Experimental investigation on binary ammonia–water

Experimental investigation on binary ammonia–water and ternary ammonia– water–lithium bromide mixture-based absorption refrigeration systems for fishing ships 1* 1 Han Yuan , Ji Zhang , Xiankun Huang1, and Ning Mei1 1. Marine Engineering, College of Engineering, Ocean University of China, 238 Songling Road, Laoshan District, Qingdao 266100, China Phone/Fax: +86-532-66781105, E-mail: [email protected] Address: 238 Songling Road, Laoshan District, Qingdao 266100, China Abstract Heat recovery of marine engine exhaust gas is an effective way of improving the onboard fuel economy and environmental compliance of fishing ships. Among such heat recovery techniques, the absorption refrigeration cycle shows potential as it can convert the exhaust thermal energy into refrigeration output and meet the onboard refrigeration requirement. However, the severe operating conditions on the shipboard poses a great challenge for its application. This paper presents an experimental investigation of an absorption refrigeration system for the heat recovery of marine engine exhaust gas. To overcome the adverse effect of the severe onboard condition on the rectification process of the absorption refrigeration system, a ternary ammonia– water–lithium bromide mixture is selected as the working fluid. A prototype of the absorption system is designed and an experimental investigation is conducted. Then, the performances of both the ternary ammonia–water–lithium bromide-based system and binary ammonia–water-based system are compared. The results show that the rectifier heat exchange area can be reduced by approximately 16% under the experimental working condition. Furthermore, the ternary system operates at a relatively lower pressure, with a refrigeration temperature of less than -15.0°C, which is higher compared to the temperature of less than -23.6°C associated with the binary system. Nevertheless, the ternary system achieves a remarkably higher cooling capacity. Moreover, by using the ternary ammonia–water–lithium bromide mixture, the heat loss of the prototype is reduced while the coefficient of performance and electric coefficient of performance are increased, indicating that the ternary system has a higher energy conversion efficiency. Keywords: Ship; Exhaust heat recovery; Absorption refrigeration; Ammonia–water– lithium bromide; ammonia–water; Experimental investigation Nomenclature Symbols Q T W X

heat input, kJ temperature, K electricity consumption, kJ ammonia concentration

cw cRM f m h h’ h” u

cooling water specific heat, kJ kg-1K-1 refrigerating medium specific heat, kJ kg-1K-1 circulation ratio mass flow rate, kg s-1 specific enthalpy, kJ kg-1 vapor-phase enthalpy, kJ kg-1 liquid-phase enthalpy, kJ kg-1 uncertainty

Subscripts I, II i=1, 2, 3…… A C G R Ref RM gas

apparatus number state points absorber condenser generator rectifier refrigeration refrigerating medium exhaust gas

Acronyms COP ECOP

coefficient of performance electric coefficient of performance

1. Introduction According to the International Maritime Organisation [1], maritime transport is responsible for 3.1% of the total global emissions of CO2 and shipping emissions are forecast to rise by 250% in 2050. In order to reduce the CO2 emissions, great efforts have been made to improve the efficiency of marine diesel engines. The thermal efficiency of such engines is approximately 50% [2], and 25% of the fuel energy cannot be converted into shaft work but is carried away by the high temperature exhaust gas [3]. The heat recovery from exhaust gas is deemed as an effective way of improving the onboard fuel economy and environmental compliance of fishing ships. Different techniques have been developed. Sun et al. [4] discussed a Sequential Turbocharging system for diesel engine heat recovery and proposed an accurate combustion model for this system. Vale et al. [5] conducted a parametric study on the thermoelectric generator (TEG) technology and the exhaust gas heat recovery efficiency reaches to 2%. Aly et al. [6] experimentally investigated the diffusion absorption refrigeration technology, the coefficient of performance (COP) of the refrigerator is approximately 0.1 and the refrigeration temperature reaches 10–14.5 °C. Yang et al. [7] investigated the organic Rankine cycle (ORC) based exhaust gas heat recovery technology for marine diesel engine and discussed the working fluid selection and effect of pre-heater on system performance. Cao et al. [8] conducted a theoretical investigation on the absorption refrigeration cycle driven by diesel engine exhaust gas of cargo ship and the COP can

