An ejector refrigeration system optimised for sub-zero temperatures on fishing vessels

On fishing vessels, large quantities of diesel fuel are traditionally used to power engines as well as the on-board refrigeration systems used for freezing harvested fishery products. In 2016, global CO₂ emissions from fuel combustion in the main engines of marine fisheries amounted to approximately 207 million tonnes of CO₂. [1] The efficiency of a diesel engine is only approximately 35–40%, since more than half of the energy is wasted through jacket water and high-temperature exhaust gas. [2] 

 

To reduce the use of diesel fuel on fishing vessels, at least three different types of waste heat-driven refrigeration systems for fishing vessels have been investigated, namely absorption refrigeration, adsorption refrigeration, and ejector-based refrigeration. The ejector-based refrigeration system, powered by the available wasted heat from jacket water or hot exhaust gases can be an environmentally friendly alternative to the diesel-oil-driven refrigeration equipment on fishing vessels. However, in spite of a high thermal efficiency, the coefficient of performance (COP) of ejector-based refrigeration systems still lags behind that of sorption systems. 

 

The entrainment capacity of an ejector is crucial for the overall performance of an ejector-based refrigeration system. In sub-zero refrigeration, a lower evaporation temperature leads to a higher pressure difference between the condenser and the evaporator. In that case, single-stage ejectors may lose their ability to boost pressure. To solve this problem and improve the entrainment ratio of the ejector under a higher pressure difference, researchers have proposed a theoretical refrigeration system based on a two-stage ejector as the pressure-boosting device and a water-cooled condenser for sub-zero refrigeration on fishing vessels. [2] 

 

Performance of a theoretical model of a sub-zero refrigeration system with a two-stage ejector  ^[2] 

 

A two-stage ejector refrigeration system typically consists of a two-stage ejector, a generator, a condenser, an evaporator, a pump, and an expansion valve. At the outlet of the condenser, the refrigerant (R134a in this study) separates into two parts. One part of the refrigerant supplies sub-zero refrigeration, acting as the secondary fluid, while the other part acts as the primary fluid. It is pumped into the generator to be heated either by waste heat or by renewable energy, in order to increase the temperature and pressure of the refrigerant. 

 

The two-stage ejector in this cycle serves as a pressure-boosting device so that the evaporated refrigerant can be sucked and compressed to a high condensation pressure. The authors opted to use seawater (at a temperature of 20-30°C) rather than air as the coolant for the condenser. The water-cooled condenser is better at reducing the condensation pressure than an air-cooled condenser. The water-cooled condenser also allows to reduce the pressure difference between the condenser and the evaporator, which improves the energy efficiency of the refrigeration system. 

 

Using a design method developed to achieve the optimal performance of entrainment, the CFD simulation results indicated that the proposed two-stage ejector could achieve an evaporation temperature as low as −25 °C with an entrainment ratio as high as 0.0961. Furthermore, the entrainment ratio of the two-stage ejector was improved by 30.57% and the COP of the sub-zero refrigeration system was improved by 28.77%, in comparison with a previous study using an air-cooled condenser. 

 

The authors therefore conclude that using the waste heat from fishing vessels is an energy-efficient method to drive a sub-zero refrigeration system with optimally designed two-stage ejector and a water-cooled condenser. 

 

 

Sources 

[1] Greer, K., Zeller, D., Woroniak, J., Coulter, A., Winchester, M., Palomares, M. D., & Pauly, D. (2019). Global trends in carbon dioxide (CO2) emissions from fuel combustion in marine fisheries from 1950 to 2016. Marine Policy, 107, 103382. https://doi.org/10.1016/j.marpol.2018.12.001 

[2] Wang, X., Wang, L., Song, Y., Deng, J., & Zhan, Y. (2021). Optimal design of two-stage ejector for subzero refrigeration system on fishing vessel. Applied Thermal Engineering, 187, 116565. https://doi.org/10.1016/j.applthermaleng.2021.116565