Analysis of a 1 kW organic Rankine cycle usinga scroll expander for engine coolant and exhaust heat recovery

Abstract

The development of engine waste heat recovery technologies attractsever increasing interests due to the rising strict policy requirementsand environmental concerns. This paper presented the study of enginecoolant and exhaust heat recovery using organic Rankine cycle (ORC).Eight working fluids were selected to evaluate and compare the performanceof the integrated waste heat recovery system. Rather than the conventionalengine ORC system mainly focusing on the utilization of exhaust energy,this work proposed to fully use the engine coolant energy by changingthe designed parameters of the ORC system. The case study selecteda small engine as the heat source to drive the ORC system using ascroll expander for power production. The evaluation results suggestthat under the engine rated condition, the solution to fully recoverthe engine coolant energy can achieve a higher power generation performancethan that of the conventional engine ORC system. The results suggestthat adding a recuperator to the ORC system can potentially improvethe system performance when the working fluids are dry and the overalldumped heat demand of the system can be reduced by 12% under optimalconditions. When the ORC evaporating and condensing temperature arerespectively set at 85°C and 30°C, the integrated engine wasteheat recovery system can improve the overall system efficiency by9.3% with R600, R600a or n-Pentaneas the working fluid.

Keywords

organic Rankine cycle / scrollexpander / coolant and exhaust recovery / internal combustion engine

Introduction

The increase in fuel prices and strictrequirements on carbon dioxide emissions limits are promoting theenhancement of engine thermal efficiency beyond the limit of in-cylindertechniques [1]. As oneof the most promising heat driven power generation technologies, theorganic Rankine cycle (ORC) has been extensively studied for engineexhaust heat recovery because the thermodynamic cycle works well forthe medium grade energy of the exhaust [2].

The market available ORC system withthe power ranges of 0.2 to 2 MW_e under thecost of around 1 and 4×103 €/kWe, and lower powers are in apre-commercial status because of the relatively long payback periodusing small scale ORC system [3]. The designed evaporation and condensation temperature determinesthe overall efficiency of a typical ORC system. A higher temperaturedifference between evaporation and condensation can result in a higheroverall ORC efficiency. Therefore, the engine coolant energy is commonlyrecognized as a heat source that is not worthy of recovering becausethe relatively low temperature ranges of the coolant is about 80°C–100°Cand can only provide a 40°C–60°C temperature differencefor on road vehicle application [4]. However, the engine coolant energy contains about 30% of the fuelenergy and has a huge potential to be utilized for the engine ORCheat recovery system in order to improve the ORC efficiency and reducethe payback period of the system [3,5]. The coolant energyis commonly used to preheat the ORC fluid as reported by Yu et al.[6]. The results suggestthat only 9.5% of the coolant heat has been recovered under engineconditions from high to low load, which means that the majority ofcoolant energy cannot be reused and is dumped to the environment [6]. Adopting a dual-loop ORC systemcould be one of the solutions to achieve high overall system efficiencyas reported by Wang et al [7,8]. Nevertheless, theproposed dual-loop ORC system requires two set of ORC system componentsand advanced controlling strategies to balance different heat sources.The cost effectively use relatively simple system is important andpromote the development of engine waste heat recovery system.

On the other hand, the selectionof a working fluid plays a key role in ORC performance [9–12]. Based on the slope of vapor saturation cures inthe T-s diagram, ORC fluids canbe classified into three groups, dry, isentropic and wet [13]. The dry and isentropic workingfluids are recommended to be used for the ORC system, because theworking conditions of the fluid for the expansion process can be designedwithin the vapor phase, which can protect the expansion machine frombeing damaged by the liquid drop of the fluid using wet working fluids[10–12]. Saleh et al. [10] have conducted a performance studyof a ORC system using 31 pure working fluids and the results indicatethat the n-butane can achieve a maximum thermal efficiency of 0.13at a heat source temperature of 120°C. The investigation on asmall-scale engine ORC waste heat recovery system using six differentworking fluids has been reported by Lu et al. [14]. The results suggest that underthe engine rated condition, the integrated ORC system can potentiallyimprove the overall energy efficiency by 11.2% and the brake specificfuel consumption (BSFC) can be improved by 10% [14]. Wang et al. [9] have conducted a study to comparethe performance of a 10 kW net power output ORC system using differentworking fluids for engine exhaust heat recovery. The results indicatethat R11, R141b, R113, and R123 manifest slightly higher thermodynamicperformances than other working fluids [9].

In this paper, a study has been conductedof a small scale ORC system to recover engine coolant and exhaustenergy. The ORC system performance using coolant energy as preheatsource and adopting coolant energy during the ORC fluid evaporatingprocess have been compared. A scroll type expander has been selectedas the expansion machine in the small scale system because the scrollexpander has the advantages of high reliability, relatively high isentropicefficiency, and broad availability [15–18]. Eight working fluids have been selected to compare the systemperformance including net power, thermal efficiency of the ORC, rotationalspeed of the scroll device, and required heat dump loads of the system.

