Realizing Ultra-High-Efficiency Engines:

Transcription

1 Realizing Ultra-High-Efficiency Engines: Understanding Limits and Overcoming Limitations Chris F. Edwards Sankaran Ramakrishnan, Matthew Svrcek, Greg Roberts, J.R. Heberle, Paul Mobley, Adelaide Calbry-Muzyka, Rebecca Pass Advanced Energy Systems Laboratory Department of Mechanical Engineering Stanford University

2 First-Law Efficiency (%). Peak Conversion Efficiency of Engines % % at 2030! Time (Years A.D.) Savery, Newcomen (<0.5%) Watt/Boulton Steam Engines Post-Watt Steam Engines Lenoir, Hugon Coal-Gas Engines Otto/Langen Coal-Gas Engines Atkinson, Tangye Coal-Gas Engines Banki Spirits Engine Priestman's Oil Engine Diesel's Oil Engines Automotive SI Engines Truck Diesel Engines Large Bore DI Diesels Steam Turbines Gas Turbine/Steam Turbine Polymer Electrolyte Membrane FC Phosphoric Acid Fuel Cells SOFC/Gas Turbine Turbo-SI,CI, NG, Diesel (48%) Turbo-CAI, Gasoline (~55%) Extreme-Compression (~60%) After three centuries of development, combined-cycle efficiency just exceeds 50%, simple-cycle remains below.

3 Engine Essentials Outside the Box All engines have three essential features they produce work (by definition) they require a resource (1 st Law) they reject entropy (and therefore energy) to their surroundings (2 nd Law) Energy Resource Engine Rejected Entropy (Surroundings) Work

4 Engine Essentials Inside the Box There are only two types of internal function transfers: moving energy around within the box transformations: energy rearrangement between storage modes or transfers Energy Resource Work Rejected Entropy

5 Engine Essentials Transfers and Storage Modes There are only four ways to transfer energy: work (entropy-free transfer of energy) heat (energy transfer due to DT ) matter (internal and external energy) radiation (thermal, non-thermal) Storage can be of only two types: radiation (cavity modes/photon density) matter (sensible, latent, chemical, nuclear, kinetic, gravitational PE, electrical PE, elastic strain PE, etc.) The design space of engines is sufficiently small and well-enumerated that it can be analyzed systematically.

6 The Exergy Limit: Energy Resource Engine Rejected Entropy (Surroundings) Work Obeys 1 st Law Constrained by 2 nd Law Combine 1 st and 2 nd law Max work iff reversible. Fuel Conversion Efficiency potential (maximum first-law efficiency based on LHV) of most fuels is ~100%.

7 An Exergetic View of Engines All engines have four essential features: they produce work (pure exergy) they require an exergetic resource (exergy balance) they destroy exergy (2 nd Law, zero only in reversible limit) they transfer exergy to the environment (zero in limit of all entropy rejected with non-exergetic energy) Exergetic Resource Transfers Transformations Destruction Work (Exergy) Non-exergetic Energy Transfers (Environment) Exergetic Energy Transfer

8 Spanning Exergy to Engines Chemical Resource Restrained Reaction Unrestrained Reaction Electrostatic Work Batch Expansion Batch Expansion Flowing Expansion Flow Work Lorentz Work (MHD) Exergy Classification Architecture Engines Limits are imposed by the resource, environment, and physics governing transfers and transformations. Limitations are introduced by the choice of devices and processes i.e., by the architecture of an engine.

9 Exergy-to-Architecture Example Choose a chemical resource (e.g., nat. gas) from which you wish to extract work by connecting it with a portion of the environment (e.g., the atmosphere). Choose unrestrained reaction (e.g., combustion) to transform the chemical energy to sensible energy, and transform that to work (e.g., via flowing expansion). Choose an architecture that incorporates use of a steady-flow burner for the first transformation and a gas turbine for the second. These two device choices require inclusion of a compressor (pressure difference), that in turn requires an internal transfer of work (turbine to compressor). Exergy Limit Classification Limits Architectural Limitations Architectural Requirements Using Horlock s notation, this architecture might be described as being of the CBT family Compressor, Burner, Turbine. NG C B T T W Air (Atm.) FG

