Organic Rankine Cycle) system for waste heat recovery from diesel engine

Organic Rankine Cycle) system for waste heat recovery from diesel engine

Energy 107 (2016) 693e706 Contents lists available at ScienceDirect Energy journal homepage: www.elsevier.com/locate/energy Experimental investigat...

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Energy 107 (2016) 693e706

Contents lists available at ScienceDirect

Energy journal homepage: www.elsevier.com/locate/energy

Experimental investigation on thermal OS/ORC (Oil Storage/Organic Rankine Cycle) system for waste heat recovery from diesel engine Gequn Shu*, Mingru Zhao, Hua Tian, Haiqiao Wei, Xingyu Liang, Yongzhan Huo, Weijie Zhu State Key Laboratory of Engines, Tianjin University, No. 92 Weijin Road, Nankai Region, Tianjin 300072, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 17 September 2015 Received in revised form 8 March 2016 Accepted 13 April 2016

With urging needs to decrease the fuel consumption and environment pollution, energy saving and emission reduction technologies in the ICE (internal combustion engine) industry are developed. A thermal OS/ORC (Oil Storage/Organic Rankine Cycle) system was constructed and preliminarily tested for WHR (Waste Heat Recovery) from exhaust gas of a 240 kW diesel engine. The heat balance test of diesel engine without OS/ORC was conducted first to investigate the varying property of exhaust gas, then the OS/ORC system was tested to show its ability against high temperature and variation of exhaust gas. The results show that thermal oil effectively dropped the working temperature of organic fluid to less than 210  C, which is much lower than the decomposition temperature of many organic fluids. Also, thermal oil brought a significant inertia to the response of system which could be positive against the variation of engine condition. In order to learn more about the operating characteristics of OS/ORC system, the impact of important parameters on each other was investigated quantitatively as well as on the performance of OS/ORC system. The results show that within the given range, higher evaporating pressure can obviously improve the performance of OS/ORC while the impact of superheat is nearly negligible. © 2016 Elsevier Ltd. All rights reserved.

Keywords: OS/ORC Diesel engine Waste heat recovery Thermal oil High temperature Transient performance

1. Introduction The oil crisis and environmental pollution have stimulated the development of energy saving and emission reduction technology in the ICE (internal combustion engine) industry. The study [1] on diesel engine found that, despite the employment of advanced engine technologies such as HCCI (Homogeneous Charge Compression Ignition), LTC (Low Temperature Combustion) or turbo-charging, approximately 60% of the energy released by fuel is still dissipated to the environment in the form of waste heat, which shows a great potential to be recovered. In comparison with other WHR (Waste Heat Recovery) technologies such as thermoelectricity and absorption cycle air-conditioning, the ORC (Organic Rankine Cycle) promises higher efficiency and is already well employed in industrial areas [2e6] such as steel and cement, solar, geothermal and biomass power plants. Recent researches show the applications in on-road vehicles are already under experiment. Honda [7] is using an Organic Rankine cycle to improve

* Corresponding author. E-mail address: [email protected] (G. Shu). http://dx.doi.org/10.1016/j.energy.2016.04.062 0360-5442/© 2016 Elsevier Ltd. All rights reserved.

the overall efficiency of a hybrid vehicle. Test results presented in 2008 showed that in 100 kph constant-speed driving, the use of the Organic Rankine cycle improved the thermal efficiency of the engine by 3.8%. In the US highway cycle, the Organic Rankine cycle system regenerated three times as much energy as the vehicle's regenerative braking system. The “Turbo Steamer” developed by BMW [8] has also been tested on its 3 series 1.8 L engine. It consists of low and high temperature cycles of which the working fluids are water and ethanol respectively. Fuel efficiency, output power and torque are improved by 15%, 10 kW and 20Nm respectively. In 2009, new configurations of the Organic Rankine cycle applied to a fourcylinder combustion engine were presented by BMW [9]. Based on bench test measurements, BMW has concluded that waste heat recovery can provide an additional power output of about 10% at typical highway cruising speeds. In 2013, a prototype was installed on a BMW 5 Series vehicle and tested on a highway [11]. The electricity recovered was used to support the on-board electric system and a fuel saving potential of 4% is reached. The report of Cummins [10,12] in 2014 shows the “Super Truck” program sponsored by DOE has made much progress. A single-stage ORC system using R245fa as working fluid is installed on the diesel engine and

