Refrigerated trucks, also known as reefers, are essential for transporting perishable goods. They ensure food, pharmaceuticals, and other temperature-sensitive items remain fresh and safe during transit. Different refrigerated trucks and containers are designed for specific needs and applications. Let&#;s explore the various types of refrigerated trucks and their unique features.
Click here to get more.
Types of Refrigerated Trucks
Insulated vans are a popular choice for transporting perishable goods over short distances. These vehicles are designed with unique insulation materials to maintain a consistent temperature inside the cargo area. Insulated vans are ideal for businesses that require frequent local deliveries of products like dairy, meat, and baked goods. They offer the advantage of being easy to work in urban environments and can be customized to meet specific temperature requirements.
Refrigerated containers, also known as &#;reefer containers &#;, are versatile units that can be used on trucks, ships, and trains. They have refrigeration systems that allow them to control precise temperatures. Refrigerated containers are commonly used for international shipping and long-distance transportation. They come in various sizes, making them suitable for transporting a wide range of products, from fresh produce to pharmaceuticals. Their robust design ensures they can withstand the severities of long journeys.
Refrigerated trucks are larger vehicles equipped with advanced refrigeration systems to transport goods long distances. These trucks are designed to maximize carrying capacity while maintaining strict temperature control. Refrigerated trucks are commonly used for intercity and interstate transportation of perishable goods. They offer superior insulation, robust design, lightweight boxes, and better fuel efficiency. These trucks are essential for industries that rely on a consistent cold chain, such as food and beverage, pharmaceuticals, and agriculture.
Special-purpose vehicle bodies are customized solutions designed to meet specific transportation needs. These vehicles can be equipped with refrigeration systems for unique applications, such as mobile medical units, blood banks, and emergency response vehicles. For instance, a mobile medical unit may require a refrigerated vehicle body to transport vaccines or organs. At the same time, a blood bank may need a refrigerated vehicle body to transport blood products. They are tailored to provide optimal temperature control and storage solutions for sensitive goods. These vehicles are essential for organizations that require specialized transportation options to ensure the safety and integrity of their products.
Advantages of Using Refrigerated Trucks and Containers
Using refrigerated trucks and containers offers several advantages for businesses with perishable goods. Here are some key benefits:
Refrigerated trucks and containers, with their ability to maintain a consistent temperature, provide a reliable solution for businesses. This ensures that goods, especially those sensitive to temperature fluctuations like food, pharmaceuticals, and chemicals, remain fresh and safe during transit, instilling a sense of security in transportation.
Refrigerated containers, with their ability to be used across various modes of transport, including trucks, ships, and trains, offer businesses a flexible transportation solution. This empowers enterprises to choose the most efficient and cost-effective transportation method, giving them a sense of control over their logistics.
Many refrigerated trucks and containers can be customized to meet specific temperature requirements and cargo sizes. This flexibility allows businesses to optimize their transportation solutions based on the nature of their products.
Modern refrigerated trucks are designed to be fuel-efficient, reducing businesses&#; operational costs. Improved fuel efficiency also contributes to a lower carbon footprint, making these vehicles a more sustainable choice for transportation.
Refrigerated trucks and containers are built to withstand the severities of long-distance transportation. Their robust design and superior insulation ensure that they can protect perishable goods from external temperature changes and physical damage.
Refrigerated trucks and containers, the backbone of transporting perishable goods, require careful consideration. Understanding the different types of refrigerated vehicles and their unique features empowers businesses to choose the best solutions for their needs. Whether it&#;s an insulated van for local deliveries or a refrigerated container for international shipping, these vehicles ensure that products arrive at their destination in optimal condition. Investing in the right refrigerated transportation solutions is not just a choice but a responsibility for maintaining the quality and safety of temperature-sensitive goods.
The vapour compression refrigeration (VCR) cycle dominates refrigeration systems used in refrigerated transport. This consists of mainly four components: evaporator, compressor, condenser and expansion device. There are mainly four types of compressor drive methods: auxiliary diesel unit, direct belt drive, auxiliary alternator unit and vehicle alternator unit (Maiorino et al., ). Most medium to large refrigerated vehicles have employed auxiliary diesel engine driven systems, a schematic diagram of which is presented in Figure 1. The other three options are commonly found in small trucks and vans.
In , the existing stock of truck refrigeration units in Australia was 21,150 (Mark Ellis & Associates Pty Ltd, ) and this number grew rapidly to 45,100 in (Brodribb & McCann, ). A comparison of the number of refrigerated vehicles in different categories are presented in Table 1. These truck refrigeration systems are estimated to consume 70-90 million litres of diesel per year, accounting for 5% of the total diesel consumption in Australia (Mark Ellis & Associates Pty Ltd, ).
Cryogenic refrigeration uses liquid nitrogen (LN2, 3 bar) or liquid carbon dioxide (LCO2, 8.6 bar) to maintain the space at the desired temperature. Commercial products are available on the market. There are three types of cryogenic systems: (1) direct injection of cryogenic fluid into the refrigerated space; (2) expanding cryogenic fluid in a heat exchanger and dispersing it into the atmosphere and (3) hybrid cryogenic refrigeration with a VCR system to allow for a quick pull down. Compared to VCR systems, cryogenic systems have the benefit of rapid pull-down of temperature, very low noise during operation, low energy consumption and reduced environmental impact. However, due to the relatively high cost of the cryogenic fluid, the market penetration of cryogenic refrigeration only accounts for 1% (Swahn, ) (based on statistics from ) In Europe, 0.5% of the sold truck and trailer refrigeration units are cryogenic refrigeration based on a personal communication with Thermo King in , reported in Air Resources Board, .
Figure 2 illustrates a liquid CO2 ST-CR 300 transport refrigeration system developed by a major refrigerated transport supplier &#; Thermo King. Even though the CO2-e emissions using the LN2 and LCO2 system is low compared to that of the VCR system during operation, the total emissions are equal in both systems if the emissions for producing diesel fuel, refrigerant and cryogenic fluid are considered
Another type of commercially available cryogenic system uses liquefied natural gas (LNG) for both refrigeration and as the fuel source for the vehicle. LNG is contained in a tank at a temperature between -140°C and -120°C and a pressure ranging from 5 to 8 bar. Prior to use, LNG transitions from liquid to gaseous state, absorbing about 900 kJ/kg of energy, which can be used for air conditioning or refrigeration in vehicles (Maiorino et al., ).
