Project Title: Model Based Control of Refrigeration System

This project is carried out under the financial support from the Danish Ministry of Science and Technology (DMST), and Center for Model Based Control (CMBC),  under Grant: 2002-603/4001-93.

Academic supervisor: Dr. Jakob Stoustrup, Dr. Henrik Rasmussen

Background:

Traditional control of refrigeration system in supermarket is to control the air temperature inside display cabinets to be within a specific hysteresis band. Activation and deactivation of refrigeration is typically based on the air temperature measured in the airflow of the cabinet.

There are a lot of reasons, historically or technically, that the focus of refrigeration has been on maintaining a desired air temperature, not an optimal product temperature.  One of the main reasons can be the complexity of product temperature measurement, and its direct application to controller.

While the strict requirement from food authority regarding safe, ongoing demand from consumers on product quality  as well as its transparency, high expectations from supermarket regarding energy saving, and storage quality improvement, all push the manufacturer of refrigeration systems to construct some new control solutions.

One of the possible solutions could be to integrate the food product models into the model of a refrigeration system,  in order to find the optimal temperature profile, such that foodstuff is stored not only for long time but also fresh. The ideal situation also includes simplifying the set up procedure and making the scientific data visible to the consumer and authorities,  at the same time, keeps the energy consumption at a low level.

Food deterioration and temperature 

Due to the nature of the food as a physicochemically and biologically active system, food quality is a dynamic state continually moving to reduced levels.

When storage temperature is lowered, the deterioration rate of foodstuff is reduced. Food deterioration is an accumulated result of temperature variation over time, not only a single temperature at one point of time. For some short period of time, such as defrost cycle, exposing to a higher than normal temperature is normally not fatal to the stored foodstuff, but the defrost profile such as  defrost frequency, its duration and temperature level will definitely affect its storage quality.

Frost and defrost

Frost formation on evaporator is a well known and undesirable phenomenon in refrigeration system.

Frost decreases the performance of the heat exchanger by decreasing the effective air flow area and increasing the thermal resistance between the warmer air and the cold refrigerant inside the evaporator.  This performance degradation will become severe with time if nothing is done, in the worst situation the system will be iced up and break down. In order to maintain a satisfactory performance, evaporators need to be defrosted regularly.

Ttraditional defrost cycle will introduce the following side effects: increase the energy consumption of system, and reduces the accuracy of the temperature control for refrigerated foodstuff.

Summing up, on one hand, we need to find an optimal temperature profile for the food storage with optimized defrost; On another hand, we need  to consider the overall energy consumption. One solution is to build these multi objectives into one cost function, and to optimize the weighed cost function.

Expected Contributions:

Identification the detrimental effect of different defrost scheme on stored food quality. For this purpose, a dynamic heat transfer model is developed to deal with phase change problem with time varying boundary condition, to convert traditionally measured air temperature to product temperature.

Proposal of a new defrost scheme, focus on ’when’, meaning schedule the defrost cycle based on 3 parameters: defrost energy, system performance, and food quality.

Proposal of a new defrost scheme, focus on ’how’, meaning by utilizing thermal mass of stored food, take pre-action before the scheduled defrost, to minimize the risk of food decay under defrosting cycle.

Identification the potential of using stored food as thermal mass to shift refrigeration load, to achieve a reduced system operating cost with guaranteed food safety and quality, and reduced peak power consumption; or improve food quality by utilizing cheaper power; or prevent food from discard under extreme condition, where traditional system fails.

Proposal of an optimal control scheme by utilizing stored food as thermal mass, to achieve one of the targets: a reduced system operating cost with guaranteed food quality, and reduced peak power consumption; Improved food quality by utilizing cheaper power; Minimized food loss under extreme condition, where traditional system fails.