Studies on design parameters and construction of parabolic trough concentrators for an industrial process heat system

Abstract

Line focus solar concentrating collector systems of the parabolic trough type with single axis tracking are being increasingly used for Industrial Process Heat (IPH) applications such as the production of process steam and high-temperature hot water. The economic feasibility of such systems is largely governed by the functional factors of the collector modules such as their optical and thermal efficiencies, reliability, and cost of manufacture and erection. If Solar Industrial Process Heat (SIPH) systems are to be efficient, reliable, and cost effective in meeting the requirements of developing countries like India, these have to be developed with local materials, skills, fabrication, and test facilities. Hence, studies have been carried out on the various subsystems of collector modules such as reflecting surfaces, reflector supporting structures, receiver tube configurations, and tracking systems. A variety of models of Parabolic Trough Concentrators (PTC) have been developed by commercial firms in the USA, Europe, and Japan and are utilized for SIPH systems. However, little published literature is available on the design and construction of PTC systems. There has been a proposal to design and develop a large solar steam generating system, the first of its kind, to be installed in India to generate process steam for processing silk fabrics at the Government Silk Factory in Mysore. The first step in such a system requires the design of an efficient line focus concentrator. Three PTC designs having semimonocoque, sandwich, and stiffened rib structures are chosen with a view to understanding certain experimental aspects of important parameters and to make an indepth analysis regarding their design considerations. A comparative study of these models under identical conditions has then been undertaken to determine the optimum conditions for building the required system.

Objectives of the Investigation The objectives of the present research investigation are therefore:

to analyse the design features of a PTC and identify design problems, to study the effect of static and dynamic loads on PTC structure, to analyse the effect of optical errors on PTC performance and develop techniques to measure contour errors of the three fullscale PTC modules, to develop techniques for measuring the nature of flux distribution around the receiver and to study its effect on the optical and thermal behaviour of a PTC, and to study the thermal performance of PTC modules developed for the investigation.

Static and Dynamic Load Studies Static load tests have been undertaken on the semimonocoque version by simulating wind loads corresponding to 48 kg/m² and 100 kg/m². Dynamic load tests are performed in an opencircuit wind tunnel for three different orientations of the PTC with respect to the wind stream, corresponding to wind speeds of 90 km/hr and 160 km/hr. The results show that the magnitude and the rate of increase of the induced moments are higher when the stiffened surface faces the wind stream than when the reflecting surface faces the wind stream. In addition, it has been observed that the higher the angle of incidence, the greater is the rate of increase of the turning moment with respect to wind speed. Windload tests have also been conducted on two scale models having rim angles of 52° and 92° respectively at a wind speed of 120 km/hr. Information collected on the distribution of normal forces and moments caused by varying angles of wind incidence is useful in the design of PTC structure.

Optical Error and Flux Distribution Studies Optical efficiency of PTCs is largely governed by errors arising in materials, manufacture, and operation. Errors due to manufacture have to be determined at various stages of fabrication and assembly. A special test rig and technique have been developed to measure profile and slope errors from rim to rim and along the length of a fullscale module (2 m × 3 m). This has provided sufficient information to determine which manufacturing technique produces a structure suited to the required profile accuracy. A novel technique has been realized for determining the distribution of flux around the receiver tube using lunar flux, leading to the computation of intercept factors and optical efficiencies.

Thermal Performance Studies The nature of heatflux distribution and its effect on heat transfer have been studied in detail to elucidate the mechanism of heat transfer and establish a design methodology for adopting solar concentrators as heat exchangers. Experimental data collected on a PTC have been compared with similar data on heat exchangers having flux patterns closely resembling those prevailing in a PTC. Test facilities with necessary instrumentation have been established for determining instantaneous and allday optical and thermal efficiencies of PTCs. Data on thermal loss and pressure drops are also recorded and compared with results reported in the literature.