337 Original paper Transactions of the Japan Society of Refrigeration and Air Conditioning Engineers, TransactionsReceived of the date Japan : September Society of 4, Refrigerating 2018 ; J-STAGE and AirAdvance Conditioning published Engineers, date : December 15, 2018 doi: 10.11322/tjsrae.18-35AC_OA Original Paper Vol.35,No.4(2018),pp.337-341, doi: 10.11322/tjsrae.18-35AC_OA Received date: September 4, 2018; J-STAGE Advance publishied date: December 15, 2018

Thermo-economic Review of Micro-scale Organic Rankine Cycle Integrated into Vehicle for Waste Heat Recovery

Jingye YANG*, Binbin YU*, Bingqing LU*, Jiangping CHEN*†

*Institute of Refrigeration and Cryogenics, Shanghai Jiao Tong University (No. 800, Dong Chuan Road, Shanghai 200240, PR China)

Summary Heavy-duty diesel engines have been widely applied in commercial vehicles. However, escalating consumption of fossil fuels and stringent legislation of CO2 emission have raised the concern about searching for viable technologies to improve the fuel energy efficiency recently. Generally, optimization technology of engines can be categorized into two types: -powertrain-applied and engine-bottoming cycles. The bottoming organic Rankine cycle is an applicable combined heat and power (CHP) technology with great potential for engine waste heat recovery. The aim is to increase the fuel energy efficiency without additional fuel consumption. In this paper, a thermo-economic overview of organic Rankine cycle system integrated into engines especially for on-the-road vehicle is presented. First of all, characteristics of various engine waste heat (e.g. exhaust gas, EGR, CAC, jacket cooling water, oil circuit) are briefly analyzed; Afterwards, special attention is paid to the screening criteria of appropriate expansion machine and working fluid selection; Subsequently, an overview of various layout of organic Rankine cycle was presented; Eventually, cost-orient economic evaluation of the synthesis cycle is overviewed, which is meant for characterizing the optimum system design.

Keywords: Review, Organic Rankine Cycle, Engine, Waste heat recovery

1. Introduction a literature review of turbo-compounding technology as waste heat recovery from ICE (Internal 11-12) Heavy-duty diesel engine and Internal Combustion Engine). Organic Rankine cycle combustion engine are both primary dynamic sources offers an interest in better use of fossil fuel through of on-the-road vehicles. However, almost 60%-70% conversion from engine waste heat to electricity, 1) which is known as combined heat and power of fuel energy is wasted through engine exhaust 13) gases. Increasing price level of fossil fuel and more generation technology. Anh Tuan Hoang conducted stringent legislation of green-house gas emission an overview of the latest technology of organic promote the evolution of energy conversion Rankine cycle for engine waste heat recovery technologies. In 2010 US EPA2) report, fossil fuel use application. Different temperatures of waste heat and industrial process are the primary source of especially the diesel engines were concentrated on. Mahmoudi et al.14) presented an overview of last four carbon dioxide (CO2) account for 65% of GHG (Green House Gas) emissions. Petroleum-based fuels, years research works on ORC application. Cycle gasoline and diesel are the main transportation energy. configurations, working fluid selection and operating 14% of global green-house gas emissions come from conditions were included in the investigation. The transportation involved with fossil fuel burned for thermo-economic overview of organic Rankine cycle road, rail, air, and marine transportation. In July 2009, implemented in this paper is concentrated on vehicle European Union leaders and the G8 announced that engines. The remainder of this review is processed green-house gas emissions should be reduced at least with four main sections. First section is the 80% below 1990 levels by 2050. This resulted in a investigation of vehicle engine background including practical guide to a low-carbon Europe called heat source characteristics of engine waste heat. Roadmap 20503). A conversion of fossil fuel to Second section presents a detailed review of organic electricity in transportation is suggested as one of the Rankine cycle technology including selection of modifications of energy systems. Engine working fluid and expansion machines. In the third manufactures can be categorized into two types: one section, a summarize of various layout of ORC system is the engine-powertrain-applied technologies. was discussed; The last section is the summary of Coupling engines with EGR (Exhaust Gas cost-orient economic evaluation criteria of organic Recirculation) for NOx reduction4), variable Rankine cycle system. timing5), engine downsizing, enhanced fuel mixing and turbocharging6-7) are main strategies; Another one 2. Characteristics of Engine Waste Heat is bottoming technology, advanced after-treatment and Recovery waste heat recovery are both engine-tailpipe strategies. Waste heat recovery is assumed as a promising Engine waste heat can be divided into two technology to improve the energy efficiency and categories according to temperature level: medium- decrease emissions effectively without additional fuel high temperature heat source (e.g. Exhaust Gas (200- consumption. Overviews of Thermo-Electric 600 °C), Exhaust Gas Recirculation (200 °C - Generators (TEG) were reported in Hamid Elsheikh 750 °C)); medium-low temperature heat source (e.g. M’s8) and Orr B’s9) researches. Aghaali H10) presented Coolant (80 °C -100 °C), Lube Oil (80 °C -120 °C),

