Modelling for Industrial Sustainability

Transcription

1 Modelling for Industrial Sustainability Dr Rick Greenough De Montfort University 19 th January 2015

2 Definition of industrial sustainability... concetualization, design and manufacture of goods and services that meet the needs of the resent generation while not diminishing economic, social and environmental oortunity in the long term. (Jansson et al, 2000)

3 Other relevant oinions (energy only) Our civilization - such as it is - rests on chea and convenient ower. (Ford, 1926) A modern industrial society can be viewed as a comlex machine for degrading high-quality energy into waste heat while extracting the energy needed for creating an enormous catalogue of goods and services (Summers, 1971)... it is energy that drives modern economic systems rather than such systems creating a demand for energy. (Sorrell, 2009)

4 Measures of industrial sustainability What we take

5 Measures of industrial sustainability What we make

6 Measures of industrial sustainability What we waste

7 Industrial sustainability in the UK What do we mean by industry? Factories, banks, universities, hositals, farms? All of the above and more Energy using sectors of UK economy (DUKES, 2013) Iron & steel (1%) Other industries (16%) Domestic sector (29%) Transort (36%) Other final users such as services and farms (12%) Non-energy use (5%)

8 Iron and steel ktoe in 2012

9 Food, drink and tobacco ktoe in 2012

10 Proosed measure of industrial sustainability Engineers talk about energy and material consumtion, yet we know that energy cannot be created or destroyed (1st Law) However, we intuitively feel that with each energy conversion something is being lost What is lost is exergy Similar to material degradation henomena such as corrosion or wear Non-reversible rocesses lead to an increase in disorder or entroy

11 Thermodynamic understanding of exergy Laws of thermodynamics 1. Increase in internal energy of a system = heat sulied to system + work done on the system Energy cannot be created or destroyed In a closed system, total energy remains the same 2. If two initially isolated systems (each in thermodynamic equilibrium with itself) are allowed to interact, the total entroy will increase Heat flows from a hotter body to a colder body Heat is a function of temerature and entroy δq = TdS Broad definition of exergy Available energy, useful energy Energy that is caable of erforming mechanical, chemical or thermal work

12 Availability according to the first law The First Law states that in every cyclic rocess either work is converted into heat or heat is converted into work. In this sense it makes no distinction between work and heat excet to indicate a means of measuring each in terms of equivalent units. Once this technique of measurement is established, work and heat become entirely equivalent for all alications of the First Law. Keenan, 1941

13 Availability according to the second law The Second Law, on the other hand, marks the distinction between these two quantities by stating that heat from a single source whose temerature is uniform cannot be comletely converted into work in any cyclic rocess, whereas work from a single source can always be comletely converted into heat. Keenan, 1941

14 Reference states for exergy calculations By convention, for thermal ower rocesses, T 0 =298.2K and P 0 =101.3 kpa By convention, for building heating, T 0 =273.2K System state System state Exergy is minimum work needed to raise system from reference state to system state Exergy is maximum work obtainable between system and reference state Reference state Reference state

15 Exergy definition Exergy is the amount of work obtainable when some matter is brought to a state of thermodynamic equilibrium with the common comonents of the natural surroundings by means of reversible rocesses, involving interaction only with the above mentioned comonents of nature Szargut et al, 1988

16 Heat without exergy An infinite exanse of warm flat desert and warm air Cannot do work therefore it is no use to us!

17 Exergy - a definition of resources? Energy and materials that exist out of equilibrium with their environment

18 Exergy, renewable and non-renewable energy Wall and Gong (1997)

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20 Exergy analysis in industry Different tyes of exergy can be identified in industrial rocesses: Kinetic and otential exergy are the same as their energy equivalents Physical exergy is the work obtainable by taking a substance through a reversible hysical rocess from temerature and ressure T and to those of the environment T 0 and 0 Chemical exergy is the work obtainable by bringing a substance having the arameters T and to a state of thermodynamic equilibrium with the datum level comonents of the environment Exergy is therefore a kind of measure of the thermodynamic distance of a system from its environment

