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

[摘要] ENGLISH ABSTRACT: Realisation of the depletable nature of fossil fuel has increased the need for its optimaluse. Increasing global pressure to reduce the emission of greenhouse gases and otherharmful gases that affect the chemical cycles or destroy the greenhouse gases in thetropospheric ozone, has attracted a increased worldwide concern. Waste heat recoverydevices have been around for more than 50 years and researches and scientists havebeen very much involved in identifying the correct type of systems to meet therequireme?ts of industries and mankind more efficiently. Waste heat can be identifiedin 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 beeninvestigated. Sets of preliminary investigations were performed to evaluate theamount of waste heat that can be extracted from the exhaust gases of typical dieselpowered truck engines. A waste heat recovery unit was designed, implemented andevaluated through simulation and experimental investigations.Preliminary calculations were performed usmg the readings presented byKoorts (1998) for a typical 6-litre diesel engine. The calculations showed that it ispossible to extract about 77kW of waste heat from the exhaust gases from such anengine. A simple Rankine cycle was then investigated to be operated on the wasteheat recovered. The optimal parameters for such a Rankine cycle was determinedusing a spreadsheet program and was found to be an optimal pressure of 800kPa witha temperature of 227.2°C and a water mass flow rate of 0.0015kgls as the workingfluid. For such a Rankine cycle, based on the efficiencies of commercially availablepumps, turbines and heat exchangers it was found that it is possible to extract 2782kWof power per unit mass flow rate of water.The next stage of the project was designing and implementing an exhaust gas pipenetwork 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 theESL 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 tothe exhaust gas supply pipes as an alternative source of exhaust gases. The waste heatexchanger designed and selected was purchased and linked into the exhaust gasstream after calibration tests were carried out on the same in the wind tunnel. Thewater 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. Thefirst set of tests was performed using exhaust gases from the continuous combustionunit 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 aturbine with the dry steam generated. With exhaust gases originating from thecontinuous combustion unit, an air fuel ratio of9.14:1 was used and exhaust gases at atemperature of 540°C were obtained with an air inflow of 1400kglh and a fuelconsumption rate of7.11 kg/h. The exhaust gases degraded to 360°C at the waste heatrecovery inlet due to losses through the bare pipes. 11.12kW of energy was extractedfrom the exhaust gases to the water stream with an efficiency of 98%. With theexhaust gases from the 10-litre diesel internal combustion engine, an exhaust gas flowrate of O.22kgls was used and with a heat transfer efficiency of 89%, 18.5kW ofpower was extracted at the waste heat recovery unit. This represents a 4.9% of thethermal content of the fuel used. A rate of energy production balance on the internalcombustion engine showed that 34% is lost in exhaust gases and 29% in coolant andother losses while only 37% is used produced as shaft power.The results obtained therefore show that there is ample room for further investigationfor 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 testingfacility and an exhaust gas pipe network and evaluation of a small scale waste heatrecovery apparatus were achieved.The tests performed can still be optimised with more waste heat removal from theexhaust gases of typical diesel truck engines and hence better recovery of waste heatand a reduction of fuel consumption.

[发布机构] Stellenbosch University