1171st Ordinary General Meeting

"New environmentally friendly approaches to cooling buildings"

Professor Geoff Smith, Department of Physics and Advanced Materials, University of Technology, Sydney

Wednesday 3 June 2009 at 7 pm
Conference Room 1, Darlington Centre, University of Sydney

The potential for energy savings in the cooling of buildings is very large and of growing importance as living standards rise, as global warming impacts, and as the "heat island" effect gets worse with increased urbanisation. There are two aspects: (i) passive systems which minimise heat gains, and (ii) active systems and strategies which minimise or eliminate the need for electrically powered cooling. This talk will examine novel materials and systems which play a role in both active and passive reductions in the demand for electrically powered cooling. It will also include results with special paints and nanostructured coatings developed at UTS.

Amplification of night sky radiative cooling using nanostructures and heat mirrors will be outlined, in which material spectral properties and system design in combination optimally exploit the spectral and directional properties of incoming atmospheric thermal radiation. Useful cooling powers under clear skies at temperatures down to ~15℃ below ambient are feasible in well engineered systems, while simple low cost systems can achieve useful cooling powers in the range 5℃ to 10℃ below ambient. There are a many ways such capabilities can be put to use.

The speaker's presentation can be found here: Geoff Smith's Talk (4.6 MB PDF).

Geoff Smith is Professor Emeritus in Applied Physics at the University of Technology, Sydney Australia. He has worked on the science and applications of nanomaterials for over 30 years. His group, in partnership with local and international industry, has pioneered developments in the fields of solar energy, energy efficient windows and paints, radiative cooling, natural lighting and LED lighting. Products and several patents have followed. Key contributions to nano-photonics, thin film optics and polymer optics feature in his work with over 180 reviewed papers and several book chapters. He is chair of the Australian Standards committee on roof glazing and skylights, helped formulate Australia's recent energy efficient building codes, and has chaired an annual International Conference in the USA (SPIE - Nanostructured Thin Films) since 2006. Geoff has a number of overseas and local awards in the renewables and energy field including an honorary doctorate from the University of Uppsala in Sweden in 2003. He is a Fellow of the AIP and of the Institute of Energy.

A summary of the July lecture by Dr Jim Franklin

There is a hole in the atmosphere that can be used to cool buildings. This is important because the electricity used for air-conditioning is a major contributor to greenhouse gas emissions and building running costs. Professor Geoff Smith from UTS explained that at wavelengths below 8 micrometers, the atmosphere is opaque because of absorption from water vapour. Above 13 micrometres it is opaque because of absorption from carbon dioxide and water vapour. So for long and short wavelengths we see a hot opaque atmosphere and no radiative cooling is possible.

However, between 8 and 13 micrometers the atmosphere is fairly transparent (opacity is 17 % at the vertical, increasing to 100 % at the horizon). So at this "wavelength hole", an object can radiate its heat away through the atmosphere into space and receive little heat back from the atmosphere. The cooling effect is greatest towards the vertical. Professor Smith has shown that net-cooling powers can in principle exceed 200 W/m2. Experimental systems developed by his group can run 10°C below ambient at night and pump 135 W/m2. Or they can achieve much lower temperatures with smaller cooling powers. The key is to use an optical design in which the radiator only sees the "cool" sky at the zenith. If these systems are shielded from the direct sun, they can also give good cooling during the day. Prof Smith has investigated special selective surfaces that radiate most efficiently in the "wavelength hole" with little emission at other wavelengths. Surprisingly, these selective surfaces offer little advantage, except when one is striving for the lowest possible temperature.

Professor Smith then discussed new building materials he has worked on that can help cool buildings. With BASF and others he has helped develop special paints that reflect the infrared part of sunlight but look like ordinary pigments to the naked eye. A special white paint developed by UTS can greatly decrease solar heating. When tested on a Queensland supermarket it cut air-conditioning power consumption by two thirds.

Another interesting material described by Prof Smith is Micronal sheeting (made by BASF). This is plasterboard with a high loading of microcapsules of an alkane wax that changes phase at room temperature. This gives the material superb heat storage capabilities. A 3 cm sheet has the same heat capacity as 18 cm of concrete or 23 cm of brick. A building using this material can have enhanced comfort and reduced costs with minimal air conditioning or heating. In summer one lets in the cold night air to chill the sheets, and then uses them to cool the building during the hot part of the day. In winter, the noon sun warms the sheets, which can heat the building at night. Clearly radiative cooling and new, high tech materials have an important future in cooling buildings.

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