A sustainable cold chain using eutectics
3 likes158 views
The document discusses alternatives to conventional diesel-powered refrigeration units for cold chain distribution. It proposes using eutectic systems, which store cooling energy through phase change materials (PCMs) instead of relying on engine operation. Eutectic systems charge overnight using electric compressors, eliminating fuel costs during distribution. They maintain temperature regardless of engine size and continue cooling even if the engine stops. The document outlines key considerations for eutectic system design, including the type of PCM, encapsulation method, and sizing the system based on storage needs and charging capacity.
A sustainable cold chain using eutectics
- 1. 8 Introduction With the world and its leaders spending more and more time and money talking about climate change, we in our daily lives are also, somehow, becoming more sensitized towards this concept. Our current cold chain is an energy guzzler and there is a need to make it more environment friendly and sustainable. Moving from sustainability to the cold chain, when we evaluated the cold chain logistics industry and the technologies used, we focused on the distribution segment, which we believe is the fastest growing given the retail and food processing growth. We identified two main issues related to conventional technology in this segment: Reefer units consume a significant amount of fuel if operated as per standard operating procedures, leading to increased costs and environmental impact Engine driven units are a little fuzzy in terms of the fuel usage, which is also extremely difficult to monitor. However, there have been some scientific and some customer studies on fuel consumption, which we are relying upon in this case. For an 8.5-10 feet vehicle, using an engine driven unit, the typical consumption will be around 1-1.5 litre per hour when running continuously. In a distribution application, since the door openings are many, the unit keeps operating for a long time and consumes much more fuel. Depending upon the usage, one may consume anywhere from 5-12 litre of fuel for refrigeration for an 8-10 hour trip. Therefore, the average cost per trip could vary from Rs.250 to 600 per day amounting to Rs.75,000 – 1,80,000 per annum (considering 300 days of use). Annual diesel consumption for these small vehicles comes to 1500-3600 litre per annum, and the number becomes staggering as we increase the fleet size. Current direct drive technologies are unable to give desired performance and reliability for secondary delivery operation About the Author Rajat Gupta is the Founder CEO at TESSOL, a Venture Capital backed cold chain technology startup in Mumbai. He is passionate about building solutions for a farm to fork, affordable and sustainable cold supply chain in the country. He has worked across continents, with a large conglomerate like Robert Bosch and several early stage startups like 3Tier, Flodesign and Promethean Power. An avid reader and a music enthusiast, he holds a mechanical engineering degree from IIT Delhi and an MBA from the Harvard Business School. By Rajat Gupta Founder CEO Thermal Energy Services Solutions Pvt. Ltd. (TESSOL), Mumbai A Sustainable Cold ChainA Sustainable Cold Chain Using EutecticsUsing Eutectics Freezers-on-wheels for ice cream vending use pouch encapsulations to hold PCMs C8 Cold Chain ? March - April 2016
- 2. 9 The nature of distribution application is very different from long distance transport – there are a large number of door openings. Given the nature and size of loads, the vehicles used, typically, have a small engine in the range of 45-50 kW and operate at lower rpm due to in-town movement. Therefore, the direct drive systems do not get enough energy between openings to pull down the temperature to desired levels. The other issue is driver discretion, i.e. at some point when the vehicle is waiting, the driver may switch off the engine, stopping the cooling. Worse, in case of any engine breakdown, the goods are at a huge risk. Alternative Solution Using Eutectics Any alternates to the existing technology will involve a different way of either energy generation or storage. While a lot of research is being done on different alternatives involving energy generation, we will focus on energy storage based solutions, which are mostly latent energy based and popularly known as Eutectic systems. Eutectic systems have been among the oldest ways of refrigerated transport with companies like Dole Refrigeration in the US making eutectic plates based ice cream carts in early 1900s. In these systems, energy is stored in the form of latent heat when the phase changes from liquid to solid and released when the reverse phase change happens. This was predominantly used for ice cream transport in the past and, with the latest developments in materials and heat exchangers, it can be used much more widely as we will discuss in this article. The ways in which Eutectic or Phase Change Material (PCM) based systems tackle the drawbacks of the conventional systems are as follows. Operating Costs and Fuel Consumption Eutectic systems, typically, have no connection to the engine and, therefore, do