# A Realistic Approach of the Maximum Work Extraction from Solar Thermal Collectors
## 4 Results--Discussion
### Impact of Design Parameters in the System Performance  
After presenting the general solution of the method, the next step is to present the impact of the collector parameters in the system performance. Specifically, the optical efficiency, the concentration ratio, and the absorber emittance were investigated. In order to make a general analysis, the rest of the parameters were kept constant. More specifically, the ambient temperature (\(T_{amb}\)) was selected to be 298 K, and the solar irradiation (\(G\)) was kept constant at 1000 W/m\({}^{2}\), which are typical values for the evaluation of solar collectors. This solar irradiation corresponds to the possible irradiation utilized from every collector in every case. More specifically, in flat technologies, the examined irradiation includes both beam and diffuse parts, while in concentrating only the beam part. At this point, it is important to state that the solar irradiation value of 100 W/m\({}^{2}\) is a high value that can be obtained close to solar noon in the summer period (for instance June or July). However, this is a typical value for the theoretical investigation of the solar thermal systems. Figure 4 shows the system efficiency for various collectors. Each collector is characterized by the concentration ration (C), the optical efficiency (\(\eta_{opt}\)), and the absorber emittance (\(\varepsilon_{r}\)). Figure 4a-c illustrates the system efficiency, while Figure 4d-f gives the optimum receiver temperature. It is obvious that higher concentration ratio increases the system efficiency and the optimum operating  
Figure 3: Maximum system efficiency as a function of parameter (p) for various optical efficiency values. Figure 2: Optimum values of parameter X for a great range of parameter q.  
temperature. Moreover, lower emittance leads to higher system efficiency and simultaneously to greater optimum temperature. The optical efficiency influences the system efficiency in a direct way, while it has the lower impact on the optimum temperature level. More specifically, lower optical efficiency causes the optimum temperature to decrease slightly, while the decrease in system efficiency is extremely great. The next step in this analysis is the determination of the optimum operating temperature for usual collectors. Table 1 includes the basic solar collector types and their optimum receiver temperature levels, according to the presented model. It is important to state that the emittance of the flat plate collector has selected to be greater in order to take into consideration the relative high convection losses. The optical efficiency of the compound parabolic collector was selected to be lower due to lack of tracking system. On the other hand, the optical efficiency of the flat plate collector is higher than others, because there are no reflection losses. The data of Table 1 have assumed the ambient temperature equal to 298 K and the solar irradiation equal to 1000 W/m\({}^{2}\). It is important to state that the emittance in the flat plate case is increased in order to include the heat convection losses. Figure 4: Maximum system efficiency (**a,b,c**) and optimum receiver temperature (**d,e,f**) for various values of the collector parameters. It is obvious that the higher concentration ratio makes the system efficiency and the optimum operating temperature greater. For no concentrating collectors, the temperature levels of the receiver should be in the range from 400 K to 450 K, while those for concentrating collectors should be much higher. Especially for the parabolic collector, the results proved that the optimum temperature was 884 K, and for the solar dish collectors it was 1110 K. In order to achieve these high temperatures, alternative working fluids as gas working fluids (air, nitrogen, and carbon dioxide), molten salts, or liquid sodium can be used. For the no concentrating technologies, the obtained temperature levels can be achieved by using thermal oils as working fluids and high-quality collectors.