CHAPTER 3: THEORETICAL ASPECTS
3.4 DIFFERENT TYPES OF SOLAR COLLECTORS
3.4.4 Concentric collectors
Concentrating collectors use mirrored surfaces to concentrate the sun's energy on an absorber called a receiver. Concentrating collectors also achieve high temperatures, but unlike evacuated-tube collectors, they can do so only when direct sunlight is available. The mirrored
surface focuses sunlight collected over a large area onto a smaller absorber area to achieve high temperatures. Some designs concentrate solar energy onto a focal point, while others concentrate the sun's rays along a thin line called the focal line. The receiver is located at the focal point or along the focal line. A heat-transfer fluid flows through the receiver and absorbs heat. These collectors reach much higher temperatures than flat-plate collectors.
However, concentrators can only focus direct solar radiation, with the result being that their performance is poor on hazy or cloudy days.
Concentrators perform best when pointed directly at the sun. To do this, these systems use tracking mechanisms to move the collectors during the day to keep them focused on the sun.
Single-axis trackers move east to west; dual-axis trackers move east and west and north and south (to follow the sun throughout the year).
Concentrators are used mostly in commercial applications because they are expensive and because the trackers need frequent maintenance. Some residential solar energy systems use parabolic-trough concentrating systems. These installations can provide hot water, space heating, and water purification. Most residential systems use single-axis trackers, which are less expensive and simpler than dual-axis trackers.
The four basic types of concentrating collectors:
Parabolic trough.
Parabolic dish.
Power tower.
Stationary concentrating collectors.
3.4.4.1 Parabolic trough
A parabolic trough is a type of solar thermal energy collector. It is constructed as a long parabolic mirror (usually coated silver or polished aluminum) with a tube running its length at the focal point. Sunlight is reflected by the mirror and concentrated on the tube. The trough is usually aligned on a north-south axis, and rotated to track the sun as it moves across the sky each day. Alternatively, the trough can be aligned on an east-west axis; this reduces the overall efficiency of the collector due to cosine loss but only requires the trough to be aligned with the change in seasons, avoiding the need for tracking motors. This tracking method works correctly at the spring the focusing of the light at other times during the year
(the magnitude of this error varies throughout the day, taking a minimum value at solar noon). There is also an error introduced due to the daily motion of the sun across the sky, this error also reaches a minimum at solar noon. Due to these sources of error, seasonally adjusted parabolic troughs are generally designed with a lower solar concentration ratio.
Parabolic trough concentrators have a simple geometry, but their concentration is about 1/3 of the theoretical maximum for the same acceptance angle for the system. Approaching the theoretical maximum may be achieved by using more elaborate concentrators based on primary-secondary designs using non imaging optics .
Fig.3.5: Parabolic trough collector 3.4.4.2 Stationary concentric collector
A type of concentrating collector that uses compound parabolic reflectors and flat reflectors for directing solar energy to an accompanying absorber or aperture through a wide acceptance angle. The wide acceptance angle for these reflectors eliminates the need for a sun tracker. This class of collector includes parabolic trough flat-plate collectors, flat-plate collectors with parabolic boosting reflectors and solar cookers.
3.4.4.3 Parabolic dish
Using parabolic dishes is a well-tested approach to concentrate solar radiation, and was an early experimental tool at many locations worldwide. The optical efficiency of parabolic dishes is considerably higher than that of trough, power tower systems because the mirror is always pointed directly at the sun, whereas the trough, power tower have a reduction in
projected area due to a frequent low angle of incidence of the solar radiation. A schematic is shown in Figure [3.6].
Fig 3.6: Parabolic dish solar collector 3.4.4.4 Central tower technology
Central Tower technology is positioned as a medium maturity solar thermal technology.
Power towers capture and focus the sun's thermal energy with thousands of tracking mirrors (called heliostats). A tower resides in the centre of the heliostat field. The heliostats focus concentrated sunlight increasing up to 600 times on a receiver which sits on top of the tower.
Heat is transferred to a fluid which is pumped to a steam generator. The steam drives a standard turbine to generate electricity.
The tower technology operation is based on three characteristic elements: the heliostats, the receiver and the tower.
1) Heliostats perform the function of focusing concentrated sunlight on a receiver which sits on top of the tower. Heliostats consist of a reflective surface, a structure that serves as a support, and sun-tracking mechanisms to follow sun's movement. Currently, glass mirrors are the most reflective surfaces used.
2) Receiver, within it the concentrated sunlight heats a fluid such as water, molten salts, etc.
The heated fluid is responsible for transferring the heat to the rest of the power plant. Then, it
flows into a thermal storage tank where it is stored, and eventually pumped to a steam generator. The steam drives a standard turbine to generate electricity.
3)Tower serves from support to the receiver, it should be placed some distance above the level of the heliostats in order to avoid or at least reduce the shadows and blockades.In the continued search to improve the efficiency, it has been advanced mostly in two fronts: to reach higher temperatures and hybridize and to improve the storage.
Fig. 3.7: Central tower solar collector
1). High temperatures to improve the efficiency. High temperatures (above 1000º C) that can be reached with this technology allow aspiring to high efficiency in electricity generation, even up to 25% in the solar radiation transformation to electricity.
2). Storage is used in solar power tower systems. Heat storage allows a solar thermal plant to produce electricity at night and on overcast days.Currently, the most used solution is to transfer the heat to a thermal storage medium in an insulated reservoir during the day, and withdrawn for power generation at night. Thermal storage media include pressurized steam, concrete, a variety of phase change materials, and molten salts. It accumulates the energy to be distributed in another moment. That is why the plant is being over measured.
3). Hybridization is another improvement used in the tower technology. It is based on using other energy sources, such as biomass, to keep the plant working even with the lack of radiation. Both of the systems, storage and hybridization look for an improvement in the number of operation hours. Normally, it does not exceed 2.500 (a year has 8760 hours).