## 4 Discussion
### Economic Efficiency  
There are several economic evaluation approaches involved in these studies, including primary energy consumption, energy production, operational costs, and other relevant factors (e.g., net present value, payback period, internal rate of return). According to the selected studies, a suitable CHP configuration will significantly affect the values of these parameters in comparison with conventional separate energy generation. Under the CHP mode, there will be a significant reduction in fuel consumption [37]. In the work of [PERSON] and [PERSON] [35], seven different types of commercial buildings (e.g., hotel, school, small office, and supermarket) in the USA were evaluated using a CPH system. Their results show that the primary energy consumptions would be reduced for most of the buildings. The maximum reduction was obtained corresponding to the highest R\({}_{h}\) value (fraction of the thermal load) by around 8.8%. [PERSON] and [PERSON] [21] compared different cogeneration units with various operating hours in a hospital in Greece. They found that, when using a cogeneration system with appropriate operational hours and backups, the annual primary energy consumption would be reduced by 28%. Additionally, with a simulated optimal CPH system, it was predicted that the consumption reduction in residential buildings in Italy could be up to 13.4% with respect to the conventional system [22]. Furthermore, through operational data and transient modeling, [PERSON] and [PERSON] [36] found that the CPH system reduced almost 14.12 GWh of primary energy consumption. The reduction of energy consumptions primarily helps to reduce net present costs over the \begin{table}
\begin{tabular}{c c} \hline \hline
**Abbreviations** & **Full Names** \\ \hline BCR & Benefit–cost ratio \\ CC & Combined cycle \\ CER & CO\({}_{2}\) emission reduction \\ CHP & Combined heat and power \\ GT & Gas turbine \\ ICE & Internal combustion engine \\ IRR & Internal rate of return \\ NA & Not applicable \\ NPV & Net present value \\ PB & Payback \\ PEC & Primary energy consumption \\ \hline \hline \end{tabular}
\end{table} 
Table 2: A list of all abbreviations and their corresponding full names. separate energy generations [38; 39], and save a great deal of money based on the demanded fuel [37]. It also crucially affects the payback period and internal rate of return on the initial investment [14]. Regarding the energy production, the cogeneration system can generate both electricity and heat energy simultaneously from a single system using a single fuel, differing from the conventional system. In the study of [PERSON], [PERSON], [PERSON], [PERSON] and [PERSON] [25], a cogeneration system, consisting of fuel cell, photovoltaic system, and battery system, was proposed to fulfill the energy demand of a selected hospital building in Malaysia. Based on the simulation results, the proposed CPH system could produce the highest energy (251,760 kWh/yr), and the excess electricity of this system (69,003 kWh/yr) could be stored in the battery. The system has the lowest levelized cost of energy ($0.0841/kWh), which will save up to 30% of the cost on power generation. The effective energy production of the CPH has also been reported by [PERSON], [PERSON] and [PERSON] [34], who showed that harnessing the resource of industrial heat via the CHP configuration can generate electricity with lower associated energy than the current configuration. Thus, an ideal system represents a more effective and efficient way to generate useful heat and electricity [24; 26] and, consequently, positive primary energy savings.  
The operational costs also benefit from the CPH implementation. Through operational data and transient modeling, [PERSON] and [PERSON] [36] reported that the CPH system reduced $3,364,066 of operational costs for a data center in San Diego, California, with overall system reductions of 46%, in comparison with the existing conventional system.