not consume any additional diesel. They are charged by using an electrically driven compressor that is operated when the vehicle is stationary at the hub using high efficiency sealed or semi-sealed compressors working mostly at night times (lower ambient conditions) and at constant duty loads. Therefore, they consume between 8-25 units per day for an 8 to 10 feet vehicle depending upon the system.This implies a daily cost of Rs.80 to 250 which is almost 60% lower than a comparable conventional unit. Therefore, one can easily save upto Rs.1 lakh per annum per vehicle using these systems. Needless to say, the unit will also displace around 15,000 litre of diesel over its lifetime. Performance and Reliability • Eutectic systems have the required amount of energy stored in the container and do not depend upon engine size or rpm. Therefore, regardless of the size of the engine, they are able to maintain the same temperatures. • These systems are, typically, at much lower temperatures compared to the desired temperatures; therefore, they are also able to pull down much faster than the conventional units. • Finally, since these systems are engine-independent, even if the driver switches off the engine or if there is a breakdown of the vehicle, the goods remain safe. We will focus the rest of the article on understanding and evaluating the eutectic technology and typical design parameters for calculating the right system to adopt. Design and Evaluation of Eutectic Systems A eutectic based application has three main aspects to the design of the system that one needs to understand and check when buying: Type of Eutectic Fluid and its Characteristics A eutectic is a phase change material – a liquid mixture that changes phase at a particular temperature. Water is one of the best eutectic materials, changing phase at 0°C and storing 330 kJ of energy per kg. Materials used for cold chain range from -35°C to +8°C. The two key aspects one needs to examine are: Phase Change Temperature Profile or History Graph of the Material Many liquids like glycols or salt mixtures are termed as eutectics by manufacturers and are available in varying price ranges in the market. In many cases, these are freezing point depressants and have a large temperature range over which they change phase. There is also a low repeatability in the graph. Figure 1 shows a typical graph of a good phase change material changing phase at -33°C. If the graph is not flat, the system will give you a good performance in the first 1-2 hours post which the temperature will rise up sharply and will not give consistent performance. Latent Heat Capacity Latent heat capacity of the liquid needs to be calculated using eitheracalorimeteroraknownreferenceatthesametemperature. The latent capacity is critical for optimizing the amount of material to be used in an application. If too much is used, the weight and cost will go up; if it is too low, the performance and use time will be compromised. Users should ask for this data in terms of kJ/kg of PCM or stored energy in the system in the form of kWh or BTU of energy. In short, the customer should ask for performance characteristics and Material Safety Data Sheet of the eutectic material used by the manufacturer (not the composition). Type of Encapsulation and its Characteristics This is as critical as the material in terms of getting the desired performance. The key aspects of encapsulation Figure 1: Phase change temperature graph Cold Chain ? March - April 2016 C9
- 3. 10 and heat exchanger to be examined are: HeatExchangerConstructionandCapability The difference between two of the most common encapsulations is shown in Figure 2. The direct PCM heat exchanger is a much superior encapsulation compared to the pouch-based unit, which has its history in the eutectic freezer industry. The pouch freezer is a cost effective option and works well for top open freezers like freezer-on-wheels or smaller compartments shown in Figure 3. For larger compartments, it does not perform as well. Some of the key differences between the two kinds of system are mentioned in Table 1. The direct PCM heat exchanger is more acceptable worldwide for large reefer applications. Material of Construction The material of construction of the encapsulation needs to be of approved food safety standard, i.e. SS304 or food grade plastics. Galvanized steel and aluminium encapsulationsarenotidealsincethefood may directly come in contact with them. In case of a direct PCM heat exchanger, these materials are not compatible with most of the eutectic liquids and, therefore, should not be chosen. Figure 2: Types of PCM encapsulation Figure 3: Freezers with pouch encapsulation in the container Table 1: Difference between pouch and direct PCM encapsulations Criteria Pouch Encapsulation Direct PCM Encapsulation Heat exchange capability Low: PCM melts in pouches and then transfers cooling to the surface inside the container. The pouches and the plates have inherent air gaps and the heat transfer rate is lower, as a result. In order to get more performance, one needs the complete compartment area. High: PCM is in direct contact with the heat exchanger surface and, in fact, develops convective currents