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Organic Rankine Cycles Low Temperature Power Cycles

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Organic Rankine Cycles Low Temperature Power Cycles Organic Rankine Cycles Low Temperature Power Cycles Some material by courtesy of Dr. Henrik Öhman, Opcon Energy Systems AB, and Atlas Copco AB Conventional Vapor Cycle Basics The goal is to find a fluid better suited for low-temperature heat sources (such as waste heat, geothermal, etc.) and often also smaller scales, than typical water-steam power applications Components: similar for any vapor power cycle Turbine Heat source Heat absorber Fluid pump Heat rejector Work Heat sink Why Organic? At first, the suggested alternative working fluids were actually organic compounds, thus the ORC denomination. However, recently some more sophisticated inorganic substances have taken over as good working fluids for low-temperature applications. In reality: ”Any-fluid-but-water” Rankine Cycle Many common light organic substances or their isomers could be used as ORC fluids, such as butane/pentane/isopentane, etc. The selection is according to cost and availability, danger of toxicity and explosivity, and of course by pressure-temperature properties. The fluid type has no effect on the max theoretical Carnot potential for power generation, but could allow for better utilization of that potential than what water-steam could achieve at the same conditions. This is especially valid for low-T heat sources and for small scales. The borderline lies around 200-300 oC and ~10 MW capacity, above which conventional steam would always be the better choice. The expander: often an alternative type Many turbine technologies are applicable to ORC applications Expanders commonly used in ORC's P1/P2 25 20 Dynamic 15 Piston Lysholm 10 Scroll 5 0 Development and Performance Analysis of a Two 1 10 100 1000 kWe Cylinder Rolling Piston Expander for Transcritical CO2 System, Yang et al, 2006 Heat exchangers for ORC: very large ORC heat exchangers: operate at low ΔT and with nasty fluids Heat exchanger design is critical for the cycle performance! And highly complex… The costs? Well developed industrial infrastructure for high volume manufacturing of vital parts, which can be shared with ORC systems: • Refrigeration industry • Air-conditioning industry • Heat pump industry • Process gas industry These are all larger than ”conventional steam industry”. Less consolidated market => wider variation and more opportunities than the large-scale steam power industry ORC thermodynamics • Thermodynamic cycle variations to match the specific heat source and heat sink parameters in each particular application. • Thereafter the choice of fluid. B H E G D C F A Thermodynamics 1 T1 HEAT SOURCE Thermal Efficiency Carnot Efficiency Q 1 W T W 1 2 th Q C Q2 1 T1 HEAT SINK But: T2 Local Carnot T2exit T2entry Efficiency 50% T2,local [˚C] A 0 T1exit T1entry 40% 20 40 30% 60 20% 80 100 10% B 0% 300 280 260 240 220 200 180 160 140 120 100 80 60 40 20 0 T1,local [˚C] Thermodynamics 2 Integrated Local Carnot efficiency T dQ 1 2,l dQ c,Il c,l 1 T 1 Q1 Q1 1,l 1 n c,Il c,l (i) n i1 The potentially available work in a low-T power cycle should be evaluated using finite heat source and finite heat sink! H. Öhman, P. Lundqvist / Applied Thermal Engineering 37 (2012) 44-50 Thermodynamics 3 1st law restriction Irreversibilities We Q1 c,Il FoC Real Power Out 2nd law restriction Where FoC th Fraction of Carnot c,Il and th We Q1 Thermal efficiency Thermodynamic potential for a LTPC c,Il It’s important to optimize for highest specific work output and maximum use of available heat source, rather than for the best We local efficiency… T1exit T2exit Utilization U Q1 Q1,ca Q1,ca Q1 Quick assessment of potential • What if we know the source and sink, but not which technology to use? • Apply a simplified evaluation • Practical engineering rather than science! Real performance of LTPCs 1,0 Market 0,9 Quotes 0,8 Technical 0,7 Science Yamada 0,6 Technical Qui 0,5 0,4 Market 0,3 Scientific 0,2 0,1 0,0 0,0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1,0 ORC, Kalina, Hybrids / 0.5kWe - 3MWe / different fluids / T1 from 55 to 300 oC Öhman. H, Lundqvist. P, ”Comparison and analysis of performance using Low Temperature Power Cycles”, Applied Thermal Engineering, 2013 What about those funky fluids? • Many