Cool White Polymer Coatings based on Glass Bubbles for Buildings.

While most selective emitter materials are inadequate or inappropriate for building applications, here we present a techno-economically viable optical coating by integrating glass bubbles within a polymer film. A controlled glass bubble volume concentration from 0 to 70% leads to a selective solar reflectivity increase from 0.06 to 0.92 while the mid-infrared emissivity remains above 0.85. Outdoor measurements show the polymer coating on a concrete surface can provide a temperature reduction up to 25 °C during the day when conduction and convection are limited and a net cooling power greater than 78 W/m2 at a cost less than $0.005/W. The impact of polymer coating on common buildings is estimated as potential annual energy savings of 2-12 MJ/m2 and CO2 emission savings of 0.3-1.5 kg/m2. More savings are expected for higher surface-area-to-volume-ratio buildings, and the polymer coating is also expected to resolve cooling issues for old buildings with no air conditioning

Investigating the potential of developing a selective nanofuid-based direct absorption solar collector

Solar thermal energy has a vital role to play in meeting tomorrow’s energy demand, particularly where heat is desired with minimum environmental impact (e.g. building HVAC, industrial process heat, refining minerals, etc.). Due to its high efficiency, solar thermal energy production significantly exceeds photovoltaic energy production today, although both technologies still have room for improvement. By bringing recent advancements of nanofabrication and nanotechnology to bear on solar thermal technology, it is possible to absorb sunlight directly inside the working fluid. This represents a simple, direct path from sunlight to useful heat. Unfortunately, the design of these beguilingly simple systems is rather complicated. Optics, heat transfer, fluid mechanics, and materials science issues are highly coupled, yielding an intricate design space. While some preliminary work on this topic has been reported in literature, a systematic modelling and design of such direct absorption collectors (DACs), using accurate tools, has not yet been performed. As such, this research work investigated the performance limits of nanofluid-based DACs and optimised them to develop a novel solar absorber which is competitive with conventional surface-based solar thermal absorbers. First a set of theoretical tools and experimental methods, essential for studying nanofluid based DACs were developed. These tools include important improvements from much of the published literature on DACs, enabling sound, detailed design and accurate determination of DAC performance. This is followed by a parametric study investigating the fundamental limits of selective solar absorption (maximising solar absorption and minimising emissive losses) by a DAC. For this, merit functions are derived that measure the selective performance of DAC components. It is clearly shown that (ideally) nanoparticles should strongly absorb solar radiation with minimum scattering while the basefluid should not cause long wavelength emissive losses. The cover material should let most of the solar radiation through while being able trap long wavelength radiation inside the collector. Using these fundamental limits as guidelines, the current study optimises and compares real materials for nanoparticles, basefluids, and covers. Optimisation processes are introduced to determine nanoparticle and cover parameters that maximises DAC performance. Multi-walled carbon nanotubes were found to be the best nanoparticle among current options, resulting in a strongly absorbing and cost effective nanofluid. Several other particles commonly considered in previous literature were deemed unsuitable when scattering is taken into account. All common heat transfer liquids were found to inherently, strongly emit long wavelength radiation, necessitating an approach which include optically selective covers. Thin film depositions of ZnS-Ag-ZnS and tin doped indium oxide were proposed as a solution to make common cover materials perform selectively, with their optimum film parameters depending on collector operating conditions. In outdoor tests with the optimum materials/designs defined herein, it was found that DACs made from current materials are still unable to compete with selective coatings like Black Chrome and TiNOx. Overall this research provides engineering tools, valuable insights, and identifies key areas for improvement in the field of solar thermal energy harvesting. It is hoped that these tools can be utilised and refined further as materials develop to further optimise DACs