Latent heat thermal energy storage employing phase change materials (PCMs), commonly known as eutectic refrigeration, has been used in combination with a VCR system. The eutectic solution can store and release thermal energy at a defined temperature, which minimises the temperature fluctuation and offers better temperature management. Both the eutectic refrigeration and VCR systems are on the vehicle. The eutectic solution is usually contained in flat, thin plates, which are located under the roof and on the front (as shown in Figure 3) to minimise the space requirements. The type of eutectic solution and the number of plates is determined by the desired temperature of the space and the refrigeration load, respectively. This system is charged using an electric refrigeration unit when the vehicle is not in service. During charging, the eutectic solution is frozen to store the energy. The stored cold energy is withdrawn by natural convection and radiation during transportation when cooling is needed.
There are a number of companies that supply the eutectic refrigerated bodies, for example, Eutectic (www.eutecticcar.com), Thermaxx (thermaxx.com.au), Carlsen Baltic (www.carlsenbaltic.com), Therma Truck (www.therma-truck.com.au) and Framec Truck (www.framectruck.com).
Eutectic refrigeration has the following advantages compared to a diesel-powered system:
&#; Reduction in diesel consumption and associated CO 2 - e emission s This is because an electric-driven refrigeration unit is used to freeze the eutectic solution during charging, eliminating diesel fuel required for refrigeration.
&#; Reduction in energy cost
Up to 86% of energy cost savings can be achieved by charging the eutectic system with off-peak grid electricity (Liu et al., ). Further reductions in cost and CO2-e emissions can be achieved if the eutectic refrigeration system is charged using renewable electricity.
&#; Low maintenance
Unlike a diesel-powered system, eutectic refrigeration is independent from the chassis and has fewer moving parts. Therefore, only very basic maintenance is required such as cleaning, lubricating and taking care of the refrigerant level.
2.2.1
Refrigerants with low global warming potential (GWP) have been used in refrigerated transport to phase out hydrofluorocarbon (HFC) refrigerants. These include:
&#; R452A
R452A (GWP of 2,139) is a non-flammable hydrofluoroolefin/hydrofluorocarbon (HFO/HFC) blend. It is now the refrigerant of choice in refrigerated transport to replace R404A (GWP of 3,922) and can help to reduce the carbon footprint of refrigerants by nearly 50%. Material testing results showed that R452A has similar material compatibility and chemical stability to R404A, indicating products utilising R452A have similar reliability to R404A products (Kujak & Schultz, ). Currently, trailer/intermodal and diesel driven single temperature trucks are rapidly transitioning from R404A to R452A with improved designs and reducing refrigerants by around 25% (Brodribb et al., ). The performance of a refrigeration system charged with R452A has demonstrated equivalent capacity and efficiency to a system charged with R404A (Kujak & Schultz, )
&#; Natural refrigerants
Recently, natural refrigerants R290 (propane, GWP of 3) and R744 (carbon dioxide, CO2, GWP of 1) have gained considerable attention because of their low environmental impact (Maiorino et al., ). Refrigeration systems using R290 have a higher COP of about 15-25% and 10-30% compared to R404A in medium and low temperature applications, respectively (Colbourne et al., ). This aligns with the result from an experimental study where a R290 refrigeration system had a 28.6% higher COP than R404A (Ramaube & Huan, ). Compared to R290, R744 has the advantage that it is non-flammable. The transcritical CO2 refrigeration cycle, which has been utilised in stationary applications, has been introduced by Carrier Transicold for refrigerated containers.
Technologies used or being developed by refrigerated transport suppliers are summarised in Table 2
&#; Battery Powered Refrigerated Vehicles
Thermo King, TKT HVAC Solution and Zanotti have already supplied a battery powered refrigeration unit on board an electric vehicle. This type of system has the benefit of being lightweight, low maintenance and low noise emissions It is very suitable for urban area delivery. However, this battery powered system has only been designed for small trucks and vans requiring low cooling capacity.
&#; Carrier ECO-DRIVE Power Module
Carrier Transicold introduced a new generation of engineless transport refrigeration technology. The refrigeration unit is powered by the ECO-DRIVE power module and the operation process is shown in Figure 4. The hydraulic pump, connecting to the truck&#;s power take-off motor, delivers electrical power to the refrigeration unit.
Supplier Technology (status) Model Type of vehicle Cooling capacity 1 /weight
Thermo King www.thermoki ng.com
Full-electric refrigeration unit (Available on market) B-100, powered by vehicle&#;s battery small trucks and delivery vans in fresh applications
Full-electric refrigeration unit for both electric and engine-powered vehicles (Available on market)
E-200, powered by vehicle&#;s alternator medium-size vans and trucks in fresh and frozen applications
Source
1 kW (fresh) https://www.thermoking.com/na /en/road/trucks-and-vans/directdrive-units.html
1.377 kW (fresh)
0.791 kW (frozen)
Carrier Transicold www.carrier.c om
CO2 refrigeration (Prototype testing under real conditions for 3 years)
All-electric refrigeration unit with ECO-DRIVE power module (Available on market)
All-electric refrigerated trailer system by using AddVolt&#;s technology (Available on market)
Vector HE19 Vector E (ECO-DRIVE25)
ICELAND 11
Iceland 18
Twincool
Semi-trailer https://www.carrier.com/carrier/ en/worldwide/news/newsarticle/carrier_transicold_natur al_refrigerant_trailer_mobiliteit srai_show_simon_loos.html
Truck, trailer, drawbar
Vector® eCool&#; Trailer
9.8 kW (-20°C)
17.65 kW (0°C) 800 kg including battery https://www.carrier.com/trucktrailer/en/uk/products/eu-trucktrailer/trailer/vector he-19/
Bosch & Carrier Transicold
Mitsubishi www.mhimth.co.jp
FresH2 project, the hydrogen fuel cell refrigerated transport (Field testing in September in France)
Semi-trailers 30 kW fuel cell H2 storage system: 350 bar & 10 kg;
TE 30 TE 30 Multi-Temp Truck https://www.carrier.com/trucktrailer/en/uk/products/eu-trucktrailer/trailer/vector-ecool/ https://www.carrier.com/trucktrailer/en/eu/news/newsarticle/fresh2-hydrogen-fuelcell-refrigerated-transportproject enters-road-testingphase.html kW (-20°C) 3.019 kW (0°C) 1.229 kW (-20°C) & 2.925 kW (0°C)
TKT HVAC solution www.tkthvac.c om
Battery powered electric van refrigeration unit (Available on market)
TKT-200E
TKT-300E Van -5°C
Z120 Van
0.55 kW (-20°C) https://www.tkthvac.com/produ cts/van-refrigerationunits/electric-cargo-vanrefrigeration-units/55.html
2.Blank cell indicates that the information is not provided in the source.
In October , Carrier Transicold launched the all-electric engineless refrigerated trailer system. Utilising AddVolt&#;s engine-agnostic technology, Vector® eCool&#; converts kinetic energy generated by the trailer into electricity, which is then stored in a battery pack to power the refrigeration unit, as illustrated in Figure 5. The battery pack has a total capacity of 19.2 kWh, weighs 325 kg and is capable of providing a maximum continuous discharge power of 19 kW. A full battery charge via the grid takes 2h.
