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ScienceDirectEnergy Procedia 00 (2017) 000–000

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IV International Seminar on ORC Power Systems, ORC201713-15 September 2017, Milano, Italy

Experimental investigations on a CO2-based Transcritical Power Cycle (CTPC) for waste heat recovery of diesel engine

Lingfeng Shi, Gequn Shu, Hua Tian*, Liwen Chang, Guangdai Huang

and Tianyu ChenTianjin University, 92 Weijin Road, Nankai District,Tianjin,China

Abstract

CO2-based transcritical Power Cycle (CTPC) could be used for engine waste heat recovery as the safety and environment-friendly characteristic of fluid, which also matches high temperature of engine exhaust gas and satisfies miniaturization demand of recovery systems. In this study, a simplified version of a CTPC system was constructed as the bottoming system and experimentally investigated to recover waste heat from exhaust gas of a heavy-duty diesel engine. The CTPC hardware was unrecuperated and the turbine was replaced with an expansion valve. By monitoring key parameters of the CTPC system and DE system, good system stability and satisfying thermal states of working fluids were observed. Investigation was based on constant operating condition of engine at speed of 1300rpm (1300ES) and 1100rpm (1100ES), constant pump condition at speed of 70rpm (70PS) and 80rpm (80PS). The CTPC system performance as a function of pressure ratio was one of the main focus points. Results indicated that the change of heat absorption and efficiency of gas heater have a clear decreasing trend with an increasing pressure ratio, mainly due the decreased mass flow rate. Compared with 1100ES, 1300ES means more heat input and more net power output, and also higher thermal efficiency at high pressure ratio range (>1.4). The advantage is feeble at the low pressure ratio range (<1.4). Up to 2.05 kW net output power was expected to be obtained at 1300ES and 80ES, and 0.043 thermal efficiency was expected at 1300ES and 70PS.

© 2017 The Authors. Published by Elsevier Ltd.Peer-review under responsibility of the scientific committee of the IV International Seminar on ORC Power Systems.

Keywords: CO2-based transcritical Power Cycle; Diesel engine; Exhaust gas;

* Corresponding author. Tel.: +86-15822683137;E-mail address: thtju@tju.edu.cn

1876-6102 © 2017 The Authors. Published by Elsevier Ltd.Peer-review under responsibility of the scientific committee of the IV International Seminar on ORC Power Systems.

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Nomenclature

η efficiencyh enthalpy (kJ/kg)m mass flow rate (kg/s)P pressure (MPa)t temperature ( )℃Wnet net power output (kW)

Abbreviations

CTPC CO2-based Transcritical Power Cycle ORC Organic Rankine CycleSWEP company name, a Swedish supplier of brazed plate heat exchangers1100ES 1100rpm engine speed 1300ES 1300rpm engine speed 70PS 70rpm pump speed 80PS 80rpm pump speed

Subscripts

f working fluidg exhaust gasin inletout outletth thermal1-5 state points

1. Introduction

Waste heat recovery is an effective technology for the energy saving of engine and has attracted more and more attention recently. Exhaust gas is the main waste heat recovery object for its great amount energy and high temperature grade. Among the waste heat recovery technologies, CO2-based transicritical Power Cycle (CTPC) is a highly effective bottoming cycle for exhaust gas recovery. Carbon dioxide (CO2) is an environment-friendly, non-flammable and non-corrosive working fluid of power cycle which makes less safety problem. It behaves high thermal stability when indirectly recycling high-temperature exhaust gas. The direct heat transfer process between CO2 and exhaust gas reduces energy and exergy loss due to a better temperature match.

Many researches paid attention to the performance comparison between CTPC system and Organic Rankine cycle (ORC) system, which is considered as a potential technology for waste heat recovery due to its high thermal efficiency, flexibility and low maintenance requirements. Due to a good heat transfer characteristics and flowing characteristics of supercritical CO2, the CTPC shows better exchanger performance and economic performance than the ORCs [1-5]. In the research of Shu et al. [6], the CTPC shows the significant advantage of high heat recovery of both the exhaust gas and engine coolant. Because the engine coolant temperature is near the CO2 critical temperature where large specific heat capacity appears, the CTPC power system can transfer more engine coolant heat to the CO2 working fluid and thus effectively reduce the thermal load on the engine. Therefore, the CTPC has its special advantage and is considered a potential technology in engine waste heat recovery.

