Broccoli processing generates a large amount of organic waste; around 50% of the product is discarded in the form of stems, leaves, and vegetable grits. The 50% fraction of broccoli residues was established based on an operational relationship reported by Eng. Ramón Romero Campos, Head of the Plant, during the field data collection. According to the plant’s operating conditions, approximately 40 t of broccoli is processed per day. Of this amount, approximately 20 t corresponds to the usable finished product (florets), while the remaining 20 t consists mainly of stems and leaves, which are considered residual biomass. Therefore, a 50% residue fraction was used to estimate the available feedstock for biogas production. Since broccoli processing depends on seasonal raw material availability, the estimated 20 t/day of residual biomass represents the availability during peak production periods and was not assumed to be continuously available throughout the year.

Similarly, mango processing generates 46% of waste in the form of stems and pits. It should be noted that the processing of these foods is seasonal, so the waste is not constant throughout the year. To estimate biogas generation, literature on the production potential of the respective waste will be reviewed to understand technical aspects such as organic matter processing, retention days, volume of biogas obtained per kg of waste, and percentage of methane in the biogas [5]. In the case of broccoli, anaerobically digested, 22 L can be produced per kg of plant material with a retention time of 18 to 20 days using a 2:1 ratio, i.e., two parts broccoli to one-part water at room temperature with a minimum methane quality of 55% in the biogas and a PC = 15,350 kJ/kg [2]. For mango waste, anaerobically digested, 7.5 ml of biogas per gram of substrate are obtained with a retention time of 29 days using a 1:1 ratio at room temperature, with an elemental analysis of 43% = C, 6% = H, and 0.6% = N. Using a stoichiometric ratio, this yields a methane quality of 26.6% and a PC = 6300 kJ/kg [3]. Using the information obtained, biogas production will be estimated in each case, taking into account the amount of waste generated. Using the volume of biogas obtained, the biodigester and the engine-generator suitable for utilizing the fuel supply will be sized, and the energy generated will be calculated to meet the energy demand. Once the energy produced is obtained, the savings will be graphed and quantified using the average kWh cost in the tariff schedule during the hours the power will be on, as well as the investment cost and return on investment.

Operational Assumptions of the Biogas System

The biodigester start-up is considered using cattle manure from an operating biodigester as inoculum, occupying 20% of the useful working volume, while the remaining volume is initially completed with water [6]. Subsequently, the biodigester is fed daily with broccoli waste and water at a 2:1. From the first day of broccoli feeding, a retention time of 20 days is considered before biogas production is available for use.

The reference used for broccoli digestion does not report an additional biogas filtration process; therefore, no specific gas-cleaning stage was included in the present proposal. The selected motor-generator is sized to consume the volume of biogas produced daily, so no long-term gas storage is considered under normal operation.

The digestate resulting from anaerobic digestion is considered suitable for subsequent use as an organic soil amendment in agricultural land. Electricity generation is planned to complement the plant’s electricity consumption during the hours with the highest electricity cost. Since the plant does not operate on Sundays, this day is considered suitable for preventive maintenance of the motor-generator. In addition, during periods in which the biodigester is not producing biogas due to the absence of broccoli waste, general maintenance of the biodigester can be performed.

To calculate the photovoltaic sizing, the average daily energy consumption (kWh/day), the power of the selected panel, the system efficiency, and the available peak solar hours in the region will be considered to quantify the number of photovoltaic modules required [7]. Various factors will be considered, such as the available area (5000 m²), coordinates and orientation (19˚58’09”N 102˚17’34”W). The module selected for the system is a 660 W “Vertex monocrystalline” module with an efficiency of 21.6%, a height of 2.38 m, and a width of 1.30 m. Considering the module dimensions, the appropriate quantity for the available area will be proposed, and the ideal distribution will be simulated to prevent the modules from being shaded and the system’s efficiency from decreasing. Using the number of modules and HSPs available throughout the year, the daily, monthly, and annual power generated will be calculated, and the inverter(s) for the system will be selected. Knowing all this data, the investment cost of the system will be estimated, considering modules, panel structure, wiring, and labor. Savings will be calculated based on the energy generated and graphed by comparing annual consumption and annual generation. The return on investment will be determined based on the investment cost and savings.

LP gas fuel consumption is 500 L on an ideal work day, processing 20 tons of broccoli in a 16-hour workday. The boiler operates at 1205 HP = 898 kW with water supply at 22˚C and steam output at 134˚C for 17 hours. The daily water volume requirement can be calculated using data obtained from the boiler, where the volumetric flow rate can be easily obtained, and the total daily water volume can be estimated using the operating hours.