Description of engine coolant and exhaust ORC system

The ORC system contains a pump, threeheat exchangers, a scroll expander, and a condenser, as illustratedin Fig. 1. The heat exchanger for the recovery of coolant energy isnamed as Heater 1 and the engine exhaust energy is recovered by Heater2, which is located at the exhaust of the engine. When the recuperatoris used the unused heat at the exit of the expander is first recoveredby the recuperator to preheat the working fluid pumped from the pumpand then dumped to the environment from the condenser. The workingconditions of the ORC fluid can be defined as

\mathrm{1}\to \mathrm{2} Isentropic compression process in the pump;

\mathrm{2}\to \mathrm{3}\to \mathrm{4} Isobaric process during the heating process of theworking fluid recovering coolant energy;

\mathrm{4}\to \mathrm{5}\to \mathrm{6} Isobaric process for the recovery of exhaust energy;

\mathrm{6}\to \mathrm{7} Isentropic expansion process in the scroll expander;

\mathrm{7}\to \mathrm{8}\to \mathrm{1} Isobaric process in the condenser.

When the recuperator is operatedin the system, the heat transfer processes of two sides from \mathrm{2}\to \mathrm{2}\text{'}and \mathrm{7}\to \mathrm{7}\text{'}are both recognized as an isobaric process.

Methodologies

The engine selected in this studyis a small scale diesel fueled internal combustion engine with themodel number YTG-6.5S. The specification and evaluated engine operatingcondition are listed in Table 1. The selected ICE is a one cylinderengine with 0.638 L engine displacement and achieved rated power of8.8 kW at 2400 r/min [20].

The isentropic efficiency of thepump is defined in Eq. (1), where h_{\mathrm{2}�s} is the specific enthalpyat the exit of the pump after isentropic expansion. The pump efficiencyin this study is set at 0.4 [21].

(1){\eta }_{\mathrm{pump}}=(h_{\mathrm{2}�s}-h_{\mathrm{1}})/(h_{\mathrm{2}}-h_{\mathrm{1}}).

The work provided to the pump can,therefore, be calculated by Eq. (2), where {\dot{m}}_{\mathrm{ORC}}is the mass flow rate of the ORCworking fluid.

(2){\dot{W}}_{\mathrm{pump}}={\dot{m}}_{\mathrm{ORC}}\cdot (h_{\mathrm{2}}-h_{\mathrm{1}}).

The heat exchanger efficiency ofHeater 1, Heater 2, and the recuperator are all set at 0.8. The heatprovided to Heater 1 and Heater 2 from the coolant and exhaust energyare respectively calculated by Eqs. (3) and (4). As stated in theintroduction, the engine coolant energy cannot be fully recoveredwhen the coolant heat is used to preheat the working fluid. Therefore,the recovered coolant heat for the engine ORC system can be representedas {\dot{Q}}_{\mathrm{coolant�rec}}.

(3){\dot{Q}}_{\mathrm{Heater1}}={\dot{m}}_{\mathrm{ORC}}\cdot (h_{\mathrm{4}}-h_{\mathrm{2}})={\dot{Q}}_{\mathrm{coolant}–\mathrm{rec}}\cdot {\eta }_{\mathrm{Heater1}},

(4){\dot{Q}}_{\mathrm{Heater}\mathrm{2}}={\dot{m}}_{\mathrm{ORC}}\cdot (h_{\mathrm{6}}-h_{\mathrm{4}})={\dot{Q}}_{\mathrm{exhaust}}\cdot {\eta }_{\mathrm{Heater}\mathrm{2}}.

When the recuperator is in operation,the heat transfer process inside the recuperator can be calculatedby Eq. (5), where {\eta }_{\mathrm{r}}is the efficiency of the recuperator.

(5){\dot{Q}}_{\mathrm{r}}={\dot{m}}_{\mathrm{ORC}}\cdot (h_{\mathrm{7}}-h_{\mathrm{7}\text{'}})\cdot {\eta }_r={\dot{m}}_{\mathrm{ORC}}\cdot (h_{\mathrm{2}\text{'}}-h_{\mathrm{2}}).

