Energy efficient CO2 refrigeration units for fishing vessels.

Reduction in greenhouse gas emissions from the fishing sector is an effort that requires the development of innovative new technology, and that requires research. This master thesis is a part of the CoolFish project, led by SINTEF Ocean, with multiple research and industrial partners. The project aims to develop environmentally friendly systems for cooling, freezing, and heating onboard fishing vessels. This thesis describes the architecture and performance of a prototype industrial CO2 transcritical system for refrigerated seawater (RSW). The design presented was developed by NTNU to be implemented at MMC First Process. The CO2 system will cover cooling demands in air conditioning (AC), RSW and low-temperature freezing. Five simulation models for the CO2 system were developed using Engineering Equation Solver (EES) and Dymola/Modelica for optimization regarding system performance, energy efficiency, and applicability for future installations: • CASE 1: Single throttling. One evaporating temperature level at -5 °C (Refrigerated sea water temperature production). • CASE 2: Double throttling with auxiliary compressor configuration. Two evaporating temperature levels at -5 °C (RSW) and 5 °C (AC). • CASE 3: Triple throttling with parallel compression. Three evaporating temperature levels at AC, RSW and low temperature frozen storage (LT) at -25 °C. • CASE 4: Similar as in CASE 2 with the utilization of a high-pressure ejector. Two evaporating temperature levels at AC and RSW. • CASE 5: Similar to CASE 2 with the utilization of a high-pressure ejector. Two evaporating temperature levels at RSW and LT. Optimal high pressure, pressure in the intermediate pressure receiver, the effectiveness of internal heat exchangers and effectiveness of ejectors were thoroughly investigated to optimize the specified cases. The research shows that the COP of the transcritical CO2 system varies with the pressure in the gas cooler; a maximum COP occurs at an optimal discharge pressure depending on the outlet temperature from the gas cooler. Based on the cycle evaluation, correlations of the optimal discharge pressure are obtained for each specified case. Further, dynamic models of the CO2 unit are presented. The simulations was done with respect to realistic operating conditions, focusing on the chilling and the temperature maintenance period. The results indicate that the length of the maintenance predominantly affects the overall system performance. During maintenance, the loads are low and primarily occurs due to the transmission losses. While the length of the maintenance period varies, it can be argued to be the most prolonged operational period for fishing vessels going far to the sea. Therefore correct system control, ensuring the best possible COP at maintenance is essential to ensure low system power consumption, hence lowering the fuel consumption onboard in the range of [7%-12%]. Further, the results show a high-performance increase utilizing a high-pressure ejector (CASE 4), especially at higher ambient temperatures. The ejector solution provides stable refrigeration capacity at 440 kW at RSW whilst achieving a COP in the range of [3.0-3.5]. The proper control of the refrigeration system should ensure efficient cooling onboard fishing vessels in warmer climates. One example of such control evaluation is the influence of internal heat exchangers (IHX) on system performance. Based on calculations performed by EES, the benefits of IHX on system COP and cooling at RSW is observed only at higher ambient temperatures (above 30 °C). Therefore, it is advised to bypass internal heat exchangers at seawater temperatures lower than 30 °C.