The FresH2® fuel cell project initiated by Bosch and Carrier Transicold, in collaboration with refrigerated vehicle builder Lamberet and temperature-controlled food transport specialist STEF, entered its road testing phase in France in early September (Carrier Transicold, b, c)
Mitsubishi Heavy Industries Thermal Systems Ltd has launched an all-electric system. This system is driven by an engine powered 24V alternator during transportation of the produce. When the vehicle engine is turned off, the management system will automatically switch power source to the backup battery (24V). The battery can be charged by plugging in to a 230V power supply, also while the vehicle engine is running. The key components of this refrigeration unit are presented in Figure 7
Temperature is the key factor that affects the shelf life of perishable foods. The proper control and management of temperature from farm through to customer is critical to ensure the food is in good quality and safe to eat. It will also contribute to a reduction in waste and energy consumption. An Australian Alliance for Energy Productivity (A2EP) report revealed that 10% of energy, costing $15 million, used in truck refrigeration can be saved by improving food condition monitoring in
Australia (A2EP, a). However, there is only limited monitoring implemented in the Australian food cold chain.
Radio frequency identification (RFID) has been demonstrated in case studies in food cold chain management (Estrada-Flores & Tanner, ). An RFID system usually comprises a sensor, a tag and a reader to gather information from the tag wirelessly. Walmart has investigated the use of RFID and digital temperature recorders in its cold supply chain inventory tracking system (A2EP, b). It allows Walmart to monitor and manage the effects of temperature conditions on perishable produce to decrease food spoilage and respond faster to equipment failure.
Remote real-time temperature monitoring already exists and has been used for high value and highly temperature sensitive products. With the falling cost of temperature monitoring and reporting technologies, real-time monitoring of food temperature is expected to become an industry standard within five years (A2EP, a). As illustrated in Figure 8, a system to enable remote real-time temperature monitoring consists of the following elements:
&#; Sensors
Low-cost sensors are commercially available to monitor temperature, humidity, and light (which indicates when doors are open or exposed to radiant heat). Pilots have indicated that at least 12 sensors per truck (ideally at least 1 sensor per pallet) are needed due to the large temperature variation in the space. It is suggested that placing much larger numbers of sensors for research/pilot purposes will help to determine the optimum numbers and locations of sensors required for alternative circumstances (A2EP, a).
&#;
Transmitters convert the measured temperature/humidity/light from the sensor into the signal and send it to the monitor/control device. They are usually packaged with a battery, sensors, and an aerial in a unit the size of a matchbox. The bulk cost is $10-15 each and the price is expected to further reduce.
IoT devices communicate over the network connection. The main constraint on ubiquitous low cost monitoring in Australia is the extent of low power wide area networks (LPWAN), but it is expected to be resolved within two years (A2EP, a).
Wireless controllers allow remote monitoring of temperature and performance of the refrigeration unit in real time from devices such as smart phones or tablets via an app and hence remote managing the refrigeration unit by changing the set temperature.
To increase payloads and reduce fuel consumption, it is necessary to use lightweight material for construction of the vehicle body. Truck bodies made of fibreglass, which is a strong and lightweight material, results in 20 to 30% less heat losses compared to truck bodies made of steel.
In Australia, currently there is no regulated insulation specification for walk-in cold rooms, refrigerated vehicles or cold storage facilities (Brodribb & McCann, ). The maximum width of refrigerated trucks in Australia and New Zealand is limited to 2.5m, compared to 2.6m in Europe and North America. Therefore, the insulation used by Australia and New Zealand refrigerated transport manufacturers has to be thinner. To achieve the same R value, better insulation material with a lower value of thermal conductivity is needed. Table 3 lists the thermal conductivities of several insulation material for cold storage (ASHRAE, ) .
Note:
In , the Australian Trucking Association has submitted recommendations to the Australian Government to increase the width of truck and trailer to 2.6 m to be consistent with international standards in Europe and North America (Australian Trucking Association, ) The thermal efficiency improvement of increasing wall insulation from the existing 2.5 m refrigerated vehicle (40mm insulation) to 2.55 m (65 mm insulation) and 2.6 m (90 mm insulation) is presented in Figure 9. The estimation is based on a 30°C difference between the internal and the outside temperature.
Vacuum insulation panels (VIP), with an excellent thermal insulation property (thermal conductivity: 0.002 &#; 0.008 W/(m K)), have been used as insulation for temperature-controlled packaging, refrigerators, freezers, refrigerated transport and insulated shipping containers. In conventional VIPs, a core material with high porosity and compressive strength is evacuated and sealed inside an envelope. Various materials commonly used as core materials include powders (e.g. fumed or pyrogenic silica, silica aerogel and expanded perlite), polyurethane and expanded polystyrene foams, glass fibre and hybrids (Resalati et al., ). The thermal conductivity of VIPs made from different core materials are summarised in Table 4.
Computational Fluid Dynamics (CFD) tools such as ANSYS Fluent allow researchers to replicate simultaneous mass and heat transfer in refrigeration systems. In a refrigerated container, heat transfer occurs between the outside and inside environments including the food itself. It is important to maintain a uniform flow and temperature inside the container to keep the quality of the produce intact during the trip until arriving at the destination. Using CFD analysis, different designs of the refrigeration and/or PCM system can be examined to optimise the heat transfer area, air and temperature distribution. Building upon the recent knowledge shared in this area, a few relevant studies are discussed providing insights into the modelling of different refrigeration systems with PCMs.
Lafaye de Micheaux et al. () used a wind tunnel to examine the infiltration flow rate through an opening door of a refrigerated truck. The results of the study showed an overall heat transfer coefficient of 0.576 W/m2·K for the insulated container. Results of a numerical study by Ben Taher et al. () showed the benefit of PCM incorporated under the roof of a truck. When doors were open for 15 minutes, the inside temperature increased from 2°C to 11°C compared to the increase from 1°C to 17°C without PCM.
A numerical study by Kayansayan et al. () investigated the impact of the length of cold air ejector inside a refrigerated truck with different aspect ratios; length/height (L/H). Results of the study showed the optimal injection velocity of 3.47 m/s, 6.94 m/s and 13.88 m/s for the L/H of 2.45, 3.33 and 5.32, respectively. The highest L/H showed the poorest performance in air circulation and temperature uniformity for different injection velocity, while L/H=3.33 demonstrated the highest performance at higher injection velocity.
From two numerical studies, Senguttuvan et al. (, ) found that directing the cold air from the refrigeration unit along the bottom of a refrigerated container (L=5.79 m, L/H=2.5) and return air to the top of the unit provides a higher heat transfer and temperature uniformity. Using different fan pressures of 50, 100 and 150 Pa showed a higher performance for 100 Pa compared with 50 Pa and a moderate improvement for 150 Pa compared with 100 Pa.