†Fax:+ (86)21 34206775 E-mail: [email protected] Paper presented at ACRA2018, June 10-13, 2018, Sapporo, Japan

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Charge Air Cooling (50 °C -70 °C)). Internal technology with high energy potential, which is combustion engine exhaust gas temperature varies assumed as the most attractive heat source. Previous from 500 °C -900 °C which is dependent on different researches revealed that heat source temperature engine types and variable operating conditions. under 80 °C has little value to recover. Although there Although exhaust gas contains large amounts of waste is an ‘apparent’ amount of heat in coolant, system heat, extremely high temperature brings difficulties in thermal efficiency can be quite low. Chammas et al.16) waste heat recovery procedure. First is the thermal reported that the exhaust gas carries waste energy match between heat source and high temperature ranging from 4.6 to 120 kW while cooling water working fluid. Organic working fluid is not suggested carries heat ranging from 9 to 48 kW for a typical here because of decompositio. Usually, an additional light duty 4- spark ignition engine. heat oil heat exchanger is introduced to cool the Organic Rankine cycle is proposed as a viable exhaust gas to a normal temperature around 200 °C. bottoming technology for engine waste heat recovery. In that case, thermal stream is more stable while Thermal match between two systems decides the considerable heat is lost as price. Second is the sealing overall coupling cycle efficiency. Off-design design of heat exchanger. High quality design of operating conditions bring time-varying heat source evaporator is demanded to avoid leakage and temperature and mass flow rate which is hard to corrosion. predict. Thus, data about variable heat source is Simon et al.15) has reviewed the thermal power of necessary for ORC performance evaluation. They are exhaust gas and EGR of four different heavy-duty usually reported with engine load or speed steady- diesel engines in Figure 1. They are single-stage state points both experimental and simulative. Torque- turbocharged engine with no EGR, a single stage speed map is also counted for describing the driving turbocharged engine with high pressure (HP) EGR, a cycle. Domingues et al.17) has investigated six two-stage turbocharged engine with HP EGR and different working points of one typical engine. Vehicle another two-stage turbocharged engine with HP EGR. speed, engine power, heat source temperature and EGR is presented as an effective aftertreatment mass flow rate are analyzed as seen in Table 1.

250 ST-TC/NO EGR ST-TC/HP EGR ST-TC/HP EGR(1) 200 ST-TC/HP EGR(2)

150

100 ThermalPower[kW] 50

0 1 2 Exhaust Gas EGR Cooler

Fig. 1 Exhaust gas and EGR thermal power for four different HDDE models at full load condition13)

fluids are usually defined with the vapor status at the 3. Screening Criteria of Scroll Expander and end of expansion process. Most wet fluids are Working Fluid Selection inorganic like water and ammonia. The slope of saturation vapor curve is negative in T-S diagram. Exhaust vapor at the outlet of the expander may Working fluid selection is dependent on exceed the two-phase region, which will damage characteristics of heat source and operating conditions. turbo type expanders. The slope of saturation dry fluid Classification of working fluids is diversified in vapor curve is positive while vertical for isentropic different aspects. In the previous literatures, working fluids. Existence of superheating is assumed

