Department of Systems and Control - Jožef Stefan Institute

Optimisation of energy cost for refrigeration systems in shopping malls

Principal investigator:

Duration:

August 1, 2013 – July 31, 2016

Funding:

ARRS - Slovenian Research Agency, L2-5476 (C), co-financed by Entia d.o.o.

Abstract:

With increased awareness of the limited energy sources and while coping with consequently constantly growing prices, great attention is being paid to electricity saving. For retail companies, energy is one of the most important parts of the costs; therefore energy efficiency is an important strategic goal. As much as 3% of the total electricity supplied in Sweden is used by large shopping malls, similar results apply to Slovenia. The reason why shopping malls consume more electricity than residential buildings is mainly the use of large refrigeration systems which are huge electricity consumers. The average energy part consumed by refrigeration systems in as high as 38.5 %.

The goal is to design the key elements of the control/management system for shopping mall refrigerating systems. Furthermore, the designed system will enable control of load on the refrigerator generator, depending on the temperature in refrigerating cycle. The proposed control system will not be installed locally; all refrigerating systems will be controlled from a remote operating center. The purpose of the proposed control system is optimization in terms of cost and energy efficiency. To achieve this the following measures will be undertaken:

Time scheduling of thawing processes for a cluster of refrigerating units
Optimal control algorithms of each refrigerating unit

A refrigerating system normally runs on constant power, except during the process of thawing of ice from the expansion valve. In that case the generator is turned off until there is no ice on the valve. This process occurs 4-6 times per day and it lasts approximately an hour. The price of electricity can vary up to 30 % in a day. It would therefore be beneficial to start the thawing process in the time of day, when electricity is expensive. The proposed control system will schedule the thawing processes in above described fashion in order to cut the electricity expenses.

To sum up, the goal is to develop an operator based concept of offsite shopping malls refrigerating systems management and control with the objective to optimally control the operation of such machinery, time schedule the thawing processes and reducing the spikes in joint usage.

To additionally optimize the energy consumption of a single refrigerating unit, advanced control algorithms based on simplified models and built in knowledge will be developed. The most critical part is controlling the start of generators right after the thawing process is over. A relatively fast response with a low overshoot (conditions the height of the spike in electricity consumption) is desired. If the start and duration time of the thawing processes is known in advance, an efficient control algorithm could be designed by using predictive control, which takes into account known setpoint demands and technical limitations. The proposed control problem is nontrivial and demands applications of the latest findings from the predictive control research field.