3.1 Literature review on phase change materials
This section presents potential phase change materials (PCMs), namely eutectic salts, to be used for cold thermal energy storage (TES) with melting/storage temperature between -40°C and 0.0°C. This will help the project team select suitable PCMs depending on the design and application of the cold TES system.
The desired thermophysical and chemical properties and economics of a potential subzero PCM candidate are (Oró et al., ; Yang et al., ):
&#; Thermophysical properties o Melting temperature in the desired operating temperature range o High latent heat of fusion per unit volume o High specific heat to provide additional significant sensible heat storage (in this case -40°C and 0.0°C) o High thermal conductivity of both solid and liquid phases o Small volume change on phase transformation o Small vapour pressure at operating temperature o Congruent melting of the PCM for a constant storage capacity of the material with each freezing/ melting cycle o Reproducible phase change
&#; Nucleation and crystal growth o High nucleation rate to avoid subcooling of the liquid phase during solidification, and to assure that melting and solidification process occurs at the same temperature o High rate of crystal growth, so that the system can meet the demand for heat recovery from the storage system
&#; Chemical properties o Complete reversible freeze/melt cycle o No degradation after a large number of freeze/melt cycles o No corrosiveness to the construction/encapsulation materials o Non-toxic, non-flammable and non-explosive
&#; Economics o Abundant o Available o Low cost o Easy recycling and treatment o Good environmental performance based on Life Cycle Assessment (LCA)
A eutectic material which is a mixture of two or more species with a specific composition is the most common PCM. This kind of material has high thermal conductivity and density together with no phase separation.
Eutectic materials can be classified as either organic or inorganic. Table 5 summarises the specifications of each group. Eutectic water salt solutions with additives have been the most investigated subzero PCMs by researchers and commercial companies (Yang et al., ). For temperatures above 0°C, organic materials have been recommended. Melting temperature is one of the most important parameters for the selection of PCMs and designing the heat exchanger. Hence, the identification of the eutectic temperature of the water-salt solution is the most important criteria that needs to be assessed by analysing the previously developed phase diagram of the hydrated salt (Oró et al., ). This helps to minimise the number of experiments for determining the melting temperatures of the eutectic salts, for the compositions which are away from the eutectic point. The studies on the eutectic NaCl-H2O PCM supports the significance of the water-salt solution phase diagram (Han et al., ). Examples of inorganic salt solution phase diagrams can be found here (Phase diagrams for binary salt solutions: Phase diagram).
Table 6 summarises a list of inorganic subzero eutectic salt solutions and their thermal properties.
Note: Blank cell indicates that the information is not provided in the source.
Methods have been suggested to improve the performance of PCMs in terms of thermal conductivity, liquid leakage due to the phase change, subcooling and thermal stability.
One method is the addition of nanomaterials (such as metal particles, carbon fibre, graphite, and nanoparticle composites). More research has been conducted with organic PCMs. Limited attention has been given to the inorganic eutectic salt solution. The effect of nanoparticle deposits on the performance of some inorganic eutectic salt solutions has been investigated and the results are given in Table 7.
Sino Air supply professional and honest service.
PCM (composition in wt.%)
Nanomaterials Phase change temperature ( ° C)
BaCl2 (22.5) TiO2 with volume fraction of 1.13%
K2HPO4 3H2ONaH2PO4·2H2O -Na2S2O3 5H2O (6-6-6) expanded graphite (MEG)
MgCl2 (23) Multi-walled carbon nanotubes (MWCNTs 1 wt%)
-8.5 (no change)
Nucleating agent Thickener Subcooling degree ( ° C) Latent heat of fusion ( k J/ k g)
Basically eliminated 254.2 (decreased by 9.57%)
Thermal conductivity (W/(m K))
0.67 (improved by 16.74%)
(13.3 times that of eutectic salt solution)
Application Year & Ref.
Refrigeration in beer industry
Zhang et al. ()
(only 10% higher than pure eutectic solution due to the use of thickener forming a gel-like structure) air conditioning refrigeration, cold chain logistics
Xie et al. ()
Cold Chain Wu et al. ()
Note: Blank cell indicates that the information is not provided in the source.
Encapsulation and shape stabilisation methods have also been studied for the improvement of the performance of PCMs. However, these methods have not been investigated for subzero inorganic eutectic salt solutions (Zhang et al., )
To combine the advantages of organic and inorganic PCMs, some studies have been conducted on organic-inorganic composite PCMs for cold TES applications (Li et al., ; Xing et al., ; Zhang et al., ). A ternary solution of organic salt of sodium formate (HCOONa) and inorganic salt of potassium chloride (KCl) in water identified from a previous study by Lu et al (Lu et al., ), has been experimentally characterised (Xing et al., ). A mass fraction of 22% / 12% / 66% showed eutectic characteristics with -23.8°C melting temperature and 257.2 kJ/kg. A 0.6 wt% of xanthan gum was suggested to supress phase separation and leakage while using nano-TiO2 0.6 wt% reduced supercooling by 67.9% (from 6.6°C to 2.6°C). The PCMs consisting of organic and inorganic water solution have been summarized in Table 8
Note: Blank cell indicates that the information is not provided in the source.
Regarding corrosion, the assessment on sub-zero inorganic salt solutions has been given little attention.
Summary
Overall, experimental-based research for the selection of suitable sub-zero eutectic salt solutions is limited. Moreover, some potential salt solutions proposed in the literature are either not at their eutectic point or there is a disagreement about the eutectic composition reported. Therefore, future research needs to be directed towards the experimental validation of potential PCMs and identification of suitable PCMs with desired properties, and hence designing an effective eutectic refrigeration system for transport application. For salt solutions which are away from their eutectic points but worth investigating due to their potential of offering better performance, particular attention needs to be paid for refrigeration system design as the system needs to deal with both latent heat and sensible heat storage.
Australian Food Cold Chain Logistic Guidelines require that chilled and frozen foods must be transported, stored and handled at temperatures never warmer than 5°C and -18°C, respectively (Australian Food and Grocery Council, ). Considering the efficiency of heat transfer when charging and discharging the eutectic system, PCM with melting temperatures in the range of -40°C up to 0°C is of interest for this project. After assessing the hazard level and economics of the chemicals, some promising candidates were selected from Table 6, as shown in Table 9.
Samples were prepared by mixing the pre-dried inorganic salt with water at desired weight ratios. The melting temperature (Tm), latent heat of fusion (ΔHm) and specific heat capacity (Cp) of those samples were measured by using a PerkinElmer differential scanning calorimetry (DSC) The thermal conductivity (k) was measured by using a Trident Thermal Conductivity Analyzer (TCA). The density (ρ) is simply calculated by the ratio of mass to volume. All measurement results are summarised in Table 9.
