*3.4. Emissions*

Greenhouse gases (GHG) include water vapor, carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), ozone (O3) and several classes of halocarbons (that is, chemicals that contain carbon together with fluorine, chlorine, and bromine). Greenhouse gases allow solar radiation to enter the Earth's atmosphere but prevent the infrared radiation emitted by the Earth's surface from escaping. Instead, this outgoing radiation is absorbed by the greenhouse gases and then partially re-emitted as thermal radiation back to Earth, warming the surface.

The most relevant greenhouse gases to the energy project analysis are CO2, CH4, and N2O. These GHG can also have a significant impact on global warming.

Table 11 shows the proposed case system with the GHG summary in comparison with a combined cycle power plant.


**Table 11.** The PVS GHG summary.

As can be seen, the system proposed has no GHG emission and has a GHG emission factor of 0.45 tCO2/MWh.

The results obtained in this study establish a relationship with the works done by Lintner [55], where he determined that a PVS installed in a university could deliver enough energy to contribute to reducing the energy demand; or with Mukherji et al. [17], where in their results they concluded that could reduce greenhouse emissions.

#### **4. Conclusions and Future Research Lines**

University sustainability covers both the set of activities aimed at the appropriate use of resources in such a way as to guarantee the permanence and development of the University as an institution and the e ffect that the university's activity can have on the sustainability of society.

Universities continue to have a major responsibility in contributing to a more sustainable world; their actions in favor of sustainability and integrity should be a model for all sectors of society.

Many previous studies have shown the importance of the use of renewable energies within universities to achieve energy, economic, and environmental sustainability.

With the implementation of PVSs in Mexican universities, UNAM contributes both to its own sustainability plan and Mexico's sustainability.

In our case study, a PVS will be installed at ENES-J, which will be interconnected to the electrical grid and will support the electric demand of the Computer Numerical Control (CNC) type Haas Automation model UMC-750 at the Orthotics and Prosthetics Laboratory (OPL). The CNC will work 5 days a week for 4 h a day, with a peak load of 22.4 kW, and an energy required of 448 kWh per week.

UNAM's sustainability plan includes energy savings. To help achieve this, in the facilities of ENES-J in Queretaro in an area of 96.7 m2, 50 solar panels of type mono-Si Advance Power API-M330 with an e fficiency of 17.83% and a capacity factor of 20.4% will be installed, and they will provide 17.25 kW of power and 345 kWh of energy. The financial assessment shows initial costs of 46,575 \$/kW, O&M costs (savings) of 569 \$/kW-year, electricity export rate-monthly of 0.10 \$/kWh, electricity exported to the grid of 21.5 MWh, and electricity export revenue of \$2,145. Using this PVS, the ENES-J will save \$12,089 per year, equivalent to 24,566 liters of fuel or 23.6 of barrels of crude oil not consumed. A sensitivity analysis, or what-if analysis, was done between machine hours and electricity price and solar irradiance and electricity exported to the grid. In the first analysis it can be proven that if the CNC machine works more hours per day, the use of PVS can save money; in the second analysis, the electricity exported to grid increases if solar irradiation does, so that if the CNC machine works more than 5 h per day and there are more than 6.1 kWh/m<sup>2</sup>/day, the PVS will contribute to saving money.

It was found that the levelized cost of electricity (LCOE) is 15.5 cents/kWh, which allows for reducing the costs during the lifetime of the project.

The gross annual reduction in GHG emissions will be 10.2 tCO2, with a GHG emission factor of 0.45 tCO2/MWh. Considering these results, the implementation of the PVS is feasible, contributing to the ENES-J sustainability plan.

Comparing this technology with a combined cycle power plant, the contribution to reducing GHG represents a reduction of CO2 emission factor of 278.9 kg/GJ, or CH4 emission factor of 0.0108 kg/GJ and N2O emission factor of 0.0072 kg/GJ.

As future research, it would be interesting to use other renewable energy sources such as biomass, wind, and thermal energy, not only for individual use, but also to improve energy sustainability at Universities.

**Author Contributions:** Conceptualization, Q.H.-E., A.R.-J., J.M.D.-G., M.-A.P.-M. and A.-J.P.-M.; methodology, Q.H.-E., A.R.-J., J.M.D.-G., M.-A.P.-M. and A.-J.P.-M.; formal analysis, Q.H.-E., A.R.-J., J.M.D.-G., M.-A.P.-M. and A.-J.P.-M.; investigation, Q.H.-E., A.R.-J., J.M.D.-G., M.-A.P.-M. and A.-J.P.-M.; resources, Q.H.-E., A.R.-J., J.M.D.-G., M.-A.P.-M. and A.-J.P.-M.; writing—original draft preparation, Q.H.-E., A.R.-J., J.M.D.-G., M.-A.P.-M. and A.-J.P.-M. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Acknowledgments:** The authors acknowledge RETScreen software for providing a license.

**Conflicts of Interest:** The authors declare no conflict of interest.
