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International Journal of Hydrogen Energy28(2003)369–375
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EXERGY
In which we reach our prize,exergy,the goal of our odysy and that something-of-value Earth mines from streams of energy.
Earlier we chuckled at the silliness of lofty,sanctimonious exhortations to conrve energy[1].Struck us like exhorting Gibraltar to stand up proudly like a rock.What could be sillier than exhorting people to follow one of NATURE’S most fundamental laws?Yet something emed amiss.Surely we do consume something-of-value when we deliver energy rvices—drive cars, y airplanes,turn on the TV.
Indeed we do.We consume exergy.In fact,exergy is what people usually mean when they say energy.
But knowing that exergy behaves the way we often think energy behaves is not good enough.That alone will not let exergy help us anticipate new technologies or have the joy of eing more clearly h
ow our world works.So we need to dig deeper.And a good place to start is to return to the idea that,“theÿrst step to wisdom is getting things by their right names[2]”.
Exergy was originally called“availability”,a name that emerged from steam engine language.In tho early days,engineers knew that only a portion of the steam’s energy was available for doing work—even if the steam were ud in a perfect steam engine.So during the early part of the20th century,engineers named that fraction of the steam’s energy its “availability”.Made n then.But later people realized there were two problems with“availability”.
高血压有哪些症状表现Theÿrst was that“availability”has a uful,everyday meaning.We shouldn’t encroach upon a word with a uful meaning by giving it another,very di erent meaning—especially when the two meanings often can’t be di erentiated by context.If I say,“the availability of steam is50%”,how will you know whether I mean,“it is available for u50%of the time”or,“50%of its energy content can be converted to work by a perfect steam engine”?Imagine you’re leaning on the garden fence talking to your neighbor,an engineer.Like many neighborly conversations this one ips from topic to topic,but now she’s telling you about the relative availability of electricity,hydrogen,gasoline and steam.You could easily be misled if she means the currency’s exergy,not how you can get it from wires,pipes or the corner store.
To be confud is one thing.To be misled,quite another.When we know we’re confud it isn’t dangerous,becau we can decide to get it right.But to be misled is dangerous.Thinking we’ve got it,we’ve no idea we’ve got it wrong. What if green lights on street corners had two meanings?One:OK to go.The other:Truck coming with failed brakes. Makes a di erence.
The cond problem arrived when people learned that,for some energy sources or currencies,the work delivered could be greater than the energy supplied to do the work.When this happened,the ratio of availability to energy(a ratio we call“energy grade”)became greater than unity.Seemed weird!How could the“availability”of something be greater than 100%?In fact,as we shall e,it turned out some currencies or sources that contained zero energy could still deliver a lot of work.That made the energy grade inÿnity.Bizarre!
For the reasons,particularly theÿrst,I much prefer“exergy”to“availability”.Happily,exergy is catching on almost everywhere.
A Slovenian Professor,Zoran Rant,ÿrst ud“exergy”back in1953.He mined Greek for the bits he needed for his better word and published the reasons for his choice in German.When placed in front of vowels,the Greek“ex”means “out”or,if you like,“out of”.The“erg”part of both en erg y and ex erg y c
ame from the Greek word erg on meaning work.1 So ex ergy means that fraction of the total energy that we can extract—excavate,exci,expel—to deliver as work.2 Now we have it.Exergy reprents the maximum work we can hope to harvest from energy.
1By the time Prof Rant came along,energy had been assigned a preci,scientiÿc deÿnition—having a broader,more inclusive meaning than work.So,in that n,Zoran’s erg u in“exergy”is more loyal to its roots than its u in“energy”.
2Ex-words don’t all come from Greek.More come from Latin,including veral I’ve ud here.Of cour many of what we consider Latin-bad words originated with the Greeks.They were imported to Latin after Roman soldiers conquered Greece and returned with Greek words,wives and mistress.In time,the wives and mistress died.But the words lived.
0360-3199/02/$30.00?2002Published by Elvier Science Ltd on behalf of the International Association for Hydrogen Energy. PII:S0360-3199(02)00040-X
370 D.S.Scott/Interna tiona l Journa l of Hydrogen Energy28(2003)369–375
It was Sadi Carnot who discovered Nature’s law that there is an upper limit to the fraction of heat tha
t can be converted to work by even a perfect engine[3].In tho days,steam engines were the only heat-to-work technology that existed.Yet Nature’s limitation is general.It applies to any heat-to-work technology—coal-ÿred electricity plants,jet engines,nuclear power plants,diel engines and so on.3
The adjective“perfect”is very important.This constraint is not impod by technological imperfections.It has nothing to do with the idea of“currency exchange booths loss”we discusd back in“Energy Currencies”[4].Loss caud by technological imperfections are superimpod on top of this fundamental limitation.
