**6. Solar thermal technologies selection**

Around the world are different types of sources of renewable energy. However, solar thermal energy is the most abundant, and the interest in their development has increased in recent years [44, 45]. This subsection describes the technologies used with solar thermal energy, advantages and disadvantages, and a guide to choose one of them depending on the project.

### **6.1. Solar radiation**

The interest in solar energy has grown since the environmental problems caused by burning fossil fuels have become severe. One of the most important parameters for the use of solar energy is the estimate of solar radiation [46]. Solar radiation is compound of three elements: *direct radiation*, which is received direct from the sun without diffusion by the atmosphere and is used by the solar energy technologies; *diffuse radiation*, formed with sunlight diffused by atmosphere when in the sky, air molecules, dust and cloud interfere the natural path of the rays, "fragmenting" the sunlight; and *albedo*, which is the radiation reflected by the floor [43]. In order to reduce the greenhouse gases pollution, there exist technologies that transform the solar energy in thermal energy, which is called "solar thermal technologies".

#### **6.2. Solar thermal technologies**

Solar energy technologies are special kind of heat exchangers that transform solar radiation energy to thermal energy storing it in a fluid. The most important component of any solar technology is the solar collector, which is a device that absorbs the incoming solar radiation, converts it into heat, and transfers that heat to a fluid flowing through the collector [48]. These collectors can be divided into two types: nonconcentrating (NC) and concentrating (CC). Then, it describes their advantages and disadvantages [48, 49].

*6.2.1. Linear Fresnel reflector*

their efficiency [50].

*6.2.2. Parabolic trough collectors*

increasing their cost and energy supply [48].

great supply of energy to the tracking solar system [52].

*6.2.3. Parabolic dish (PD)*

*6.2.4. Solar tower (ST)*

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

Design and Construction for Hydroxides Based Air Conditioning System with Solar Collectors…

http://dx.doi.org/10.5772/intechopen.72188

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PTCs have a parabolic shape and are covered by a bending sheet of reflective material. A metal black tube, covered with a glass tube to reduce heat losses, is placed along the focal line of the receiver. Then, parallel ray's incident on the reflective material and are reflected onto the receiver where a HTF absorbs the solar power [48]. Each collector is connected together in long lines, and a system tracks the Sun´s path throughout the day along a single axis, usually east to west [49]. These technologies have the following advantages: they are feasible commercial options to transform heat, and they are the most advanced of STT because of considerable experience and have several applications [51]. One of their principal disadvantages is the tracking mechanism because it must be reliable and able to follow the Sun with a certain degree of accuracy, returning the collector to its original position at the end of the day,

The parabolic dish (PD) is one of the most important methods in solar power generation. It is a point-focus collector that tracks the Sun in two axes, concentrating solar energy onto a receiver. The receiver absorbs solar radiation, transforming it into thermal energy. The thermal energy can either be transported through piped to a central power-conversion system or it can be transformed into electricity [48]. Solar dish presents some advantages such as high efficiency, hardness against deflection and wind load, modularity, versatility, durability against moisture and temperature changes, long-term low maintenance operation, and long lifetime. There are some disadvantages such as conversion of heat into electricity is needed in the system to have moving parts, increasing maintenance and cost, and is necessary for a

This is a technology composed of multiple mirrors called heliostats distributed on a field, ordered and oriented automatically using a solar tracking system to reflect the direct radiation

of reflective surface

to a receiver situated a great height. Usually, each heliostat has 50–150 m<sup>2</sup>

## Advantages


### Disadvantages


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 (SF) [48, 49].

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 temperatures above 2000°C [48, 49].