reach to 0.6. Among the above techniques, the absorption refrigeration cycle shows potential as it can convert the exhaust thermal energy into refrigeration output, which can meet the refrigeration requirement of fishing ships. The major advantage of the absorption refrigeration cycle from the compression refrigeration cycle is that the former can be driven by the marine diesel engine exhaust gas and thus requires less electricity. In addition, the fuel consumption and CO2 emissions can be reduced. A case study of the B. Delta37 bulk carrier [9] shows that a 70% theoretical potential of electricity consumption of the compressor can be reduced by utilizing the absorption refrigeration cycle in ISO conditions. Palomba et al. [10] experimentally investigated the performance of an onboard absorption system driven by a 195 kW marine engine. The results show that up to 3500 kg/y of fuel can be saved, and CO2 emissions can be reduced by up to three tons a year. The lithium bromide absorption system can be applied on ships for cooling. Great efforts have been made to improve its performance. Ochoa et al. [11] investigated the transient performance of a single-effect lithium bromide absorption chiller as the thermal load varies. Ibrahim et al. [12] introduced a solar-assisted lithium bromide absorption refrigeration system and the results show that integrating the absorption energy storage with the absorption chiller is feasible. Wang et al. [13] combined the thermal recovery from the jacket water into an exhaust gas-driven absorption chiller, which effectively improved the cycle efficiency. Yan et al. [14] developed an enhanced single-effect or double-lift configuration for the absorption refrigeration cycle. Compared with the ammonia–water based absorption system, the lithium bromide absorption system obtains a relatively higher coefficient of performance (COP) [15]. However, the refrigeration temperature of the lithium bromide absorption system is restricted at a relatively high level (> 0°C); thus, it cannot meet the requirement for aquatic product preservation (< -18°C). In order to reach a much lower refrigeration temperature, the ammonia–water working pair should be selected as the working fluid. Cao et al. [16] modelled the whole structure of a ship including a waste heat-powered absorption cooling system. The simulation results indicate that this recovery system could help reduce the total energy by 8.23%. Furthermore, Cao et al. [17] introduced a cascaded absorption-compression configuration into the waste heat-powered absorption cooling system. They concluded that this configuration can remarkably reduce the life cycle cost of the cooling system. The challenge for the ammonia–water absorption refrigeration technique is that this type of system is very difficult to reliably operate on shipboard. In contrast to industrial heat wastage, the exhaust gas discharged from the diesel engine is not stable because it varies with the engine load. A severe fluctuation of the heating source condition will significantly affect the rectification process and results in insufficient removal of the absorbent in the rectifier. Moreover, the rectifier of the onboard system needs to handle the severe rolling, pitching, and yawing motion of the ship. As pointed out by Fernández-Seara et al. [18], ultimately, this would significantly aggravate the performance of the absorption refrigeration system. It is important to note that a rectifier with a smaller size can be less influenced by the

abovementioned effects [19] and shows a promising way of solving the problem. A smaller size means that the heat exchange area along with the rectification heat output should be reduced. The ternary working fluid of NH3–H2O–LiBr is deemed to be a potential working combination that can reduce the rectifier size. According to the characteristics of the NH3–H2O–LiBr ternary solution at temperatures ranging from 15°C to 200°C and at pressures up to 2.0 MPa obtained by Wu [20] and Peters et al. [21], this ternary working fluid can match the marine engine exhaust gas heated refrigeration system. Furthermore, Mclinden et al. [22] conducted an experimental research by comparing the rectifier performance in both binary ammonia–water and ternary ammonia–water–lithium bromide mixture absorption refrigeration cycles. The results indicate that the rectification heat output demand is reduced by utilizing the ternary ammonia–water–lithium bromide mixture. The research carried out by Peters et al. [23] indicates that this performance can be explained by the ‘salting-in’ effect. Owing to ion formation and complexing in the mixture, lithium bromide is supplied as a non-volatile salt which can therefore absorb water. This effect can salt-in water and result in a higher ratio of ammonia to water in the generated ammonia–water mixture vapour. Therefore, the water vapour that is required to be rectified back into the generator becomes less and the rectifier heat exchange area can ultimately be reduced by using the ternary ammonia–water–lithium bromide mixture as the working fluid. Although the ammonia vapour purity at the rectifier inlet is higher as the ternary ammonia–water–lithium bromide mixture is used, the ammonia vapour purity at the rectifier outlet is unchanged for different working fluids [22]. This phenomenon indicates that the ammonia and water vapour discharged from the generator can be fully rectified with sufficient rectifier heat exchange area, no matter which mixture is utilized in the system. Moreover, an oversized rectifier heat exchange area will not contribute to improving the ammonia purity at the outlet. Since the rectification heat output of a ternary mixture-based system is much lower, a small rectifier can meet the demands of both the heat exchange in the rectification process and the purification of ammonia water vapour simultaneously. This is of great significance to the application of the absorption refrigeration system on shipboard. This study develops an absorption refrigeration system driven by the marine engine exhaust gas. A prototype of the absorption refrigeration system with two different sizes of rectifiers is designed and built. The effect of the ternary ammonia–water–lithium bromide mixture on the rectifier size is experimentally investigated. In addition, the ternary system performance is experimentally compared with that of the binary ammonia–water system. 2. System description The schematic of the absorption refrigeration cycle for fishing ships is shown in Figure 1 and the Dühring diagram of the absorption refrigeration cycle is shown in Figure 2. The absorption refrigeration system utilizes the exhaust gas from the marine diesel engine as the heating source and the circulating water as the cooling source. A cooling tower maintains the circulating water inlet temperature between 24°C to 27°C. Generally, the cycle consists of three main systems: the solution circulation system,

water circulation system, and instrumentation system.

