## 2 Parabolic Trough Solar Collectors  
These days, different types of solar collectors are widely used to harvest solar energy. Solar collectors can be separated into two essential types including stationary and tracking concentrating [13]. Stationary or non-tracking collectors include flat plate solar collectors (FPT) [14], evacuated tube collectors (ETC) [15], compound parabolic collector and bowl collector [16], which are mostly used for heating liquids. Other types of solar collectors are based on heating air, including the through-pass air collector [17] and unglazed transpired solar collector [18]. These types of collectors work based on capturing radiation from the sun and transferring thermal energy to air via conduction heat transfer. Sun tracking concentrating collectors are other types of solar thermal collectors, which are able to track the sun by either one or two axes and concentrate the sunlight on the focal point (or line). Oneaxisincludes the parabolic trough [19], cylindrical trough, and linear Fresnel collector [20], and two axes includes parabolic dish [21], parabolic tower [19], and circular Fresnel [22]. Figure 1 shows the classification of solar collectors.  
Parabolic trough solar collectors consist of a parabolic reflector. This parabolic sheet can be made by bending a sheet to parabolic shape. The sheet should be highly reflective and be constructed as a long parabolic reflecting mirror. In this type, the solar collector should be pointed directly into the Sun, and since the solar radiation is parallel, all light waves can reflect on focal from all parts of the trough-shaped as shown in Figure 2. This consequently causes the central pipe to heat up.  
The parabolic trough solar collectors consist of different parts:  
* Parabolic reflectors
* Receiver tube
* Support structure
* Tracking system  
Figure 1: Solar collectors’ classification.  
Figure 2: General conceptual form of a parabolic trough collector (PTC) [23].  
The receiver tube which is located at the focal line of parabolic trough reflective surface (Figure 3) can transfer the absorbed solar energy to the fluid flowing inside. The tube usually consists of two layers to make it more efficient. An outer layer is made out of glass which is transparent and anti-reflecting to solar radiation and can reduce convection and radiation losses and maintain strength and transmittance under high temperatures [24], as well as an inner layer of copper or selective blackened nickel which, inside of it, circulates the heat transfer fluid [25].  
Coating of the inner layer makes it capable of reducing thermal radiation losses by absorbing the short length solar radiation and also low emissivity for long wave energy spectrum [24].  
The device also includes one-axis solar tracking to align the trough with sunlight and make sure the maximum amount of radiation will be reflected on the focal line [26]. Parabolic troughs are rotated to track the Sun as it moves across the sky each day from morning to night. This is one of the crucial sections which can increase the efficiency and maximize the solar heat gain.  
The critical section of parabolic trough solar collector is the heat transfer fluid flowing inside the tube, which is usually a mixture of water or thermal oil and other additives like nanofluids which enhance thermal conduction. The fluid is pumped through the tube and absorbs the solar heat reaching temperatures of over 200 \({}^{\circ}\)C. The hot water is then directed to a heat exchanger where it can heat a storage tank for different applications.  
As mentioned above, PTC is used in large scales as solar concentrating technology. Nanofluid-based concentrating parabolic solar collectors (NCPSC) provide better energy absorption and conductive heat transfer compared with conventional parabolic trough concentrators [27]. Recently, the use of nanofluids as the working fluid in parabolic trough solar collectors (PTC) is getting more common.