The fuel cost per ton of production per day, month, and year will be estimated. In this case, energy will be analyzed in terms of heat Q = kJ, where the energy supplied by the LP gas to heat the water from 22˚C to 134˚C will be calculated. The energy input for preheating using solar collectors, where the water temperature reaches 90˚C, will also be determined. Knowing the volume of water and the region’s irradiation = 5.76 kWh/m² [4], the preheating system can be sized, therefore suitable equipment will be selected and the investment cost will be determined. Knowing the heat supplied by the solar heater, the equivalence in liters of LP Gas will be calculated. With these equivalent liters and considering the cost of LP Gas, the savings generated by preheating can be calculated. Finally, the return on investment will be estimated and the heat supply in both cases will be graphed.

Biogas production from mango waste is ruled out due to the low methane content. For biodigester dimensions, the volume is calculated using the kilograms of broccoli waste and its water ratio (2:1) over the retention days.

According to the literature, the amount of Biogas per day and per month is calculated depending on the amount of waste after the first 20 days of retention.

For the selection of the biodigester, the total volume of the mixture during the 20 days of retention is considered, as can be seen in Table 3.

Table 3. Mixing volume to Biodigestor sizing (20 days of retention).

A continuous flow biodigester from JINGCHENG WEIYE company with a capacity of 650 m³ was selected.

With the available volume of biogas, its Heat Value (HV) = 15,350 kJ/kg, its density = 1.57 kg/m³, working hours, and 25% efficiency of a biogas engine, the potential for electrical generation will be obtained to then choose the engine-generator that adapts to the fuel supply.

Knowing the generation potential, a motor-generator is proposed that meets the daily fuel supply capacity, thus determining the engine’s operating hours and the energy generated at rated power. Motor-generator selection. Model TG7.IDG 70 kW, 15-year life. All this is done with the help of the engine’s technical data.

The generator motor requires a supply of 0.6 m³/kWh, so that it works at its nominal power of 70 kW (Figure 3), considering the volume of biogas produced per day to know how long it is able to keep the motor running and how much energy it generates (Table 4).

To calculate the savings, the average cost of kWh is calculated in the GDMTH tariff scheme (Table 5) and the schedule that the engine-generator is intended to be kept on, which is from 11:30 am to 10:00 pm.

Figure 3.Parameters and supply details for 70 kW Biogas Generator.

Table 4.Electric generation with biogas.

The economic savings from biogas electricity generation were calculated according to the GDMTH tariff periods corresponding to the proposed operating schedule of the motor-generator. From Monday to Friday, 6.48 h/day of generation corresponds to the intermediate period at a cost of $2.006 MXN/kWh and 4 h/day to the peak period at $2.253 MXN/kWh, considering 155 operating days. On Saturdays, 8.48 h/day corresponds to the intermediate period and 2 h/day to the peak period, considering 32 operating days. Based on the 70 kW generator capacity, the weighted calculation results in an equivalent electricity cost of approximately $2.0922 MXN/kWh. Therefore, this value represents the weighted cost of the electricity displaced during the actual generation periods rather than a general average electricity tariff.

Table 5.Average cost per kWh during electricity generation hours.

Figure 4 shows the energy that can be generated by the biogas engine generator compared to the total energy consumption during the year. Knowing the savings and investment cost (Table 6) allows to estimate the return on investment and the economic benefit over time.

Figure 4.Consumption vs. electricity generation with biogas.

Table 6.Biogas engine generator investment costs.

Sensitivity Analysis

The biogas production proposal was evaluated considering the main parameters involved in the calculation. The available data establish a biogas yield of 22 L/kg of broccoli waste, with a minimum methane content of 55% and a retention time of 18 - 20 days. For the design calculations, a retention time of 20 days was adopted.

The system considers 20,000 kg/day of broccoli waste mixed with 10,000 kg/day of water, resulting in a daily mixture volume of approximately 30 m³. With a 20-day retention time, the required volume is 600 m³, for which a 650 m³ continuous-flow biodigester was selected.

For electricity generation, 25% generator efficiency was considered. The proposed 70 kW motor-generator requires approximately 0.6 m³ of biogas per kWh generated.

The number of operating days varies according to broccoli availability, with the available records considering 15 days in November, 25 in December, 26 in January, 24 in February, 26 in March, 26 in April, 25 in May, and 15 in June. Therefore, annual biogas and electricity production are directly conditioned by the seasonal availability of broccoli waste and the actual operating days of the plant.

