# Concentrating Solar Collectors for a Trigeneration System--A Comparative Study
## 2 Materials and Methods  
### The Investigated Trigeneration Unit  
In this paper, a solar-driven trigeneration configuration was examined, and Figure 1 depicts this system. There was an ORC, a single-stage absorption machine, a solar field with PTC and a sensible storage tank device. More specifically, the storage was performed by using an insulated tank which included thermal oil. The ORC is an advantaged technological cycle with a regenerative heat exchanger in order to achieve high efficiency. Generally, the use of a regenerator exploits a part of the waste energy after the turbine, and so it reduces the heat input needs in the heat recovery system. An enhancement of around 5% in efficiency can be achieved with this advantaged configuration in comparison to the simple ORC. The organic fluid was toluene and there was superheating of 20 \({}^{\circ}\)C. The pinch point (smallest temperature deviation) in the recuperator was about 10 \({}^{\circ}\)C, while in the heat recovery system (HRS) the pinch point was about 5 \({}^{\circ}\)C. Typical values were selected for the efficiency values of the turbine and the generator. More specifically, the isentropic efficiency in the turbine was equal to 85% and the electromechanical efficiency in the generator was about 97%. The single-stage absorption chiller operated with the H\({}_{2}\)O/LiBr as the working pair. The cooling product was taken by the evaporator device which operated at 5 \({}^{\circ}\)C, while the heating was produced by the condenser device which operated at 60 \({}^{\circ}\)C. The absorber device rejected the ambient and it operated at 40 \({}^{\circ}\)C. The heat exchanger effectiveness was 70% and the generator temperature was selected at 115 \({}^{\circ}\)C (after a sensitivity analysis), while the pinch point in the generator was 5 \({}^{\circ}\)C. For more details about this system, the reference [13] can be useful. The solar field was 100 m\({}^{2}\) in all the cases, and there was a storage tank of 4 m\({}^{3}\)[29]. The working fluid was thermal-oil, and more specifically the Therminol VP1, in order to operate up to 400 \({}^{\circ}\)C safely [30]. The solar collector mass flow rate was 2 kg s\({}^{-1}\), and this value was calculated by using a typical value of 0.02 kg s\({}^{-1}\) m\({}^{-2}\) for the specific flow rate [31]. The storage tank included thermal oil, and it had a cylindrical shape and a height equal to the diameter. Moreover, the storage tank thermal losses coefficient was about 0.5 W m\({}^{-2}\) K\({}^{-1}\)[32] which is a reasonable value for an insulated tank. Table 1 summarizes all the aforementioned data of the studied configuration. At this point, it is important to state that the selection of the collecting area in a specific value (100 m\({}^{2}\)) is an assumption of this work and it determines the plant size because this parameter is associated with the heat input by the sun in the system. The values of nominal electricity, heating and cooling powers are dependent on the heat input value in the system. \begin{table}
\begin{tabular}{c c} \hline
**Parameters** & **Values** \\ \hline
**Solar field and storage tank** & \\ \hline Solar field area & 100 m\({}^{2}\) \\ Collector mass flow rate & 2 kg s\({}^{-1}\) \\ Maximum operating temperature & 400 \({}^{\circ}\)C \\ Storage volume of the tank & 4 m\({}^{3}\) \\ Tank thermal loss coefficient & 0.5 W m\({}^{-2}\) K\({}^{-1}\) \\ \hline
**Organic Rankine Cycle** & \\ \hline Superheating in the turbine & 20 \({}^{\circ}\)C \\ Recuperator pinch point & 10 \({}^{\circ}\)C \\ HRS pinch point & 5 \({}^{\circ}\)C \\ Turbine isentropic efficiency & 85\% \\ Electromechanical efficiency & 97\% \\ \hline
**Absorption machine** & \\ \hline Cooling production temperature & 5 \({}^{\circ}\)C \\ Heating production temperature & 60 \({}^{\circ}\)C \\ Effectiveness of the solution heat exchanger & 70\% \\ Generator temperature & 115 \({}^{\circ}\)C \\ Absorber temperature & 40 \({}^{\circ}\)C \\ \hline \end{tabular}
\end{table} 
Table 1: Main data of the studied system.