single-molecule fluids are possible • Choices grow to hundreds if mixtures are used • Various mixing ratios are also possible • Zeotropic/Non-zeotropic mixtures Main challenges for the ORC fluids: • Stability over time ? (often unstable at high T) • Safety, toxicity, corrosivity, degradation compatibility ? • Separation or undesired stratification ? • Customer acceptance ? Thermodynamic choice of fluids Comfy vapor pressure and crooked double-phase dome of ORC fluids allow for efficient expansion process but carry less power per unit mass of fluid Good reference for 105 fluids: Namal Joumal ”Comparative studies and analysis of working fluids for Organic Rankine Cycles”, M.Sc. Thesis report, KTH, 2012 Adaptation to new heat sources • Smaller scale technology • Accept large variation in operating conditions • Short cycling ability (intermittent operation depending on the heat source availability) • Generally speaking – no chance for high efficiencies when using low-T heat sources, therefore hard to operate economically Feasibility of ORCs? • Different fluids match different temperatures, equipment and investment cases Operating fluid Global Safety Cost/kg relative Warming Class to NH3 (Sept- Potential 2011) NH3 0 B2 1 R134a 1300 A1 8.4 R236fa 9400 A1 42.9 R245fa 950 B1 25.2 R407c 1650 A1 5.3 R410a 1980 A1 5.9 Misconceptions are common due to simplifications (a bad example below) ??? This comparison is totally misleading unless proper accounting for the heat source type and temperature is given, for instance via the exergetic efficiency (fraction of Carnot) Are ORC’s new? Rumor goes that Carnot himself has been experimenting with organic fluids, 200 years ago… He wanted to find out if the efficiency potential is dependent on the fluid – and he managed to prove that it isn’t – but via his theoretical analysis, without realizing fully the impact it had ORC engines for cars have been suggested and tested in the 1970-ies… The largest heat exchanger would be the condenser, here shown on the roof of the car (left) or below the floor (the picture above) Sample 1: Infinity ORC Turbine • 3.8 x 4 meter front • 10 kWe • ~0.7kW/m3 (nominal) Sample 2: Opcon PowerBox • Opcon Marine (utilizing the waste heat from a ship engine) • 11 x 3.5 x 3 meter • 37 tonnes • 775 kWe • 7kW/m3 (nominal) Large Size, Little Power GE: 3MW BarberNichols: 380kW Pratt&Whitney: 275kW Ormat: 350kW Applications (Waste heat) • ICEs (jacket water, charge air, exhaust gas) Adding ~5-to-8 %-points of efficiency • Process waste heat (Paper industry, Metallurgic, Chemical, Cement industry, etc) Usually around 10% thermal efficiency of ORC • CHP – ORC could even use district heating as energy input Often there is a surplus of district heat that could be converted to power via ORC Applications (Prime heat) • Small-scale distributed CHP Replacing steam cycles in <5MWe local plants • Solar heat powered ORC 1kWe to 5MWe Solar-ORC plants • Geothermal heat powered ORC Up to 15MWe multiple-unit Geo-ORC plants Highly sensitive to the heat sink properties Jacket Cooling Scav air cooling 74.000 tonnes RoRo carrier Figaro 230 meter 8000 cars 13 decks ORC system integration example m/v Figaro 74.000 ton LCTC • 19MW 2-stroke diesel engine • Operating load 2-16 MWshaft • Multiple waste heat sources • Variable waste heat source temperature & flow • Theoretical potential for efficiency improvement by waste heat recovery: 17% • Economic potential for improvement: 6-8% Waste heat recovery from ship engine Fuel savings of 5 – 10% Shaft power Electric power 50% 2-4% 3-6% Steam Rankine 25% 14% ORC Cooling water 6% 4% 1% Sample system layout Saturated RANKINE steam, 8 bar Dump valve G OPB-WST – Genset Boiler onboard heat Dump 8 bar abs loads Condenser Electric power to main switchboard ORC Exhaust gas LT Cooling by seawater economizer G Jacket cooling OPB–ORC equipment Main Engine Scavenging (charge air) cooling Profitability • Value of electric energy: 0.035 – 0.3 Euro/kWh • Typical ORC unit cost : 1000-2000 Euro/kWe • PayOff: Best: 1000 / 0.3 = 3,300 hours Worst: 2000 / 0.035 = 57,000 hours Do’s and Dont’s with ORC Do’s: Dont’s: • Investment case focus • Technology focus • System integration • Cycle type priority • ”Good enough” • Endless Optimization • Standardize • Customize • Maximize the • Maximize the specific Running Hours cycle efficiency Thank you !.
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