Establishment and verification of energy consumption model of fruits and vegetables fresh-keeping transportation container with controlled atmosphere by liquid nitrogen injection

Abstract: China is a country that produces and consumes large amounts of fruits and vegetables. During fresh-keeping transportation, energy consumption rises with the increase of traffic volume of fruits and vegetables. The fresh-keeping transportation container with controlled atmosphere by liquid nitrogen injection is an advanced and efficient equipment for transporting fruits and vegetables. However, there is little research on the energy consumption regulations of this kind of fresh-keeping transportation container so this article puts forward a research method. Fresh-keeping transportation with controlled atmosphere by liquid nitrogen injection for fruits and vegetables keeps the temperature, relative humidity, and oxygen volume fraction of the transportation container in a state of relative balance, which could meet the demand for fruit and vegetable fresh-keeping. However, due to the influence of heat transferring towards the container, cold consuming of electrical appliances, cold consuming of fruits and vegetables, heat transferring by door opening, aperture heat leaking, container pre-cooling, and solar radiation in transportation, the equilibrium states of fresh-keeping environment in the container is broken. In addition, heat and mass transferring between the inside and outside of the container has begun. At this time, the fresh-keeping equipments started to keep the equilibrium states of fresh-keeping environment, and then energy consumption was generated. The fresh-keeping equipments of controlled atmosphere fresh-keeping transportation container by liquid nitrogen injection consist of refrigeration, humidification, and controlled atmosphere system, and the energy consumption generated from these three equipments. Finally, the energy consumption model was established based on the analysis of the heat and mass transfer and energy consumption in refrigerating, humidifying, and injecting. After the energy consumption model was set up, the verification experiment was carried out using litchi as testing material and was based on fresh-keeping transportation platform with controlled atmosphere by liquid nitrogen injection for fruits and vegetables. The fresh-keeping transportation platform can adjust the temperature, relative humidity, and oxygen volume fraction in the container automatically and intelligently through refrigeration, humidification, and controlled atmosphere system, creating a suitable fresh-keeping environment for the litchi. Results indicated that the energy consumption was mainly composed of the controlled atmosphere energy, refrigerating energy consumption, and humidifying energy. The theoretical energy consumption obtained through the energy consumption model was basically consistent to the experimental energy consumption with the average relative error of 11.86%±4.29%. The theoretical liquid nitrogen consumption value was basically consistent to the experimental liquid nitrogen consumption mass with the average relative error of 11.60%±3.51%. The total refrigerating capacity from the controlled atmosphere process was associated with the controlled atmosphere volume of the container. What's more, by consuming less energy, the controlled atmosphere could produce a large total refrigerating capacity that accounted for about 22% of the total theoretical refrigerating capacity in the experimental verification. This research provides a reference for optimizing the equipments of controlled atmosphere transportation by liquid nitrogen injection and saving energy of fruits and vegetables during fresh-keeping transportation.