extent the exergy is consumed.

The following are three distinguished ways of exergy transfer :

Exergy transfer with work (Work exergy) :

max 1QaW Ex Q T T

Ex W Exergy transfer with heat interaction, (Thermal exergy) :

Exergy associated with mass transfer or flow. (flow exergy) :

o

oh T s

The first law of thermodynamics or equation of energy balance for steady flow process of an open system is given :

1

n

in i outi

E Q E W

OR

Qin out W

Qin out W

Ex Ex Ex Ex I

I Ex Ex Ex Ex

The second law of thermodynamics or equation of exergy balance for steady flow process of an open system is given by :

Destruction of availability = irreversibility

Net rate of exergy supplied to a process = Net rate of exergy output from the process + Net rate of exergy loss in effluents +Net rate of exergy consumption by the process

Exergy in Exergy outExergy product

Exergy LossWaste

Exergy and Anergy

All type of energy (work, heat,..) are always composed

of Exergy (sometimes known as availability) and Anergy

Energy = Exergy + Anergy

exergy is that part of the energy that can be converted

without restrictions to all other types of high grade

energy or work

During energy conversion processes exergy may be

converted to anergy but anergy can never be converted

to exergy

Exergy –optimization: Real economic representation

thermodynamic optimization implies, exergy

optimization and not energy conservation, which

according to first law is never consumed or

destroyed the exergy analysis is an useful step on the

way to achieve economically optimum solution

which must always correspond to an

economically reasonable irreversibility

Second Law Efficiency

supplied

( )

II

II

II

II

Actual output

Maximum desirable output

Exergy output

Exergy input

Availability of desired output

availability

First law efficiency

Ideal efficiency Carnot efficiency

TransformationprocessMaterial inputs

Energy inputs

Wastes & emissions

Useful outputs

ThermodynamicsThermodynamics and Material Flows in the Economy and Material Flows in the Economy

1. Law of Thermodynamics:1. Law of Thermodynamics:Conservation of energyIn non-nuclear processes energy can neither be created nor destroyed. Energy can only be transformed from one form into another. The total amount of energy input to a non-nuclear transformation process is thus equal to the total amount of energy output.

Conservation of massThe total mass of material inputs into a (non-nuclear) material transformation process is equal to the total mass of material outputs.

Conservation of mass per chemical elementThe total mass of each chemical element is conserved during every (non-nuclear) material transformation process.

1. Law of Thermodynamics: Quantity of energy during transformations stays the same.

2. Law of Thermodynamics: Quality of energy decreases during transformations (what matters is exergy not energy).

2. Law of Thermodynamics2. Law of ThermodynamicsShort form: In a closed system, entropy (disorder) will increase with time until it reaches

its highest possible value.

What does this mean for material transformation processes (which are open systems):

• Every order-increasing material transformation processes requires low-entropy energy inputs.

• Order-increasing material transformation processes turn low-entropy energy inputs into high-entropy energy outputs.

• Every production process creates waste and/or emissions.

• Without low-entropy energy inputs materials tend to dissipate during use and disposal.

ThermodynamicsThermodynamics and Material Flows in the Economy and Material Flows in the Economy

The first law efficiency is based on law of conservation of energy, which is valid whether a process is physically possible or not. The second law efficiency is based on law of degradation of available energy i.e. quality of energy.Which is more realistic, rational & true measure of the deviation of actual system from ideal system. The first law efficiency can be even more than 100% ,while second law efficiency is always lower than 100% . Energy management on the whole, a first law practice which is a necessary but insufficient. As per first law energy conservation programs are unnecessary. Energy conservation should be done by using the concept of exergy. Where exergy is maximum work, which can be obtained from that form of energy . This presentation establishes a thermodynamics for energy efficiency of various thermal systems and numerical appreciation is also given. Energy = Exergy + Anergy

Energy Exergy

It is dependent on the parameters of matter or energy flow only, and independent of the environment parameters.

It is dependent both on the parameters of matter or energy flow and on the environment parameters.

It is governed by the FLT for all the processes.

It is governed by the FLT for reversible processes only (in irreversible rocesses it is destroyed partly or completely).

It is limited by the SLT for all processes (incl. reversible ones).

It is not limited for reversible processes due to the SLT.

It is always conserved in a process, so can neither be destroyed nor produced .

It is always conserved in a reversible process, but is always consumed in an irreversible process

It is a measure of quantity only.

It is a measure of quantity and quality due to entropy.

