**3. Case Study: Renewable Energy Supply Scenarios for Mainau GmbH**

In the present study, we analyze four scenarios that result from an energy system optimization carried out by the ENsource partner Fraunhofer Institute for Solar Energy systems (ISE) by means of the optimization tool KomMod [28]. All scenarios imply the coupling of heat and electricity sector. First, an analysis of the renewable energy potential is conducted for the island of Mainau to identify relevant constraints. For instance, due to lack of space for wind turbines on the island, wind energy is excluded from the energy supply portfolio. KomMod then minimizes operational and investment costs under the assumption that no energy infrastructure exists. This greenfield approach shall assure a fair comparison of existing with newly installed energy technologies.

#### *3.1. Energy System Scenario Description*

The "Business as usual scenario" (BAU) corresponds to the current energy system of Mainau GmbH. Scenario 1 (S1) integrates a power-to-liquid plant to generate methanol as fuel for the company's vehicle fleet. It uses surplus energy of a photovoltaic plant (PV) installed on the car park roof on the nearby shore. This requirement corresponds to a constraint of an additional 870 MWh of electric energy (Export in Figure 2) in the optimization model. In order to operate the electrolysis continuously, battery storage is used to compensate for the fluctuations in PV electricity generation. Scenario 2 (S2) exclusively uses the PV potential available on the island. Scenario 3 (S3) additionally comprises the PV potential on the car park but—in contrast to S1—without operating the power-to-liquid plant. Table 2 summarizes the underlying potentials and constraints. The electronic supplementary material (S3\_Parameter and modeling) provides further details on capacity, generation, and full load hours per technology and scenario.

**Figure 2.** Energy supply and storage technology for all scenarios (CHP = combined heat and power plant; PV = photovoltaic plant).


**Table 2.** Potentials and restrictions for optimization of the case study scenarios.

Figure 2 illustrates the energy technology portfolios for the target year of *t* = 2050 for all scenarios that result from cost optimization. Additionally, Figure 3 provides an overview over the system configurations by means of Sankey diagrams for the energy flows.

**Figure 3.** Sankey diagrams of the system configuration.