Latent heat energy density (ΔHm/ρ) vs. melting temperature of the PCM candidates are plotted in Figure 10.
Note: Blank cell indicates that the sample has not been measured as it has been excluded based on previous testing Differences in PCM-33 composition are confidential.
Refrigerated trucks incorporating eutectic plates have been commercially available for over 20 years.
Our industry partner, Aldom Transport Engineering owns a recently retired eutectic refrigeration truck that was manufactured in . The truck was used for delivering ice-creams during the day, while the eutectic plates were charged at night-time, taking about 12 hours with a charging power of about 3 kWh The truck and the eutectic plates were examined, and a few conclusions were made:
1. The eutectic plates were made of stainless steel. It was observed that the steel was corroded by the eutectic solution, resulting in leakage (as shown in Figure 11). One plate was detached from the truck. The eutectic solution and refrigerant were carefully drained out of this plate. Then the metal plate was cut open. As can be seen from Figure 12, localised/pitting corrosion was found on the metal, which leads to the creation of small holes causing the leakage through the plate
2. The plates and tubes were made of very thick steel, in particular the thickness of tube was approximately 3 mm. It is believed that much thinner material can be used to contain the eutectic solution if the selected material is more corrosion resistant As a result, the weight of the plates can be reduced.
3. The eutectic plates cool the space by means of radiation and natural convection heat transfer. This &#;passive&#; heat transfer process is uncontrollable. If the space temperature rises due to door openings or high ambient temperature, the truck will be cooled down very slowly. This poses the risk of exposing the produce to high temperatures, which will increase the risk of food spoilage.
A techno-economic analysis has been conducted to evaluate four types of transport refrigeration systems for a medium-size refrigerated vehicle (dimensions of 4.2m (L) × 2.2m (W) × 1.9m (H)) used for both fresh and frozen produce delivery in Adelaide climate conditions (8 h from 9am to 5pm, 7 days per week) This is a two-compartment vehicle with frozen zone at -20°C in the front and chilled zone at 2°C in the back, accounting for 20% and 80% of the volume, respectively. The size of vehicle and operating conditions were selected by the industry partner Aldom to target supermarket customers.
The refrigeration systems under investigation are: type i. Self-powered diesel VCR system (mostly common for medium to large vehicles). type ii. Battery powered VCR system. type iii. Eutectic refrigeration indirectly charged by an off-board electric refrigeration unit (as shown in Figure 13a). type iv. Eutectic refrigeration directly charged by an on-board electric refrigeration unit (as shown in Figure 13b).
Other than type i, all systems are operated using electricity, which reduces the energy cost and carbon emissions. Zero emissions could be achieved if the charging is correlated with renewable electricity generation. In both type iii and type iv, the eutectic system is charged by the refrigeration unit whenever the vehicle is not in use, which is generally at night since trucks are generally in use during the day (9am until 5pm, 7 days per week). With type i and ii, the refrigeration unit will supply the cooling as required to maintain the set temperature during the whole delivery period using diesel fuel and battery power respectively. The assumptions and estimated values used in this analysis are listed in Table 10.
Table 10. Assumptions and estimated values used in techno-economic analysis.
Note: 1 Communication with Aldom.
2 Thermo King T-90 Model T-590.
3 Estimation based on the data from https://en.wikipedia.org/wiki/Lithium-ion_battery
4 https://www.thunderstruck-ev.com/tsm-x2-and-charge-controller.html
5 Measured at UniSA.
6 Based on experience at UniSA. Effectiveness = energy extracted from the storage system / total energy storage capacity.
7 Based on AGL Standard Retail Contract Rates off-peak usage in South Australia from https://www.agl.com.au//media/aglmedia/documents/help/rates-contracts/market-contracts//07/-my-pcp agl-sa-elec-websitepricing-v7.pdf
8 https://gosolarquotes.com.au/solar-panel-dimensions/
The parameters to calculate the heat gain through doorways from air exchange are presented in Table 11.The R-value of the insulation material is 2.5 K·m2/W. Estimation of peak refrigeration load was made using the method in the ASHRAE Handbook &#; Refrigeration: Chapter 24 Refrigerated-Facility Loads (ASHRAE, ). The outdoor design temperature for Adelaide is 42°C in the hottest month and this temperature was used to calculate the peak refrigeration load. It is assumed that the products enter the frozen and chilled space at -20°C and 2°C, respectively, and the average heat of respiration rate of chilled products (1,500 kg) is 40 mW/kg. Heat gain from the fan motor is assumed to be 680 W. The effect of the weight on the total energy consumption is not included in the analysis but will be included in Phase 2 of the project.
The peak refrigeration load is estimated to be 2,680 W, following the calculation procedure in ASHRAE Handbook &#; Refrigeration, Chapter 24: Refrigerated-facility Loads (ASHRAE, ). In this case, the refrigeration load consists of transmission load (heat transfer into the refrigerated space through the truck envelope), product load (respiration heat from fresh product), internal load (heat produced by fan motors) and infiltration load (warm ambient air entering the refrigerated space). For the same size truck, Thermo-King recommends using its T-90 series of self-powered truck refrigeration unit model T-590, which has a cooling capacity of 4,543 W and 2,931 W at 2°C and -18°C, respectively, based on an ambient temperature at 37.8°C.
To calculate the total energy required during the daily delivery period and hence to estimate the amount of PCM required, an hour-by-hour load calculation was carried out using historical Adelaide climate data obtained from Lowa Environmental Mesonet (mesonet.agron.iastate.edu). The weather data between 1 January and 1 July was examined. The weather data from 9am to 5pm was used from 30th January , which was the hottest day over the period. The hour-by-hour weather data and refrigeration load are presented in Figure 14
The daily refrigeration energy is 20.22 kWh, thus approximately 360 kg of PCM is required assuming candidate #1 in Table 9 is used due to its high energy storage density. Due to the low thermal efficiency of the diesel combustion engine (~35%), 10.9 litres of diesel will be consumed on an extremely hot day, generating 29.21 kg CO2-e emissions If the eutectic system in type iii and type iv is charged using grid electricity in South Australia (with an emission factor of 0.43 kg CO2-e/kWh), only 8.69 kg CO2-e emissions will be produced. This represents 70% emissions reduction from a standard diesel-powered refrigeration option (type i).
