Published Date 12/10/17 08:00
Undoubtedly, the expansion of commercial refrigeration is likely to continue due to the growing importance of food preservation.
The food chain from “field to fork” must be able to guarantee food quality and safety, and above all for fresh products with a short shelf-life this can only be ensured by low temperature transport and storage, so as to prevent bacterial growth: indeed, the lack of an efficient cold chain causes food wastage that, according to the International Institute of Refrigeration (IIR), accounts for 20% of global production, and an average of 23% in emerging countries, with enormous social and economic consequences.
Furthermore, global urbanisation - totalling around 75 million people a year, mainly in developing countries - makes it necessary to improve the cold chain so as to ensure distribution from production to retail whilst minimising waste.
Lastly, it should be stressed that the increase in female employment in emerging countries will lead to a considerable rise in the consumption of frozen or ready-to-eat foods.
In addition to 1.5 billion home refrigerators, there are 90 million commercial refrigeration systems (including condensing units, stand-alone appliances and installations comprising compressor racks and showcases) and 47 millions of beverage coolers and refrigerated vending machines, all of which are growing significantly (with an average CAGR of over 5%).
The expansion of the cold chain will not necessarily exacerbate its environmental impact: technical evolutions will in fact tend to reduce the cold chain’s environmental impact.
The two main sources of environmental pollution responsible for the “greenhouse effect” are the introduction of carbon dioxide into the atmosphere due to the generation of electricity from fossil fuels (indirect emissions), and the release of refrigerants into the atmosphere due to leakage (direct emissions).
According to the UNFCCC (United Nations Framework Convention on Climate Change), indirect emissions currently account for 67% of greenhouse gas emissions due to commercial refrigeration, while the remaining 33% is attributable to refrigerant leaks.
In 2015, worldwide commercial refrigeration power consumption totalled 517 billion kWh, equal to around 2.5% of global consumption, with estimated emissions of approximately 390 million tonnes of CO2 annually.
Refrigerant leaks relating to the currently used gas blends total about 200 million tonnes of CO2-equivalent.
However, improving system efficiency and introducing new low-GWP refrigerants, as mandated by regulations either in force or about to be introduced in most countries, will significantly reduce the environmental impact of food storage, despite an increase in the number of systems.
Replacing traditional refrigerants with natural or low-GWP fluids has a significant impact on the greenhouse effect. As the pollution caused by leakages from a refrigeration system is the product of four factors:
a) refrigerant GWP;
b) total refrigerant charge;
c) the extent of leakages, expressed as a % of total content per year of operation;
d) number of years of operation.
For example, in a traditional supermarket, replacing the R404A (still the most commonly used refrigerant, with a GWP of 3922 kg CO2eq), with an almost equivalent HFO whose GWP is much lower (≈ 1300 kg CO2eq), brings an approximately 20% reduction in potential total environmental impact.
In the case of natural refrigerants (HC, CO2, ammonia), any leaks are irrelevant in terms of greenhouse effect, even though the overall assessment of the system’s cost-effectiveness also needs to take into account other factors, such as - depending on the chosen fluid - flammability, total cost, overall energy efficiency and toxicity.
Despite the right moves being made to abolish high-GWP refrigerants, it is still surprising to note that regulators and industry have not been very vigorous in reducing gas charge per unit cooling capacity, and that the requirements or recommendations intended to limit leakages in commercial refrigeration are still very high (statistically up to 25% of the total content per year).
It is undoubtedly possible to significantly reduce leaks from commercial refrigeration systems.
For instance, it is worth underlining that, according to IIASA researchers, overall leakage from the 600 million HVAC systems operating around the world, including chillers, is almost equivalent to the total from all commercial refrigeration installations, despite their power consumption being 7 times higher (3650 billion kWh/year) and cooling capacity 15 times higher!
Traditional commercial refrigeration systems are made by connecting the various units (showcases or cold rooms) using long and complex pipelines, developed manually on site on a case-by-case basis. These pipelines contain almost all of the refrigerant and represent the primary source of leakages, while on the other hand almost all HVAC units are packaged and therefore assembled, pre-charged and tested in the factory.
The use of self-contained showcases with built-in compressors can reduce total refrigerant charge by 80%, with the probability of annual leaks falling from 15-25% to just 1%-2%: for the same fluid used, this means a 98%-99% reduction in direct emissions; moreover, when leaks do occur, they are minor and only affect the unit in question, without any consequences on the others.
The heat of condensation can be rejected outside via a closed water loop made using low-pressure, low-cost piping.
Such system architecture has already been successfully used in stores specialising in the sale of frozen products, where independent stand-alone units are installed, albeit with low capacity.
It should be remembered, however, that together with reducing direct emissions, it is essential to pay significant attention to energy efficiency, which has a decisive influence on air pollution, as well as naturally on running costs.
The solutions to improve system energy efficiency are many. As well as carefully designing the various system components, innovative technologies can be adopted in terms of both hardware - such as, just to give a few examples, motorised expansion valves for superheat control and ejectors in transcritical CO2 systems - and software, including continuous floating condensing temperature and floating suction pressure control, the most appropriate sequence for starting the compressors, optimised defrost intervals, and so on.
One important innovation for energy savings is today offered by the use of inverter-controlled compressors with brushless motors, combining the intrinsically high efficiency of these motors with the possibility to modulate rotation speed (from 15-25% to 100%), depending on instant cooling demand.
With this technology, maximum efficiency is obtained by using self-contained showcases on which the compressor is automatically set to the optimum speed for each operating condition, independently of all the others: in the same conditions, power consumption is typically reduced by 20%-25%, with obvious environmental and economic benefits.
The characteristics of the system bring additional benefits: with independent, unit-level control, each unit can monitor real-time compressor consumption, energy efficiency, instant cooling capacity, temperature and pressure and, using these data, signal any existing and even expected problems (including low refrigerant charge) based on predictive algorithms.