As a volumetric expansion machine,the geometry of the equipment mainly determines the suitable operatingregion of the expander. The selected 1 kW scroll expander has beengeometrically studied to obtain the key parameters as shown in Fig.2. Based on the calculated results from the geometric scroll expandermodel, the exhaust and suction volume of the scroll expander is 41.38cm³ and 11.82 cm³, respectively. The relationships between the ORC fluid mass flowrate and the expander rotational speed can, therefore, be calculatedby Eq. (6), where V_{\mathrm{SV}} is the swept volume of the scroll expander, v_{\mathrm{su}} is the specific volume at the inlet of the expander, and the expanderrotational speed is represented as

(6)N\cdot {\dot{m}}_{\mathrm{ORC}}={\displaystyle \frac{N}{\mathrm{60}}}\cdot {\displaystyle \frac{V_{\mathrm{SV}}}{v_{\mathrm{su}}}}.

The power output from the expandercan be obtained from Eq. (7), where h_{\mathrm{su}} is the specific enthalpy at the inlet of the expander, h_{\mathrm{d}�s} is the designed exhaust specific enthalpy of thescroll expander after the isentropic expansion process, P_{\mathrm{d}�s} is the designed exhaust pressure, and v_{\mathrm{d}} is the designed specific volume. The expansion efficiency {\eta }_{\exp } of the expander is assumed to be 0.8 and the isentropicexpander efficiency is written as {\eta }_{\mathrm{iso�exp}}.

(7){\dot{W}}_{\exp }={\dot{m}}_{\mathrm{ORC}}\cdot \left[(h_{\mathrm{su}}-h_{\mathrm{d}�s})+(P_{\mathrm{d}�s}-P_{\mathrm{ex}})\cdot v_{\mathrm{d}}\right]\cdot {\eta }_{\exp }={\dot{m}}_{\mathrm{ORC}}\cdot (h_{\mathrm{6}}-h_{\mathrm{7}})\cdot {\eta }_{\mathrm{iso�exp}}.

The thermal efficiency of the ORCsystem can, therefore, be defined as

(8){\eta }_{\mathrm{ORC}}=({\dot{W}}_{\exp }-{\dot{W}}_{\mathrm{p}\mathrm{ump}})/\left({\dot{Q}}_{\mathrm{coolant�rec}}+{\dot{Q}}_{\mathrm{exhaust}}\right).

The ORC system dumps heat from thecondenser in order to maintain the low pressure condition of the expanderand provide liquid phase working fluid to the pump. The dumped heatfor vehicle application is commonly released from the engine radiatorsystem. Therefore, the evaluation of the required dumped heat usingthe ORC system for engine application is important. The dumped heatusing the engine integrated ORC waste heat recovery can be definedand calculated by Eq. (9).

(9){\dot{Q}}_{\mathrm{dump}}={\dot{Q}}_{\mathrm{con}}+{\dot{Q}}_{\mathrm{coolant}}-{\dot{Q}}_{\mathrm{coolant�rec}}.

The boundary conditions and standardparameters of the simulation model are summarized in Table 2.

Four key performance parameters includingnet power output, thermal efficiency, expander rotational speed, anddumped heat requirement of the engine coolant and exhaust heat recoveryORC system under the engine rated power condition have been studied.Three different ORC operating methods for engine coolant and exhaustrecovery are studied to investigate the system performance by usingcoolant as preheater source, by reducing ORC evaporating temperatureto fully recover coolant energy, and by adding a recuperator to thefull coolant energy recover ORC system. Eight working fluids havebeen selected and the performance of ORC system using different workingfluids are analysed. The properties of selected working fluids canbe found in Table 3.

Results and discussion

Engine coolant and exhaust heat recovery ORC using coolantheat as preheat source

The engine coolant can only providea temperature of about 85°C [4] and the superheated temperature in this study is set at 5°C.Therefore, the performance of the engine coolant and exhaust heatrecovery ORC system can be evaluated by identifying the effects ofthe expander inlet temperature. The results suggest that the maximumpower output from the ORC is around 0.77 kW, when R134a is used asworking fluid and the expander inlet temperature is set around 100°C.However, the thermal efficiency of the system using R134a is the lowestof the eight selected working fluids. At the same expander inlet temperature,the ORC system using working fluids R134a, R152a, R124, and R600acan achieve a relatively higher output than that of other workingfluids, which are mainly caused by the relatively higher mass flowrate of the four working fluids as indicated in Fig. 3(e). The maximumORC thermal efficiency of the selected working fluids is about 0.086,when the ORC system adopts n-Pentaneas the working fluid at the expander inlet temperature between 120°Cto 130°C. The calculation results of the scroll expander rotationalspeed as plotted in Fig. 3(c) can provide a reference for the ORCsystem under no load condition. The maximum range plotted in Fig.3(c) is 4500 r/min. Toluene is not suggested to be used as the workingfluid for this small scale ORC system because the scroll expanderrotational speed is over 4500 r/min, which is not within the desirableoperational condition of this type expansion machine. Moreover, thepower output of the ORC system using toluene is the lowest of otherseven working fluids. The dumped thermal load of the system rangesfrom 8.5 to 8.7 kW. When R600 is used as the working fluid, the minimumdumped heat of the ORC system is about 8.5 kW at an expander inlettemperature of 110°C. The increase of required dumped thermalload with the increase of expander inlet temperature is mainly contributedby the coolant energy, because only limited coolant heat can be recoveredwith the increase of designed ORC evaporating temperature.

