Nanoparticles and nanofluids, air cycle refrigeration and magnetocaloric refrigeration were three of the technologies studied as part of the four-year EU-funded FRISBEE (Food Refrigeration Innovations for Safety, consumer Benefit, Environmental impact and Energy optimisation along cold chain in Europe) project.
The alternative technologies work was just a part of the broad range of research carried out by the Frisbee team. Ultimately, the project developed a comprehensive database of the cold chain in Europe, identified refrigeration needs and available current technologies in the food industry, and investigated consumer needs and expectations in the respect to the food cold chain.
Nanoparticles in the form of nanofluids is one method being researched to enhance heat transfer. Within Frisbee work was carried out to identify existing studies on nanofluids, to examine the properties of the fluid, especially in terms of heat transfer and to simulate and validate the influence of the nanoparticles on the overall energy efficiency of refrigeration plant.
While heat transfer coefficients were found to significantly increase with the increase of nanoparticle concentrations, the pressure drop, which is directly related to the pumping power, also increased at the same time. Also, calculations revealed that the nanofluids Al2O3, TiO2, SiO2 were clearly less efficient than others, like Co, Fe and CuO.
The work is said to have confirmed the potential of nanofluids to intensify heat transfer when used in heat exchangers and improve refrigeration efficiency when used in refrigeration plant.
However, the project identified that many questions remain regarding the safety of nanoparticles. While the FRISBEE project provides recommendations for the use of nanofluids in refrigeration plants, it also insisted that it was crucial for effective rules to be quickly put in place to regulate nanotechnology. Those concerns centre around a growing debate as to the potential for nanoparticles to penetrate the skin and attack sensitive lung tissues if inhaled.
The project also found that air cycle refrigeration has long term possibilities, comparing favourably to existing low temperature applications. This is said to be particularly the case where there is an associated need for heating which can be served by the relatively high-temperature heat rejected from the air cycle system.
Within Frisbee, existing mathematical models of air cycle combined heating and cooling systems for cooling at chilled, frozen and very low temperatures were developed. This enabled the potential of optimised and matched components rather than existing non-ideal components to be investigated.
The creation of a mathematical model allowed comparison of temperature performance, energy consumption and environmental impact of air cycle systems with vapour compression refrigeration and liquid nitrogen freezing.
Air cycle refrigeration was found to be uncompetitive on grounds of both carbon dioxide emissions and operating costs in comparison to vapour compression technology in chilled or frozen storage applications. However, for applications in the low (-80˚C) and very low (-120˚C) temperature ranges, which are currently served by cascade refrigeration systems and total loss refrigerants, air cycle is competitive – particularly where use can be made of the relatively high temperature heat produced by air compression.
Much has been written about magnetic refrigeration which exploits the magnetocaloric effect – the temperature change observed when certain materials are exposed to a rapidly changing magnetic field.
The real challenge in magnetic refrigeration has been to increase the temperature span of the refrigeration cycle. The typical change in temperature of the magnetocaloric material is between 2ºC-4ºC, inadequate for any practical room temperature application.
A key innovation within Frisbee has been the creation of a regenerative cooling cycle, which extends the span of a magnetic refrigerator. By using an innovating technique, the span of the basic (and benchmark) refrigerant and the gadolinium alloys can be extended from the typical 2.5ºC in a 1T magnetic field to over 20ºC.
Optimisation performed on the system allowed a reduction in the mass of refrigerant, and a consequent reduction in the required magnetic field volume and consequently lower cost (field and refrigerant being the two primary cost elements in the technology). High-frequency operation was found to be able to be achieved by careful regenerator design, using advanced geometries rather than a powdered refrigerant, to allow the rapid flow of exchange fluid and heat. The magnetic refrigeration work within Frisbee lead to the construction of a high-efficiency magnetic cooling engine and quantified how efficiently the technology delivers cooling when embedded inside a domestic refrigerator.
A summary of the final Frisbee report can be read and downloaded here.