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Dynamic tests of CO2-Based waste heat recovery system with preheating process
Energy 171 (2019) 270e283
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Energy journal homepage: www.elsevier.com/locate/energy
Dynamic tests of CO2-Based waste heat recovery system with preheating process Lingfeng Shi a, b, Gequn Shu b, *, Hua Tian b, **, Tianyu Chen b, Peng Liu b, Ligeng Li b a b
Department of Thermal Science and Energy Engineering, University of Science and Technology of China, Hefei 230027, China State Key Laboratory of Engines, Tianjin University, 92 Weijin Road, Nankai District, 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 3 July 2018 Received in revised form 30 September 2018 Accepted 18 December 2018 Available online 21 December 2018
For engine waste heat recovery, CO2-based Transcritical Rankine Cycle (CTRC) system with preheating process has a strong capability of combining exhaust gas and engine coolant, thereby leading to improvement of output, efficiency improvement and the reduction in cooling load reduction. In previous researches, all the benefits were achieved and investigated mainly under ideal and stable engine conditions in previous researches. But in this study, various engine conditions were considered for a preheated CTRC test bench, namely starting and ‘warm up’ condition, stopping and idling condition, restarting condition and random condition. After system parameters (e.g. pressure, temperature) were measured, heat absorption quantity was calculated, and meanwhile net power output and thermal efficiency were estimated. The results show that the preheating process can improve the dynamic performance of the CTRC at various engine conditions. Firstly, the preheating process is beneficial for continuous output capacity and safe operation of CTRC. Besides, the preheating process also can avoid the sudden rise in the expansion inlet pressure under the stopping condition of engine, and meanwhile it plays an important energy supplement role in the random conditions of the engine, resulting in the cross change in the waste heat absorption from engine coolant and exhaust gas. © 2018 Elsevier Ltd. All rights reserved.
Keywords: Engine waste heat recovery CO2-based transcritical Rankine cycle (CTRC) Dynamic test Various engine conditions Preheating benefits
1. Introduction At present, the thermal efficiency of diesel engine is commonly less than 45% [1], and that of the gasoline engine is even lower [2], with over half of fuel energy wasted, mainly existing in exhaust gas and engine coolant. Therefore, engine waste heat recovery (EWHR) technologies show large potential to improve engine efficiency. Among E-WHR technologies, power cycle is a research hot due to its high efficiency, such as Organic Rankine Cycle (ORC) [3] and CO2-based Transcritical Rankine Cycle (CTRC) [4]. Generally, exhaust gas is the primary goal of E-WHR due to its high grade of temperature and large quantity of energy. Exhaust temperature could approach over 700 C in a gasoline engine and over 400 C in a diesel engine, and its energy accounts for over 30% of total fuel combustion energy [5]. Many organic fluids have low thermal destruction temperature so they cannot achieve direct heat transfer with high-temperature exhaust gas. So far, a few modified
* Corresponding author.; ** Corresponding author. E-mail addresses: [email protected] (G. Shu), [email protected] (H. Tian). https://doi.org/10.1016/j.energy.2018.12.123 0360-5442/© 2018 Elsevier Ltd. All rights reserved.
and novel ORCs have been investigated to solve this problem. Shu et al. [6] adopted high-temperature organic fluids (alkanes) and studied the energy and exergy performance for waste heat recovery of a diesel engine, finding that cyclohexane and cyclopentane were the most suitable fluids, but the flammability of alkanes is the main problem during application. Vaja [7] conducted a medium circuit (oil) between the exhaust gas and organic fluids, not only for safety but also for stabilizing the transient operation of the ORC system. However, addition of the big heat exchangers leads to the increase of complexity and investment for E-WHR, especially for automotive application. Currently, CTRC is attracting more and more attention, as another suitable choice for E-WHR. Except the good environmental performance and down-sized potential due to good heat transfer and flowing property, the stable chemical property of CO2 makes direct heat transfer with the high-temperature exhaust gas. Chen et al. [8] conducted a CTRC and a CO2 brayton cycle for exhaust gas recovery of a vehicle engine, and the results indicate that the CTRC is more suitable for the exhaust gas recovery than the CO2 brayton cycle. Engine coolant is another valuable waste heat with similar amount of thermal energy versus exhaust gas. However it is a low-
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Nomenclature H h m P Power Q q t W
liquid level (cm) specific enthalpy (kJ/kg) mass flow rate (kg/s) pressure (MPa) power output (kW) heat flow rate (kW) calorific value (kJ/kg) temperature ( C) work (kW)
Greek letters h efficiency Subscripts c ec est f g
cooling water engine coolant estimation working fluid exhaust gas
grade waste heat (less than 100 C), which can only lead to low thermal efficiency when used as single heat source for power cycles. Hence, there are few researches on single recovery of engine coolant. Peris et al. [9] simulated six ORC configurations with ten non-flammable working fluids, finding the most efficient pair was double regenerative configuration using SES36 as working fluid, which provided net electrical efficiency of 7.15%. Yang et al. [10] investigated six fluids in a regenerative ORC system and found that R245ca obtained the highest thermal efficiency at 4.56%. Compared with single recovery, a more reasonable and potential solution is to use engine coolant as a preheat source on the basis of exhaust gas recovery. Boretti [11] indicated that the single recovery of exhaust gas and engine coolant acquired an increase in engine efficiency up to 6.4% and 2.8%, respectively, and the increase could reach up to 8.2% when engine coolant was used as a preheat source from a combined recovery. Yang et al. [12] compared ORC performance with six fluids by engine coolant preheating and then indicated that R1234yf performed the best in the optimal economic evaluation. After the scavenge air cooling water was used to first preheat engine coolant, a preheated R1234yf-based ORC obtained 2.96% lower levelized energy cost and 21.6% lower CO2 reduction than the common preheated cycle [13]. However, most ORCs cannot obtain high utilization rate of engine coolant when it is used as a preheat source [14e16]. For common preheated ORCs, Kim et al. [14] showed that only 40% of the waste heat from the engine coolant was utilized at most because the heat absorption from engine coolant was limited by the small temperature increase in preheating process. There was wide temperature region for fluids to absorb heat from hightemperature exhaust gas, but narrow one from low-temperature engine coolant. Therefore, to largely utilize both exhaust gas and engine coolant, a large specific heat capacity is required in the preheating process and a smaller one is required in further heating process. CO2 at the supercritical state just meets the requirement. Due to low critical temperature, there is a large peak of specific heat capacity of supercritical CO2 located in the temperature region of preheating process. Therefore, CTRC is more suitable selection than ORC for high utilization rate of exhaust gas and engine coolant, both of which could obtain such rate above 0.9. Relatively, R123-based
gen gh in net out preh res th turb w, 1-3 1e7 4, ideal
271
generator gas heater inlet net output outlet preheater residue thermal turbine state point of cooling water state point of CO2 state points for the ideal case
Abbreviations C1eC4 CTRC actions CTRC CO2-based Transcritical Rankine Cycle E1-E15 engine conditions EC engine coolant E-WHR engine waste heat recovery ORC Organic Rankine Cycle
transcritical ORC realized a little higher utilization rate of exhaust gas but a much lower utilization rate of engine coolant (less than 0.3) [16]. Shi et al. [17] summarized design conditions of CTRC for both complete recovery of exhaust gas and engine coolant, which called ‘ideal point’. For the direction for engineering design, a prediction formula of ‘ideal point’ was proposed. Available researches on preheated CTRC mainly focus on the thermodynamic performance of system at stable and constant engine conditions. At rated condition of a gasoline engine, Shu et al. [18] analyzed four CTRC configurations and obtained their selection maps showing their respective preponderant region of condition. The selection maps showed that the design condition of preheated CTRC was suitable at high turbine inlet pressure (higher than about 11 MPa) and low turbine inlet temperature (lower than about 250 C), where it presented better performance than other configurations. Shi et al. [19] tested the four CTRC configurations at a stable engine condition (1100 rpm, 50% load) and indicated that the preheater made more contributions to improvement of power output than regenerator, and also was beneficial for total cooling load of engine-CTRC combined system. Generally, engines, especially for automotive engines, operate at various conditions. It is significant to study dynamic performance of CTRC at various engine conditions. Except the improvement of output and the reduction of cooling load, the preheating process by engine coolant may keep the system stable and long-lasting under variable engine operating conditions, because of the slight change in the coolant temperature when engine condition changes when engine condition changes. Preheated CTRC may still have output potential as it benefits from engine coolant even if engine stops. However, researches on preheated CTRC at various engine conditions are rare and are mainly concentrated on regularly incremental engine conditions and corresponding steady performance. Farzaneh-Gord et al. [20] analyzed system performance of P-CTRC at 11 incremental speeds of gas engine, and showed that the power output performance increased with engine speed. Shu et al. [21] compared the CTRC performance at 5 incremental loads of gasoline engine and diesel engine and found that the power output performance increased with engine load. The engine load had slighter effect on the CTRC performance of gasoline engine, compared with diesel engine.