Figure 1 Schematic of the absorption refrigeration cycle using binary ammonia–water and ternary ammonia–water–lithium bromide mixtures

Figure 2 Dühring diagram of the absorption refrigeration cycle The solution circulation system consists of the generator, rectifier, condenser, ammonia tank, throttle valve, evaporator, absorber, heat exchanger, and solution pump. Both the

binary ammonia–water and the ternary ammonia–water–lithium bromide mixtures are utilized as working fluids. The working fluids (rich solution) are heated by the exhaust gas and the ammonia/water vapour mixtures are desorbed in the generator. Two rectifiers (Rectifier I and Rectifier II) with different heat exchange areas are designed for the binary and ternary mixture-based absorption cycles, respectively. The ammonia vapour is separated in the rectifier before condensing into liquid in the condenser and flowing into the ammonia tank. The ammonia liquid then passes into the throttle valve and evaporates in the evaporator. The refrigeration output is obtained and supplied for the cold storage. In the absorber, the evaporated ammonia vapour is absorbed by the weak solution discharged from the generator. The heat exchanger is introduced between the generator and the absorber to recover the heat from the weak solution.

Figure 3 Test facility of the absorption system: (a) front view, with insulated hightemperature components; (b) left rear view A test bench was designed and built, as shown in Figure 3, to conduct the experimental investigation. In this study, two rectifiers were designed: Rectifier I for the binary ammonia–water mixture-based system and Rectifier II for the ternary ammonia–water– lithium bromide mixture-based system. The valves, marked in Figure 1 as a, a’, b, and b’, are used to control the working fluid and the cooling water flow into Rectifiers I and II, respectively. When the binary ammonia–water mixture is chosen as the working fluid, valves a and b remain open while valves a’ and b’ remain closed, and the prototype functions as an ammonia–water-based absorption system. When the ternary ammonia– water–lithium bromide mixture is chosen as the working fluid, valves a and b remain

closed while valves a’ and b’ remain open, and the prototype functions as an ammonia– water–lithium bromide-based absorption system. It is important to note that the rectifier heat exchange area for the ternary mixture-based system is based on the experimental results. This experiment is pre-tested and the experimental procedure is described in the experimental plan section in this paper. The water circulation system consists of a cooling tower, water pump, and pipes. The water is circulated by the water pump and exchanges heat in the condenser, absorber, and rectifier. The cooling tower maintains the circulating water inlet temperature between 24°C to 27°C. In this study, a 0–475 kW gas burner is selected and installed in the absorption refrigeration system to produce the exhaust gas. The exhaust gas temperature at the outlet of the burner is controlled. By monitoring the exhaust gas flow rate and temperature at both the inlet and outlet of the absorption refrigeration system, the entire heat input is obtained. The main components of the experimental system are listed in Table 1. Table 1 Components of the experimental system Component Material Description Tube side: working fluid Generator TC4 Shell side: exhaust gas Tube side: working fluid Condenser TC4 Shell side: cooling water Tube side: refrigerating medium Evaporator 316L Shell side: working fluid Ammonia tank 316L Medium: working fluid Tube side: working fluid Absorber TC4 Shell side: cooling water Heat exchanger 316L Plate type Tube size: ø10 × 4800 mm; Rectifier I 316L Heat transfer area: 0.30 m2 Tube size: ø10 × 4000 mm; Rectifier II 316L Heat transfer area: 0.25 m2 Gas burner 0–475 kW, < 500°C

Structural feature Vertical tube Shell and coil tube Fin tube Shell Bubbling absorber Plate heat exchanger Shell and coil tube Shell and coil tube -

The detailed specifications of these instruments are listed in Table 2. Generally, the instrumentation system mainly includes the pressure gauges, flowmeters, and K-type thermocouples. Furthermore, the components of the exhaust gas at the inlet and outlet of the generator are determined by the flue gas analyser, while the gas enthalpy can be calculated. An electric parameter measuring instrument is used to determine the total electricity consumption of the prototype. A data acquisition system is used to collect the pressure, flow rate, and temperature at each state point of the absorption refrigeration system. Table 2 Specifications of instruments

K-type thermocouple Pressure meter Flowmeter for liquid ammonia Flowmeter for water Flowmeter for refrigerating medium Flowmeter for exhaust gas (Pitot tube-based) Flue gas analyser Electric parameter measuring instrument