in the heat exchanger volume. This enables a much higher rate of heat exchange leading to rapid cooling with lesser number of plates. System size Smaller: Ideal for smaller compartments of 200-250 litre with small doors or top open freezers. Larger: Required for high performance units of greater than 2500 litre. Scalable upto 24 feet containers. Ease of Integration Low: Integrated in the container body itself during PUF insulation. It cannot be integrated by a third party or the user himself. High: Modular unit that can be filled with the required PCM and installed on any standard container. Even container manufacturers and refrigeration contractors can install the unit with basic instructions. Cooling mechanism Natural Convection: A large number of panels installed across the body and cooling is through natural convection. Hybrid Convection: Lesser number of panels installed in the container and cooling through a combination of forced and natural convection. Application segments Specific: Mostly for dairy, ice cream and chilled meat applications. Diverse: Works for all applications including bakery, vegetables and pharmaceuticals,wherethereisanarrowtemperaturerangetobemaintained. Maintenance and retrofit Tedious: Integrated into the insulated body and more difficult to maintain in case of a PCM leak. Cannot be retrofitted. Simpler: Modular and easy to maintain. The heat exchanger can be simply exchanged with a new one. Can be installed by someone remotely and can also be retrofitted in an existing container. Weight Same: Almost same weight as the direct PCM heat exchanger due to more PCM. Same: Lower PCM weight but higher encapsulation weight. Cost Cheaper: Cheaper option for a eutectic box. More Expensive: More expensive due to additional heat exchangers and material. A Sustainable Cold Chain Using Eutectics C10 Cold Chain ? March - April 2016
- 4. 11 Some examples of units with direct PCM encapsulation are given in Figure 4. System Design and Capacity Calculation System design using eutectics is a critical parameter for achieving the desired results based on one’s own application needs. This requires the heat transfer study of the heat exchanger being used. Such a study can be done using a CFD software like Ansys or CFX. At the same time, the energy storage capacity calculation of the unit and the average charging time are critical parameters that should be verified by customers during selection and purchase. Some companies have selection tools that simulate the data based on inputs and suggest the optimized system for a customer’s application. A sample calculation for such a system is given in Table 2. Conclusion As mentioned in the article, a eutectic based system can be an economically advantageous and environment friendly solution for the cold chain logistics industry currently reeling under margin pressures. The solution can also be applied to stationary applications in India given the low grid reliability both at rural and urban levels. This solution is being further implemented in cold storages combined with renewable technologies like solar. But, it is critical for us to make a practice of using and building/ eval- uating products using this technol- ogy so that system performance can be easily evaluated by customers and integrated by contractors. The customer needs to know the char- acteristics of the PCMs or eutectics used, the technical data sheet of the heat exchangers and the energy con- sumption data of his equipment. This article is an initial effort to educate customers to ask specific questions regarding the systems they are in- vesting in. Figure 4: Containers with eutectic units built on direct PCM heat exchangers (pictures courtesy PLUGnCHILLTM ) Table 2: Calculation of energy storage and charger capacity for a eutectic system Input Data Internal size of the container = 2300mm x 1500mm x 1700mm (for a 8 Ō vehicle) External size of the container = 2500 mm x 1700mm x 1900mm PUF insulaƟon = 100mm on all sides K value of the container = 0.4 W/sq m/K (depends upon the construcƟon and insulaƟon of the container. The best containers are with K value of 0.3). Ambient temperature = 40o C Internal temperature = -18o C # Hours of operaƟon = 8 hours Factor of safety = 1.75 as per ATP norms for normal duty, about 2 in case of high number of openings, 1.35 for low duty Desired charging Ɵme = 8 hours Energy Storage Heat Loss (waƩs) = FOS x K x External Area x (Tambient – Tinternal) = 1.75 x 0.4 x 24.46 x (40 – (-18)) = 1.75 x 568 W = 994 W Total minimum usable energy storage or Energynet = # hours x Heat loss = 8 hours x 994W = 7.95 kWh Based on the type of heat exchanger, the efficiency could range from 60% to 80%. Therefore, design energy storage should be: Energynet Heat exchanger efficiency between 9.9 to 13.5 kWh depending upon heat exchanger type. Charging Time Charging Time (Tcharging) = Energy storage + loss during charging Charger capacity at required evaporaƟng temperature or Charger capacity = Energy Storage + Heat Loss (without safety factor) Tcharging = 9.9kWh + .568kW = 1.83 kW 8 Therefore, for a -30o C PCM, one will need to select a charger with 1.83kW capacity at -35o C evaporaƟng. Cold Chain ? March - April 2016 C11