## 2 Materials and Methods
### Case Studies Design  
#### 2.3.1 Overview  
In order to compare various combinations of RE and RO desalination plants a big number of case studies were deployed. The main points to highlight are as follows:  
* Three main water consumption profiles were considered, 100, 600, and 2000 m\({}^{3}\)/day. These profiles were chosen since they cover the range of the small-scale desalination systems. The most commonly used technology for energy recovery is applicable from systems rated beyond 80 m\({}^{3}\)/day [42] and this is the lower of the three profiles chosen. The 600 m\({}^{3}\)/day water profile is roughly in the middle of the small scale systems and is where the prices have shown a decrease, as depicted in Figure 3. Finally, the 2000 m\({}^{3}\)/day is the highest water profile considered and in line with the real systems present in the Aegean Sea, Greece [43].
* Net-metering was considered for utilizing RE technologies when grid-connected. Under the current regulatory framework of Greece, PV net-metering installations up to 3 MW and wind turbines up to 60 kW are allowed and as such the wind turbine option was investigated only for the 100 m\({}^{3}\)/day case. It has to be highlighted that under the net-metering scheme a cost of USD 0.04/kWh still has to be paid by the end-user as a tariff. It is considered that the grid is able to accommodate any given renewables installation up to the maximum allowed by the regulatory framework.
* In order to calculate the net present cost of water, optimizations based on simulations take place (see Section 2.3.3 for more details).
* In all case studies, the size of the PV array, number of wind turbines, and capacity of the battery bank was determined through optimization. For case studies 11, 15, and 24 (see Table 1), the size of the desalination plant and the size of the water tank were also optimized.
* To see the impact of the water tank size, for the 100 m\({}^{3}\)/day group of case studies, a tank equal to 1, 3, and 10 days was considered. To make the comparison realistic, the cost of the water tank was included. Typical water tanks suitable for potable water were considered.
* One case study utilizing low-enthalpy geothermal power generation through an ORC engine has been included. Geothermal energy is not available anywhere, but where it is, it might be a very interesting choice to investigate. The ORC engine and conclusions of [44] have been utilized in order to implement this case study.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Case Study No.**} & **Water Consumption (m\({}^{3}\)/Day)** & **Nominal Water Production of Desalination Plant (m\({}^{3}\)/Day)** & **Renewable Energy Technologies** & **Type of Interconnection** & **Water Tank (Days)** \\ \hline
1 & 100 & 100 & No renewables & Grid-connected & 1 \\ \hline
2 & 100 &
\begin{tabular}{c} 100 No Energy Recovery \\ \end{tabular} & No renewables & Grid-connected & 1 \\ \hline
3 & 100 & 100 & PV &
\begin{tabular}{c} Grid-connected/Net- \\ metering \\ \end{tabular} & 1 \\ \hline
4 & 100 & 100 & Wind &
\begin{tabular}{c} Grid- \\ connected/Net- \\ metering \\ \end{tabular} & 1 \\ \hline
5 & 100 & 100 & PV & Autonomous & 1 \\ \hline
6 & 100 & 100 & Wind & Autonomous & 1 \\ \hline
7 & 100 & 100 &
\begin{tabular}{c} Hybrid (PV and \\ Wind) \\ \end{tabular} & Autonomous & 1 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Case studies.