Modeling and Optimization of Parabolic Trough Collector

Introduction

Parabolic trough collector is a concentrating collector of line focusing type widely used for steam cooking, water heating, and also steam power generation and desalination work (Lupfert, et al. 2011). Concentrated solar energy is a promising source of energy that is currently attracting many targeted efforts to enhance its exploitation. In particular PTC for low enthalpy processes is an emerging technology. This technology converts the solar beam radiation into useful thermal energy at the receiver unit. The PTC consists of an absorber tube, a concentric transparent cover (in order to prevent convective heat loss by maintaining thermal insulation over the absorber tube surface), and a parabolic reflector. The absorber, also called receiver tube is fixed permanently at the focus of the parabolic reflector. The generation of thermal energy for some industrial processes requires temperatures in the range 80^o–400°C. And the temperature of the PTC absorber tube can be as high as 350^o–400°C. The parabolic trough collector is placed on a rigid structure and the Sun tracking mechanism is placed on the rigid structure to track the Sun rays by the parabolic trough (or reflector). Sun tracking is an important feature of commercial PTC systems. This is done to ensure that solar radiation falls normally, i.e. at zero incidence angle, onto the aperture plane of the parabolic trough.

The first parabolic trough collector was built in the year 1936, which produced a power of 0.37 kW through a steam engine (Spencer, 1989). The parabolic trough is made from a polished sheet bent into a parabolic shape. The receiver tube is made up of copper or stainless steel having good absorptivity and less emissivity. The tube is sometimes enclosed within an outer transparent cylindrical cover to prevent thermal loss due to convection and re-radiation. It can also be coated with selective coating that has a high absorptance for solar radiation, but low emittance for thermal radiation loss. When the parabolic trough is pointed towards the Sun, solar radiation incident on the parabolic trough is reflected onto the receiver tube located at the focal plane of the trough. The working fluid (here it is water) when passes through the tube gets heated up due to conduction and convective heat transfer (i.e. conjugate heat transfer), and then the hot water is stored in the insulated hot water storage tank. Hot water is stored in the storage tank by means of thermal stratification of water, which means that the hot water temperature is lower in the bottom layers and higher in the top layers, i.e. to say that hot water temperature increases from bottom to top layers. The intermixing of hot water in the storage tank destroys the thermal stratification and after mixing uniform temperature distribution can be obtained in the hot water storage tank. The PTC system is called a line-focusing system if the collector tube is assumed to be replaced with a line at the focal plane. The parabolic trough can be oriented along East-West direction so that it can track the Sun during its traverse from East to West in the sky due South direction for any location in the Northern hemisphere, like India. The tracking can be a single axis or multi-axis tracking type (Roy, et al., 2016). Multi-axis tracking is required for point focusing systems. However, point focusing requires a complicated electronic circuitry for tracking the Sun in all directions. Obviously the cost also increases with the complex construction of the tracking system.

On the other hand, a programmed motorized unit can provide single-axis tracking. After the design of PTC and investigation of PTC performance is also essential and it depends on the various independent and dependent parameters. Improper selection of these parameters results in an error in the results which directly and indirectly the performance of PTC. Therefore, optimization of PTC parameters is essential to get higher outlet temperature (T_o), higher heat useful gain (H) and higher thermal efficiency (η_th). Further, optimization of PTC is considered to be an MCDM problem because PTC involves any number of independent parameters like alternatives and the number of dependent parameters like criteria.