J. Cent. South Univ. (2018) 25: 1464-1474

DOI: https://doi.org/10.1007/s11771-018-3840-6

Energy and exergy recovery from exhaust hot water using organic Rankine cycle and a retrofitted configuration

SUN Wen-qiang(孙文强)^{1, 2}, YUE Xiao-yu(岳晓宇)^{1, 3}, WANG Yan-hui(王彦辉)¹, CAI Jiu-ju(蔡九菊)²

1. Department of Thermal Engineering, School of Metallurgy, Northeastern University,Shenyang 110819, China;

2. State Environmental Protection Key Laboratory of Eco-Industry, Northeastern University,Shenyang 110819, China;

3. State Key Laboratory of Multiphase Flow in Power Engineering, Xi’an Jiaotong University,Xi’an 710049, China

Central South University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2018

Abstract: Exhaust hot water (EHW) is widely used for various industrial processes. However, the excess heat carried by EHW is typically ignored and discharged into the environment, resulting in heat loss and heat pollution. An organic Rankine cycle (ORC) is an attractive technology to recycle heat from low-temperature energy carriers. Herein, ORC was used to recycle the heat carried by EHW. To investigate the energy and exergy recovery effects of EHW, a mathematical model was developed and a parametric study was conducted. The energy efficiency and exergy efficiency of the EHW-driven ORC system were modeled with R245fa, R113 and R123 as the working fluids. The results demonstrate that the EHW and evaporation temperatures have significant effects on the energy and exergy efficiencies of the EHW-driven ORC system. Under given EHW conditions, an optimum evaporation temperature exists corresponding to the highest exergy efficiency. To further use the low-temperature EHW, a configuration retrofitted to the ORC by combining with flash evaporation (FE) was conducted. For an EHW at 120 °C and 0.2 MPa, the maximum exergy efficiency of the FE-ORC system is 45.91% at a flash pressure of 0.088 MPa. The FE-ORC performs better in exergy efficiency than the basic FE and basic EHW-driven ORC.

Key words: exhaust hot water (EHW); organic Rankine cycle (ORC); energy efficiency; exergy efficiency; flash evaporation (FE)

Cite this article as: SUN Wen-qiang, YUE Xiao-yu, WANG Yan-hui, CAI Jiu-ju. Energy and exergy recovery from exhaust hot water using organic Rankine cycle and a retrofitted configuration [J]. Journal of Central South University, 2018, 25(6): 1464–1474. DOI: https://doi.org/10.1007/s11771-018-3840-6.

1 Introduction

Water is widely used in industrial processes, e.g., in steelmaking and petrochemical plants for moistening, dedusting, stripping, liquid–liquid extraction, steam generating, heating, and cooling [1, 2]. The temperature of some exhaust hot water (EHW) is generally higher than the environmental temperature. However, the excess heat carried by the EHW is typically ignored and directly discharged into the environment, leading to heat loss and heat pollution [3]. Given the depletion of fossil fuels and the increasing global energy costs, the EHW energy recovery and utilization should be emphasized from the energy, environmental, and economic perspectives because EHW also serves as an important energy carrier [4].

Excess heat recovery from energy carriers can save the consumption of primary fuels and reduce emissions into the environment; thus, it is regarded as the fifth fuel followed by coal, oil, natural gas, and hydro [5, 6]. A case study conducted by TOGAWA et al [7] indicated that heat recovery could provide more environmental benefits than the individual system. Therefore, it is a significant measure for industrial plants to reuse the excess heat resources. According to their temperatures, energy carriers can be divided into three types: high, medium, and low temperature. EHW is a low-temperature heat from industrial-emitted or -recycled water, low pressure steam, and some cooling media such as cooling water or oil. The organic Rankine cycle (ORC) is a low-temperature heat recovery technology [8]. The heat sources that can be used in the ORC primarily include flue gas [9, 10], solar energy [11–13], biomass [14], and geothermal energy [15–18]. Thus, conceptually, the excess heat carried by EHW, as a kind of low-temperature heat, can be recycled using the ORC. Further, an ORC used for the slag-washing water of a blast furnace power station has been reported by XIAO et al [19].

