06 exergy analysis gs

8/9/2019 06 Exergy Analysis GS

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The Two Laws Combined: The

Destruction of Exergy


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A problem:

Some researchers decide to write a proposal to harvestthermal energy produced in buildings for electrical

generation. They argue that building uses 41 % of primaryenergy in US, 38% on heating and air conditioning.

Is it a good idea?, is it a bad idea?. Why?


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Key Concept

The actual tendencies for analyzing thermodynamics systemsimply the understanding of the relationship between entropygeneration and the destruction of available work becausework is seen for the engineering thermodynamics as a

commodity.

Energy quality Exergy/available work Entropy Generation

Minimization

Potential of usable work


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Concepts

Exergy or availability analysis consist in using first andsecond laws together, for analyzing the performance of thethermodynamic system in the reversible limit and his departurefrom this limit. (I.e. the estimation of the theoretically idealoperating conditions of a proposed installation)

Entropy generation minimization (EGM or thermodynamicoptimization) consist in combining thermodynamics withprinciples of heat transfer, fluids mechanics and other transportphenomena. It is use for modeling and optimizing real devices andinstallations improving thermal designs.

In EGM the analyst:Composes a realistic model for the system.Constructs an expression for the entropy generation.Minimizes the entropy generation expression.


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First law

If elevation and speed changes between inlet and outlet aredepreciable.

Where n is the numbers of reservoir.


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Second law

To, Po are temperature and pressure of the atmosphere actinglike a reservoir.

Eliminating between first and second law equations andsolving for


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Reversible work

From the analysis presented above we have:

Where is the limiting value of (the upper or lower limitdepending on whether the system is designed to produce orabsorb work, respectively).


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Lost available work Lost exergy)

(entropy generated by the system is a measure of the availablework that has been destroyed).

Work transfer and lost available work arent thermodynamic

properties of the system because both depend of the path(design, constitution, functioning) follow for it


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Available work Exergy)

If the atmospheric reservoir exchanges work with the system

there is a fraction of work transferred to the atmosphereand the remainder constitutes the rate of available work

In the reversible limit(the maximum rate of available work).

is an algebraic ceiling value for the available worktransfer rate.


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Cycles

When the system is working in a cycle we must remember thatits working in a steady state so the terms d/dt are zero and Toof the atmosphere is changing for TH or TL depending of thecycle (Heat-engine cycle, refrigeration cycle, heat-pump cycle).

Exergy content of the heat transfer


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Heat-Engine Cycles


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Refrigeration Cycles


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Heat-Pump Cycles


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First and Second Law Efficiencies


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Mechanisms of entropy generation or

exergy destruction

There are three features that always contribute to theirreversibility

Heat transfer across a finite temperature difference


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Flow with friction


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Mixing

Even when the mixing process involves only two substances, therecan be two types of mixing apparatuses that can be modeled asadiabatic and zero-work:

1. A flow device in which two streams are mixed.2. A nonflow device in which mixing occurs between two batches.

Furthermore two substances that can be mixing can be dissimilarbecause of their respective temperatures, pressures, or chemical

constitutions (chemical potentials). Considering the mixing of twostreams that carry the same substance into a third, mixed stream


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The entropy generation depends on the degree of dissimilaritybetween the inflowing streams, it becomes visible if we placethe analysis in the limit of small changes or marginal mixing


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Entropy Generation Minimization

-Isolated publications in 1950s and 1960s.

-1970 EGM emerges as a self-standing method and field in engineering, withapplications in cryogenics, heat transfer engineering, and solar energyconversion.

-These first developments were reviewed in 1982 where EGM was firstdescribed as a modeling and optimization method and as a special engineering

course. To make an thermodynamics optimization we must first develop an expression

for Sgen, this requires the use of relations between temperatures differencesand head transfer rates, and between pressure differences and mass flow rates.Later we must relate the degree of thermodynamic nonideality of the design tothe physical characteristics of the system, namely to finite dimensions, shapes,

materials, finite speeds, and finite-time intervals of operation. For this theanalyst must rely on heat transfer and fluid mechanics principles, in addition tothermodynamics.


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Exergetic modeling and performance

evaluation of solar water heating

systems for building applications.

Exergy analysis method is employed to detect and to evaluatequantitatively the causes of the thermodynamic imperfection

of the process under consideration. It can, therefore, indicatethe possibilities of thermodynamic improvement of theprocess under consideration, but only an economic analysiscan decide the expediency of a possible improvement.


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Experimental system tested at Ege University, Izmir, Turkey. This SWH system consists ofmainly three parts:

1. The flat plate solar collector (2 m2 aperture area)

2. The circulating pump and

3. The heat exchanger with water storage tank.

Water is circulated through the closed collector loop to a heat exchanger, where its heat istransferred to the potable water. The collector is oriented facing towards south, inclined atan angle equal to 45 at Bornova in Izmir,Turkey (latitude 3828N: longitude 2715E).


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Thermal efficiency

The solar collectors instantaneous thermal efficiency can bedefined as a ratio of the actual useful energy collected ( u)to the solar energy intercepted by the collector gross area(Ascol)


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Instantaneous exergy efficiency

The instantaneous exergy efficiency of the solar collector can be

defined as the ratio of the increased water exergy to the exergy of thesolar radiation. In other words, it is a ratio of the useful exergydelivered to the exergy absorbed by the solar collector.

