Design and evaluation through simulation and experimental apparatus of a small scale waste heat recovery system

Lotun, Devprakash (2001-12)

Thesis (MScEng)--Stellenbosch University, 2001.

Thesis

ENGLISH ABSTRACT: Realisation of the depletable nature of fossil fuel has increased the need for its optimal use. Increasing global pressure to reduce the emission of greenhouse gases and other harmful gases that affect the chemical cycles or destroy the greenhouse gases in the tropospheric ozone, has attracted a increased worldwide concern. Waste heat recovery devices have been around for more than 50 years and researches and scientists have been very much involved in identifying the correct type of systems to meet the requireme?ts of industries and mankind more efficiently. Waste heat can be identified in the form of unburned but combustible fuel, sensible heat discharges in drain water, and latent and sensible heat discharge in exhaust gases. In this project the feasibility of a small scale waste heat recovery system has been investigated. Sets of preliminary investigations were performed to evaluate the amount of waste heat that can be extracted from the exhaust gases of typical diesel powered truck engines. A waste heat recovery unit was designed, implemented and evaluated through simulation and experimental investigations. Preliminary calculations were performed usmg the readings presented by Koorts (1998) for a typical 6-litre diesel engine. The calculations showed that it is possible to extract about 77kW of waste heat from the exhaust gases from such an engine. A simple Rankine cycle was then investigated to be operated on the waste heat recovered. The optimal parameters for such a Rankine cycle was determined using a spreadsheet program and was found to be an optimal pressure of 800kPa with a temperature of 227.2°C and a water mass flow rate of 0.0015kgls as the working fluid. For such a Rankine cycle, based on the efficiencies of commercially available pumps, turbines and heat exchangers it was found that it is possible to extract 2782kW of power per unit mass flow rate of water. The next stage of the project was designing and implementing an exhaust gas pipe network from the engine test cells at the Centre for Automotive Engineering (CAE) located on the ground floor to the Energy Systems Laboratory (ESL) at the first floor. This pipe network was equipped with a valve system that can be operated from the ESL and allows the selection of the route of the exhaust gases and two bellows to compensate for thermal expansion. A continuous combustion unit was also linked to the exhaust gas supply pipes as an alternative source of exhaust gases. The waste heat exchanger designed and selected was purchased and linked into the exhaust gas stream after calibration tests were carried out on the same in the wind tunnel. The water supply and a steam separator were then connected to the waste heat exchanger. In the final experimental stage of the project, two sets of tests were carried out. The first set of tests was performed using exhaust gases from the continuous combustion unit and the second using exhaust gases from the internal combustion engines in CAE. Superheated steam was obtained in both cases indicating the possibility of operating a turbine with the dry steam generated. With exhaust gases originating from the continuous combustion unit, an air fuel ratio of9.14:1 was used and exhaust gases at a temperature of 540°C were obtained with an air inflow of 1400kglh and a fuel consumption rate of7.11 kg/h. The exhaust gases degraded to 360°C at the waste heat recovery inlet due to losses through the bare pipes. 11.12kW of energy was extracted from the exhaust gases to the water stream with an efficiency of 98%. With the exhaust gases from the 10-litre diesel internal combustion engine, an exhaust gas flow rate of O.22kgls was used and with a heat transfer efficiency of 89%, 18.5kW of power was extracted at the waste heat recovery unit. This represents a 4.9% of the thermal content of the fuel used. A rate of energy production balance on the internal combustion engine showed that 34% is lost in exhaust gases and 29% in coolant and other losses while only 37% is used produced as shaft power. The results obtained therefore show that there is ample room for further investigation for the use afwaste heat in exhaust gases of typical diesel engines. It can therefore be concluded that the aims of the project that were to set up a testing facility and an exhaust gas pipe network and evaluation of a small scale waste heat recovery apparatus were achieved. The tests performed can still be optimised with more waste heat removal from the exhaust gases of typical diesel truck engines and hence better recovery of waste heat and a reduction of fuel consumption.

