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Offshore Rankine Cycles Offshore Rankine Cycles Jo Brandsar Master of Energy and Environmental Engineering Submission date: June 2012 Supervisor: Trygve Magne Eikevik, EPT Co-supervisor: Armin Hafner, SINTEF Energy Research Norwegian University of Science and Technology Department of Energy and Process Engineering Abstract This paper summarizes my master thesis during the spring semester 2012 at the Institute of Energy and Process Engineering, NTNU, Norway. The project was given to me as a collaboration with an ongoing project at SINTEF Energy Re- search. The focus represent electricity production offshore by applying a Rankine cycle with surplus heat as the energy source. Investigations concern foremost a comparison between steam and organic cycles and the effect the waste heat recov- ery unit (WHRU) has on the cycle as a whole. I would like to thank; my supervisors, Prof. Trygve M. Eikevik, Department of Energy and Process Engineering, NTNU, Armin Hafner and Daniel Rohde, SIN- TEF Energy, for technical guidance; my fellow inmates at room B432 for moral support and Atle Brandsar for final proofreading. I Sammendrag Tittelen p˚adenne oppgaven - "Offshore Rankine Cycles" - er svært generell og dekker et stort spekter av tekniske felt, f.eks. termodynamiske sykluser (Rankine, ORC, Bray- ton, Kalina, etc.), mekanisk utstyr (gass-/dampturbin, varmevekslere og ekstra utstyr) og sikkerhetsaspekter (brennbar og/eller giftig væske, høye temperaturer og trykk) for ˚anevne de viktigste. Avhandlingen forsøker ˚agi en kortfattet oversikt over alle kri- tiske punkter og alternativer, og over implementering av en varmegjenvinningsprosess p˚a offshore-anlegg, selv om fokus har vært p˚ato mer spesifiserte tilfeller, nemlig: Sammenligning av en dampsyklus vs. en organisk syklus (ORC) ved høye varmeg- • jenvinningstemperaturer. Studie av varmevekslerparametere med hensyn til den helhetlige prosessen. • Disse to punktene er studert sammen med en analyse av et modulært ekspanderoppsett kontra en enkelt-ekspander for ˚ase om bedre effekt kan oppn˚aesfor et modulært oppsett med hensyn til frekvente variasjoner i gassturbinbelastningen. For ˚asammenligne en dampsyklus med en organisk syklus s˚am˚attedet gjøres en analyse av hvilke arbeidsmedium som kunne brukes. Toluen ble valgt p˚agrunn av at det er en "vanlig" væske med kjente egenskaper og den ble ansett for˚avære et godt alternativ for varmegjenvinning ved høye temperaturer, b˚adeved sub-kritisk og superkritisk operasjon. Siden vann i dette tilfellet er begrenset til sub-kritiske trykk s˚able en CO2-syklus brukt i sammenligning til den superkritiske toluensyklusen. Hovedfokuset for sammenligningen var eksergitapet i varmeoverføringen og total effekt levert av syklusen. Studien av varmevekslerparametere ble utført med en s˚akallt "printed circuit"-varmeveksler (PCHE) som eksempel. Studien av varmevekslerparameterene p˚aden helhetlige pros- essen har nære forbindelser til størrelsen p˚avarmeveksleren, siden dette er en viktig pa- rameter med hensyn til offshore-drift p˚agrunn av kostbart "fotavtrykk". Prosessen sin avhengighet av varmeveksleren er hovedsakelig gjennom varmeoverføringen, eller varme- belastningen p˚aførtprosessen. Varmebelastningen er gitt av varmevekslerens evne til ˚a redusere temperaturen p˚aavgassene. Denne evnen er avhengig av de to væskene som er involvert og geometrien til varmeveksleren. Mens valget av arbeidsmedium og "pinch points" fastsetter hvor mye varmeenergi som overføres, vil den resterende analysen hvile p˚avarmeovergangskoeffisienten (UA) som skal balansere varmebelastningen. N˚arfluide- genskapene er fastsatt s˚aavhenger UA-verdien p˚avarmevekslergeometrien, og videre vari- asjon av disse parameterene gir oss størrelsen p˚avarmeveksleren. N˚aret arbeidsmedium er valgt p˚aden kalde siden av varmeveksleren s˚akan varmevekslervolumet optimaliseres ved en gitt varmebelastning. En VBA-makro er laget der ekspanderparametere (effekt, og virkningsgrad vs volum- strøm) kan implementeres til ˚aberegne effekten av to ekspandere i et modulært oppsett i forhold til en enkel ekspander brukt som referanse. II Summary The title of the thesis - ”Offshore Rankine Cycles" - is very general and cover a large range of engineering fields, e.g. thermodynamic cycles (Rankine, ORC, Brayton, Kalina, etc.), mechanical equipment (gas/steam turbine, heat exchangers and additional equipment) and safety concerns (flammable and/or toxic fluids, high temperature and pressures), to name the most important. The thesis try to give a brief overview of all critical points and alternatives, concerning employment of a waste heat recovery machine on offshore facilities, although focus has been on three more specified cases, namely: Comparison of a steam cycle vs. an organic Rankine cycle for high temperature • operating conditions. Study of heat exchanger parameters on total cycle performance. • Investigation of a modular expander setup versus a single expander. • To compare a steam