The initial capital cost of the refrigeration system (CapEx), daily cost of energy (OpEx) (recharging using off-peak electricity), total cost of ownership over 10 years, weight of the four types of systems and daily CO2-e emissions (from the refrigeration system, applied to the analysis in this chapter) were estimated using the values in Table 10 The results are presented in Figure 15 Total cost of ownership is determined by the CapEx, OpEx and discount rate (15% is used in this study). When calculating the OpEx, only the cost of diesel and electricity were considered. Other OpEx includes insurance, maintenance, repair, and employee training It is worth noticing that the cost of maintenance for dieselpowered system is higher than that of the other types of system Compared to the diesel-powered and electric-powered VCR refrigeration system, the eutectic refrigeration systems have the lowest cost of ownership, including CapEx and OpEx, and generate the least amount of CO2-e emissions The eutectic refrigeration system with an off-board electric refrigeration unit (type iii) requires infrastructure to be set up in the warehouse and it is more complex to assemble and operate. It is more suitable to a fleet of refrigerated vehicles that could share the same infrastructure Therefore, the system with an on-board electric refrigeration unit (type iv) was selected as the first prototype to be constructed and tested on a single truck body in the Phase 2 of the project.
o R-value
The R-value of the insulation panel has a significant impact on the transmission load. When the R-value is 2.5 K·m2/W, approximately half of the energy provided by the refrigeration system is for the transmission load. Peak refrigeration load and daily/transmission energy required at R-values of 2, 3, 4, 5 and 20 K·m2/W were calculated, and the results were plotted in Figure 16. A high R-value could be achieved by using VIP (in Section 2.2.4). For example, 50 mm VIP with an aerogel core (thermal conductivity of 0.01 W/(m K)) and 100 mm VIP with glass fibre core (thermal conductivity of 0.005 W/(m·K)) has an R-value of 5 and 20 K·m2/W, respectively.
The amount of daily energy consumption (diesel for type i and electricity for the other three) and the daily CO2-e emissions for the four types of refrigeration system were calculated and are shown in Figure 17.Both will drop by 25% and 43.7% if the insulation is improved from 2.5 K·m2/W to 5 K·m2/W and 20 K·m2/W, respectively, for all types. Compared to the diesel driven VCR system (type i), the battery driven VCR system (type ii) and eutectic refrigeration system (type iii and type iv) produce less CO2-e emissions by 38.8% and 69.4% if they are charged by the South Australian grid, respectively, regardless of the R-value.
With improved insulation, the required amount of PCM and battery capacity will also be less. The weight of the battery powered VCR system and eutectic refrigeration system were presented in Figure 18.The battery system is lighter in most cases, but the discrepancy of weight between the battery and eutectic systems becomes smaller with increasing R-value. It is worth noting that when the R-value is 20 K·m2/W, the difference between type ii and type iii is small.
Batteries are classified as lead-acid, nickel-based, ambient-temperature lithium, high-temperature, metal-air and flow batteries, as presented in Figure 19. Lithium-ion batteries have the advantages of high specific energy (Wh/kg) and energy density (Wh/l), long cycle life and high safety. Therefore, they not only dominate the battery market of portable electronics but also have widespread application in the booming market of automotive and stationary energy storage (Liu et al., ). Researchers (Liu et al., ) have recently reviewed the types of electric vehicle batteries currently available, which are summarised in Table 12. Based on the value range, four specific energies of low (100 Wh/kg), medium (200 Wh/kg), high (250 Wh/kg) and very high (300 Wh/kg) were selected to study the impact of specific energy on the weight of the battery powered VCR system (type ii). The weight of battery and refrigeration system at different specific energies are shown in Figure 20. The weight reduction is large (~50%) when the specific energy of the battery increases from 100 Wh/kg to 200 Wh/kg.
Since , the price of batteries has fallen by about 89% The current cost of lithium-ion per kWh is AUD$216 (US$137), which is predicted to drop to AUD$92 (US$58) by (Gaton, ) (US$1 = AUD$1.58). König et al () have given the range of battery pack costs (&#;/kWh) in Figure 21, with the numbers after year being the forecast. The current cost is already at the lower end in the range, therefore, the minimum costs in years , , and (1&#;/kWh = 1.5 AUD$/kWh) were selected for this analysis (red dots on Figure 21) The CapEx cost of battery powered VCR system (type ii) and battery pack were estimated and plotted in Figure 22. With the falling cost of batteries, the battery powered refrigeration system will gradually become cheaper over time However, it will always be more expensive than the eutectic refrigeration system as long as the cost of battery is higher than AUD$2.1/kWh.
o Sensitivity analysis
A sensitivity analysis has been carried out to study the impact of key parameters on the cost of refrigeration system (CapEx), cost of energy (OpEx), total cost of ownership over 10 years, weight of refrigeration system (on vehicle) and CO2-e emissions for the four types of refrigeration systems The parameters investigated in this analysis include: COP of the refrigeration unit used in product delivery, COP of the refrigeration unit used for charging the eutectic system, insulation R-value (K·m2/W), specific energy of battery (Wh/kg), latent heat of fusion of PCM (kJ/kg), cost of battery ($/kWh), cost of refrigeration unit ($), cost of on-board charger ($, for battery-powered refrigeration system), cost of secondary heat exchanger ($, for type iii eutectic refrigeration system) and discount rate. The values of those parameters were varied by ±20%. Changes were made one parameter at a time, keeping all other parameters fixed to their baseline values in Table 10. The results obtained (CapEx, OpEx per day, cost of ownership, CO2-e emissions per day and weight of on-board refrigeration system) were compared with the results from the baseline case presented in Figure 15 The results are listed in Table 13
o CapEx: the COP of the refrigeration unit and cost of battery are the two most critical variables that affect the CapEx of battery-powered refrigeration systems (type ii), while the insulation Rvalue and cost of the refrigeration unit has less impact The CapEx of eutectic systems is influenced more by the cost of the refrigeration unit than the insulation R-value, latent heat of fusion of the PCM and the cost of secondary heat exchanger (type iii only).
o OpEx and CO2-e emission: a decrease of 20% in COP for the refrigeration unit and R-value implies an increase of 25% and 12.5% of OpEx & CO2-e emissions for all types of refrigeration systems, respectively An increase of 20% in COP and R-value will lead to a decrease of 16.7% and 8.3% of OpEx & CO2-e emissions, respectively.
o Cost of ownership: same as the CapEx and OpEx, the cost of ownership is affected primarily by the COP of the refrigeration unit regardless of the type of system, followed by the R-value and discount rate for type i and CapEx for types ii-iv, respectively.
o Weight: compared to the R-value, the COP of the refrigeration unit has more influence on the weight of the battery-powered refrigeration system. If the latent heat of fusion of the PCM is 20% more than that of PCM candidate #1, the weight of the eutectic refrigeration system type iii and type iv can be reduced by 15% and 11.8%, respectively.
