David Jafrancesco

Absorption and scattering properties of carbon nanohorn-based nanofluids for direct sunlight absorbers.

In the present work, we investigated the scattering and spectrally resolved absorption properties of nanofluids consisting in aqueous and glycol suspensions of single-wall carbon nanohorns. The characteristics of these nanofluids were evaluated in view of their use as sunlight absorber fluids in a solar device. The observed nanoparticle-induced differences in optical properties appeared promising, leading to a considerably higher sunlight absorption with respect to the pure base fluids. Scattered light was found to be not more than about 5% with respect to the total attenuation of light. Both these effects, together with the possible chemical functionalization of carbon nanohorns, make this new kind of nanofluids very interesting for increasing the overall efficiency of the sunlight exploiting device.PACS78.40.Ri, 78.35.+c, 78.67.Bf, 88.40.fh, 88.40.fr, 81.05.U.

Evaluation of Surface Slope Irregularity in Linear Parabolic Solar Collectors

The paper describes a methodology, very simple in its application, for measuring surface irregularities of linear parabolic collectors. This technique was principally developed to be applied in cases where it is difficult to use cumbersome instruments and to facilitate logistic management. The instruments to be employed are a digital camera and a grating. If the reflector surface is defective, the image of the grating, reflected on the solar collector, appears distorted. Analyzing the reflected image, we can obtain the local slope of the defective surface. These profilometric tests are useful to identify and monitor the mirror portions under mechanical stress and to estimate the losses caused by the light rays deflected outside the absorber.

Pointing Sensors and Sun Tracking Techniques

Every optical system for sunlight concentration requires following the sun in its movement. The sun tracking method is essentially chosen on the base of collection geometry and optical system configuration. A simple, useful, and original technique to realise sun tracking is proposed. It is based on a double guiding system using two complementary procedures. A passive tracking device performs a preliminary collector orientation. Then an active tracking system realises its fine positioning and adjustments exploiting an optical pointing sensor. The core of this active tracking device is the sun finder. Pointing sensors for fibre-coupled, CPV (Concentrating Photo voltaic), and linear collectors are presented, illustrating in detail the working principle and practical use. All sensors were optically characterised in laboratory, under controlled and reproducible conditions. Some field tests completed the experimentation evaluating the sensors performance in outdoor working conditions.

Optical properties of boride ultrahigh-temperature ceramics for solar thermal absorbers

Abstract. It is a known rule that the efficiency of thermodynamic solar plants increases with the working temperature. At present, the main limit in temperature upscaling is the absorber capability to withstand high temperatures. The ideal solar absorber works at high temperatures and has both a low thermal emissivity and a high absorptivity in the solar spectral range. The present work reports on the preparation and optical characterization of hafnium and zirconium diboride ultrahigh-temperature ceramics for innovative solar absorbers operating at high temperature. Spectral hemispherical reflectance from the ultraviolet to the mid-infrared wavelength region and high-temperature hemispherical emittance reveal their potential for high-temperature solar applications. Boride samples are compared with silicon carbide (SiC), a material already used in solar furnaces.

Solar Divergence Collimators for Optical Characterisation of Solar Components

Experimentation and laboratory optical tests on solar components are central aspects of the research on renewable energies. The key element of the proposed testing systems is a solar divergence collimator, which exactly reproduces in laboratory the sunlight divergence, while commercial solar simulators are mainly aimed to replicate intensity and spectrum of the sun. Precise solar divergence reproduction is essential to correctly assess the optical properties and to simulate the operative conditions of a solar collecting device. Optical characterisation and experimentation can give information about production quality and homogeneity; moreover, specific tests can address the serial production of solar components detecting defects type and location. For Concentrating Photovoltaic systems, appropriate tests can analyze solar concentrators of various shapes, dimensions, and collection features. Typically, to characterise a solar component the most important and commonly examined quantities are collection efficiency, image plane analysis, and angle dependence.