cycle to an organic cycle, a choice of working fluid for the organic cycle had to be made. After some investigation, toluene was chosen as it is a "common" fluid with known properties and was found to be a viable option for high temperature heat sources, both for subcritical and supercritical operation. Due to water being constricted to subcritical operation a CO2 cycle was implemented as a comparison to the supercritical toluene cycle. The main focus of the comparison was exergy losses during heat transfer and power output. The heat exchanger parameter study was conducted with a printed circuit heat ex- changer as an example. The study of overall cycle performance has close connections to the heat exchanger size, since it is an important parameter concerning offshore employ- ment due to costly "footprint". The cycle's dependency on the heat exchanger is mainly by the heat transfer rate, or heat load, which the heat exchanger applies to the cycle. The heat transfer rate is given by the heat exchanger`s ability to reduce the temperature of the exhaust gases. This ability depends on the two fluids involved and the geometry of the heat exchanger. While the choice in working fluid and pinch points sets the amount of heat transferred, the remaining analysis rest on the overall heat transfer coefficient (UA) to balance the heat load. When fluid properties are determined, the UA - value is again dependent on heat exchanger geometry and further variation of these parameters will in turn reveal the size of the heat exchanger. When imposing a working fluid to the cold side of the heat exchanger an optimization in heat exchanger volume could be found at specified heat load. A VBA macro has been made where expander parameters (rated power and efficiency vs. volumetric flow rate values) could be used as inputs to calculate the power output of two expanders in a modular setup relative to a single expander as reference. III Limitations Calculations conducted on the working fluid side of the WHRU are done with the • use of constant properties due to the complicated physics involving evaporation of a fluid (both sub- and supercritical) in a heat exchanger. Analysis concerning the condenser was neglected due to the assumed impact on the • cycle, both in power and size, was small relative to the importance of the WHRU. This, in addition to the aforementioned use of constant properties (low precision) when evaluating the heat transfer on the working fluid side of the heat exchanger, resulted in less focus on the condenser. Investigations on a cycle with OTSG as evaporator was not performed. • Cycle comparison is limited to a single pressure, i.e. it does not concern the effect • e.g. reheating has on the cycle. Reliable information regarding expander efficiencies and gas turbine load variation • was difficult to obtain. Expander manufacturers do not easily give away classified material and information from the literature are rather sparse. Due to the limited information gathered, the focus of the expander module calculations were of creating a functional program, were reliable inputs could be implemented when collected. IV Contents Sammendrag II Summary III List of figures IX List of tables X Nomenclature XII 1 Background 1 1.1 EFFORT . .1 1.2 Energy sources . .2 1.2.1 Gas turbine exhaust heat . .2 1.2.2 Gas compressor intercooling/aftercooling . .3 1.2.3 Internal combustion engine exhaust heat . .3 2 Theory 4 2.1 1st Law of Thermodynamics . .4 2.1.1 Open system . .4 2.1.2 1st law efficiency . .5 2.2 2nd Law of Thermodynamics . .5 2.2.1 Entropy generation . .6 2.2.2 Control volume . .6 2.2.3 Irreversibilities . .6 2.3 Exergy . .7 2.3.1 2nd law efficiency for heat cycles . .7 2.3.2 Flow exergy . .8 2.4 Rankine Cycle . .8 2.4.1 Principle . .8 2.4.2 1st and 2nd law applied on component level . .9 2.4.3 Thermal efficiency . 10 2.4.4 Carnot cycle . 11 2.4.5 Trilateral cycle . 11 2.4.6 Additions/Improvements . 12 2.4.7 Supercritical Rankine cycle . 14 2.4.8 Kalina cycle . 15 2.5 ORC working fluid . 15 2.6 Machinery . 16 2.6.1 Pump . 16 2.6.2 Expander devices . 17 2.6.3 Turboexpander . 17 2.6.4 Number of expander stages . 18 2.6.5 Other expander solutions . 18 2.6.6 Expander selection . 20 V 2.7 Gas turbine . 21 2.7.1 Brayton cycle . 21 2.7.2 Brayton cycle efficiency . 21 2.7.3 Variable load strategies for gas turbines . 22 2.7.4 Water-augmented gas turbine . 22 2.8 Heat transfer . 23 2.8.1 Convective heat transfer . 23 2.8.2 Optimal division of heat transfer conductance between WHRU and condenser . 24 2.8.3 Internal irreversibilities, heat transfer vs friction in heat exchanger 24 3 Cycle comparison and heat exchanger parameter study 25 3.1 Comparison criteria and important parameters for evaluation . 26 3.2 Physical system . 27 3.2.1 Heat recovery steam generator - HRSG . 27 3.2.2 Deaerator . 30 3.2.3 Pinch point and approach temperature . 30 3.2.4 Summary HRSG . 31 3.2.5 Heat exchangers . 31 3.2.6 Plate heat exchangers (PHE) .
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