ENERGY EFFICIENCY STRATEGIES APPLIED TO REFRIGERATION SYSTEMS IN SUPERMARKETS

1. INTRODUCTION

With recent developments in the energy field, the impact of energy consumption and is relationship with sustainable development is one aspect of primordial importance in engineering systems. The concept of sustainable communities with its three components (economy, society and environment) is directly connected with the rational utilization of energy, both at a local level, and at a global level. These three aspects of sustainability are in most case difficult to evaluate and the aspect of energy consumption should be carefully considered.

With the recent increase in the number of large supermarkets (Hypermarkets), this sector of activity had increased is importance and weight in energy consumption related to other economic activities. A study made in Portugal by INETI [2] concludes that in this kind of superficies there are no energy consumptions below 500 Tep/year, and in most cases the energy consumption is above 1500 Tep/year. These results had been achieved after a fieldwork based in inquiries and energetic judgeships.

 The same study [2] concludes that electrical energy is the main kind of energy used (95,2%) followed by far by the second type of energy (1,2%) natural gas.

One of the most important energy consumption sectors in a hypermarket is the energy used related with the refrigeration activity. This cost represents a mean value about 30% of the final energy used in this activity [2]. So the scope of this article is to discuss the possibility of energy savings related to the refrigeration systems and equipment in supermarkets and hypermarkets. Nevertheless, many of the aspects discussed in this article, can be extended and applied to refrigeration systems of other refrigeration plants, industrial and commercial.

2. ENERGY SAVING STRATEGIES IN REFRIGERATION

With this scenario of high dependence of electric energy in great supermarkets, two strategies are primordial to achieve our goal of sustainability and environment respect in these installations. We remember that these articles focus only the refrigeration systems, and the other systems like HVAC, lights, hot sanitary water, bakeries, ovens, are out of the scope of this article. So, to achieve our goal, related with the refrigeration systems we can:

i) Try to replace the electric consumption by alternative forms of energy especially renewable energies.

ii) Try to reduce the global electric consumption in hypermarkets using more efficient systems.

The first line of action – trying to replace the electric consumption by alternative forms of energy - is today maybe the most difficult of both strategies. In fact nowadays, the great majority of the actual refrigeration systems use the vapour compression cycle with one stage or multistage compressors. This kind of systems uses electric energy to drive the compressors, essentially because of the substantial amount of power needed when compression is made. In fact Lindhart [3] sustain that in current supermarkets compressor are responsible for almost 50% of the total energy consumption due to refrigeration. The only two alternatives that the refrigeration “state of art” knows are the absorption cycles and the adsorption cycles. But these cycles have some practical problems when the engineers try to implement them. The usage of other forms of energies like wind, solar, geothermal and seas have nowadays to many limitations to be considered as valid solutions. In fact the costs associated to this systems associated with the inexistence of tested equipment that can be used using this forms of energy, jointly with the specific difficulties associated with this systems when implemented in real installations, are sufficient to discourage the refrigeration engineer.

The second line of action – trying to reduce electric consumption using more efficient systems – is maybe the easiest and more realistic way of acting at short term in refrigeration systems of supermarkets. In this field many things can be made to achieve the purpose of optimising the installation and reduce electric consumption. First, we can try to use more efficient equipment, namely evaporators, condensers, pumps and above all more efficient compressors. It is known that usually the compressor is the greatest source of electric consumption in the refrigeration systems. So, it should be our goal to choose not only the most efficient compressors, but also try to step the capacity of the refrigeration central using a correct number of compressors, and also if possible, using different capacities in the individual compressors that compose the central. This will increase the number of possible steps capacity, due to an optimised combination of the compression work. In some systems it is interesting also to use scroll compressors instead of alternative ones. This is more interesting when we can use capacity variators namely with variation in velocity of rotation. This variation of velocity of rotation it is possible also in some pumps.

Another important item that we can try to reduce to achieve energy savings in our installation is trying to use more efficient isolations. This is especially important if we work with very low temperatures namely in ultra-freezing systems and freezing tunnels. There are two items that we can consider the isolation of the cold stores and climatized rooms and the isolation of the piping. So, it is important to choose correct isolation materials, correct isolation thickness and correct construction and application of these isolations.