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tested on road, which improved engine's brake thermal efficiency from 47.5% to over 51%. Considering the large mass flowrate and high temperature of exhaust gas, it's usually adopted as the heat source of WHR system. However, there are two major challenges. First is the high temperature of exhaust gas which exceeds the decomposition temperature of most organic fluid. Siloxanes, alkanes and toluene are studied by many researchers for their high decomposition temperatures. Researches conducted by Xiaoning LI et al. [13] and F.J. ndez et al. [14] show that Alkane-based ORC and SiloxaneFerna based ORC are suitable for high temperature heat source and ensure good thermal stability of the working fluid. However, the toxicity of siloxanes and toluene as well as the flammability of alkanes have limited their application. Non-azeotropic mixtures are introduced to retard the flammability of alkanes. Despite of its advantages, the proportion of non-azeotropic mixtures is easily affected by the leakage of the system and the decomposition of constituents, which largely weaken its ability of heat recovery even flame retardance. The second challenge is the varying condition of engine on road which leads to the variation of exhaust gas. When engine condition is switched from heavy duty to light duty, the expander in WHR system has to be shut down since the energy absorbed from exhaust gas is unable to generate enough vapors. Hui Xie et al. [15] tested the operating performances of a designed ORC system under an actual driving cycle. The results indicate that the on-road system efficiency is less than the half of designed efficiency at the rated operating point because of the variation of the engine condition, which causes safety issues to the expander. The main solution now is to develop a control strategy which keeps track of the engine condition meanwhile adjusts WHR system. Paolino Tona et al. [16] have conducted a survey about the most promising control solutions presented by 2015 for Organic Rankine Cycle Systems on board Heavy-Duty Vehicles. The conclusion is that as with the existing solutions, a lot of improvements still need to be done to deal with the disturbance represented by highly-varying exhaust gas conditions. They [17] also came up with a control strategy which aims to ensure the continuity and safety of operation. The strategy is tested on the motorway condition under which engine mostly runs at high speed and changes little. The performance shows that however the continuity and safety of operation is obtained, the global efficiency is largely reduced. The similar results of other researchers [18,19] show that it is hard to set up control strategies which promise both maximum power output and system safety. Heat storage technology has also been brought up to deal with this problem. Tao Chen et al. [20] designed an evaporator with heat storage material inside for on-board ORC, which can effectively smooth the fluctuation of exhaust gas. The simulation result shows that the ideal heat storage material requires both high heat capacity and low heat resistance. However, the suitable material is still absent by far, which limits the development of such evaporators. Thermal oil is believed as an alternative solution for the two challenges above. In Europe, there are over 120 ORC plants in the industrial applications with sizes between 0.2 and 2.5 MW electric. In these facilities, thermal oil is usually used as heat transfer medium, demonstrating a number of advantages, including low pressure in boiler, large inertia against the variation of heat sources and simple adaptability to load changes, automatic as well as safe control and operation. Moreover, the transferred working temperature of ORC is lower, which ensures a very long working life of organic fluid. The utilization of a thermal oil boiler also allows operation without requiring the presence of licensed operators as for steam systems in many European countries. Several companies like Turboden, Pratt&Whitney have already made great achievements at biomass plants, of which the flue gas from boiler has the

temperature as high as 1000  C and the net electric efficiency is above 18%. So the thermal oil were introduced by some researchers to deal with the high temperature and variation of exhaust gas. Yu et al. [21] did a theoretical study of an ORC system combined with thermal oil cycle for waste heat recovery of a 243 kW diesel engine. R245fa was chosen as working fluid. The system shows that the maximum potential output power and recovery efficiency are as high as 14.5 kW and 9.2% separately. Jacek Kalina et al. [22] compared the ORC containing thermal oil with the single and dual loop ORC when studying the WHR for gas engine theoretically. The results show that the recovery ability of ORC containing thermal oil is between the single and dual loop ORC. What's more, thermal oil can avoid the use of gasegas heat exchanger which is difficult to manufacture. Generally speaking, because of the extra weight and complexity, thermal oil was not considered suitable by some researchers as heat transfer media in vehicle, therefore not adopted in the typical WHR system of ICE. However, a thermal OS/ORC (Oil Storage/ Organic Rankine Cycle) system was constructed and preliminarily tested in this study. The result shows that the OS/ORC system could operate against the decomposition problem of organic fluid and the frequently changing conditions of engine on road. The characteristics of exhaust gas from diesel engine and the potential recovery ability of OS/ORC system were also investigated. What's more, the operating characteristics of OS/ORC system are investigated quantitatively.

