Experimental study on transcritical Rankine cycle (TRC) using CO2/R134a mixtures with various composition ratios for waste heat recovery from diesel engines

https://doi.org/10.1016/j.enconman.2020.112574Get rights and content

Highlights

  • CO2/R134a mixtures as working fluid in TRC are studied by experiment.

  • Effect of composition ratio of the CO2/R134a mixtures is investigated.

  • Heat transfer coefficients of supercritical CO2/R134a mixtures were provided.

  • Optimal composition ratio of the studied mixture for best performance was obtained.

Abstract

A carbon dioxide (CO2) based mixture was investigated as a promising solution to improve system performance and expand the condensation temperature range of a CO2 transcritical Rankine cycle (C-TRC). An experimental study of TRC using CO2/R134a mixtures was performed to recover waste heat of engine coolant and exhaust gas from a heavy-duty diesel engine. The main purpose of this study was to investigate experimentally the effect of the composition ratio of CO2/R134a mixtures on system performance. Four CO2/R134a mixtures with mass composition ratios of 0.85/0.15, 0.7/0.3, 0.6/0.4 and 0.4/0.6 were selected. The high temperature working fluid was expanded through an expansion valve and then no power was produced. Thus, current research focused on the analysis of measured operating parameters and heat exchanger performance. Heat transfer coefficients of various heat exchangers using supercritical CO2/R134a mixtures were provided and discussed. These data may provide useful reference for cycle optimization and heat exchanger design in application of CO2 mixtures. Finally, the potential of power output was estimated numerically. Assuming an expander efficiency of 0.7, the maximum estimations of net power output using CO2/R134a (0.85/0.15), CO2/R134a (0.7/0.3), CO2/R134a (0.6/0.4) and CO2/R134a (0.4/0.6) are 5.07 kW, 5.45 kW, 5.30 kW, and 4.41 kW, respectively. Along with the increase of R134a composition, the estimation of net power output, thermal efficiency and exergy efficiency increased at first and then decreased. CO2/R134a (0.7/0.3) achieved the maximum net power output at a high expansion inlet pressure, while CO2/R134a (0.6/0.4) behaves better at low pressure.

Introduction

Heavy-duty trucks are one of the main consumers of fossil fuel. In China, heavy trucks, which account for 13.9% of vehicles, consume 49.2% of the total fuel consumed by transport. It is expected that by 2040, heavy-duty trucks will become the largest contributor of carbon dioxide (CO2) emissions in the transport sector [1]. China and United States are the two largest markets for commercial vehicles in the world. In 2017, China and the United States jointly launched a five-year research in the field of commercial trucks. This program aims to demonstrate technical solutions that could achieve a goal of increasing truck efficiency by 50% over a 2016 baseline. In this program, waste heat recovery (WHR) technology by means of thermodynamic cycles, including Organic Rankine cycle (ORC) and non-organic power cycle, is considered as a promising solution to achieve this goal [2].

Extensive theoretical studies have been involved in ORC-WHR for waste heat recovery from heavy-duty diesel (HDD) engines [3], [4], [5]. Recently, several researchers and vehicle manufacturers have developed ORC prototypes, which have been tested in the laboratory and on-road vehicles [6], [7], [8]. Alshammari et al. [6] evaluated the potential of fuel efficiency improvement of an ORC prototype for an HDD engine using R245fa and Fluid A as working fluids. Guillaume et al. [9] experimentally discussed the possibility of R1233ze as substitute of R245fa for ORC-WHR on truck applications. AVL [10] presented the results of ORC-WHR for a long-haul truck from test bench and public road testing and indicated that fuel consumption benefits for different real-life cycles in the United States and Europe reach between 2.5% and 3.4%. Cummins [7] has installed an ORC prototype (R245fa as working fluid) on a heavy-duty truck, and the test results showed that brake thermal efficiency of diesel engine increased by 3.6%. Although refrigerants as working fluids of ORC is promising in terms of fuel efficiency improvement, there are still some drawbacks for them in high-temperature ORC field, including low thermal decomposition temperature, unsatisfied thermo-economic performance and relatively high ozone depletion potential (ODP) and global warming potential (GWP).

