# Techno-Economic Assessment of an Innovative Small-Scale Solar-Biomass Hybrid Power Plant
## 2 Materials and Methods
### Technical Characterization  
Below are the most characteristic parameters of the plant. In terms of the power cycle, since the gross electrical power of the plant was 2 MW, it is worth noting that a non-regenerative Rankine cycle without intermediate reheating was employed. Table 1 summarizes the main characteristics of the Rankine cycle considered. Next, the parameters corresponding to the solar field of the parabolic trough collectors are presented. The solar field consisted of loops of 4 collectors with a length of 150 m each, and their main characteristics are shown in Table 2[33]. The collectors were aligned along the north-south axis. Although the final size of the solar field was determined through a techno-economic optimization process, it was initially considered that the design DNI of the solar field was 850 W/m\({}^{2}\) and was the optical efficiency corresponding to solar noon on the summer solstice. The heat transfer fluid used in the solar field was a synthetic oil. Specifically, a eutectic mixture of 73.5% diphenyl oxide and 26.5% diphenyl was considered [34]. Due to the temperature limitations of the heat transfer fluid in the boundary layer of the absorber tube, a minimum DNI of 150 W/m\({}^{2}\) was required for the operation of the solar field. The analyzed plant did not have a thermal energy storage system, as one of the aims was to demonstrate the production management capability derived from the proposed process scheme. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Parameter** & **Value** & **Units** \\ \hline Live steam temperature & 400 & \({}^{\circ}\)C \\ Live steam pressure & 41 & bar \\ Live steam flow rate & 2.8 & kg/s \\ Turbine isentropic efficiency & 0.85 & - \\ Condenser pressure & 0.1 & bar \\ Feedwater temperature & 105 & \({}^{\circ}\)C \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Main characteristics of the Rankine cycle. Figure 4: Hourly evolution of direct normal irradiance (DNI, W/m\({}^{2}\)) for 4 representative days. In terms of the hybridization configuration for steam generation, the solar steam generator (SSG) consisted of shell-and-tube heat exchangers, while the boilers used for the proposed plant were based on the cooled grate burner technology. Given that the proposed hybrid scheme involved operating the biomass boilers at partial load, it was necessary to characterize their performance. Within the range from their nominal power to 75% load, the biomass boilers are capable of generating steam at nominal conditions, with only the mass flow rate being modified. In the range from 75% to 50% load, in addition to reducing the steam mass flow rate, the boilers are unable to generate steam at nominal conditions. Instead, it was assumed that there is a linear decrease in temperature from the nominal temperature (at 75% load) to the saturation temperature (at 50% load). Finally, within the range from 50% load to the minimum load (20%), the biomass boilers generate saturated steam. The design inputs for the biomass boilers were completed with the assumption of a combustion efficiency of 88%, an excess air factor of 30%, and a furnace exit gas temperature (FEGT) of 700 \({}^{\circ}\)C. This temperature could be reached because the chosen biomass type was forest residue, which has a lower heating value of 14.63 MJ/kg on a dry basis. The main decision regarding the technical characterization of the plant in this study was determining the hybridization configuration of the two technologies used for steam generation. Traditionally, hybrid plants have been based on either series or parallel designs. Below are some of the most relevant advantages and disadvantages of these two types of hybrid configuration:  
* The main advantage of series hybridization is that it allows for an increase in the live steam temperature of the Rankine cycle, which contributes to improving the thermal efficiency of the plant [35].