The appropriate working fluid that matches the application is one of the most important steps in designing an EWH-driven ORC system. GUO et al [20] reported a method of selecting working fluids that match with different heat sinks, and compared the performance characteristics of pure and mixed working fluids. The results demonstrated that when the initial temperature of a heat source is lower and its temperature gradient is higher, and the temperature gradient of the heat sink is higher, mixed working fluids have better performance than pure working fluids. However, for the opposite heat source and heat sink situations, pure working fluids perform better. BARAL et al [21] investigated 15 organic fluids to evaluate their fitness and performance as working fluids in an ORC system. In the investigated application, R134a and R245fa were found to be the most appropriate working fluids for medium- and low-temperature heat recovery; while RC318 and R123 offer attractive performances but require environmental precautions owing to their high ozone depletion potential (ODP) and high global warming potential (GWP). MAGO et al [22] conducted an investigation on R134a, R113, R245ca, R245fa, R123, isobutane, and propane, with boiling points between 243 and 48 °C. The results demonstrated that an ORC using R113 showed the maximum efficiency among the evaluated organic fluids for temperatures >430 K. The results in the study of HUNG et al [23] indicated that wet fluids, with very steep saturated vapor curves in the T–s diagram, have a better performance in energy conversion efficiencies than dry fluids.

Several criteria are used to assess the performance of an ORC system, including energy efficiency, exergy efficiency, exergy destruction, net power output, and even cost index. BRYSZEWSKA–MAZUREK et al [13], KARELLAS et al [24], WANG et al [25], and PIEROBON et al [26] employed energy efficiency to evaluate the system performance. WANG et al [25], WEI et al [27], and MAGO [28] presented an exergy efficiency analysis for an ORC system that converts heat to power from low- and medium- grade heat sources. SPAYDE et al [29] investigated how hourly temperature change affects the exergy destruction rates of the ORC in Jackson, USA. In addition, the net power output was employed by KHENNICH et al [30] to judge the performance of a low-temperature ORC system, concluding that the best working fluid for the specified conditions is R141b. Further, the ratio of the heat transfer area to the net power and the heat recovery efficiency were used by WANG et al [31] to optimize the ORC system. Additionally, WU et al [32, 33] established an exergo-economic evaluation model to evaluate the cost of an ORC system.

The primary focus of this study is modeling the excess heat recovery effect of an EHW-driven ORC system. A parametric study is conducted by changing the evaporation temperature, condensation temperature, degree of superheat, and heat source temperature of the ORC system. WANG et al [34] reported that the heat recovery space is still available for energy carriers discharged from conventional ORC; thus, ORC-based combined cycles have begun to attract more attention. However, the reported ORC and ORC-based combined cycles primarily involve gaseous waste heat [8–10] and geothermal energy [15–18], while few were based on EHW [19]. This work developed an EHW-driven cycle that combines flash evaporation (FE) and ORC, and analyzed the excess heat recovery effect of the combined cycle.

2 EHW heat recovery system description

2.1 ORC system

EHW-driven ORC system consists of four primary components: an evaporator, an expander, a condenser, and a pump. The followings are the four primary stages (see Figures 1 and 2):

Isobaric heating in the evaporator (6–1). In this stage, liquid organic working fluid from the pump enters the evaporator to absorb heat from the EHW at a constant pressure, through which the organic working fluid is converted into steam of high temperature and high pressure.

Isentropic expansion in the expander (1–2). In this stage, the organic working fluid, from the evaporator, with high pressure and high temperature enters the expander, through which it exports mechanic work during the isentropic expansion process.

Isobaric cooling in the condenser (2–5). In this stage, the expanded steam from the expander flows into the condenser, in which the working fluid steam is cooled and condensed into a liquid via an isobaric heat transfer.

Isentropic compression in the pump (5–6). In this stage, the organic working fluid of liquid state from the condenser is adiabatically compressed in the pump, and is sent into the evaporator for the next cycle.

It is noteworthy that in practical ORC applications, however, the heat exchanges in the expansion and compression stages are not strictly isentropic.

