*6.2.1. Linear Fresnel reflector*

These collectors can be divided into two types: nonconcentrating (NC) and concentrating

• Heat transfer fluid (HTF) can achieve higher temperatures because the solar beam is fo-

• The thermal efficiency is greater because of the small heat-loss area relative to the receiver area. • With these systems, the HTF reduces exergetic losses through the STT; it means that the interchange between the fluid temperature and environment is less according to the smaller

• For a CC, the cost per unit area of the solar collecting surface is cheaper than of a NC collector.

• In a cloudy day, it may not work; it means, without solar radiation, these systems do not

• Concentrator systems collect little diffuse radiation depending on the concentration ratio. • Some form of tracking system is required to follow the Sun, generating more monetary and

• Solar reflecting surfaces may lose their reflectance with time and require periodic cleaning

Even with these characteristics, not all solar collectors have the same design, depending on different concentrators and receivers; for that reason, they are divided into: linear fresnel reflector (LFR), parabolic trough collector (PTC), parabolic dish reflector (PDR) and central receiver (CR); finally, they are subdivided into solar tower (ST) and solar furnace

The **Figure 6(a)** shows the focus method to achieve the recollected energy of sun and increase the efficiency of collector; the PTC and PD are divided into mobile receivers; it means that the collector and the receiver need a tracking sun technology. On the other hand, LFR, SF, and ST are classified as fixed receivers because the receiver is static. In **Figure 6(b)**, a graph is shown comparing the concentration ratio with the temperature that can be achieved. LFR has a concentration ratio from 10 to 40 units, and a temperature ranges from 60 to 250°C. PTC increases its concentration compared with LFR, between 15 and 45 units; thus, the temperature range can reach from 60 to 300°C. On one hand, ST achieves a concentration ratio of 300–600 and can achieve a temperature above 800°C. On the other hand, PD has a concentration ratio between 100 and 1000 and a temperature range from 100 to 1600°C. Furthermore, SF has the highest concentration range of all STT, reaching 10,000 units; thus, this technology can arrive

(CC). Then, it describes their advantages and disadvantages [48, 49].

Advantages

cused on a point.

54 Sustainable Air Conditioning Systems

area of the receiver.

transform energy.

energetic costs.

and refurbishing.

temperatures above 2000°C [48, 49].

(SF) [48, 49].

Disadvantages

LFR is a type of solar collector that collects sunlight by adapting long, narrow or slightly curved mirrors to reflect the Sun's ray into an absorber tube and concentrate that energy. They usually use water as HTF, which passes through the receiver and change to steam. Considering that the focal line in the LFR can be distorted by astigmatism of the mirror, usually a secondary mirror is placed above the receiver to refocus the Sun´s ray [6]. They have the advantages of easy manufacture and maintenance, and low cost comparing with other solar thermal technologies. However, these systems have the disadvantage of losing some portion of reflector aperture because the arrangement between each other blocks part of the sunlight and affecting their efficiency [50].
