Ecosystem exergy theory and applications

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Ecosystem exergy theory and applications

• Bart MUYS
• K.U.Leuven University Belgium
• bart.muys_at_biw.kuleuven.be

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What is exergy?

• energy able to do work, entropy-free energy,
  energy concentration
• a consumable, unlike energy (1st Law of
  Thermodynamics or Conservation Law)
• what every day language calls energy

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Ecosystems and second law

• Life develops towards increased order in apparent
  contradiction with the 2nd Law (evolution long
  term at species level succession shorter term
  at ecosystem level)
• Explanation ecosystems are open systems feeding
  on external exergy sources
• They are like a dam on a river temporarily
  storing water at intermediate elevation and
  feeding a hydro-electric power plant

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Ecosystem exergy concept

• Four key elements
• Ecosystems are open systems that receive external
  exergy fluxes (mainly solar exergy)
• Ecosystems use part of that external exergy to
  increase their internal exergy level in terms of
  biomass, structure and information (order from
  disorder)
• Ecosystems increase and maintain their capability
  to build up order through a process of learning
  and memorizing by genetic selection and transfer
  (order from order)
• Ecosystems with high exergy level are more
  successful in dissipating external exergy flows
  it means that they are better buffered and thus
  have higher stability

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The exergy concept illustrated

• Dead systems
• low exergy level
• Low exergy dissipation
• Living systems
• High exergy level
• High exergy dissipation

Legend
Young or disturbed system
Mature system
Dead system, desert
Living system, ecosystem
Low entropy High exergy
High entropy Low exergy
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Relationship between exergy and ecosystem
succession (modified after E. Odum, 1969) (1/2)
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Relationship between exergy and ecosystem
succession (modified after E. Odum, 1969) (2/2)
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Pitfalls

• Exergy maximization or entropy maximization?
• Goal function
• Information exergy and Shannon entropy

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Pitfall1 entropy or exergy maximization

• It is not in contradiction
• Carnot cycle the more exergy a system includes
  the more (dissipative) work it can perform
• Entropy machine of Kay Dewar for large complex
  systems the state of maximum entropy production
  is the most probable sum of its microscopic
  parts (an attractor). Local exergy maximization
  will contribute to overall maximum entropy
  production.

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Pitfall2 is exergy maximization a goal function?

• No, the process is darwinistic. Yes, the result
  is lamarckian
• In fact evolution of life is a two-step process.
  The first step (mutation) is stochastic, and it
  increases entropy (more information, more chaos).
  In a second step (natural selection as a result
  of competition, cooperation, environmental
  pressure) there is an emergence of order from
  chaos Complex species combinations emerge from
  random species richness.

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Pitfall3 Shannon entropy

• There is an apparent contradiction between the
  information entropy (which increases with
  increasing species diversity) and the
  thermodynamic entropy (which is lower at higher
  species diversity).
• Information entropy is not the same, only an
  analogon of thermodynamic entropy.
• See evolution model order (complexity) emerges
  from a random species pool if the species
  combination makes sense. Information can be
  either entropy or exergy depending on the order
  (predictability) in that information. Information
  is contextual.
• Exergy content of information is much lower than
  the formula proposed by Bendorichio Joergensen.
  The information exergy of BJ can be regarded as
  the probability of emergence of exergy
  dissipating structures.Therefore I propose to
  call it potential exergy.

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Application 1 Role of biodiversity (biodepth)

• Ethics and commerce form poor basis for
  biological conservation
• Biodiversity has clear function in getting and
  keeping ecosystem exergy high complementarity
• selection result of most successful pathways of
  exergy degradation
• insurance for stability (resistance and
  resilience)
• finetuning to maximize exergy dissipation
  (complementarity)

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BIODEPTH experiment

• Strongly significant relationship between number
  of plant species, productivity and surface
  temperature
• Hector et al. (1999), Science
• Bulteel et al. (submitted), Ecology

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BIODEPTH experiment

• Overcooling (i.e. mixtures cool more than the
  coolest monoculture) demonstrate species
  complementarity
• Overcooling is strongest and most permanent with
  intermediate species richness

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Application 2 increasing understanding relation
man/nature
Fossil fuels,nuclear
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Application 2 increasing understanding relation
man/nature

• Ecosystems
• goal function maxbuffer exergy flows by
  maxexergy level
• buffering activity temperature change, nutrient
  loss, water runoff, sediment loss, wind damage
• Exergy storage biomass, DNA
• main exergy source solar exergy
• Memory DNA

• Human society
• goal function idem
• Buffering external threats
• Exergy storage food reserves, houses, bank
  accounts, other comforts
• main exergy sources ecosystems and fossil fuels
• Memory DNA, oral and written information, bits
  and bytes

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Application 2 a better definition of Sustainable
Development

• Bruntland definition is vague and equivocal and
  therefore misused
• Need for operational definition, offering a basis
  for indicator development
• Definition of Sustainable development Increasing
  the exergy level of the human society not
  provoking a significant decrease of ecosystem
  exergy level at all relevant scale levels

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Application 3. Development of a universally
applicable indicator method for land use impact
assessment

• Method Muys and Garcia Quijano (2002) has 17
  quantitative indicators
• Compares exergy level of present land use with
  exergy level of climax system at the same site
• Indicators cover 4 themes (soil, water,
  vegetation structure and biodiversity)

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Application 3 results of a land use impact
comparison
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Application 4 thermal land use impact assessment
Surface Temperature image derived from DAIS
airborne thermal sensor (Land use transect,
Sint-Truiden, Belgium)
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Application 4 thermal ecosystem restoration
status assessment

• Project area degraded highland of northern
  Ethiopia
• Aim revegetation
• Strategy establishment of closed areas near
  ancient woodland remnants

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Application 4 some results

• Ancient woodland remnant (high living biomass,
  complex structure, high SOM and high
  biodiversity) stays the coolest throughout the
  day (?T (7-11am) 7.7C)
• Closed area (medium biomass and plant species
  diversity) heats up quicker (?T (7-11am) 9.2C)
• Grazing land (low biomass, low SOM and low plant
  species diversity heats up fast (?T (7-11am)
  12.5C)

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Application 4 some results

• Closed area with Eucalypts is coolest until mid
  afternoon. Exergy increase by enrichment is
  explained by increase in gene information,
  structure, biomass and transpiration capacity.
• From late afternoon closed area without
  enrichment becomes coolest. Explanation water
  shortage stops evapo-transpiration and
  photosynthesis of Eucalypts.
• Grazing land is hottest. Impact of firewood
  extraction ?T (7-11am) 14.3C in grazing land
  with extraction versus 10.6 in grazing land with
  ban

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Challenges

• Demonstrate exergy concept in microcosms,
  controlled experiments and succession series
• Operationalize thermal indicators for ecosystem
  integrity/complexity
• Analyse relationship between exergy content and
  exergy dissipation capacity
• Quantify exergy fluxes and exergy contents of
  ecosystems