Modeling and Optimization of Evacuated Tubular Solar Thermal Collector

Introduction

The solar water heaters using solar thermal collectors are primarily the energy harvesters used worldwide, and the evacuated-tube designs of the collector are the most popular due to their simplicity and best overall performance over their counterpart flat-plate collectors especially in adverse weather conditions. Like flat plate collectors, evacuated tube collector utilizes both direct and diffuse part of the solar radiation. The evacuated tubular collector is more efficient than flat plate collector as the heating time required by this collector is less than its counterpart flat plate collector and also due to evacuation of the space in the annulus thereby reducing convection and conduction heat losses. Thus, an evacuated tube collector can operate at a higher temperature than a flat plate collector. Further, it can provide the same heating effect in less time than the latter collector. It has higher efficiency at low incidence angles, which tends to give it an advantage over flat plate collector in day-long performance. And in cloudy weather, the performance of ETSTC is better than solar flat plate collector. Development in evacuated tube design is important among which the water-in-glass design is very popular because of its low cost and simple manufacturing and also simple installation procedures. One more design uses a heat-pipe system with an intermediate fluid used to carry the heat from the heating elements to the tank (Kalogirou, 2004). A water-in-glass tube-based collector consists of a set of glass tubes connected to a shell or tank. Each tube is surrounded by a second glass tube of a higher diameter. The annular space between the tubes is evacuated in order to reduce the convective heat losses. Another variant of this design is the use of parabolic mirror reflectors attached underneath each tube for reflecting and concentrating radiant flux onto the tubes. The working medium in the evacuated tubes, generally water (or any better heat transfer fluid, like glycol), flows from the tank to the tubes, captures heat, and then return back to the tank by a natural flow mechanism (Morrison et al., 2004). Several types of research have been conducted to illustrate the overall performance of water-in-glass evacuated tube collectors and the results show that the overall efficiency is in the range of 50- 60% (Morrison, et al., 2005; Kim, & Seo, 2007; Zhang, & Yamaguchi, 2008). Innovative mathematical methods have been also used to investigate the performance and to find possible ways to improve existing designs of evacuated tubes (Hayek, 2009). A schematic diagram of this stationery collector based water heating system is shown in Figure 1 (a). The collector is formed into the shape of a number of tubes through which water is passed. The tubes are of two types- all glass type for e.g. Figure 1(b), and a heat pipe (copper pipe) design hermetically sealed in a vacuum-sealed borosilicate glass tube, as shown in Figure 1 (b).

Figure 1a.

Photograph of a standard evacuated

Figure 1b.

Schematic of an evacuated tube tubular collector (Courtesy MNRE website)