Commentaries

Commentary: Environmental nanophotonics and energy

[+] Author Affiliations
Geoff B. Smith

University of Technology, Department of Physics and Advanced Materials and Institute of Nanoscale Technology, P.O. Box 123, Broadway NSW, 2007 Sydney Australia

J. Nanophoton. 5(1), 050301 (February 03, 2011). doi:10.1117/1.3549225
History: Received November 29, 2010; Revised December 20, 2010; Accepted December 23, 2010; Published February 03, 2011; March 03, 2011
Text Size: A A A

Open Access Open Access

Abstract

The reasons nanophotonics is proving central to meeting the need for large gains in energy efficiency and renewable energy supply are analyzed. It enables optimum management and use of environmental energy flows at low cost and on a sufficient scale by providing spectral, directional and temporal control in tune with radiant flows from the sun, and the local atmosphere. Benefits and problems involved in large scale manufacture and deployment are discussed including how managing and avoiding safety issues in some nanosystems will occur, a process long established in nature.

Figures in this Article

Solutions to the urgent challenges of environment degradation, resource depletion, growth in population, and cities, and in energy use, will rely heavily on nanoscience.1 New mindsets are needed if we are to solve these critical problems expeditiously. Nature has shown us how the complexity inherent in nanostructures enables harmony with the environment. Safety must be a concern and some nanomaterials, such as carbon nanotubes, and some uses of ZnO and TiO2 nanoparticles, may turn out to be hazardous in specific contexts. Scientists need a good overview to rationally handle recent reports about nanotechnology, claiming that the hazards to humans and the environment outweigh the benefits. An example is that just out from Friends of the Earth.2 Such cases are built around a limited set of problems, and ignore solutions, using mainly opinion-type articles as references interspersed with a few regular scientific articles. They would not survive peer review but may appear to nonexperts to cover the whole field, while ignoring the big picture. An intrinsic feature of nanotechnology, as nature has also “discovered” is diversity, so when problems arise, safe alternatives evolve. A scientific approach to identifying, avoiding, and managing technological risk has always been central to progress and will be for nanosystems.

Select bio-mimicking and replicating of natural structures can fast track some energy related developments. One example is replicating insect eyes for efficient collection of obliquely incident rays3 in various solar energy collectors. Spectral control and/or high reflectance with one-dimensional all polymer photonic crystals4 is another. Developments in nanoscience, including computer simulation, combined with greater freedom in component choice and processing conditions, means we will both match and surpass nature's ability to harmonize materials responses to environmental energy flows.

Nanophotonics is central. It can best manage the sharp spectral demarcations which occur between various key components of the primary radiative energy flows in the environment. To manage or utilize such flows efficiently we have to switch optical response, say reflectance or transmittance, between one spectral extreme to another over specific very short wavelength intervals. This process is called spectral selectivity. The full spectral range of interest covers the UV ∼300 nm to the tails of the Planck thermal radiation spectrum for surface temperatures down to around 40°C subambient (∼35 μm). Two ideal spectral goals are shown in Fig. 1. That for a solar thermal collector is well known, but much less attention has been paid to its exact opposite counterpart for cool roofs. Both ideals have one large amplitude switch, near 2.5 μm, and two widely different spectral responses. Ideal windows and skylights can have one or two transmittance switches depending on climate. Some may switch from high transmittance at the edge of the visible (∼700 nm) to high reflectance at NIR and Planck wavelengths. Others switch twice from high visible transmittance, to high NIR absorptance, to high Planck reflectance.

Graphic Jump LocationF1 :

Ideal spectral reflectance for a solar absorber (black) and a painted cool roof (green). An option for the latter with color (orange) is shown.

Night sky radiative cooling to lower than ∼7°C below ambient, needs special covers which limit convective gains but transmit IR, and requires sharp surface spectral switching inside the Planck range where the atmosphere is significantly transmitting (7.9 to 13 μm – the “sky window”).1,5 Composites of the low cost nanoparticles SiO2 and SiC in select polymer matrices or polymeric paint binders [e.g., polyethylene (PE), polyvinyl fluoride (PVF), and polyvinylidene fluoride (PVDF)] provide surface phonon resonances (SPR) which together nicely fill the sky window6 as seen in Fig. 2.

Graphic Jump LocationF2 :

Radiation emitted (blue) and atmospheric radiation absorbed (green) for 25 μm thick PE foil doped with 10% SiC nanoparticles on aluminum under a PE cover. Most absorption is in the 25 μm PE foil.

