David R. Mills

Compact Linear Fresnel Reflector solar thermal powerplants

This paper evaluates Compact Linear Fresnel Reflector (CLFR) concepts suitable for large scale solar thermal electricity generation plants. In the CLFR, it is assumed that there will be many parallel linear receivers elevated on tower structures that are close enough for individual mirror rows to have the option of directing reflected solar radiation to two alternative linear receivers on separate towers. This additional variable in reflector orientation provides the means for much more densely packed arrays. Patterns of alternating mirror inclination can be set up such that shading and blocking are almost eliminated while ground coverage is maximised. Preferred designs would also use secondary optics which will reduce tower height requirements. The avoidance of large mirror row spacings and receiver heights is an important cost issue in determining the cost of ground preparation, array substructure cost, tower structure cost, steam line thermal losses, and steam line cost. The improved ability to use the Fresnel approach delivers the traditional benefits of such a system, namely small reflector size, low structural cost, fixed receiver position without moving joints, and non-cylindrical receiver geometry. The modelled array also uses low emittance all-glass evacuated Dewar tubes as the receiver elements. Alternative versions of the basic CLFR concept that are evaluated include absorber orientation, absorber structure, the use of secondary reflectors adjacent to the absorbers, reflector field configurations, mirror packing densities, and receiver heights. A necessary requirement in this activity was the development of specific raytrace and thermal models to simulate the new concepts.

Very low‐emittance solar selective surfaces using new film structures

New cermet film structures suitable for selective solar absorbers, composed of two cermet sublayers with identical metal volume fractions in each sublayer, on metal reflectors with an antireflection dielectric layer coating are described. The absorbing cermet sublayers have thickness and volume fractions such that solar radiation is absorbed internally and by phase cancellation interference. They are substantially transparent in the thermal infrared region. Modeling of the new cermet layer structures has revealed a number of different selective surfaces, involving a variety of practical cermet materials, with better performance than recent published results. For example, the best predicted ratio of absorptance to normal emittance α/e at room temperature is 46 for a Cu‐SiO2 cermet, 49 for a Cu‐Al2O3 cermet and a Au‐Al2O3 cermet. The best laboratory results of α/e at room temperature thus far are 46 for a Cu‐SiO2 cermet, and 45 for a Au‐Al2O3 cermet. From computer modeling, using published experimental diel...

High efficiency MoAl2O3 cermet selective surfaces for high-temperature application

Abstract Highly efficient MoAl2O3 cermet solar absorbers have been designed with a numerical model and deposited experimentally. The typical film structure is an Al2O3 anti-reflection layer on a double MoAl2O3 cermet layer on a Mo or Cu metal thermal reflector. In numerical calculations of the thermal emittance at high temperature for these selective surfaces, the temperature dependencies of the complex refractive indices of the metal reflector and cermet in the infrared region have been considered, and the dielectric functions of the cermet materials are evaluated using Shengs approximation. An optimization calculation yields a photothermal conversion efficiency as high as 0.914 at 350°C for a concentration factor of 26 for the film structure consisting of a double cermet layer on a Mo metal thermal reflector with an Al2O3 anti-reflection coating. The corresponding normal absorptance and hemispherical emittance at 350°C are 0.96 and 0.11, respectively. MoAl2O3 cermet selective surfaces using the double cermet layer structure were deposited by vacuum co-evaporation, and an absorptance of 0.955 and near normal emittance of 0.032 at room temperature have been achieved. An emittance of 0.08 at 350°C is estimated based upon room temperature experimental data for the film structure of a double cermet layer on a Cu metal thermal reflector with an Al2O3 anti-reflection coating.

New cermet film structures with much improved selectivity for solar thermal applications

A new cermet film structure of solar thermal absorber is presented. The typical film is similar to that of the single cermet layer case, except the single cermet layer is replaced by two isotropic cermet sublayers. The calculated results have been shown that low‐ and high‐temperature performances using this new selective surface structure are excellent. The value of the ratio of absorptance to normal emittance α/e, 46, for deposited film has been achieved.

High solar performance selective surface using bi-sublayer cermet film structures

Numerical modelling calculations have been used to design solar selective absorbers with new cermet film structures, composed of two cermet sublayers, each having a different metal volume fraction, located between a conventional metal reflector and a dielectric anti-reflection layer. These selective surfaces may use a variety of practical cermet materials and all achieve better solar thermal performance than any published results. For example, our best predicted ratio of absorptance to normal emittance α/e at room temperature is 46 for a CuSiO2 cermet. The best experimental result of α/e at room temperature is also 46 for the same cermet. From a computer optimazation using published experimental dielectric functions of CoAl2O3 cermets, we clearly show improved performance is obtained by using two cermet layers rather than one. For example, an absorptance of 0.90 and normal emittance of 0.024 at 50°C (α/e = 37) could be obtained for a film composed a two cermet sublayers on a Mo reflector with an Al2O3 antireflection layer. For an optimized low emittance double cermet coating, hemispherical emittance at 350–400°C is very close to hemispherical emittance at room temperature.