Figure 1 Schematic of EHW-driven ORC system

2.2 FE-ORC system

The retrofitted system, FE-ORC, combines FE with the ORC to further utilize the heat contained in EHW. The basic principle of FE is schematically depicted in Figure 3. The pressure of the EHW entering the flash tank is p_E, and its specific enthalpy is h_E. The pressure of the saturated water from the flash tank is p_w, and its specific enthalpy is h_s. Because p_E>p_w and h_E>h_w, part of the sensible heat is then released and absorbed by water to evaporate, and the flashed steam has the pressure of p_s and specific enthalpy of h_s. The water and steam from the flash tank are both saturated at the corresponding pressures. The lower the flash pressure, the lower is the flash temperature, and the more is the flashed saturated steam [35, 36].

Figure 2 T–s diagram of EHW-driven ORC

Figure 3 Schematic of FE process

As described in Figure 4, the FE-ORC system consists of FE section and ORC section. The ORC section contains an evaporator, expander 1, condenser 1, and pump; meanwhile, the FE section primarily involves a flash tank, expander 2, and condenser 2. EHW first enters the flash tank, in which its volume dilates and its pressure drops. The generated saturated steam flows into expander 2 in the FE section to perform work; further, the saturated water, as a heat source, enters the ORC section to exchange heat with the organic working fluid through the evaporator. The next stages are the same as those of the basic ORC, as shown in Figure 1.

Figure 4 Schematic of EHW-driven FE-ORC

3 Energy and exergy analysis models

3.1 ORC system

The followings are some assumptions before establishing the mathematical model:

The expansion and compression processes are assumed to be isentropic.

The energy loss and pressure drop across the system are neglected for simplification.

The ORC system is investigated under a steady-state operation.

As shown in Figure 2, the isobaric heating stage (6–1), can be divided into three substages: preheating (6–7), evaporating (7–8), and superheating (8–1). The amount of heat absorbed by the organic working fluid in the isobaric heating stage is

                         (1)

where m_f is the flow rate of the organic working fluid, and h is the specific enthalpy of the corresponding point.

Stage 1–2 is a process of entropy increment due to the irreversibility of the expansion. The specific enthalpy of point 2 can be obtained by

                       (2)

where η_t is the expander efficiency.

The power output of the expander is

                          (3)

Similarly, the isobaric heat release of the organic working fluid in the condenser, stage 2–5, can also be divided into three substages: precooling (2–3), condensing (3–4), and supercooling (4–5). The heat released from the organic working fluid in the condenser is

                         (4)

Stage 5–6 is also entropy-increased as a result of the irreversible loss in the compression through the pump. The specific enthalpy of point 6 can be calculated as

                      (5)

where η_p is the pump efficiency.

The power consumed in the pump is

                          (6)

From Eqs. (3)–(6), the net power output of the EHW-driven ORC system is

                            (7)

Energy efficiency is derived from the first law of thermodynamics, which reflects the ratio of the available energy to the consumed heat in terms of quantity. The energy efficiency of the EHW-driven ORC system is defined as the ratio of the net power output to the amount of heat absorbed from the EHW, and is expressed as

                    (8)

Exergy efficiency is derived from the second law of thermodynamics, which reflects the heat utilization degree from the perspective of both quantity and quality. The exergy efficiency of the EHW-driven ORC system is defined as the ratio of the net power output to the original exergy of the EHW, and can be written as

                   (9)

where T₀ is the environmental temperature, and T_l is the temperature of the EHW.

3.2 FE-ORC system

The T–s diagram of the EHW-driven FE-ORC combined cycle is shown in Figure 5.

The energy balance in the flash tank can be written as

                  (10)

where m_l and m_s are the mass flow rates of the EHW and saturated steam, respectively.