In order to predict the amount of refrigeration-related CO2-e emitted by refrigerated vehicles, a number of assumptions were made: o The stock of refrigerated vehicles increased dramatically from 21,150 in to 45,100 in (Brodribb & McCann, ; Brodribb et al., ; Mark Ellis & Associates Pty Ltd, ). No recent data after was found. It is assumed the total stock will keep rising at a similar rate, as shown by the solid green line in Figure 23. o As described in Table 1, diesel-powered truck refrigeration units are classified into three categories depending on the vehicle size. It is assumed the medium-size vehicle investigated in this work represents the average size, which has been applied to the whole stock, along with its working conditions for 365 days per annum. Thereby, the amount of CO2-e was calculated and considered as the baseline case (solid red line in Figure 23). These assumptions will be investigated in the next phase of the project. o This project hopes to develop a new and improved eutectic refrigeration system by . It is assumed that this product will start penetrating the market with a linear penetration rate of 9.1% Thus, by , this product is assumed to account for 15% of the market, the rest of which will likely still be dominated by diesel-powered refrigeration (indicated by the dotted green line in Figure 23) The same penetration rate was applied to the electric-powered refrigeration (type ii). o Electricity is assumed to have an emission factor of 0.63 t CO2-e/MWh (Scope 2 emissions), averaging the emission factors of all the states/territories (Clean Energy Regulator, ) This was applied to calculate the amount of CO2-e by charging the battery (type ii) or the eutectic system (type iii and iv) by the grid. A standard emission factor of 0 was applied if those systems would be charged by electricity from renewable sources, such as from solar PV and wind power.
The predicted amount of CO2-e emissions is presented in Figure 23. Four scenarios were considered in this analysis: (1) baseline, in which all the transport refrigeration systems are powered by diesel engines; (2)battery-powered VCR system will take up the market; (3) eutectic refrigeration systems will take up the market and (4) the battery-powered and eutectic refrigeration systems are charged by electricity from renewable sources while they are charged by grid electricity in scenarios (2) and (3). In the baseline case with only diesel-powered refrigeration units, CO2-e emissions are predicted to increase dramatically and reach 940 kilo tonnes in . Compared to diesel-powered refrigeration, eutectic (type iii or type iv) and battery-powered refrigeration technologies (type ii) could achieve a CO2-e reduction of 56.3% and 13.6% for a single vehicle, respectively. Accordingly, those two technologies will contribute to a CO2-e reduction of 79.4 kilo tonnes (8.4%) and 17.8 kilo tonnes (1.9%) by , respectively. Under a scenario where charging is accomplished by using renewable electricity, a reduction of 15% could be achieved.
The following conclusions can be drawn from the techno-economic analysis:
&#; Benefiting from the higher COP of the refrigeration unit in colder night-time temperatures, eutectic refrigeration systems of types iii (off-board electric VCR system) and iv (on-board electric VCR system) have the lowest energy consumption and operating cost. Diesel consumption is high in the type i system (diesel-powered refrigeration) due to the very low efficiency of the diesel engine (~35%) and low COP of the refrigeration unit.
&#; Compared to the diesel driven VCR system (type i), the battery driven VCR system (type ii) and eutectic refrigeration systems (type iii and type iv) produce less CO2-e emissions by 39% and 69% respectively. This assumes they are charged from the grid in South Australia.
&#; Improved insulation will allow for smaller and lighter refrigeration systems. The weight difference between the battery and eutectic system becomes smaller with increasing R-value.
&#; When the specific energy of a battery is greater than about 150 Wh/kg, battery-powered VCR systems is lighter than the eutectic refrigeration system as indicated in Figure 20.
&#; Even though the price of batteries has dropped by 89% over the last 10 years, the battery-powered refrigeration system (type ii) is still the most expensive option, followed by the diesel system. Eutectic refrigeration has a cost advantage over the other two systems; however, it adds extra weight on the vehicle. It will slightly increase the energy consumption for driving the vehicle.
&#; The amount of the CO2-e emissions estimated in this report only accounts for the emissions from the refrigeration system, not from powering the vehicle. However, the heavier the refrigeration system, the more energy will be needed for driving the vehicle, leading to a larger amount of CO2-e emissions. This research did not study the effect of the weight on the total energy consumption and CO2-e emissions of the vehicle, which will be investigated in the Phase 2 of the project.
&#; The eutectic refrigeration system with an on-board electric refrigeration unit (type iv) will be further studied and a prototype will be constructed and tested in the Phase 2 of the project.
Although eutectic refrigeration systems have the advantages of low cost of ownership, including CapEx and OpEx, compared to battery-driven vehicle refrigeration systems, the commercial eutectic vehicle has the shortcomings of ineffectiveness in its thermal performance due to door openings as well as being too heavy. The problems stem from the design in regard to containment and location of the PCM, as well as the type of PCM used This project addresses these problems by developing high performing PCM(s) in Chapter 3 and a new design of eutectic system in Chapter 5.
To overcome the shortcomings of the existing commercial product, a novel eutectic refrigeration system needs to be designed. Unlike the commercial eutectic truck where the PCM plates are distributed under the roof and on the front of the refrigerated space as shown in Figure 4, in the new design the PCM is centralised and all the PCM containers are assembled in one case that is located in the front of the space, as illustrated in Figure 24. Also, the system used fan-driven forced convection of the air to ensure more uniform temperature distribution. A commercially available eutectic refrigerated truck was used as a base to calculate the heat transfer area and the weight of PCM needed. This vehicle has an internal dimension of 5.4 m length by 2 m width by 1.57 m height. To enable frozen food delivery for 8 hours, seven eutectic plates are installed with a total amount of PCM of about 450 kg (estimated) and a total plate heat transfer area of 17 m2
Five potential configurations were proposed and only two of them are illustrated in the drawings in Table 14 for confidentiality reasons. In the first configurations, the PCM is encapsulated in rectangular containers. The coolant tubes are embedded inside the PCM container, in which the refrigerant flows and freezes the PCM during charging A fan is installed on the top of the PCM case to draw warm air from the bottom of the compartment. Passing through the void space between PCM containers, the warm air is cooled down and released from the top of the compartment. Configuration #2 is a typical shell-and-tube heat exchanger, in which the PCM is kept in the shell space and a secondary heat transfer fluid flows inside the tubes to transfer energy in/out of the PCM. Specifications are also given for indication purposes only and further optimisation of the design will be conducted when a specific design is selected The advantages and disadvantages of the proposed configurations are discussed in Table 15.
