Moving toward sustainability in refrigeration applications for refrigerated warehouses

My HVAC&R Research editorial deals with refrigeration technology applied to refrigerated warehouses (or cold stores, as we say in Europe). I learned as a student that this topic is one of the oldest and most reliable fields of application of refrigeration technology. Still I am writing this editorial to address new research topics on this kind of plant. My motivation is twofold: social and energetic, that, to some extent, merge together within the framework of sustainability.

Social impact

We are around 7 billion world inhabitants and the UN projections are for some 9 or 10 billion by 2050 or so. It is estimated that 85% of this population growth will be in developing countries. According to the Food and Agriculture Organization of the United Nations (FAO; 2013), in 2012 one out of eight inhabitants lived in a chronic, undernourishment state and, by far, the largest percentage lived in developing countries. The International Institute of Refrigeration (IIR; 2009) claimed that “greater use of refrigeration technology would ensure better worldwide nutrition in terms of both quantity and quality.” IIR estimates that 25% of worldwide food yearly production is lost during the postharvesting chain to the final consumer. In developed countries, the percentage is some 10% and rises to 28% in developing countries.

Some 20% of total food losses is due to the lack of refrigeration: the percentage falls to 9% in developed countries and rises to 23% in developing countries. There is an estimation that if developing countries could reach the same level of refrigeration penetration per inhabitant of the developed countries, there would be the possibility of saving about 200 million tonnes of fresh food that represent some 14% of the current consumption in developing countries (IIR 2009). The following picture can reinforce the previous estimations and highlight the need for a remarkable increase of cold store volumes available in the future: developed countries have an average cold store volume of 200 m³ per 1000 inhabitants (300 m³ in the United States), whereas developing countries have an average of 19 m³ per 1000 inhabitants. Furthermore, it is foreseen that already agriculturally vulnerable regions will suffer earlier and more than the temperate areas because of global warming, and so there will be further need of new and larger cold stores for the preservation of imported perishable foodstuff.

I have to admit that the need for refrigerated warehouses is only the first step to the establishment of a reliable cold chain in some developing countries. I believe this is also a moral duty for all of us, especially when I think to the tragedy of more than three million deaths per year because of hunger.

Energetic impact

This social motivation has an impact mainly on developed economies, but it also represents an option for creating sustainable increases of available cold stores volume in emerging countries.

The total energy consumption of refrigeration in food industry is about 1300 TWh (Guilpart 2008; i.e., about the 8% of the worldwide electric energy consumption). In terms of carbon footprint, it equates to about 2.5% of the world emissions on equivalent CO₂.

There is a limited amount of research available about cold stores energy consumption. Recently, I was involved in a European Commission funded project (through the Executive Agency for Competitiveness and Innovation), Improving Cold Storage Equipment in Europe (ICE-E; www.ice-e.eu), together with seven institutions and partners. Data from 329 different cold stores (chilled, frozen, mixed, small volume, large plants, etc.) showed specific energy consumption (SEC) per unit of cold store volume between 4 to 250 kWh/m³/year. Other studies in the United States displayed numbers between 15 and 132 kWh/m³/year (Elleson and Freund 2004; Singh 2008). SEC values up to 379 kWh/m³/year were found in New Zealand (Werner et al. 2006). The large scattering of SEC values is due to very different cold stores typologies and variable operating and environmental conditions (including set-point temperatures, types of refrigeration plant, system management and maintenance, type of food, and related throughput).

Based on the previous figures and considering that as a rough estimate, the cold stores volume installed in the world is about 300 million m³, the reader may speculate about the yearly worldwide energy consumption for cold stores operation. In refrigerated warehouses about 60% to 70% of this energy is consumed by refrigeration.

Through the energy audits of 38 cold stores, ICE-E found possible energy savings up to 72% (the worse one, fortunately!). Very often, energy savings issues were easy to identify and solve (e.g., air infiltration because of damaged doors or strip curtains). In most cases, the major issues were linked to the lack of controls of the refrigerating plant.

I have to admit that ICE-E has been the first large project on cold stores I have been involved in. I previously used to operate in that part of refrigeration technology applied to air conditioning (positive temperature delivery of the cooling medium) or retail and supermarkets. I was surprised to see only a limited number of recent scientific papers devoted to some relevant aspects of refrigeration plants for cold stores, in comparison with the impressive number of works published about air conditioning and supermarket applications in the last decade. For example, there are dozens of papers introducing, analyzing, and testing robust control algorithms for water chillers and air conditioning units. I have serious difficulties in finding similar comprehensive studies even for small equipment operating with HydroFluoroCarbons (HFC) refrigerants for a single cold room, not to speak of large ammonia plants. Besides, we can find rather complex and detailed applications of cold storage strategies (including phase change materials) for building air conditioning but almost no evidence is given to similar strategies in cold stores (where, by the way, the chilled and, especially, the frozen foodstuff can operate itself as a cold storage for a limited time).

Photovoltaic panels have recently started to be installed on the rooftop of cold stores in Italy, thanks to governmental grants. This is in my opinion a smart option: it is possible to shade a remarkable part of the solar heat load on the cold store roof and to transform it in energy for the refrigeration plant. This idea would be strategic in the warmest countries. Still there is plenty of work to be done for making efficient the synergies between solar load and cooling demand and I cannot find any comprehensive and exhaustive study on how to manage the off-peak loads and how to store the “cold” for nighttime operations.

The ICE-E team found that a significant percentage of the “small” stores in Europe still operate with R22. The use of this HydroFluoroChloroCarbons (HCFC) molecule will not be anymore possible starting from 2015. I cannot find a reliable and consistent number of published case studies to address the crucial choice for cold stores operators and owners: to replace R22 with a transition mixture or to install a new plant. In both cases, which is the best refrigerant to adopt?

In the last decade, detailed CFD studies on air distribution in several types of confined spaces have been proposed, but rather poor emphasis has been put on cold stores where the distribution of chilled air from the evaporator or the brine coil has a direct impact on food preservation quality, frost formation, defrosting cycles and the related energy consumption.

The above mentioned points represent only some sparks that I'm proposing according to my personal experience. I believe it is time to broaden the competencies and studies that have so far been developed for air-conditioning and supermarket refrigeration applications to display new efficiency enhancement opportunities and to increase the sustainability of the “old” and still reliable technology adopted in refrigerated warehouses. The outcome will be appreciable on a worldwide scenario from the social, environmental, and economic points of view.