Figure 5 T–s diagram of EHW-driven FE-ORC:

The net power output of the FE section is

                     (11)

Therefore, the exergy efficiency of the FE section is

          (12)

According to Eqs. (3), (6) and (7), the net power output of the ORC section is

              (13)

Further, the exergy efficiency of the ORC section is

         (14)

Thus, the total exergy efficiency of the FE-ORC combined system is

(15)

4 Results and discussion

Working fluids that can be used in low- temperature EHW-driven ORC involve alkanes, CFCs, HFCs, HCFCs, etc. They are categorized into three groups based on their slope of saturation vapor curves in the T–s diagram (see Figure 6). The fluids, having positive, negative and nearly infinitely large slopes of saturated vapor curves are called dry, wet, and isentropic fluids, respectively [37]. The most commonly used dry fluids are R113, R114, R123, R245fa and R600; the wet fluids are R12 and R22; the isentropic fluids are R11 and R142b [38]. According to the selecting principles reported by ANDERSEN [39], and GUO et al [20], R245fa, R113 and R123 were chosen in the present work. The involving parameters of the organic working fluids are listed in Table 1. The parameters of the expander and condenser investigated in this work are shown in Table 2.

Figure 6 Classification of organic working fluids

Table 1 Parameters of selected organic working fluids

Table 2 Parameters of expander and condenser

Figure 7 shows the influence of evaporation temperature on energy efficiency. The energy efficiencies of the EHW-driven ORC using R245fa, R113 and R123 all increase with the increasing evaporation temperature. We found that the energy efficiency of R123 rises from 11.31% to 17.48% when the evaporation temperature increases from 86 °C to 147 °C. This is because the average temperature of heat absorption increases as the evaporation temperature increases, resulting in the net power output increase per unit working fluid under given EHW conditions. Among the three working fluids, R245fa has the smallest energy efficiency, while the energy efficiencies of R113 and R123 are approximately equal.

Figure 7 Relationship between energy efficiency and evaporation temperature

Figure 8 shows the relationship between exergy efficiency and evaporation temperature. The exergy efficiencies of R245fa, R113 and R123 all present a trend of increasing first and decreasing later, with increasing evaporation temperature, i.e., a maximum exergy efficiency occurs for each organic working fluid. Figure 8 shows that the maximum exergy efficiencies of the EHW-driven ORC system with R245fa, R113 and R113 as the working fluids are 52.61%, 46.79% and 46.79%, respectively, when the evaporation temperatures of each working fluid are 117 °C, 108 °C and 109 °C, respectively. This is because the flow rate of the working fluids and the corresponding net power output per unit working fluid decrease with the increasing evaporation temperature. Therefore, the total net power output increases at the first stage and decreases later, leading to a maximum exergy efficiency at the optimum evaporation temperature. Among the three working fluids, R245fa has the highest exergy efficiency while R113 has the lowest.

Figure 9 presents the effect of condensation temperature on energy efficiency. The energy efficiencies of R245fa, R113, and R123 all decrease linearly with the increase in condensation temperature. The energy efficiency of R123 reduces from 15.74% to 13.32% when the condensation temperature increases from 31 to 47 °C. The increase in condensation means the increase in the average temperature of heat release, resulting in a linear reduction in the net power output per unit working fluid. Among the three working fluids, R245fa is the smallest energy efficiency, and the energy efficiencies of R113 and R123 are approximately equal at a fixed condensation temperature.

Figure 8 Relationship between exergy efficiency and evaporation temperature

Figure 9 Relationship between energy efficiency and condensation temperature

Figure 10 depicts the influence of condensation temperature on the exergy efficiency of the EHW-driven ORC system. The exergy efficiencies of R245fa, R113 and R123 also reduce linearly with the increasing condensation temperature. For R245fa, the exergy efficiency decreases from 51.41% to 39.73% when the condensation temperature increases from 31 to 47 °C. This is because when the condensation temperature increases, the specific enthalpy of the organic working fluid entering the evaporator will increase, and the heat absorbed by the working fluid from the EHW will reduce correspondingly, leading to a reduction in the net power output of the ORC system. Among the three working fluids, the exergy efficiency of the EHW-driven ORC system with R245fa as the working fluid is the highest, while that of using R113 is the lowest.

