## 5 Conclusions  
With the further increase of renewable energies costs, flexible and large-scale electricity storage capacities are needed, allowing an increase of volatilities disposability and the participation in different markets. Adiabatic compressed air energy storage power plants extended by P2H options offer high potentials to fulfill the ambitious requirements for costs and flexibility. Based on an original two-stage A-CAES system, studies considering large variations related to thermal energy storage efficiencies, electrical heating location, power, as well as central process variables, were conducted and evaluated in order to identify suitable solutions with high potential of cost reduction and moderate losses in round trip efficiency. The results show clearly that the electrical heating system must be located in the first pressure stage to allow significant cost reduction potentials \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **Reference Concept** & \multicolumn{2}{c}{**Extended Concept**} & \multicolumn{2}{c}{**Alternative Concept**} \\ P2H **Location** & & **xLP2H = 100\%** & **xLP2H = 0\%** & **xLP2H = 100\%** \\ \hline \(\Delta\eta\)/\(\Delta\)P\({}_{\text{P2H}}\) [\%/MW\({}_{\text{P2H}}\)] & - & \(-\)0.34 & \(-\)0.34 & \(-\)0.114 \\ \(\Delta\)(Q/V)\({}_{\text{LP-TES}}\)/\(\Delta\)P\({}_{\text{P2H}}\) & - & 2.3 & \(-\)1.3 & 3.4 \\ \(\Delta\)(Q/V)\({}_{\text{LP-TES}}\)/\(\Delta\)P\({}_{\text{P2H}}\) & - & 0 & 1.65 & - \\ \(\Delta\)(Q/V)\({}_{\text{LP-TES}}\)/\(\Delta\)P\({}_{\text{P2H}}\) & - & \(-\)18 & \(-\)18 & 0.003 \\ \(\Delta\)m\({}_{\text{P2H}}\) [t/MW\({}_{\text{P2H}}\)] & - & \(-\)20.4 & \(-\)20.4 & \(-\)29.5 \\ \(\Delta\)P\({}_{\text{Compressor}}\)/\(\Delta\)P\({}_{\text{P2H}}\) & - & \(-\)0.5 & \(-\)0.5 & \(-\)0.75 \\ \([\)MW/MW\({}_{\text{P2H}}\)] & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 6: P2H specific results related to the data in Table 5. \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **Reference Concept** & \multicolumn{2}{c}{**Extended Concept**} & \multicolumn{2}{c}{**Alternative Concept**} \\ P2H **Location** & & **xLP2H = 100\%** & **xLP2H = 0\%** & **xLP2H = 100\%** \\ \hline HTS [\%] & 90 & 90 & 90 & 90 \\ P1P [bar] & 8.1 & 12.4 & 5.3 & 12.4 \\ \(\eta_{0}\) [\%] & 73.7 & 73.7 & 73.7 & 58 \\ (Q/V)\({}_{\text{UF-TES},0}\) [kWh/m3] & 66.2 & 96.3 & 48.4 & 129.7 \\ (Q/V)\({}_{\text{HP-TES},0}\) [kWh/m3] & 63 & 46.8 & 79.9 & - \\ (kQ)\({}_{\text{TES-HTX},0}\) [kW/K] & 3100 & 3100 & 3100 & 1480 \\ \({}_{\text{P2G}}\) [t] & 3450 & 3450 & 3450 & 3420 \\ \({}_{\text{CP}}\)Compressor [MW] & 88.2 & 88.2 & 88.2 & 87.3 \\ P\({}_{\text{P2H,min}}\) [MW] & - & 0 & 0 & 24.7 \\ P\({}_{\text{P2H,max}}\) [MW] & - & unbounded & 11.3 & unbounded \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Central results related to the maximum total round trip efficiency for TES efficiencies of 90%. for all components without drawbacks inside the cyclic operation. Simultaneously, the pressure level in the first stage must be chosen as high as possible in order to minimize the specific losses in total round trip efficiency through the electrical heating option. On the basis of these aspects, an alternative concept was elaborated offering, at lower total round trip efficiencies, highly efficient energetic utilization and additional cost reductions, which can be treated as a novel base for upgrading the existing CAES power plants. On the basis of the assumed large-scale specifications, P2H-dependent results for both systems were obtained, allowing the identification of promising configurations, with adequate cost models und considering revenues. Further benefits can be provided by adaptations in the thermal energy storage efficiencies, but this requires an overall techno-economic optimization. Additionally, detailed development studies were done in order to elaborate design concepts regarding the P2H option coupled with the regenerator-based thermal energy storage inside the low-pressure stage.