# Sustainable Energy Solutions: Utilising UGS for Hydrogen Production by Electrolysis
## 2 Methodology and Concept Description
### Exergy Process Description in Pressure Reduction Station at UGS Facility  
During the gas withdrawal cycle, the high-pressure gas (19.6 MPa) from the reservoir was pressure reduced via regulating valves before it entered the low-pressure gas transportation network. Each year, approximately 450 \(\times\) 10\({}^{6}\) Sm\({}^{3}\) of high-pressure gas passed through three reduction valves, with each reduction valve located on a separated and parallel gas reduction line, each with 80,000 Sm\({}^{3}\)/h of gas flow capacity. The highest feasible pressure before the reduction valve was 17 MPa with constant pressure after the reduction valve at 4.5 MPa. Prior to pressure reduction, the gas was heated to a temperature of 30-50 \({}^{\circ}\)C using gas heaters. By installing a turboexpander with a power generator in parallel to the existing pressure reduction valves lines, electricity could be generated by expanding the gas in the turboexpander using the exergy process. Turboexpanders are devices that make it possible to convert the potential energy of the high gas pressure into mechanical work and thus into electricity. They are manufactured as single-stage expanders (for smaller pressure reduction condition) or multi-stage (up to four reduction stages) for greater pressure reduction. The gas enters the expander impeller radially and exits axially. In most process plants, the work generated at the expander wheel is absorbed by a single-stage centrifugal compressor as compression is almost always required downstream of the expander to restore the gas to sale gas pressure [44]. In cases where compression is not required, the expander can drive a generator to produce electricity. Expander efficiencies are quite high, approaching 85% (isentropic) for new installations operating under design conditions. The theoretical thermodynamic path through an expander is isentropic. The theoretical work is calculated on the basis of an isentropic path and corrected to the actual using an expander efficiency (Figure 2). \begin{table}
\begin{tabular}{c c} \hline \hline \multicolumn{3}{c}{**Water Properties**} \\ \hline Salinity (NaCl) & 7352 mg/dm\({}^{3}\) \\ Electric resistivity & 0.862 Ohm-m \\ pH & 6.31 \\ Density & 1005 kg/dm\({}^{3}\) \\ TDS (Total dissolved solids) & 8720 mg/dm\({}^{3}\) \\ Cations (Na\({}^{+}\), Ca\({}^{2+}\), Mg\({}^{2+}\), Fe\({}^{3+}\)) & 3260 mg/dm\({}^{3}\) \\ Anions (Cl\({}^{-}\), HCO\({}_{3}\)\({}^{-}\), SO\({}_{4}\)\({}^{2-}\)) & 5550 mg/dm\({}^{3}\) \\ Fe (ICP) & 284 mg/dm\({}^{3}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: List of water properties. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Month/Year** & **2020** & **2019** & **2018** & **2017** & **2016** & **2015** & **2014** & **2013** \\ \hline November & 2.4 & 0 & 0.6 & 0 & 0 & 0 & 0.53 \\ \hline December & 78.2 & 0 & 53.19 & 0 & 8.2 & 0 & 0.92 & 30.03 \\ \hline January & 5  
In the case of multi-component flows and condensation across the expander, the calculation is iterative and is almost always performed on a computer. The expander work could be expressed as follows [44]:  
\[\text{W}=\text{m}\times\Delta\text{h}_{\text{isen}}\times\text{E}_{\text{isen}},\]  
where  
\(\text{E}_{\text{isen}}\)--isentropic efficiency,  
\(\text{m}\)--mass flow rate, kg/s  
\(\Delta\text{h}_{\text{isen}}\)--isentropic enthalpy change (head), kJ/kg  
W--work, kW  
A proposed model of power generation system and hydrogen production and utilisation is shown in Figure 3. The turbo-expander was installed parallel to the existing pressure reduction automatic valve ensuring that the reliability and operational efficiency of the gas withdrawal process was not compromised. During expansion in a TE, natural gas experienced rapid cooling due to the Joule-Thomson effect, with the temperature dropping by approximately 15-20 \({}^{\circ}\)C per MPa [32].