Pattern of Demand for conventional energy in percent[1]

Year 1979-80 1984-85 1989-90 2000 A.D.(Conventional energy only)

2000 A.D.(including

Non- conventional

energy)

Household 15.70 18.20 14.24 22.00 36.50

Agriculture 9.40 9.80 10.36 5.60 4.25

Industry 38.20 36.40 55.00 39.00 32.10

Transport 32.80 31.40 16.80 25.60 20.40

Others 3.90 4.20 3.60 7.80 6.25

Total 100.00 100.00 100.00 100.00 100.00

Type of Application Percent Energy Use

Space conditioning 30

Refrigeration 25

Water Heating 15

Lighting 10

Cooking 7

Entertainment (T.V. etc) 5

Grinding 5

Washing and others 3

Percentage energy use in various applications [1]

Desired output energy

Input energy suppliedI

Desired output energy

Maximum possible output energyII

max 1QoW E Q T T

Qin out WE E E E I

minII A A

Exergy of steady stream of matter is sum of kinetic, potential and physical exergy. The physical exergy is given by ,where 0 oh T s

o genI T S And

               

To , Qo

Surrounding

Ein

Exergy in

EW

Qi ,Ti

Reservoirs

Eout

Exergy outIrreversibility, I

                

First and Second law efficiencies for energy conversion systems

By input

Shaft work

heat

input

from

reservoir

at

i

Produce work oW

iW

iA W

min oA W

I o iW W

I II

Electric motor

oWMotoriW

fQ

fT

1f o f

A Q T T

min oA W

I o fW Q

1II I o f

T T

(Heat engine cycle)

oT

oQ

E oW

fT

fQ

Add

heat

to a

Reservoir

at

By input

Shaft work

heat

input from

reservoir at

uQ

uT

iW

fQ

fT

iA W

min1

u o uA Q T T

I a iQ W

1II I o u

T T

Carnot heat pump

1f o fA Q T T

min 1u o uA Q T T

I u fQ Q

1 1II I o u o f

T T T T

Water heater

oToQPiW

uT

uQ

uT

atSpace

fT

oT

Extract

heat

from a

cold

reservoir

At

By input

Shaft work

heat

input

from

reservoir

at

cQ

cT

iW

fT

fQ

iA W

min1

c o cA Q T T

I c iQ W

1II I o c

T T

Carnot refrigerator

1f o fA Q T T

min 1c o cA Q T T

I c fQ Q

1 1II I o c o f

T T T T

Vapour absorption system

cTcQR

oT

oQiW

,oQ

cQ

''oQ

E PW

fT

Generator

fQ

oT

Absorber

oT

Condensor

eT

Evaporator

It is apparent that demand of energy is growing steadily which is being fulfilled by conventional sources.Among all sectors,major consumption is in domestic& industrial. [1]. In these sectors, maximum consumption is in space conditioning, refrigeration,water heating and process heating. Energy conservation and replacement of existing energy sources with renewable energy should be the present need. As the Extraction cost of fossil fuels is sky rocketing so switching over to solar energy is need of today. It is shown by Petla that sun’s radiation is exergy rich but for higher collection temperature it will not be economical.

In this paper, a comparison of solar energy and fossil fuel resources is also made from the point of view of second law efficiency

4

1 44 1* 1 1 cos

3 3o o

xs s

T Te q

T T

,2

For

rad

0.931 *xe q

*

*

1 ox

Te q

T

* *

*

3000.931 1q q

T

* 4350oT K

Exergy of Sun’s radiation[5-6]

283 283.6 1 1 0.0323298 4350II

.9 1 283 298 1 283 2300 .0515II

.6 1 283 298 1 283 318 .274II

1 1o oII I

u f

T TT T

An illustration:-For the building space heating

For solar collector heating source at 45oC

For gas space heating system

For sun’s fuel source temperature

Second law efficiency for industrial processes requiring heat below using fossil fuel source and solar heat sources assuming

280oC0300oT K

Temperature in oC

uT

II % Solar radiation

Source 04350fT K

II % Using fossil fuel

02300fT K

II % Solar collector

Source

20f uT T

35 1.67 2.68 18.25 50 4.58 7.37 34.08 70 8.07 12.97 43.34 85 10.44 16.76 47.10

100 12.61 20.25 49.62 125 15.86 25.48 52.33 150 18.74 30.09 54.04 190 22.68 36.43 55.74 230 26.00 41.77 56.78 250 27.47 44.13 57.16 270 28.84 46.31 57.47

Solar heat is unique in that its temperature can be obtained by choosing suitable collector to provide an excellent second law match between the resource collection and end-use temperatures. From an end use point of view, solar thermal energy finds its application as given below.

1. Water heating:-Use of 1000 water heater can save 1 MW power.

2. Cooking :- Saving of fire wood and LPG, saving to consumer and nation.

3. Industrial applications :--In Food, Textile, Pulp and Paper,Rubber, Glass and Chemical industries for process heat application at various temperatures.

4. Power generation [7]:-- Solar farm systems and feed water heating in place of bleed steam ( Jodhpur )

The use of solar heat can conserve the high quality fossil fuel and increases the efficiency of energy use.

