## 4 Results
### Reference Concept  
On the basis of the compact formulated thermal models in Section 3, variation studies regarding the thermal storage efficiencies (\(\eta_{\text{LIP-TES}}\), \(\eta_{\text{HP-TES}}\)) are conducted for the reference concept, allowing to determine the total round trip efficiency, volumetric thermal storage densities, HP-TES heat exchanger size, charging-discharging fluid masses, and compressor powers.  
For this purpose, the pressure in the first stage (p\({}_{\text{LP}}\)) is set to 8.1 bar, leading to equal pressure ratios in all stages during compression. The resulting total round trip efficiencies as well as the thermal storage densities are illustrated in Figure 3 for variations simulations relating to the thermal storage efficiencies.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Temperature** & **Pressure** & **In-Pressure** & **Isentropic** & **Outlet** & **Charging–Discharging** & **Turbine** \\
**Cavern.** & **Loss Cavern** & & **Turbomach.** & **Safety Coolers** & **Duration** & **Power** \\ \hline
10 \({}^{\circ}\)C & 4 bar & 65 bar & 0.86 & 10 \({}^{\circ}\)C & 6 h/6 h & 65 MW \\ \hline \hline \end{tabular}
\end{table}
Table 1: Central specifications for system simulations.  
The results show an increase in total round trip efficiency (\(\eta_{0}\)) with increasing thermal storage efficiencies. Assuming a highly efficient HP-TES with \(\eta_{\text{HP-TES}}\) = 90%, up to 74.2% total round trip efficiency is achievable with LP-TES efficiencies of \(\eta_{\text{LP-TES}}\) = 95%. Simultaneously, a dramatic decrease in thermal storage density is visible for the regenerator-based LP-TES starting at \(\eta_{\text{LP-TES}}\) efficiencies of more than 90%. This characteristic is based on increasing requirements for high temporal averaged discharging fluid outlet temperatures, leading to a significant increased ratio of the iteratively calculated dimensionless parameters \(\Pi\) and \(\Lambda\) defined in Equations (1) and (2).  
As illustrated in Figure 3, higher LP-TES thermal storage densities are reached with lower HP-TES efficiencies. This system-related effect results from decreasing LP-TES inlet temperatures during discharging--thus increasing the total temperature spread--caused by lower HP-TES and HP-turbine outlet temperatures, respectively. At the same time, lower HP-TES efficiencies lead to decreasing HP-TES heat exchanger sizes (\(\text{kO}_{\text{TES-HTX,0}}\)) from 3100 kW/K at \(\eta_{\text{HP-TES}}\) = 90% to 860 kW/K at \(\eta_{\text{HP-TES}}\) = 70% and to decreasing HP-TES thermal storage densities from 63 kWh/m\({}^{3}\) at \(\eta_{\text{HP-TES}}\) = 90% to 49 kWh/m\({}^{3}\) at \(\eta_{\text{HP-TES}}\) = 70%, due to reduced temperature spread inside the liquid storage medium. Further effects are associated with TES efficiencies, for example, the resulting mass flow rates--and, thus, the compressor and cavern sizes--and need to be optimized in a techno-economic way.  
In order to investigate the influence of the additionally implemented P2H option on total round trip efficiency and cost reduction potential, thermal storage efficiencies of 90% are chosen for the reference case. Central results regarding the round trip efficiency (\(\eta\)), volumetric thermal storage densities (Q/V), HP-TES heat exchanger size (\(\text{kO}_{\text{TES-HTX}}\)), charging-discharging fluid masses (\(\text{m}_{\text{F}}\)), and compressor powers (\(\Sigma\text{P}_{\text{Compressor}}\)) are summarized in Table 2.