Where Tsr is the solar radiation temperature and taken to be 6000 K.


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Assumptions.

(a) All processes are steady state and steady flow with negligiblepotential and kinetic energy effects and no chemical ornuclear reactions.

(b) The directions of heat transfer to the system and worktransfer from the system are positive.

(c) The pressure losses in the pipelines connecting thecomponents are ignored, since their lengths are short.

(d) The circulating pump mechanical (hpump,mech) and thecirculating pump motor electrical (hpump,elec) efficiencies

are 82% and 88%, respectively. These values are based on anelectric power of 0.03 kW obtained from the pumpcharacteristic curve.


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Results


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It may be concluded that exergy analysis is a useful tool fordetermining the locations, types and true magnitudes ofenergy losses, and therefore help in the design of more

efficient energy systems. It is also a way to a sustainabledevelopment and reveals whether or not (and by how much)it is possible to improve SWH systems by reducinginefficiencies.


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This work provides a theorical and idealized entropygeneration analysis due to flow and heat transfer in a mixture ofWater + MWCNT + MPCM

Thermal and Flow properties were proposed using theoretical

and experimental models.

Effects of MPCM and MWCNT mass fractions are studied andan optimal proportion with minimal entropy generation ispresented.


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In this case, reduced pump power and increased heattransfer in a heat exchaging process.

Entropy

generation

minimization

Reducing

Irreversibilities=

Improving a

process=


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Increpases apparent heat capacity in a tempeture rage due to latent heatrecovery: Cp (Positive effect)

Increase slightly viscocity: (Negative effect)

Reduce slightlyThermal conductivity: K (Negative effect)
http://www.google.com.ec/url?sa=i&rct=j&q=MPCM+slurry&source=images&cd=&cad=rja&docid=c2qo468f3M2pWM&tbnid=rSBqUir2hzFrtM:&ved=&url=http://www.sciencedirect.com/science/article/pii/S0378778808000583&ei=kC_sUZ2LL4my9gSHkYFw&bvm=bv.49478099,d.eWU&psig=AFQjCNHGcF4jk0i5vVys2qGv6oqvmEfKPQ&ust=1374519569233527http://www.google.com.ec/url?sa=i&rct=j&q=MPCM+slurry&source=images&cd=&cad=rja&docid=c2qo468f3M2pWM&tbnid=rSBqUir2hzFrtM:&ved=&url=http://www.sciencedirect.com/science/article/pii/S0378778808000583&ei=kC_sUZ2LL4my9gSHkYFw&bvm=bv.49478099,d.eWU&psig=AFQjCNHGcF4jk0i5vVys2qGv6oqvmEfKPQ&ust=1374519569233527


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Highly IncreaseThermal conductivity: K (Posistive effect)

Highly Increase viscocity: (Very Negative effect)

Deviates from newtonian behaviour in medium and high concentration onthe base fluid. (Very Negative effect)


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Important Asumptions

Valid Linear blending Models

Newtonian Fluid in all range of fraction

For blends Density it is applied the simple mixingtheory for three componets

For Especific heat the equation is proposed by Mulligan(Mulligan et al. 1996) derivedfrom an energy balance.


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For Conductivity and Viscosity a linear model (Singh et al. 2010) in whichboth the MPCM and NP contribute to the mixtures conductivity and viscosityaccording to their volume fraction

The value of the MPCM and NP coeficients (CKmpcm CKnp CUmpcm CUnp) are chosen such thateffective conductivity or viscosity found in literature mach according to the fraction ofMPCM or NP in the blend.

The second model to predict final blending properties is not shown.

The final properties results are similar in both models


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It consists in the blend flowing through a circular channel under laminar or turbulent regime

under constant heat flux conditions.

Three different diameters (1,5, 10 mm)

two different heat fluxes (1000, 10000 W/m2)

Same Heat Capacity flow for blend and water. (Q* * Cp = constant)

Using Bejans Equation to compute the entropy generation

Friction factor f

Reynolds Re

Prandtl Pr

Nusselt Nu.

Can be obtained due to flowconditions (Laminar or turbulent)and heat transfer conditions.


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The work shows models to predict properties of a blend of MPCM,NP and water.

Further experimental research for themophysical properties isneeded to:

Understand the behavior of this type of fluids, Verify the mathematical models

That implies more accuracy in heat transfer, pressure drop andentropy generation models.

This study highlight the importance of diameter pipe, heat flux andflow regime in irreversibilitys of the process and in the best

composition to minimize entropy.


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Conclusions

For reduced diameters and low heat fluxes (for laminar an turbulentregimes) it is recommended MPCM. The reduced flow minimizefriction losses.

For greater diameters and heat fluxes it is recommended the NPusage especially for laminar flow in which it is important an increasein conductivity.

In turbulent flow, and increased diameters abs heat fluxes it is alwayspreferable water than blend.


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Task

Analyze from a second law perspective, the following waterheating technologies: High efficiency resistance water heater.

LPG water heater.

Solar water heater. Heat pump water heater.

Assume you need to heat water to 45 C, and you are going touse 200 lts/per day. Assume an average outside temperatureof 20 C. Assume in the case of solar 3000 W/m2h

Due date

06 exergy analysis gs

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