Concentrating (focusing) collectors
Logically, in order to collect the maximum radiation for each unit surface area of the collector, it is necessary to direct its surface at right angles to direct radiation direction continuation of maximum benefit by the collector during a day is possible by keeping the collector surface perpendicular to the incident solar radiation throughout the daylight. It is theoretically simple to show that a moving collector compared to the horizontal ones at the same site can collect p/2 times more energy per day. However, in practice this factor is around 1.5 times. Of course, the more the direct radiation, the better is the energy generation from the sun radiation.
If high temperatures are needed then the collector surface is manufactured as a curve for concentrating (focusing) the solar radiation at certain points by mirror or lens. Mirrors are cheaper to construct than lenses. The mirror collectors may have spherical or linear parabolic shapes as shown in Fig. 32. In a parabolic mirror solar radiation is concentrated at a point, and therefore, the concentration ratio is approximately 40,000. The concentration of one-dimensional linear parabolic system is around 200. So far as the lenses are concerned there are single surface or equivalent Fresnel types as shown in Fig. 33. Although in flat plate collectors diffuse solar radiation also makes contribution in the radiation collection, but the concentrating collectors focus the incident sunlight on the collector surface, leaving the contribution of diffuse
radiation a side. Another disadvantage of the concentrating collectors is that they must track the sun in order to obtain the optimal benefit. Concentrating collectors cause temperature rises in the heater up to 300-6000 °C. These collectors must be aligned with sufficient accuracy to ensure that the focus coincides with the collector surface. The greater the degree of concentration, the more accurate is the alignment required. Typical uses of solar radiation collectors can be grouped into four different categories depending on the purpose.
1. as a heat source for low temperatures, which may be used for domestic hot water or crop drying purposes,
2. in order to power heat engines, relatively high heat collectors can be used,
3. depending on the climate, the collectors can be used as high temperature heat to power refrigerators and air conditioners, and finally,
4. in the photovoltaic cells operation for direct electricity production.
Although the flat collectors make use of diffuse solar radiation also, but this is not possible with the concentrators where the temperatures can reach to 300-600 °C. In a concentrator collector the position must be aligned such that sufficient accuracy is ensured by directing the focus with the collector surface.
Most low-temperature solar collectors are dependent on the properties of glass which is transparent to visible light and short-wave infrared, but opaque to long wave infrared reradiated from a solar collector or building behind it. In order to benefit from daylight, and especially solar radiation as energy source, manufacturers strive for making glass as transparent as possible, by keeping the iron content down.
Optical properties of commonly used glazing materials
In Table 5, the optical properties of some commonly used glazing materials are indicated.
Line focus collectors focus the solar radiation on to a pipe running down the center of a trough which is mainly used for generating steam for electricity. To get the maximum benefit, it is necessary that the trough is pivoted to track the sun’s movement in any direction.
Point focus collectors as shown in Fig. 28 also track the sun but in two dimensions and these also generate steam for electricity. If the solar radiation is concentrated trough mirrors or lenses then over boiling temperatures may be reached for water. It is possible to use such high temperature trough in steam production for mechanical work, for instance, for water pumping or electricity generation. These are already named as high-temperature collectors. Most often parabolic mirrors are used for solar radiation concentrations. As shown in Fig. 34, all sun’s rays directed parallel to the axis of such a mirror are reflected to one point. It is necessary that the mirror tracts the sun, otherwise slightly off-axis solar beams will make inconvenient reflections, and the intensity of radiation concentration onto a point or line will be weakened. In the line focus form the sun radiation can be concentrated on a small region running along the length of the mirror. For the maximum focusing of the sun radiation, it is necessary to tract the sun in elevation only that is up and down. However, in the point
focus form, the sun radiation is reflected on a boiler in the mirror center. For optimum performance, the axis must be pointed directly at the sun all times, so it needs to tract the sun both in elevation and in azimuth.
On the other hand, another technology of centralized electricity generation is solar-thermal power. In order to reflect the sun’s rays onto an oil-filled tube this is produced by using large mirror troughs, which in turn superheats water to produce the steam that drives an electricitygenerating turbine. Since mid-1980s, about 350 MW of solar systems are installed across 3 mile2 of southern California desert, which is enough to electrify 170,000 homes. Especially, in areas of extensive pollution control, solar-thermal electricity substitution is required for the pollution protection. In order to have sufficiency and success, solar-thermal electricity production is practical in areas where there are intense and direct sunlight conditions such as the arid regions of the world.