Massachusetts Institute of Technology (MIT) startup Promethean Power is bringing India milk chillers that quickly drop the temperature of milk to reduce bacterial growth, even without electricity. Powering the chillers is a novel thermal battery that stores thermal energy when the grid is available and releases the energy without need of electricity. So far, Promethean has installed about 100 chillers for top dairies around India.
India is the world’s leading milk producer, with many of its people relying on milk as a primary source of income. Indian dairies buy milk from local farmers at village collection centres, and then sell the milk or use it to make dairy products. However, with rural India’s limited electric grid, often available for only several hours daily, keeping milk fresh – it must be refrigerated within a few hours of milking – becomes very difficult. Many dairies use expensive diesel generators for refrigeration, or risk high percentages of spoiled product. Of the roughly 130 million tonnes of milk produced by India each year, millions of tonnes go to waste or reach the market as low-quality dairy products that pose safety threats.
Promethean systems are set up by dairies in collection stations, where farmers can bring milk within half an hour after milking a cow. Grama, inventor of the Promethean power, says: “This also dramatically improves the quality of milk collected, which improves the product for Indian consumers. On the dairy processor side, the system drops operating costs, because dairies no longer need to buy and transport diesel fuel to villages.
Additionally, dairies can gather milk from remote villages that had not had collection routes before. Overall, it offers a lot of advantages for all stakeholders – dairy processors, farmers, and dairy consumers.” The system’s thermal battery – co-invented by Grama after he launched Promethean
at MIT – is not a battery in the traditional sense, but a tank that contains two types of materials: a phase-changing material (PCM) that freezes and liquefies inside a series of tubes, submersed in a heat-transfer fluid (HTF) that never freezes. The battery is attached to a refrigeration compressor and a 3-foot-high, stainless-steel heat exchanger that rapidly chills milk that is poured over the exchanger’s surface.
In the battery’s ‘charging’ process, using grid electricity, the HTF is cooled by the compressor to a temperature that allows the PCM inside the tubes to freeze, and, therefore, absorb huge amounts of thermal energy. The thermal energy later released from the melting PCM, as it changes from solid to liquid, is captured by the HTF and circulated around the custom heat exchanger. Cold energy is essentially stored in the battery and released to cool and preserve milk. Milk is poured over the cylindrical heat exchanger and collected in a basin at the bottom. By the time the milk drips from the top of the cylinder to its basin, it receives a ‘thermal shock’ that instantly cools it from 35 degrees Celsius to 4 degrees Celsius.
The battery takes about 5 hours to charge fully, and the system can be used to cool up to 1,000 litres of raw milk a day. Chilled milk is stored inside an
insulated tank for pick-up by dairies•