Table 1 Working points of 2.8 L VR6 spark ignition engine15) Operating Vehicle Engine Exhaust gas Exhaust gas Recoverable exhaust heat point speed Power mass flow rate temperature (°C) (kW) (km/h) (kW) (g/s) 1 0 0 12.8 457.8 5.0 2 23.5 13.0 25.9 595.1 14.3 3 47.2 26.4 43.0 716.7 18.4 4 67 37.2 59.7 779.2 46.1 5 80 44.1 71.9 800.7 57.2 6 100 54.4 92.4 804.1 73.9

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unnecessary to dry fluids since over superheated performance. The modified scroll expander is widely vapor of expander will increase the cooling load of utilized in experimental investigation. Woodland et condenser, thus an additional is suggested al.23) experimentally investigated an open-drive scroll to improve the system thermal efficiency. In expander integrated into a micro-scale ORC system. accordance to the origin source, working fluids can be The peak of expander isentropic efficiency occurs categorized into nature fluids (e.g. water, carbon when the filling factor is near a unity and the dioxide, ammonia) and organic fluids (e.g. alcohol, expansion volume ratio is near the built-in volume refrigerants). Based on the heat source temperature, ratio. The deviation between the expansion volume working fluids can be classified into medium-high ratio and the designed value results in drastic decrease temperature working fluids (e.g. alcohol, steam water, in expansion performance. This explains why the hydrocarbon) and medium-low temperature working specifically designed expansion machine is more fluids (e.g. R245fa, R134a, R1234yf). Pure fluids can efficient than the modified one, theoretically. The hardly satisfy the thermal match between working optimization of scroll expander based on a simulative fluids and heat sources since various waste heat exist model is relevant to improve the system performance. in vehicle engines. Mixtures including water soluble Usually, there are two ways of modeling: matter and zeotropic fluids are proposed to bring the thermodynamic modeling and mathematical modeling. temperature glide to better match the heat source and For thermodynamic modeling, there is no need to decrease the exergy loss. know the exact geometry of expansion machine. However, the downside of zeotropic fluids is the Operating conditions are paid more attention to. insufficient phase change that weakens the heat Lemort et al.24) had proposed a simplified transfer process. Screening criteria of working fluid thermodynamic modeling method of open-drive scroll selection is important since appropriate working fluid expander. Pressure-drop and internal leakage are both is beneficial to improve cycle performance. Thermo- modelled with an isentropic fluid flow through a physical properties of working fluids, global safety nozzle. The heat transfer procedure is modelled as a criteria and environmental impact are main heat exchanger. Filling factor and expander isentropic considerations. Thermo-physical properties are efficiency are used to fully characterize the expansion referred to the critical temperature/pressure of performance. Yanagisawa25) experimentally tested a working fluids, saturation slope after expansion modified scroll expander in 1988. The expansion (ds/dT), condensation pressure and molecular weight. adiabatic efficiency can be reached between 60%-75% Environmental impact indicators include GWP for the speed range of 1000 -4000 rpm. (Global Warming Potential) and ODP (Ozone Mathematic modeling of expander is much more Depletion Potential). Global safety criteria are complicated since bunch of geometric parameters are concerned with flammability, toxicity and corrosivity. demanded. Large amount of experimental data is Actually, considering the significant impact of HFCs demanded to validate the mathematical model. to climate change, several investigations have been Although the modeling procedure is more focused on the refrigerant substitute procedure. In complicated, the geometric optimization can be 2014, Datla and Brasz18) first-time carried on an realized based on such simulative model. experimental study with a 75-kW basic ORC using radial inflow turbine. They revealed that R1233zd(E) 4. Layout of Organic Rankine Cycle System led to 8.7% higher net cycle efficiency and more 19) output electricity than R245fa. Matthais Welzl et al. Apart from the single-pressure organic Rankine proposed a comprehensive evaluation of organic cycle system, there are also different layouts of ORC Rankine cycle combining nucleate boiling heat developed by researchers, which is aimed to improve transfer coefficient (HTC) with output electricity. The the cycle thermal efficiency. Several literatures have higher HTC and better performance of R245fa than been concentrated on the various configurations of R1233zd(E) at the same saturation temperature can be organic Rankine cycle system. Kevin Rosset et al26) explained by the higher pressure and density. 20) theoretically evaluate the various cycle configurations Sebastian Eyerer et al. reported that R1233zd(E) and working fluids of ORC system for passenger car performed approximately 7% higher thermal internal combustion engine application. The available efficiency but with 12% lower gross power output. space in the engine and the condenser were Scroll expander is chosen as an appropriate considered as the most important factors that expansion machine for small-scale organic Rankine influence the system performance. Xuan wang et al.27) cycle because of fewer moving parts, reliability, lower 21) Compared four forms of organic Rankine cycle with noise and vibration . Usually, the categorization of the dynamic math model on the SIMULINK. The aim scroll expander is dependent on the operation way: is to reveal the effect factors of part-load performance. one is velocity expansion machine, another is In this section, four different types of organic Rankine volumetric machine. However, in this paper, two types cycle were introduced, as shown in Figure 2. Upper of scroll expander are characterized by the left is the basic ORC with only four main components. manufacture: One is design expander, another is the The whole system is simple, compact but the cycle modified expander from commercial compressor. 22) thermal efficiency may be unsatisfying considering George et al. designed an open-drive scroll large heat load was waste in condenser side. The expander and integrated it into ORC system. upper right is a basic ORC with an additional Theoretically, the specifically designed expander is preheater. The waste heat in radiator and coolant were more adapted to the operating pressure ratio. However, recovered to preheat the coming cold refrigerant. The the isentropic efficiency is only 25-40%, which is low-grade waste heat can be taken advantage using lower than expected. The main reason is the limited this method. The down left is a regenerate ORC expander rotational speed (500 rpm) that leads to system. The regenerator takes advantage of the large internal leakage and decrease the expansion

