A new anodised coating has radical implications for more efficiently gathering alternative energy and improving energy efficiency generally. Tom Shelley reports

A new black anodised coating uses a random structure to greatly improve radiative heat transfer over a wide range of temperatures and can be applied to a wide range of aluminium alloys as well as iron and steel.

Acting as an almost perfect black body surface over the wavelengths from 2 to 1,000 microns, it greatly improves the efficiency of ovens, radiators and solar heaters and hot water heaters.

The efficiency by which a radiator emits radiation, or a solar heater absorbs radiation is decided by its surface texture and colour. The quantity by which this quality is measured is the emissivity. The best possible value for emissivity is 1, which represents the performance of a hole in a hollow body, painted black on its inside. Emissivity of conventional surfaces is decided by colour and texture, but whereas black paint may look black and absorb all visible light, it may not perform so well at the longer infra red wavelengths associated with radiated heat.

It is well known that one of the most effective radiating and absorbing coatings is black anodised aluminium. Anodising is a process in which a film of aluminium oxide is grown on the surface of aluminium and its alloys by making them the positive electrode in an electrolytic bath. Black anodised coatings have for some time been available from companies such as Fujikuro, who call theirs, 'Alumite'.

The surface of this material shows a regular pore structure, typical of anodised films. But a new process, called Fujihokka produces a complex, irregular pore structure, which absorbs infra red heat radiation over the entire spectrum from 2 to 1,000 microns with an emissivity of around 0.9.

The company is best able to produce this type of coating on one particular aluminium alloy, which they call Super Grade Fujihokka. However, they are also able to form the coating on other aluminium alloys and iron and steel. Suitable aluminium alloys for what they call, Premium Grade Fujihokka include the 1000 series, (99% min Al), 5000 series aluminium magnesium alloys but not 5056, and 6000 series aluminium magnesium silicon alloys. Some 3000 series aluminium manganese and 4000 series aluminium silicon alloys are suitable but 2000 series aluminium copper and 7000 series aluminium zinc alloys are unsuitable. The surfaces of the alloys have to be modified to accept the treatment. SG Fujihokka is typically grey in colour while PG is typically black. Infra red emissivity for PG is about 0.85.

Most remarkable of all, the company has developed a process to produce a specially formulated alloy coating on iron and steel. They decline to reveal how this is done and will only say that it is a chemical and not a physical bonding process. They call this process, HR or heat resistant Fujihokka. The emissivity of untreated steel is typically 0.2 to 0.3 at 5 microns, whereas the HR process raises this to 0.8, even in the far infra red.

Applications include: domestic and industrial ovens, furnaces and infra red heating devices, hot plates, burners, domestic radiators, refrigerator condensers, heat sinks, heat shields (especially in cars), infra red lamps, and solar heat gathering systems for domestic and horticultural use. The company says that in a far infra red drying oven application, energy consumption was reduced by over 50%, drying times, by over 60%, and oven length by 50%. More information can be found at: www.fujikura.co.uk

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