Some polymers, polymer composites, and polymer nanostructures can by themselves provide excellent sky window spectral selectivity. SiC nanoparticles are unique from an environmental radiation perspective as their SPR lies in that section of the sky window which is almost totally transmitting (10 to 13 μm). This spectral zone is thus the clearest for IR earth-based astronomy, which is why the early literature on SiC nanoparticles (common in space) is to be found in astronomy related journals.

Applications including air-conditioning where cooling is adequate to above ∼7°C below the coldest ambient of the night have higher net heat pumping. Then high emittance works best. Such surfaces also radiate strongly through the sky window, but also absorb nearly all incoming atmospheric radiation. 7°C is a rough guide as the change-over temperature from sky window spectral selectivity to high emittance depends on humidity. Lower humidity, hence lower incoming radiation, means a high emittance surface works best to even lower temperatures because sky window spectral selective surfaces benefit little from a reduction in incoming radiation as they absorb little of it in the first place as in Fig. 2. In contrast, a high emittance surface pumps a lot more net heat as the intensity of incoming thermal radiation falls. Spectral choice thus depends on application and local atmospheric conditions.

It is usually preferable to also have high solar reflectance to minimize daytime heating so cooling gets underway faster as evening approaches. Collecting heat in the day for hot water or space heating in winter, and summer cooling at night is feasible also but then no spectral switch occurs from 400 nm to 30 μm. If color is needed in combination with “cooling capability,” additional spectral switching occurs in the visible/NIR zones. Aesthetics and hence color is important in the built environment, especially in suburbia. Nanostructures, both via special sometimes resonant pigments or very thin layers on metal, have a key role in combining color, solar reflectance and high emittance.7

A simple example of nanostructure-based spectral switching is the narrow resonances in plasmonic nanoparticles whose plasma frequency is lower than that in Ag and Au, if in polymer foils or membranes for solar control glazing.8 Such is the strength of these resonances that in typical sub-millimeter foil thicknesses concentrations under 0.1% to 0.5% by weight may be sufficient, depending on exact peak location. Even lower concentrations can be used in thicker membranes and polymer sheets. An added attraction is low cost and ease of production at commodity scales. The downside is that NIR blocking is via absorption while the best thin film stacks reflect NIR and thermal radiation.

An interesting possibility is switching the nanoparticle resonance on and off according to solar conditions, for example in using VO2-based nanoparticles which switch at a phase transition temperature from plasmonic to semiconducting. Such systems have other advantages over dense VO2 thin films. They have higher visible transmittance and greater transmittance modulation.9 Insufficient visible transmittance has been a main drawback to window suppliers adopting thermochromic films for energy efficient glazing. High or even moderate optical modulation, via very fast on–off switching of plasmonic resonant response in nanoparticles, could have many uses in optics and photonics, with the added attractions of low cost and ease of implementation over large areas.

Solar energy and daylight dynamics over a year and a day means considerable changes in incident ray directions, while sky dynamics from overcast to partly cloudy to clear mean a varying admix of diffuse and specular rays coming in. Thus apart from spectral selectivity we are also interested for environmental control in three additional classes of optical responses for which nanostructures also have much to offer. These are:

  • High transmittance diffusers1 which utilize forward scattering. Nanostructures in polymer can provide the slight local index shifts needed.
  • Angular selectivity,1,10 This usually also involves polarization and optical anisotropy. Oblique columnar nanostructures can provide such responses, with details depending on whether they are all dielectric, or involve metals.
  • Switchable or “smart” surfaces (chromogenics) in which optical response changes according to glare or solar intensity. Chromogenics1 covers electrochromic, photochromic, thermochromic, thermotropic, and gasochromic materials. Response is shifted by applied electric field, UV irradiation intensity, temperature, and amount of hydrogen gas present.

The intensity of thermal radiation incident from the atmosphere also increases with angle of incidence, so oblique incidence IR response is also important. The spectrum of incoming radiation also varies with angle to the zenith. It barely changes at non-sky window IR wavelengths as the atmosphere is essentially black there at the zenith. The increase comes mainly within the sky window whose average clear sky absorption is ∼0.13 at the zenith and intensifies as the atmosphere thickens at directions closer to the horizontal. This combination of spectral and angular behavior is an interesting challenge with phonon response in nanostructures probably useful.

A strategic approach to nanophotonics solutions for this urgent problem will expedite developments and multiscale aspects of environmental nanophotonics is part of this. Another issue is excessive hype which can siphon off effort from productive avenues and is common today.11 Granting agencies thus need skills at looking behind today's inevitable “sales pitches.” Issues relevant to applications at the scales required for global impact need to be addressed. Multiscale aspects of the responses to environmental stimuli of nanobased materials is important and goes beyond the usual “near field” and “far field” issues to increasingly larger scales in various respects when deployed within the environment.

