Ice would be slow to melt if external heat were kept out, says S.Ananthanarayanan.
Ice is something that readily melts. As ice has a very large role in preserving food, much energy is expended in production of ice. Melting of ice when exposed to sunlight is thus a factor that increases world energy consumption.
In the context of ecology, melting affects the ice cover at the poles, or on mountain tops. This ice is what regulates winds and rainfall, and importantly, locks away a huge quantity of water. This persistent store of ice is fast melting, leading to changes in climate and dangerous rise of the sea-level because of the water released.
Jinlei Li, Yuan Liang, Wei Li, Ning Xu, Bin Zhu, Zhen Wu, Xueyang Wang, Shanhui Fan, Minghuai Wang and Zia Zhu, from Nanjing University, China, Stanford University and the Chinese Academy of Sciences, write in the journal, Science Advances, of an ecologically safe material that can retard melting of ice by keeping away the energy of sunlight from small and large surfaces of ice, while allowing the ice to keep losing energy by radiation.
As a first step in examining how to minimize the melting of ice in sunlight, the authors say, one needs to understand the energy flows that are involved. On the surface of ice, exposed only to sunlight, the principal inflow of energy, in the form of heat, is sunlight - infra-red radiation in the wavelength range of 0.3 to 2.5 micrometres. While this heat flows in, the surface also radiates energy, heat energy, also in the infra-red, in the wavelength range, 2.5 to 18 micrometres.
As shown in the diagram, the main input of heat is from sunlight, and there is more at lower latitudes, nearer the equator, than at high latitudes, nearer the poles. In contrast, the heat that is radiated out is much less than what is received. A method of preventing ice from melting, hence, should minimize the sun’s heat that reaches the ice surface and, at the same time, allow the largest part of radiation from the ice to escape.
A number of candidate solutions are now available. Most of these rely on ‘selective emissivity’, or getting a surface to radiate heat within a narrow wavelength window. The reason the earth does not chill to the icy coldness of outer space soon after sunset is that heat is trapped by the atmosphere, which keeps the earth warm. The same property of the air around us captures radiation from an object and prevents it from cooling as fast as we would like it to. Except that there is a narrow range of wavelengths at which the atmosphere is transparent to infra-red radiation. And the part of the heat the object gives off, which lies in the window, passes through the atmosphere, without warming the surroundings, into outer space.
One such arrangement, reported in 2014, consisted of a combination of silica (silicon dioxide) and hafnium dioxide, with a polished silver backing, which captured incoming radiation and turned it around as radiation in the 9-12 micrometre wavelength range, which passes out, through the atmosphere. And a year later, silicon crystal architecture was adapted so that heat was sent out, as before, but visible light was allowed in, to allow solar cells to function, and to stay efficient, because they were kept cool.
And another year later, in 2016, a wearable fabric was developed that allowed body heat to pass through, but was opaque to light, so that it performed the ‘social’ role of personal clothing. The fabric was a polyethylene, which is normally transparent. But the fibres were modified to have dimensions that were comparable to the wavelength of visible light. Visible light was hence scattered, but not infra-red radiation, which has longer wavelength. A ’wicking’ treatment, that drew perspiration away from the inner surface, made the fabric comfortable to use for clothing.
While these were arrangements that required complex construction and were useful to cover smaller objects, a low-cost film that which could be laid over larger structures was reported in 2019. The film was of a common silicone, a transparent, soft and pliable material that is found in contact lenses, as a lubricant, a water repellant coating, even in shampoos. The material was found to selectively emit at the wavelength where the atmosphere was transparent, and with a promise of passive cooling by 12°C, under the night sky. And on the same lines, we now have super-reflective paints, with selective emission, to paint buildings and keep the interior cool, during the day. There has been cooling to the extent of 40 to 100 W per square metre, with temperature drop as high as 13⁰C, the Science Advances paper says.
Preventing the melting of ice, however, presents unique requirements, the paper says. For one, ice is at a lower temperature than other objects that need cooling. The rate of loss of heat hence needs to be greater, by some 70 to 110 W per square metre. And then, there is huge quantity of ice to be protected - with widespread use, as in food preservation, and if used to help glaciers that are in danger of melting, hundreds of square kilometres would be required. “…it is critical that the radiative cooling materials be extremely abundant and scalable (~million metric tons) with minimized environmental impact,” the paper says.
The material developed by the authors is based on eco-friendly, cellulose acetate (AC), which is sourced abundantly from vegetable matter. The CA is worked into a layered film, where the AC molecules, for one, create a surface of very high reflectivity, and for another, radiate heat strongly at the wavelength range that leads to maximum cooling. The material develops pores from 0.5 to 3 micrometers. This enables reflection of light in the wide range from 0.3 to 2.5 micrometers, which includes the visible and near infra-red. And CA molecules can be ‘tailored’ for emission at wavelengths that suit different conditions. The result is a ‘reduced thermal load’ on ice, the paper says……..and the material is non-polluting, as it can be digested by micro-organisms in nature.
Trials were conducted with common food materials, like ice cream, wrapped in the CA based film and in the usual way with paper, aluminium foil, PET or PE. It was found that the heat load with usual materials was some 150 to 300 W per square metre, against a net heat loss in the case of the CA based film. Ice cream wrapped in the film stays 98% intact after 80 minutes in the sun, against only 50% with usual wrapping, the paper says.
As for preservation of ice in bulk, as in icebergs, trails showed that even with a load of 700 W per square metre, which is nearly twice what is found in higher latitudes, the CA based film could keep the ice temperature 7⁰C below the ambient. In a trial, samples of ice were exposed to natural sunlight for five consecutive days. Ice covered with CA based film stayed unchanged, while bare ice shrank and disappeared, the paper says. The CA based material, which can be spread as a powder over ice, is equally effective over snow, the paper says.
The development is exciting, as a solution both to conserving huge energy that is consumed to manufacture ice, as well as to help stave the melting of glaciers, in the face of global warming.
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