Multi-tower Line Focus Fresnel Array Project

As an alternative to conventional tracking solar thermal trough systems, one may use line focus Fresnel reflector systems. In a conventional Fresnel reflector design, each field of reflectors is directed to a single tower. However, efficient systems of very high ground utilisation can be setup if a field of reflectors uses multiple receivers on different towers. This paper describes a line focus system, called the compact linear fresnel reflector system and a project to produce an initial 95 MWth solar array. The array will be used as a retrofit preheater for a coal fired generating plant.

Heliostats for maximum ground coverage

Most conventional heliostats consist of a rectangular reflector which is moved around a fixed vertical axis tracking the azimuth of the sun and a second moving horizontal axis which rotates around the vertical axis to allow tracking the elevation of the sun. The maximum ground coverage possible of fields of such heliostats without colliding neighbouring reflectors is 58%. Applications of a Multi Tower Solar Array (MTSA) may need heliostat fields with ground coverage over 90%. We propose a new type of heliostat which can be set up in fields with a ground coverage of up to 100%. The maximum possible ground coverage depends on the unimpeded space volume needed by the reflector of each heliostat. The shape and the orientation of this space volume depends on the orientation of the tracking axes and on the shape of the reflector. A horizontal orientation of the fixed axis allows higher ground coverage and heliostats with rectangular reflectors can be lined up in rows with the highest density. The row density increases the longer the reflector becomes relative to its width and approximates 100% maximum ground coverage for infinite long and slim heliostats. With specially shaped hexagonal reflectors, ground coverage up to 100% is possible.

Optical constants of amorphous hydrogenated carbon and silicon-carbon alloy films and their application in high temperature solar selective surfaces

Abstract A study was made of the alloy series a-SixC1−xHy for 1 ⩾ x ⩾ 0 prepared by a dc magnetron glow discharge in argon/silane/acetylene mixtures. Optical constants of a-CH-y and a-SixC1−xHy for three values of x were determined. The changes in the optical constants caused by heat treatment at 500°C in vacuum were investigated. Solar selective coatings with exceptionally low emittance and good high temperature stability were studied theoretically and experimentally using both single layer and multilayer coatings. A stagnation temperature of over 500°C with no concentration was predicted for some coatings, and in preliminary experiments 480°C was attained.

Relative cost-effectiveness of CPC reflector designs suitable for evacuated absorber tube solar collectors

Abstract Specular reflectors of the fixed CPC type are compared in terms of yearly energy collection and relative cost-effectiveness. The reflector designs used are designed for use with a circular-cylindrical evacuated tubular absorber, and a gap is allowed between reflector and absorber to accommodate the tubular glass envelope and evacuated space. Stainless steel, aluminum, and thin, back-silvered glass mirrors were modelled. The results show that the choice of acceptance angle of a reflector for use with a moderately priced evacuated tube at water heating temperatures is not critical; almost any reflector acceptance angle will do so long as the aperture is carefully chosen, and both North-South or East-West orientations have approximately similar performance. Under such conditions, other factors such as mirror self-cleaning and manufacturing ease may be decisive in the choice of design. At temperatures above 100°C or for high tube costs, an East-West reflector design of concentration >1.4 is strongly indicated. At the time of writing, polished stainless steel is as cost-effective a choice as any other for a mirror material, and is probably more durable and amenable to mass production.

Design of the Heliostat Field of the CSIRO Solar Tower

A close-packed heliostat field of more than 800 m 2 reflector area has been installed by Solar Heat and Power for the CSIRO solar tower at the Energy Centre in Newcastle, Australia. The heliostat field has been designed with significantly greater field packing density than normally associated with heliostat fields. It can be shown that even though a heliostat field with a high ground coverage exhibits more blocking and shading, a higher annual performance can be achieved up to a certain point. The optimum ground coverage calculated for the CSIRO solar tower configuration is in the range of 53%. Other heliostat field designs usually have ground coverage below 30%. The annual optical performance of the CSIRO field per square meters of deflector is about 9% higher than a radial stagger field of 30% ground coverage for a research tower, which was optimized to have the highest perfor- mance for the time frame from 10 a.m. until 2 p.m.