At present, the investigations on the CTPC mainly focus on the theoretical aspect. Construction research has attracted a few researchers to conduct comparison studies. Chen et al. [7] concluded that the CTPC was more suitable for the exhaust gas recovery than CO2 Brayton cycle. Farzaneh-Gord et al. [8] and Shu et al. [9] conducted

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four CTPC constructions and made a comparison study. The selection map of Reference [9] could be used as a construction selection reference in different applied conditions, which considered three parts: net power output, exergy efficiency and electricity production cost.

While, there are not many experimental investigations on the CTPC due to the high pressure of the CTPC system (over 10 MPa). Pan et al. [10] conducted a test bench of the CTPC using oil as the simulative heat source. The work device is a rolling piston expander. The system thermal efficiency is only 5.0% with an expander isentropic efficiency of 21.4% when the high and low pressures are about 11 MPa and 4.6 MPa, respectively. Echogen Power Systems [11, 12] did preliminary tests of a 250 kW CTPC system using steam as the heat source. System performed as expected and proved to be applied for recovering the waste heat of engine exhaust gas with a wide temperature range from 200 to 540 . This research provided the application possibility of the CTPC on the large static℃ ℃ engine waste heat recovery. Korea Institute of Energy Research [13] developed three supercritical carbon dioxide power cycle experimental loops, namely the 1 kWe-class, 10 kWe-class and 80 kWe-class experimental loop. Among them, the 1 kWe-class experimental loop is a transcritical cycle at a maximum temperature of 200 °C.

The investigations on the CTPC test introduced above mainly used simulative heat sources instead of practical waste heat source. Using a practical waste heat source is necessary and meaningful for the study of waste heat recovery, especially for the engine’s field. Except for the high pressure problem of the CTPC system, the engine condition and high temperature exhaust gas are also the two important and difficult points. Therefore, a CTPC system was constructed as the bottoming system practically recovering waste heat from exhaust gas of a heavy-duty diesel engine. The effect of engine operating condition and the CTPC operating condition were both considered in this study. Investigation was based on constant operating condition of diesel engine at two different operating conditions. The CTPC system performance changed with pressure ratio at constant pump speed. Observations of key exergy states as well as estimations and comparisons of potential output power were carried out stepwise.

2. System layout and test bench setup

In this research, the topping system is an 8.4 L 6-cylinder heavy-duty diesel engine, of which rated power is 240 kW. 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. The exhaust gas from the diesel engine flows into the gas heater of the CTPC system.

CO2 is pressurized by a reciprocating plunger pump and it is from liquid state to supercritical state. A damper is set up to weaken the effect of interval operating of pump and make a more accurate measurement of supercritical flowmeter later. Then, CO2 is heated by the exhaust gas in the gas heater. The gas heater is serpentine pipe sleeve type exchanger with exhaust gas flowing in the sleeve side and CO2 in the pipe side. The CO2 flows through a expansion valve and has a pressure drop. Due to difficult manufacture of a small-scale CO 2 expander or turbine, the expansion valve is used to replace and served as a device of pressure control. To make an enough cooling load, two SWEP plate heat exchangers are used as the precooler and condenser, respectively. The liquid CO2 then flows into a tank. These processes form a complete circle of the CTPC. Water is the cooling source of the CTPC, which dissipates heat by a refrigerating unit. Thermal resistance thermometers and thermocouple thermometers are adopted to measure the temperature of working fluid and exhaust gas, respectively. It depends on their temperature range. Monocrystalline silicon resonant pressure sensors are adopted for the pressure measurement of exhaust gas and working fluids. Especially, the contact segment of exhaust gas in pressure sensors was made of hastelloy alloy, which avoid the corrosion by exhaust gas. The mass flow rate measurement of supercritical CO2 uses a Coriolis flowmeter, and a turbine flowmeter is used for the volume flow rate of cooling water.

