On 13th September 1833, an American ship ‘The Clipper Tuscany’ landed at Calcutta, carrying about 100 tons of Ice shipped from the frozen lakes of New England, USA. This was to be the first of many voyages from the USA to bring Ice to the Indian subcontinent, which was stored away in Ice houses and either consumed or used in storage of fruits and wine in ships.
The Indian segment would be the most profitable part of a prosperous business called the Ice trade. Ice harvested from the ponds of New England during winter, was sold far and wide including the Caribbean, Europe, India and South East Asia. The person who envisioned and embarked on this venture Frederic Tudor- was made a multi-millionaire within a decade through the sheer audacity of a vision to be a seller of a ubiquitous and universal commodity, but which had also been infeasible enough to readily obtain. In another sense, the success of this business might also be called a starter to the feast of industrialisation, economic materialism and globalization. A demand for colder and versatile temperatures- for human consumption or storage of temperature sensitive materials was going to be a unwavering reality. Prospering during 1850s, ice trade was to inevitably die out by 1900s. The well known revolutions in refrigeration had happened by then and were replacing natural ice used in most areas.
However, two things from this story from the Victorian era may be of relevance today-
Demand for colder temperatures for human consumption or cold chain would ultimately be satisfied by mechanical refrigeration systems.
The then refrigeration plants were also used to freeze water in bulk and ice would continue to have a major influence in cold chain and refrigeration. For ice as a phase change material would fill in the gaps and shortfalls of mechanical refrigeration.
The current refrigeration industry
A refrigerator is a common household appliance that consists of a thermally insulated compartment and a mechanical heat pump that transfers heat from the inside of the fridge to its external environment to cool the inside of the fridge to a temperature below the ambient temperature of the room.
The commercial refrigeration industry comprises a large variety of products, including:
Cold displays or plug-in commercial refrigerators
Refrigerating chambers, cold counters and reach-in refrigerated displays
Refrigerated automatic sales machines
Ice machines
Refrigerator parts and other products, such as cryogenic equipment and liquid refrigerators.
The demand for chilled and frozen food and beverages is being boosted by increases in disposable income, and change in eating habits. Improvements in the standard of living allow more people to eat out. The growing concentration of the developing world’s population in major urban centres and an increased participation of women in the work force also contribute to changes in eating habits.
One such change is the higher consumption of frozen foods and refrigerated drinks. The key categories of food and beverage include: (i) non-alcoholic beverages, such as soft drinks, juices and water (ii) alcoholic beverages (iii) ice cream; and (iv) chilled and frozen food.
According to Euromonitor, the ice cream industry in India in 2014- 15, valued at about $31 billion is growing at a rapid pace of 15% annually. The ~$35 billion worth Indian alcoholic beverage market notes a growth rate of around 7%, while soft drinks valued at ~$50 billion are also growing at a similar pace.
These positive trends reverberate positively on demand for commercial refrigeration equipment used for the conservation of food and beverage. Simultaneously, there is also a growing ecological concern to minimise dependence on mechanical cooling, and to ensure optimal power consumption by improving efficiency of refrigeration systems.
Most stored frozen and chilled foods are sensitive to temperature fluctuations, which are caused by heat penetrating the walls of the freezer. The refrigeration system removes this heat load, but if in case of a power failure, cooling is not provided to the stored product. A Thermal Energy Storage (TES) system, with a high capacity for thermal storage can maintain the required temperature for a longer period in case of a power cut. Traditionally, glycol, coolants and water based gels have been used as TES, but Phase Change Materials (PCMs) can provide a far better thermal backup.
The International Institute of Refrigeration (IIR/IIF) observes that 28% of food produced in developing countries goes to waste due to lack of refrigeration. Less than 4% of India’s fresh produce is transported by cold chain, compared to more than 90% in the UK. In an agrarian economy like India, more reliable and efficient cold chains would not only lead to reduction of wastes but also ensure that farmers get a better return for their labour.
Integrating phase change materials in refrigeration
Desirable qualities for a material to be used in refrigeration TES system are:
High latent heat of fusion per unit volume so that a lesser amount of material stores a given amount of energy.
A melting point in desired operating temperature range.
High thermal conductivity so that the temperature difference required for charging can be small and charging is more efficient.
Non-toxic, non-flammable, non-explosive and non-corrosive Reproducible properties over many cycles, to ensure the use of the material as many times for storage and release of heat as required by an application.
Economically viable to make the system cost effective.
The above characteristics are fulfilled by a category of highly specialized materials called Phase Change Materials (PCM). PCMs can release or absorb large amount of heat energy in the form of latent heat while maintaining a constant temperature. Compared to other technologies like ethylene glycol, they not only have better thermal storage capacity but are also non-toxic, non-hazardous, temperaturespecific and reusable over a substantial number of cycles (~ 3000 or more). The benefits of the above features are precise temperature control (allowing not more than +/- 1°C of error); longer duration of retention period (upto 18 hours due to high latent heat), and reduction in the overall weight of the freezer due to high energy storage to weight ratio of the PCMs. Hence PCMs with a suitable melting temperature may be used to provide thermal capacity for maintaining suitable recommended internal temperature during power failure. Integration of PCMs in the evaporator of a domestic refrigerator helps increase its COP by providing better thermal conduction while the compressor is active. PCM may also be used in load shedding applications to shift electricity usage to an optimum time.
Applications with phase change materials
Freezers for ice-creams are designed to maintain a temperature between – 18°C to -23°C while beverages are maintained between 2°C to 8°C in a cooler. The PCM chosen for both of these applications are different due to the different temperature requirements. The selected PCM gets charged passively when the power is available within 9 -10 hours.
The selection of the phase change materials is important as one of the critical factors is the minimum temperature that the refrigeration unit can offer to ensure that the PCM is completely charged. Once fully charged the PCM is in ready state to release the energy in event of a power or equipment failure. This freezer is designed to maintain a temperature of -18°C to -19°C at an ambient of 40°C for a minimum of 16 hours and in spite of several openings and closing of the lid. Innovative products using PCMs strive to provide better off grid cold storage.
Solar based micro cold storage units launched by Ecozen Solutions are helping farmers to store their products in un-electrified areas with poor access to cold storage. Besides reducing crop wastage, the product helps enhance margins by allowing farmers to sell their produce over a period of time as prices increase.
In an attempt to make refrigerated trucks more cost efficient, the company, TESSOL relies on frozen PCMs to maintain a chilled temperature. Instead of having a diesel based compressor to run throughout the truck’s transport and delivery time, the PCMs can be chilled or ‘charged’ at the start of the journey using a refrigerantsystem based on grid electricity, which is much cheaper than relying upon a diesel based system.
The field of grocery e-commerce, once held back by expensive logistics, has been making rapid strides, helped by investments made by large companies like Amazon and Google. Integration of phase change materials in e-commerce chains to keep fruits and vegetables cool for a short duration, is ongoing and is expected to increase with time.
There are several refrigeration companies in India and outside which have now introduced deep freezers and chest coolers incorporated with these phase change materials to cater to the market in low and middle income countries where power availability is an issue.
The variety of PCMs available now allows OEMs to manufacture a range of refrigeration equipment to cater to different temperature requirements. The introduction of these materials have proved to be a boon to the refrigeration industry.