Various feedbacks occur into the response at the nanoscale from the environment itself. Optical materials after they interact with primary flows such as solar energy, can change chemically and structurally which can degrade or improve responses. This is well known but not so the reverse, in that they can alter both local and global environments, thermally, sometimes spectrally, and for humans visually. “Local” may mean within a room, around a single building, or across a neighborhood precinct. Such issues which include the urban heat island have been rarely addressed to date but are of high importance.1 As an example, coatings on a roof cannot only cool the building directly by limiting solar heat gains and enhancing radiative outflows, but indirectly by creating a colder microclimate adjacent to its walls and roof. With enough such buildings, a cooler and more pleasant suburb is possible. Both effects also reduce air conditioning demand, and may improve performance of roof-based solar cells and roof and wall-based air-conditioning units. Global environmental benefits occur if large area deployment results, which must be the ultimate goal anyway.12

The second “scale” issue concerns maintaining or creating the desired optical responses under likely production and deployment conditions. Nanophotonic and related optical systems required will need to be produced and deployed on scales of 10 to 100 million m2 per year. Nanostructures can be sensitive to the temperature and pressure changes occurring in large scale production. A well-known example is on annealing noble metal nanoparticles change shape, while noble metal thin films may develop surface nanostructures. The chemical environment present during processing may introduce problems including admission of traces of water vapor. Forming composites in polymer foil with nonspherical nanoparticles in an extrusion molding system will tend to align major axes creating distinct optical anisotropy. Such changes on processing can also be put to use if controlled, to create desired structures and responses.

In conclusion nanophotonic developments, past and future, will play a central role in saving energy and supplying renewable energy on the scales needed to combat global warming. They provide low cost options for harmonizing materials and coating responses to environmental energy flows by enabling large range spectral switching over short wavelength ranges. Safety, durability, and large scale manufacturing problems must be considered. However, the intrinsic diversity at the nanoscale is such that options which are safe, durable, and at desired scales will become available. Finally, with the right spectral responses, there appear to be multiplier benefits, not only to the quality of interiors, but to microclimates around buildings, and within whole suburbs. This is a relatively new research field with much promise, involving environmental and human responses.

References

Smith  G. B., and Granqvist  C. G,  Green Nanotechnology: Solutions for Sustainability and Energy in the Built Environment. , Chaps. 2 and 7,  CRC Press ,  Boca Raton, FL  ((2010)).
“ Nanotechnology, climate and energy: over-heated promises and hot air?. ,” http://www.foe.org, http://www.foe.org.au.
Martın-Palma  R. J., , Pantano  C. G., , and Lakhtakia  A., “ Replication of fly eyes by the con-formal-evaporated-film-by-rotation technique. ,” Nanotechnology. 19, , 355704  ((2008)).
Weber  M. F., , Stover  C. A., , Gilbert  I. R., , Nevitt  T. J., , and Ouderkirk  A. J., “ Giant birefringent optics in multilayered polymer mirrors. ,” Science. 287, , 2451–2456  ((2000)).
Smith  G. B., “ Amplified radiative cooling via optimised combinations of aperture geometry and spectral emittance profiles of surfaces and the atmosphere. ,” Sol. Energy Mater. Sol. Cells. 93, , 1696–1701  ((2009)).
Gentle  A. R., and Smith  G. B., “ Radiative heat pumping from the earth using surface phonon resonant nanoparticles. ,” Nano Lett.. 10, , 373–379  ((2010)).
Smith  G. B., , Gentle  A., , Swift  P., , Earp  A., , and Mronga  N., “ Colored paints based on coated flakes of metal as pigment for enhanced solar reflectance and cooler interiors: Description and theory. ,” Sol. Energy Mater. Sol. Cells. 79, , 163–177  ((2003)).
Schlem  S., and Smith  G. B., “ Dilute LaB6 nanoparticles in polymer as optimised clear solar control glazing. ,” Appl. Phys. Lett.. 82, , 4346–4348  ((2003)).
Li  S. Y., , Niklasson  G. A., , and Granqvist  C. G., “ Nanothermochromics: Calculations for VO2 nanoparticles in dielectric hosts show much improved luminous transmittance and solar energy transmittance modulation. ,” J. Appl. Phys.. 108, , 063525  ((2010)).
Smith  G. B., , Dligatch  S., , Sullivan  R., , and Hutchins  M. G., “ Thin Film Angular Selective Glazing (A review). ,” Sol. Energy. 62, , 229–244  ((1998)).
Lakhtakia  A., “ Editorial: Very unique omelet concocted by a head chef is a revolutionary breakthrough. ,” J. Nanophotonics. 4, , 049902  ((2010)).
Akbari  H., , Menon  S., , and Rosenfeld  A., “ Global cooling: increasing world-wide urban albedos to offset CO2. ,” Clim. Change. 94, , 275–286  ((2009)).