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The above discussions on regularly incremental engine conditions were still based on a stable heat source, which could indicated the output potential of CTRC but could not represent dynamic performance under real engine conditions. In fact, the heat sources change and follow with dynamic engine conditions. The aim of current study is to test response performance of a preheated CTRC prototype at various engine conditions, including starting, stopping, restarting, and random operating condition. Based on the dynamic response results, the benefits from the preheating are the focus of this study.
2. Descriptions of test bench To utilize exhaust gas and engine coolant of a diesel engine, a test bench of preheated CTRC was constructed, whose photo and diagram are shown in Figs. 1 and 2 respectively. Table 1 presents main parameters of the CTRC and the diesel engine. In this study, operating the diesel engine at low and medium loads can provide enough waste heat for the CTRC, which is a small scale system. As a turbine was not available, the expansion valve was adopted temporarily to realize high and low pressures of the test bench. Table 2 lists types and specifications of main components. The high-pressure CO2 from pump outlet is preheated in the preheater by engine coolant, and then further heated in the gas heater by exhaust gas; the obtained high-temperature CO2 gas flows through the expansion valve; then CO2 gas enters into the precooler and condenser and becomes CO2 liquid; and finally, after collected in the CO2 tank, the CO2 liquid is pumped to high-pressure side and completes a cycle. A branch scheme was adopted for engine coolant system, instead of the direct scheme with engine coolant flowing through engine and preheater in succession. A part of flow driven by the EC preheating pump was introduced to preheat CO2 from the EC tank. The scheme can stably cool engine and provide enough engine coolant for preheating the CTRC, and is suitable for controlling the
engine-CTRC combined system at laboratory conditions. In this study, four kinds of engine conditions were operated, including starting and ‘warm up’, idling and stopping, restarting, and random conditions. During experimental process, the CTRC was operating at a fixed opening of expansion valve. The EC preheating pump, EC pump, and cooling water pump were operating at fixed speed. Due to ambient temperature changing from 13 C to 27 C for different experiment in this study, the temperature of cooling water was unstable over a wide range of 7e16 C. Parameters of CTRC can be recorded in every second, such as pressure and temperature of each state, mass flow rate of CO2, and liquid level. Analysis of response performance is conducted through the change data of the parameters. Table 3 lists types and precisions of main components. Mass flow rate of CO2 is measured by a coriolis flowmeter, which adopts a high-pressure type specially. The liquid and gas flow rate are both measured as a volume data by general flowmeter. For maintaining high pressure of CO2, liquid level is measured by a magnetic flap type liquidometer. Resistance temperature detector is adopted for CO2 and water, whose tempereure range mainly below 300 C. While for the high-tempereure exhaust gas, thermocouple is adopted instead of resistance temperature detector. By a detailed error analysis of data referring to the error models in our previous paper [22], the maximum uncertainty of heat transfer quantity in CO2 side, exhaust gas side and engine coolant side is 5.3%, 4.7%, and 6.2% respectively. 3. Experiments and results This research involves various engine conditions, referring to starting, stopping, restarting, and random operating condition. Specially, detailed analysis of heat balance were conducted and illustrated in Appendix A as a convenient reference of engine conditions and waste heat quantity. In generally speaking, engine power, waste heat quantity of exhaust gas and engine coolant increase with an increase in engine speed or engine torque. 3.1. Response under starting and ‘warm up’ condition of engine
Fig. 1. Photo of the test bench.
To represent practical starting and‘warm up’ process of engine, the diesel engine operates from start, small load, medium load then stably operates at a constant condition in this test. The engine speed and engine load are controlled by an engine controller, and all changes of engine conditions are named as E1-E9. The operating changes of the CO2 pump and valve are named as C1eC4, and operating change of the EC preheating pump is named as EC1. The main parameters are displayed in Figs. 3 and 4. Among the system parameters, pressure is an important indicator of CTRC performance, which mainly decides the power ability of the system, and has quicker response than temperature when external conditions change, such as temperature or flow rate of heat sources and pump speed. As shown in Fig. 3, the inlet (state 3) and outlet (state 4) pressures between the expansion valve vary significantly with changes in the engine, the CTRC, and the EC preheating pump. As there is no enough heat to drive the CTRC when the engine starts at idle and low-load conditions, the CTRC does not have to be started at the early starting process of engine. Hence, the CO2 pump starts (C1) after over 20 min of engine condition ascension (E1-E5) until the exhaust inlet temperature (tg,in) reaches to about 200 C, as shown in Fig. 4. After the CO2 pump starts, a steep increase appears for P3 and P4, and there is pressure difference between them. The obtained temperature of CO2 from the gas heater (t3) has a convex-shaped change after the CO2 pump starts. It is because that the small flow comes into the gas heater at the starting moment of pump, which makes the temperature rise for a while.
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Fig. 2. Diagram of the test bench.
Table 1 Main parameters of the CTRC and the diesel engine. Parameter of P-CTRC (unit)
Values
Parameter of diesel engine (unit)
Values
Expansion inlet pressure [MPa] Expansion outlet pressure [MPa] Expansion inlet temperature [ C] Mass flow rate of CO2 [kg/s] Power generation magnitude [kW]
10 6 231 0.18 4.5
Total displacement [L] Rated power [kW] Rated speed [rpm] Maximum torque [N.m] Cylinders []
8.424 243 2200 1280 6
Table 2 Types and specifications of main components. Components
Types
Specifications
Gas heater Preheater, Precooler, Condenser Expansion valve CO2 tank CO2 pump Refrigerating unit
Double-pipe Brazed plate Needle valve Vertical Reciprocating plunger R22
3.09 m2 1.56 m2 0e100% opening/(±1%) 10 L 1.7 m3/h e
Then, with the follow-up increase of engine condition (E6-E9), temperature of the exhaust gas increases gently, which means more generation of waste heat energy. Hence, the P3 and the t3 rise sequentially, and the P3 have step increase after each action of the engine, and compared with pressure changes, temperature changes are milder. Besides, increasing CO2 pump speed (C3) also leads to
sharp increase in P3 and sharp decrease in t3. The response performance of pressure and temperature conforms to the characteristic of a transcritical power cycle. At the early ‘warm up’ process of engine, the temperature of engine coolant is too low to be used as a preheat source. What's more, the low-temperature engine coolant is unfit for efficient
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L. Shi et al. / Energy 171 (2019) 270e283 Table 3 Types and precisions of measuring devices. Measuring devices
Types
Precision
CO2 flowmeter Air flowmeter Fuel consumption meter EC flowmeter 1 Cooling water flowmeter Liquidometer for the CO2 tank Thermocouple (exhaust gas) Resistance temperature detector (CO2 and water) Pressure transmitter (gas and water) Pressure transmitter (CO2 at low pressure side) Pressure transmitter (CO2 at high pressure side)
Coriolis Laminar flow e Turbine Turbine Magnetic flap type Armoured Armoured P100 Low pressure type High pressure type High pressure type
0e0.3 kg/s (±0.2%) 0e1350 kg/h (±0.5%) 5e2000 kg/h (±0.8%) 0e10 m3/h (±0.5%) 0e12 m3/h (±1%) 0e30 cm (±3.3%) 60-650 C/(±1%) 200-500 C/(±0.15%) 0e0.5 MPa (±0.065%) 0e12 MPa (±0.065%) 0e14 MPa (±0.065%)
Fig. 3. Change of CTRC pressure, liquid level and mass flow rate during engine starting.
