Experimental investigation on binary ammonia–water and ternary ammonia–water–lithium bromide mixture-based absorption refrigeration systems for fishing ships

https://doi.org/10.1016/j.enconman.2018.04.013Get rights and content

Highlights

  • An absorption refrigeration system for shipboard energy recovery is presented.

  • Binary ammonia-water and ternary ammonia-water-lithium-bromide are used.

  • A prototype with two rectifiers is designed and tested.

  • The rectifier size is reduced by using ternary mixture as the working fluid.

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.

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. [8] 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. [16] 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. [17], 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 [18] 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 [19] and Peters et al. [20], this ternary working fluid can match the marine engine exhaust gas heated refrigeration system. Furthermore, Mclinden et al. [21] 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. [22] 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 [21]. 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.

Section snippets

System description

The schematic of the absorption refrigeration cycle for fishing ships is shown in Fig. 1 and the Dühring diagram of the absorption refrigeration cycle is shown in Fig. 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 and 27 °C. Generally, the cycle consists of three main systems: the solution

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

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 [25] and the details are provided in the Appendix A. 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%.

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.

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

A

The Root-Sum-Square method is chosen to conduct the uncertainty analysis and the equations are listed as follows:uCOPCOP=uQRefQRef2+uQGQG2uECOPECOP=uQRefQRef2+uWQW2uQGQG=umgasmgas2+uh15mh152+uh16mh62uQRefQRef=umRMmRM2+uT17T172+uT18T182uh=(h(T+uT,ξ)-h(T,ξ))2+(h(T,ξ+uξ)-h(T,ξ))2

Acknowledgements

The authors acknowledge the support provided by National Natural Science Foundation of China (51706214) and Fundamental Research Funds for the Central Universities (201713034).

References (27)

Cited by (35)

View all citing articles on Scopus
View full text