A Sustainable Cold Chain Using Eutectics
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
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.
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. Figure 1: Phase change temperature graph 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 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
- 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. 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 either a calorimeter or a known reference at the same temperature. 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).