We can also try to use more efficient refrigeration fluids. Not only by optimising the type of refrigerant in use related with the evaporating and condensing pressure, working temperatures and refrigeration capacity, but also studying the possibility of using two fluids (one primary and one secondary) in our system. To eliminate refrigerant leaks is one of easiest ways of reducing energy consumption but it is also one of the most efficient. Some studies made in the sector of food and drinks [4] permit us to estimate a medium value between 1% and 15% of losses due to leaks, in refrigerant piping and others. A very important measure that can be made to reduce energy consumption is the implementation of heat recovery in the refrigeration systems. This is even more interesting when can be conjugated with other systems existing in the hypermarket. One example is the utilisation of heat recovery in refrigeration is the usage of hot gas leaving the compressor to pre-heating water to sanitary hot water usage. Other example is the utilisation of high pressure hot gas leaving compressor to defrost evaporators. Also the implementation of thermal accumulation in some times attractive namely one it is possible to produce cold energy at periods when compressors operation are more efficient. Nevertheless, this measure needs to be very well studied, because some time conduces to reduction in overall efficiency of the global installation.

Optimising installation layout is other very important measure that can help us achieving energy savings in our refrigeration system. In fact a correct layout design of our hypermarket, should consider the grouping together of the cold store refrigeration areas [5], the minimisation of paths that the refrigerated goods need to pass through, the minimisation of distances of piping, and the optimisation of the localisation of refrigeration centrals and condensers. The optimisation of working parameters is another very important tool to achieve energy savings. Two of the most important parameters that are possible to optimise are the condensing pressure and the aspirating pressure. In condensing pressure instead of keeping it in a fixed value (like the design value) it’s very efficient to vary it as a function both of the exterior temperature and the instant charge of the installation. That is possible by varying the velocity of rotation of the condenser ventilators or by varying the number of condenser ventilators in use. Also in the evaporating pressure instead of keeping it in a fixed value it is possible to vary it using electronic expansion valves associated to an efficient control system. So, a optimised set of working parameters will be the conjugation of both reducing to minimum the condensing pressure and maximise the aspirating pressure in  every moments of installation work. This will permit to achieve the minimum value of energy consumption in compressor due to compression work.

Another very important aspect when we talk about energy savings in hypermarkets, is the possibility of using velocity variators in mechanical equipment, namely compressors, pumps and ventilators. The basic principle is simple, when we have a variation in the solicitation charge of our mechanical equipment, we can vary is velocity of running to adapt the equipment to the demand, trying that the equipment work near his maximum efficiency. This is possible namely using frequency variators when electric motors are prepared to that. With this action we can also achieve more stable working temperatures and pressures, and lower levels of noise. A recent improvement in our goal of optimising the operation of refrigeration systems is the possibility of implement specific systems that monitories and control our installation. With these, we can define a group of working parameters that can be adapted to minimise energy consumption, namely working temperatures and pressures, defrosts, alarms, working hours and others. Finally, it is very important to keep our refrigeration system with efficient operation and maintenance during its lifetime.

5. conclusions

A set of energy saving strategies to refrigeration installations were presented and discussed. Some examples of vantages and disadvantages of some of these strategies were described. Particular importance was given to the usage of more efficient equipment, more efficient isolations, and more efficient refrigeration fluids. The importance of eliminate refrigerant leaks was also discussed. Other strategies like the implementation of heat recovery, thermal accumulation, and optimising installation layout were also refereed. Special importance was given to some operating aspects like the usage of correct capacity stages, the usage of optimised working conditions and velocity and capacity variations, and efficient installation monitorize and control systems.

REFERENCES

[1] Hart, M. (2001), “Guide to Sustainable Community Indicators”. Sustainable measures

[2] INETI, (2000), “Estudo sobre as condições de utilização de energia nos grandes espaços comerciais”. INETI

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

[4] CCE, (1999), “Indústria de alimentação e bebidas – estudos sectoriais”. Centro de Conservação de Energia

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

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

[7] 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