-40 to 1200°C ±0.5% 0–1.5 MPa ±0.5% 10–100 L/h ±1.0% 0.10–10 m3/h ±1.5% 0.10–10 m3/h ±1.5% 0–120 m/s ±1.0% O2, CO2, N2 (±1.0%) ±0.5%

3. Experimental plan and calculation In the experiments, the gas burner is used to provide the controllable exhaust gas for the prototype. The exhaust gas temperature is controlled from ambient level to 500°C. The exhaust gas inlet temperature is chosen as the dominant factor that affects the system performance. To simulate the actual working condition of the prototype, the temperature is gradually increased from a low level of approximately 250°C to a relatively high level of approximately 350°C. The electricity consumption is measured using the electric parameter measuring instrument. Moreover, the pressure, flow rate, and temperature measurements are collected and recorded using the data acquisition system. All collected experimental data are analysed and equalized into one value every 5 min. Before conducting the experiments, the solution concentration of the ternary ammonia– water–lithium bromide mixture should be determined. According to reference [22], both the purity and mass flow rate of the refrigerant (ammonia) at the outlet of the rectifier remain almost unchanged as the lithium–water ratio in the ternary ammonia– water–lithium bromide mixture varies. This experiment result suggests that the performance of the system (e.g. the refrigeration output) is insensitive to the proportion of the components. Moreover, Sathyabhama [24] investigated the effect of the component proportion on the heat transfer behaviour of a ternary ammonia–water– lithium bromide mixture and a similar result was obtained when the generation pressure was greater than 0.6 MPa. Therefore, in this study the component proportion of the ternary ammonia–water–lithium bromide mixture was fixed at a single value instead of selecting various component proportions. More importantly, a high concentration of lithium bromide could be evaporated from the generator and could flow into the absorber, which ultimately prevents the absorption of NH3 molecules by the weak solution in the absorber [25]. Thus, a relatively low concentration of lithium bromide is mixed in the binary ammonia–water mixture. Owing to the reasons stated above, the mass concentration of ammonia in the binary ammonia–water mixture was set as 25% and the mass concentration of lithium bromide in the ternary ammonia–water–lithium bromide mixture was set as 10%. The exhaust gas heat input is calculated by the enthalpy difference of the components between the inlet and outlet of the prototype. According to the test results, the fly ash can be neglected while N2, O2, CO2, and H2O constitute approximately 99.7% of the volume of the exhaust gas. The volume percentage of N2, O2, and CO2 can be directly determined using the flue gas analyser, and the volume percentage of H 2O can be

calculated based on the conservation of oxygen elements. The enthalpy of N2, O2, and CO2 can be obtained based on the component percentage. In this way, the exhaust gas heat input is calculated. The rectification heat output, condensation heat output and absorption heat output of the system are calculated using the following equations, respectively: QR  cw m12 (T12  T11 ) (1) QC  cw m13 (T13  T11 ) (2) QA  cw m14 (T14  T13 ) (3) where cw represents the specific heat of cooling water, whereas m12 , m13 , and m14 are the mass flow rates of the cooling water. The refrigeration is calculated as follows: (4) where cRM and mRM are the specific heat and mass flow rate of the refrigerating medium flowing between the evaporator and cold storage, respectively. The generation heat input is calculated as follows: QG  mgas  h15  h16  (5) where mgas represents the mass flow rate of the exhaust gas. The enthalpies of the exhaust gas h15 and h16 are determined based on both the components and temperature measured using the flue gas analyser and K-type thermocouple respectively. In this study, both the COP and the electric COP (ECOP) are used to evaluate the refrigeration system, calculated as follows: QRef  cRM mRM (T17  T18 )

COP 

QRef QG

ECOP 

QRef

W where W is the electricity consumption of the entire system.

(6) (7)

To match the prototype that is operating with different working fluids, two rectifiers with different sizes need to be designed. As discussed, the rectification heat output demand can be reduced by utilizing the ternary ammonia–water–lithium bromide mixture. This is an important phenomenon because the rectifier heat exchange area for ternary mixture-based systems can be smaller in the same prototype. In order to verify this prediction and determine the exact heat exchange area of Rectifier II for the ternary mixture-based prototype, a comparison experiment should be conducted first. The general experimental plan was as follows: First, the rectification heat output for both the binary and ternary working fluid-based systems were tested in the same prototype with the same rectifier, and comparative tests were controlled at the same condition. The purpose of the experiment is to determine the reduction ratio of heat output in the ternary mixture-based system. Rectifier I for the ammonia–water binary mixture-based prototype was first designed, and the parameters are given in Table 1. Then, two different working fluids (binary/ternary mixtures) were used in the prototype and the rectification heat output was tested separately. In this way, the required rectifier heat exchange area for the ternary mixture-based prototype was fixed. Moreover, the