2. System layout and test bench setup In this research, the heat source is the exhaust gas from an 8.4 L 6-cylinder heavy-duty diesel engine, of which rated power is 240 kW. This kind of engine was turbo-charged and widely used on the long-haul heavy trucks which are very likely to apply WHR techniques. The engine bench was equipped with a whole set of controlling and measurement device, which can keep the engine working steady under any specific condition with all the performance data recorded at the same time. As shown in the Fig. 1, the thermal oil cycle was added in OS/ORC system as the heat transfer medium, which can serve as a buffer to decrease the impact from the fluctuation of exhaust gas. Besides, since the highest temperature of exhaust gas can be close to 500  C, the thermal oil cycle can effectively drop the working temperature of organic fluids below their decomposition temperature, however at the cost of lower quality of heat source, lower efficiency, higher irreversibility and longer response time. DBT (Dibenzyl-toluene) is chosen as thermal oil for its boiling point as high as 390  C, as well as its non-corrosiveness to metal which makes heat exchangers easy to manufacture. What's more, DBT has a low desirably viscosity which minimizes the required pump work and increases the heat transfer. Besides, DBT can resist oxidation well. Considering back pressure of engine, a shell-and-tube heat exchanger as shown in the Fig. 2 is chosen for thermal oil to absorb heat from exhaust gas. All the other heat exchangers in the OS/ORC system are plate heat exchangers as shown in Table 1. The oil pump is a 0.75 kW centrifugal pump. Since Coriolis flow meter is unable to withstand the high temperature of thermal oil despite of its higher accuracy and direct measurement of mass flowrate, a VSF (Vortex Shedding Flowmeter) is installed to measure the volume flowrate of oil. Considering the volume of thermal oil can expand by as much as 25% under high temperature, a 40 L oil storage tank is installed above the whole system preparing to contain the expanded oil. When thermal oil get cold and shrink, they will flow back to system from tank by gravity.

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Diesel Engine

T P

Gas to Oil Heat Exchanger

T P

T

T

Pump T

T

T P

Damper

T P

Tank

Evaporator

Expansion Valve T P

Condenser T

Tank

T

T P

Flowmeter

T P

Pump

Pump

Tank

Refrigerating Unit

Exhaust gas Thermal oil Organic fluid

T P

T

Tank

Temperature Pressure Sensor Temperature Sensor Pressure meter

Cooling water

Valve

Fig. 1. Structure of the OS/ORC system.

Fig. 2. Gas to oil heat exchanger.

Table 1 Important parameters of heat exchangers. Name

Type

Heat transfer area

R123 evaporator R123 condenser

plate heat exchanger plate heat exchanger

4.61㎡ 5.18㎡

The OS/ORC system was set up with the thermal oil cycle and ORC as a whole test bench. Fig. 3 shows the test bench before thermal insulated. The chosen pump for ORC is an 2 kW plungerdriven diaphragm pump, for the membrane placed between the

Fig. 3. OS/ORC system.

fluid chamber and the plunger zone ensures a perfect sealing of the circuit toward environment and allows reaching pressure as high as 3 MPa, in spite of the low viscosity feature of the pumped liquid and the absence of any lubricant properties. Also, the mass flow rate can be controlled by adjusting the stroke length of plunger. A Coriolis flow meter was set before the pump inlet to measure the mass flow rate, while a damper was set after the pump to smooth the flow. An expansion valve was employed to temporarily take the place of the expander which is under design. By controlling the expansion valve, the evaporating pressure can be adjusted. Since the mass flow rate of organic fluid is slightly influenced, the expansion valve was kept widely open during the whole condition test to make sure the mass flow rate can be controlled by ORC pump more accurately and be large enough to absorb more heat from thermal oil. As for organic fluids, the temperature of thermal oil under the whole test is controllable at a low-and-mid level from 81  C to 222.5  C, which makes R123 the suitable working fluid. Although with micro toxicity and GWP of 120, the study of Wang et al. [23] show that R123 has the best performance among the several working fluids when recovering low-and-mid temperature waste heat. Also, R123 was selected because of its non-flammable properties, low boiling temperature, chemical stability and low cost. Moreover, it has high thermal conductivity as well as moderate working temperature and pressure. Zhou et al. [24] set up an ORC bench using R123 to recover waste heat from low-temperature flue gas. The maximum cycle efficiency reaches over 8.5%. So R123 is the first choice to investigate the maximum potential of the OS/ORC system. However, considering R123's drawbacks, R245fa is more widely chosen by researchers for its lower toxicity and less damage to environment while still high efficiency close to R123. Tian et al. [25] compared 20 working fluids searching for the suitable ones for waste heat recovery from one popular commercial diesel engine. Results show that R123 and R245fa present the best thermodynamic performance as well as the best economical efficiency bastien Declaye et al. [26] and Reberto Bracco et al. [27] sepaSe rately set up test benches to test the performance of R245fa under low to medium temperature heat sources, and the cycle efficiency are all above 8%. The biggest problem of R245fa is its low decomposition temperature compared with R123. At this research, both R123 and R245fa were chosen, however the comparison between them is absent since the impact of thermal oil on OS/ORC system is the top priority to investigate. A refrigerating unit using R22 as refrigerant was set up to supply cooling water whose temperature and flow rate can be adjusted. Therefore the condensing temperature and pressure of ORC system can be maintained in a certain range.

