# A Realistic Approach of the Maximum Work Extraction from Solar Thermal Collectors
## 3 Suggested Model  
### General Description  
In this study, a modified model about the maximum work production from the solar energy is suggested. The suggested model does not calculate the exergy efficiency of the solar irradiation, but it gives a possible maximum limit of the work production of a coupled thermal engine to the solar collector. This model takes into consideration some important limitations of the solar thermal collectors as the optical and the thermal losses. These losses have been investigated theoretically, and the minimum possible values of these losses are investigated in order to give a possible upper limit. This model is mainly focused on solar concentrating technologies (parabolic trough collector for instance), and it includes the following parameters:  
* The optical efficiency of the collector (\(\eta_{opt}\)), which is the product of the receiver absorbance, the cover transmittance, the concentrator reflectance; it also includes the impact of the incident angle.
* The radiation thermal losses of the receiver are taken into account using the emittance of the receiver (\(\epsilon_{r}\)). This parameter usually takes low values (close to 0.05-0.10) for high-quality collectors. It has been assumed the collector exchanges irradiation with the ambient and not with the sky.
* The solar irradiation is considered as heat and not irradiation in the model, something that is not performed in similar studies [13].
* The convection losses are neglected because by using vacuum between glass cover and absorber (in parabolic trough collectors, for instance), these thermal losses become negligible. This assumption is reasonable because the present work aims to determine the maximum work extraction using a real concentrating solar collector. So, this assumption corresponds to a high quality concentrating collector. In any case, an increase in the emittance (about 0.1) can be performed in order to take into account the convection thermal losses.  
Figure 1 illustrates the energy flow between the used components of the suggested model. The solar energy amount delivered to the receiver is partially absorbed due to optical losses. The receiver loses energy to the environment due to its high temperature. The useful energy obtained by the receiver is the input energy in a thermal engine, which produces useful work. This engine is assumed as a Carnot cycle in order to determine the maximum possible work extraction.  
Moreover, the heat losses of the receiver are assumed to be only from radiation. The heat convection losses have been neglected, an assumption that is more common in concentrating collectors investigation in which the radiation losses play a significant role. As it has been stated above, in case the convection losses are important, an increase in the emittance can be inserted in order to simulate the real situation. It can be assumed that the receiver is inside a huge vacuum dome. This idea is close to the evacuated tubes in which the heat convection losses are close to zero. The cover of the collector, which is a huge dome, has a temperature close to the environment, and for this reason we can assume that the receiver exchanges radiation with the environment. However, the optical losses due to the cover are taken into account in the optical loss efficiency (\(\eta_{opt}\)).