performance of the prototype with the ternary mixture and Rectifier II was tested. Finally, the experimental results of the ternary mixture-based system were compared with those of the binary mixture-based system. The effects of the two working fluids on the key parameters and performance criteria are discussed. 4. Experimental uncertainty The precisions of the measurement instruments are listed in Table 2. An uncertainty analysis is made based on the method proposed by Moffat [26] and the details are provided in the Appendix. The resulting uncertainty in the COP was 2.29%. The uncertainty in the ECOP was 1.94%, whereas the uncertainty in the refrigeration was 1.66%. 5. Experimental results and discussion In this experimental investigation, a comparison experiment was first conducted to determine the exact heat exchange area of Rectifier II for the ternary mixture-based prototype. Then, an experimental investigation on the absorption systems with ammonia–water and ammonia–water–lithium bromide mixtures as working fluids was performed respectively. The results obtained are discussed in the following section. 5.1. Reduction in rectifier heat exchange area

Ammonia-water binary mixture

Ammonia-water-lithium bromide mixture Proportion of Rectifying heat output between binary and bromide mixtures

Rectifying heat output (kW)

12 10 8 6 4

Ratio of QR= 0.84

2 0 230

250

270 290 310 330 350 Heating source temperature (oC)

370

Figure 4 Comparison of rectification heat output for binary ammonia–water and ternary ammonia– water–lithium bromide mixture

The rectification heat outputs of both binary and ternary working fluid-based systems were tested in the same prototype with Rectifier I (the heat exchange area is given in Table 1), and comparative tests were controlled at the same condition. The experimental

results are compared as shown in Figure 4, and the required rectifier heat exchange area is obtained. It is observed that the rectification heat output is obviously reduced by using the ternary mixture as working fluid. This observation corresponds well to the prediction, and the average ratio of QR in the bromide system to the binary system is 0.84. This shows that the heat exchange area of Rectifier I is larger than that required when the ternary ammonia–water–lithium bromide mixture is used. Based on this result, Rectifier II is designed and constructed with the heat exchange area reduced from 0.30 mm2 to 0.25 mm2, as indicated in Table 1. 5.2. System operating pressure Ammonia-water binary mixture

Ammonia–water–lithium bromide mixture

Generation pressure (MPa)

1.6

1.2

0.8

0.4

0 230

250

270 290 310 330 350 Heating source temperature (°C)

Figure 5 Generation pressure of two absorption systems

370

Ammonia-water binary mixture

Ammonia–water–lithium bromide mixture

Absorption pressure (MPa)

0.16

0.12

0.08

0.04

0 230

250

270 290 310 330 350 Heating source temperature (°C)

370

Figure 6 Absorption pressure of two absorption systems

The generation pressure is significantly different when binary and ternary working fluids are used in the absorption system. According to Figure 5, the binary system operates at a relatively higher generation pressure within the investigated heating source temperature. A similar phenomenon was also found in the experimental research made by Mclinden [22] and the mechanism can be explained by the ‘salting-in’ effect of lithium bromide as discussed by Peters et al. [23]. Since lithium bromide behaves as a non-volatile salt and water can be more easily absorbed in it than in ammonia, this ultimately leads to a lower bubble point pressure in the ternary ammonia–water–lithium bromide mixture. Thus, the equilibrium pressure of the ternary ammonia–water–lithium bromide mixture is much lower than that of the binary ammonia–water mixture. Figure 6 shows the absorption pressure in the two systems. For the ternary system, it is observed that the absorption pressure remains at a lower level of less than 0.04 MPa, which indicates that lithium bromide could also flow into the absorber and the bubble point pressure in the absorber is decreased. According to the above results, the ternary system operates under a lower pressure. This is beneficial to its shipboard application as the absorption system is exposed to harsh working conditions and the components and connecting fittings are prone to the risk of breakdown. With a lower operating pressure, the absorption system can be safer and more reliable. 5.3. Heat load of components

Generation heat input (kW)

Ammonia-water binary mixture

Ammonia–water–lithium bromide mixture

50

40

30

20 230

250

270 290 310 330 Heating source temperature (°C)

350

370

Figure 7 Generation heat input of two absorption systems

Figure 7 compares the generation heat input in the two absorption systems. It is observed that the heat input of the system is almost the same even when using different working fluids. Therefore, the ternary mixture can satisfactorily fit the prototype and the generator does not need to be redesigned. Moreover, a higher heating source temperature leads to a higher generation heat input with a linear growth as shown in the figure.

