A MONITORING STUDY OF ENERGY CONSUMPTION IN A HYPERMARKETS

1. INTRODUCTION

This article describes a monitoring study of energy consumption, which was made in a large hypermarket in Portugal. The hypermarket has a total sailing area of 11000m² and represents a investment of 34 millions of Euros. The objective of this monitoring study was to verify the energy conception considering a conventional parameterization, and an optimized parameterization.

The owner of the installation wanted to know if some purposes made in the time of the project’s installation were correct and how far was the performance of the refrigeration installation from this plans. They wanted also to know how the refrigeration energy consumption in this hypermarket was. The refrigeration substructures are formed by 7 cold stores (Congelation), 17 cold stores (Refrigeration), 18 Climatized working rooms, 118 Refrigeration display cases, 43 Congelation display cases and 1 ice machine (3000 kg/day). 

The refrigeration system used, is a vapour compression cycle using R404A with three subsystems, one for the refrigeration cold stores, one for the refrigeration display cases and one for the freezers (cold stores and display cases). Each subsystem is composed by one refrigeration central with three our four alternative or scroll compressors, one feeding piping, two or three condensers and the respective number of evaporators with electronic expansion valves. The defrosting is by hot gas in the freezers and by stoppage in the refrigerators. The refrigeration installation is monitored and controlled by the Adap-kool Danfoss system.

This monitoring study as made in two different periods (January and July), corresponding to winter and summer working conditions. In these periods each refrigeration central was monitored, and the values of electric energy consumption and refrigeration capacities were registered. In each period of one week, two operating conditions were tested – running of the refrigeration installation with optimised parameters and running of the refrigeration installation with fixed design parameters. The time periods and working parameters considered are shown in table 4.

During this one week periods the values of electric consumption of each refrigeration central were registered, using a impulse counter and a AKL111A Danfoss controller, associated with the Adap-kool Danfoss control system. The values of the refrigeration capacity in each moment were obtained through of the values of the condensing pressure and evaporating pressure, the values of the level of compressor’s usage in each moment and the performance curves of each compressor. Finally, the value of real Coefficient of Performance (COP_real) was obtained using the values of electric power consumption and refrigeration power capacity using the [1] formulae:  

4. RESULTS

A complete set of data results were obtained with this monitoring study. The values of condensing pressure evaporating pressure, compressors capacity, exterior temperature, capacity of condensers, electric energy consumption, instant electric power and COP, were recorded each five minutes. After this registration, integration and mean data values were calculated. Some of the obtained results are shown in figures 15 to 18. In this graphs we can see the values of the real COP obtained by formulae (1) for the situations of summer operating conditions. The values of COP are shown in the vertical axis, for all week data (horizontal axis). A resume of the obtained results are also shown in table 5 and 6. 

 

By the analysis of the obtained results we can conclude that in a general matter the installation is operating under very satisfactory conditions. We can also conclude that o correct choose of the optimised parameters conduces to a higher value in the COP of the refrigeration centrals. It is also visible that, the centrals operation under optimised conditions (variation of condensing pressure and evaporating pressure) permit to achieve higher COP’s, reduces the electric energy consumption and reduces the number of working hours in compressors, than the conventional working conditions. It is also visible that in a general way the alternative compressors central (refrigeration cold stores) achieve higher COP’s than the screw compressor one (refrigeration display cases). This could possible change if we use 100% capacity variations instead of three steps capacity variation in the screw compressors central.   

5. conclusions

An example of a monitoring study realised in a real hypermarket was presented and discussed. It is possible to conclude that in the specific case of this installation, the centrals operation under optimised conditions (variation of condensing pressure and evaporating pressure) had achieved higher COP’s, less electric energy consumption. Also the number of working hours in compressors was reduced, compared with the conventional working conditions. It is also visible that in a general way the alternative compressors central (refrigeration cold stores) achieve higher COP’s than the screw compressor one (refrigeration display cases). This is explained by the fact that the variation of capacity is by three steps and not a 100% capacity variations.

ACNKOWLEDGMENTS

The authors want to express their acknowledgements to Eng. Paulo Monteiro (Danfoss) and Mr. Fernando Neves (Santos & Soares) for their collaboration in this study.

REFERENCES

[1] Lindhard, B. (2001), “Control inteligente de refrigeración – una herramienta potente para el ahorro energético en los supermercados”. The danfoss Jornal. Nº54, pag.10-11

[2] Coca, E.L. (2000), “Poupanças energéticas em instalações frigoríficas”. O Instalador. Nº48, pag.49-58

[3] Whitman, W.C., Johnson, W.M. (1991), “Refrigeration and Air Conditioning Technology”. Delmar Publishers Inc, New York

[4] Garcia, J.M., Coelho, L.M.R. (2000), “Refrigeration cycles characterization parameters”. ETHERACE As a result of the European project PHETRA. EC DG Enterprise/Innovation and services