**1. Introduction**

Climate change is the greatest challenge facing humanity. All the agents involved, from leaders to citizens, must become aware of the problem and join efforts in the fight against global warming. All countries must agree on mitigation measures against climate change that involve ambitious targets for reducing greenhouse gases. To reach the Paris agreemen<sup>t</sup> and limit the temperature increase to 2 ◦C by the end of the century, or to 1.5 ◦C if possible, a major shift is needed in current energy policy towards a low-carbon economy, where on the one hand the share of renewables in the current energy mix is enhanced and on the other hand the energy e fficiency of energy systems is increased [1–3]. Universities, within the framework of their social and environmental responsibility, cannot stay away from these objectives [4,5]. Universities, research centers, and institutions are the starting point of research in renewable energy knowledge and practice. These institutions have carried out several projects on renewable energy sources (RES), like the proposal on compressed air as reserve energy, made at the University of Auckland in New Zealand [6]. Zhou et al. [7] and the Ocean University of China (OUC) developed a methodology based on the study of the dynamic characteristics of an actual O ffshore Wind Turbines (OWT) in di fferent operational states based on sea tests to determine the negative impact on their structures. In Tokat Gaziosmanpasa University, Emeksiz and Cetin [8] analyzed the e ffects of tower shadow disturbance and wind shear variations. In this study, they determined that the x distance is the most correlated parameter on the tower shadow disturbance problem. Karasmanaki and Tsantopoulos [9] researched the attitude of RES students in the Department of Forestry and Management of the Environmental and Natural Resources at the Democritus University of Thrace in Greece since they are possible experts in RES, and the results show that students support renewables and have awareness about current polluting energy systems. Tran and Smith [10] developed an analysis on renewable energy systems integration and uncertainty to meet the three major types of energy consumption: Electricity (solar photovoltaic and wind), heating (combined heat and power) and cooling (electricity) at facilities on the campus of the University of Utah. They found that the uncertainty of energy loads and power generation from renewable energy heavily a ffects the operating cost of the district energy system.

Arnaout et al. [11] explain that the Heriot-Watt University Malaysia (HWUM) has a unique roof design that could be utilized as part of the Building-integrated photovoltaics (BIPV) system to generate electricity, thereby reducing the carbon footprint of the facility. This system is an innovative green solution that makes possible energy generation on the building facade with modification of the building material or architectural structure.

Sierra et al. [12], during characterization of an 840Wp BIPV installed at the Faculty of Sciences at the National University of Colombia, found that the energy generation with coal has a greater negative environmental impact (84%) compared with the photovoltaic system (PVS) (6%), and the use of PVS represents an emission factor of 35 gCO2eq/kWh.

A techno-economic and environmental analysis of a PVS conducted by Sulukan [13] at the Turkish Naval Academy of the National Defense University was done. This study was carried out using the RETScreen software considering performance, e fficiency, inverter e fficiency, and temperature on PVS. The results of emission were a 93% reduction in greenhouse gases and a saving of 721.1 tons of crude oil.

Bilcik et al. [14] dealt with the impact of temperature on module surfaces The experiment was conducted at the University of Life Sciences in Prague and found that the performances of photovoltaic modules depend on climatic conditions.

In Morocco, Ameur et al. [15] evaluated di fferent solar photovoltaic technologies (amorphous silicon (a-Si), Polycrystalline silicon (pc-Si) and Monocrystalline silicon (mc-Si)) connected to a low voltage three phases electrical grid of Al Akhawayn University. The results show that the polycrystalline panel is the most cost-e ffective technology.

Silveira et al. [16] presented an economic study about electric power generation using PSV in a Brazilian university with the aim of reducing the electric consumption. The results show that with the PSV, the tari ff can be reduced by 39.9%.

Mukherji et al. [17] present a techno-economic and ecological analysis of a 50 kWp rooftop solar photovoltaic plant installed at ICFAI University, Jaipur where the plant produced

106.9 kWh/kWp/month, and the reductions in greenhouse gases obtained were 102tCO2, 128 kg SO2, 268 kg NOx and 7033 kg ash.

Oh and Park [18] analyzed the optimal orientation of solar panels, and the analysis was performed regarding demand and supply electricity at the Seoul National University. Their results show that orientation is very important and depends on the building's architecture. The output generation presented a low electricity demand in the evening, but monthly demand shows a pattern opposite to solar power generation.