Figure 10 Relationship between exergy efficiency and condensation temperature

Figure 11 shows the relationship between energy efficiency and degree of superheat. Different working fluids have different change trends with increasing degree of superheat. Among the three working fluids, the energy efficiency of R113 decreases with the increasing degree of superheat, while the energy efficiencies of R245fa and R123 increase. This is because with the increase in superheat degree, both the heat absorbed and net power output per unit working fluid increase, whereas their increase rates are different. However, Figure 11 shows that the energy efficiency variation amplitudes of the three working fluids are relatively small. When the degree of superheat increases from 3 °C to 12 °C, the energy efficiency of R123 simply rises from 15.85% to 15.90% with a growing rate of 0.05%, while the energy efficiency of R113 reduces from 15.83% to 15.78% with a reducing rate of only 0.05%. That is, the degree of superheat has little impact on the energy efficiency of an EHW- driven ORC.

Figure 12 illustrates the influence of superheat degree on the exergy efficiency of the EHW-driven ORC system. As shown, a relatively small reduction in exergy efficiency occurs when the degree of superheat increases for each working fluid. Among the three working fluids, the exergy efficiency of R245fa is the highest while that of the R113 is the lowest. When the degree of superheat increases from 3 °C to 12 °C, the exergy efficiency of the EHW-driven ORC system with R245fa as the working fluid decreases from 52.98% to 49.65%, while that with R113 as the working fluid decreases from 43.43% to 42.10%, which is not a significant change.

Figure 11 Relationship between energy efficiency and degree of superheat

Figure 12 Relationship between exergy efficiency and degree of superheat

Figure 13 describes the relationship between energy efficiency and evaporation temperature under different EHW temperatures, with R245fa as the working fluid. We observed that the energy efficiency of the ORC system maintains the same value with the increasing temperature of the EHW at a fixed evaporation temperature. Although the EHW temperature differs, the evaporation temperature, condensation temperature, and degree of superheat is unchanged, which leads to the same energy efficiency.

Figure 13 Relationship between energy efficiency and evaporation temperature under different EHW temperatures

Figure 14 shows the impact of EHW temperature on exergy efficiency of the system with R245fa as the working fluid. For a certain EHW temperature, the exergy efficiency first increases and then decreases with increasing evaporation temperature, and a maximum exergy efficiency exists at the optimum evaporation temperature. However, the maximum exergy efficiency and the optimum evaporation temperature vary with the EHW temperature. For an EHW temperature of 140 °C, the optimum evaporation temperature is 86 °C and the maximum exergy efficiency is 42.00%. For an EHW temperature of 160 °C, the optimum evaporation temperature is 100 °C with the maximum exergy efficiency of 46.70%. For an EHW temperature of 180 °C, the optimum evaporation temperature is 117 °C and the maximum exergy efficiency is 52.35%. The relationship between maximum exergy efficiency of the ORC system and the EHW temperature is shown in Figure 15. Figures 14 and 15 indicate that the maximum exergy efficiency increases with the increase in EHW temperature; further, the lower the EHW temperature, the faster the exergy efficiency decreases after the optimum evaporation temperature.

Figure 14 Relationship between exergy efficiency and evaporation temperature under different EHW temperatures

Figure 15 Relationship between maximum exergy efficiency and EHW temperatures

The system efficiency analyses in thermodynamics involve energy efficiency analysis and exergy efficiency analysis, as discussed above. As presented by the first and second laws of thermodynamics, the energy efficiency analysis provides no information regarding the irreversibility of the heat recovery process. However, the exergy efficiency analysis of the system provides insight into the inefficiency and presents opportunities for minimizing the exergy destruction of unit operations [18]. The purpose of the EHW-driven ORC system is to achieve an efficient and green water and energy utilization; therefore, it is of significance to investigate the exergy efficiency of the system that can be used to reflect the available energy utilization, sustainability, and environment issues. Thus, exergy efficiency is selected as the evaluation criterion in the following configuration retrofit.

The investigated EHW flowed at a rate of 72 t/h. Its temperature and pressure were 120 °C and 0.2 MPa, respectively. R245fa was selected as the organic working fluid used in the ORC section, whose condensation temperature was 30 °C. The local environmental temperature was 20 °C.

Figure 16 presents the exergy efficiency of the FE section under different flash pressures. The exergy efficiency of the FE section increases first and decreases later with increasing flash pressure; thus, an optimum flash pressure exists. This is because with the increase in flash pressure, the specific enthalpy drop of the saturated steam from the flash tank increases through expander 2; correspondingly, the flow rate of the saturated steam reduces, and therefore the net power output increases first and then decreases. We found that the optimum flash pressure is 0.0338 MPa, and the corresponding maximum exergy efficiency is 25.91%.