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Fig. 4. Change of temperature in the preheater and the gas heater during engine starting.
engine operation, hence, it is suitable to introduce the engine coolant after its temperature reach to a relatively high value (>60 C). The CO2 pressures further increase after EC preheating pump starts (EC1). Due to residual cool coolant existing in EC pipes before EC1, the response of CO2 pressure and temperature has about 1 min delay after EC1. Actions of both the CO2 pump and the EC preheating pump would cause the significant increase in pressure and then a slight reduction. For system safety, the operating system should pay attention to the sudden rise in pressure. With the addition of preheating process, the inlet temperature of CO2 (t2) increases. There is good following relationship between the outlet temperature of exhaust gas (tg,out) and the t2. The temperature difference between them is within a range of 6e23 C after the CO2 pump starts, which is about 20 C at most time.
CO2 liquid level is another important parameter for stable operation of the CTRC, which has to stop if there is no liquid in the tank. As shown in Fig. 3, after the CO2 pump runs (C1), the liquid level has a rapid and drastic drop and then rises steadily with engine condition (E6-E9). It is because that the CTRC requires enough CO2 to fill the heat exchangers and pipes at the starting, and the more energy input accelerates CO2 from high-temperature side to low-temperature side. Then with the induction of engine coolant, the liquid level obtains a large increase and reaches a relative safe value for safe operation of the CTRC. The mass flow rate of CO2 mainly depends on the CO2 pump speed, and has varies little due to the system pressures. On the one hand, low-side pressure decides the saturated density of liquid CO2, thereby affecting the pump output of CO2. On the other hand, the increase of high-side pressure
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reduces pump efficiency, which in turn reduces the pump output. To understand the thermodynamic performance of the CTRC system, progressive pressure conditions are achieved by adjusting the expansion valve (C4) for several times when the engine operates at stable condition (1100 rpm, 601 N m). After the expansion inlet parameters (state 3) are obtained, an estimation method is used for representing potential of power output and thermal efficiency, which assumes turbine efficiency of 0.7 and generator efficiency of 0.9 [14,17]. Main equations are listed as below:
Qpreh ¼ mf $ðh2 h1 Þ
(1)
Qgh ¼ mf $ðh3 h2 Þ
(2)
Wturb;est ¼ mf $ðh3 h4;ideal Þ$hturb
(3)
Powerest ¼ Wturb;est $hgen
(4)
Wpump ¼ mf $ðh1 h5 Þ
(5)
Wnet;est ¼ Powerest Wpump
(6)
Wnet;est Qgh þ Qpreh
(7)
hth;est ¼
Fig. 5. Estimation of net power output and thermal efficiency with variation of pressure ratio.
wherein, 4,ideal is an assumed state that shall have the same pressure as state 4 and it is assumed that CO2 expands from state 3 in an isentropic manner. Table 4 gives the main results at different pressure conditions, and Fig. 5 presents the variation line of an estimation of net power output and thermal efficiency. Estimation of net power output and thermal efficiency increases directly with an increase in pressure ratio and expansion inlet pressure. Therefore, expansion inlet pressure has a direct impact on output and efficiency performance of the CTRC system in this study, which is also the focus in the following analysis. The preheating process can improve system performance as it obtains higher pressure. In addition, it can also result in higher liquid level to address the risk of liquid lack and maintains the operation of system.
Fig. 6. Change of expansion inlet pressure under engine stopping and idling.
preheating, and idling without preheating, in which stopping or idling represent engine condition, and with or without preheating represent CTRC configuration. After the steady running period at constant and same engine condition (1100 rpm, 601 N m) and CO2 pump speed (80 rpm), similar expansion inlet pressures are obtained for the four groups within a range of 10.2e10.5 MPa, which are considered as similar initial CTRC conditions for the four groups. To mitigate the small difference, a fair comparison is made using a percentage change in pressure relative to the initial data. Fig. 7 presents the treated results of 20% reduction. After stopping or idling of engine, the four groups show different time period from 100% to 80% pressure change. The pressure of the CTRC maintains for a longer time under idling condition of engine, compared with that under stopping condition of engine. After all, there is still a
3.2. Response under stopping and idling condition of engine For an automotive engine, short stopping and idling are usually conditions under road conditions. Exhaust gas flow stops and reduces to a minimum at stopping condition and idling condition, respectively. Hence, it is significant to know how the CTRC performs under such low conditions. In this study, sudden stopping and idling conditions are conducted after similar stable conditions are maintained. Two sets, with and without preheating, are provided for a comparative study by controlling the EC preheating pump on and off. Fig. 6 presents four groups of pressure results, including stopping with preheating, stopping without preheating, idling with Table 4 Main parameters and system performance at different pressure conditions. Pressure ratio [-]
P3 [MPa]
P4 [MPa]
t2 [ºC]
t3 [ºC]
tg,in [ºC]
tec,in [ºC]
Qgh [kW]
Qpreh [kW]
Powerest [kW]
Wnet,est [kW]
hth,est [%]
1.25 1.28 1.34 1.40 1.50 1.59 1.67
7.52 7.74 8.10 8.52 9.15 9.72 10.28
5.98 6.01 6.04 6.06 6.07 6.10 6.11
34.5 36.0 38.5 41.3 45.0 48.3 51.2
137.0 140.3 141.4 142.2 142.0 146.2 149.6
430.3 432.3 433.6 434.8 435.6 436.4 436.9
67.3 67.3 67.3 67.4 67.8 68.2 68.5
32.5 33.0 32.7 32.3 31.7 31.5 31.6
22.6 22.1 21.6 21.1 20.5 19.9 19.6
1.64 1.81 2.07 2.37 2.77 3.10 3.43
1.20 1.37 1.60 1.82 2.15 2.38 2.63
2.20 2.60 3.10 3.50 4.30 4.80 5.40
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Fig. 7. Percentage change of expansion inlet pressure under engine stopping or idling.
small amount of exhaust gas at the idling condition. On the other hand, as the CTRC with preheating keeps flow of engine coolant after stopping and idling, preheated CTRC extends more time than the CTRC without preheating. Therefore, the group of idling without preheating keeps the longest time (739 s) among the four, followed by the groups of idling without preheating (278 s), stopping with preheating (273 s), and stopping without preheating (127 s). The preheating process provides more than double time for the CTRC operating at high pressure. The results also can provide a reference of practical operation of the CTRC. For example, the automotive engine changes to idling condition only for a short period (e.g. situation of traffic light), it can continue to drive the CTRC and has no occasion to cut off the CTRC system. The sudden rise in pressure appears for the CTRC system without preheating at stopping condition. Such sudden fluctuant change is bad for system operation. It is significant to analyze the causes and avoid it. Firstly, as the other groups had no such phenomenon, the study conducted a repetitive experiment to confirm the phenomenon. As shown in Fig. 8, similar phenomenon occurs at two repetitive tests and thus it is confirmed.
Fig. 8. Repetitive results for sudden rise in pressure phenomenon without preheating under engine stopping.