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3. Experiments and results analysis

3.1. Experimental conditions

The CTPC test bench is designed at magnitude of 4 kW power output, thus is much smaller than the full waste heat magnitude of diesel engine. Hence, the diesel engine operates at two low and middle load conditions to match the CTPC system in this research: 1100 rpm rotate speed, 606 N.m torque and 1300 rpm rotate speed, 636 N.m torque. 1100ES (1100rpm engine speed) and 1300ES (1300rpm engine speed) are used to represent these two conditions. The main parameters of 1100ES and 1300ES are listed in the Table 1.

Fig. 1. (a) Structure and layout of the CTPC and diesel engine system.

Table 1. Operating parameters of diesel engine.

Operating conditions Rotate speed (rpm)

Torque (N.m)

Engine power (kW)

Fuel injection (kg/h)

Air intake (kg/h)

Exhaust temperature ( )℃

1100ES 1100 606 69.0 14.96 348.6 465

1300ES 1300 636 86.4 18.78 443.9 485

The exhaust gas consists of several gas components (H2O, CO2, N2, O2 and et al.). The property of exhaust gas is average calculated by mass proportion of each component. The mass proportion of each component is calculated by the model of Reference [14], which used the same diesel engine by our group.

The pump is a positive displacement type. The mass flow rate of CO 2 is mainly related to the rotate speed of pump. In this research, 70 and 80 pump speed (PS) are selected to investigate the pressure effect, simplified as 70PS (70rpm pump speed) and 80PS (80rpm pump speed). Under a constant pump speed and engine condition, the expansion valve inlet pressure is changed by opening the valve in six steps. The maximum flow occurs with the valve fully open and the pressure is lowest at about 7.5 MPa. Minimum flow occurs through the valve when it is

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most closed and the inlet pressure is greatest (about 10.5 MPa). Six valve settings were used to divide the pressure range from (7.5-10.5 MPa) for each group of pump speed and engine speed. Different valve open has impact on the CO2 mass flow rate. The measurement data of the high and low pressure with variation of CO2 mass flow rate is present in Fig. 2. The low pressure P3 has a little change with the expansion valve opening. It is mainly related to the condensation temperature of CO2. The difference between four groups is the result of the cooling water with a little temperature change (9.0-12.7 ). ℃

At the experimental process, engine speed and load are set to a fixed value. The parameters are shown in Fig. 3 to illustrate the stability of the engine operating condition. The maximum difference of the temperature between the same engine conditions is less than 1%. And the maximum difference of mass flow rate is less than 0.2%. It can be considered that heat source is changeless when the engine speed and torque are fixed. It is important to note that, the exhaust temperature in Fig. 3 is the gas heater inlet temperature. A heat loss of exhaust gas occurs on the pipe from engine outlet to gas heater inlet. The loss heat is 2.5%-4.0% of total exhaust gas energy in this research.

Fig. 2. Pressure of CTPC under each measuring point. Fig. 3. Temperature and mass flow rate of exhaust gas under each measuring point.

3.2. Experimental results analysis

In this research, performance variation with system pressure is selected as the study point. The low pressure is different among four groups. For a reasonable comparison between four groups, a relative value (pressure ratio between high and low pressure P2/P3) is selected as dependent variable to instead of the high pressure. The pressure difference is generated with expansion valve, thus is associated with CO2 mass flow rate. It does not mean the pressure is the main driving function of system performance, mass flow rate also plays a significant role. Fig. 4 shows pump performance of the CTPC system, namely mass flow rate of CO2 and pumping power. With an increase in pressure ratio, the change of CO2 mass flow rate has a little decrease trend for all groups. The change of pumping power has an obvious increase with an increase in pressure ratio. Maximum pump power is 0.93 kW. Normally, more mass flow rate is produce at 80PS than that at 70PS. The difference between the two groups at the same pump speed is mainly because the different liquid CO2 density at the pump inlet. The liquid CO2 density at the pump inlet is mainly linked to the low pressure.

