Refrigeration in transition

Toward eco-friendly technologies for the cold chain

Much of the information of this article was sourced, with thanks, from https://www.newfoodmagazine.com/

Production, transportation, sales, and household consumption is not easy to achieve without refrigeration in the supply chain. Chilling and freezing techniques are commonly used in the food industry to cool products to temperatures below 0°C and between -18°C and -35°C respectively, helping slow down the natural processes that cause food to spoil. 

However, these methods also contribute significantly to environmental issues through direct and indirect emissions of greenhouse gases. To address this problem, extensive developments are continuously ongoing to improve existing refrigeration systems and create new technologies to lessen the impact on our environment.

A brief history. The dominant use of the vapour compression refrigeration cycle can be traced back to the 1930’s when chlorofluorocarbon (CFC) and hydrofluorocarbon (HCFC) refrigerants were first introduced. As environmental concerns grew in the 1980’s, it became apparent that CFC emissions were harming the ozone layer. This led to international agreements banning their use and phasing out HCFCs over time. While new refrigerants such as HFCs have been developed with zero Ozone Depletion Potential (ODP), they have high Global Warming Potential (GWP) and contribute significantly to greenhouse gas emissions through leakage and energy consumption. There is now renewed interest in thermally driven technologies for cooling, which offer potential economic and environmental advantages over traditional methods.

Sorption refrigeration

Sorption refrigeration is a heat-driven refrigeration process that uses a sorbent to absorb and release a refrigerant, producing a cooling effect. Specifically an adsorption system, it is a method of cooling that utilises the process of sorption to achieve needed temperatures. Sorption refrigeration technologies, such as absorption and adsorption, operate using a thermal compressor and a sorbent. This sorbent, which can be either solid or liquid depending on the system, is heated to release refrigerant vapour at the condenser pressure. The vapour then goes through the condenser to liquefy before passing through an expansion valve into the evaporator.

As the sorbent cools down, it reabsorbs vapour, maintaining low pressure in the evaporator. Then the liquefied refrigerant absorbs heat from the cooled space and turns into vapour again, creating a cooling effect. In contrast to absorption and vapour compression methods, adsorption refrigeration involves a cyclical process that requires multiple adsorbent beds to maintain continuous capacity.

Certain systems have capacities ranging from 70kW to 1300kW and can be powered by low grade heat of 50-90°C, achieving COPs of approximately 0.71. Prototypes have already been developed or are in the works for refrigeration at temperatures as low as -25°C. In terms of applications in the food industry, the primary focus is moving on to using waste heat to power adsorption systems in food factories and transportation refrigeration. Additionally, there is potential for use in tri-generation with combined heat and power systems to provide refrigeration capabilities.

Ejector refrigeration

Ejector or jet pump refrigeration is a long-standing thermally driven technology commonly used for cooling purposes. The advantage lies in its ability to generate refrigeration using waste, heat or solar energy at temperatures above 80°C. These systems come in a range of sizes, from a few kilowatts to 60 MW. However, despite significant development efforts, the COP (refrigeration effect to boiler heat input ratio) remains relatively low at less than 0.2. In the food industry, this technology can be best utilised in areas where waste heat is readily available to power the ejector system. Examples of such applications can be found in food processing factories, where it can effectively cool products and processes and provide transport refrigeration.

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