277
The reasons can be explained from the aspect of P4, whose percentage change is presented in Fig. 9. Due to sudden reduction of flow of heat sources, the P3 decreases, and then P4 would decrease firstly but rise again, limited by cooling waste at the condensation side. Hence, stopping and idling of engine produce concave-shaped changes of P4. The scale extent of the concaveshaped changes is different for the four groups, which has different reduction degree of heat sources. For the group of engine stopping without preheating, an extreme reduction group without both exhaust gas and engine coolant, has an acute concave-shaped change, i.e. it drops first and then rises. The resulting large increase in the expansion outlet pressure only results in a pressure surge in the expansion outlet pressure. For the other groups, the concaveshaped change is small due to preheating or idling, and even there is no such change with both preheating and idling. Therefore, the preheating process can avoid the pressure's sudden rise phenomenon of the expansion inlet pressure. Besides, different reduction degree of heat sources also affects different reduction of liquid level. By a comparison shown in Fig. 10, the CTRC with preheating achieves smaller change in liquid level
Fig. 9. Percentage change of expansion outlet pressure under engine stopping and idling.
Fig. 10. Change of expansion outlet pressure under engine stopping and idling.
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than that without preheating. The experiment of stopping without preheating has the fast reduction of liquid level, which results in the shortest time for effective operation. Therefore, the preheating process is also beneficial to maintain safe liquid level under engine stopping and idling.
3.3. Response under restarting condition of engine As analyzed above, the preheated CTRC can maintain high expansion inlet pressure for a while after stopping and idling of engine. In this section, a restarting condition of engine is conducted for the preheated CTRC as follows: after stable operation of the CTRC for a while under certain engine condition (1100 rpm and 601 N.m), the engine condition changes to idling and stopping successively, then restarts at idling with the increase to 1100 rpm and 601 N.m again. Some parameters under the restarting condition of engine are presented in Fig. 11 and Fig. 12. The expansion inlet pressure and temperature decrease from 8.98 MPa, 192.3 C to 7.72 MPa, 75.9 C after 4.3 min idling and 4.2 min stopping of engine, successively. Then they climb up to 8.82 MPa, 172.4 C after the engine restarts again within 2.2 min idling and 12.0 min middle condition (1100 rpm and 601 N m). The CO2 liquid level is full in the tank for most lengths of time. During the restarting process of engine, the engine coolant maintains a small temperature change within 59.7e70.1 C, which provides persistent heat source for the CTRC although there is no exhaust gas under engine stopping condition. As shown asⅠregion in Fig. 12, the engine coolant, instead of exhaust gas, becomes the main heat source instead of exhaust gas after engine idling for a while (about 1 min). Although the capacity of preheating also reduces, its reduction speed is more slower than gas heating due to the large heat capacity of engine coolant. Hence, the exhaust gas
makes more contribution again after engine restarts for a while (about half of mins), as shown asⅡregion in Fig. 12. The decreased capacity of engine coolant in the stopping condition would increase again in theⅡregion. The cross change between heat absorption quantity of exhaust gas and engine coolant is well adapted to stopping (or idling) erestarting condition of engine. The preheating process plays an important compensation role at the low-load engine conditions.
3.4. Response under random conditions of engine In view of complicated operating conditions in practice, a group at random conditions of engine was conducted in this study for the preheated CTRC. As shown in Fig. 13, the random conditions involve 15 conditions (E1-E15) with change of engine speed or engine torque at intervals of about 1e2 min, with a wide range of engine power (18.4e95.2 kW), engine speed (700e1400 rpm) and engine torque (100e700 N m). As illustrated in Section 3.1-3.3, the preheating process makes for maintaining pressure and liquid level. Therefore, under the 22.4 min and big-change random conditions, the expansion inlet pressure of the preheated CTRC still keeps stable within a range of 9.20e10.06 MPa as presented in Fig. 14, as well as CO2 liquid level with only 1 cm change and CO2 mass flow rate with ±1.5% change, as shown in Fig. 15. Small difference in pressure, liquid level and mass flow rate would be beneficial for stable and efficient operation of turbine and system (see Fig. 16). The random conditions influence large change of exhaust gas, and then affect the temperature and amount of exhaust gas, which result in larger change of expansion inlet temperature from 126.4 to 179.5 C than that of expansion inlet pressure. Hence, it greatly influences the heat absorption quantity (25.4e34.9 kW) and the estimation of net power output (1.71e2.88 kW), both of which have
Fig. 11. Change of CO2 pressures and liquid level under engine restarting condition.
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Fig. 12. Change of temperatures and heat absorption quantity under engine restarting condition of engine.
Fig. 13. Speed and torque under random engine conditions.
the similar variations trends with the exhaust temperature. As continuous preheating provides stable heat absorption quantity (19.5e22.8 kW) from engine coolant, the CO2 has a quite high temperature (46.8e53.0 C) into the gas heater all the way.
Therefore, the preheating process can maintain CO2 pressure, temperature and liquid level and improve the heat absorption and the output performance under the random conditions of engine, as well as it can under the other engine conditions in Section 3.1-3.3.
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Fig. 14. Change of main pressures and temperatures under random engine conditions.
Fig. 15. Change of liquid level and mass flow rate under random engine conditions.
4. Conclusions Based on a preheated CTRC test bench for waste heat recovery of exhaust gas and engine coolant, a detailed experimental study was
conducted under various engine conditions, i.e. starting and ‘warm up’ condition, stopping and idling condition, restarting condition and random conditions. The results show that the preheating process can improve the dynamic performance of the CTRC at
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Fig. 16. Change of estimation of net power output and heat absorption quantity under random engine conditions.
various engine conditions. On the one hand, it is beneficial for continuous output capacity by maintaining high pressure and temperature of CO2 for extended time, and it is beneficial for safe operation by keeping high liquid level of CO2 on the other hand. Besides, the preheating process also can avoid the sudden rise in the expansion inlet pressure under the stopping condition of engine, and meanwhile it plays an important energy supplement role in the random conditions of the engine, resulting in the cross change in the waste heat absorption from engine coolant and exhaust gas.
Qfuel;total ¼ mfuel $qfuel
(A.1)
wherein, qfuel is the calorific value of diesel fuel that equals to 42500 kJ/kg in this study. Except engine power (We), the other three parts are calculated as follows:
Qg ¼ mg $ðhg;in hg;0 Þ
(A.2)
Acknowledgements
Qec ¼ mec $ðhec;obtained hec;back Þ
(A.3)
The authors would like to acknowledge the National Natural Science Foundation of China (No. 51636005) for grants and supports.
Qres ¼ Qfuel;total We Qg Qec
(A.4)
Appendix A A heat balance test of diesel engine with CTRC system was conducted to investigate waste heat capacity of the engine, which can be a reference for various engine conditions mentioned above. During the test, the engine speed varied from 1000 rpm to 1500 rpm with a 100 rpm interval, while the load varied from 20% to 60% load with a 10% interval at every speed with a total torque range from 229 N m to 770 N m. Four parts are included in heat balance, i.e. engine power, waste heat quantity of exhaust gas, waste heat quantity of engine coolant, and residue energy (e.g. intercooling, oil cooling). The total combustion energy of fuel is calculated as
The property of exhaust gas is calculated by an approximation method, which is decided and averaged by the proportion of gas composition. The detailed method can be found in the precious research by our group [22]. Different from the heat balance test in Ref. [22] without recovery system, the test in this study is on the basis of connecting CTRC system. Only in this way, can the results have reference for various engine conditions in this study. The maps of the four parts are presented in Fig. 17. Meanwhile, the energy distribution percentage at 60% load and 1500 rpm speed is shown in Fig. 18. Obviously, engine power and waste heat of exhaust gas have clear increase with an increase in engine speed or engine torque. However, the waste heat of engine coolant is only significantly related to the engine load. At 1500 rpm and 60% load, the engine power occupies 38.1%, while waste heats of exhaust gas and engine coolant occupy 30.6% and 22.3%, respectively.
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Fig. 17. Heat balance performance of engine.