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All the hot parts of the facility have been thermal insulated after first trial, and the test bench has been equipped with the necessary auxiliaries such as measurement devices, particular thermocouples and pressure sensors. Since the main purpose is to investigate the properties of OS/ORC system, the overall weight and the charge amount of thermal oil as well as organic fluid are not concerned.

3. Experiments on the heat recovery ability of OS/ORC system 3.1. Characteristics of exhaust gas from diesel engine The heat balance test of diesel engine without OS/ORC was conducted first to investigate the varying properties of exhaust gas. During the test, the speed of engine varied from 1,200 rpm to 2,200 rpm with a 200 rpm interval, while the load varied from 20% to 100% with a 20% interval under every speed. The results are shown in the figures. Fig. 4 shows the varying trends of the exhaust temperature which directly determine the quality of the exhaust heat. It's clearer that the temperature rises along with the load under the same speed. However with the speed changing, the temperature didn't change uniformly. Under 100% Load, the temperature curve takes a counter “N” pattern with the increase of the speed. That's because, under low speed of engine, intake air is much less due to the low speed of turbo-charger, which leads to a small air-fuel ratio. Considering the usually oxygen-enriched combustion of diesel engine, a small air-fuel ratio means the energy released by burned fuel is distributed to less mass of combustion products, which causes a high average temperature. As speed climbs up, the temperature is more dependent on the load, which obtains the maximum value at 1,800 rpm. The varying trend of temperature tends to be flat under light duty. However the high temperature indicates the high quality of heat source, the temperatures under most conditions have exceeded the decomposition temperature of R123 as 327  C, not to mention the decomposition temperature of R245fa as 167  C. It would require special attention on control equipment or strategy. Fig. 5(a) shows the varying trends of the exhaust mass flowrate which mainly affects the amount of exhaust heat. From the figure, the mass flowrate remains monotonically increasing along with both the speed and the load.

As with the recoverable heat contained in the exhaust gas denoted as Q ideal, it's usually assumed as all the heat released from the exhaust gas when they are cooled down to acid dew point (120  C). The exhaust temperature is better above acid dew point after heat transfer to protect the heat exchanger from corrosion. Q ideal is calculated using an approximation method. CxHyOz can denote the average molecular composition for common diesel fuel because the petroleum-derived diesel fuel is a very complicated mixture of alkane, alkene and arene, with minimal amount of sulphur and nitrogen which can be neglected. In this composition, x, y, and z respectively represent the moles of the C, H, O. Their mole ratio is shown as the Eq. (1) below:

x : y : z ¼ 0:87 : 0:126 : 0:004

(1)

The air is presumed as the mixture of nitrogen and oxygen with the mole ratio of 3.762:1. Hypothetically, the combustion products only consist of carbon-dioxide, water, nitrogen, and oxygen. Nitrogen comes from air and oxygen is residual from the usually oxygen-enriched combustion of diesel engine. Based on the atomicity balance principle, the mass fraction of combustion products is obtained by the Eq. (2) below:

 y z y Cx Hy Oz þ x þ  ðO2 þ 3:762N2 Þ þ xCO2 þ H2 O 4 2 2  y z þ 3:762 x þ  N2 4 2

(2)

According to the measured temperature and pressure, the waste energy in exhaust gas can be calculated by the Eqs. (3)e(4) below:

hexh ðT; PÞ ¼ uCO2 hCO2 ðT; PÞ þ uH2 O hH2 O ðT; PÞ þ uO2 hO2 ðT; PÞ þ uN2 hN2 ðT; PÞ (3)   Q ideal ¼ mexh ðhexh ðTexh ; Pexh Þ  hadp Tadp ; P0

(4)

From the Fig. 5(b), the maximum range occurs within the heavy duty conditions, which are believed to be the suitable conditions to recover waste energy. However the quality of the exhaust heat is high at low speed, the amount of recoverable heat is still less than heavy duties because of the insufficient of exhaust mass flowrate. The maximum Q ideal is obtained at 2,000 rpm and 100% load as 142.2 kW.

3.2. Ability of OS/ORC system against high temperature of exhaust gas

Fig. 4. Exhaust temperature.