Although this ramiÿcation of Nature’s entropy law wasÿrst ud to design steam engines,and then applied to any heat-to-work engines,the principles govern all energy conversion process—from nuclear power plants to photosynthesis and metabolism.
Energy contains exergy when—and only when—that energy is not in equilibrium with its environment.We considered the concept of environmental equilibrium back in“Smelling Land”[5].But we’ve voyaged some distance since then.So let’s review the idea.
Nature always moves things towards equilibrium.She moves rainwater from your roof to be in equilibrium with water in your yard,and then moves the water in your yard to ultimately be in equilibri
um with water in the oceans.
Or consider a chemical fuel like methane,CH4,the primary component of natural gas.Methane is not in chemical equilibrium with its environment.But when Nature oxidizes methane—or we burn it—the waste products are water and carbon dioxide.And becau water and carbon dioxide are part of the environment,they are in chemical equilibrium with the environment”.4
Exergy is energy that’s out of equilibrium with its environment.
How do we extract exergy from energy?We put cleverly designed technologies between the energy we want to harvest and the rvice we want to deliver.
Think back to our afternoon on a hillside.When we extract exergy from the energy of a river(today often as electricity but historically for sawing timber or grinding wheat)we do so by putting technologies in the path Nature us to pull the river’s water towards its oceanic fate.We u transformer technologies—intervention technologies—like dams,turbines and generators.The higher the water behind the dam,the greater the exergy available for harvesting.After the intervention technologies have extracted the exergy—or at least most of it—the water is relead to continue its voyage hunting the ocean.A couple of things jump out from our hydraulic example.
(1)The amount of exergy depends upon how much the energy deviates from environmental he elevation
淡奶油of water behind the dam relative to the water below the dam).Therefore,to know how much exergy is in some quantity of energy,we need to know more than just the properties of the energy;we must also know the properties of its he elevation of the water below the dam).
(2)Nature’s entropy law guarantees that energy will eventually degrade to environmental equilibrium,no matter what
we do.But left to themlves,equilibration process don’t yield uful5work.So to deliver civilization’s energy rvices we intervene in equilibration process with technologies that can harvest exergy—and its cousin“structure”. Dams,heat engines and fuelcells are all intervention technologies.So too are mitochondria,which dip into our body’s energy ows to power our running,jumping and thinking.And photosynthesis technologies,which intervene in sunlight’s incoming trajectory,guiding its exergy towards powering life on Earth.
Not all cleverly designed technologies were designed by people.6
We know hydraulic potential energy is one-way Nature stores exergy.But we should have a more complete list of Nature’s exergy storage tricks.Recalling that engineering is a great career for the logic-gifted but memory-challenged,it shouldn’t surpri that the number of tricks are few.
3Indeed,it’s di cult toÿnd power generation technologies that aren’t heat engines.Electric motors,hydraulic turbines,batteries and fuelcells are a few examples,but there aren’t many others.
4While the CO2is always in species equilibrium,it might not be in concentration equilibrium.To be fully in equilibrium with the environment,material must not only be one of the environmental reference materials it must also exist in environmental concentrations. 5I’ve ud the adjective“uful”to mean uful to us.Unconstrained rivers can certainly do work—the work of moving riverbed boulders or scouring out the landscape.But that’s not uful to us.Indeed,scouring may cau us problems,especially if we’ve built homes by the riverbank.
6Of the many books on how evolution“designs”living systems,Richard Dawkins’The Blind Watchmaker,Longman1986,paperback by Penguin1988,is one of the best.
D.S.Scott/Interna tiona l Journa l of Hydrogen Energy28(2003)369–375371过新年日记
Fig.1.Cylinder and atmospheric pressure equal.
Fig.2.Cylinder pressure above atmospheric pressure.
In“Conrvation,Confusion and Language”[1]we found the main ways Nature stores energy are kinetic,potential, electromagnetic,chemical and thermal.Electromagnetic storage is less important for civilization’s energy system.7Since exergy is simply energy not in equilibrium with its environment,we’re left with three key modes of exergy storage:•Mechanical non-equilibrium(gravitational potential,kinetic and pressure8di erences),
•Thermal non-equilibrium(temperature di erences),and
•Chemical(including electrochemical)non-equilibrium.
Mechanical non-equilibrium often results from pressure di erences,while thermal non-equilibrium always results from temperature di erences.Becau both temperature and pressure are manifestations of internal molecular motion(internal energy),it’s natural to name the combination thermomechanical exergy.But let’s keep them parate for now.