Fig. 18. Variation of energy distribution with engine load and speed.
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Contents lists available at ScienceDirect
Energy journal homepage: www.elsevier.com/locate/energy
Dynamic tests of CO2-Based waste heat recovery system with preheating process Lingfeng Shi a, b, Gequn Shu b, *, Hua Tian b, **, Tianyu Chen b, Peng Liu b, Ligeng Li b a b
Department of Thermal Science and Energy Engineering, University of Science and Technology of China, Hefei 230027, China State Key Laboratory of Engines, Tianjin University, 92 Weijin Road, Nankai District, 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 3 July 2018 Received in revised form 30 September 2018 Accepted 18 December 2018 Available online 21 December 2018
For engine waste heat recovery, CO2-based Transcritical Rankine Cycle (CTRC) system with preheating process has a strong capability of combining exhaust gas and engine coolant, thereby leading to improvement of output, efficiency improvement and the reduction in cooling load reduction. In previous researches, all the benefits were achieved and investigated mainly under ideal and stable engine conditions in previous researches. But in this study, various engine conditions were considered for a preheated CTRC test bench, namely starting and ‘warm up’ condition, stopping and idling condition, restarting condition and random condition. After system parameters (e.g. pressure, temperature) were measured, heat absorption quantity was calculated, and meanwhile net power output and thermal efficiency were estimated. The results show that the preheating process can improve the dynamic performance of the CTRC at various engine conditions. Firstly, the preheating process is beneficial for continuous output capacity and safe operation of CTRC. Besides, the preheating process also can avoid the sudden rise in the expansion inlet pressure under the stopping condition of engine, and meanwhile it plays an important energy supplement role in the random conditions of the engine, resulting in the cross change in the waste heat absorption from engine coolant and exhaust gas. © 2018 Elsevier Ltd. All rights reserved.
Keywords: Engine waste heat recovery CO2-based transcritical Rankine cycle (CTRC) Dynamic test Various engine conditions Preheating benefits
1. Introduction At present, the thermal efficiency of diesel engine is commonly less than 45% [1], and that of the gasoline engine is even lower [2], with over half of fuel energy wasted, mainly existing in exhaust gas and engine coolant. Therefore, engine waste heat recovery (EWHR) technologies show large potential to improve engine efficiency. Among E-WHR technologies, power cycle is a research hot due to its high efficiency, such as Organic Rankine Cycle (ORC) [3] and CO2-based Transcritical Rankine Cycle (CTRC) [4]. Generally, exhaust gas is the primary goal of E-WHR due to its high grade of temperature and large quantity of energy. Exhaust temperature could approach over 700 C in a gasoline engine and over 400 C in a diesel engine, and its energy accounts for over 30% of total fuel combustion energy [5]. Many organic fluids have low thermal destruction temperature so they cannot achieve direct heat transfer with high-temperature exhaust gas. So far, a few modified
* Corresponding author.; ** Corresponding author. E-mail addresses: [email protected] (G. Shu), [email protected] (H. Tian). https://doi.org/10.1016/j.energy.2018.12.123 0360-5442/© 2018 Elsevier Ltd. All rights reserved.
and novel ORCs have been investigated to solve this problem. Shu et al. [6] adopted high-temperature organic fluids (alkanes) and studied the energy and exergy performance for waste heat recovery of a diesel engine, finding that cyclohexane and cyclopentane were the most suitable fluids, but the flammability of alkanes is the main problem during application. Vaja [7] conducted a medium circuit (oil) between the exhaust gas and organic fluids, not only for safety but also for stabilizing the transient operation of the ORC system. However, addition of the big heat exchangers leads to the increase of complexity and investment for E-WHR, especially for automotive application. Currently, CTRC is attracting more and more attention, as another suitable choice for E-WHR. Except the good environmental performance and down-sized potential due to good heat transfer and flowing property, the stable chemical property of CO2 makes direct heat transfer with the high-temperature exhaust gas. Chen et al. [8] conducted a CTRC and a CO2 brayton cycle for exhaust gas recovery of a vehicle engine, and the results indicate that the CTRC is more suitable for the exhaust gas recovery than the CO2 brayton cycle. Engine coolant is another valuable waste heat with similar amount of thermal energy versus exhaust gas. However it is a low-
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Nomenclature H h m P Power Q q t W
liquid level (cm) specific enthalpy (kJ/kg) mass flow rate (kg/s) pressure (MPa) power output (kW) heat flow rate (kW) calorific value (kJ/kg) temperature ( C) work (kW)
Greek letters h efficiency Subscripts c ec est f g
cooling water engine coolant estimation working fluid exhaust gas
grade waste heat (less than 100 C), which can only lead to low thermal efficiency when used as single heat source for power cycles. Hence, there are few researches on single recovery of engine coolant. Peris et al. [9] simulated six ORC configurations with ten non-flammable working fluids, finding the most efficient pair was double regenerative configuration using SES36 as working fluid, which provided net electrical efficiency of 7.15%. Yang et al. [10] investigated six fluids in a regenerative ORC system and found that R245ca obtained the highest thermal efficiency at 4.56%. Compared with single recovery, a more reasonable and potential solution is to use engine coolant as a preheat source on the basis of exhaust gas recovery. Boretti [11] indicated that the single recovery of exhaust gas and engine coolant acquired an increase in engine efficiency up to 6.4% and 2.8%, respectively, and the increase could reach up to 8.2% when engine coolant was used as a preheat source from a combined recovery. Yang et al. [12] compared ORC performance with six fluids by engine coolant preheating and then indicated that R1234yf performed the best in the optimal economic evaluation. After the scavenge air cooling water was used to first preheat engine coolant, a preheated R1234yf-based ORC obtained 2.96% lower levelized energy cost and 21.6% lower CO2 reduction than the common preheated cycle [13]. However, most ORCs cannot obtain high utilization rate of engine coolant when it is used as a preheat source [14e16]. For common preheated ORCs, Kim et al. [14] showed that only 40% of the waste heat from the engine coolant was utilized at most because the heat absorption from engine coolant was limited by the small temperature increase in preheating process. There was wide temperature region for fluids to absorb heat from hightemperature exhaust gas, but narrow one from low-temperature engine coolant. Therefore, to largely utilize both exhaust gas and engine coolant, a large specific heat capacity is required in the preheating process and a smaller one is required in further heating process. CO2 at the supercritical state just meets the requirement. Due to low critical temperature, there is a large peak of specific heat capacity of supercritical CO2 located in the temperature region of preheating process. Therefore, CTRC is more suitable selection than ORC for high utilization rate of exhaust gas and engine coolant, both of which could obtain such rate above 0.9. Relatively, R123-based
gen gh in net out preh res th turb w, 1-3 1e7 4, ideal
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generator gas heater inlet net output outlet preheater residue thermal turbine state point of cooling water state point of CO2 state points for the ideal case
Abbreviations C1eC4 CTRC actions CTRC CO2-based Transcritical Rankine Cycle E1-E15 engine conditions EC engine coolant E-WHR engine waste heat recovery ORC Organic Rankine Cycle
transcritical ORC realized a little higher utilization rate of exhaust gas but a much lower utilization rate of engine coolant (less than 0.3) [16]. Shi et al. [17] summarized design conditions of CTRC for both complete recovery of exhaust gas and engine coolant, which called ‘ideal point’. For the direction for engineering design, a prediction formula of ‘ideal point’ was proposed. Available researches on preheated CTRC mainly focus on the thermodynamic performance of system at stable and constant engine conditions. At rated condition of a gasoline engine, Shu et al. [18] analyzed four CTRC configurations and obtained their selection maps showing their respective preponderant region of condition. The selection maps showed that the design condition of preheated CTRC was suitable at high turbine inlet pressure (higher than about 11 MPa) and low turbine inlet temperature (lower than about 250 C), where it presented better performance than other configurations. Shi et al. [19] tested the four CTRC configurations at a stable engine condition (1100 rpm, 50% load) and indicated that the preheater made more contributions to improvement of power output than regenerator, and also was beneficial for total cooling load of engine-CTRC combined system. Generally, engines, especially for automotive engines, operate at various conditions. It is significant to study dynamic performance of CTRC at various engine conditions. Except the improvement of output and the reduction of cooling load, the preheating process by engine coolant may keep the system stable and long-lasting under variable engine operating conditions, because of the slight change in the coolant temperature when engine condition changes when engine condition changes. Preheated CTRC may still have output potential as it benefits from engine coolant even if engine stops. However, researches on preheated CTRC at various engine conditions are rare and are mainly concentrated on regularly incremental engine conditions and corresponding steady performance. Farzaneh-Gord et al. [20] analyzed system performance of P-CTRC at 11 incremental speeds of gas engine, and showed that the power output performance increased with engine speed. Shu et al. [21] compared the CTRC performance at 5 incremental loads of gasoline engine and diesel engine and found that the power output performance increased with engine load. The engine load had slighter effect on the CTRC performance of gasoline engine, compared with diesel engine.