Strain-dependent Optical and Thermal Properties of CNT films

Passive surface thermal regulation is an area of critical interest, with many potential applications ranging from the design of energy efficient building coatings to applications such as thermo camouflage. The thermoregulation of a surface requires the dynamic modulation of emissivity and reflectivity of the material; these changes are achieved via the manipulation of the surface morphology of the material. The work presented in this thesis used a strain driven CNT film to achieve the said morphology changes. In the experimental characterization of CNT films, a 0.047 increase in the average solar reflectivity was observed when strain up to 80%. When exposed to direct sunlight, this change in reflectivity was manifested as a temperature change of 2.8℃. Following the experimental work, a 3D model was developed to represent the CNT film. The developed model was simulated using raytracing under varying levels of strain, these results were used to develop a hypothesis. The developed hypothesis viewed the surface of the CNT films to be composed of many pit-like structures; the shallowing of the pit depth when strained was determined to be the driving mechanism of increased reflectivity. Later based on the developed hypothesis a simpler periodic model was created to provide an enhanced strain-dependent reflectivity increase of 0.145 in the solar region. This model was further improved by adding a silver backed layer to provide an increased averaged solar reflectivity of 0.6826 resulting in a surface temperature change of nearly 61℃

Strain-dependent Optical and Thermal Properties of CNT films

Passive surface thermal regulation is an area of critical interest, with many potential applications ranging from the design of energy efficient building coatings to applications such as thermo camouflage. The thermoregulation of a surface requires the dynamic modulation of emissivity and reflectivity of the material; these changes are achieved via the manipulation of the surface morphology of the material. The work presented in this thesis used a strain driven CNT film to achieve the said morphology changes. In the experimental characterization of CNT films, a 0.047 increase in the average solar reflectivity was observed when strain up to 80%. When exposed to direct sunlight, this change in reflectivity was manifested as a temperature change of 2.8℃. Following the experimental work, a 3D model was developed to represent the CNT film. The developed model was simulated using raytracing under varying levels of strain, these results were used to develop a hypothesis. The developed hypothesis viewed the surface of the CNT films to be composed of many pit-like structures; the shallowing of the pit depth when strained was determined to be the driving mechanism of increased reflectivity. Later based on the developed hypothesis a simpler periodic model was created to provide an enhanced strain-dependent reflectivity increase of 0.145 in the solar region. This model was further improved by adding a silver backed layer to provide an increased averaged solar reflectivity of 0.6826 resulting in a surface temperature change of nearly 61℃

Cool White Polymer Coatings based on Glass Bubbles for Buildings.

While most selective emitter materials are inadequate or inappropriate for building applications, here we present a techno-economically viable optical coating by integrating glass bubbles within a polymer film. A controlled glass bubble volume concentration from 0 to 70% leads to a selective solar reflectivity increase from 0.06 to 0.92 while the mid-infrared emissivity remains above 0.85. Outdoor measurements show the polymer coating on a concrete surface can provide a temperature reduction up to 25 °C during the day when conduction and convection are limited and a net cooling power greater than 78 W/m2 at a cost less than $0.005/W. The impact of polymer coating on common buildings is estimated as potential annual energy savings of 2-12 MJ/m2 and CO2 emission savings of 0.3-1.5 kg/m2. More savings are expected for higher surface-area-to-volume-ratio buildings, and the polymer coating is also expected to resolve cooling issues for old buildings with no air conditioning

Limits of selectivity of direct volumetric solar absorption

Direct volumetric absorption of solar radiation is possible with fluids which have controlled optical properties. As with conventional surface absorbers, it is possible to make direct absorbing collectors 'selective' where short wavelength absorption is maximised, but long wavelength emission is minimised. This work investigates the fundamental limits of this concept as it pertains to nanofluid-based direct absorbing collectors. This is especially important at higher operating temperatures (100-600. °C) where radiative losses increase significantly.A study of optical parameters of collector components is conducted herein to investigate the best theoretically (and practically) achievable 'selective' nanofluid-based direct absorbing collectors. When the effect of the short wavelength optical properties was investigated, a short wavelength optical depth of 3 was found to be sufficient for efficient absorption of solar radiation while scattering is minimised. It is also advantageous to use a base fluid which shows weak absorption at long wavelengths to reduce emission losses.Overall, this study directs future research of direct absorption by underlying theoretical and real-world limitations of a selective direct absorbing collector - an emerging receiver technology that can be used for efficient solar thermal harvesting