We’ll start with mechanical exergy.We’ve already talked a lot about hydraulic potential,so we’ll try something di erent. Let’s return to the piston=cylinder technology weÿrst considered in“The Trouble
太阳系英语with Micro states”[6].We’ll designate atmospheric pressure as p o,and the pressure inside cylinder as p c.To ensure we consider only mechanical non-equilibrium e ects,we’ll imagine the piston=cylinder is thermally insulated(to preclude e ects of thermal non-equilibrium),and doesn’t leak material(to preclude e ects of chemical non-equilibrium).We’re left with pressure non-equilibrium as the only possible source of exergy.
We’ll consider the piston=cylinder in three di erent situations.In Fig.1the gas within the piston=cylinder is at atmo-spheric pressure.In Fig.2it’s at a higher pressure.In Fig.3it’s lower.
Starting with Fig.1,the pressure inside the piston-cylinder is identical to the pressure outside,that is p c=p o.So although the gas within the cylinder contains energy,it contains zero exergy becau the inside and outside pressures are
7Energy is often moved electromagnetically—for example through electricity networks—but electromagnetic energy storage is restricted to capacitors and(indirectly via electrochemistry)batteries.Compared with other storage forms,batteries and capacitors hold trivial quantities of exergy.
8Positive and negative“pressure”exergy exists in solids as well as gas and liquids—for example a
s tension or compression in a spring.
372 D.S.Scott/Interna tiona l Journa l of Hydrogen Energy28(2003)369–375
Fig.3.Cylinder pressure zero.
in equilibrium.The piston will just sit there,impotent,unable to deliver any work,unable to extract exergy becau there isn’t any to extract.
Fig.2shows the ca when the inside pressure is greater than the outside pressure,that is p c¿p o.Becau the piston-cylinder we know best lives inside our car,a higher pressure inside the cylinder than outside is what we normally expect.Now there is an inside-to-outside pressure di erential that can push the piston out,to turn a crankshaft or lift a weight.At least it can push the piston out until the inside pressure drops(as it must)to equal the outside pressure,when p c=p o.Then the work stops.This reinforces two features of all forms of exergy:
•The exergy existed becau,initially,the gas in the cylinder was not in equilibrium with the environment.
•After all the exergy has been harvested,the gas in the piston=cylinder may still contain energy.
The real surpri comes with Fig.3.Now the cylinder contains a perfect vacuum.All the gas is out.Nothing is left, which means there is zero energy within the cylinder.But lo!There is a di erential pressure across the piston.Our equation for pressures is now,p o¿p c=0.So this time the atmosphere can push the piston in,until it reaches the end of its stroke or hits the cylinder head.This time three things are interesting:
•Although there was initially no energy in the cylinder and none at the end,we were still able to harve
st exergy.•Exergy existed becau,on Earth’s surface,a vacuum is not in environmental equilibrium.
•Although the energy within the cylinder was zero,the exergy was substantial.This is a ca where the energy grade (exergy=energy ratio)is inÿnity.
贝德福德
When introducing the idea of exergy,I often ask my engineering students,“How much exergy is contained within a piston=cylinder when it’sÿlled with air at atmospheric pressure?”When you throw a class any question,most stu-dents are silent—most fear being called to be the designated hitter.Some suspect a trick question.One or two might step-up-to-the-plate and venture an answer,to this question it’s usually“zero”.
Of cour the proper answer is not an answer.Rather it’s another question:“Where is this cylinder of gas located?”If it’s in our backyard the exergy is zero.
But if it’s on the moon the air in the piston is not in equilibrium with the environment on the moon’s surface.On the moon,there is no atmosphere to prevent the piston from moving outwards doing work.So the piston-cylinder will contain quite a bit of exergy.
Now,imagine the piston-cylinder is immerd within the dark depths of ocean.The immen surround
ing pressure will create an equally immen di erential pressure across the piston—and we can harvest a corresponding amount of exergy as the a pushes the piston in.In the ocean’s bament the cylinder not only contains exergy,it contains a lot more exergy than energy.
This leads to an important subtlety.While exergy exists becau an energy medium is out of environmental equilibrium, that doesn’t require that the work come from the medium itlf.In some cas,the energy comes from the environment. When our piston-cylinder sits in an ocean trench,it is the kilometers-deep ocean,not the air in the cylinder,that does the work.Yet the ocean couldn’t have done that work if the cylinder had not been sitting on the bottom.The ocean did the work.But the exergy in the piston allowed the work to be done.
It may have emed strange that a medium can contain more exergy than energy.But at least now it’s not mysterious.