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The above discussions on regularly incremental engine conditions were still based on a stable heat source, which could indicated the output potential of CTRC but could not represent dynamic performance under real engine conditions. In fact, the heat sources change and follow with dynamic engine conditions. The aim of current study is to test response performance of a preheated CTRC prototype at various engine conditions, including starting, stopping, restarting, and random operating condition. Based on the dynamic response results, the benefits from the preheating are the focus of this study.
2. Descriptions of test bench To utilize exhaust gas and engine coolant of a diesel engine, a test bench of preheated CTRC was constructed, whose photo and diagram are shown in Figs. 1 and 2 respectively. Table 1 presents main parameters of the CTRC and the diesel engine. In this study, operating the diesel engine at low and medium loads can provide enough waste heat for the CTRC, which is a small scale system. As a turbine was not available, the expansion valve was adopted temporarily to realize high and low pressures of the test bench. Table 2 lists types and specifications of main components. The high-pressure CO2 from pump outlet is preheated in the preheater by engine coolant, and then further heated in the gas heater by exhaust gas; the obtained high-temperature CO2 gas flows through the expansion valve; then CO2 gas enters into the precooler and condenser and becomes CO2 liquid; and finally, after collected in the CO2 tank, the CO2 liquid is pumped to high-pressure side and completes a cycle. A branch scheme was adopted for engine coolant system, instead of the direct scheme with engine coolant flowing through engine and preheater in succession. A part of flow driven by the EC preheating pump was introduced to preheat CO2 from the EC tank. The scheme can stably cool engine and provide enough engine coolant for preheating the CTRC, and is suitable for controlling the
engine-CTRC combined system at laboratory conditions. In this study, four kinds of engine conditions were operated, including starting and ‘warm up’, idling and stopping, restarting, and random conditions. During experimental process, the CTRC was operating at a fixed opening of expansion valve. The EC preheating pump, EC pump, and cooling water pump were operating at fixed speed. Due to ambient temperature changing from 13 C to 27 C for different experiment in this study, the temperature of cooling water was unstable over a wide range of 7e16 C. Parameters of CTRC can be recorded in every second, such as pressure and temperature of each state, mass flow rate of CO2, and liquid level. Analysis of response performance is conducted through the change data of the parameters. Table 3 lists types and precisions of main components. Mass flow rate of CO2 is measured by a coriolis flowmeter, which adopts a high-pressure type specially. The liquid and gas flow rate are both measured as a volume data by general flowmeter. For maintaining high pressure of CO2, liquid level is measured by a magnetic flap type liquidometer. Resistance temperature detector is adopted for CO2 and water, whose tempereure range mainly below 300 C. While for the high-tempereure exhaust gas, thermocouple is adopted instead of resistance temperature detector. By a detailed error analysis of data referring to the error models in our previous paper [22], the maximum uncertainty of heat transfer quantity in CO2 side, exhaust gas side and engine coolant side is 5.3%, 4.7%, and 6.2% respectively. 3. Experiments and results This research involves various engine conditions, referring to starting, stopping, restarting, and random operating condition. Specially, detailed analysis of heat balance were conducted and illustrated in Appendix A as a convenient reference of engine conditions and waste heat quantity. In generally speaking, engine power, waste heat quantity of exhaust gas and engine coolant increase with an increase in engine speed or engine torque. 3.1. Response under starting and ‘warm up’ condition of engine
Fig. 1. Photo of the test bench.
To represent practical starting and‘warm up’ process of engine, the diesel engine operates from start, small load, medium load then stably operates at a constant condition in this test. The engine speed and engine load are controlled by an engine controller, and all changes of engine conditions are named as E1-E9. The operating changes of the CO2 pump and valve are named as C1eC4, and operating change of the EC preheating pump is named as EC1. The main parameters are displayed in Figs. 3 and 4. Among the system parameters, pressure is an important indicator of CTRC performance, which mainly decides the power ability of the system, and has quicker response than temperature when external conditions change, such as temperature or flow rate of heat sources and pump speed. As shown in Fig. 3, the inlet (state 3) and outlet (state 4) pressures between the expansion valve vary significantly with changes in the engine, the CTRC, and the EC preheating pump. As there is no enough heat to drive the CTRC when the engine starts at idle and low-load conditions, the CTRC does not have to be started at the early starting process of engine. Hence, the CO2 pump starts (C1) after over 20 min of engine condition ascension (E1-E5) until the exhaust inlet temperature (tg,in) reaches to about 200 C, as shown in Fig. 4. After the CO2 pump starts, a steep increase appears for P3 and P4, and there is pressure difference between them. The obtained temperature of CO2 from the gas heater (t3) has a convex-shaped change after the CO2 pump starts. It is because that the small flow comes into the gas heater at the starting moment of pump, which makes the temperature rise for a while.
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Fig. 2. Diagram of the test bench.
Table 1 Main parameters of the CTRC and the diesel engine. Parameter of P-CTRC (unit)
Values
Parameter of diesel engine (unit)
Values
Expansion inlet pressure [MPa] Expansion outlet pressure [MPa] Expansion inlet temperature [ C] Mass flow rate of CO2 [kg/s] Power generation magnitude [kW]
10 6 231 0.18 4.5
Total displacement [L] Rated power [kW] Rated speed [rpm] Maximum torque [N.m] Cylinders []
8.424 243 2200 1280 6
Table 2 Types and specifications of main components. Components
Types
Specifications
Gas heater Preheater, Precooler, Condenser Expansion valve CO2 tank CO2 pump Refrigerating unit
Double-pipe Brazed plate Needle valve Vertical Reciprocating plunger R22
3.09 m2 1.56 m2 0e100% opening/(±1%) 10 L 1.7 m3/h e
Then, with the follow-up increase of engine condition (E6-E9), temperature of the exhaust gas increases gently, which means more generation of waste heat energy. Hence, the P3 and the t3 rise sequentially, and the P3 have step increase after each action of the engine, and compared with pressure changes, temperature changes are milder. Besides, increasing CO2 pump speed (C3) also leads to
sharp increase in P3 and sharp decrease in t3. The response performance of pressure and temperature conforms to the characteristic of a transcritical power cycle. At the early ‘warm up’ process of engine, the temperature of engine coolant is too low to be used as a preheat source. What's more, the low-temperature engine coolant is unfit for efficient
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L. Shi et al. / Energy 171 (2019) 270e283 Table 3 Types and precisions of measuring devices. Measuring devices
Types
Precision
CO2 flowmeter Air flowmeter Fuel consumption meter EC flowmeter 1 Cooling water flowmeter Liquidometer for the CO2 tank Thermocouple (exhaust gas) Resistance temperature detector (CO2 and water) Pressure transmitter (gas and water) Pressure transmitter (CO2 at low pressure side) Pressure transmitter (CO2 at high pressure side)
Coriolis Laminar flow e Turbine Turbine Magnetic flap type Armoured Armoured P100 Low pressure type High pressure type High pressure type
0e0.3 kg/s (±0.2%) 0e1350 kg/h (±0.5%) 5e2000 kg/h (±0.8%) 0e10 m3/h (±0.5%) 0e12 m3/h (±1%) 0e30 cm (±3.3%) 60-650 C/(±1%) 200-500 C/(±0.15%) 0e0.5 MPa (±0.065%) 0e12 MPa (±0.065%) 0e14 MPa (±0.065%)
Fig. 3. Change of CTRC pressure, liquid level and mass flow rate during engine starting.
