Maria João Carvalho

Truncation of CPC solar collectors and its effect on energy collection

Abstract Analytic expressions are derived for the angular acceptance function of two-dimensional compound parabolic concentrator solar collectors (CPCs) of arbitrary degree of truncation. Taking into account the effect of truncation on both optical and thermal losses in real collectors, we also evaluate the increase in monthly and yearly collectible energy. Prior analyses that have ignored the correct behavior of the angular acceptance function at large angles for truncated collectors are shown to be in error by 0–2% in calculations of yearly collectible energy for stationary collectors.

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.

Accuracy of the European solar water heater test procedure. Part 2: Prediction of long-term performance

This paper continues the evaluation of the European test procedure for solar water heaters, by considering what consequences the errors of the parameters (derived in Part 1, the companion paper) will have for the prediction of the long-term performance. A simple criterion is derived for the required length of the test period, as a function of climatic variables and desired accuracy of the long-term performance prediction. Typically a duration of 10 to 50 days will suffice. Since the actual conditions during a test cannot be foreseen, it will be more efficient to calculate the error of the long-term performance prediction on line at the end of each test day and terminate the series whenever acceptable accuracy is reached.

Economic Optimization of Stationary Nonevacuated CPC Solar Collectors

ABSTRACT Stationary, non-evacuated CPC solar collectors are a promising alternative to corresponding flat plate collectors in that they offer superior yearly energy delivery at comparable cost for low-temperature thermal applications. For realistic cost scenarios, we determine optimal concentrator configurations and concentration ratios, and calculate their sensivity to variations in relative component costs, climate, orientation and collector operating temperature. For CPCs that are to have the flexibility to either east-west or north–south orientation, optimized collectors are shown to have low concentrations of around 1.2, achieved by truncation from acceptance half angles of about 45°.

An experimental comparison of operating strategies for solar water systems

Abstract Three identical DHW (Domestic Hot Water) solar energy systems are experimentally compared in terms of the amount of energy collected during two extended periods. The goal of the experiment is to compare three different modes of operating the systems: (1) a single-pass mode, in which the circulating fluid passes once and only once through the collector; (2) a conventional forced circulation multipass mode, in which storage contents passes many times through the collector during each day, and (3) a thermosyphon mode. It is shown that the single-pass system performs better than the other two, with the multipass system being the worst. The thermal advantage of the single-pass mode is specially significant on days with poor radiation and can be as high as 16%, in comparison with the multipass system, being on the average 7% better. The thermosyphon system is shown to perform slightly better than the multipass, although the 1.4% difference found is within the experimental errors.

State of the art of performance evaluation methods for concentrating solar collectors

For the development and establishment of concentrating solar thermal collectors a reliable and comparable performance testing and evaluation is of great importance. To ensure a consistent performance testing in the area of low- temperature collectors a widely accepted and commonly used international testing standard (ISO 9806:2013) is already available. In contrast to this, the standard ISO 9806:2013 has not completely penetrated the testing sector of concentrating collectors yet. On that account a detailed literature review has been performed on published testing procedures and evaluation methodologies as well as existing testing standards. The review summarizes characteristics of the different steady-state, quasi-dynamic and fully dynamic testing methods and presents current advancements, assets and drawbacks as well as limitations of the evaluation procedures. Little research is published in the area of (quasi-) dynamic testing of large solar collectors and fields. As a complementary a survey has been ...

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.

Optical and thermal testing of convection reduction mechanisms in a new 1.2X CPC solar collector

A new non-evacuated solar collector of the CPC type, developed and manufactured in Portugal, is now commercially available. Its design features are unique and deserve a careful study, both of its optical and of its thermal characteristics. The optics is interesting given the unusual shape and the opportunity to test different convection suppression schemes and determine their impacts on the collectors optical performance. As for the collectors thermal behavior it is very interesting to test how simple (and potential marketable) different convection suppression ideas can improve an already very good collector from the heat loss point of view (FUL equals 4.0 W/( degree(s)C.m2)). In the course of the paper a brief description of the collector is given and testing results are presented for the testing carried out in the following situations: (1) (i) measurement of its optical and thermal performance (instantaneous efficiency curve) measured both in E.W. and N.S. collector orientation (the collector has a very wide acceptance angle allowing it to work in N.S. orientation and, thus, function in a thermosyphon mode like any regular flat plate collector, (ii) measurement of its angular acceptance function; (2) measurement of the instantaneous efficiency curve after the introduction of (i) a thin Teflon high transmissivity film below the glass cover, (ii) transparent insulation of the capillary type, inserted also under the glass cover, (iii) measurement of the acceptance angle function in this last situation. In this paper it is shown that the addition of the film reduces the heat loss coefficient by a factor of 1.3 W/( degree(s)C.m2) and the transparent insulation leads only to an improvement of 1.0 W/( degree(s)C.m2) in that same coefficient.

UNISOL: solar combistore evaluation and optimization

Abstract In the frame of UNISOL project, a test bench was installed to perform tests of a combistore which includes a two-way heat exchanger already submitted to a national patent application. The present work describes the main characteristics of the test bench installed and the tests performed with the objective of optimization of the configuration of the inner storage tank of the combistore (two way heat exchanger), used for DHW pre-heating or as back-up of the space heating. Tests according to EN 12977-3:2012 were performed in order to evaluate distinct configurations of the inner storage tank. Tests according to EN 12977- 4:2012 were also performed for characterization of the complete combistore. Long-term performances of solar thermal systems using these combistore are presented. Long-term performance prediction based on testing results according to EN 12977-3:2012, showed how an increased active volume of the heat exchanger enhances the energy available for space heating, although it may decrease solar collector thermal performance and increase the energy losses of the combistore. Long-term performance prediction based on testing results according to EN 12977-4:2012, considering a lower heat loss coefficient since a better insulation of the combistore is expected in future prototypes, showed fsav values for Davos and Wurzburg of 39.3% and 25.3%, respectively.