Next we’ll carry our understanding of mechanical exergy over to thermal exergy.
Just as mechanical exergy results from mechanical non-equilibrium,thermal exergy results from thermal non-equilibrium. An energy source or currency contains thermal exergy,if its temperature di ers from the environmental temperature. Normally we expect it to be hotter.Like steam is hotter.But t
hermal exergy will also exists if it’s colder.Liquid hydrogen(LH2)contains thermal exergy becau it’s colder than the environment.
D.S.Scott/Interna tiona l Journa l of Hydrogen Energy28(2003)369–375373 Once again—now it’s almost tedious—“how much”thermal exergy is t by“how much”hotter or colder.9So to know the thermal exergy content,we need to know the temperature of both the environment and the energy source or currency.
When we u an absolute temperature scale(a scale tting zero at absolute zero,like the Kelvin or Rankine scales), quantifying thermal exergy is eerie in simplicity.10What could be simpler than that the total thermal energy of material be proportional to its temperature,T?What could be simpler than to have the thermal exergy be proportional the di erence between material temperature,T,and the environmental temperature,T o?
Now since(T−T o)is proportional to“how much”exergy the thermal energy contains and T is proportional to“how much”energy it contains,the energy grade(the ratio of exergy to energy)is:
Thermal energy grade=|(T−T o)=T|:
生腰果怎么做好吃
The symbol“||”means“the absolute value of”—and it’s needed to take care of the situation when T is below T o,becau energy grade can be zero or positive but never negative.
We harvest thermal exergy from heat sources—like coal,gasoline or uranium—by placing heat engines between the thermal source and the environment.A heat engine takes heat from burning(orÿssioning)the fuel,converts less than |(T−T o)=T|of it into work,and dumps the rest into the environment as waste heat.
淘宝虚假交易It’s happening all about us,all the time—ever since James Watt walked upon the Green of Glasgow,thinking upon the engine.
Now chemical exergy.
We’ve talked about how fuels like methane,CH4,contain exergy becau the fuel is not in equilibrium with its environment.To determine the chemical exergy of a fuel—or,for that matter,of any material—we need to know the chemical composition of both the material and the environment.Determining the chemical environment is tougher than identifying the thermal environment,or the pressure environment.That’s becau,to deÿne the chemical environment we must identify a ries of compounds that,together,contain all possible chemical elements and also exist in the environment at
their lowest possible energy levels.Usually we take the atmosphere as the reference chemical environment. Moreover,it’s not enough to merely identify the compounds.We must also know their environmental concentrations. For example,oxygen is included in the Earth’s reference environment.But pure oxygen is not in equilibrium with our atmospheric environment and so it contains exergy.So chemical exergy has two components.We’ll call them“species”and“concentration”exergy.
Chemical exergy leads to a kind of upside-down insight.
We already know exergy is the maximum work that can be delivered as we bring a commodity into environmental equilibrium.But the inver is this:If we start with a commodity that’s in environmental equilibrium,exergy tells us the minimum work required to extract the material from the environment so to have it in its concentrated,pure form. Take pure oxygen,which we often manufacture by parating it from air.The exergy of pure O2reprents the minimum work that must be put into an oxygen paration machine.So by comparing the actual work of oxygen paration with the minimum possible work deÿned by exergy,we know the real e ciency of the machine.Helps block manufacturers from pulling the wool over our eyes.
The same idea will give us the minimum energy needed to harvest iron from iron ore,or clean water from dirty.Uful design targets
Let’s zoom to potable drinking water.
It’s a clo-run-thing,whether climate volatility or the scarcity of clean fresh water will turn out to be the environmental issue of the21st century.My impression is,throughout most of the developed world climate disruption will overwhelm that curious race towards infamy.Still a dearth of fresh water will be very rious in large regions of the developing world—and perhaps in California.Increasingly,a big part of our energy rvices will be providing fresh water—or,as people in countries like Germany and the Netherlands say,“sweet”water.
The exergy of clean fresh water(relative to a a water environment)reprents the minimum work needed to harvest potable water from the oceans.Exergy can also tell us the minimum work required to clean up polluted water—or to harvest aluminium from bauxite.
9In this article,we repeatedly describe the same principle.But that’s engineering.If we work with fundamental principles,the few principles do recur repeatedly.It’s just a matter of applying the principles to di erent circumstances.In this ca,we’re applying the same principles of exergy to the d
i erent forms of exergy storage.Are you becoming persuaded that engineering is a great career for the logic gifted but memory challenged?
10We can’t u arbitrarily normalized scales,like Fahrenheit or Celsius.