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Fig. 4. Change of temperature in the preheater and the gas heater during engine starting.
engine operation, hence, it is suitable to introduce the engine coolant after its temperature reach to a relatively high value (>60 C). The CO2 pressures further increase after EC preheating pump starts (EC1). Due to residual cool coolant existing in EC pipes before EC1, the response of CO2 pressure and temperature has about 1 min delay after EC1. Actions of both the CO2 pump and the EC preheating pump would cause the significant increase in pressure and then a slight reduction. For system safety, the operating system should pay attention to the sudden rise in pressure. With the addition of preheating process, the inlet temperature of CO2 (t2) increases. There is good following relationship between the outlet temperature of exhaust gas (tg,out) and the t2. The temperature difference between them is within a range of 6e23 C after the CO2 pump starts, which is about 20 C at most time.
CO2 liquid level is another important parameter for stable operation of the CTRC, which has to stop if there is no liquid in the tank. As shown in Fig. 3, after the CO2 pump runs (C1), the liquid level has a rapid and drastic drop and then rises steadily with engine condition (E6-E9). It is because that the CTRC requires enough CO2 to fill the heat exchangers and pipes at the starting, and the more energy input accelerates CO2 from high-temperature side to low-temperature side. Then with the induction of engine coolant, the liquid level obtains a large increase and reaches a relative safe value for safe operation of the CTRC. The mass flow rate of CO2 mainly depends on the CO2 pump speed, and has varies little due to the system pressures. On the one hand, low-side pressure decides the saturated density of liquid CO2, thereby affecting the pump output of CO2. On the other hand, the increase of high-side pressure
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reduces pump efficiency, which in turn reduces the pump output. To understand the thermodynamic performance of the CTRC system, progressive pressure conditions are achieved by adjusting the expansion valve (C4) for several times when the engine operates at stable condition (1100 rpm, 601 N m). After the expansion inlet parameters (state 3) are obtained, an estimation method is used for representing potential of power output and thermal efficiency, which assumes turbine efficiency of 0.7 and generator efficiency of 0.9 [14,17]. Main equations are listed as below:
Qpreh ¼ mf $ðh2 h1 Þ
(1)
Qgh ¼ mf $ðh3 h2 Þ
(2)
Wturb;est ¼ mf $ðh3 h4;ideal Þ$hturb
(3)
Powerest ¼ Wturb;est $hgen
(4)
Wpump ¼ mf $ðh1 h5 Þ
(5)
Wnet;est ¼ Powerest Wpump
(6)
Wnet;est Qgh þ Qpreh
(7)
hth;est ¼
Fig. 5. Estimation of net power output and thermal efficiency with variation of pressure ratio.
wherein, 4,ideal is an assumed state that shall have the same pressure as state 4 and it is assumed that CO2 expands from state 3 in an isentropic manner. Table 4 gives the main results at different pressure conditions, and Fig. 5 presents the variation line of an estimation of net power output and thermal efficiency. Estimation of net power output and thermal efficiency increases directly with an increase in pressure ratio and expansion inlet pressure. Therefore, expansion inlet pressure has a direct impact on output and efficiency performance of the CTRC system in this study, which is also the focus in the following analysis. The preheating process can improve system performance as it obtains higher pressure. In addition, it can also result in higher liquid level to address the risk of liquid lack and maintains the operation of system.
Fig. 6. Change of expansion inlet pressure under engine stopping and idling.
preheating, and idling without preheating, in which stopping or idling represent engine condition, and with or without preheating represent CTRC configuration. After the steady running period at constant and same engine condition (1100 rpm, 601 N m) and CO2 pump speed (80 rpm), similar expansion inlet pressures are obtained for the four groups within a range of 10.2e10.5 MPa, which are considered as similar initial CTRC conditions for the four groups. To mitigate the small difference, a fair comparison is made using a percentage change in pressure relative to the initial data. Fig. 7 presents the treated results of 20% reduction. After stopping or idling of engine, the four groups show different time period from 100% to 80% pressure change. The pressure of the CTRC maintains for a longer time under idling condition of engine, compared with that under stopping condition of engine. After all, there is still a
3.2. Response under stopping and idling condition of engine For an automotive engine, short stopping and idling are usually conditions under road conditions. Exhaust gas flow stops and reduces to a minimum at stopping condition and idling condition, respectively. Hence, it is significant to know how the CTRC performs under such low conditions. In this study, sudden stopping and idling conditions are conducted after similar stable conditions are maintained. Two sets, with and without preheating, are provided for a comparative study by controlling the EC preheating pump on and off. Fig. 6 presents four groups of pressure results, including stopping with preheating, stopping without preheating, idling with Table 4 Main parameters and system performance at different pressure conditions. Pressure ratio [-]
P3 [MPa]
P4 [MPa]
t2 [ºC]
t3 [ºC]
tg,in [ºC]
tec,in [ºC]
Qgh [kW]
Qpreh [kW]
Powerest [kW]
Wnet,est [kW]
hth,est [%]
1.25 1.28 1.34 1.40 1.50 1.59 1.67
7.52 7.74 8.10 8.52 9.15 9.72 10.28
5.98 6.01 6.04 6.06 6.07 6.10 6.11
34.5 36.0 38.5 41.3 45.0 48.3 51.2
137.0 140.3 141.4 142.2 142.0 146.2 149.6
430.3 432.3 433.6 434.8 435.6 436.4 436.9
67.3 67.3 67.3 67.4 67.8 68.2 68.5
32.5 33.0 32.7 32.3 31.7 31.5 31.6
22.6 22.1 21.6 21.1 20.5 19.9 19.6
1.64 1.81 2.07 2.37 2.77 3.10 3.43
1.20 1.37 1.60 1.82 2.15 2.38 2.63
2.20 2.60 3.10 3.50 4.30 4.80 5.40
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Fig. 7. Percentage change of expansion inlet pressure under engine stopping or idling.
small amount of exhaust gas at the idling condition. On the other hand, as the CTRC with preheating keeps flow of engine coolant after stopping and idling, preheated CTRC extends more time than the CTRC without preheating. Therefore, the group of idling without preheating keeps the longest time (739 s) among the four, followed by the groups of idling without preheating (278 s), stopping with preheating (273 s), and stopping without preheating (127 s). The preheating process provides more than double time for the CTRC operating at high pressure. The results also can provide a reference of practical operation of the CTRC. For example, the automotive engine changes to idling condition only for a short period (e.g. situation of traffic light), it can continue to drive the CTRC and has no occasion to cut off the CTRC system. The sudden rise in pressure appears for the CTRC system without preheating at stopping condition. Such sudden fluctuant change is bad for system operation. It is significant to analyze the causes and avoid it. Firstly, as the other groups had no such phenomenon, the study conducted a repetitive experiment to confirm the phenomenon. As shown in Fig. 8, similar phenomenon occurs at two repetitive tests and thus it is confirmed.
Fig. 8. Repetitive results for sudden rise in pressure phenomenon without preheating under engine stopping.