In this investigation, the whole condition test was conducted with R123 as working fluid to show the ability of OS/ORC system against high temperature of exhaust gas. In the Fig. 8, the highest temperature of thermal oil and R123 are denoted as Toil and TR123 while the temperature of exhaust gas at the inlet of heat exchanger as Texhaustin. Fig. 6(a) shows that at 1,800 rpm, the temperature of exhaust gas rises from 230  C to 474  C along with the load, however the temperature of thermal oil rises from 97  C to 220  C. The temperature of R123 is mainly controlled by its mass flow rate and only required to be above the evaporating temperature in this test. Fig. 6(b) shows their temperatures at full-load conditions. The highest temperature of thermal oil is around 220  C, even though the temperatures of exhaust gas are all above 400  C. The temperature of thermal oil at every condition is shown in the Fig. 6(c). The temperature range is from 81  C to 222.5  C, which is much lower than the decomposition temperature of R123.

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Fig. 5. (a). Exhaust mass flowrate; (b).Q ideal

However it requires extra caution when R123 is replaced by R245fa, since decomposition temperature of R245fa is only 167  C. Performance of OS/ORC system was also investigated in the whole condition test since the working temperature of R123 has been controlled below 222.5  C. During this investigation, the waste energy is usually unable to be fully recovered. Based on the experiment data and similar equations like Eqs. (3)e(4), the waste energy absorbed by OS/ORC system from exhaust gas can be obtained as shown in the Fig. 7(a) and denoted as Q real. It's clear that the trends of the absorbed exhaust heat is practically the same with that of Q ideal. The maximum Q real is 72.63 kW at 1,800 rpm and 100% load. The heat absorption efficiency as habs is defined as Eq. (5) below. The Fig. 7(b) shows that the trends of habs is totally opposite to the trends of Q ideal and Q real, which means the more recoverable heat in this condition, the lower absorption efficiency it has. Values greater than 100% mean the temperature of exhaust gas after heat exchanger is below acid dew point, which also mean the available heat have been absorbed completely.

habs ¼

Q real $100% Q ideal

(5)

The exhaust heat absorbed by R123 as Q e is calculated by the Eq. (6) below and shown in the Fig. 7(c). Affected by the thermal oil cycle, the maximum waste heat absorbed is 69.14 kW at the condition of 1,800 rpm and 100% load.

Q e ¼ mR123 ðhR123 ðTR123in ; PR123in Þ  hR123 ðTR123out ; PR123out ÞÞ (6) Although the expander is unavailable yet, the potential power ability of R123 after evaporator can still be studied with an expansion valve. The Fig. 7(d) is the T-s map of R123, point 3 to point 4 is the isentropic expansion process with point 3 and 4 as the states before and after expansion, of which the temperatures and pressures are measured. The potential power output ability during expansion can be denoted as We and calculated by the Eq. (7) below:

We ¼ mR123 $ðh3  h4 Þ

(7)

Fig. 8(a) shows that the trend of We is basically like that of Q e for the obvious reason that the more heat the system absorbed, the more power it can potentially output. However, the trends is slightly different such as at the 1,800 rpm and 60% load condition. That's because the condensing pressure, which determines the enthalpy of R123 after expansion, affects We too. Higher condensing pressure usually yields less output power. The maximum potential power output ability of R123 as 9.67 kW is obtained at 2,000 rpm, 100% load condition. The potential thermal efficiency denoted by he at every condition is plotted below in Fig. 8(b) and calculated by the following Eq. (8):

he ¼

We $100% Qe

(8)

Among all the performance parameters, the potential thermal efficiency he is the most important one to evaluate the performance of the system. The figure clearly shows that the amount of absorbed heat is not the only decisive factor for system performance, which requires further investigation. 3.3. Response of OS/ORC system against variation of engine condition The inertia that thermal oil brings to the OS/ORC system can be helpful against the variation of engine condition, despite of the difficulty when stabilizing the system. To investigate its positive impacts, the response of OS/ORC system when engine condition varies is studied. The first test was conducted with R245fa as working fluid while the second test using R123. Only the condition of engine is adjusted, while the parameters of OS/ORC system remain unvaried due to its large inertia. The comparison between these two organic fluids is absent because the inertia of thermal oil is the dominating influence factor on the response of OS/ORC system. In the first test, the response of OS/ORC system when engine condition varies is studied. Heavy duty of 2,200 rpm and 100% load was maintained until OS/ORC system was steady. Then engine condition was quickly switched to light duty of 1,200 rpm and 20% load. After 2 min, the engine condition was quickly switched back

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Fig. 6. (a) Temperature of exhaust, oil and R123 at 100% load; (b) Temperature of exhaust, oil and R123 at 1800 rpm; (c)Temperature range of thermal oil.

