# Concentrating Solar Collectors for a Trigeneration System--A Comparative Study
## 2 Materials and Methods
### Simulation Strategy  
In this study, the thermodynamic model of the studied unit was created in EES [28] and it was used for extracting useful results about the system performance. The steady-state analysis was conducted for solar irradiation at 700 W m\({}^{-2}\), for the sun angle of incidence at 30\({}^{\circ}\) and the PTC as the solar technology. The selected solar irradiation input was an energetic equivalent for operation of 2500 h per year, as it was the real net operation time of the concentrating systems in the examined location. These nominal conditions have also been used in other literature studies about Athens [18; 38], and these are representative values that take into consideration the typical operating conditions of a real operation. In other words, it has been calculated that the steady-state simulation in these conditions can lead to performance results close to the performance results by a detailed dynamic study, and thus these conditions are reasonable default conditions. At this point, it is important to state that the used model in EES has been validated in previous studies. More specifically, in Ref [13], the ORC and the absorption chiller components have been validated separately by using literature reviews by others. Concerning the solar collector performance of this work, literature equations were used and so they were adopted as valid.  
It was found that in the nominal operating conditions, the electricity, heating and cooling rates were 7.72 kW, 20.14 kW and 18.63 kW, respectively, while the heat input in the HRS was 34.25 kW. These values were assumed to be the nominal design conditions, and they were inserted in the dynamic model which was developed in the FORTRAN programming language. This language is appropriate for dynamic problems because it leads to a reasonable computation time when there is a need for many iterations for simulation of the whole year in many cases. The dynamic model was practically based on solving the energy-balance differential equations of the tank for 12 days. These were the 12 typical days of the year (1 for each month), and this method is a usual one in the literature about the proper simulation of a solar-driven system. Then, the performance of the total year was found by using only the sunny days of each month. More details about the following methodology and the used weather data of Athens (Greece) can be found in the references [39; 40]. Table 2 summarizes the aforementioned values for the nominal operating point.  
In the economic investigation, the SPP was selected as the used criterion. The specific cost of the parabolic trough collector was EUR 250/m\({}^{2}\), of the linear Fresnel reflector EUR 200/m\({}^{2}\) and of the solar dish EUR 300/m\({}^{2}\)[36]. The specific ORC cost per electricity production was selected at EUR 300/kWel, the absorption chiller specific cost per cooling production at EUR 1000/kWcool and the tank specific cost at EUR 1000/m\({}^{3}\). Furthermore, it is important to state that the electricity price was selected at EUR 0.2/kWhel, the heating price at EUR 0.1/kWhheat and the cooling price at EUR 0.067/kWhcool [41].  
Furthermore, it is worth to noting that the operation and maintenance cost was 1%, which was a reasonable value. Lastly, the input data of this financial analysis can be found in Table 3.