Joseph J. O'Gallagher

Design and test of non-evacuated solar collectors with compound parabolic concentrators

The intermediate range of concentration ratios (1.5X–10X) which can be achieved with CPCs without diurnal tracking provides both economic and thermal advantages for solar collector design even when used with non-evacuated absorbers. The present paper summarizes more than 3 yr of research on non-evacuated CPCs and reviews measured performance data and critical design considerations. Concentrations in the upper portions of the practical range (e.g. 6X) can provide good efficiency (40–50 per cent) in the 100–160°C temperature range with relatively frequent tilt adjustments (12–20 times per year). At lower concentrations (e.g. 3X) performance will still be substantially better than that for a double glazed flat plate collector above about 70°C and competitive below, while requiring only semi-annual adjustments for year round operation. In both cases the cost savings associated with inexpensive reflectors, and the optimal coupling to smaller, simple inexpensive absorbers (e.g. tubes, fins, etc.) can be as important an advantage as the improved thermal performance. The design problems for non-evacuated CPC collectors are entirely different from those for CPC collectors with evacuated receivers. For example, heat loss through the reflector can become critical, since ideal CPC optics demands that the reflector extend all the way to the absorber. Recent improvements in reflector surfaces and low cost antireflection coatings have made practical a double-glazed non-evacuated CPC design. It is calculated that a 1.5X version of such a collector would have an optical efficiency ηo = 0.71, a heat loss coefficient U = 2.2 W/m2°C and a heat extraction effciency factor F′ ≥ 0.98, while requiring no tilt adjustments.

Performance characterization of the SERI High-Flux solar furnace

Abstract This paper describes a unique new solar furnace at the Solar Energy Research Institute (SERI) that can generate a wide range of flux concentrations to support research in areas including materials processing, high-temperature detoxification and high-flux optics. The furnace is unique in that it uses a flat, tracking heliostat along with a long focal length-to-diameter (f/D) primary concentrator in an off-axis configuration. The experiments are located inside a building completely outside the beam between the heiostat and primary concentrator. The long f/D ratio of the primary concentrator was designed to take advantage of a nonimaging secondary concentrator to significantly increase the flux concentration capabilities of the system. Results are reported for both the single-stage and two-stage configurations.

A stationary evacuated collector with integrated concentrator

Abstract A comprehensive set of experimental tests and detailed optical and thermal models are presented for a newly developed solar thermal collector. The new collector has an optical efficiency of 65 per cent and achieves thermal efficiencies of better than 50 per cent at fluid temperatures of 200°C without tracking the sun. The simultaneous features of high temperature operation and a fully stationary mount are made possible by combining vacuum insulation, spectrally selective coatings, and nonimaging concentration in a novel way. These 3 design elements are “integrated” together in a self contained unit by shaping the outer glass envelope of a conventional evacuated tube into the profile of a nonimaging CRC-type concentrator. This permits the use of a first surface mirror and eliminates the need for a second cover glazing. The new collector has been given the name “Integrated Stationary Evacuated Concentrator”, or ISEC collector. Not only is the peak thermal efficiency of the ISEC comparable to that of commercial tracking parabolic troughs, but projections of the average yearly energy delivery also show competitive performance with a net gain for temperatures below 200°C. In addition, the ISEC is less subject to exposure induced degradation and could be mass produced with assembly methods similar to those used with fluorescent lamps. Since no tracking or tilt adjustments are ever required and because its sensitive optical surfaces are protected from the environment, the ISEC collector provides a simple, easily maintained solar thermal collector for the range 100–300°C which is suitable for most climates and atmospheric conditions. Potential applications include space heating, air conditioning, and industrial process heat.

Test of a “trumpet” secondary concentrator with a paraboloidal dish primary

Abstract Secondary concentrators properly designed according to the principles of nonimaging optics can increase the achievable concentration ratio of a point focus solar concentrator substantially above the level for a focussing primary alone. One such secondary is the so called “trumpet” concentrator whose hyperbolic shape is derived from the theory of the geometrical vector flux. A practical version was designed, fabricated and tested recently on the large 11-meter Paraboloidal Test Bed Concentrator operated by the Jet Propulsion Laboratory. The secondary concentration ratio was 2.1 x . That is, it reconcentrated the energy in a focal spot of 20.3 cm diameter into a circle of 14.0 cm diameter corresponding to an area reduction of 2.1 to 1. The trumpet performed as predicted by optical models. When operating with a receiver corresponding to a gross geometrical concentration ratio of 4800:1, addition of the trumpet increased the intercepted energy by 30%. Characterized in another way the trumpet increased the geometric concentration ratio from 2200:1 for the primary alone to nearly 4800:1 with an efficiency of >96%. The results of the test demonstrate that with a properly cooled secondary one can either improve the achievable concentration ratio or relax the primary tolerance requirements for essentially negligible increase in system cost or complexity.