21 Consequences of exergy analysis for industry Industrial resources should therefore only be discharged into the environment once their exergy has been minimised They have similar temerature and ressure They are chemically inert (note that this is not the same as non-toxicity) Definition of reference environment is critical Relatively easy for hysical exergy (see revious examles) Not so easy for chemical exergy, so standard tables have been created by Szargut et al (1988)

22 The value of exergy analysis to industry Industrial sustainability requires: Careful accounting of resource consumtion Careful accounting of waste emission Exergy analysis combines analysis of mass flows and energy flows in one method Exergy analysis takes account of: First law (energy conservation) and Second law (entroy increase) Identifies clearly any irreversibility and rocess inefficiencies Allows identification of riority areas for efficiency imrovements in terms of: Thermal efficiency Material choices Process choices

23 Quality and thermal exergy (Hermann, 2006)

24 Exergy conversion efficiencies (Ayres and Warr, 2005)

25 Exergy accounting B in Process B out B lost = B in B out B lost It can be shown that: B = H T0 S where: H = C T (at constant ressure) and: S = C T T 0 1 dt T = C ln T T 0 (at constant ressure)

26 Exercise: mixing hot and cold water 1 2 1kg at 100 C 1kg at 0 C For each mass of water, we have: B = C T T T0C ln T 0 3 2kg at 50 C Assuming T 0 =25 C and C = kj/kg C at 100 C C = kj/kg C at 0 C C = kj/kg C at 50 C Calculate lost exergy For water mass 1, we have: B 1 B 1 B 1 = = C C T = 8.1C T C 0 ln T T (75) 298.2C 0 ln

27 Exercise: mixing hot and cold water 1 2 1kg at 100 C 1kg at 0 C For each mass of water, we have: B = C T T T0C ln T 0 3 2kg at 50 C Similarly: B B and: B B = C = 1.1C = 2C Assuming T 0 =25 C and C = kj/kg C at 100 C C = kj/kg C at 0 C C = kj/kg C at 50 C Calculate lost exergy ( 25) 298.2C = 2 C (25) C ln ln 298.2

28 Exercise: mixing hot and cold water 1 2 1kg at 100 C 1kg at 0 C 3 2kg at 50 C Assuming T 0 =25 C and C = kj/kg C at 100 C C = kj/kg C at 0 C C = kj/kg C at 50 C For each mass of water, we have: B = C T T T0C ln T 0 Lost exergy = 7.2 C

29 Grassman diagram Temerature = 100 C Exergy = kj Temerature = 0 C Exergy = 4.63 kj Mixing rocess with heat exchange Temerature = 50 C Exergy = 8.36 kj Lost exergy = kj Similar to a Sankey diagram in as much as widths of arrows reresent sizes of flows

30 Grassman diagram exercise Temerature = 60 C Exergy =? Temerature = 40 C Exergy =? Mixing rocess with heat exchange Temerature = 50 C Exergy =? C = kj/kg C at 60 C C = kj/kg C at 40 C C = kj/kg C at 50 C Lost exergy =? In this diagram, widths of arrows do not reresent sizes of flows That would suggest the answer!

31 Grassman diagram for industrial rocess

32 Good exergy strategy Decrease exergy loss Use high quality energy for high grade uroses only Serve low grade uroses with low quality energy only Use cascading to make otimal use of waste heat flows

33 Energy cascading rimary energy ower lant cascade of waste heat heavy industry storage waste electricity horticulture hotel and catering offices dwellings waste agriculture Institute SUSTAINABLE, of Energy LOW-EX and Sustainable SYSTEM Develoment

34 Comarison of English and Chinese kilns English kiln Chea energy Chea to build Chea to run Relatively inefficient Chinese kiln More exensive energy More exensive to build More labour intensive Relatively efficient Allen, 2011