*4.1. Primary Energy Consumption*

Final primary energy (PE) consumption is calculated according to Equation (6) where the exported electricity has a positive effect because of the decrease in PE consumption (negative in the equation). Figure 7 groups the graphs with the simulation results for the PE reduction in the vertical axis as the percentage value of the PE consumed with the CHP operative over the PE consumption calculated for the dataset (without an operative CHP installation). Figure 7a1,a2 resume the simulation results under the "Natural gas scenario" for single dwellings and 10-dwelling buildings, respectively. Similarly, Figure 7b1,b2

summarize the results for the same dwellings when the "green gas scenario" is simulated, while Figure 7c1,c2 correspond to the results for the "expected scenario".

## *4.2. Carbon Emissions Results*

Figure 8a–c show the percentage of CO2 emissions over current emissions. Values are determined by Equations (7) and (8). These plots are similar to PE reduction graphs due to the linear relationship between the two variables, but it should be noted the relative value in each scenario.

## *4.3. Cogeneration and Energy Demand Rates*

The graphs on this section are independent of the scenarios because the energy balances depend on the consumption and the CHP parameters but not on the fuel. Figure 9a,b show the importation and exportation of electrical energy from/to the utility over the electrical demand of each individual and building dwellings respectively.

**Figure 7.** Primary energy consumption reduction percentage over the initial PE per technology and dataset. (**a**) Natural gas scenario simulation results; (**b**) green gas scenario; (**c**) expected scenario. Plots on the left, labeled with (**1**), correspond to the single unit dwellings. Plots on the right, labeled with (**2**), are for the 10-dwelling buildings datasets.

**Figure 8.** Relative CO2 emission over the current situation of the dwellings (**a**) using natural gas-fueled CHP; (**b**) considering all the systems powered with green fuel as hydrogen (carbon-free); (**c**) expected scenario, each CHP system with the corresponding fuel and back-up heaters powered with natural gas. Plots on the left, labeled with (**1**), correspond to the single unit dwellings. Plots on the right, labeled with (**2**), are for the 10-dwelling buildings datasets.

**Figure 9.** Electrical energy import/export over the electrical demand Results of the electrical energy exchange with the grid over the demanded energy for (**a**) the single-dwelling simulation; (**b**) the 10-dwelling buildings simulation.

**Figure 10.** Results for the relative electrical production over the electrical energy demanded. (**a**) Individual dwellings. (**b**) 10-dwelling buildings.

**Figure 11.** Relative thermal energy produced with the CHP vs. thermal demand. (**a**) Individual dwellings. (**b**) 10-dwelling buildings.

**Figure 12.** Operating time over the total year-hour rate. (**a**) Results for individual dwelling simulation. (**b**) Results for 10-dwelling building simulation.

In Figure 10a,b, the results for the relative electrical production over the electrical energy demanded are depicted. The plots show the capacity of the CHP system to meet the electrical energy demand of the consumers. Values above 100% mean that the system exceeds the electrical demand resulting in an energy surplus.

Figure 11a,b show the percentage of thermal energy produced with the CHP against the thermal demand. When the system is well-sized the value is 100% because the entire thermal demand is met with the CHP production. When the system is under-sized, the thermal production has to be compensated with some thermal energy produced with the back-up boiler. There is not a thermal energy surplus because the system control was set to feed the thermal demand without exceeding it.

The use of CHP systems has to be economically viable. This viability depends not only on the facility cost, but also on the rate of use. Figure 12a,b show the percentage of the year-hour that each technology will operate in each dwelling and building respectively. The time is calculated with Equation (1) as a function of the thermal demand and the production capacity of the CHP system. As can be observed in Figure 12a for the individual dwellings simulation, where the fuel-cell-based CHP systems present a higher duty cycle, the smaller the rated power, the higher the duty cycle. A similar behavior is observed in the results for the 10-dwelling building simulation (Figure 12b).