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resume heat of the exhaust vapor at the end of the review, an opinion is drawn as: considering the trade- expansion to preheat the cold fluid flow before off between the component cost and the improvement evaporator. The last one is a conceptual schematic of in effectiveness, the basic ORC was assumed more dual loop ORC with two expanders and pumps. It was appropriate since the available room in vehicles is considered as the most complicated configuration limited. Moreover, the integral weight of the system since the number of each component was doubled should be controlled in a smaller value. than the basic ORC. In that case, the experimental cost will be increased. Based on the previous literature

Basic ORC BORC with preheater

Regenerate ORC Dual-loop ORC

Fig.2 Layouts of organic Rankine cycle28)

5. Cost-orient Economic Evaluation 6. Conclusion

There are plenty of economic analysis of ORC Engine waste heat is presented as an appropriate system in previous literatures. In this paper, a heat source to recovery. Large amount of energy is summary of several cost-orient evaluation indicators contained in exhaust gas and EGR. Organic Rankine is presented. Before implementing the evaluation cycle is supposed to be used for low-grade waste heat criteria, an economic modeling is demanded. APR, recovery with great potential. Screening criteria of SIC and LCOE are the most commonly used working fluid selection and expansion machine indicators. APR is defined as the ratio of heat selection are paid attention to in previous researches. exchanger area to net power output; SIC is referred to Thermo-physical properties, environmental impact the specific investment cost; LCOE is the levelized and safety concern are three main considerations for cost of electricity. Lecompte et al.29) proposed SICpl working fluid selection. Modified scroll expander is for part load consideration of the system. It’s defined proved applicable in ORC system with potential. as the ratio of investment cost to annual average net Considering the trade-off between the experimental power output. Such indicator not only takes the heat cost and the cycle thermal efficiency, the single- transfer procedure into account but also the ambient pressure basic ORC was considered the most conditions. Economic evaluation methods presented appropriate for engine waste heat recovery. Cost- above are all related to the systematic performance. orient economic evaluation is important as well for Expander size factor (SF) is used to quantify the cost further consideration of commercial production in the of expansion machine. It is defined as a ratio between future. exhaust specific volume and heat transfer rate in evaporator. Minimizing the SF value is one of the References optimization objectives of expander. The total heat transfer capacity is specific for economic evaluation 1) T. Endo, S. Kawajiri, Y. Kojima, K. Takahashi, T. Baba, of heat exchangers, which is related to the geometric S. Ibaraki, T. Takahashi, et al., Study on Maximizing design of heat exchangers and working fluid selection. Exergy in Automotive Engines, SAE Int. Publication 2007-01-0257, 2007. 2) https://www.epa.gov/climate%20change/ghg%20emiss

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