Optical design of a light-emitting diode lamp for a maritime lighthouse

Traffic signaling is an emerging field for light-emitting diode (LED) applications. This sustainable power-saving illumination technology can be used in maritime signaling thanks to the recently updated norms, where the possibility to utilize LED sources is explicitly cited, and to the availability of high-power white LEDs that, combined with suitable lenses, permit us to obtain well-collimated beams. This paper describes the optical design of a LED-based lamp that can replace a traditional lamp in an authentic marine lighthouse. This source recombines multiple separated LEDs realizing a quasi-punctual localized source. Advantages can be lower energy consumption, higher efficiency, longer life, fewer faults, slower aging, and minor maintenance costs. The proposed LED source allows us to keep and to utilize the old Fresnel lenses of the lighthouse, which very often have historical value.

Internal Lighting by Solar Collectors and Optical Fibres

Sunlight concentration on small surfaces is widely studied [1-3], experimented and mostly applied to photovoltaic power generation [4-6]. More rarely these solar collectors are coupled to optical fibres [7-9], with the advantage of always having a circular absorber shape. On the contrary the photovoltaic (PV) cell is typically squared and therefore it requires a secondary optical system to reshape the image and to improve the light distribution uniformity. The introduction of optical concentrators, especially high concentration systems, has two positive effects: it reduces the area of expensive solar cells and it increases their efficiency. The main reasons for this development are enhanced efficiency of CPV (concentrating photovoltaic) systems due to new solar cells, improved size of PV installations and increasing interest in alternative technologies, both due to government incentives and to the poor Silicon availability. In general it can be assumed that an improvement in the volume of the collection system reduces the costs, given that the system provides a higher production of energy. This chapter presents optical systems exploiting the sunlight using optical collectors and fibres to illuminate building interiors. In particular Sect. 5 describes in details a solar plant demonstrator installed to provide illumination of museum showcases [10]. This daytime lighting system was developed from design to production, installation and testing in working conditions. The light focused by the solar collector can be used either for direct illumination or to accumulate power [11] for lighting at times when there is no sunlight. The first function is obtained coupling an optical fibre to a solar collector. The second consists in focusing the solar light on a PV cell, which converts the light into electrical energy. This function has been suggested by the long closing times, typical of the museum. Due to the fact that the internal illumination was not required for hours or entire days (museum closure day), during these periods the PV cells can exploit the solar light. A solar collector with optimised features and collection performance was specifically designed for mass production to reduce costs. The evolution of solar concentrators for fibre coupling [12-13] is discussed in Sections 2-3, with theoretical and experimental comparisons based on optical tests [14] performed on the realised samples. The field tests, with direct exposition to the sun, required to design and built suitable mechanical systems to support and move the concentrators: examples of tracking systems [15-16] are reported in Sect. 4.

Optical characterisation of Carbon-Nanohorn based nanofluids for solar energy and life science applications

Single Wall Carbon Nanohorns (SWCNHs) are innovative carbon nanostructures belonging to the thriving family of carbon nanotubes and fullerenes. They have being studied up to now mainly as non-cytotoxic drug carriers [1], but they are interesting for a huge number of applications such as electronics [2], hydrogen and methane storage [3] and renewable energies. In this paper we investigate their optical properties in view of their use as direct sunlight absorbers in solar thermal devices and as reference standard in Biological Tissue Optics.

New Strategies and Simulation Tools to Optically Design a Field of Heliostats

A heliostats field is an array of mirrors concentrating the solar power on a receiver, typically placed on a tower. Our research experience in the optical design of heliostats fields for Concentrating Solar Power plants suggested using apposite simulation procedures, which were especially developed to simplify the reproduction of heliostats fields and to support in the analysis of the tower plant performances. Our most practically useful simulation tools are presented in this paper with exemplificative application results. The proposed strategies are addressed to solve specific problems and to evidence particularly crucial features. The main program facilitates the heliostats field analysis, exploiting the integration between our software code and Zemax. A side program permits to assess effects of seasonal and daily variations of solar irradiation. Another dedicated side code simplifies the selection of a Compound Parabolic Concentrator as secondary optics.

Color-coded methodology for deformable mirrors

We describe an experimental procedure for the reconstruction of the geometrical parameters of a reflecting surface. The method is based on the projection of a luminous pattern constituted by colored points. The successive chromatic selection reduces the complexity of the acquired image. We show that the colors of the points comprising the pattern are modified by the combined effect of the CCD camera and the projection system. In some situations, the chromatic components of the pattern points can generate ambiguity. Consequently, the number of colors to be projected and their chromatic separation must be studied to enable the simultaneous projection of an elevated number of points and colors.