10 Optimal Architecture Optimal in what sense? For us, efficiency. (In exergy terms, minimizing total irreversibility.) But always as a trade-off with specific work. (Think carpet plots or Pareto fronts.) Subject to what constraints? Choices of resource, classification, and environment Device availability (i.e., must actually exist) Device metrics (e.g., polytropic efficiency) Device limitations (e.g., temperature cap)

11 The CBT Family Optimal: CB(TB) n T

12 CB(TB) n T Efficiency vs. PR

13 Directed Evolution of Architectures Specify resource and environment (Exergy) Specify transfers and transformations to be invoked (Classification) and an initial set of devices and system configuration (Architecture) Optimize architecture using a combination of analytical and numerical techniques Selectively introduce new degrees of freedom (Classification) or new devices (Architecture) and re-optimize.

14 The CBTX Family Optimal: CX in B(TB) n TX out

15 Effect of Heat Exchanger Temperature Limit

16 The CBTXI Family Optimal: (CI) m X in B(TB) n TX out

17 Modern Art

18 Take-Home Messages A systematic approach to engine design is possible. It is no longer a matter of inspiration or invention. A clear understanding of the limits from physics for energy transfers and transformations is essential. These lead to viewing engine design as an exercise in exergy mgt. Intermediaries between (abstract) exergy and (concrete) engines, such as Classification and Architecture can be useful tools in understanding engine design. A combination of architecture Optimization and Directed Evolution seems to provide both a systematic and useful path for identifying architectural limitations and thereby providing a path toward ultra-high-efficiency engines.

19 Commercials (Three posters)

20 Efficiency (%) Extreme Compression: Initial Results First law (per LHV), = % first law Combustion data Theoretical efficiency with air losses Low blowby Diesel #2 Losses in air experiments %, 20 C walls Additional losses due to combustion Confident we can demonstrate 60% indicated Speculate 70% is achievable regeneratively Compression Ratio

21 Extreme Compression: New Data Diesel #2 = Have demonstrated 60% indicated Speculate 70% is achievable regeneratively

22 Combustion in Supercritical Water

23 T, K LP Air Oxygen Nitrogen MP HP HX 1 HX 2 LOX Pump High Press. Col. Low Press. Col. Energy Intensity, MJ/kg O SRCCS low-pressure range GOX ASU LOX ASU Oxygen Exit Pressure, Bar AIR ASU Oxygen LP Desal. Regenerator Products Stream NITROGEN MP Pump MP Desal. HP Pumps To SCWO System LP Pump Injection Pump INJECTANT Lift Pump AQUIFER BRINE Slurry Pump COAL Slurry Prep Combustion Products 400 Regenerator Cold Streams MP Desal. & Preheat 350 MP Brine & Vapor LP Desal. & Preheat Products-Specific Enthalpy, kj/kg-products

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25 h-h i (MJ/kg mix ) 1.5 The CBTQ Family T cap 1800 K, 0.43, n 20, PR 250:1 poly CB(TB) n T CB(QB) n (TQ) n T s-s i (kj/kg mix K)

26 Relative Merit of Internal, Forward Heat Transfer

27 Merit of Internal, Backward Heat Transfer Optimal architecture: CQ(QB) n (TQ) n TQ or CQB(TB) n TQ All work must be extracted prior to heat transfer

28 Merit of External, Environmental Heat Transfer Intercooling

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30 Intercooled Attractor

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32 Experimental Apparatus

33 33 Operating Space

34 Combustion Visualization CR = 30:1 CR = 100:1 #2 Diesel, 1 ms injection duration, finishing at TDC

35 Species Concentration (ppm) Soot Signal (-ln(i/i 0 )) Diesel-Style Combustion Isooctane, 35:1 CR NO x (ppm) HC (ppmc 1 )

36 Species Concentration (%) Combustion efficiency (%) Diesel-Style Combustion Isooctane, 35:1 CR CO 2 CO O

37 Soot Signal (-ln(i/i 0 )) Soot Signal (-ln(i/i 0 )) Diesel-Style Combustion Isooctane, 35:1 CR Percent CO Percent of Fuel Carbon in Gas Emissions