At the Marmara University in Istanbul, a study done by Akpolat et al. [19] demonstrates that the installation of grid-connected rooftop solar photovoltaic systems of 84.75 kW can produce several benefits and an annual electrical savings of 90,298 kWh.

Another study was done by Al-Najideen et al. [20] at the faculty of Engineering-Mu'tah in University of Jordan in order to reduce the electricity demand with a 56.7 kW grid-connected PVS. The results show that this PVS will produce 97.02 MWh per year, with an investment of 117,000 USD and a payback period of 5.5 years.

A hybrid system composed by a photovoltaic (PV) panel, wind turbine, and storage batteries installed in Yildiz Technical University is analyzed by Arikan et al. [21] and determines the most accurate system sizing using the maximum power point tracking controller, a hybrid controller, and an inverter.

An Artificial Neural Network (ANN) methodology for studying and modeling the soiling e ffect on solar PV glass has been done by Laarabi et al. [22]. They exposed outdoor solar PV glazing at Mohammed V University in Rabat (Morocco), and found that the most influential parameter for the PV soiling rate was relative humidity, followed by wind direction.

An ensemble approach to predict solar PV power production has been proposed by Al-Dahidi et al. [23]. This ensemble approach has the capability of handling the intermittent nature of solar energy. They demonstrated it using a grid-connected solar PV system of 231 kW of capacity installed on the rooftop of the Faculty of Engineering at the Applied Science Private University of Jordan and it was determined that this methodology could allow for balancing power supplies. Since the installation of rooftop PV arrays is increasing, many standards have been designed, like the study done by Bender et al. [24] at Central Washington University (CWU) where they present one calibration done at the rooftop of CWU.

According to Wen [25], the interaction between the University-Firm-Government linkage has been discussed several times. In the special case of the solar photovoltaic industry in Taiwan, he found that this industry is essential for the development of the University-Firm-Government linkage.

As it can be seen in many universities studies about RES which have been done, with these e fforts, institutions contribute to developing these systems, because in the end, the main objective of renewable energy is to create development where there is none and increase it where there is. RES faces some problems. One of them is this premise: where there is no price for carbon emissions, there is no reason to reduce carbon [26]. This premise is not related to a very important decision-making variable called sustainability, which it can be defined as the use of today's resources without compromising the ability of future generations to be at least as well o ff as we are [27]. Several authors coincide that sustainability is a concept with many interpretations, including economic, environmental and social ones, and completesustainabilityrequiresallofthem.Thisisalsoreferredtoasthetriple-bottom-line[28–30].

 In 1990, in France, a statement made by university presidents, chancellors, and rectors commited to environmental sustainability in higher education. This statement was called the Talloires Declaration (TD) and has ten actions to incorporate sustainability into institutions [31–34].

There is a ranking that grouped 619 worldwide universities and awards the best sustainable policies incorporated in these universities. This is the *UI GreenMetric World University Ranking,* which gives basic information about the university's policy towards a green environment. Its aim is to trigger the participating university to provide more space for developing sustainable energy [35]. From the results of this ranking, 11 Mexican universities are in di fferent positions; the most important Mexican University is the National Autonomous University of Mexico (UNAM) and is in 101st place from 619 universities worldwide.

UNAM developed a macro project for the transformation of the University into a model of efficient and intelligent energy use, by which it is expected to obtain electricity savings ranging from 20 and 30 percent by using solar, biomass, and hydrogen energy, as well as the promotion of green culture.

With the title of "The University City and Energy", the macro project, which can serve as an example to other communities in the country, includes 21 projects contained in 6 research lines, highlighting the creation of public lighting with solar energy, saving electric energy with the use of photovoltaic technology, and using the prototype of a multifunctional ecological vehicle and a virtual classroom for learning and teaching on the subject, among other things.

In this work, the software RETScreen is used, which will help to determine both the technical and economic feasibility of the study. RETScreen is a Clean Energy Management Software system for energy efficiency, renewable energy, and cogeneration project feasibility analysis, as well as ongoing energy performance analysis. It is developed by Canada's governmen<sup>t</sup> through the Natural Resources Canada office.

UNAM has some campuses outside of Mexico City. One of them is the National School of Higher Studies Juriquilla (ENES-J) in the state of Queretaro, where there is the Orthotics and Prosthetics Laboratory (OPL), in which has been installed a Computer Numerical Control (CNC) machine type Haas Automation model UMC-750, which has 5-axis and is an effective means to reduce setups and increase accuracy for multi-sided and complex parts. This machine will be used to design, build, and assess human prosthesis.