The sizing was based on the average daily energy consumption, Peak Sun Hours (PSH), power of the selected panel, and system efficiency. According to the guidance, the efficiency drops to 85%. Various estimates were made using minimum, average, and maximum consumption to determine the number of modules needed (Table 7) to cover the energy supply in each case (Table 8), and these were graphed along with the annual consumption (Figure 5).

Table 7.Number of panel.

Table 8. Simulated photovoltaic generation for a typical local year.

Figure 5.Total consumption vs. photovoltaic generation: minimum, average, maximum.

For a real sizing, important factors were taken into account such as the available and optimal area for the installation of the system, in this case 100 m × 50 m, the dimensions of the photovoltaic module with a height of 2.38 m and 1.30 m wide and the optimal distance between panel sheets to avoid shadows and guarantee the efficiency of the system. The sizing for the optimal distribution of the panels was carried out with the help of the PVsyst software [8] that simulates the solar path as can be seen in Figure 6 and Figure 7, while the resulting monthly percentage values are compiled in Table 9.

Figure 6.Dimension and simulation (software PVsyst) [8].

The electricity generated from biogas is intended for on-site consumption during plant operation. As shown in Figure 6, the monthly electricity generation from biogas remains below the plant’s electricity consumption; therefore, no electricity export to the grid or generation curtailment is considered in the proposed operating scheme, integrated with the complete monthly photovoltaic yield performance detailed in Table 10.

Table 9.Efficiency losses due to shadows (Software PVsyst) [8].

Table 10.Photovoltaic generation (PVsyst) [8].

Figure 7.Photovoltaic generation (PVsyst) [8] vs. consumption.

For the investment cost of the photovoltaic system, 962 Vertex modules, model TSM-DE21 660 W, will be taken into account. The height is 2.50 m at the highest point and 0.3 m at the lowest point, therefore a manufactured aluminum structure is considered. To select the inverters, the installed power is taken into account, in this case 508 kW. Three inverters of 125 - 150 kW Mod. MAX 124 - 150KTL3 and one inverter of 50 - 70 kW Mod. MAC 50 - 70KTL3-X LV/MV; of the Growatt brand were selected. Another important part of the investment is the wiring used, cabinets, trays and/or pipes, accessories, connectors, switches, electrical protections and finally the cost of labor, which accounts for 30% of the total investment cost (Table 11). The lifespan of a high-efficiency photovoltaic system is approximately 25 years.

Table 11.Photovoltaic system investment costs.

The boiler operates at 1200 HP with a feedwater temperature of 22˚C and generates steam at 134˚C, which is supplied directly to a water vat to maintain it at 95˚C and be used in the broccoli precooking process. To determine the heat supplied, the volume of water required during the day must be determined, considering that the boiler is fired for 17 hours, i.e., 1 hour for the vat water to reach the ideal process temperature and 16 hours to maintain the water temperature (Table 12). The fuel consumption log records a consumption of 500 L of LP gas for a daily production of 20 tons of broccoli. Figure 8 shows the heat input of the preheating system compared to the current LP gas input. This calculation methodology was obtained from the book Solar Engineering of Thermal Processes [9] and standard solar thermal design fundamentals [10].

To know the volume of water needed during the day.

Thermal efficiency of the boiler is 85%, calorific value (HV) of LP gas equal to 46,000 kJ/kg and a liquid state density of 500 kg/m³.

Knowing the volume of water per day, it is possible to size the number of solar collectors to heat the water up to 90°C (Table 12).

To select the preheating system, the volume of water required for steam generation is taken into account; in this case, the volume is 20,925.6 L/day. A parabolic solar collector system was chosen. This calculation methodology was obtained from the book Solar Engineering of Thermal Processes [9]. The economic savings are shown in Table 12 and Figure 8.

Table 12.Heat supplied by solar preheating and LP gas.

Figure 8.Heat supplied LP Gas vs. solar preheating heat.

Collectors of the brand “Shandong Gaia PT-30” with a useful life of 15 years and a 15,000 liters’ storage tank with thermal insulation coating, were selected since the production day is 16 hours a day and water has to be stored for the hours when there is no solar radiation. With the savings generated by the preheating system and the investment cost (Table 13) the return on investment is estimated.

Table 13. Investment cost.

Considering the sum of the two proposals for electricity generation (Table 14), a significant part of the energy consumption can be satisfied, as we can see in Figure 9.

Table 14. Total generation with biogas and photovoltaic panels.

Figure 9.Consumption vs. total generation.