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Fig. 4. Mass flow rate of working fluid and pumping power. Fig. 5. CO2 temperature at gas heater inlet and outlet

As shown in Fig. 5, different CO2 temperature at the gas heater outlet (t2) is obtained by four groups at the base of an approximate same inlet temperature. For the four groups, t2 increase gently with an increase in pressure ratio, mainly reasonable to the small decrease of CO2 mass flow rate. Because of a smaller mass flow rate and more heat input, the temperature of gas heat outlet is highest at 1300ES and 70PS. 66.7 kW and 50.1 kW waste heat is contained in the exhaust gas at engine condition of 1300ES and 1100ES, respectively. The heat input performance is present in Fig 6. At the same engine condition, the heat absorption is approximate. The heat source has an direct effect on the heat absorption, as shown on the big difference between 1100ES and 1300ES. The decrease of heat absorption with an increase in pressure ratio is produced by the decrease trend of mass flow rate. The gas heater efficiency is present at in the right side of Fig. 6, which is defined as:

(1)

Wherein, hg,out,ideal is referred to the gas enthalpy at the CO2 inlet temperature (t1). It can obviously conclude that, 1100ES and 80PS can obtain high heat exchanger efficiency. Compared with 70PS, 80PS means more mass flow rate participating in heat absorption process. Then, more heat flowing trends to cold working fluid side instead of cold environment side. As shown in Fig. 7, 1100ES has a smaller heat loss in gas heater than 1300ES. Therefore, the highest gas heater efficiency is achieved at 1100ES and 80PS. It is worth mentioning, heat insulation work should be more perfect in this experimental setup to protect the heat at the fuel gas side and gas heater.

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Fig. 6. Heat absorption by working fluid and gas heater efficiency. Fig. 7. Average heat loss exhaust gas.

Net power output and thermal efficiency is necessary to evaluate a thermodynamic system. In this research, expansion valve replaces CO2 expander or turbine and has no power output. But the inlet parameters are obtained in the experimental process. Therefore, 0.6 turbine isentropic efficiency and 0.9 generator efficiency is set to predict the power output by the measuring parameter (P2, P3 and T2). In Fig. 8, forecast net power output and thermal efficiency are present in the left and right side, respectively. Forecast net power output is the difference between forecast power output of turbine and practical pumping power. The thermal efficiency is present at in the right side of Fig. 6, which is defined as:

(2)

For the condition of 1300ES, the forecast net power output is preponderant than that at the condition of 1100ES. But the difference between them in the thermal efficiency becomes smaller. For this CTPC system at a certain pressure ratio, more heat input means larger net power output but has little effect on the thermal efficiency, especially at low pressure ratio range (<1.4). With an increase in pressure ratio, the forecast net power output and thermal efficiency both increase obviously. This test bench expects to get 2.05 kW largest net power output at the condition of 1300ES and 80PS, and 0.043 highest thermal efficiency at the condition of 1300ES and 70PS.

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Fig. 8. Forecast of net power output and thermal efficiency.

4. Conclusions

This paper conducts an experimental research on the CTPC for waste heat recovery of diesel engine. A test bench of the CTPC as the bottoming cycle and diesel engine as the topping cycle are built. The CTPC performance variation with pressure ratio, engine speed and pump speed is the study object of this paper. Main conclusions are got as following:

The change of pumping power, forecast net power output and thermal efficiency have a clear increase trend with an increase in pressure ratio. The change of heat absorption and efficiency of gas heater have a clear decrease trend with an increase in pressure ratio, mainly due the decreased mass flow rate.

Compared with 1100ES, 1300ES means more heat input and more net power output, and also higher thermal efficiency at high pressure ratio range (>1.4). The advantage is feeble at the low pressure ratio range (<1.4).

This test bench expects to get 2.05 kW largest net power output at the condition of 1300ES and 80PS, and 0.043 highest thermal efficiency at the condition of 1300ES and 70PS.

Acknowledgements

The authors would like to acknowledge the National Natural Science Foundation of China (No. 51636005 and No. 51676133) for grants and supports.

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