KEYWORDS: Solar concentrators, Solar radiation, Optical fibers, Solar thermal energy, Numerical analysis, Solar energy, Absorption, Epoxies, Cladding, Nonimaging optics
A challenge in high-temperature solar thermal applications is transfer of concentrated solar radiation to the load with minimum energy loss. The use of a solar concentrator in conjunction with optical fibres has potential advantages in terms of transmission efficiency, technical feasibility and cost-effectiveness compared to a conventional heat transfer system employing heat exchangers and a heat transfer fluid. For transferring higher levels of concentrated flux it is necessary to employ multiple optical fibres or fibre bundles. However, the losses at the incident plane of a bundle due to absorption by the epoxy and cladding between the individual fibres in a bundle are substantial, typically over 60% of the overall transmission loss. The optical transmission of the system can thus be enhanced by employing a coupler between the concentrated solar radiation and the entrance to the bundle that reflects all incident light into the cores of individual fibres rather than allowing it to strike the interstitial spaces between the cores. This paper describes the design for such couplers based on multiple compound parabolic (CP) reflectors each with its exit aperture coinciding with the core of an individual fibre within the bundle. The proposed design employs external reflection from a machined metallic aluminium surface. This CP arrangement has the additional benefit of increasing the concentration ratio of the primary solar concentrator used. Simulation modeling using LightTools is conducted into a parabolic Cassegrain solar concentrator employing these CP couplers prior to a fibre bundle. The dependence of overall transmission and total optical efficiency of the system over lengths of the bundle up to 100 m are investigated quantitatively. In addition, the influence on transmission of the angular distribution of radiation intensity at the aperture of the couplers is studied.
KEYWORDS: Manufacturing, Solar radiation, Optical fibers, Light sources, Signal attenuation, Solar energy, Spectroscopy, Objectives, Solar concentrators, Computer simulations
Employing optical fibres for transferring concentrated radiation from solar concentrators has potential advantages in terms of transmission energy efficiency, technical feasibility and cost-effectiveness compared to a conventional heat transfer system employing heat exchangers and a heat transfer fluid. The basic investigated system comprised a broadband source, collimator lens, objective lens and optical fibre as the carrier of energy to the receiver. The relationship between transmission and length of fibre is studied via simulation using the ray tracing model, LightTools®. Two different sources were defined in the system setup including a white light source and the solar simulator with similar spectral distribution as solar spectrum. The effects on transmission of varying the hydroxyl content, and the core size of the fibres are also investigated experimentally. The experimental results are then compared with simulations. The initial results indicate that the selected low OH unjacketed bulk fibre with NA=0.22 is capable of transmitting approximately 92% of the concentrated solar energy over lengths up to 10 m with less loss compared to conventional methods for direct transferring of concentrated solar radiation.
This paper reviews the different methods of directly transferring solar radiation from concentrated solar collectors to
medium to high temperature thermal absorbers, at temperatures ranging from 100 to 400°. These methods are divided
into four main categories associated with the radiation transfer medium: optical fibres, photonic crystal fibres, metal
waveguides and light guides. The reviewed methods are novel compared to most rooftop solar concentrators that have a
receiver and a thermal storage unit coupled by heat transfer fluids. Bundled optical fibres have the capability of
transferring concentrated solar energy across the full wavelength spectrum with the maximum optical efficiency. In this
study two different types of optical bundle, including hard polymer cladding silica (HPCS) and polymer clad silica (PCS)
fibres are introduced which offer a broad spectrum transmission range from 300 to 1700 nm, low levels of losses through
attenuation and the best resistance to heating. These fibres are able to transmit about 94% of the solar radiation over a
distance of 10 m. The main parameters that determine the overall efficiency of the system are the concentration ratio, the
acceptance angle of the fibres, and the matching of the diameter of the focus spot of the concentrator and the internal
diameter of the fibre. In order to maximize the coupling efficiency of the system, higher levels of concentration are
required which can be achieved through lenses or other non-imaging concentrators. However, these additional
components add to the cost and complexity of the system. To avoid this problem we use tapered bundles of optical fibres
that enhance the coupling efficiency by increasing the acceptance angle and consequently the coupling efficiency of the
system.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.