Ammonia-water binary mixture

Ammonia–water–lithium bromide mixture

Absorption heat output (kW)

30 25

20 15 10 5 0 230

250

270 290 310 330 350 Heating source temperature (°C)

370

Figure 8 Absorption heat output of two absorption systems A comparison of the absorption heat output for the two absorption systems is shown in Figure 8. Generally, it is observed that the ternary system requires a higher absorption heat output. In addition, the absorption heat outputs of both systems rise as the heating source temperature increases. To explain these performances, the first law of thermodynamics should be utilized. The absorption heat output can be calculated using the following equation: QA  m6 h6''  h10'  f (h10'  h7 ' )  f=

(8)

X 1' -X 2

(9)

X1  X 2

where m6 is the mass flow rate of the ammonia vapour at state point 6, h6'' is the vapour-phase enthalpy of the ammonia vapour at state point 6, h10' is the liquid-phase enthalpy of the ammonia weak solution at state point 10, h7' is the liquid-phase enthalpy of the ammonia–water rich solution at state point 7, and f is the circulation ratio of the absorption system as defined in equation (9), where X1 is the ammonia '

concentration at state point 1’, X 1 and X 2 are the ammonia concentration at state point 1 and 2, respectively. First, as shown in Figure 9, the mass flow rate of refrigerant in the ternary system is much higher than that in the binary system, which means that m6 is much higher. Thus, the absorption heat output in the ternary system is ultimately higher. Second, with the increase in heating source temperature, the ammonia–water weak solution temperature at state point 9 will increase, which leads to a higher temperature at state point 10 and a corresponding increase in h10' . Meanwhile, h6'' and h7' remain the same because the absorption/evaporation processes are determined by the absorption/evaporation pressures, which are not affected by the fluctuation of heating source temperature. Therefore, the monotonicity of equation (8) is determined by f . As X1  X1 , which '

means that f  1 , the function on the right side of equation (8) is monotonically increasing and the absorption heat output of both systems will increase with increasing heating source temperature. According to above-mentioned experimental results, the heat exchange capacity of the absorber for the ternary system should be higher than that of the binary system, and a design margin in the absorber is necessary to match the full load of the marine diesel engine.

5.4. Performance evaluation of binary and ternary mixture-based systems Ammonia-water binary mixture

Ammonia–water–lithium bromide mixture

Refrigerant flow rate (L/h)

60 50 40 30 20 10

0 230

250

270 290 310 330 350 Heating source temperature (°C)

370

Figure 9 Refrigerant flow rate of two absorption systems

Figure 9 shows the comparison of refrigerant flow rate in the binary and ternary mixture-based absorption systems. Generally, the ternary mixture-based system has a higher refrigerant flow rate. To determine the reason for this phenomenon, an important parameter should be noted: X 1''  X 1’'' R （X 1'  X 1’''） R

(10)

where R is the reflux ratio in the rectification process, which represents the rectified liquid that refluxes into the generator, X 1'' is the gas-phase ammonia concentration at the rectifier inlet, X1' is the liquid-phase ammonia concentration at the rectifier inlet (which is equal to the ammonia concentration in the initial rich solution), X 1 '' is the ’

gas-phase ammonia concentration at the rectifier outlet, and R is the rectification efficiency. According to reference [22], the ammonia vapour purity at the rectifier outlet is unchanged for both binary and ternary mixtures. Meanwhile, compared with the binary system, the ternary ammonia–water–lithium bromide mixture-based system has a higher X 1 '' owing to the salting-in effect. Because the ammonia concentration in the ’

initial rich solution remains the same, the reflux ratio R in the ternary system is lower than that in the binary system. With a lower reflux ratio, a higher flow rate of ammonia

vapour will be obtained at the outlet of the rectifier in the ternary system. Obviously, the increase in the mass flow rate of the refrigerant is beneficial for improving the refrigerating capacity. Ammonia-water binary mixture

Ammonia–water–lithium bromide mixture

Refrigerating capacity (kW)

12

10

8

6

4 230

250

270 290 310 330 350 Heating source temperature (°C)

370

Figure 10 Refrigerating capacity of two absorption systems

The refrigerating capacity of the two absorption systems is shown in Figure 10. Generally, a higher exhaust gas inlet temperature leads to a higher refrigerating capacity in both systems. The refrigeration of the binary ammonia–water-based absorption system ranges from 5.3 to 7.1 kW. By comparison, it is found that the ternary ammonia–water–lithium bromide-based system has a better performance and produces a much higher refrigeration of 6.3–9.2 kW. This indicates that the refrigerant flow rate performs a major role in determining the refrigerating power.

