PRINCIPAL INVESTIGATOR: Prof. Linus Opara
CONTACT DETAILS: +27 21 808 4064/8 / +27 78 562 6446 / opara@sun.ac.za / Skype: umezuruike
DURATION: 20 months
PHI-2 CONTRIBUTION: R380 080
LEAD INSTITUTION: Stellenbosch University (Faculty of AgriSciences)
BENEFICIARY: The entire fresh fruit industry
FOCUS AREA: Carbon footprint / Energy efficiency in the supply chain
HUMAN CAPITAL DEVELOPMENT: One post-doctorate and one PhD student
PRESENTATIONS AND PAPERS DELIVERED: Two
PUBLICATIONS: Two
The beauty of intermodal freight transport is that containers are closed at the start of their journey and only opened when they reach their destination.
As far as fruit export is concerned, this is also the system’s downside. There is no way of monitoring that the precious cargo is cold enough for quality to be preserved.
Keeping fruit from spoiling depends on maintaining the integrity of the cold chain, which is an energy-intensive and expensive undertaking.
Several initiatives are currently underway to reduce the energy used in the cold chain. Most of these depend on site visits and manual testing to determine energy usage and waste in cold chain components such as containers. This approach is expensive, tedious, time-consuming and, perhaps most importantly, not repeatable and scalable.
It is fortuitous, therefore, that Prof. Linus Opara, Research Professor and South African Chair in Post‑harvest Technology at Stellenbosch University, has applied his mind to a solution. The result is a project to develop an experimentally validated mathematical model that can do the groundwork involved in improving energy efficiency in the fruit cold chain.
“The improper design and operation of cold chain systems cause a significant waste of energy,” says Prof. Opara. “Not only does this cause unnecessary economic losses and an increased burden on electricity supply systems, but it is entirely avoidable.”
With this in mind, Prof. Opara’s team set out to develop a validated mathematical model of a cold chain handling system for fruit. The model will be used to evaluate the effect on energy consumption of different design and operational parameters of containers. At the end of the project the model will be applied as an industry training tool, called an energy health check, to demonstrate and recommend possible energy saving mechanisms in the cold chain.
Although similar modelling has been done before, none has ever been applied to improve the energy efficiency of the cold chain. The innovation in this project lies in its ability to predict heat transfer inside a refrigerated container, and to quantify the contributions of the different components of the refrigeration system to the total energy use.
WHAT HAPPENS BEHIND CLOSED DOORS?
“Our starting point was to understand how air flows inside a shipping container,” says Prof. Opara. “Air is used to cool the fruit but if it does not get to where the heat is, cooling does not take place.”
The first phase of the project therefore entailed modelling the airflow in an empty container by using
computational fluid dynamics (CFD), a powerful software tool. The model showed that even in an empty container, the air does not flow evenly. A large area at the door, which is furthest away from the refrigeration system, is cut off from the flow. Similarly, airflow is minimal in the middle of the container and is prone to turbulence due to the mixing of hot and cold air.
The model suggests that, depending on where the pallet is placed in the container, the fruit will cool at different rates.
The next step is to physically test the model. This will entail the placement of temperature, air velocity and humidity sensors inside the container and mapping its interior to validate the computer model.
“Once we have completed the validation, and we have made the necessary adjustments to our model, we will place a pallet of fruit inside the container near the cooling system,” elaborates Prof. Opara. Temperature, humidity and airflow data will be logged before the refrigeration system is switched on and recorded during the cooling process. The process will be repeated for pallets placed in the middle and at the door of the container. Throughout this process the readings will be tested against the CFD model.
The final step is to fill the container with pallets containing mostly plastic balls filled with a water solution to mimic the biophysical properties of fruit.
Real fruit with probes inserted will be placed between the balls in order to ensure reliable data collection without wasting fruit unnecessarily. Again, the data logged will be compared to the model predictions.
THE PRACTICAL APPLICATION
“Once we understand airflow in the container, we can model the cooling process by measuring heat transfer,” says Prof. Opara. “This will tell us how long it takes to cool fruit from the pick temperature of about 25°C to -0,5°C during storage, which is the optimum cold chain temperature for many types of fruit.”
Armed with all this information, the model can work its predictive magic. “We will be able to calculate how much energy is used to cool down both the fruit inside the box and the air inside the container. This will be used to calculate the efficiency of the refrigeration system.”
Prof. Opara intends the model to be a tool for industry to test different scenarios. By simply running the model, the impact of different carton designs on cooling time can be determined. The same applies to different ways of stacking the boxes and aligning their vent holes.
Containers have a standardised design, but boxes and pallets come in different shapes and sizes.
Therefore, in order to define the ideal operation of cooling systems, it is necessary to start with the packaging, says Prof. Opara. “The cost of cooling relates directly to the refrigeration unit, the type of packaging material and how the boxes and pallets are stacked. The model we are working towards will be able to tell a producer exactly how much it costs to have fruit in a refrigerated container.”
It is not difficult to see why different cold chain participants are so excited about this project. Fruit producers, packaging and transport companies are eagerly awaiting the results and researchers in Europe and the United States have expressed interest in a knowledge exchange programme.
The power of the project is in the crystal ball.
“This innovative tool allows us to ask the ‘What if’ questions that are expensive to answer by doing physical experiments,” says Prof. Opara. “In future, with this model, we will be able to assess box designs and advise people on how best to load a container. And I see no reason why we can’t change container designs too …”
THE CFD-MODEL PROJECT OBJECTIVES
1 Develop a CFD model of airflow and heat transfer in a typical South African fruit export container.
2 Conduct experiments to validate the model.
3 Apply the validated CFD model to analyse the effects of different container design and operational parameters on the system’s energy efficiency and recommend energy saving measures.
THE BEAUTY OF THE MODEL
The mathematically validated CFD model can:
• Provide a quick and in-depth understanding of the factors that contribute to energy usage and waste.
• With minimum data on the characteristics of a container, accurately predict different scenarios without the need for expensive data collection and extended monitoring.