Second law efficiency of various applications using other sources and solar thermal sources

A p p l i c a t i o n T e m p e r a t u r e s O t h e r s o u r c e

I I ( % )

S o l a r c o l l e c t i o n c a s e

I I ( % )

S p a c e h e a t i n g B y f o s s i l f u e l s

. 7I

B y e l e c t r i c i t y

. 9I

0 2 8 3 1 0o oT K C

2 9 8 2 5o ouT K C

4 . 0 1 . 5

2 4 . 4 [ 5 0 ]

. 6

of

I

T C

S p a c e c o o l i n g B y e l e c t r i c i t y ( V a p o u r c o m p r e s s i o n s y s t e m )

2I

B y f o s s i l f u e l s ( V a p o u r a b s o r p t i o n s y s t e m )

. 8I

0 3 1 8 4 5o oT K C

2 9 5 2 2o ouT K C

5 . 2 7 . 2 3

2 7 . 3 3 [ 1 1 0 ]

. 6

of

I

T C

W a t e r h e a t i n g B y f o s s i l f u e l s

. 7I

B y e l e c t r i c i t y

. 9I

0 2 8 3 1 0o oT K C

3 3 5 6 2o ouT K C

1 2 . 3 9 4 . 6 6

5 5 . 5 5 [ 6 7 ]

. 6

of

I

T C

C o o k i n g B y f u e l s

. 6I

2 9 8 2 5o ooT K C

3 9 3 1 2 0o ouT K C

1 6 . 6 7

5 5 . 6 6 [ 1 3 0 ]

. 6

of

I

T C

R e f r i g e r a t i o n B y e l e c t r i c i t y ( V a p o u r c o m p r e s s i o n s y s t e m )

1 . 8I

2 9 8 2 5o ooT K C

2 7 0 3o ouT K C

6 . 2 2

1 9 . 9 3 [ 1 6 0 ]

. 6

of

I

T C

D e h u m i d i f i c a t i o n o f a i r i n h o t a n d h u m i d c l i m a t e ( e l e c t r i c i t y )

3 1 3 4 0o ooT K C

2 8 3 1 0o ouT K C

5

2 5 . 6 7

[ 9 0 ]ofT C

( U s i n g D e s i c c a n t )

Conclusion The second law efficiency has been shown to be a very useful

source index, which is a measure of the exergy of fuel. The exergy of a given fuel is utilized to its fullest when the entropy generated in the fuel-user temperature gaps is minimized.

For proper utilization of exergy there must be proper match between source and use temperature or higher temperature source should first be used for higher temperature applications and heat rejected from these applications should be cascaded to applications at lower temperatures.

In future if costs permit then the solar radiation collection and utilization should be done at higher temperature.

The use of solar thermal heat not only saves the precious exergy rich fuel but also improves the economy, as investment in the fuel saving technology may cost less than efforts to increase fuel supplies in the future.

References

1. S.C.Tripathy, “Electric Energy Utilization and Conservation”, TataMcGrawHill Pub.,1991

2. A.Bejan, “Advanced Engineering Thermodynamics,” Wiley Interscience Pub. 1988

3. Van Wylen et al, “Fundamentals Of Classical Thermodynamics”, John Wiley & Sons, Fourth edition, 1994

4. Jan. F. Kreider, “ Second law analysis of solar thermal processes” Energy Research,vol.3, 325-331,1979

5. J.E. Parrot, “ Theoretical upper limit to the conversion efficiency of solar energy, Solar Energy, 21, 227, 1978

6. Petela R.,Exergy of heat radiation , J. Heat Transfer, vol. 86, 187-192, 1964,.

7. S.C.Kaushik et al “Second law analysis of a thermal power system”,Int. J. of Solar Energy,Vol. 20, 239- 253, 2000

Second law efficiency of various applications using other sources and solar thermal sources

Application Temperatures Other source

II ( %)

Solar collection case

II ( %)

Space heating By fossil fuels

.7I

By electricity

.9I

0 283 10o oT K C

298 25o ouT K C

4.0 1.5

24.4 [ 50 ]

.6

of

I

T C

Space cooling By electricity (Vapour compression system)

2I

By fossil fuels (Vapour absorption system)

.8I

0 318 45o oT K C

295 22o ouT K C

5.2 7.23

27.33 [ 110 ]

.6

of

I

T C

A p p l i c a t i o n T e m p e r a t u r e s O t h e r s o u r c e

I I ( % )

S o l a r c o l l e c t i o n c a s e

I I ( % )

W a t e r h e a t i n g B y f o s s i l f u e l s

. 7I

B y e l e c t r i c i t y

. 9I

0 2 8 3 1 0o oT K C

3 3 5 6 2o ouT K C

1 2 . 3 9 4 . 6 6

5 5 . 5 5 [ 6 7 ]

. 6

of

I

T C

C o o k i n g B y f u e l s

. 6I

2 9 8 2 5o ooT K C

3 9 3 1 2 0o ouT K C

1 6 . 6 7

5 5 . 6 6 [ 1 3 0 ]

. 6

of

I

T C

R e f r i g e r a t i o n B y e l e c t r i c i t y ( V a p o u r c o m p r e s s i o n s y s t e m )

1 . 8I

2 9 8 2 5o ooT K C

2 7 0 3o ouT K C

6 . 2 2

1 9 . 9 3 [ 1 6 0 ]

. 6

of

I

T C

D e h u m i d i f i c a t i o n o f a i r i n h o t a n d h u m i d c l i m a t e ( e l e c t r i c i t y )

3 1 3 4 0o ooT K C

2 8 3 1 0o ouT K C

5

2 5 . 6 7 [ 9 0 ]o

fT C

( U s i n g D e s i c c a n t )