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The reasons can be explained from the aspect of P4, whose percentage change is presented in Fig. 9. Due to sudden reduction of flow of heat sources, the P3 decreases, and then P4 would decrease firstly but rise again, limited by cooling waste at the condensation side. Hence, stopping and idling of engine produce concave-shaped changes of P4. The scale extent of the concaveshaped changes is different for the four groups, which has different reduction degree of heat sources. For the group of engine stopping without preheating, an extreme reduction group without both exhaust gas and engine coolant, has an acute concave-shaped change, i.e. it drops first and then rises. The resulting large increase in the expansion outlet pressure only results in a pressure surge in the expansion outlet pressure. For the other groups, the concaveshaped change is small due to preheating or idling, and even there is no such change with both preheating and idling. Therefore, the preheating process can avoid the pressure's sudden rise phenomenon of the expansion inlet pressure. Besides, different reduction degree of heat sources also affects different reduction of liquid level. By a comparison shown in Fig. 10, the CTRC with preheating achieves smaller change in liquid level
Fig. 9. Percentage change of expansion outlet pressure under engine stopping and idling.
Fig. 10. Change of expansion outlet pressure under engine stopping and idling.
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than that without preheating. The experiment of stopping without preheating has the fast reduction of liquid level, which results in the shortest time for effective operation. Therefore, the preheating process is also beneficial to maintain safe liquid level under engine stopping and idling.
3.3. Response under restarting condition of engine As analyzed above, the preheated CTRC can maintain high expansion inlet pressure for a while after stopping and idling of engine. In this section, a restarting condition of engine is conducted for the preheated CTRC as follows: after stable operation of the CTRC for a while under certain engine condition (1100 rpm and 601 N.m), the engine condition changes to idling and stopping successively, then restarts at idling with the increase to 1100 rpm and 601 N.m again. Some parameters under the restarting condition of engine are presented in Fig. 11 and Fig. 12. The expansion inlet pressure and temperature decrease from 8.98 MPa, 192.3 C to 7.72 MPa, 75.9 C after 4.3 min idling and 4.2 min stopping of engine, successively. Then they climb up to 8.82 MPa, 172.4 C after the engine restarts again within 2.2 min idling and 12.0 min middle condition (1100 rpm and 601 N m). The CO2 liquid level is full in the tank for most lengths of time. During the restarting process of engine, the engine coolant maintains a small temperature change within 59.7e70.1 C, which provides persistent heat source for the CTRC although there is no exhaust gas under engine stopping condition. As shown asⅠregion in Fig. 12, the engine coolant, instead of exhaust gas, becomes the main heat source instead of exhaust gas after engine idling for a while (about 1 min). Although the capacity of preheating also reduces, its reduction speed is more slower than gas heating due to the large heat capacity of engine coolant. Hence, the exhaust gas
makes more contribution again after engine restarts for a while (about half of mins), as shown asⅡregion in Fig. 12. The decreased capacity of engine coolant in the stopping condition would increase again in theⅡregion. The cross change between heat absorption quantity of exhaust gas and engine coolant is well adapted to stopping (or idling) erestarting condition of engine. The preheating process plays an important compensation role at the low-load engine conditions.
3.4. Response under random conditions of engine In view of complicated operating conditions in practice, a group at random conditions of engine was conducted in this study for the preheated CTRC. As shown in Fig. 13, the random conditions involve 15 conditions (E1-E15) with change of engine speed or engine torque at intervals of about 1e2 min, with a wide range of engine power (18.4e95.2 kW), engine speed (700e1400 rpm) and engine torque (100e700 N m). As illustrated in Section 3.1-3.3, the preheating process makes for maintaining pressure and liquid level. Therefore, under the 22.4 min and big-change random conditions, the expansion inlet pressure of the preheated CTRC still keeps stable within a range of 9.20e10.06 MPa as presented in Fig. 14, as well as CO2 liquid level with only 1 cm change and CO2 mass flow rate with ±1.5% change, as shown in Fig. 15. Small difference in pressure, liquid level and mass flow rate would be beneficial for stable and efficient operation of turbine and system (see Fig. 16). The random conditions influence large change of exhaust gas, and then affect the temperature and amount of exhaust gas, which result in larger change of expansion inlet temperature from 126.4 to 179.5 C than that of expansion inlet pressure. Hence, it greatly influences the heat absorption quantity (25.4e34.9 kW) and the estimation of net power output (1.71e2.88 kW), both of which have
Fig. 11. Change of CO2 pressures and liquid level under engine restarting condition.
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Fig. 12. Change of temperatures and heat absorption quantity under engine restarting condition of engine.
Fig. 13. Speed and torque under random engine conditions.
the similar variations trends with the exhaust temperature. As continuous preheating provides stable heat absorption quantity (19.5e22.8 kW) from engine coolant, the CO2 has a quite high temperature (46.8e53.0 C) into the gas heater all the way.
Therefore, the preheating process can maintain CO2 pressure, temperature and liquid level and improve the heat absorption and the output performance under the random conditions of engine, as well as it can under the other engine conditions in Section 3.1-3.3.
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Fig. 14. Change of main pressures and temperatures under random engine conditions.
Fig. 15. Change of liquid level and mass flow rate under random engine conditions.
4. Conclusions Based on a preheated CTRC test bench for waste heat recovery of exhaust gas and engine coolant, a detailed experimental study was
conducted under various engine conditions, i.e. starting and ‘warm up’ condition, stopping and idling condition, restarting condition and random conditions. The results show that the preheating process can improve the dynamic performance of the CTRC at
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Fig. 16. Change of estimation of net power output and heat absorption quantity under random engine conditions.
various engine conditions. On the one hand, it is beneficial for continuous output capacity by maintaining high pressure and temperature of CO2 for extended time, and it is beneficial for safe operation by keeping high liquid level of CO2 on the other hand. Besides, the preheating process also can avoid the sudden rise in the expansion inlet pressure under the stopping condition of engine, and meanwhile it plays an important energy supplement role in the random conditions of the engine, resulting in the cross change in the waste heat absorption from engine coolant and exhaust gas.
Qfuel;total ¼ mfuel $qfuel
(A.1)
wherein, qfuel is the calorific value of diesel fuel that equals to 42500 kJ/kg in this study. Except engine power (We), the other three parts are calculated as follows:
Qg ¼ mg $ðhg;in hg;0 Þ
(A.2)
Acknowledgements
Qec ¼ mec $ðhec;obtained hec;back Þ
(A.3)
The authors would like to acknowledge the National Natural Science Foundation of China (No. 51636005) for grants and supports.
Qres ¼ Qfuel;total We Qg Qec
(A.4)
Appendix A A heat balance test of diesel engine with CTRC system was conducted to investigate waste heat capacity of the engine, which can be a reference for various engine conditions mentioned above. During the test, the engine speed varied from 1000 rpm to 1500 rpm with a 100 rpm interval, while the load varied from 20% to 60% load with a 10% interval at every speed with a total torque range from 229 N m to 770 N m. Four parts are included in heat balance, i.e. engine power, waste heat quantity of exhaust gas, waste heat quantity of engine coolant, and residue energy (e.g. intercooling, oil cooling). The total combustion energy of fuel is calculated as
The property of exhaust gas is calculated by an approximation method, which is decided and averaged by the proportion of gas composition. The detailed method can be found in the precious research by our group [22]. Different from the heat balance test in Ref. [22] without recovery system, the test in this study is on the basis of connecting CTRC system. Only in this way, can the results have reference for various engine conditions in this study. The maps of the four parts are presented in Fig. 17. Meanwhile, the energy distribution percentage at 60% load and 1500 rpm speed is shown in Fig. 18. Obviously, engine power and waste heat of exhaust gas have clear increase with an increase in engine speed or engine torque. However, the waste heat of engine coolant is only significantly related to the engine load. At 1500 rpm and 60% load, the engine power occupies 38.1%, while waste heats of exhaust gas and engine coolant occupy 30.6% and 22.3%, respectively.
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Fig. 17. Heat balance performance of engine.
Fig. 18. Variation of energy distribution with engine load and speed.
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