## 4 Case Study
### Economic Analysis of the MSTS Retrofit Scheme for Thermal Power Units Under RLVs
Additionally, since thermal power units cannot provide sufficient flexibility reserves during DPR, the cost of flexibility reserves increases. To maximize the overall economic benefits of the CS, a small amount of wind and PV curtailment is necessary, as illustrated in Figure 6.  
When thermal power units are retrofitted solely with MSTS technology, their maximum ramping rate between adjacent time periods is 3.28 MW/min, resulting in weaker real-time response to renewable energy fluctuations. However, the coal consumption and operational maintenance costs are significantly reduced. Figure 7 shows that the MSTS system charges during DPR valley periods, participating in DPR ancillary services, and discharges during normal or full-load operation. This leads to a significant increase in PR revenue. Additionally, the net load fluctuations of the thermal power unit are substantially reduced, thereby decreasing flexibility reserve costs and significantly increasing total revenue compared to Scheme 1. When the TPUMSTSR under RLV model is used, the maximum ramping rate is 5.10 MW/min, which is higher than in Scheme 2, thereby enhancing the unit's ability to accommodate renewable energy in real time and further reducing flexibility reserve costs. Figure 8 shows that the overall energy storage output in Scheme 3 is lower, leading to a reduction in energy storage costs. However, due to RLVs, the coal consumption costs are slightly higher than in Scheme 2. At the same time, the RLV technology compensates for the limitations of thermal efficiency and storage capacity in the MSTS system, allowing the unit to participate more frequently in DPR ancillary services. This increases PR revenue, leading to higher total revenue compared to Scheme 2. In summary, the proposed TPUMSTSR under RLV model enhances the economic efficiency of the CS, yielding higher overall operational revenue, improving renewable energy accommodation capability, and reducing the peak-valley difference of the net load for thermal power units. In this case study, the total profit improvement of Scheme 3 compared to Scheme 2 is relatively small. This is mainly due to the relatively small fluctuations and peak-valley differences in the current load demand curve, which does not fully reflect the advantages of RLV retrofitting. Additionally, the mechanical wear and tear costs of thermal power units are not included in the day-ahead optimization model for a short time scale. However, the proposed retrofitting scheme is beneficial in reducing mechanical wear and extending the service life of thermal power units. From a long-term perspective, considering the increasing requirements for real-time power output adjustments in future coal-fired flexibility retrofits, TPUMSTSR under RLVs will demonstrate greater advantages over extended time scales. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Category** & **Operation and Maintenance Costs** & **Flexibility Reserve Costs** & **Pollution Costs** \\ \hline Scheme 1 & 5,626,987.09 & 41,115.37 & 1900.02 \\ Scheme 2 & 5,280,215.66 & 40,272.32 & 1901.94 \\ Scheme 3 & 5,298,865.02 & 38,006.62 & 1896.77 \\ \hline Category & Thermal Storage Costs & PR Benefits & Total Benefits \\ \hline Scheme 1 & 0 & 6,492,766.83 & 4,346,768.08 \\ Scheme 2 & 105,089.79 & 6,674,692.46 & 4,826,702.50 \\ Scheme 3 & 68,817.64 & 6,727,512.16 & 4,899,415.86 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: The optimized revenue and cost results for the CS under the three schemes (CNY).