Greening: from nanostructures to buildings, cities, and farms

What is it about nanotechnology that ensures it will underpin a wide range of ‘green’ products, and what exactly are green products? In our book,¹ Claes Granqvist and I systematically address such important questions. These nanotechnology efforts involve a great deal more than solar cells. Although important, solar cells are consuming disproportional intellectual effort and investment with respect to their environmental impact and cost, relative to many other beneficial options. Alternatively, I have worked with Ian Edmonds² to rigorously quantify the physical impact of a wide range of approaches to global warming mitigation year-on-year from implementation.² We indicate that a rebalancing of research, development, and investment is needed. For example, new approaches to albedo management, radiative cooling, water supply, farm salinity management, and select geo-engineering exemplify areas requiring more effort. Each approach can supply key needs, global cooling, and superior financial returns on investment, rendering them compelling scenarios.

We take an integrated research philosophy, from optics and nanoscience, through commodity scale products and processes of global impact (see Figure 1). Micro- and nanophotonics have enabled new materials and coatings, which transform the properties of everyday systems. The goal of such technologies is integration of environmental benefits with provision of basic human needs. Spinning off from our recent work on improved approaches to cooling,^3,4 our philosophy now includes a fourth tier, urban and global systems.^5,6 Relevant to this issue is controlling the external and internal impacts of buildings, urban layouts, and energy supply options, which are of increasing importance.⁷ External issues include land use changes, local microclimate, power source locations (e.g. rooftops or rural) and type, the urban heat-island problem, and (with cities growing in population and area) global warming.⁸

Figure 1. Integrated approach to green technologies and their impact: core science, new materials, products, cities and ‘whole-of-earth’ response.

Nanoscience enables holistic harmonization of material properties with the physical attributes of the environment, as well as responses of the human body to light and heat. Nature has achieved such harmonization for living organisms over millennia. Nanoscience, despite its many restraints, will facilitate further harmonization in the coming decades. The solar spectrum, the role of gases in the atmosphere on thermal emission, emission back to earth, and the response of our eyes to diffuse and direct light are all relevant to this effort. Ideally we need materials that exhibit rapid switching between spectral extremes, for which nanostructures are ideal. A well-known example is energy efficient windows for warm climates. Such windows use one or two spectral switches by transmitting daylight, absorbing or reflecting 50% of invisible, near IR solar energy and reflecting thermal radiation. Two distinct classes of nanostructured materials are used in such designs.

Less appreciated (yet equally valuable) are roof coatings with very high solar reflectance. These can be combined with either very low reflectance of thermal radiation or low IR reflectance only in the spectral region in which the atmosphere is transparent to thermal radiation (7.9μm to 13μm). These coatings enable cooler buildings and cooler cities. It is not necessary for the paint (coating) to be white. High solar reflectance with color, even black, is possible (as we first demonstrated some years ago).⁹ Colored coatings use either nanostructures or dyes and pigments with narrow absorption bands, and are now commercially available. Some of our coatings collect and store cool at night⁵ for use the next day (see Figure 2).

Figure 2. Schematic of a radiative cooling system. The type of coating or surface used on the heat exchanger is governed by the application and local climate. The cover has previously unused features that resolve many practical problems.

The convection suppressant cover in our radiative coating system was devised by Angus Gentle (University of Technology, Sydney), and it is crucial to such coatings. It combines the mechanical, water management, thermal, IR, solar, and durability properties needed for this particular outdoor task. Design optimization is challenging for most renewable and many energy saving technologies, and many fail on some of these criteria. Areas of ∼3m² are now cooling 100 liters of water to approximately 5°C below the coldest ambient temperature of the night, and this is used to keep a sizeable volume of air quite cool the next day. This type of system is scalable to buildings at low cost.

Figure 3. Thermal image at night (left) and satellite day-lit image (right) of central Sydney, plus nearby suburbs, parks, warehouses, and factory facilities near the airport (with permission of the City of Sydney), illustrating intra-city heat differentials. Red is hottest, blue is coolest, and white is medium warm.

The local benefits of having low mass, suitably coated roofs can be seen in a thermal map of Sydney (see Figure 3). The warehouse factory area is much cooler at night than the residential areas, and the central business district stays very warm. Adding practical convection management would allow elimination of cooling power altogether in many buildings. Much future work is needed to optimize approaches and further understand the microclimate and global warming implications of nanotechnology options for cooling buildings.