Biography and photograph of the author not available.

© 2011 Society of Photo-Optical Instrumentation Engineers (SPIE)

Citation

Geoff B. Smith
"Commentary: Environmental nanophotonics and energy", J. Nanophoton. 5(1), 050301 (February 03, 2011). ; http://dx.doi.org/10.1117/1.3549225


Figures

Graphic Jump LocationF1 :

Ideal spectral reflectance for a solar absorber (black) and a painted cool roof (green). An option for the latter with color (orange) is shown.

Graphic Jump LocationF2 :

Radiation emitted (blue) and atmospheric radiation absorbed (green) for 25 μm thick PE foil doped with 10% SiC nanoparticles on aluminum under a PE cover. Most absorption is in the 25 μm PE foil.

Tables

References

Smith  G. B., and Granqvist  C. G,  Green Nanotechnology: Solutions for Sustainability and Energy in the Built Environment. , Chaps. 2 and 7,  CRC Press ,  Boca Raton, FL  ((2010)).
“ Nanotechnology, climate and energy: over-heated promises and hot air?. ,” http://www.foe.org, http://www.foe.org.au.
Martın-Palma  R. J., , Pantano  C. G., , and Lakhtakia  A., “ Replication of fly eyes by the con-formal-evaporated-film-by-rotation technique. ,” Nanotechnology. 19, , 355704  ((2008)).
Weber  M. F., , Stover  C. A., , Gilbert  I. R., , Nevitt  T. J., , and Ouderkirk  A. J., “ Giant birefringent optics in multilayered polymer mirrors. ,” Science. 287, , 2451–2456  ((2000)).
Smith  G. B., “ Amplified radiative cooling via optimised combinations of aperture geometry and spectral emittance profiles of surfaces and the atmosphere. ,” Sol. Energy Mater. Sol. Cells. 93, , 1696–1701  ((2009)).
Gentle  A. R., and Smith  G. B., “ Radiative heat pumping from the earth using surface phonon resonant nanoparticles. ,” Nano Lett.. 10, , 373–379  ((2010)).
Smith  G. B., , Gentle  A., , Swift  P., , Earp  A., , and Mronga  N., “ Colored paints based on coated flakes of metal as pigment for enhanced solar reflectance and cooler interiors: Description and theory. ,” Sol. Energy Mater. Sol. Cells. 79, , 163–177  ((2003)).
Schlem  S., and Smith  G. B., “ Dilute LaB6 nanoparticles in polymer as optimised clear solar control glazing. ,” Appl. Phys. Lett.. 82, , 4346–4348  ((2003)).
Li  S. Y., , Niklasson  G. A., , and Granqvist  C. G., “ Nanothermochromics: Calculations for VO2 nanoparticles in dielectric hosts show much improved luminous transmittance and solar energy transmittance modulation. ,” J. Appl. Phys.. 108, , 063525  ((2010)).
Smith  G. B., , Dligatch  S., , Sullivan  R., , and Hutchins  M. G., “ Thin Film Angular Selective Glazing (A review). ,” Sol. Energy. 62, , 229–244  ((1998)).
Lakhtakia  A., “ Editorial: Very unique omelet concocted by a head chef is a revolutionary breakthrough. ,” J. Nanophotonics. 4, , 049902  ((2010)).
Akbari  H., , Menon  S., , and Rosenfeld  A., “ Global cooling: increasing world-wide urban albedos to offset CO2. ,” Clim. Change. 94, , 275–286  ((2009)).

Some tools below are only available to our subscribers or users with an online account.

Related Content

Customize your page view by dragging & repositioning the boxes below.

Related Book Chapters

Topic Collections

Advertisement
  • Don't have an account?
  • Subscribe to the SPIE Digital Library
  • Create a FREE account to sign up for Digital Library content alerts and gain access to institutional subscriptions remotely.
Access This Article
Sign in or Create a personal account to Buy this article ($20 for members, $25 for non-members).
Access This Proceeding
Sign in or Create a personal account to Buy this article ($15 for members, $18 for non-members).
Access This Chapter

Access to SPIE eBooks is limited to subscribing institutions and is not available as part of a personal subscription. Print or electronic versions of individual SPIE books may be purchased via SPIE.org.