Investigation of Small Scale Solar Concentrating Parabolic Dish with Heat Storage: [Low to Medium Temperature Applications]

Abstract

This PhD thesis focuses on the development and testing of a small scale concentrating parabolic dish with heat storage for low to medium temperature applications. The system consists of a parabolic dish solar concentrator that concentrates solar radiation, a fibrous mat solar absorber that captures concentrated solar rays and converts them to thermal energy and a packed bed with pebble rock as a thermal energy storage unit. Air is used as a heat transfer medium between the absorber and the heat storage unit. This research has targeted several issues in which there is a lack of knowledge on small scale concentrating solar energy technologies, with the work summarized in eight papers.

Paper 1 concerns experimental measurement of the dynamic temperature profiles along a rock bed heat storage unit during thermal charging and degradation. The study examined both finned and non-finned types of rock bed storages. The effects of the long fins, which are incorporated to transport heat from the bottom to the top surface of the heat storage, were investigated in relation to temperature distribution.

As an extension of Paper 1, the performance of a rock bed fitted with long fins was studied as a heat storage unit and a cooking device. The bed charging efficiency, as well as the capacity to store thermal energy and extract heat for boiling of water was discussed.

Paper 3 describes the implementation of a 1D numerical model in the MATLAB environment to simulate the transient temperature profiles of rock bed heat storage units. Conservation equations were formulated for the air, rock pebble and fins. The equations were solved on a staggered grid, and the model predicts the experimental results reasonably well.

The thesis also investigates two types of volumetric solar absorbers (a fibrous wire mesh and a ceramic) that could be incorporated with a small scale solar concentrating parabolic dish system. Both the fibrous mesh and ceramic type absorbers display a better performance, as discussed in Paper 4.

Another contribution of the PhD work is to investigate a 1D sun tracking system that could be integrated with a small scale concentrating parabolic dish. This includes the design and testing of a polar mounted electronic-based analogue and digital type sun tracking system. A PhotoVoltaic (PV) based direct sun tracking system that actuates the tracking motor based on the shading effect was also developed and tested. As described in Paper 5, the direct type of sun tracking system was found to be a very simple system which provided a sufficient tracking accuracy.

Paper 6 describes ray tracing program simulations to study the optical performance of concentrating solar reflections and absorption systems. The program is used to study the effect of the tracking error, the size of the receiver and flux distribution on different absorbers that could possibly be integrated with a parabolic dish,. The program was further extended to study the possibility of transporting concentrated solar rays from the focus of a parabolic dish to an application area via a mirror channel guide (Paper 7).

Finally, an overall test was conducted to investigate the performance of prototype for the small scale parabolic dish solar concentrator with rock bed as a heat storage unit, with the results presented in Paper 8. They show that the small scale system can capture and store solar thermal energy. As a heat transfer medium, air requires careful design and construction of the receiver, circulating fan and heat transport pipe. The small scale system yields modest storage temperatures. A scaled system with a higher concentration ratio is necessary in order to achieve higher temperatures