Rooftop solar cells may become more acceptable if they are colored, e.g., red or bluish green, which requires that a certain part of the incoming solar spectrum be reflected. We implemented and optimized an optoelectronic model for Cu<sub>2</sub>ZnSn(S<sub>ξ</sub>Se<sub>1-ξ</sub>)<sub>4</sub> (CZTSSe) solar cells containing (i) a conventional 2200-nm-thick CZTSSe layer with homogeneous bandgap, or (ii) an ultrathin CZTSSe layer with optoelectronically optimized sinusoidally nonhomogeneous bandgap, or (iii) a CZTSSe layer with optoelectronically optimized linearly nonhomogeneous bandgap. Either complete or partial rejection of either red or bluish green photons was incorporated in the model. Calculations show that on average, the efficiency of a typical solar cell will be reduced by about 9% if 50% red photons are reflected or by about 13% if 50% blue-green photons are reflected. The efficiency reduction increases to about 18% if all red photons are reflected or about 26% if all blue-green photons are reflected.
Silicon photovoltaic solar cells generally have a black or blue appearance that makes them aesthetically very different from traditional red roofs that either comprise burned-clay tiles or composite-material shingles. Rooftop solar cells may become more acceptable if they are similar in appearance to traditional roofs. This objective requires that the red part (620–700 nm wavelength) of the incoming solar spectrum be reflected so that it becomes unavailable for photovoltaic generation of electricity. Complete reflection of red photons would result in the reduction of useful solar photons (300– 1200 nm wavelength) by 12.5%. Calculations show that the optical short-circuit density will then decline by: 17% for 100-μm-thick crystalline-silicon solar cells, 20–22% for triple-junction tandem thin-film solar cells of amorphous silicon, 15-16% for 2.2-μm-thick CIGS solar cells, and 16–20% for ultrathin CIGS solar cells. On average, the efficiency of a typical solar cell will have to be multiplied by a factor of 0.8 if all red photons were reflected. This reduction in efficiency can be offset by wider adoption of rooftop solar cells. Red-rejection filters can be made of particulate composite materials containing, say, silica nanospheres. Typically, the solar cells will be iridescent then, which may not be aesthetically pleasing to many. Non-iridescent red-rejection filters can be fabricated by upscaling the linear dimensions of biomimetic filters nano-imprinted to reproduce the Morpho blue, this possibility being guaranteed by the scale invariance of the Maxwell equations and the weak dispersion of the refractive indexes of numerous polymers in the visible spectral regime. Non-uniformly red rooftop solar cells would also become feasible.
Biologically inspired design is attracting increasing interest since it offers access to a huge biological repository of well proven design principles that can be used for developing new and innovative products. Biological phenomena can inspire product innovation in as diverse areas as mechanical engineering, medical engineering, nanotechnology, photonics, environmental protection and agriculture. However, a major obstacle for the wider use of biologically inspired design is the knowledge barrier that exist between the application engineers that have insight into how to design suitable products and the biologists with detailed knowledge and experience in understanding how biological organisms function in their environment. The biologically inspired design process can therefore be approached using different design paradigms depending on the dominant opportunities, challenges and knowledge characteristics. Design paradigms are typically characterized as either problem-driven, solution-driven, sustainability driven, bioreplication or a combination of two or more of them. The design paradigms represent different ways of overcoming the knowledge barrier and the present paper presents a review of their characterization and application.
The stinging proboscis in mosquitos have diameters of only 40-100 μm which is much less than the thinnest medical needles and the mechanics of these natural stinging mechanisms have therefore attracted attention amongst developers of injection devises. The mosquito use a range of different strategies to lower the required penetration force hence allowing a thinner and less stiff proboscis structure. Earlier studies of the mosquito proboscis insertion strategies have shown how each of the single strategies reduces the required penetration force. The present paper gives an overview of the advanced set of mechanisms that allow the mosquito to penetrate human skin and also presents other biological mechanisms that facilitate skin penetration. Results from experiments in a skin mimic using biomimetic equivalents to the natural mechanisms are presented. This includes skin stretching, insertion speed and vibration. Combining slow insertion speed with skin tension and slow vibration reduces the penetration force with 40%.
Self-organisation appeals to humans because difficult and repeated actions can be avoided through automation via
bottom-up nonhierarchical processes. This is in contrast to the top-level controlled action strategy normally applied in
automated products and in manufacturing. There are many situations where it is required that objects perform an action
dependent on external stimuli. An example is automatic window blinds that open or closes in response to sunlight level.
However, simpler and more robust designs could be made using the self-organising principles for movement found in
many plants. Plants move to adapt to external conditions, e.g. sun-flower buds tracking the sun, touch-me-not Mimosa
and Venus fly trap responding to mechanical stimuli by closing leaves to protect them and capture insects respectively.
This paper describes 3 of the basic biomimetic principles used by plants to track the sun; i) light causing an inhibiting
effect on the illuminated side causing it to bend, ii) light inducing a signal from the illuminated side that causes an action
on the darker side and iii) light illuminating a number of sensing plates pointing upwards at an angle activate an
expansion on the same side. A concept for mimicking the second principle is presented. It is a very simple and possible
reliable self-organising structure that aligns a plate perpendicular to the source of illumination.
A range of different beetles exhibits brilliant colours and metallic sheen. One of the most spectacular species is the
Plusiotis resplendens from Central America with gold metal appearance. The beetle shells are made from chitin and have
a number of unique properties that apart from spectacular aesthetic effects include metal sheen from non-metal surfaces
combined with electric and thermal insulation. The reflection mechanism has been studied by a number of authors and is
well understood. Basically there are 2 different reflection principles. One is the multilayer reflector where alternating
layers have high and low refractive index. The other is the Bouligand structure where birefringent chiral nanofibres are
organised in spiral structures. The paper describes work done to explore different approaches to mimic these structures
using polymer based materials and production methods that are suitable for more complex double curved geometry. One
approach is to use alternating layers of 2 different polymers applied by dipping and another is applying cholesteric liquid
crystals in paint. However, none of them can yet make the desired metal-looking free-form surfaces.