Solar thermal collectors and applications

Parabolic trough collectors

In order to deliver high temperatures with good efficiency a high performance solar collector is required. Systems with light structures and low cost technology for process heat applications up to 400 °C could be obtained with parabolic through collectors (PTCs). PTCs can effectively produce heat at temperatures between 50 and 400 °C.

PTCs are made by bending a sheet of reflective material into a parabolic shape. A metal black tube, covered with a glass tube to reduce heat losses, is placed along the focal line of the receiver (Fig. 7). When the parabola is pointed towards the sun, parallel rays incident on the reflector are reflected onto the receiver tube. It is sufficient to use a single axis tracking of the sun and thus long collector modules are produced. The collector can be orientated in an east-west direction, tracking the sun from north to south, or orientated in a north-south direction and tracking the sun from east to west. The advantages of the former tracking mode is that very little collector adjustment is required during the day and the full aperture always faces the sun at noon time but the collector performance during the early and late hours of the day is greatly reduced due to large incidence angles (cosine loss). North-south orientated troughs have their highest cosine loss at noon and the lowest in the mornings and evenings when the sun is due east or due west.

Over the period of one year, a horizontal north-south trough field usually collects slightly more energy than a horizontal east-west one. However, the north-south field collects a lot of energy in summer and much less in winter. The east-west field collects more energy in the winter than a north-south field and less in summer, providing a more

SHAPE * MERGEFORMAT

Sun rays

cover

Tracking.

mechanism

Receiver tube

Fig. 7. Schematic of a parabolic trough collector.

constant annual output. Therefore, the choice of orientation usually depends on the application and whether more energy is needed during summer or during winter [64].

Parabolic trough technology is the most advanced of the solar thermal technologies because of considerable experi­ence with the systems and the development of a small commercial industry to produce and market these systems. PTCs are built in modules that are supported from the ground by simple pedestals at either end.

PTCs are the most mature solar technology to generate heat at temperatures up to 400 °C for solar thermal electricity generation or process heat applications. The biggest application of this type of system is the Southern California power plants, known as solar electric generating systems (SEGS), which have a total installed capacity of 354 MWe [65]. More details on this system are given in Section 5.6.1. Another important application of this type of collector is installed at Plataforma Solar de Almeria (PSA) in Southern Spain mainly for experimental purposes. The total installed capacity of the PTCs is equal to 1.2 MW [66].

The receiver of a parabolic trough is linear. Usually, a tube is placed along the focal line to form an external surface receiver (Fig. 7). The size of the tube, and therefore the concentration ratio, is determined by the size of the reflected sun image and the manufacturing tolerances of the trough. The surface of the receiver is typically plated with selective coating that has a high absorptance for solar radiation, but a low emittance for thermal radiation loss.

A glass cover tube is usually placed around the receiver tube to reduce the convective heat loss from the receiver, thereby further reducing the heat loss coefficient. A disadvantage of the glass cover tube is that the reflected light from the concentrator must pass through the glass to reach the absorber, adding a transmittance loss of about 0.9, when the glass is clean. The glass envelope usually has an antireflective coating to improve transmissivity. One way to further reduce convective heat loss from the receiver tube and thereby increase the performance of the collector, particularly for high temperature applications, is to evacuate the space between the glass cover tube and the receiver.

In order to achieve cost effectiveness in mass production, not only the collector structure must feature a high stiffness to weight ratio so as to keep the material content to a minimum, but also the collector structure must be amenable to low - labour manufacturing processes. A number of structural concepts have been proposed such as steel framework structures with central torque tubes or double V-trusses, or fibreglass [67]. A recent development in this type of collectors is the design and manufacture of EuroTrough, a new PTC, in which an advance lightweight structure is used to achieve cost efficient solar power generation [68,69]. Based on environ­mental test data to date, mirrored glass appears to be the preferred mirror material although self-adhesive reflective materials with 5-7 years life exists in the market.

The design of this type of collector is given in a number of publications. The optimization of the collector aperture and rim angle is given in Ref. [59]. Design of other aspects of the collector is given in Refs. [70,71].

A tracking mechanism must be reliable and able to follow the sun with a certain degree of accuracy, return the collector to its original position at the end of the day or during the night, and also track during periods of intermittent cloud cover. Additionally, tracking mechanisms are used for the protec­tion of collectors, i. e. they turn the collector out of focus to protect it from the hazardous environmental and working conditions, like wind gust, overheating and failure of the thermal fluid flow mechanism. The required accuracy of the tracking mechanism depends on the collector acceptance angle. This is described in detail in Section 4.3.

Various forms of tracking mechanisms, varying from complex to very simple, have been proposed. They can be divided into two broad categories, namely mechanical [72-74] and electrical/electronic systems. The electronic systems generally exhibit improved reliability and tracking accuracy. These can be further subdivided into the following:

1. Mechanisms employing motors controlled electronically through sensors, which detect the magnitude of the solar illumination [75-77].

2. Mechanisms using computer controlled motors with feedback control provided from sensors measuring the solar flux on the receiver [78-80].

A tracking mechanism developed by the author uses three light dependent resistors which detect the focus, sun/cloud, and day or night conditions and give instruction to a DC motor through a control system to focus the collector, to follow approximately the sun path when cloudy conditions exist and return the collector to the east during night. More details are given in Ref. [81].

New developments in the field of PTC aim at cost reduction and improvements of the technology. In one system the collector can be washed automatically thus reducing drastically the maintenance cost.

After a period of research and commercial development of the PTC in the 80s a number of companies entered into the field producing this type of collectors, for the temperature range between 50 and 300 °C, all of them with one-axis tracking. One such example is the solar collector produced by the Industrial Solar Technology (IST) Corporation. IST erected several process heat installations in the United States with up to 2700 m2 of collector aperture area [82].

The IST parabolic trough has thoroughly been tested and evaluated by Sandia [83] and the German Aerospace Centre (DLR) [82] for efficiency and durability. Improvements of the optical performance, which recently have been discussed [84], would lead to a better incident angle modifier and a higher optical efficiency.

The characteristics of the IST collector system are shown in Table 5.