Compared with subcritical Rankine cycle (SRC), transcritical Rankine cycle (TRC) has been proven to achieve better thermal matching and exergy efficiency [3], [11]. Previous studies by our group [3] compared the SRC and TRC for diesel engine waste heat recovery, concluding that TRC could obtains a 5.4% increase in thermal efficiency. Yang et al. [12], [13] proposed a TRC to recover waste heat from marine diesel engine and compared economic performance of TRCs using various working fluid. Yağlı et al. [14] designed SRC and TRC for waste heat recovery from biogas fueled combined heat and power engine, indicating that the net power output of TRC is higher than that of SRC by 2.9%.

Nowadays, CO2 transcritical Rankine cycle (C-TRC) has been drawing more and more attention as an application in the waste heat recovery field. The main advantages of using CO2 as working fluid are desirable thermodynamic performance [15], small size with compact heat exchangers [16], [17] and turbines [18], and outstanding environmental performance with zero ODP and extremely low GWP. Specific to WHR from a diesel engine, C-TRC is also beneficial for simultaneous utilization of high and low temperature waste heat sources [19] and presents a good dynamic characteristic [20]. Several prior research studies have discussed and demonstrated the potential of C-TRC for diesel engine [21], [22], [23]. Shu’s group [24] has proposed a C-TRC prototype to recover exhaust gas and engine coolant waste heat from a heavy duty diesel engine. Experimental results indicated that thermal efficiency of the studied diesel engine could be increased from 39.4% to 41.4% by such WHR system. However, due to low critical temperature of CO2, the application of C-TRC, especially for WHR from diesel engine, faced the challenge of low-temperature condensation. In addition, the cycle efficiency of C-TRC is relatively low due to its high expansion outlet pressure.

To overcome these disadvantages, CO2 based mixtures, composed of CO2 and a chemical with high critical temperature, were investigated in solar plants [25], [26], geothermal plants [27], [28] and WHR plants [29], [30]. Manzolini et al. [25] discussed the potential of a CO2 mixture in improving thermodynamic and thermo-economic performance for a solar power plant, indicating that the CO2 mixture outperformed pure CO2 in conversion efficiency by 2%. Dai et al. [31] studied several CO2 mixtures in TRC for low temperature heat conversion, demonstrating that CO2 mixtures are capable of improving thermal efficiency and decreasing operating pressure. Shu et al. [29] considered CO2 mixtures as working fluid in TRC for diesel engine WHR and concluded that CO2 mixtures can help expand condensation pressure range. The same conclusions were also drawn in Wu et al. [27].

Identifying a proper mixture composition ratio is essential to optimize the thermodynamic performance of a power system. Thus, the effect of composition ratio was fully considered and discussed in previous theoretical research studies. The majority of previous studies looked at the optimal composition ratio of mixtures using thermodynamic analysis with various criterions, such as net power output [32], exergy performance [30] and economic performance [29], [33]. Generally, mixtures only with an appropriate composition ratio behave better than their pure-component counterparts [34], [35]. Optimization methods, including generic algorithms [36], sequential quadratic programming [37], mixed-integer nonlinear programming [38] and stochastic optimization [39] also were adopted to determine the optimal composition ratio. computer-aided molecular design (CAMD) also has been used to design and identify the composition ratio of mixture [40].