Performance model for two-stage optical concentrators for solar thermal applications

Abstract A performance model has been developed for evaluating benefits associated with the addition of a nonimaging secondary concentrator to a conventional paraboloidal solar dish. The model uses a Monte Carlo ray-trace procedure to determine the focal plane distribution as a function of optical parameters and, by evaluating the trade-off between thermal losses and optical gain, calculates the corresponding optimized concentration and thermal efficiency as a function of temperature, both with and without the secondary. These comparative optimizations, carried out over a wide range of design parameters, show that the efficiency of a two-stage concentrator is always greater than that of a single stage if all other design parameters are the same. For example, for a reference design corresponding to a dish with a focal length to diameter ratio of 0.6 and a characteristic slope error of 5 milliradians operated at a receiver temperature of 1000°C, the optimized efficiency with a secondary is 0.70 compared to 0.59 for the primary alone. At fixed focal ratio, the relative performance advantage with a secondary increases, if either the temperature or the primary slope error or both, are increased, whereas it decreases if they are decreased. However, the advantage remains significant at temperatures above 400°C, even in the “high performance limit” of slope errors

Thermal and optical performance test results for compound parabolic concentrators (CPCs)

Abstract The primary objective of the present study was to evaluate the performance characteristics (thermal and optical) of a properly truncated CPC that could be used in two-stage solar thermal power generation systems. The CPCs selected for testing were the 5:1 cones with a 25° acceptance angle and an untruncated concentration ratio of 5.6×. Experiments were carried out at the Advanced Components Test Facility of the Georgia Tech Research Institute. Several cones of the same dimensions but with different shell materials, reflector surfaces, and employing various heat removal methods were tested. It has been demonstrated experimentally for the first time that the CPCs with high reflectivity surfaces can have optical efficiencies in the range of 90% and above. In order to verify these results, a computer ray-trace analysis was also performed. These tests have shown that passive cooling alone is adequate for small-scale, low-power systems.

Nonimaging solar concentrator with near-uniform irradiance for photovoltaic arrays

We report results of a study our group has undertaken to design a solar concentrator with uniform irradiance on a planar target. This attribute is especially important for photovoltaic concentrators. We find that a variety of optical mixers, some incorporating a moderate level of concentration, can be quite effective in achieving near uniform irradiance.

High-brightness-solar-pumped Nd:YAG laser design

We have designed a Nd:YAG laser to be pumped by the High-Flux Solar Furnace (HFSF) at the National Renewable Energy Laboratory. Based on the unique features of the HFSF, the design objectives are high brightness and superior efficiency in primary mirror area utilization. The HFSF has a primary mirror of 11.5 m2 and a 1.85 f-number. With such a high f-number, the target is set off-axis and does not block incoming solar flux. Moreover, large f-number enables concentration which approaches the theoretical limit, and a two- dimensional non-imaging concentrator deposits the solar flux onto the internal part of a 10 mm diameter laser rod. For high brightness, we plan a wide low-loss fundamental mode and a laser rod aperture that suppresses high order modes. To get a fundamental mode, of up to a 2.5 mm waist, we have designed a convex-concave resonator, following well-known g1g2 equals 0.5 design for resonators with internal beam focusing. We have used the edge ray principle to design the concentrator, and ray traced the deposited power inside the laser rod. A 1.3% Nd doping level supports a maximal power deposition inside a 5 mm diameter.

Approaching the irradiance of the sun through nonimaging optics

Through the use of nonimaging techniques, a solar concentrator is designed that increases the solar flux density achievable. (AIP)

Comparative performance features of different nonimaging secondary concentrators

In this paper, we discuss the features of different types secondary concentrators used in solar energy for dish- thermal and high flux applications. We include a preliminary comparison of a new type of nonimaging concentrator with the more traditional ideal concentrators.