to heavy duty and maintained for 18 min. During this process, the data was recorded every 20 s and the result is plotted in the Fig. 9. Fig. 9(a) shows that the only quickly responsive parameter is Q real, which denotes the waste energy absorbed from exhaust gas. The PID control for the temperature of intercooler results in the fluctuating temperature and mass flowrate of intake air, further in the varying energy of exhaust gas which can be proven by Q real in the figure. The highest temperatures of thermal oil and R245fa are denoted as Toil and TR245fa, while the superheat degree of R245fa after evaporation is denoted as DT, which means vapor state only when it's above zero. All these temperatures as well as Q e went down as soon as the engine was switched to light duty except that TR245fa

and DT decreased in a larger extent than Toil, which means the inertia of thermal oil cycle as well as its thermal capacity is much larger than that of ORC. The engine was switched back to heavy duty before DT went below 0. However, 3 min later, DT went below 0 anyway because of the inertia. That brought a drastic change to Q e as shown in the figure. The sudden decline results from the weaker ability of heat transfer and thermal capacity when organic fluid is in two-phase zone. As soon as DT increased back to above 0, Q e bounced to high level immediately. We as well as he was affected due to the same reason as shown in the Fig. 9(b). That's to say, if carefully adjusted to make sure DT always above 0, the system can keep outputting power even when engine condition is changing vastly.

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Fig. 7. (a). Q real; (b). habs; (c). Q e; (d). T-s map of R123.

Fig. 9(c) shows the variation of evaporating and condensing pressure denoted as Pevp and Pcond during this period. Unlike other parameters, Pevp and Pcond went through little variation with no more than 5% for Pevp and 1% for Pcond. It's an important conclusion since ORC without thermal oil cycle usually suffers a large-scale oscillation of working pressure which not only affects the performance but also endangers the expander [15]. This result shows that these two parameters can be almost immune to the variation of engine conditions, which ensures the continuous operation of OS/ORC system. What's more, the whole period also clearly shows that warming up process is much slower than the cooling down process which may requires more attention on heat insulation. In the second test, the response of OS/ORC system when engine shuts down is studied. The light duty condition of 1,200 rpm and 20% load is chosen instead of the heavy duty because there could be serious damage to engine when it shuts down at a high temperature level. The Fig. 10(a) shows the performance of ORC system

when the engine shut down at 100th second and the data was also recorded every 20 s TR123 denotes the highest temperature of R123. During the time from 100th second to 680th second, TR123 slowly decreased from 69.73 Cat first, then quickly went down to 59.37  C and DT from 15.03  C to 4.03  C likewise, while Q e only decreased from 7.452 kW to 7.108 kW and We from 0.2858 kW to 0.2422 kW, which means the thermal oil had been transferring heat to R123 even though the engine was shut down. At around 700th second, DT eventually dropped down below zero which means R123 was in two-phase state with little ability to absorb heat and output power. Q e went down sharply to 1.423 kW and We to 0.01364 kW, which means the system lose the ability to output power at 10th minute after engine shuts down. During this period, the temperature of thermal oil slowly dropped down from 76.16  C to 68.84  C which means ORC only consumed a small part of energy in thermal oil. Same with the conclusion above, Pevp and Pcond remained almost unchanged during the whole process, as shown in the Fig. 10(b).

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Fig. 8. (a). We; (b). he.

Form these two investigations, DT is proven to be a reliable indicator for safety that a moderate superheat not only protects the expander blade from erosion, but also ensures the effective heat absorption and power output. However, the effect of the amount of thermal oil on the response of OS/ORC system is still expecting further study. 4. Experiments on the operating characteristics of OS/ORC system OS/ORC system proves to be effective on the two challenges of WHR from exhaust gas. However, with one more cycle than typical ORC system, the operating characteristics of OS/ORC system is more complicated which needs more researches. There are four adjustable components within the whole test bench: engine, ORC pump, expansion valve and refrigerating unit. Five important parameters are corresponded to them as show in the Fig. 11. mR245fa stands for the mass flowrate of R245fa which was chosen as working fluid. From the previous experience, some connections between them are apparent, shown as blue broad arrow in the figure. To be specific, the highest temperature of thermal oil mainly responds to the engine condition, DT mainly to the mass flowrate of R245fa adjusted by the stroke of ORC pump, evaporating pressure mainly to the opening of expansion valve, and condensing pressure mainly to the condensing temperature adjusted by refrigerating unit. However, the other links between these components and parameters are not that obvious, therefore requires more researches. In this study, ORC pump and expansion valve are chosen as separate variables because engine condition affects too many parameters and the large inertia of refrigerating unit makes it inconvenient to adjust. As a result, the influence of DT and Pevp on other parameters, shown as the red narrow arrows in the figure, and on the performance of OS/ORC system are investigated below. Engine was under medium duty since the thermal load is beyond the capacity of engine cooling system if it was running a long time under heavy duty. Firstly, DT was adjusted by ORC pump with all other components unchanged. The results are plotted in the Fig. 12. During this time, mR245fa decreased by 11.4%, which naturally brought a