Ammonia-water binary mixture

Ammonia–water–lithium bromide mixture

0.3

COP

0.2

0.1

0 230

250

270 290 310 330 350 Heating source temperature (°C)

370

Figure 11 COP of two absorption systems

Figure 11 shows the COP of the two absorption systems. It is observed that the COPs for both systems decline as the exhaust gas temperature increases. For the absorption system, COP 

QRef QG

. A higher heating source temperature leads to an increase in both

QRef and QG , as shown in Figures 9 and 7. Therefore, this result shows that the increase

rate of QG is higher than that of QRef . This phenomenon also indicates that the energy conversion in both the absorption systems become less efficient when the exhaust gas temperature rises to a high level. This is because of the heat loss in the prototype, which is shown in Figure 13. Furthermore, according to the results, the ternary system performs better with a higher COP of 0.18–0.24 compared with 0.14–0.23 of the binary system. Because the new Rectifier II is designed to adapt to the thermodynamic property of the ternary ammonia– water–lithium bromide mixture, the heat exchange area in Rectifier II is reduced by approximately 16% and the vapour discharged from the generator will not be over rectified. Thus, less working fluid is condensed in the reflux flow and more refrigerant is obtained at the outlet of the rectifier. This ultimately results in a higher refrigerating capacity of the absorption system and contributes to a higher COP for the ternary system.

Ammonia-water binary mixture

Ammonia–water–lithium bromide mixture

10

ECOP

8 6 4 2 0 230

250

270 290 310 330 350 Heating source temperature (°C)

370

Figure 12 ECOP of two absorption systems

A comparison of the ECOP of the binary and ternary systems is shown in Figure 12. In contrast to the COP, the ECOP values of both systems rise as the heating source temperature increases. This is because the pump work in these two systems changes only slightly, but the refrigeration power can be greatly increased by the higher temperature of exhaust gas. It is also found that the ternary system obtains a higher ECOP of 4.6–7.0 compared with 4.1–6.0 of the binary system.

Ammonia-water binary mixture

Ammonia–water–lithium bromide mixture

16

Total heat losses (kW)

14 12 10 8 6 4 2 0 230

250

270 290 310 330 350 Heating source temperature (°C)

370

Figure 12 Heat losses of two absorption systems

Figure 12 shows the heat losses of the binary and ternary systems. With a higher heating source temperature, the heat losses become severe in both systems and thus, the energy conversion of both the absorption systems become less efficient as the exhaust gas temperature rises to a high level, which is shown in Figure 10. However, owing to the lower evaporation temperature in the evaporator and the smaller rectifier size, the temperature difference between the ternary system and the environment is decreased and thus, the ternary system performs better with less heat losses. 5.5. Performance comparison of different absorption refrigeration systems After performing the experimental investigation on absorption systems with binary ammonia–water and ammonia–water–lithium bromide mixtures as working fluids, the results are compared with those obtained by Koehler [27] and Manzela [28], as presented in Table 3. All of these studies use the engine exhaust gas as the heating source to drive the absorption refrigeration cycle and produce refrigeration. Therefore, these cycles are comparable because the refrigeration outputs have a similar level. Table 3 Performance comparison of experimental studies for absorption refrigeration systems Koehler [27]

Manzela [28]

This study (binary cycle)

Cycle type

This study (ternary cycle)

Absorption

Absorption

Absorption

Absorption

refrigeration

refrigeration

refrigeration

refrigeration

cycle

cycle

cycle

cycle

Working fluid

NH3–H2O

NH3–H2O

NH3–H2O

NH3–H2O–LiBr

Heating source

Engine exhaust

Engine exhaust

Engine exhaust

Engine exhaust

gas

gas

gas

gas

Ammonia concentration (%)

18%

-

25%

25%

Heating source temperature (°C)

440 ~ 490

-

230 ~ 370

230 ~ 370

Refrigeration temperature (°C)

-20 ~ 0

4 ~ 30

-23.6 ~ -27.5

-15.0 ~ -16.7

Refrigeration output (kW)

6~7

3 ~ 18

5~7

5~9

COP

0.23 ~ 0.26

0.015 ~ 0.047

0.15 ~ 0.22

0.18 ~ 0.24

Heat transfer area reduction in

0

0

0

16

rectifier (%)

Compared with the experiment results obtained by Manzela [28], the cycle in this study performs better: the refrigeration temperature is much lower and the COP is much higher. Compared with the experiment results in reference [27], the refrigeration temperature, refrigeration output, as well as COP are all similar, which suggests that the experimental investigation in this study is reasonable. Moreover, the main purpose of this study is to determine a method of reducing the rectifier size, so that the absorption refrigeration system can be effectively applied on shipboard. According to the experimental results, by utilizing the ternary ammonia–water–lithium bromide mixture as the working fluid, the heat transfer area of the rectifier can be reduced by 16%. Furthermore, with a similar heating source condition, the performances of the systems are compared in Table 4. Generally, the binary system reached a much lower evaporation temperature of -23.6°C to -27.5°C compared with -15.0°C to -16.7°C of the ternary system. It should be noted that the refrigeration temperature is still sufficient for the fishing ship to produce and store the sea ice. In addition, the ternary system achieved a remarkably higher value of cooling capacity, COP, and ECOP, which indicates that this system has a competitive performance in energy conversion. Table 4 Experimental performance of two absorption systems Working