The Kyoto Protocol has established three mechanisms: The Clean Development Mechanism (CDM), Joint Implementation (JI), and Emissions Trading (ET), which allow parties to pursue opportunities to cut emissions or enhance carbon sinks abroad. The cost of curbing emissions varies considerably from region to region, and therefore it makes economic sense to cut emissions where it is cheapest to do so, given that the impact on the atmosphere is the same [36]. It is important to mention that a project can be evaluated as CDM if the project is planned in a developed country.

The main objective of this work is to contribute to sustainability policies at the ENES-J from UNAM implementing a solar photovoltaic system to deliver electricity to the grid and contribute to reducing the electricity load at the Orthotics and Prosthetics Laboratory (OPL), as well to propose new research lines to support the sustainability policies in universities and proposing a financial analysis.

In the following section, the materials and methods are described. In Section 3, the results and discussion are presented. Finally, Section 4 presents the main conclusions and future research lines.

#### **2. Materials and Methods**

The average solar irradiation recorder in the area studied is 6.1 kWh/m<sup>2</sup>/day with an average global horizontal irradiance (GHI) of 800 <sup>W</sup>/m2. The PVS has been designed to contribute with the CNC machine type Haas Automation model UMC-750 electric load and could be applied to any site when data are known.

Figure 1 shows both, the Computer Numerical Control (CNC) machine type Haas Automation model UMC-750 arriving at the Orthotics and Prosthetics Laboratory (OPL) and an image obtained from the machine's website.

**Figure 1.** Computer Numerical Control (CNC): (**a**) CNC installed at the Orthotics and Prosthetics Laboratory (OPL); (**b**) CNC Haas Automation model UMC-750.

All the electric energy at the National School Higher Studies in Queretaro is delivered by the electric grid; in this case, the PVS show both economic and technical benefits.

The electric load demanded by the CNC machine is presented in Table 1.


**Table 1.** The CNC Haas Automation model UMC-750 electric load.

With the electric load presented in Table 1, the electric rate will be the highest one, named Domestic High Consumption Tariff (DHCT), which means that it is the one with the highest prices; the load is in AC.

The ENES-J belongs to UNAM and is located in the city of Queretaro in central Mexico. This city is a semidesert zone, with an average temperature of 16.4 ◦C during the year. The maximum temperature is 28 ◦C in April and May, and the minimum is 6 ◦C in January [37]. In Figure 2 is shown the geographic position of Queretaro Mexico.

**Figure 2.** Queretaro's geographic location.

Data are extracted from the Synoptic Meteorological Station (SMS) that belongs to the National Weather Service (NWS) of Mexico. In the state of Queretaro, there is one SMS, and this meteorological station delivers data every 10 minutes. The process of recording data is as follows: the data acquirer is programmed to record data every 2 seconds and after 10 minutes delivers an average. The variables registered are wind speed, wind direction, air temperature, atmospheric pressure, rain, relative humidity, and solar radiation. This last one is our focus of study. Table 2 shows the main characteristics of these meteorological stations.



#### *2.1. Modelling the Energy*

The software RETScreen has been applied in several studies on renewable energy to determine their feasibility [38–42]. RETScreen rapidly identifies, assesses and optimizes the technical and financial viability of potential clean energy projects. This decision intelligence software platform also allows managers to easily measure and verify the actual performance of their facilities and helps find additional energy savings/production opportunities [43].

Figure 3 shows how the software looks and the variables taken into the assessment.

**Figure 3.** The workflow used in the RETScreen software.

The workflow shown in Figure 3 represents the model used by the software. The data set is the first variable needed, followed by location and facility, the energy project, costs, and emission analysis, then both financial and risk analyses are done.

Figure 4 shows the data used and technology selected in the software.

**Figure 4.** Initial characteristics: (**a**) Meteorological variables used; (**b**) technology used in the software.

Data used in the study are included in the location module (see Figure 4a). The technology used is added in the energy module, as seen in Figure 4b. As we can see in this last figure, the manufacturer, model, number of solar panels, initial costs, and other variables must be included.

#### *2.2. Solar Assessment*

The solar global irradiance assessment considers the variable which is most important to calculate a PVS [44]. Through the photoelectric principle, solar irradiation is converted into electrical energy and can be delivered to consumers. An inverter alternative current (AC) can be used, or a direct current (DC).

The PVS equipment includes a PV panel, a load regulator to regulate the voltage generated by the PV panel (this load regulator can perform a charge cycle on deep cycle batteries), and the protection systems.