significant increase of DT from 1.2  C to 47.13  C in Fig. 12(a). Pevp was slightly affected with a decrease by 6.7%, which in turn leads to a larger DT. Pcond and Toil, by contrast, were almost insusceptible with a variation less than 1.03% and 0.08% separately, as shown in Fig. 12(b). The influence of DT on system performance was plotted in the Fig. 12(c). Surprisingly, Q e and We were only slightly affected with DT increasing in the given range, which is because more energy is carried by per unit mass of working fluid even though the mass flowrate declines. What's more, he only dropped down a little, from 11.76% to 11.29%. The result is important for the operation of the system, since dry fluids like most organic fluids are proven to be less efficient when DT increases, according to the typical simulation results [28e30]. However, system performance is not that susceptible to DT from this investigation since he only dropped by a negligible content. It would relieve the challenge of system controller since DT could be maintained within a wider range. Then, Pevp was adjusted by closing expansion valve bit by bit with all other components unchanged. The results are plotted in the Fig. 13. During this time, Pevp increased from 1,196 kPa to 1,590 kPa. mR245fa was nearly unaffected with a decrease by 1.8% as shown in the Fig. 13(a). However DT decreased by 33% because higher Pevp corresponds to higher saturated temperature. Pcond and Toil were still almost insusceptible with a variation less than 1.9% and 1.37% separately in Fig. 13(b). The influence of Pevp on system performance was shown in the Fig. 13(c). Unlike the influence of DT, Q e and We were affected with an obvious linear relation with Pevp increasing in the given range. This is similar with the simulation results of other people [21]. The reason why Q e and We are slightly reduced and increased respectively is investigated below. Q e and We are actually determined by the states of three places which are the inlet and the outlet of heat exchanger as well as the inlet of condenser, separately denoted by subscripts “in”, “out” and “cond”. To be specific, there are only five involved parameters: Tin, Tout, mR245fa, Pevp and Pcond. Fig. 13(d) shows that Tin and Tout remained almost unvaried when Pevp changes. Considering the similarly steady mR245fa and Pcond, Pevp is no doubt the stimulus to Q e and We.

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Fig. 9. Variation of system when engine condition changes.

Further investigation is conducted by numerical calculation. Pevp is assumed as from 1200 kPa to 1600 kPa with a 100 interval. Other parameters are assumed as shown in Table 2 which is the average of experimental data. The specific enthalpies at those three places are plotted in Fig. 14 and their relations with Q e and We are shown in the following Eqs. (9) and (10):

Q e ¼ mR245fa ðhout  hin Þ

(9)

We ¼ mR245fa ðhout  hcond Þ

(10)

Figure shows that the increase of Pevp leads to the decrease of hout since the state of R245fa is vapor here, while hin remains unchanged where R245fa is liquid. That's why Q e is declining both in Figs. 13(c) and 14. Also, because of the quicker decrease of hcond, We is increasing as shown in the figures. As a result, he in Fig. 13(c) increased obviously from 12.117% to 14.1%.

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Fig. 10. Variation of system when engine shuts down.

According to this investigation, same as the typical simulation results, Pevp is proven to be a big help when improving the system performance, therefore needs to be as high as possible. 5. Analysis of the uncertainties in the test

5.3. Flowrates measurements The air flow meter for diesel engine can measure from 0 to 1,350 kg/h with inaccuracy less than ±0.5%. The fuel consumption meter can measure from 5 to 2,000 kg/h with inaccuracy less than ±0.8%. The Coriolis mass flowmeter for ORC an measure from 0 to 25,000 kg/h with inaccuracy less than ±0.1%.

5.1. Temperature measurements The temperature of exhaust gas is measured by thermocouple sensors with first-class precision. The measurement range is from 60 to 650  C with inaccuracy less than ±1%. All the other sensors are pt100 thermo-resistive type with A-class precision. The measurement range is from 200 to 500  C with inaccuracy less than ±0.15%. 5.2. Pressure measurements The pressure measurement is conducted using pressure transmitters, with a range of 0e50 bar for ORC and 0e5 bar for thermal oil cycle. The inaccuracy is less than ±0.065%.