Figure 17 compares the changes in the exergy efficiencies of the basic ORC and FE-ORC driven by EHW under different flash pressures. As shown, the exergy efficiency of the EHW-driven FE-ORC at any flash pressure is higher than that of the basic ORC driven by EHW. According to Figure 16, the exergy efficiency of the EHW-driven FE-ORC can also be higher than that of the basic FE within certain evaporation pressure ranges. The maximum exergy efficiencies of the basic FE and EHW-driven basic ORC are 25.91% and 37.21%, respectively. However, the maximum exergy efficiency of the EHW-driven FE-ORC is 45.91% when the flash pressure is 0.088 MPa. We conclude that the exergy performance of the EHW-driven FE-ORC is better than that of the basic FE and basic ORC.

Figure 16 Relationship between exergy efficiency of FE section and flash pressure

Figure 17 Relationship between exergy efficiency and evaporation pressure

5 Conclusions

To achieve an efficient and green water and energy utilization, a method to recycle the excess heat of EHW using the ORC was studied herein. Based on the energy analysis and exergy analysis, the mathematical model of the EHW-driven ORC was established. Several factors influencing the energy and exergy recovery performances of the ORC were discussed. Further, an FE-ORC combined cycle was retrofitted for the further utilization of the excess heat from the EHW. Finally, the exergy efficiency of the FE-ORC was studied. The impacts of operational parameters on the energy and exergy efficiencies of the EHW-driven ORC system were investigated, with R245fa, R113, and R123 as the working fluids. The observed operational parameters include evaporation temperature, condensation temperature, degree of superheat, and EHW temperature. Further, we observed that the evaporation temperature and EHW temperature, rather than the degree of superheat, have a significant effect on the system energy and exergy efficiencies. An optimum evaporation temperature exists, corresponding to the highest exergy efficiency under given EHW conditions. The exergy efficiency of R245fa is the highest, whereas its energy efficiency is the lowest. Concerning the low-temperature EHW, an FE-ORC system was built. Under fixed EHW conditions, the exergy efficiency of the EHW-driven FE-ORC system increases first and decreases later with the increasing evaporation pressure of the ORC section. Compared with the basic FE and basic ORC, the EHW-driven FE-ORC shows better exergy recovery performance.

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(Edited by FANG Jing-hua)

中文导读

利用有机朗肯循环及其改进结构回收余热水的能量和量

摘要：工业过程中广泛存在着余热水，依附于余热水的热量经常被忽略而排放到环境中，造成了热量的散失和环境的污染。有机朗肯循环是一项从低温热载体中回收热量的技术，本文利用有机朗肯循环回收依附于余热水的热量。为了考察余热水的热回收和回收效果，建立数学模型并进行参数研究。以R245fa、R113和R123为工质，模拟余热水驱动的有机朗肯循环系统的热效率和效率。结果表明，余热水温度和蒸发温度对余热水驱动的有机朗肯循环系统的热效率和效率影响较大。在一定的余热水参数下，有一个对应于最高效率的最佳蒸发温度。为了对低温余热水进行深度回收，建立了一个联合闪蒸与有机朗肯循环的改进结构。对于本研究中的120 °C和0.2 MPa的余热水，当闪蒸压力为0.088 MPa时，闪蒸–有机朗肯联合循环系统的最大效率为45.91%。余热水驱动的闪蒸–有机朗肯联合循环系统的效率优于纯闪蒸系统和简单朗肯循环系统。

关键词：余热水；有机朗肯循环；热效率；效率；闪蒸

Foundation item: Projects(51704069, 51734004, 71403175) supported by the National Natural Science Foundation of China; Project(N162504011) supported by the Fundamental Research Funds for the Central Universities, China

Received date: 2016-12-15; Accepted date: 2017-03-06

Corresponding author: SUN Wen-qiang, PhD; Tel: +86–24–83686994; E-mail: neu20031542@163.com, sunwq@mail.neu.edu.cn; ORCID: 0000-0002-3885-9254