## 4 Results and Discussion
For the simplicity of analysis, the annual maintenance cost is assumed to be 15 USD. Both the battery and inverter are not considered during the life cycle cost analysis; therefore, the annual replacement cost of the components is taken as zero. Other information for the cost analysis such as the domestic cost of energy or electricity, the discount rate, and the inflation rate specific to a region has been taken from the local government and private departments and is given in Table 4. \begin{table}
\begin{tabular}{c c c} \hline \hline  & **Dual-Fluid PV/T** & \\  & **System** & **Single-Fluid PV/T System** \\ \hline
**Components** & **Price (USD)** & **Price (USD)** \\ PV module & 150 & 150 \\ Heat exchanger & 530 & 290 for water type and 260 for air type \\ Charge controller & 150 & 150 \\ Digital flow meter & 120 & 120 \\ Pump & 90 & - \\ Air blower & 60 & - \\ Pump or Air blower & - & 90 or 60 \\ Water Tank (for water-type only) & 100 & 100 \\ Piping or ducting & 90 & 60 for piping and 50 for ducting \\ Installation & 150 & 150 \\ Total & 1440 & 1100 for water type and 940 for air type \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Total Initial Investment. Figure 9: Monthly variations of thermal and electrical powers of the PV/T systems across the year. In finance, a discounted cash flow (DCF) is a widely used method because it estimates the current value of an investment based on expected outcomes that investment can produce. In fact, DCF values a project considering the concept of the time value of money. Variations of DCF across the 25-year life span of the PV/T system with dual and single fluid heat exchangers are presented in Figure 10. There is no component to be replaced, therefore, the value of DCF for all systems increases linearly without any decrease throughout the 25-year life. The performance of each system is reflected in the form of life cycle cost; this means higher energy output will lower the cost payback time. In addition, Figure 11 compares the net present value (NPV) of the aforementioned systems from the 1 st year to the 25 th year. The point where the line crosses the \(x\)-axis depicts the DPBT for the corresponding system. The DPBT for the PV/T system with dual-fluid, water, and air are 3, 5, and 6 years, respectively. It is observed that the dual-fluid PV/T system produces additional thermal and electrical energy due to efficient cooling performance, which results in a higher present value and a lower cost payback period compared to the water and air-based PV/T systems. \begin{table}
\begin{tabular}{c c} \hline \hline
**Description** & **Values** \\ \hline Total investment & 1440 USD for dual-fluid, 1100 USD for water type, and 940 USD for air type \\ Annual maintenance cost & 15 USD \\ Cost of electricity [21] & 0.077 USD per kW/h \\ Domestic electricity inflation rate & 2.0\% \\ Inflation rate [22] & 1.5\% \\ Discount rate [22] & 0.5\% \\ Annual degradation of PV \& heat exchanger performances [1] & 1\% and 1.5\% \\ Life of system & 25 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Parameters for life cycle cost analysis. Figure 10: Variations of the discounted cash flow (DCF) of the PV/T systems across 25 years. The energy production factor (EPF) is an important energy matrix that defines the relationship between the overall energy yields by the system and the invested energy over the 25 years of the system's life. It is defined as the ratio of energy gain to input energy. Furthermore, from an environmental perspective, the PV/T systems can also earn carbon credits by mitigating CO2 emissions into the atmosphere. EPF, CO2 mitigation, and carbon credits considering the life of the PV/T system with dual-fluid, water, and air heat exchangers for 20 and 25 years are given in Figures 12 and 13. For the dual-fluid, water, and air type PV/T systems considering a 25 year lifetime, the highest values of EPF and carbon credits are 5.1 and 175.9; 4.4 and 142.4; and 3.8 and 127.7; respectively. It is observed that the dual-fluid PV/T system has the highest EPF and carbon credits.