Pedro Horta

Incidence Angle Modifiers: A General Approach for Energy Calculations

The calculation of the energy (power) delivered by a given solar collector, requires special care in the consideration of the way it handles the incoming solar radiation. Some collectors, e.g. flat plate types, are easy to characterize from an optical point of view, given their rotational symmetry with respect to the incident angle on the entrance aperture. This in contrast with collectors possessing a 2D (or cylindrical) symmetry, such as collectors using evacuated tubes or CPC collectors, requiring the incident radiation to be decomposed and treated in two orthogonal planes.

Ray-tracing software comparison for linear focusing solar collectors

Ray-Tracing software tools have been widely used in the optical design of solar concentrating collectors. In spite of the ability of these tools to assess the geometrical and material aspects impacting the optical performance of concentrators, their use in combination with experimental measurements in the framework of collector testing procedures as not been implemented, to the date, in none of the current solar collector testing standards. In the latest revision of ISO9806 an effort was made to include linear focusing concentrating collectors but some practical and theoretical difficulties emerged. A Ray-Tracing analysis could provide important contributions to overcome these issues, complementing the experimental results obtained through thermal testing and allowing the achievement of more thorough testing outputs with lower experimental requirements. In order to evaluate different available software tools a comparison study was conducted. Taking as representative technologies for line-focus concentrators the Parabolic Trough Collector and the Linear Fresnel Reflector Collector, two exemplary cases with predefined conditions – geometry, sun model and material properties – were simulated with different software tools. This work was carried out within IEA/SHC Task 49 “Solar Heat Integration in Industrial Processes”.

ESCTP: Évora solar concentrators testing platform

When applied to line-focus concentrators, the current version of ISO/FDIS 9806:2013 implies the use of the collector tracking device after a collector mounting enabling performance measurements up to 60° incidence angles along the relevant directions: transversal and longitudinal planes, in biaxial collectors. Also, the collector must be tested at near normal incidence conditions. Whereas for a Parabolic Trough collector (PTC) both conditions are met with an EW orientation, that might not be the case when dealing with e.g. Linear Fresnel Reflector collectors (LFR). For such concentrators, testing conditions require the collector to be tilted according to the latitude and mounted both in the EW and NS directions. A solar thermal concentrators testing bench, open to industry and R&D institutions, was designed and constructed at the University of Evora, having in mind the experimental testing of line-focus concentrator modules under the conditions of ISO9806:2013. The testing bench enables the application of...

Energy cost based design optimization method for medium temperature CPC collectors

CPC collectors, approaching the ideal concentration limits established by non-imaging optics, can be designed to have such acceptance angles enabling fully stationary designs, useful for applications in the low temperature range (T < 100°C). Their use in the medium temperature range (100°C < T < 250°C) typically requires higher concentration factors in turn requiring seasonal tracking strategies. Considering the CPC design options in terms of effective concentration factor, truncation, concentrator height, mirror perimeter, seasonal tracking, trough spacing, etc., an energy cost function based design optimization method is presented in this article. Accounting for the impact of the design on its optical (optical efficiency, Incidence Angle Modifier, diffuse acceptance) and thermal performances (dependent on the concentration factor), the optimization function integrates design (e.g. mirror area, frame length, trough spacing/shading), concept (e.g. rotating/stationary components, materials) and operation (...

The Use of Collector Efficiency Test Results in Long Term Performance Calculations: Revisions and Clarifications in View of Proper Collector Characterization and Inter Comparison

There are a growing number of solar thermal collector types: flat plates, evacuated tubes with and without backing reflectors and different tubular spacing, low concentration collectors, using different types of concentrating optics. These different concepts and designs all compete to be more efficient or simply cheaper, easier to operate, etc. at ever higher temperatures, and even to extend the use of solar thermal energy in other applications beyond the most common water heating for domestic purposes.

SOLAR THERMAL COLLECTORS IN POLYMERIC MATERIALS: A NOVEL APPROACH TOWARDS HIGHER OPERATING TEMPERATURES

The increasing demand for low temperature solar thermal collectors, especially for hot water production purposes in dwellings, swimming pools, hotels or industry, has lead to the possibility of high scale production, with leading manufacturers presenting yearly productions of hundreds of thousands of square meters. In such conditions, the use of polymeric materials in the manufacturing of solar collectors acquires particular interest, opening a full scope of opportunities for lower production costs, by means of cheaper materials or simpler manufacturing operations. Yet, the use of low cost materials limits the maximum operating temperatures estimated for the collectors (stagnation) to values around 120 oC, easily attainable by any simple glazed solar collector. Higher performances, leading to higher stagnation temperatures as those observed for regular metal-based solar thermal collectors, would require high temperature polymers, at a much higher cost. The present paper addresses the manufacturing of a high performance solar thermal collector based in polymeric materials and includes a base thermal study, highlighting the different possibilities to be followed in the production of a polymeric collector, as well as a description of different temperature control strategies.