38 Species Concentration Combustion efficiency (%) Diesel-Style Combustion Diesel #2, = HC (ppmc 1 ) CO (ppm*10) Diesel, =.48 i-octane, =.48 (interpolated) CR Compression ratio

39 NO x Concentration (ppm) Diesel-Style Combustion Diesel #2, = CR

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42 Spanning Exergy to Engines Chemical Resource Restrained Reaction Unrestrained Reaction Electrostatic Work Batch Expansion Batch Expansion Flowing Expansion Flow Work Lorentz Work (MHD) Exergy Classification Architecture Engines Engine design is an exercise in exergy management. Classification and Architecture can be useful intermediates in bridging from exergy to engines.

43 Chemical Exergy of Some Fuels Fuel Chemical Chem. Exergy DH Reaction* DG Reaction* DS Reaction* Exergy Species+ Formula MJ per fuel MJ per fuel MJ per fuel kj/k per fuel to LHV kmol kg kmol kg kmol kg kmol kg Ratio Methane CH Methanol CH3OH Carbon Monoxide CO Acetylene C2H Ethylene Ethane C2H4 C2H Ethanol Propylene C2H5OH C3H Propane C3H Butadiene C4H i-butene C4H i-butane C4H n-butane n-pentane C4H10 C5H i-pentane C5H Benzene C6H n-heptane C7H i-octane C8H n-octane C8H Jet-A Hydrogen C12H23 H All species taken as ideal gases. Environment taken as: 25 C, 1 bar, 363 ppm CO 2, 2% H 2 O, 20.48% O 2, balance N 2. *Reaction with stoichiometric air at 25 C, 1 bar. All products present as ideal gases, including water. Fuel Conversion Efficiency potential (maximum first-law efficiency based on LHV) of most fuels is ~100%.

44 Two Approaches to Reaction Unrestrained Reactants are initially internally restrained, i.e., frozen in chemical non-equilibrium (e.g. combustion, fuel reforming). Internal restraint is released, allowing reaction to proceed. Reaction stops when equilibrium is achieved or kinetics are so slow as to be negligible (frozen again). Inherently irreversible. Restrained Reactants are initially externally restrained, i.e., in chemical equilibrium (e.g. electrochemistry, solution chemistry). External restraints are changed, allowing reaction to proceed. Never stops; always dynamically balanced. Reversible only in the limit of infinitesimal rate and constrained chemical pathway (chemical reversibility).

45 Restrained vs. Unrestrained Restrained (SOFC) Unrestrained (DI Diesel*) Efficiency declines with load Irreversibility reduced via facile kinetics (reaction and transport) Efficiency improves with load Irreversibility reduced by reaction at extreme states * After Primus, et al. Proceedings of International Symposium on Diagnostics and Modeling of Combustion in Reciprocating Engines, (1985) p

46 Exergy Destruction via Reaction Stoichiometric propane/air mixture modeled as ideal gases. Includes the effects of variable specific heats, reaction, & dissociation.

47 Restrained/Unrestrained Expansion W out 47 W max Wout W max

48 Restrained/Unrestrained Reaction Restrained Reaction Unrestrained Reaction Wout W 48 max Wout W max

49 Implementing Restrained Reaction W X T S lost destroyed 0 gen 2 2 i i A i Sgen d d T T dx if piston A T 1 1 d is small for a given d S gen will be small 49 The rate of change of the restraint must be slow compared to the internal relaxation time of the resource in order to be fully restrained (reversible).

50 Efficiency (per LHV, %) Efficiency Achievable by Compression Otto 1st-Law Limit* 60 Sulzer RT-flex Duratec HE Prius (engine) VW TDI VW TDI HCCI Jumo 205 (1934) FPEC 2010 Our Goal max / 0 (Effective Compression Ratio) * Premixed, stoichiometric, ideal gas i-octane and air, including variable properties, dissociated products, and equilibration during expansion.

51 Efficiency vs. number of stages

52 Efficiency vs. equivalence & pressure ratios