AFRIKAANSE OPSOMMING: Met die besef van die kwynende beskikbaarheid van fosielbrandstof het die behoefte vir die optimale benutting van die brandstof toegeneem. Toenemende globale druk om die emissies van groenhuis gasse en ander gevaarlike gasse wat chemiese siklusse beïnvloed in die troposfeer te verrniner, geniet wêreldwye aandag. Oorskotenergie-toestelle is alreeds beskikbaar die afgelope 50 jaar en navorsers en wetenskaplikes was tot op hede betrokke met die identifisering van die korrekte tipe sisteme om meer effektief aan die industrie en samelewing se behoeftes te voldoen. Oorskotenergie bestaan uit onder andere onverbrande maar brandbare brandstof, voelbare warmte in dreinwater, en latente en voelbare warmte in uitlaatgasse. In hierdie projek word die lewensvatbaarheid van 'n kleinskaal oorskotenergie herwinningsisteem ondersoek. Voorlopige ondersoeke was gedoen om die hoeveelheid oorskotenergie te bepaal wat herwin kan word uit die uitlaatgasse van 'n tipiese 6 liter vragmotor dieselenjin. 'n oorskotenergie herwinningseenheid was ontwerp, geïmplimenteer en ge-evalueer deur similasies en eksperimentele ondersoeke. Voorlopige berekeninge was uitgevoer op data wat deur Koorts (1998) saamgestel is vir 'n tipiese vragmotor dieselenjin. Die berekeninge toon dat dit moontlik is om ongeveer 77kW oorskotenergie van die uitlaatgasse van so enjin te onttrek. Die moontlikheid was toe ondersoek om die herwinne energie te gebruik om 'n eenvoudige Rankine siklus aan te dryf. Die optimale parameters vir die Rankine siklus was bereken deur van 'n sigblad program gebruik te maak en dit was gevind dat die optimale druk is 800kPa, die optimale temperatuur is 227.2°C teen 'n water massa vloeitempo van 0.0015kg/s. Vir so 'n Rankine siklus, gebaseer op die effektiwiteit van kommersiële beskikbare pompe, turbines en warmteruilers, was dit gevind dat dit moontlik is om 2782kW drywing per eenheidsmassa vloeitempo van water, te onttrek. Die volgende stadium van die projek was die ontwerp en implimentering van 'n uitlaatgas pypnetwerk vanaf die toetsselle van die Centre for Automotive Engineering (CAE) op die grondvloer na die Energy Systems Laboratory (ESL) op die eerste vloer. Die pypnetwerk was toegerus gewees met 'n kleptstelsel wat vanaf ESL bedryf kan word en wat dit moontlik maak om die roete van die uitlaatgasse te beheer. Twee samedrukbare koppelstukke was ook ingesluit in die lang reguit pypseksie om vir termiese uitsetting te kompenseer. 'n Aaneenlopende verbrandingseenheid was ook gekoppel met die uitlaatgasse toevoerpype as 'n alternatiewe bron van uitlaatgasse. Die oorskotenergie warmteruiier wat ontwerp en geselekteer was, was aangekoop en opgekoppel met die uitlaatgas-stroom nadat kalibrasie toetse op die warmteruiier gedoen was in 'n windtonnel. Die watertoevoer en 'n stoomskeier was gekoppel aan die oorskotenergie warmteruiler. Twee toetse was uitgevoer in die finale eksperimentele stadium van die projek. Die eerste stel toetse was uitgevoer deur gebruik te maak van die uitlaatgasse van die aaneenlopende verbrandingseenheid en met die tweede toets is van die uitlaatgasse van die interne verbrandingsenjins van CAE gebruik gemaak. Oorverhitte stoom was verkry in beide gevalle en wys dus dat daar 'n moontlikheid is om 'n turbine met droë stoom aan te dryf. 'n Lug tot brandstof verhouding van 9.14 : 1 was gebruik gewees in die aaneenlopende verbrandingseenheid om uitlaatgasse te verskaf teen 540°C. Die massavloeitempo van die lug was 1400kg/h en die brandstof 7.11kg/h. Die uitlaatgasse se temperatuur het afgeneem tot 360°C tot voor die oorskotenergie herwinningseenheid as gevolg van hitteverliese vanaf die ongeïsoleerde pypnetwerk. 11.12kW energy was onttrek vanaf die uitlaatgasse en oorgedra aan die waterstroom met 'n effektiwiteit van 98%. Die 10 liter diesel interne verbrandingsenjin het uitlaatgas gelewer met 'n massa vloeitempo van O.22kg/s. 18.5kW energie was herwin gewees met 'n effektiwiteit van 89%. Dit verteenwoording 4.9% van die termiese inhoud van die brandstof gebruik. 'n Energie balans op die interne verbrandingsenjin het getoon dat 34% energie gaan verlore in die uitlaatgasse, 29% word aan die verkoelingsmiddeloorgedra en 37% is bruikbare meganiese drywing. Die resultate wat verkry is, wys daarop dat daar nog groot ruimte is vir verdere ondersoeke in die gebruik van oorskotenergie in uitlaatgasse van tipiese vragmotor dieselenjins. Die gevolgtrekking kan dus gemaak word dat die doelwitte van die projek naamlik die opstel van 'n toetsfasiliteit, installering van 'n uitlaatgasse pypnetwerk en die toets van a kleinskaalse oorskotenergie herwinningseenheid, bereik was. Die toetse wat uitgevoer was kan nog ge-optimeer word om meer energie te herwin vanaf die uitlaatgasse van 'n tipiese vragmotor dieselenjin om sodoende beter brandstofverbruik te bewerkstellig.

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