Exhaust gas

Exhaust gas

Exhaust

Heating

Cooling

Evaporation

Cooling

COP

ECOP

fluid of the

inlet

outlet

gas flow

capacity

water

temperature

capacity

cycle

temperature

temperature

rate

(kW)

temperature

(°C)

(°C)

(m3/h)

(°C)

(kW)

NH3–H2O

245.2

141.7

856

29.65

mixture

299.6

147.1

899

37.90

26.0

-23.6

27.0

-26.6

5.54

0.21

4.14

6.01

0.16

4.52

343.3

149.9

903

48.21

26.3

-27.5

7.12

0.15

5.42

NH3–H2O–

247.3

134.7

862

28.01

25.8

-15.0

6.30

0.22

4.75

LiBr

298.3

140.2

896

38.59

26.4

-14.9

7.30

0.19

5.41

mixture

349.5

151.7

928

46.78

26.3

-16.7

8.62

0.18

6.61

inlet

(°C)

6. Conclusion The major advantage of the absorption refrigeration cycle from the compression refrigeration cycle is that the former can be driven by the marine engine exhaust gas and thus, needs less electricity. Moreover, the fuel consumption and CO2 emissions of the ship can be significantly reduced. However, the severe operational conditions on shipboard presents a great challenge to its application. This paper presents an experimental investigation of an absorption refrigeration system for the heat recovery of marine engine exhaust gas. To overcome the adverse effects of the severe onboard

condition on the rectification process of the absorption refrigeration system, the ternary ammonia–water–lithium bromide mixture is selected as the working fluid. As the rectification heat output of a ternary mixture–based system is much lower, a small rectifier can simultaneously meet the demands of both the heat exchange in the rectification process and the purification of ammonia water vapour. This is of great significance to the application of the absorption refrigeration system on shipboard. A prototype is established and two rectifiers are designed and installed on both the binary and ternary systems. The actual performance of the prototype is tested. Based on the experimental investigation, the following conclusions can be made:  By using the ternary ammonia–water–lithium bromide mixture as the working fluid, the rectifier size in the ternary system can be decreased and the heat exchange area can be reduced by approximately 16% under the experimental working condition, which is significantly beneficial for the application of the absorption system on fishing ships.  The ternary system operates at a relatively lower pressure in both generation and absorption processes with a cooling capacity of 6.3–9.2 kW.  The binary system obtains a lower refrigeration temperature of less than -23.6°C, compared with the ternary system of less than -15.0°C. This is sufficient for the fishing ship to produce and store the sea ice.  The ternary system achieves a remarkably higher value of cooling capacity, COP, and ECOP as well as a lower heat loss, which indicates that this system has a higher energy conversion efficiency. Acknowledgements The authors acknowledge the support provided by National Natural Science Foundation of China (51706214) and Central University of basic scientific research business fees special of China (201713034). Appendix The Root-Sum-Square method is chosen to conduct the uncertainty analysis and the equations are listed as follows: 2

 uQ u COP   Ref Q COP  Ref

  u QG        QG 

2

2

 uQ u ECOP   Ref Q ECOP  Ref

  uW        QW 

2

2

2

u QG

  u h15   u h16            mh15   mh 6 

u QRef

 u m   uT   uT    RM    17    18   mRM   T17   T18 

 um   gas m QG  gas

QRef uh 

2

 h T  u

T

2

2

2



,    h(T ,  )   h T ,   u   h(T ,  ) 2

[1] M. Traut. Third IMO Greenhouse Gas Study 2014.



2

(2014).

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Figure captions Figure 1 –Schematic of the absorption refrigeration cycle using binary ammonia– water and ternary ammonia–water–lithium bromide mixtures Figure 2 – Dühring diagram of the absorption refrigeration cycle Figure 3 –Test facility of the absorption system: (a) front view, with insulated hightemperature components; (b) left rear view Figure 4 –Comparison of rectification heat output for binary ammonia–water and ternary ammonia–water–lithium bromide mixture Figure 5 – Generation pressure of two absorption systems Figure 6 – Absorption pressure of two absorption systems Figure 7 – Refrigerant flow rate of two absorption systems Figure 8 – Refrigeration of two absorption systems

Figure 9 – Generation heat input of two absorption systems Figure 10 – COP of two absorption systems Figure 11 – ECOP of two absorption systems Figure 12 – Heat losses of two absorption systems Table captions Table 1 – Components of the experimental system Table 2 – Specifications of instruments Table 3 – Performance comparison of experimental studies for absorption refrigeration cycle Table 4 – Experimental performance of two absorption systems