The temperature of heat transfer fluids from solar collectors can be increased if the heat loss of their receivers is reduced and if a large amount of solar radiation can be concentrated on a relatively small receiver area (high concentration ratio). Concentrating collectors have certain advantages over non-concentrating collectors. Table 1.3 shows the pros and cons of concentrating collectors.
Table 1.3: pros and cons of concentrating collectors
Pros Cons
Can have higher thermal efficiency Do not collect diffuse radiation
Able to supply high temperature heat Require a tracking system to track the sun
Have smaller cost per unit area of reflector compared to the cost of others for same energy
Reflecting surfaces lose their reflectance with time and require periodic cleaning and renewing
Require small area of receiver i.e. economically feasible to apply selective surface treatment and vacuum insulation to reduce heat losses and
Nowadays many designs of concentrating collectors are existing in different applications.
These designs can be reflector/refractor type concentrator, cylindrical/parabolic type, and continuous/segmented. These collectors may also use convex, flat, cylindrical, or concave type of receiver and the receiver may be covered with glazing to reduce heat loss. The concentration ratio of these collectors may vary from unity to high values about 10,000 [33]. However, higher concentration ratio system requires extreme precision in optical quality and positioning of the optical system. Practical concentration ratios are often in the range of 10-100.
Concentrating collector systems may apply for solar power generation and process heat production because of their capability of higher temperature energy delivery. In spite of the huge potentials for solar thermal concentrators in industrial heat supply, between 50 and 1,500°C, so far they have not been applied for more than 400°C for this purpose [34]. Among the many types of concentrators, this part of the study only covers the literature related to parabolic trough, parabolic dish and heliostat collectors and offset reflectors (Scheffler type reflector).
a) Offset (Scheffler) collector
Most parabolic concentrators have a rigid structure and their focus moves as the reflector follows the direction of the sun. This design feature has complicated and limited their applications.
Scheffler concentrator is a modified parabolic reflector design of Wolfgang Scheffler to collect solar energy with a fixed focus [35]. This collector has a primary reflector that tracks the sun and focus the solar radiation onto a fixed receiver. The focused radiation generates heat, which can be used to boil water, generate steam, cooking, bread baking and incineration. Scheffler reflector is either standing or laying type depending on the direction of its reflector’s face [36]. A standing reflector faces towards south in the northern hemisphere, and north in the southern hemisphere and gives ground level focus. However, a laying reflector faces north in the northern hemisphere, and south in southern hemisphere and gives an elevated focus. Scheffler uses a telescopic clamp mechanism to track the reflector by half of the change of the solar declination angle and to attain the required shape of the parabola for any day of the year. Figure 1.13 gives the schematic of this reflector and its tracking system.
Figure 1.13: Installation and daily tracking details of Scheffler reflector [36].
b) Parabolic trough collector
Parabolic trough collector can be made by bending a sheet of reflector into a parabolic shape to have a line focus. The line focus commonly uses a black coated pipe receiver that is sometimes covered with a vacuum glass tube to reduce heat losses. This concentrator needs one axis tracking to collect parallel incident rays and reflect them onto the receiver tube. The concentrated reflected radiation reaching the receiver tube converts in to heat and start heating the fluid that circulates through it. Figure 1.14 shows a schematic of a parabolic collector with vacuum glass covered receiver.
Figure 1.14: Schematic of a parabolic trough collector and receiver [33]
c) Parabolic dish collector
A parabolic dish reflector is a point-focus collector that tracks the sun in two axes to concentrate solar radiation onto a receiver located at its focal point. The dish fully tracks the sun to collect beam radiation and reflect them onto the receiver. The receiver then absorbs the radiant energy and converts it into thermal energy in a circulating fluid. This thermal energy can be used directly or converted into electricity. This concentrator can achieve temperatures in excess of 1,500°C on its receiver [33]. Figure 1.15 gives the schematic of parabolic dish concentrator. Compared to other concentrators parabolic dish concentrators have several advantages as shown below [33]:
1. They are the most efficient of all collector systems because of beam radiation collection.
2. Have higher concentration ratios (600–2000) and are efficient thermal energy absorber and convertor.
3. They are modular collectors that can function independently/as part of a larger system of dishes.
Figure 1.15: Schematic of parabolic dish collector [33]
d) Heliostat solar collector
Central receiver system is a module of flat or slightly curved mirrors that track the sun and focus on a central receiver as shown in Fig. 1.16. This system gives extremely high inputs of radiant energy on the receiver, which heats a working fluid that can be stored and used for continuous power production. Small-scale heliostat collectors can be an ideal collector to consider its application for large-scale solar thermal applications to satisfy industrial and households demand. For example, industrial steam supply, centralized hot water supply in highland areas of
developing countries for building heating and large scale cooking centers with heat storage. Some advantages of heliostat are [33]:
1. The single receiver minimizes thermal energy transport requirements.
2. Has higher concentration ratios (300–1500) and is highly efficient in energy collection and conversion to electricity.
3. It stores thermal energy conveniently
4. Large scale system (> 10 MW) that is economically feasible
Figure 1.16: Schematic of central receiver system [33]