5.4. Analysis of the uncertainties The error transfer formula of mathematical statistics is used to verify the uncertainty of the parameters. Provided the measurements (x1, x2, x3, …, xi)are all uncorrelated and slightly varied, if parameter F is the function of the measurements as Eq. (11), then the uncertainty wF on the variable F is calculated as a function of the uncertainties wi on each measured variables xi by Eq. (12) below: F ¼ F(x1, x2, x3, …, xi)

Fig. 11. Connections between adjustable components and important parameters.

(11)

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703

Fig. 12. Variation of system when DT changes.

vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u   uX vF 2 wF ¼ t w2i vxi i

_ T; PÞ ¼ m½h _ 1 ðT1 ; P1 Þ  h2 ðT2 ; P2 Þ Q ¼ W ¼ f ðm;

wF F

The uncertainty is calculated by the Eqs. (15)e(17) below:

(12)

wQ ¼wW sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi       vh1 vh2 2 2 vh1 vh2 2 2 ¼ ðh1 h2 Þ2 w2m_ þ m_  wT þ m_  wP vT1 vT2 vP1 vP2

Relative uncertainty hF is calculated by the Eq. (13):

hF ¼

(14)

(13)

The waste heat absorbed from the exhaust gas as Q real and Q e and Power output W such as We is calculated by the following Eq. (14):

(15) 

vh vP



 ¼vT T

vv vT

 ¼v P

T$R P

(16)

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Fig. 13. Variation of system when Pevp changes.

Table 2 Assumed value of involved parameters.



vh vT

Involved parameters

Assumed value

Tin Tout mR245fa Pcond

20  C 140  C 612 kg/h 232 kPa

Recovery) from exhaust gas of a 240 kW diesel engine. The heat balance test of diesel engine without ORC was conducted first to investigate the varying property of exhaust gas, then the OS/ORC system was tested to show its ability against high temperature and variation of exhaust gas. In order to learn more about the characteristics of OS/ORC system, the impact of important parameters on each other is also investigated as well as on the performance of OS/ ORC system. The main conclusions are listed below:

 ¼ cP

(17)

P

The relative uncertainties of Q real, Q e and We are calculated as 6.4%, 8.6% and 3.6% separately.

6. Conclusion A thermal OS/ORC (Oil Storage/Organic Rankine Cycle) system was constructed and preliminarily tested for WHR (Waste Heat

(1). The results from heat balance test show that the temperature of exhaust gas is at a high level from 202  C to over 480  C, which exceeds the decomposition temperature of most organic fluids. The recoverable heat Q ideal in exhaust gas varies in a large extent. The maximum range occurs within the heavy duty conditions, which are believed to be the suitable conditions to recover waste energy. The maximum Q ideal is obtained at 2,000 rpm and 100% load as 142.2 kW. (2). Thermal oil can not only drop the working temperature of organic fluid effectively, but also bring a significant inertia to the OS/ORC system which could be positive. The temperature range

G. Shu et al. / Energy 107 (2016) 693e706

h DT

705

efficiency superheat degree,  C

Subscripts 0 environmental state 1e4 state points ideal recoverable heat in exhaust real actual heat absorbed abs absorption adp acid dew point e properties of organic fluid evp evaporating cond condensing oil thermal oil exhaust-in exhaust gas at the inlet of the evaporator in inlet of heat exchanger out outlet of heat exchanger References Fig. 14. Variation of the specific enthalpies when Pevp changes.

of exhaust gas is from 200  C to 480  C, however that of thermal oil is from 81  C to 222.5  C, which is way below the decomposition temperature of R123. The maximum heat absorbed by the OS/ORC system at whole condition is 72.63 kW and the maximum potential power output is 9.67 kW. The response test shows that, thanks to the inertia of thermal oil, the OS/ORC system is able to keep outputting power when the engine condition changes vastly even shuts down. During this investigation, DT is proven to be a reliable indicator for safe operation. Pevp and Pcond are found almost unchanged during the variation of engine condition which is important to the system safety. (3). The impacts of Pevp and DT on OS/ORC system is investigated quantitatively. The results show that within the given range, higher Pevp can obviously improve the performance of ORC while the impact of DT is nearly negligible, which would relieve the challenge of system controller since DT could be maintained within a wider range. What's more, Toil and Pcond are almost immune to the variation of Pevp and DT. Acknowledgments This work was supported by the National Basic Research Program of China (973 Program) (No.2011cb707201). The authors gratefully acknowledge them for support of this work. Nomenclature cP F Q R W h

u

m, m_ T P w x v

specific heat at constant pressure, kJ/(kg K) function of the measurements heat, kW gas constant, J/(mol K) Power, kW specific enthalpy, kJ/kg mass fraction mass flowrate, kg/s temperature,  C pressure, kPa uncertainty measurement specific volume, m3/kg

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