Engine coolant and exhaust heat recovery ORC to fully recovertwo heat sources

When the ORC system is operated tofully recover both heat sources from coolant and exhaust under enginerated condition, the working fluid mass flow rate is stable with thechanges of expander inlet temperature as illustrated in Fig. 4(e).The power output and thermal efficiency of the ORC system increasewith the increase of designed expander inlet temperature as shownin Fig. 4(a) and (b). Among the selected working fluids, the optimalthermal efficiency can be achieved at expander inlet temperaturesranging from 85°C to 90°C. At the designed expander inlettemperature of 90°C, the power output from the ORC system usingR152a can be as high as 0.8 kW, which is higher than the maximum powerproduction from the engine coolant and exhaust ORC system using coolantenergy as the preheat source as described in Subsection 4.1. Moreover,the dumped heat load of the proposed solution can be lower than theprevious solution. At the scroll expander inlet temperature of 85°C,the ORC system using R600 rejects about 8.3 kW thermal loads to theenvironment, which is lower than the previous solution.

Engine coolant and exhaust heat recovery ORCR to fully recovertwo heat sources

The purpose of adding a recuperatorto the ORC system is to recover the unused energy from the exit ofthe expander in order to improve the ORC overall performance and potentiallyreduce the dumped heat load from the system. The results suggest thatexcept for using R152a as working fluid, adding a recuperator to theORC system can potentially improve the system performance as demonstratedin Fig. 5. As a wet type of working fluid, R152a is not suggestedto be used under the designed conditions. The reason for this is thatafter the expansion process, the temperature condition at the exitof the scroll expander is already lower than the condensation temperature.The results also indicate that when dry working fluids are used inthe ORC system, the dumped heat load can be effectively reduced whena recuperator is introduced. Among the selected eight working fluids,the maximum power output under the designed can be reached 0.82 kW,when R600, R600a, and n-Pentaneare used at an expander inlet temperature of 90°C. This is animprovement of about 9.3%, when the waste heat recovery system isintegrated with the engine under the rated power conditions (8.81kW). It can be observed that by adding a recuperator to the ORC system,when n-Pentane is the working fluid,the system dumped heat load can be reduced from 8.3 kW to 7.3 kW atan expander inlet temperature of 85°C, which is a reduction ofabout 12%.

Conclusions

This paper reports the study of asmall scale engine coolant and exhaust heat recovery system usingthe ORC system. Eight working fluids have been selected in the analysisto investigate the system performance. The purpose of this study isto point out the potential of using simple ORC waste heat recoveryto fully recover coolant energy rather than the conventional engineheat recovery solution mainly focusing on the utilization of exhaustenergy. The main conclusions of this study can be summarized as follows.

When the engine coolant heat is usedas a preheat source, the maximum thermal efficiency of the ORC systemis about 8.6% by using n-Pentaneas a working fluid at an expander inlet temperature of 120°C.However, the maximum power output from the system is not obtainedunder the maximum ORC thermal efficiency condition, because the enginecoolant energy has not been effectively recovered. Toluene can achievea good overall thermal efficiency, but the net power output from thesystem is the worst of the selected working fluids.

Because of the limitation of suppliedtemperature from the coolant energy, the thermal efficiency of theengine coolant and exhaust heat recovery ORC system is lower thanthat of the system using coolant energy as a preheater source. However,the power output from the system can be higher than that of the systemusing coolant for preheating. Moreover, the overall dumped heat demandof the engine can be slightly reduced when the ORC system is operatedas fully recovering coolant energy. The conventional engine exhaustheat recovery ORC system can be easily modified into the proposedsolution, which can produce a similar power output with reduced dumpedheat requirement.

Adding a recuperator to the ORC systemcan recover the unused heat at the exit of the expander, when theworking fluids are isentropic or dry. When wet ORC working fluid isoperated in the system, it is generally suggested that the recuperatoris not to be used. When R600, R600a and n-Pentane are selected as the working fluids, the maximum power outputfrom the ORCR system can be as high as 0.82 kW at the designed expanderinlet temperature of 90°C. Under the optimal designed conditions,introducing the recuperator into the engine coolant and exhaust heatrecovery ORC system can effectively reduce the dumped heat load ofthe system by 12%. Considering the performance of the selected engineunder rated power condition, the integrated waste heat recovery systemcan potentially improve the overall system performance by 9.3%.

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