Indeed, these theoretical analysis can help gain a preliminary understanding of the effect of composition ratio on cycle performance, but the conclusive results still need to be obtained by experiment [34]. Published experimental data relating to power cycle using mixtures are extremely rare. A few mixtures, including R234fa/R123 [41], R245fa/R134a [42], R245fa/R152a [43], R245fa/R365mfc [44], R245fa/R600a [45] and R600a/R601a [46], have been investigated by experiment. Existing publications focused on the performance comparison between the mixture and its pure component counterpart. Only one publication by Wang et al. [46] discussed the effect of mixture composition on cycle performance by experiment. However, the mixtures outlined above were designed for low-temperature application; the application of CO2 mixtures in high-temperature applications has not yet been experimentally investigated. Furthermore, there are almost no experimental results on the heat transfer coefficient for mixtures within the temperature range of a power cycle [34], [35].

Thus, this paper presents the detailed experimental results in a TRC system using CO2/R134a mixtures with various composition ratios. A regenerative TRC test bench based on an expansion valve was used to recover waste heat of engine coolant and exhaust gas from a heavy-duty diesel engine. With the measured experimental data, the potential of power output was estimated numerically. The main unique contribution of this paper lies in three aspects:

  • 1.

    Effect of composition ratio of CO2/R134a mixtures on system performance was investigated by experiment for the first time.

  • 2.

    Heat transfer coefficients of various heat exchangers using supercritical CO2/R134a mixtures are provided and discussed, which may give reference for cycle optimization and heat exchanger design for CO2 mixtures application.

  • 3.

    Optimal composition ratio of CO2/R134a mixtures for the maximum net power output, thermal efficiency and exergy efficiency were determined by experiment.

Section snippets

CO2/R134a mixture

Carbon dioxide, as the working fluid for TRC system, is difficult to be condensed at ambient temperature cold source due to its low critical temperature. Hence, CO2-based mixture is supposed to have a relatively high critical temperature to overcome that issue. Additionally, a CO2-based mixture should be environmentally friendly and safe, with good thermodynamic performance. R134a is a non-toxic and non-flammable hydrofluorocarbon with insignificant ozone depletion potential (ODP) and a

Experimental test bench

A typical regenerative TRC (R-TRC) test bench was built to recover waste heat of exhaust gas and engine coolant from a diesel engine. Fig. 2 shows the schematic diagram of the whole test bench. The waste heat sources were provided by a six-cylinder, four-stroke heavy-duty diesel engine, equipped with complete measuring and controlling instruments to ensure it operates stably at a specific condition. Its main parameters are listed in Table 2. The cooling water was provided by a refrigeration

Results and discussion

In this section, the experimental results based on the experimental methodology outlined above are presented. The effect of the composition ratio of CO2/R134a mixture on measured operating parameters, heat exchanger performance and cycle performance was investigated carefully. The difference in the cycle behavior among CO2/R134a mixtures was explained from the point of thermophysical properties.

Starting with the results of pressure variations shown in Fig. 5 and Table 5, as the mass fraction of

Conclusion

In order to improve cycle performance and alleviate the low-temperature condensation issue encountered with CO2 in transcritical Rankine cycles (TRC), the CO2/R134a mixture was presented as working fluid in a TRC test bench to recover waste heat of exhaust gas and engine coolant from a heavy-duty diesel engine. Four CO2/R134a mixtures with various mass composition ratios, CO2/R134a (0.85/0.15), CO2/R134a (0.7/0.3), CO2/R134a (0.6/0.4) and CO2/R134a (0.4/0.6), were investigated. The effect of

CRediT authorship contribution statement

Peng Liu: Conceptualization, Investigation, Visualization, Writing - original draft. Gequn Shu: Supervision, Funding acquisition. Hua Tian: Writing - review & editing. Wei Feng: Writing - review & editing. Lingfeng Shi: Investigation. Xuan Wang: Investigation.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors would like to acknowledge the National Key Research and Development Plan of China (2017YFE0102800) for grants and supports. The financial support from China Scholarship Council (CSC) to the first author is also gratefully acknowledged.

The U.S. authors recognize Lawrence Berkeley National Laboratory’s support from Department of Energy – The United States under Contract No. DE-AC02-05CH11231 and supports from the Energy Foundation. The U.S. Government retains a non-exclusive, paid-up,

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