Configuration 1 - vertical medium - sized plates
Specifications
Number of plates: 18
Dimension of plate (length x width x thickness):
1.2 5m x 0.35 m x 0.045 m
Heat transfer area: 15.75 m2
Quantity of PCM: 457 kg
Dimension of case (length x width x height): 1.75 m x 0.425 m x 1.25 m
Configuration 2 - shell -and - tube, PCM contained in the case
Specifications
Number of heat transfer fluid (HTF) tubes: 28
Dimension of each HTF tube: 6.8 m (length)
Outer tube diameter: 0.018 m
Inner tube diameter: 0.015 m
Heat transfer area: 9.8 m2
Quantity of PCM: 500 kg
Dimension of box (length x width x height): 1.75 m x 0.2 m x 1.25 m
Advantages Disadvantages
Configuration 1 (vertical medium-sized plates)
Configuration 2 (shell-and-tube, PCM contained in the case) o Medium heat transfer area to PCM mass/ volume ratio. o Low number of vessels initially needing to be manufactured for prototypes. o Half volume of PCM case compared to the others. o Only one vessel needing to be manufactured for prototypes.
5.2.1 Modelling of current commercial eutectic truck o Intermediate impact of individual vessel failure in relation to both thermal performance and spillage volume. o Intermediate individual vessel mass and likely associated stresses on each vessel. o Relatively high potential impact of vessel failure in relation to both thermal performance and spillage volume. o Intermediate stresses on the vessel. o Need a fan coil unit to supply cold air and a secondary HTF to charge the eutectic system.
Commercially available eutectic trucks (Truck) have PCM plates installed under the roof and on the front of the container as shown in Figure 4. To understand the current performance benchmark of eutectic trucks it is important to understand the internal air pattern, thermal performance, and temperature uniformity under different indoor/outdoor conditions. Modelling the current design can help identify possible enhancements or alternative designs.
The modelling was conducted for a medium size commercial container of L=5.4 m, W=2 m, H=1.57 m with an aspect ratio (AR) of L/H = 3.3. The literature has identified this size as offering the best thermal performance (Kayansayan et al., ). A symmetrical model was prepared as shown in Figure 25a. The number and size of PCM plates were selected to replicate the commercial system, two plates with dimensions of L=1.60 m, W=0.49 m, H=0.045 m, and five plates with dimensions of L=1.75 m, W=0.7 m, H=0.045 m. The PCM plate included in the model had a refrigeration tube immersed in PCM as shown in Figure 25b.
The impact on air and temperature profiles inside the refrigerated container can be examined using different outlet pressures and fan velocity. If a fan is excluded, due to its electricity consumption and maintenance requirement, the natural convection prevails which may not be optimal for containers longer than four metres. The performance of natural convection diminishes with higher aspect ratio (L/H), and without a fan and forced convection, hot spots are more likely to form across a container.
CFD modelling assists with determining an optimum outlet pressure for each specific configuration. Figure 26 shows the air velocity when the outlet pressure from a fan at the front of the container is set to 50 Pa. The higher the pressure outlet from the fan, the higher the air velocity and circulation around the container. Higher air velocity results in a more intense air curtain at the back which is desired to reduce air infiltration particularly when doors are open. This can be seen in Figure 26a where the vector density covers the back door.
Air circulation can also impact the temperature distribution inside the refrigerated container (Figure 27a) and heat transfer from the walls of the container and PCM surfaces (Figure 27b). Figure 27b shows a higher heat transfer rate from the PCM plates closer to the fan which can reduce the temperature uniformity in the container. These results are from the preliminary study which shows the capability of the CFD study to examine the impact of different parameters on the flow, and thermal performance of the PCM system. In the next stage of this study, more modelling will be conducted to identify the impact of different parameters to assist in optimisation and detailed design.
From the concept designs introduced in section 5.1, one centralised configuration was selected and modelled. This configuration has shown higher heat transfer area and the lowest individual size and mass of vessel.
A small size container with dimensions of L=4.2 m, W=2.2 m, H=1.9 m has been selected for the first size to be built by the industry partner, Aldom, for modelling the proposed configuration Polyurethane was considered as insulation with thicknesses of 0.075 m for the top and 0.045 m for the back and side of the container. The insulation type and thicknesses were selected according to the recommendation by ASHRAE Refrigeration Handbook Using polyurethane with thermal conductivity of 0.022 W/(m·K) results in less heat loss.
The results of preliminary transient modelling are shown in Figure 28 and Figure 29 Transient modelling is necessary to accommodate the melting of PCM.
Figure 28a shows the axial velocity vector with higher velocity at the fan outlet which reduces towards the back of the container. This velocity pattern is the result of fan outlet pressure equal to 50 Pa (relevant for small container with L=4 m considering the results of a previous study) which resulted in maximum velocity of 5 m/s. A higher velocity may be required for more air coverage at the back of container to prevent a high rate of air infiltration through the back doors If so, a fan with outlet pressure higher than 50 Pa would be required.
Figure 28b shows the axial velocity contours and gradients further from the fan at the top and return to the PCM cassette entrance at the bottom The air velocity is higher at the top and lower at the bottom of the container due to the fan location, however, the indoor velocity distribution is more uniform elsewhere. This model can provide more insights with the inclusion of the packages and pallets in the container, which will assist in the selection of all parameters to achieve a more uniform air velocity around the packages to keep the temperature low enough and preserve the product quality.
in the middle of the container. All the results are from the early stages of transient modelling after about 100 seconds which shows a low and uniform indoor temperature. Figure 29b shows the heat flux from the outer surfaces which is low at the top and side while it is higher at the back considering the cooling load due to air infiltration from the outside. Modelling of phase change processes is computationally demanding and requires a long time. In the next stage of the project, more insight will be acquired by modelling the proposed system during the melting process of the PCM.
Conventional refrigerated containers with PCM plates and a few alternative designs with PCM in a cassette were compared in this study. The distributed PCM plate system (commercially available) and the alternative PCM centralised system has the potential to be an optimum system for a specific application. Therefore, a detailed study including modelling and a parametric analysis is required to propose a suitable PCM system for each application.
Preliminary modelling of the commercially available and the proposed centralised system was conducted to show the capability of modelling to replicate indoor conditions inside a refrigerated container with different PCM systems. The preliminary design and modelling provided some insights into conventional refrigerated containers with PCM plates and the alternative design. The results indicate the following:
Air circulation impacts the temperature distribution inside the refrigerated container and the heat transfer from the walls of the container and PCM surfaces.
A higher fan outlet pressure results in higher outlet air velocity and an intense air curtain at the back which is desired to reduce air infiltration particularly when doors open.
The heat transfer from the PCM plate closer to a fan outlet can be higher than the PCM plates closer to the back of the container.
Modelling the selected centralised configuration showed a uniform air velocity distribution except close to the top and bottom of the container due to the fan location at the top.
However, a parametric study is required for the optimisation of the PCM system in the next stage of the project. The optimisation study will identify a design where a uniform air circulation and temperature provides minimum heat loss to the outside and a more uniform temperature inside preserving the food quality during transport.
The company is the world’s best Truck Electric Refrigeration Units supplier. We are your one-stop shop for all needs. Our staff are highly-specialized and will help you find the product you need.