A phase change material (PCM) based passively cooled container for integrated road-rail cold chain transportation – An experimental study

Highlights

• An experimental study on a PCM-based passively cooled container for cold chain.

• The internal temperature can be maintained 2~8 °C uniformity for 94.6 h.

• The system COP was 1.84, with an energy-saving by 86.7% than diesel-powered reefer.

• The cost and emission reduced by 91.6% and 78.5% than diesel ones, respectively.

Abstract

This paper reports a phase change material (PCM) based passively cooled container for integrated rail-road cold chain. It was equipped with cold energy storage plates containing the PCM. A separate charging facility was built to charge the plates. Four kinds of fresh vegetables and fruits were used for integrated rail-road transportation. The charging time and efficiency, the internal temperature and relative humidity of the container during delivery, as well as the coefficient of performance (COP) of the system, were obtained. The results were compared with a diesel-powered reefer in terms of energy consumption, operation costs and environmental impact. The quality of products before and after the cold transportation was also compared. The results showed that the new container had a discharging time of up to 94.6 h and the system COP could be as high as 1.84. The results also demonstrated that the energy consumption, the operational cost, and the emission were reduced by 86.7%, 91.6%, and 78.5%, respectively.

Introduction

Over 40% of all foods require refrigeration and cold chain transportation [1], which accounts for a considerable portion of world energy consumption [2]. Energy consumption and associated carbon emission reduction in the cold chain has therefore been regarded as a global challenge [3]. The current cold chain transportation relies mainly on a diesel engine-driven vapour compression system, a major source of carbon and particulate matter emissions [4], [5]. The fuel consumption of these engines varies between 1 and 5 L per hour depending on the size of the unit but only 20–25% is converted into work due to the low efficiency of small diesel engines for conventional vapour compression transportation refrigeration units [6]. Additionally, refrigerant leakage has also been a source of concern [7]. Apart from the above challenges, fast-changing ambient and limited size and capacity make the road transportation refrigeration units operate a relatively low efficiency, which has to be operated reliably in a much harsher environment than usual stationary refrigeration equipment [8]. These challenges have led the research communities to seek alternatives to vapour compression refrigeration units for cold chain transportation [9]. This has further been strengthened by new regulations for reducing the use of diesel generators and fuel consumption [10].

This work concerns the use of phase change material (PCM) based latent heat storage for integrated rail-road cold chain transportation. This offers an attractive solution to address some of the major challenges outlined above. Such an approach takes the advantages of the high latent heat and almost constant temperature of PCMs during phase transition[11], [12] and has therefore been explored for reducing energy consumption and the environmental impact of refrigeration systems [13], [14], energy efficiency improvement in domestic refrigerators[15] and coolers [16], [17], refrigerated trucks [18], industrial refrigeration plants [19] as well as the off-peak refrigeration system [20]. A brief review is given in the following in two categories of conventional refrigeration systems and cold chain transportation.

• Conventional refrigeration systems Fioretti et al. studied the thermal performance of a reefer container with an external PCM (Rubitherm RT35 HC paraffin wax) layer inside the walls [21] and observed a 1–2 °C decrease in the internal wall surface temperature and a reduced peak heat transfer rate of 8.57%, compared with the conventional container. Khan et al. examined the effect of the use of a Polyethylene glycol-400 PCM on the temperature fluctuation of a household refrigerator during door opening [22]. A lower temperature fluctuation by about 2 °C was observed inside the refrigerator cabin, whereas the air temperature rise in the cabin was about 3–5 °C lower. In case of a power failure, the presence of the PCM was able to maintain the low temperature for up to 2 h. Ben-Abdallah et al. experimentally investigated thermal management of an open display cabinet with PCM [23]. Their results showed that the PCMs could maintain the air and product temperature for up to 2 h when the compressor was off. Yan et al. modified a vapour compression refrigeration system of a freezer using a PCM [24] and found that the modified system had an improved COP by up to 10.5%, with the volumetric cooling capacity enhanced by 25.4%.

• Cold chain transportation Liu et al. experimentally tested a refrigeration system containing a PCM with a melting point of −26.8 °C in a lab-scale environmental test chamber to simulate a truck compartment [24]. The cold storage unit was proposed to be located outside the compartment and charged by a stationary refrigeration unit through a circulating heat transfer fluid. Their results suggested that the energy cost for the PCM-based system was up to 86.4% lower than the conventional system. Additionally, the PCM-based system generates much fewer emissions and has improved temperature control and reduced noise. Ahmed et al. incorporated a PCM into the walls of a refrigerated truck trailer simulator, aimed to reduce the heat flux [18]. They found an average reduction in peak heat transfer rate of 29.1 and up to a reduction of 16.3% in the overall average daily heat flow into the refrigerated compartment.

The above summary demonstrates the benefits of PCM integration. The integration could also offer other benefits including quality enhancement of refrigeration [25] and renewable energy utilisation [26], [27]. Besides, thermal energy storage can integrate well with large-scale systems, such as building ventilation[28] and air conditioning[29].

These studies, however, have not studied the dynamic performance of PCM based transport refrigeration systems. The studied refrigeration systems could not be operated without a power supply. The limited charging rate in these previous studies also presents a key obstacle for real-world applications [23], [30]. We report here a study on a PCM-based TES container without a power supply nor a refrigeration unit. The container was a 40ft ISO (International Organization for Standardization) standard container. Ten TES plates containing a total of 1260 kg PCM were installed inside the container. A mobile off-vehicle charging facility was established, which could charge the container through a fast-connection charging loop. The container was designed for integrated road and rail cold chain transport applications, carrying real items for a long duration and a long distance. The charging performance (charging rate and efficiency) and the discharging performance (cooling duration and internal humidity under the dynamic conditions) were studied. The system COP based on long-distance transportation was evaluated. A comparison between the new PCM based container with conventional diesel-powered refrigeration conditioner was compared in terms of energy consumption, cost and emission using real data. Four typical fresh vegetables and fruits were used to evaluate the refrigeration performance. To the best of our knowledge, this represents the first of such a study on a real scale PCM-based passively cooled contain for integrated road-rail cold chain transportation applications.